Application of tl494 in voltage converters. Control chip TL494
Description
- Full range of PWM control functions
- Output sinking or sinking current of each output is 200mA
- Can be operated in push-pull or single-stroke mode
- Built-in double pulse suppression circuit
- Wide adjustment range
- Output reference voltage 5V +-05%
- Easy to organize synchronization
Domestic equivalent: 1114EU3/4.
Specially created for the construction of secondary power supplies (SPS), TL493/4/5 microcircuits provide the developer with expanded capabilities when designing SPS control circuits. The TL493/4/5 includes an error amplifier, a built-in variable oscillator, a dead-time comparator, a control trigger, a 5V precision ionizer, and an output stage control circuit. The error amplifier produces a common mode voltage in the range of –0.3...(Vcc-2) V. The dead time comparator has a constant offset that limits the minimum dead time duration to about 5%.
It is possible to synchronize the built-in generator by connecting the output R to the reference voltage output and applying an input ramp voltage to the pin WITH, which is used for synchronous operation of several IVP schemes. Independent output drivers on transistors provide the ability to operate the output stage using a common emitter circuit or an emitter follower circuit. The output stage of the TL493/4/5 microcircuits operates in single-cycle or push-pull mode with the ability to select the mode using a special input. The built-in circuit monitors each output and prohibits the issuance of a double pulse in push-pull mode. Devices with a suffix L, guarantee normal operation in the temperature range –5…85С, with suffix C guarantee normal operation in the temperature range 0…70С.
Block diagram of TL494
Pin layout
Parameter Limits
Supply voltage 41V
Amplifier input voltage (Vcc+0.3)V
Collector output voltage 41V
Collector output current 250mA
Total power dissipation in continuous mode 1W
Operating ambient temperature range:
With suffix L -25..85С
With suffix С..0..70С
Storage temperature range -65…+150С
Description of work
The TL494 chip is a PWM controller for a switching power supply, operating at a fixed frequency, and includes all the blocks necessary for this. The built-in sawtooth voltage generator requires only two external components R and C to set the frequency. The generator frequency is determined by the formula: F osc =1.1/R*C
Modulation of the output pulse width is achieved by comparing the positive sawtooth voltage obtained across the capacitor WITH, with two control signals (see timing diagram). NOR gates drive the output transistors Q1 And Q2 only when the built-in trigger clock line is in LOW logical state. This occurs only during the time when the amplitude of the ramp voltage is higher than the amplitude of the control signals. Consequently, an increase in the amplitude of the control signals causes a corresponding linear decrease in the width of the output pulses. Control signals refer to the voltages produced by the dead time adjustment circuit (pin 4), error amplifiers (pins 1, 2, 15, 16) and the feedback circuit (pin 3).
The dead time comparator input has a 120mV offset, which limits the minimum output dead time to the first 4% of the ramp voltage cycle duration. This results in a maximum duty cycle of 96% when pin 13 is grounded and 48% when pin 13 is referenced.
You can increase the duration of the dead time at the output by applying a constant voltage in the range of 0..3.3V to the dead time adjustment input (pin 4). The PWM comparator regulates the width of the output pulses from the maximum value determined by the potential at the dead time adjustment input to zero when the feedback voltage changes from 0.5 to 3.5V. Both error amplifiers have a common-mode input range of –0.3 to (Vcc-2.0)V and can be used to read voltage or current values from the output of a power supply. The outputs of the error amplifiers have an active HIGH voltage level and combined by function OR at the non-inverting input of the PWM comparator. In this configuration, the amplifier that requires minimal time to turn on the output dominates the control loop. During capacitor discharge WITH a positive pulse is generated at the output of the dead time adjustment comparator, which clocks the trigger and blocks the output transistors Q1 And Q2. If a reference voltage is applied to the operating mode selection input (pin 13), the trigger directly controls two output transistors in antiphase (push-pull mode), and the output frequency is equal to half the generator frequency. The output driver can also operate in single-ended mode, where both transistors turn on and off simultaneously, and when a maximum duty cycle of less than 50% is required. This mode is recommended for use when the transformer has a ringing winding with a clamping diode used to suppress transients. If high currents are required in single-ended mode, the output transistors can be operated in parallel. To do this, you need to short the input of the OTS operating mode selection to ground, which blocks the output signal from the trigger. The output frequency in this case will be equal to the generator frequency.
The TL494 has a built-in 5V reference that can provide up to 10mA of current to bias external circuit components. The reference voltage allows an error of 5% in the operating temperature range from 0 to 70C.
(not TDA1555, but more serious microcircuits) require a power supply with bipolar power supply. And the difficulty here arises not in the UMZCH itself, but in the device that would increase the voltage to the required level, transferring a good current to the load. This converter is the heaviest part of a homemade car amplifier. However, if you follow all the recommendations, you will be able to assemble a proven PN using this scheme, the diagram of which is given below. To enlarge it, click on it.
The basis of the converter is a pulse generator built on a specialized widespread microcircuit. The generation frequency is set by the value of resistor R3. You can change it to achieve the best stability and efficiency. Let's take a closer look at the design of the TL494 control chip.
Parameters of the TL494 chip
Upp.chip (pin 12) - Upp.min=9V; Upit.max=40V
Permissible voltage at input DA1, DA2 no more than Upit/2
Acceptable parameters of output transistors Q1, Q2:
Uus less than 1.3V;
Uke less than 40V;
Ik.max less than 250mA
The residual collector-emitter voltage of the output transistors is no more than 1.3V.
I consumed by the microcircuit - 10-12mA
Allowable power dissipation:
0.8W at ambient temperature +25C;
0.3W at ambient temperature +70C.
The frequency of the built-in reference oscillator is no more than 100 kHz.
- sawtooth voltage generator DA6; the frequency is determined by the values of the resistor and capacitor connected to the 5th and 6th pins;
- stabilized reference voltage source DA5 with external output (pin 14);
- voltage error amplifier DA3;
- error amplifier for current limit signal DA4;
- two output transistors VT1 and VT2 with open collectors and emitters;
- dead zone comparator DA1;
- comparator PWM DA2;
- dynamic push-pull D-trigger in frequency division mode by 2 - DD2;
- auxiliary logic elements DD1 (2-OR), DD3 (2ND), DD4 (2ND), DD5 (2-OR-NOT), DD6 (2-OR-NOT), DD7 (NOT);
- constant voltage source rated 0.1B DA7;
- DC source with a nominal value of 0.7 mA DA8.
IRFZ44N field-effect transistors swing the load (power transformer). Inductor L1 is wound on a ferite ring with a diameter of 2 cm from a computer power supply. It contains 10 turns of double wire with a diameter of 1 mm which are distributed throughout the ring. If you don’t have a ring, you can wind it on a ferite rod with a diameter of 8 mm and a length of a couple of centimeters (not critical). Board drawing in Lay format - download in .
We warn you, the robotic capability of the converter unit greatly depends on the correct manufacturing of the transformer. It is wound on a 2000NM ferite ring with dimensions of 40*25*11 mm. First you need to round off all the edges with a file and wrap it with linen tape. The primary winding is wound with a bundle that consists of 5 cores 0.7 mm thick and contains 2 * 6 turns, that is, 12. It is wound like this: we take one core and wind it with 6 turns evenly distributed around the ring, then we wind the next one close to the first and so on 5 cores The wires are twisted at the terminals. Then, on the wire-free part of the ring, we begin to wind the second half of the primary winding in the same way. We get two equal windings. After this, we wrap the ring with electrical tape and wind the secondary winding with 1.5mm wire 2*18 turns in the same way as the primary. To ensure that nothing burns out during the first start-up, you need to turn on the transformer primary through a 40-60 W lamp through 100 Ohm resistors in each arm, and everything will hum even with random errors. A small addition: there is a small defect in the filter block circuit; parts c19 r22 should be swapped, since when the phase is rotated, attenuation of the signal amplitude appears on the oscilloscope. In general, this step-up voltage converter can be safely recommended for repetition, since it has already been successfully assembled by many radio amateurs.
Nikolay Petrushov
TL494, what kind of “beast” is this?
TL494 (Texas Instruments) is probably the most common PWM controller, on the basis of which the bulk of computer power supplies and power parts of various household appliances were created.
And even now this microcircuit is quite popular among radio amateurs who are building switching power supplies. The domestic analogue of this microcircuit is M1114EU4 (KR1114EU4). In addition, different foreign companies produce this microcircuit with different names. For example IR3M02 (Sharp), KA7500 (Samsung), MB3759 (Fujitsu). It's all the same chip.
Its age is much younger than TL431. It began to be produced by Texas Instruments somewhere in the late 90s - early 2000s.
Let's try to figure out together what she is and what kind of “beast” this is? We will consider the TL494 chip (Texas Instruments).
So, first, let's see what's inside.
Compound.
It contains:
- sawtooth voltage generator (SPG);
- dead time adjustment comparator (DA1);
- PWM adjustment comparator (DA2);
- error amplifier 1 (DA3), used mainly for voltage;
- error amplifier 2 (DA4), used mainly for the current limit signal;
- stable reference voltage source (VS) at 5V with external pin 14;
- control circuit for the operation of the output stage.
Then, of course, we will look at all its components and try to figure out why all this is needed and how it all works, but first we will need to give its operating parameters (characteristics).
| Options | Min. | Max. | Unit Change |
| V CC Supply voltage | 7 | 40 | IN |
| V I Amplifier input voltage | -0,3 | V CC - 2 | IN |
| V O Collector voltage | 40 | IN | |
| Collector current (each transistor) | 200 | mA | |
| Feedback current | 0,3 | mA | |
| f OSC Oscillator frequency | 1 | 300 | kHz |
| C T Generator capacitance | 0,47 | 10000 | nF |
| R T Generator resistor resistance | 1,8 | 500 | kOhm |
| T A Operating temperature TL494C TL494I |
0 | 70 | °C |
| -40 | 85 | °C |
Its limiting characteristics are as follows;
Supply voltage................................................ .....41V
Amplifier input voltage...................................(Vcc+0.3)V
Collector output voltage................................41V
Collector output current...................................................250mA
Total power dissipation in continuous mode....1W
Location and purpose of microcircuit pins.
Conclusion 1
This is the non-inverting (positive) input of error amplifier 1.
If the input voltage on it is lower than the voltage on pin 2, then there will be no error at the output of this amplifier, there will be no voltage (the output will have a low level) and it will not have any effect on the width (duty factor) of the output pulses.
If the voltage at this pin is higher than at pin 2, then at the output of this amplifier 1, a voltage will appear (the output of amplifier 1 will have a high level) and the width (duty factor) of the output pulses will decrease the more, the higher the output voltage of this amplifier (maximum 3.3 volts).
Conclusion 2
This is the inverting (negative) input of error signal amplifier 1.
If the input voltage on this pin is higher than on pin 1, there will be no voltage error at the output of the amplifier (the output will be low) and it will not have any effect on the width (duty factor) of the output pulses.
If the voltage at this pin is lower than at pin 1, the amplifier output will be high.
The error amplifier is a regular op-amp with a gain of the order of = 70..95 dB at DC voltage (Ku = 1 at a frequency of 350 kHz). The op-amp input voltage range extends from -0.3V to the supply voltage, minus 2V. That is, the maximum input voltage must be at least two volts lower than the supply voltage.
Conclusion 3
These are the outputs of error amplifiers 1 and 2, connected to this pin through diodes (OR circuit). If the voltage at the output of any amplifier changes from low to high, then at pin 3 it also goes high.
If the voltage at this pin exceeds 3.3 V, then the pulses at the output of the microcircuit disappear (zero duty cycle).
If the voltage at this pin is close to 0 V, then the duration of the output pulses (duty factor) will be maximum.
Pin 3 is usually used to provide feedback to amplifiers, but if necessary, pin 3 can also be used as an input to provide changes in pulse width.
If the voltage across it is high (> ~ 3.5 V), then there will be no pulses at the MS output. The power supply will not start under any circumstances.
Conclusion 4
It controls the range of variation of the “dead” time (English Dead-Time Control), in principle it is the same duty cycle.
If the voltage on it is close to 0 V, then the output of the microcircuit will have both the minimum possible and maximum width pulses, which can accordingly be set by other input signals (error amplifiers, pin 3).
If the voltage at this pin is about 1.5 V, then the width of the output pulses will be around 50% of their maximum width.
If the voltage at this pin exceeds 3.3 V, then there will be no pulses at the MS output. The power supply will not start under any circumstances.
But you should not forget that as the “dead” time increases, the PWM adjustment range will decrease.
By changing the voltage at pin 4, you can set a fixed width of the “dead” time (R-R divider), implement a soft start mode in the power supply (R-C chain), provide remote shutdown of the MS (key), and you can also use this pin as a linear control input.
Let's look (for those who don't know) what "dead" time is and what it is needed for.
When a push-pull power supply circuit operates, pulses are alternately supplied from the outputs of the microcircuit to the bases (gates) of the output transistors. Since any transistor is an inertial element, it cannot instantly close (open) when a signal is removed (supplied) from the base (gate) of the output transistor. And if pulses are applied to the output transistors without “dead” time (that is, a pulse is removed from one and immediately applied to the second), a moment may come when one transistor does not have time to close, but the second has already opened. Then all the current (called through current) will flow through both open transistors, bypassing the load (transformer winding), and since it will not be limited by anything, the output transistors will instantly fail.
To prevent this from happening, it is necessary that after the end of one pulse and before the start of the next, some certain time has passed, sufficient for the reliable closing of the output transistor from whose input the control signal was removed.
This time is called "dead" time.
Yes, if we look at the figure with the composition of the microcircuit, we see that pin 4 is connected to the input of the dead time adjustment comparator (DA1) through a voltage source of 0.1-0.12 V. What is this done for?
This is precisely done to ensure that the maximum width (duty factor) of the output pulses is never equal to 100%, to ensure the safe operation of the output (output) transistors.
That is, if you “connect” pin 4 to the common wire, then at the input of the comparator DA1 there will still not be a zero voltage, but there will be a voltage of just this value (0.1-0.12 V) and pulses from the sawtooth voltage generator (RPG) will appear at the output of the microcircuit only when their amplitude at pin 5 exceeds this voltage. That is, the microcircuit has a fixed maximum threshold of the duty cycle of the output pulses, which will not exceed 95-96% for the single-cycle mode of operation of the output stage, and 47.5-48% for the push-pull mode of operation of the output stage.
Conclusion 5
This is the GPG output; it is intended for connecting a timing capacitor Ct to it, the second end of which is connected to the common wire. Its capacitance is usually selected from 0.01 µF to 0.1 µF, depending on the output frequency of the GPG pulses of the PWM controller. As a rule, high quality capacitors are used here.
The output frequency of the GPG can be controlled at this pin. The generator output voltage swing (amplitude of output pulses) is somewhere around 3 volts.
Conclusion 6
This is also the GPN output, intended for connecting to it a time-setting resistor Rt, the second end of which is connected to the common wire.
The values of Rt and Ct determine the output frequency of the gas pump, and are calculated using the formula for single-cycle operating mode;
For push-pull operating mode, the formula is as follows;
For PWM controllers from other companies, the frequency is calculated using the same formula, with the exception that the number 1 will need to be changed to 1.1.
Conclusion 7
It connects to the common wire of the device circuit on the PWM controller.
Conclusion 8
The microcircuit contains an output stage with two output transistors, which are its output switches. The terminals of the collectors and emitters of these transistors are free, and therefore, depending on the need, these transistors can be included in the circuit to work with both a common emitter and a common collector.
Depending on the voltage at pin 13, this output stage can operate either in push-pull or single-cycle mode. In single-ended operating mode, these transistors can be connected in parallel to increase the load current, which is what is usually done.
So, pin 8 is the collector pin of transistor 1.
Conclusion 9
This is the emitter pin of transistor 1.
Conclusion 10
This is the emitter pin of transistor 2.
Conclusion 11
This is the collector of transistor 2.
Conclusion 12
The “plus” of the TL494CN power supply is connected to this pin.
Conclusion 13
This is the output for selecting the operating mode of the output stage. If this pin is connected to the common wire, the output stage will operate in single-ended mode. The output signals at the terminals of the transistor switches will be the same.
If you apply a voltage of +5 V to this pin (connect pins 13 and 14), then the output switches will operate in push-pull mode. The output signals at the terminals of the transistor switches will be out of phase and the frequency of the output pulses will be half as much.
Conclusion 14
This is the output of the stable AND drain ABOUT porn N voltage (ION), With an output voltage of +5 V and an output current of up to 10 mA, which can be used as a reference for comparison in error amplifiers, and for other purposes.
Conclusion 15
It works exactly the same as pin 2. If the second error amplifier is not used, then pin 15 is simply connected to pin 14 (reference voltage +5 V).
Conclusion 16
It works in the same way as pin 1. If the second error amplifier is not used, it is usually connected to the common wire (pin 7).
With pin 15 connected to +5V and pin 16 connected to ground, there is no output voltage from the second amplifier, so it has no effect on the operation of the chip.
The principle of operation of the microcircuit.
So how does the TL494 PWM controller work?
Above, we examined in detail the purpose of the pins of this microcircuit and what function they perform.
If all this is carefully analyzed, then from all this it becomes clear how this microcircuit works. But I will once again very briefly describe the principle of its operation.
When the microcircuit is typically turned on and power is supplied to it (minus to pin 7, plus to pin 12), the GPG begins to produce sawtooth pulses with an amplitude of about 3 volts, the frequency of which depends on the C and R connected to pins 5 and 6 of the microcircuit.
If the value of the control signals (at pins 3 and 4) is less than 3 volts, then rectangular pulses appear at the output switches of the microcircuit, the width of which (duty factor) depends on the value of the control signals at pins 3 and 4.
That is, the microcircuit compares the positive sawtooth voltage from the capacitor Ct (C1) with any of the two control signals.
The logic circuits for controlling the output transistors VT1 and VT2 open them only when the voltage of the sawtooth pulses is higher than the control signals. And the greater this difference, the wider the output pulse (the greater the duty cycle).
The control voltage at pin 3 in turn depends on the signals at the inputs of operational amplifiers (error amplifiers), which in turn can control the output voltage and output current of the power supply.
Thus, an increase or decrease in the value of any control signal causes a corresponding linear decrease or increase in the width of the voltage pulses at the outputs of the microcircuit.
As mentioned above, the voltage from pin 4 (dead time control), the inputs of error amplifiers, or the feedback signal input directly from pin 3 can be used as control signals.
Theory, as they say, is theory, but it will be much better to see and “touch” all this in practice, so let’s assemble the following circuit on a breadboard and see with our own eyes how it all works.
The easiest and fastest way is to assemble it all on a breadboard. Yes, I installed the KA7500 chip. Pin “13” of the microcircuit is connected to the common wire, that is, our output switches will operate in single-cycle mode (the signals on the transistors will be the same), and the repetition frequency of the output pulses will correspond to the frequency of the sawtooth voltage of the GPG.
I connected the oscilloscope to the following control points:
- The first beam to pin “4”, to control the constant voltage at this pin. Located in the center of the screen on the zero line. Sensitivity - 1 volt per division;
- The second beam to pin “5”, to control the sawtooth voltage of the GPG. It is also located on the zero line (both beams are combined) in the center of the oscilloscope and with the same sensitivity;
- The third beam to the output of the microcircuit to pin “9”, to control the pulses at the output of the microcircuit. The sensitivity of the beam is 5 volts per division (0.5 volts, plus a divider by 10). Located at the bottom of the oscilloscope screen.
I forgot to say, the output switches of the microcircuit are connected to a common collector. In other words - according to the emitter follower circuit. Why repeater? Because the signal at the emitter of the transistor exactly repeats the base signal, so that we can clearly see everything.
If you remove the signal from the collector of the transistor, it will be inverted (upside down) in relation to the base signal.
We supply power to the microcircuit and see what we have at the terminals.
On the fourth leg we have zero (the trimmer resistor slider is in the lowest position), the first beam is on the zero line in the center of the screen. The error amplifiers don't work either.
On the fifth leg we see a sawtooth voltage of the GPN (second ray), with an amplitude of slightly more than 3 volts.
At the output of the microcircuit (pin 9) we see rectangular pulses with an amplitude of about 15 volts and a maximum width (96%). The dots at the bottom of the screen are exactly the fixed duty cycle threshold. To make it easier to see, let’s turn on the stretch on the oscilloscope.
Well, now you can see it better. This is precisely the time when the pulse amplitude drops to zero and the output transistor is closed for this short time. The zero level for this beam is at the bottom of the screen.
Well, let's add voltage to pin "4" and see what we get.
At pin “4” I set a constant voltage of 1 volt using a trimming resistor, the first beam rose by one division (straight line on the oscilloscope screen). What do we see? The dead time has increased (the duty cycle has decreased), this is the dotted line at the bottom of the screen. That is, the output transistor is closed for about half the duration of the pulse itself.
Let's add one more volt with a trimming resistor to pin "4" of the microcircuit.
We see that the first beam has risen one more division, the duration of the output pulses has become even shorter (1/3 of the duration of the entire pulse), and the dead time (the closing time of the output transistor) has increased to two thirds. That is, it is clearly visible that the logic of the microcircuit compares the level of the GPG signal with the level of the control signal, and passes to the output only that GPG signal whose level is higher than the control signal.
To make it even clearer, the duration (width) of the output pulses of the microcircuit will be the same as the duration (width) of the sawtooth voltage output pulses located above the level of the control signal (above the straight line on the oscilloscope screen).
Let's go further, add another volt to pin "4" of the microcircuit. What do we see? At the output of the microcircuit there are very short pulses, approximately the same in width as the peaks of the sawtooth voltage protruding above the straight line. Let's turn on the stretch on the oscilloscope so that the pulse is better visible.
Here, we see a short pulse, during which the output transistor will be open, and the rest of the time (lower line on the screen) will be closed.
Well, let's try to increase the voltage at pin "4" even more. We use a trimming resistor to set the voltage at the output above the level of the sawtooth voltage of the GPG.
Well, that’s it, our power supply will stop working, since the output is completely “calm”. There are no output pulses, since at the control pin “4” we have a constant voltage level of more than 3.3 volts.
Absolutely the same thing will happen if you apply a control signal to pin “3” or to any error amplifier. If anyone is interested, you can check it yourself experimentally. Moreover, if the control signals are on all control pins at once and control the microcircuit (prevail), there will be a signal from the control pin whose amplitude is greater.
Well, let's try to disconnect pin "13" from the common wire and connect it to pin "14", that is, switch the operating mode of the output switches from single-cycle to push-pull. Let's see what we can do.
Using a trimming resistor, we again bring the voltage at pin “4” to zero. Turn on the power. What do we see?
The output of the microcircuit also contains rectangular pulses of maximum duration, but their repetition frequency has become half the frequency of sawtooth pulses.
The same pulses will be on the second key transistor of the microcircuit (pin 10), with the only difference being that they will be shifted in time relative to these by 180 degrees.
There is also a maximum duty cycle threshold (2%). Now it is not visible, you need to connect the 4th beam of the oscilloscope and combine the two output signals together. The fourth probe is not at hand, so I didn’t do it. Anyone who wants to, check it out practically for yourself to make sure of this.
In this mode, the microcircuit operates in exactly the same way as in single-cycle mode, the only difference being that the maximum duration of the output pulses here will not exceed 48% of the total pulse duration.
So we won’t consider this mode for a long time, but just see what kind of pulses we have when the voltage at pin “4” is two volts.
We raise the voltage with a trimmer resistor. The width of the output pulses decreased to 1/6 of the total pulse duration, that is, also exactly two times than in the single-cycle mode of operation of the output switches (1/3 times there).
At the output of the second transistor (pin 10) there will be the same pulses, only shifted in time by 180 degrees.
Well, in principle, we have analyzed the operation of the PWM controller.
Also on pin “4”. As mentioned earlier, this pin can be used for a “soft” start of the power supply. How to organize this?
Very simple. To do this, connect an RC circuit to pin “4”. Here is an example fragment of the diagram:
How does "soft start" work here? Let's look at the diagram. Capacitor C1 is connected to the ION (+5 volts) through resistor R5.
When power is applied to the microcircuit (pin 12), +5 volts appears at pin 14. Capacitor C1 begins to charge. The charging current of the capacitor flows through resistor R5, at the moment of switching on it is maximum (the capacitor is discharged) and a voltage drop of 5 volts occurs across the resistor, which is supplied to pin “4”. This voltage, as we have already found out experimentally, prohibits the passage of pulses to the output of the microcircuit.
As the capacitor charges, the charging current decreases and the voltage drop across the resistor decreases accordingly. The voltage at pin “4” also decreases and pulses begin to appear at the output of the microcircuit, the duration of which gradually increases (as the capacitor charges). When the capacitor is fully charged, the charging current stops, the voltage at pin “4” becomes close to zero, and pin “4” no longer affects the duration of the output pulses. The power supply returns to its operating mode.
Naturally, you guessed that the startup time of the power supply (it reaches operating mode) will depend on the size of the resistor and capacitor, and by selecting them it will be possible to regulate this time.
Well, this is briefly all the theory and practice, and there is nothing particularly complicated here, and if you understand and understand the work of this PWM, then it will not be difficult for you to understand and understand the work of other PWMs.
I wish everyone good luck.
The pulse generator is used for laboratory research in the development and adjustment of electronic devices. The generator operates in a voltage range from 7 to 41 volts and has a high load capacity depending on the output transistor. The amplitude of the output pulses can be equal to the value of the supply voltage of the microcircuit, up to the limiting value of the supply voltage of this microcircuit +41 V. Its basis is known to everyone and is often used in.
Analogues TL494 are microcircuits KA7500 and its domestic clone - KR1114EU4 .
Parameter limit values:
Supply voltage 41V
Amplifier input voltage (Vcc+0.3)V
Collector output voltage 41V
Collector output current 250mA
Total power dissipation in continuous mode 1W
Operating ambient temperature range:
-c suffix L -25..85С
-with suffix С.0..70С
Storage temperature range -65…+150С
Schematic diagram of the device
Square pulse generator circuit
Generator printed circuit board TL494 and other files are in a separate one.
Frequency adjustment is carried out by switch S2 (roughly) and resistor RV1 (smoothly), the duty cycle is adjusted by resistor RV2. Switch SA1 changes the generator operating modes from in-phase (single-cycle) to anti-phase (two-cycle). Resistor R3 selects the most optimal frequency range to cover; the duty cycle adjustment range can be selected using resistors R1, R2.
Pulse generator parts
Capacitors C1-C4 of the timing circuit are selected for the required frequency range and their capacity can be from 10 microfarads for the infra-low subrange to 1000 picofarads for the highest frequency.
With an average current limit of 200 mA, the circuit is able to charge the gate fairly quickly, but
It is impossible to discharge it with the transistor turned off. Discharging the gate using a grounded resistor is also unsatisfactorily slow. For these purposes, an independent complementary repeater is used.
- Read: "How to make it from a computer."
General Description and Use
TL 494 and its subsequent versions are the most commonly used microcircuit for building push-pull power converters.
- TL494 (original development of Texas Instruments) - PWM voltage converter IC with single-ended outputs (TL 494 IN - package DIP16, -25..85C, TL 494 CN - DIP16, 0..70C).
- K1006EU4 - domestic analogue of TL494
- TL594 - analogue of TL494 with improved accuracy of error amplifiers and comparator
- TL598 - analogue of TL594 with a push-pull (pnp-npn) repeater at the output
This material is a generalization on the topic of the original technical document Texas Instruments, publications International Rectifier (“Power semiconductor devices International Rectifier”, Voronezh, 1999) and Motorola.
Advantages and disadvantages of this microcircuit:
- Plus: Developed control circuits, two differential amplifiers (can also perform logical functions)
- Cons: Single-phase outputs require additional mounting (compared to UC3825)
- Minus: Current control is not available, relatively slow feedback loop (not critical in automotive PN)
- Cons: Synchronous connection of two or more ICs is not as convenient as in the UC3825
1. Features of TL494 chips
ION and undervoltage protection circuits. The circuit turns on when the power reaches the threshold of 5.5..7.0 V (typical value 6.4V). Until this moment, the internal control buses prohibit the operation of the generator and the logical part of the circuit. The no-load current at supply voltage +15V (output transistors are disabled) is no more than 10 mA. ION +5V (+4.75..+5.25 V, output stabilization no worse than +/- 25mV) provides a flowing current of up to 10 mA. The ION can only be boosted using an NPN emitter follower (see TI pp. 19-20), but the voltage at the output of such a “stabilizer” will greatly depend on the load current.
Generator generates a sawtooth voltage of 0..+3.0V (the amplitude is set by the ION) on the timing capacitor Ct (pin 5) for the TL494 Texas Instruments and 0...+2.8V for the TL494 Motorola (what can we expect from others?), respectively, for TI F =1.0/(RtCt), for Motorola F=1.1/(RtCt).
Operating frequencies from 1 to 300 kHz are acceptable, with the recommended range Rt = 1...500 kOhm, Ct = 470pF...10 μF. In this case, the typical temperature drift of frequency is (naturally, without taking into account the drift of attached components) +/-3%, and the frequency drift depending on the supply voltage is within 0.1% over the entire permissible range.
To turn off the generator remotely, you can use an external key to short-circuit the input Rt (6) to the output of the ION, or short-circuit Ct to ground. Of course, the leakage resistance of the open switch must be taken into account when selecting Rt, Ct.
Rest phase control input (duty cycle) through the rest phase comparator, sets the required minimum pause between pulses in the arms of the circuit. This is necessary both to prevent pass-through current in power stages outside the IC, and to stable operation trigger - the switching time of the digital part of the TL494 is 200 ns. The output signal is enabled when the saw exceeds the voltage at control input 4 (DT) by Ct. At clock frequencies up to 150 kHz with zero control voltage, the resting phase = 3% of the period (equivalent bias of the control signal 100..120 mV), at high frequencies the built-in correction expands the resting phase to 200..300 ns.
Using the DT input circuit, it is possible to set a fixed resting phase ( R-R divider), soft start mode (R-C), remote shutdown (key), and also use DT as a linear control input. The input circuit is assembled using PNP transistors, so the input current (up to 1.0 μA) flows out of the IC rather than into it. The current is quite large, so high-resistance resistors (no more than 100 kOhm) should be avoided. See TI, page 23 for an example of surge protection using a TL430 (431) 3-lead zener diode.
Error Amplifiers- in fact, operational amplifiers with Ku = 70..95 dB at constant voltage (60 dB for early series), Ku = 1 at 350 kHz. The input circuits are assembled using PNP transistors, so the input current (up to 1.0 μA) flows out of the IC rather than into it. The current is quite large for the op-amp, the bias voltage is also high (up to 10 mV), so high-resistance resistors in the control circuits (no more than 100 kOhm) should be avoided. But thanks to the use of pnp inputs, the input voltage range is from -0.3V to Vsupply-2V.
The outputs of the two amplifiers are combined by diode OR. The amplifier whose output voltage is higher takes control of the logic. In this case, the output signal is not available separately, but only from the output of the diode OR (also the input of the error comparator). Thus, only one amplifier can be looped in line mode. This amplifier closes the main, linear feedback loop at the output voltage. In this case, the second amplifier can be used as a comparator - for example, when the output current is exceeded, or as a key for a logical alarm signal (overheating, short circuit, etc.), remote shutdown, etc. One of the comparator inputs is tied to the ION, and a logical signal is organized on the second OR alarm signals (even better - logical AND normal state signals).
When using an RC frequency-dependent OS, you should remember that the output of the amplifiers is actually single-ended (series diode!), so it will charge the capacitance (upward) and will take a long time to discharge downward. The voltage at this output is within 0..+3.5V (slightly more than the generator swing), then the voltage coefficient drops sharply and at approximately 4.5V at the output the amplifiers are saturated. Likewise, low-resistance resistors in the amplifier output circuit (feedback loop) should be avoided.
Amplifiers are not designed to operate within one clock cycle of the operating frequency. With a signal propagation delay inside the amplifier of 400 ns, they are too slow for this, and the trigger control logic does not allow it (side pulses would appear at the output). In real PN circuits, the cutoff frequency of the OS circuit is selected on the order of 200-10000 Hz.
Trigger and output control logic- With a supply voltage of at least 7V, if the saw voltage at the generator is greater than at the DT control input, and if the saw voltage is greater than at any of the error amplifiers (taking into account the built-in thresholds and offsets) - the circuit output is allowed. When the generator is reset from maximum to zero, the outputs are switched off. A trigger with paraphase output divides the frequency in half. With logical 0 at input 13 (output mode), the trigger phases are combined by OR and supplied simultaneously to both outputs; with logical 1, they are supplied in phase to each output separately.
Output transistors- npn Darlingtons with built-in thermal protection (but without current protection). Thus, the minimum voltage drop between the collector (usually closed to the positive bus) and the emitter (at the load) is 1.5 V (typical at 200 mA), and in a circuit with a common emitter it is a little better, 1.1 V typical. The maximum output current (with one open transistor) is limited to 500 mA, the maximum power for the entire chip is 1 W.
2. Features of application
Work on the gate of an MIS transistor. Output repeaters
When operating on a capacitive load, which is conventionally the gate of an MIS transistor, the TL494 output transistors are switched on by an emitter follower. When the average current is limited to 200 mA, the circuit is able to quickly charge the gate, but it is impossible to discharge it with the transistor turned off. Discharging the gate using a grounded resistor is also unsatisfactorily slow. After all, the voltage across the gate capacitance drops exponentially, and to turn off the transistor, the gate must be discharged from 10V to no more than 3V. The discharge current through the resistor will always be less than the charge current through the transistor (and the resistor will heat up quite a bit, and steal the switch current when moving up).
Option A. Discharge circuit through an external pnp transistor (borrowed from Shikhman’s website - see “Jensen amplifier power supply”). When charging the gate, the current flowing through the diode turns off the external PNP transistor; when the IC output is turned off, the diode is turned off, the transistor opens and discharges the gate to ground. Minus - it only works on small load capacitances (limited by the current reserve of the IC output transistor).
When using the TL598 (with a push-pull output), the function of the lower bit side is already hardwired on the chip. Option A is not practical in this case.
Option B. Independent complementary repeater. Since the main current load is handled by an external transistor, the capacity (charge current) of the load is practically unlimited. Transistors and diodes - any HF with a low saturation voltage and Ck, and sufficient current reserve (1A per pulse or more). For example, KT644+646, KT972+973. The “ground” of the repeater must be soldered directly next to the source of the power switch. The collectors of the repeater transistors must be bypassed with a ceramic capacitance (not shown in the diagram).
Which circuit to choose depends primarily on the nature of the load (gate capacitance or switching charge), operating frequency, and time requirements for pulse edges. And they (the fronts) should be as fast as possible, because it is during transient processes on the MIS switch that most of the heat losses are dissipated. I recommend turning to the publications in the International Rectifier collection for a complete analysis of the problem, but I will limit myself to an example.
A powerful transistor - IRFI1010N - has a reference total charge on the gate Qg = 130 nC. This is no small feat, because the transistor has an exceptionally large channel area to ensure extremely low channel resistance (12 mOhm). These are the keys that are required in 12V converters, where every milliohm counts. To ensure that the channel opens, the gate must be provided with Vg=+6V relative to ground, while the total gate charge is Qg(Vg)=60nC. To reliably discharge a gate charged to 10V, it is necessary to dissolve Qg(Vg)=90nC.
2. Implementation of current protection, soft start, duty cycle limitation
As a rule, a series resistor in the load circuit is asked to act as a current sensor. But it will steal precious volts and watts at the output of the converter, and will only monitor the load circuits, and will not be able to detect short circuits in the primary circuits. The solution is an inductive current sensor in the primary circuit.
The sensor itself (current transformer) is a miniature toroidal coil (its internal diameter should, in addition to the sensor winding, freely pass the wire of the primary winding of the main power transformer). We pass the wire of the primary winding of the transformer through the torus (but not the “ground” wire of the source!). We set the rise time constant of the detector to about 3-10 periods of the clock frequency, the decay time to 10 times more, based on the response current of the optocoupler (about 2-10 mA with a voltage drop of 1.2-1.6V).
On the right side of the diagram there are two typical solutions for TL494. The Rdt1-Rdt2 divider sets the maximum duty cycle (minimum rest phase). For example, with Rdt1=4.7kOhm, Rdt2=47kOhm at output 4 the constant voltage is Udt=450mV, which corresponds to a rest phase of 18..22% (depending on the IC series and operating frequency).
When the power is turned on, Css is discharged and the potential at the DT input is equal to Vref (+5V). Css is charged through Rss (aka Rdt2), smoothly lowering the potential DT to the lower limit limited by the divider. This is a "soft start". With Css = 47 μF and the indicated resistors, the circuit outputs open 0.1 s after switching on, and reach operating duty cycle within another 0.3-0.5 s.
In the circuit, in addition to Rdt1, Rdt2, Css, there are two leaks - the leakage current of the optocoupler (not higher than 10 μA at high temperatures, about 0.1-1 µA at room temperature) and the base current of the IC input transistor flowing from the DT input. To ensure that these currents do not significantly affect the accuracy of the divider, Rdt2=Rss is selected no higher than 5 kOhm, Rdt1 - no higher than 100 kOhm.
Of course, the choice of an optocoupler and a DT circuit for control is not fundamental. It is also possible to use an error amplifier in comparator mode, and to block the capacitance or resistor of the generator (for example, with the same optocoupler) - but this is just a shutdown, not a smooth limitation.
Generator on TL494 with adjustable frequency and duty cycle
A very useful device when carrying out experiments and tuning work is a frequency generator. The requirements for it are small, you only need:
- frequency adjustment (pulse repetition period)
- duty cycle adjustment (duty factor, pulse length)
- wide range
The frequency adjustment range of the generator is extremely high - from tens of hertz to 500 kHz, and in some cases up to 1 MHz, depending on the microcircuit; different manufacturers have different real values of the maximum frequency that can be “squeezed out”.
Let's move on to the description of the scheme:
Pit± and Pit~ - power supply of the digital part of the circuit, with direct and alternating voltage, respectively, 16-20 volts.
Vout is the supply voltage of the power unit, it will be at the output of the generator, from 12 volts. To power the digital part of the circuit from this voltage, it is necessary to connect Vout and Pit±, taking into account the polarity (from 16 volts).
OUT(+/D) - power output of the generator, taking into account polarity. + - power supply plus, D - field-effect transistor drain. The load is connected to them.
G D S - screw block for connecting a field-effect transistor, which is selected according to parameters depending on your frequency and power requirements. Wiring printed circuit board made taking into account the minimum length of conductors to the output switch and their required width.
Controls:
Rt is a variable resistor for controlling the frequency range of the generator; its resistance must be selected to suit your specific requirements. An online calculator for calculating the frequency of TL494 is attached below. Resistor R2 limits the minimum resistance value of the timing resistor of the microcircuit. It can be selected for a specific instance of the microcircuit, or it can be installed as shown in the diagram.
Ct - frequency-setting capacitor, referring, again, to online calculator. Allows you to set the adjustment range to suit your requirements.
Rdt is a variable resistor for adjusting the duty cycle. With resistor R1 you can precisely adjust the adjustment range from 1% to 99%, and instead of it you can put a jumper first.
| Ct, nF: | |
| R2, kOhm: | |
| Rt, kOhm: |
A few words about the operation of the circuit. By applying a low level to pin 13 of the microcircuit (output control), it is switched to single-cycle mode. The lower transistor of the microcircuit is loaded onto resistor R3 to create an output for connection to the generator of a frequency meter (frequency meter). The upper transistor of the microcircuit controls the driver on a complementary pair of transistors S8050 and S8550, whose task is to control the gate of the power output transistor. Resistor R5 limits the gate current; its value can be changed. Inductor L1 and a capacitor with a capacity of 47n form a filter to protect the TL494 from possible interference created by the driver. The inductance of the inductor may need to be adjusted to suit your frequency range. It should be noted that transistors S8050 and S8550 were not chosen by chance, since they have sufficient power and speed, which will provide the necessary steepness of the fronts. As you can see, the scheme is extremely simple and, at the same time, functional.
The variable resistor Rt should be made in the form of two series-connected resistors - single-turn and multi-turn, if you need smoothness and accuracy of frequency control.
The printed circuit board, following tradition, is drawn with a felt-tip pen and etched with copper sulfate.
Almost any field-effect transistors that are suitable for voltage, current and frequency can be used as a power transistor. These could be: IRF530, IRF630, IRF640, IRF840.
The lower the resistance of the transistor in the open state, the less it will heat up during operation. However, the presence of a radiator on it is mandatory.
Assembled and tested according to the diagram provided by flyer.
Only the most important things.
Supply voltage 8-35V (it seems possible up to 40V, but I haven’t tested it)
Ability to operate in single-stroke and push-pull mode.
For single-cycle mode, the maximum pulse duration is 96% (not less than 4% dead time).
For the two-stroke version, the duration of the dead time cannot be less than 4%.
By applying a voltage of 0...3.3V to pin 4, you can adjust the dead time. And carry out a smooth launch.
There is a built-in stabilized reference voltage source of 5V and a current of up to 10mA.
There is built-in protection against low supply voltage, turning off below 5.5...7V (most often 6.4V). The trouble is that at this voltage the mosfets already go into linear mode and burn out...
It is possible to turn off the microcircuit generator by closing the Rt pin (6), the reference voltage pin (14) or the Ct pin (5) to ground with a key.
Operating frequency 1…300 kHz.
Two built-in “error” operational amplifiers with gain Ku=70..95dB. Inputs - outputs (1); (2) and (15); (16). The outputs of the amplifiers are combined by an OR element, so the one whose output voltage is greater controls the pulse duration. One of the comparator inputs is usually tied to the reference voltage (14), and the second - where it is needed... The signal delay inside the Amplifier is 400 ns, they are not designed to operate within one clock cycle.
The output stages of the microcircuit, with an average current of 200 mA, quickly charge the input capacitance of the gate of a powerful mosfet, but do not ensure its discharge. in a reasonable time. Therefore, an external driver is required.
Pin (5) capacitor C2 and pin (6) resistors R3; R4 - set the frequency of the internal oscillator of the microcircuit. In push-pull mode it is divided by 2.
There is a possibility of synchronization, triggering by input pulses.
Single-cycle generator with adjustable frequency and duty cycle
Single-cycle generator with adjustable frequency and duty cycle (ratio of pulse duration to pause duration). With single transistor output driver. This mode is implemented by connecting pin 13 to a common power bus.
Scheme (1)
Since the microcircuit has two output stages, which in this case operate in phase, they can be connected in parallel to increase the output current... Or not included... (in green on the diagram) Also, resistor R7 is not always installed.
Measuring operational amplifier voltage across resistor R10, you can limit the output current. The second input is supplied with a reference voltage by divider R5; R6. Well, you see, the R10 will heat up.
Chain C6; R11, on the (3) leg, is placed for greater stability, the datasheet asks for it, but it works without it. The transistor can also be used as an NPN structure.
Scheme (2)
Scheme (3)
Single-cycle generator with adjustable frequency and duty cycle. With two transistor output driver (complementary repeater).
What can I say? The signal shape is better, transient processes at switching moments are reduced, load capacity is higher, and heat losses are lower. Although this may be a subjective opinion. But. Now I only use a two transistor driver. Yes, the resistor in the gate circuit limits the speed of switching transients.
Scheme (4)
And here we have a circuit of a typical boost (boost) adjustable single-ended converter, with voltage regulation and current limitation.
The circuit is working, I assembled it in several versions. The output voltage depends on the number of turns of coil L1, and on the resistance of resistors R7; R10; R11, which are selected during setup... The reel itself can be wound on anything. Size - depending on power. Ring, Sh-core, even just on the rod. But it should not become saturated. Therefore, if the ring is made of ferrite, then it needs to be cut and glued with a gap. Large rings from computer power supplies will work well; there is no need to cut them, they are made of “pulverized iron”; the gap is already provided. If the core is W-shaped, we do not install a magnetic gap; they come with a short medium core - these already have a gap. In short, we wind it with a thick copper or mounting wire (0.5-1.0 mm depending on the power) and the number of turns is 10 or more (depending on what voltage we want to get). We connect the load to the planned voltage of low power. We connect our creation to the battery through a powerful lamp. If the lamp does not light up at full intensity, take a voltmeter and an oscilloscope...
We select resistors R7; R10; R11 and the number of turns of coil L1, achieving the intended voltage at the load.
Choke Dr1 - 5...10 turns with thick wire on any core. I’ve even seen options where L1 and Dr1 are wound on the same core. I haven't checked it myself.
Scheme (5)
This is also a real boost converter circuit that can be used, for example, to charge a laptop from a car battery. The comparator at inputs (15); (16) monitors the voltage of the “donor” battery and turns off the converter when the voltage on it drops below the selected threshold.
Chain C8; R12; VD2 - the so-called Snubber, is designed to suppress inductive emissions. A low-voltage MOSFET saves, for example IRF3205 can withstand, if I’m not mistaken, (drain - source) up to 50V. However, it greatly reduces the efficiency. Both the diode and the resistor get quite hot. This increases reliability. In some modes (circuits), without it, a powerful transistor simply burns out immediately. But sometimes it works without all this... You need to look at the oscilloscope...
Scheme (6)
Push-pull master generator.
Various design and adjustment options.
At first glance, the huge variety of switching circuits comes down to a much more modest number of ones that actually work... The first thing I usually do when I see a “cunning” circuit is to redraw it in the standard that is familiar to me. Previously it was called GOST. Nowadays it’s not clear how to draw, which makes it extremely difficult to perceive. And hides mistakes. I think that this is often done on purpose.
Master oscillator for half-bridge or bridge. This is the simplest generator. The pulse duration and frequency are adjusted manually. You can also adjust the duration using an optocoupler on the (3) leg, but the adjustment is very sharp. I used it to interrupt the operation of the microcircuit. Some “luminaries” say that it is impossible to control using (3) pin, the microcircuit will burn out, but my experience confirms the functionality of this solution. By the way, it was successfully used in a welding inverter.
Scheme (10)
Examples of implementation of current and voltage regulation (stabilization). I liked what I did in picture No. 12 myself. You probably don’t have to install blue capacitors, but it’s better to have them.
Scheme (11)
All electronic engineers involved in the design of power supply devices sooner or later face the problem of the lack of a load equivalent or the functional limitations of the existing loads, as well as their dimensions. Fortunately, the appearance of cheap and powerful field-effect transistors on the Russian market has somewhat corrected the situation.
Amateur designs of electronic loads based on field-effect transistors began to appear, more suitable for use as electronic resistance than their bipolar counterparts: better temperature stability, almost zero channel resistance in the open state, low control currents - the main advantages that determine the preference for their use as regulating component in powerful devices. Moreover, a wide variety of offers have appeared from device manufacturers, whose price lists are replete with a wide variety of models of electronic loads. But, since manufacturers focus their very complex and multifunctional products called “electronic loads” mainly on production, the prices for these products are so high that only a very wealthy person can afford the purchase. True, it is not entirely clear why a wealthy person needs an electronic load.
I have not noticed any commercially manufactured EN aimed at the amateur engineering sector. This means that you will have to do everything yourself again. Eh... Let's begin.
Advantages of Electronic Load Equivalent
Why, in principle, are electronic load equivalents preferable to traditional means (powerful resistors, incandescent lamps, thermal heaters and other devices) often used by designers when setting up various power devices?
Citizens of the portal who are involved in the design and repair of power supplies undoubtedly know the answer to this question. Personally, I see two factors that are sufficient to have an electronic load in your “laboratory”: small dimensions, the ability to control the load power within large limits using simple means (the same way we regulate the sound volume or the output voltage of the power supply - with a regular variable resistor and not by powerful switch contacts, rheostat motor, etc.).
In addition, the “actions” of the electronic load can be easily automated, thus making it easier and more sophisticated to test a power device using an electronic load. At the same time, of course, the engineer’s eyes and hands are freed, and the work becomes more productive. But the delights of all possible bells and whistles and perfections are not in this article, and, perhaps, from another author. In the meantime, let's talk about just one more type of electronic load - pulsed.
Features of the pulsed version of EN
Analog electronic loads are certainly good, and many of those who used electronic loads when setting up power devices appreciated its advantages. Pulse power supplies have their own peculiarity, making it possible to evaluate the operation of a power supply under a pulsed load, such as, for example, the operation of digital devices. Powerful amplifiers audio frequencies also have a characteristic effect on power supply devices, and therefore it would be nice to know how the power supply, designed and manufactured for a specific amplifier, will behave under a certain specified load.
When diagnosing power supplies being repaired, the effect of using pulsed EN is also noticeable. For example, with the help of pulsed EN, a malfunction of a modern computer power supply was found. The declared malfunction of this 850-watt power supply was as follows: the computer, when working with this power supply, turned off randomly at any time when working with any application, regardless of the power consumed at the time of shutdown. When tested for a normal load (a bunch of powerful resistors of +3V, +5V and halogen bulbs of +12V), this power supply worked with a bang for several hours, despite the fact that the load power was 2/3 of its declared power. The malfunction appeared when connecting a pulsed electric power supply to the +3V channel and the power supply began to turn off as soon as the ammeter needle reached the 1A mark. In this case, the load currents on each of the other positive voltage channels did not exceed 3A. The supervisor board turned out to be faulty and was replaced with a similar one (fortunately, there was the same power supply unit with a burnt-out power unit), after which the power supply unit worked normally at the maximum current allowed for the pulsed power supply instance used (10A), which is the subject of the description in this article.
Idea
The idea of creating a pulse load appeared quite a long time ago and was first implemented in 2002, but not in its current form and on a different element base and for slightly different purposes, and at that time there were not sufficient incentives and other grounds for me personally to develop this idea. Now the stars are aligned differently and something has come together for the next incarnation of this device. On the other hand, the device initially had a slightly different purpose - checking the parameters of pulse transformers and chokes. But one does not interfere with the other. By the way, if anyone wants to research inductive components using this or a similar device, please: below are archives of articles by venerable ones (in the field power electronics) engineers dedicated to this topic.
So, what is a “classical” (analog) EN in principle? Current stabilizer operating in short circuit mode. And nothing else. And the one who, in a fit of any passion, will be right will close the output terminals of the charger or welding machine and say: this is an electronic load! It is not a fact, of course, that such a short circuit will not have detrimental consequences, both for the devices and for the operator himself, but both devices are indeed sources of current and could, after some fine-tuning, claim to be an electronic load, like any another arbitrarily primitive current source. The current in the analog EN will depend on the voltage at the output of the power supply being tested, the ohmic resistance of the field-effect transistor channel, set by the voltage value at its gate.
The current in a pulsed electric power supply will depend on the sum of parameters, which will include the pulse width, the minimum resistance of the open channel of the output switch and the properties of the power supply being tested (capacitance of capacitors, inductance of power supply chokes, output voltage).
When the switch is open, the EN forms a short-term short circuit, in which the capacitors of the tested power supply unit are discharged, and the chokes (if they are contained in the power supply unit) tend to saturate. A classic short circuit, however, does not occur, because The pulse width is limited in time by microsecond values that determine the magnitude of the discharge current of the power supply capacitors.
At the same time, testing a pulsed power supply is more extreme for the power supply being tested. But such a check reveals more “pitfalls”, including the quality of the supply conductors supplied to the power supply device. Thus, when connecting a pulsed electric power supply to a 12-volt power supply with connecting copper wires with a core diameter of 0.8 mm and a load current of 5A, the oscillogram on the electric power supply revealed ripples, which were a sequence of rectangular pulses with a swing of up to 2V and sharp spikes with an amplitude equal to the supply voltage. At the terminals of the power supply itself there was practically no pulsation from the electric power supply. On the EN itself, ripples were reduced to a minimum (less than 50 mV) by increasing the number of cores of each conductor supplying the EN - up to 6. In the “two-core” version, a minimum ripple comparable to the “six-core” version was achieved by installing an additional electrolytic capacitor with a capacity of 4700 mF at the connection points supply wires with load. So, when building a power supply, pulsed power supply can be very useful.
Scheme
EN is assembled using popular (thanks to the large number of recycled computer power supplies) components. The EN circuit contains a generator with adjustable frequency and pulse width, thermal and current protection. The generator is made on PWM TL494.
Frequency adjustment is carried out by variable resistor R1; duty cycle - R2; thermal sensitivity - R4; current limit - R14.
The generator output is powered by an emitter follower (VT1, VT2) to operate on the gate capacitance of field-effect transistors of 4 or more.
The generator part of the circuit and the buffer stage on transistors VT1, VT2 can be powered from a separate power source with an output voltage of +12...15V and a current of up to 2A or from the +12V channel of the power supply being tested.
The output of the EN (drain of the field-effect transistor) is connected to the “+” of the power supply being tested, the common wire of the EN is connected to the common wire of the power supply. Each of the gates of field-effect transistors (in the case of their group use) must be connected to the output of the buffer stage with its own resistor, leveling the difference in the gate parameters (capacitance, threshold voltage) and ensuring synchronous operation of the switches.
The photographs show that the EN board has a pair of LEDs: green - load power indicator, red indicates the operation of the microcircuit error amplifiers at a critical temperature (constant light) or when the current is limited (barely noticeable flickering). The operation of the red LED is controlled by a key on a KT315 transistor, the emitter of which is connected to a common wire; base (through a 5-15 kOhm resistor) with pin 3 of the microcircuit; collector - (through a 1.1 kOhm resistor) with the cathode of the LED, the anode of which is connected to pins 8, 11, 12 of the DA1 microcircuit. This node is not shown in the diagram, because is not absolutely mandatory.
Regarding resistor R16. When a current of 10A passes through it, the power dissipated by the resistor will be 5W (with the resistance indicated on the diagram). In the actual design, a resistor with a resistance of 0.1 Ohm is used (the required value was not found) and the power dissipated in its body at the same current will be 10 W. In this case, the temperature of the resistor is much higher than the temperature of the EN keys, which (when using the radiator shown in the photo) do not heat up much. Therefore, it is better to install the temperature sensor on resistor R16 (or in the immediate vicinity), and not on the radiator with EN keys.
The TL494 chip is a PWM controller, perfect for building switching power supplies of various topologies and powers. It can operate in both single-stroke and two-stroke modes.
Its domestic analogue is the KR1114EU4 microcircuit. Texas Instruments, International Rectifier, ON Semiconductor, Fairchild Semiconductor - many manufacturers produce this PWM controller. Fairchild Semiconductor calls it, for example, KA7500B.
If you just look at the pin designations, it becomes clear that this microcircuit has quite a wide range of adjustment capabilities.
Let's look at the designations of all pins:
- non-inverting input of the first error comparator
- inverting input of the first error comparator
- feedback input
- dead time adjustment input
- output for connecting an external timing capacitor
- output for connecting a timing resistor
- common pin of the microcircuit, minus power supply
- collector pin of the first output transistor
- emitter pin of the first output transistor
- emitter pin of the second output transistor
- collector pin of the second output transistor
- supply voltage input
- input for selecting single-cycle or push-pull operating mode
microcircuits - built-in 5 volt reference output
- inverting input of the second error comparator
- non-inverting input of the second error comparator
On the functional diagram you can see the internal structure of the microcircuit.
The top two pins on the left are for setting the parameters of the internal ramp voltage generator, which is labeled here as “Oscillator”. For normal operation of the microcircuit, the manufacturer recommends using a timing capacitor with a capacity in the range from 470 pF to 10 μF, and a timing resistor in the range from 1.8 kOhm to 500 kOhm. The recommended operating frequency range is from 1 kHz to 300 kHz. The frequency can be calculated using the formula f = 1.1/RC. So, in operating mode, pin 5 will have a sawtooth voltage with an amplitude of about 3 volts. It may differ for different manufacturers depending on the parameters of the internal circuits of the microcircuit.
For example, if you use a capacitor with a capacity of 1nF and a resistor of 10kOhm, then the frequency of the sawtooth voltage at output 5 will be approximately f = 1.1/(10000*0.000000001) = 110000Hz. The frequency may differ, according to the manufacturer, by +-3% depending on temperature regime components.
Dead time adjustment input 4 is designed to determine the pause between pulses. The dead time comparator, designated “Dead-time Control Comparator” in the diagram, will give permission to the output pulses if the saw voltage is higher than the voltage supplied to input 4. Thus, by applying a voltage from 0 to 3 volts to input 4, you can adjust the duty cycle of the output pulses, in this case, the maximum operating cycle duration can be 96% in single-cycle mode and 48%, respectively, in push-pull mode of operation of the microcircuit. The minimum pause here is limited to 3%, which is provided by a built-in source with a voltage of 0.1 volts. Pin 3 is also important, and the voltage on it also plays a role in resolving output pulses.
Pins 1 and 2, as well as pins 15 and 16 of the error comparators can be used to protect the designed device from overcurrent and voltage overloads. If the voltage supplied to pin 1 becomes higher than the voltage supplied to pin 2, or the voltage supplied to pin 16 becomes higher than the voltage supplied to pin 15, then the PWM Comparator input (pin 3) will receive signal to inhibit pulses at the output. If these comparators are not planned to be used, then they can be blocked by shorting the non-inverting inputs to ground, and connecting the inverting inputs to the reference voltage source (pin 14).
Pin 14 is the output of a stabilized 5-volt reference voltage source built into the chip. Circuits that consume current up to 10 mA can be connected to this pin, which can be voltage dividers for setting up protection circuits, soft starting, or setting a fixed or adjustable pulse duration.
Pin 12 is supplied with a supply voltage of the microcircuit from 7 to 40 volts. As a rule, 12 volts of stabilized voltage are used. It is important to eliminate any interference in the power circuit.
Pin 13 is responsible for the operating mode of the microcircuit. If a reference voltage of 5 volts is applied to it (from pin 14), then the microcircuit will operate in push-pull mode, and the output transistors will open in antiphase, in turn, and the switching frequency of each of the output transistors will be equal to half the frequency of the sawtooth voltage at pin 5. But if you close pin 13 to the power supply minus, then the output transistors will work in parallel, and the frequency will be equal to the frequency of the saw at pin 5, that is, the frequency of the generator.
The maximum current for each of the output transistors of the microcircuit (pins 8,9,10,11) is 250mA, but the manufacturer does not recommend exceeding 200mA. Accordingly, when operating the output transistors in parallel (pin 9 is connected to pin 10, and pin 8 is connected to pin 11), the maximum allowable current will be 500mA, but it is better not to exceed 400mA.
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