Switched-mode power supplies (UPS) are very common. The computer you're using now has a multi-voltage UPS (+12, -12, +5, -5, and +3.3V at least). Almost all such blocks have a special PWM controller chip, usually of the TL494CN type. Its analogue is the domestic microcircuit M1114EU4 (KR1114EU4).
Producers
The microcircuit under consideration belongs to the list of the most common and widely used integrated electronic circuits. Its predecessor was the Unitrode UC38xx series of PWM controllers. In 1999, this company was bought by Texas Instruments, and since then the development of a line of these controllers has begun, leading to the creation in the early 2000s. TL494 series chips. In addition to the UPSs already noted above, they can be found in DC voltage regulators, in controlled drives, in soft starters, in a word, wherever PWM control is used.
Among the firms that cloned this chip, there are such world-famous brands as Motorola, Inc, International Rectifier,Fairchild Semiconductor, ON Semiconductor. They all give a detailed description of their products, the so-called TL494CN datasheet.
Documentation
Analysis of the descriptions of the considered type of microcircuit from different manufacturers shows the practical identity of its characteristics. The amount of information given by different firms is almost the same. Moreover, TL494CN datasheet from brands such as Motorola, Inc and ON Semiconductor repeat each other in its structure, figures, tables and graphs. The presentation of the material by Texas Instruments is somewhat different from them, however, upon careful study, it becomes clear that an identical product is meant.
Assignment of the TL494CN chip
Let's traditionally start describing it with the purpose and list of internal devices. It is a fixed frequency PWM controller primarily designed for UPS applications, containing the following devices:
- sawtooth voltage generator (SPG);
- error amplifiers;
- source of the reference (reference) voltage +5 V;
- dead time adjustment circuit;
- output transistor switches for current up to 500 mA;
- scheme for selecting one-stroke or two-stroke operation.
Limits
Like any other microcircuit, the description of the TL494CN must contain a list of maximum permissible performance characteristics. Let's give them based on data from Motorola, Inc:
- Power supply: 42 V.
- Collector voltageoutput transistor: 42 V.
- Output transistor collector current: 500 mA.
- Amplifier input voltage range: -0.3V to +42V.
- Power dissipation (at t< 45°C): 1000mW.
- Storage temperature range: -55 to +125°C.
- Ambient operating temperature range: from 0 to +70 °С.
It should be noted that parameter 7 for the TL494IN chip is somewhat wider: from -25 to +85 °С.
TL494CN chip design
Description in Russian of the conclusions of its case is shown in the figure below.
The microcircuit is placed in a plastic (this is indicated by the letter N at the end of its designation) 16-pin package with pdp-type leads.
Its appearance is shown in the photo below.
TL494CN: functional diagram
So, the task of this microcircuit is pulse-width modulation (PWM, or English Pulse Width Modulated (PWM)) of voltage pulses generated inside both regulated and unregulated UPSs. In power supplies of the first type, the pulse duration range, as a rule, reaches the maximum possible value (~ 48% for each output in push-pull circuits, widely used to power car audio amplifiers).
The TL494CN chip has a total of 6 output pins, 4 of them (1, 2, 15, 16) are inputs of internal error amplifiers used to protect the UPS from current and potential overloads. Pin 4 is the inputsignal from 0 to 3 V to adjust the duty cycle of the output square wave, and3 is the output of the comparator and can be used in several ways. Another 4 (numbers 8, 9, 10, 11) are free collectors and emitters of transistors with a maximum allowable load current of 250 mA (in continuous mode, no more than 200 mA). They can be connected in pairs (9 to 10, and 8 to 11) to drive high-power field-effect transistors (MOSFETs) with a current limit of 500 mA (max. 400 mA in continuous mode).
What is the internals of the TL494CN? Its diagram is shown in the figure below.
The microcircuit has a built-in reference voltage source (ION) +5 V (No. 14). It is usually used as a reference voltage (with an accuracy of ± 1%) applied to the inputs of circuits that consume no more than 10 mA, for example, to pin 13 of the choice of one- or two-cycle operation of the microcircuit: if +5 V is present, the second mode is selected, if there is a minus of the supply voltage on it - the first one.
To adjust the frequency of the sawtooth voltage generator (GPN), a capacitor and a resistor are used, connected to pins 5 and 6, respectively. And, of course, the microcircuit has terminals for connecting the plus and minus of the power source (numbers 12 and 7, respectively) in the range from 7 to 42 V.
The diagram shows that there are a number of internal devices in the TL494CN. A description in Russian of their functional purpose will be given below in the course of the presentation of the material.
Input terminal functions
Like anyother electronic device. The microcircuit in question has its own inputs and outputs. We'll start with the first. A list of these TL494CN pins has already been given above. A description in Russian of their functional purpose will be given below with detailed explanations.
Output 1
This is the positive (non-inverting) input of error amplifier 1. If the voltage on it is lower than the voltage on pin 2, the output of error amplifier 1 will be low. If it is higher than on pin 2, the error amplifier 1 signal will go high. The output of the amplifier essentially replicates the positive input using pin 2 as a reference. The functions of the error amplifiers will be described in more detail below.
Conclusion 2
This is the negative (inverting) input of error amplifier 1. If this pin is higher than pin 1, the output of error amplifier 1 will be low. If the voltage on this pin is lower than the voltage on pin 1, the output of the amplifier will be high.
Conclusion 15
It works exactly the same as 2. Often the second error amplifier is not used in the TL494CN. Its switching circuit in this case contains pin 15 simply connected to the 14th (reference voltage +5 V).
Conclusion 16
It works the same as 1. It is usually connected to common 7 when the second error amplifier is not being used. With pin 15 connected to +5V and 16 connected to common, the output of the second amplifier is low and therefore has no effect on the operation of the chip.
Conclusion 3
This pin and each internal amplifier TL494CNconnected to each other via diodes. If the signal at the output of any of them changes from low to high, then at number 3 it also goes high. When the signal on this pin exceeds 3.3V, the output pulses turn off (zero duty cycle). When the voltage on it is close to 0 V, the pulse duration is maximum. Between 0 and 3.3V, the pulse width is 50% to 0% (for each of the PWM controller outputs - on pins 9 and 10 on most devices).
If required, pin 3 can be used as an input signal or can be used to provide damping for the pulse width change rate. If the voltage on it is high (> ~ 3.5V), there is no way to start the UPS on the PWM controller (there will be no pulses from it).
Conclusion 4
It controls the duty cycle of the output pulses (eng. Dead-Time Control). If the voltage on it is close to 0 V, the microcircuit will be able to output both the minimum possible and the maximum pulse width (which is set by other input signals). If a voltage of about 1.5V is applied to this pin, the output pulse width will be limited to 50% of its maximum width (or ~25% duty cycle for a push-pull PWM controller). If the voltage on it is high (> ~ 3.5V), there is no way to start the UPS on the TL494CN. Its switching circuit often contains No. 4, connected directly to the ground.
Important to remember! The signal at pins 3 and 4 should be below ~3.3V. What if it's close to, say, +5V? Howthen TL494CN will behave? The voltage converter circuit on it will not generate pulses, i.e. there will be no output voltage from the UPS
Conclusion 5
Serves to connect the timing capacitor Ct, and its second contact is connected to the ground. Capacitance values are typically 0.01 µF to 0.1 µF. Changes in the value of this component lead to a change in the frequency of the GPN and the output pulses of the PWM controller. As a rule, high quality capacitors with a very low temperature coefficient (with very little change in capacitance with temperature change) are used here.
Conclusion 6
To connect the time-setting resistor Rt, and its second contact is connected to the ground. The Rt and Ct values determine the frequency of FPG.
f=1, 1: (Rt x Ct)
Conclusion 7
It connects to the common wire of the device circuit on the PWM controller.
Conclusion 12
It is marked with the letters VCC. The "plus" of the TL494CN power supply is connected to it. Its switching circuit usually contains No. 12 connected to the power supply switch. Many UPSs use this pin to turn the power (and the UPS itself) on and off. If it has +12 V and No. 7 is grounded, the FPV and ION chips will work.
Conclusion 13
This is the operation mode input. Its operation has been described above.
Functions of output terminals
Above they were listed for TL494CN. A description in Russian of their functional purpose will be given below with detailed explanations.
Conclusion 8
On thisThe chip has 2 npn transistors which are its output keys. This pin is the collector of transistor 1, usually connected to a DC voltage source (12 V). However, in the circuits of some devices, it is used as an output, and you can see a meander on it (as well as on No. 11).
Conclusion 9
This is the emitter of transistor 1. It drives the high power UPS transistor (field effect in most cases) in a push-pull circuit, either directly or through an intermediate transistor.
Output 10
This is the emitter of transistor 2. In single-cycle mode, the signal on it is the same as on 9. on the other it is low, and vice versa. In most devices, the signals from the emitters of the output transistor switches of the microcircuit in question drive powerful field-effect transistors, which are driven to the ON state when the voltage at pins 9 and 10 is high (above ~ 3.5 V, but it does not refer to the level of 3.3 V on No. 3 and 4).
Conclusion 11
This is the collector of transistor 2, usually connected to a DC voltage source (+12V).
Note: In devices on the TL494CN, the switching circuit may contain both collectors and emitters of transistors 1 and 2 as outputs of the PWM controller, although the second option is more common. There are, however, options when exactly pins 8 and 11 are outputs. If you find a small transformer in the circuit between the IC and the FETs, the output signal is most likely taken from them.(from collectors)
Conclusion 14
This is the ION output, also described above.
Working principle
How does the TL494CN chip work? We will give a description of the order of its work based on materials from Motorola, Inc. The pulse width modulation output is achieved by comparing the positive sawtooth signal from the capacitor Ct to either of the two control signals. The output transistors Q1 and Q2 are NOR gated to open them only when the trigger clock input (C1) (see TL494CN function diagram) goes low.
Thus, if at the input C1 of the trigger the level of a logical unit, then the output transistors are closed in both modes of operation: single-cycle and push-pull. If a clock signal is present at this input, then in the push-pull mode, the transistor switches open one by one upon arrival of the clock pulse cutoff to the trigger. In single-cycle mode, the trigger is not used, and both output keys open synchronously.
This open state (in both modes) is possible only in that part of the FPV period when the sawtooth voltage is greater than the control signals. Thus, an increase or decrease in the magnitude of the control signal causes a linear increase or decrease in the width of the voltage pulses at the outputs of the microcircuit, respectively.
Voltage from pin 4 (dead time control), error amplifier inputs or feedback signal input from pin 3 can be used as control signals.
First steps in working with a microcircuit
Before doingany useful device, it is recommended to learn how the TL494CN works. How to check if it works?
Take your breadboard, put the IC on it and connect the wires according to the diagram below.
If everything is connected correctly, the circuit will work. Leave pins 3 and 4 not free. Use your oscilloscope to check the operation of the FPV - at pin 6 you should see a sawtooth voltage. The outputs will be zero. How to determine their performance in TL494CN. Checking it can be done like this:
- Connect feedback output (3) and dead time control output (4) to ground (7).
- Now you should detect the square wave at the outputs of the IC.
How to amplify the output signal?
The output of the TL494CN is rather low current, and you certainly want more power. Thus, we must add some powerful transistors. The easiest to use (and very easy to get - from an old computer motherboard) are n-channel power MOSFETs. At the same time, we must invert the output of the TL494CN, because if we connect an n-channel MOSFET to it, then in the absence of a pulse at the output of the microcircuit, it will be open for DC flow. In this case, the MOSFET can simply burn out … So we take out the universal npn transistor and connect it according to the diagram below.
Powerful MOSFET in thisthe circuit is passively controlled. This is not very good, but for testing purposes and low power it is quite suitable. R1 in the circuit is the load of the npn transistor. Select it according to the maximum allowable current of its collector. R2 represents the load of our power stage. In the following experiments, it will be replaced by a transformer.
If we now look at the signal at pin 6 of the microcircuit with an oscilloscope, we will see a “saw”. On 8 (K1) you can still see square wave pulses, and on the drain of the MOSFET pulses of the same shape, but larger.
How to raise the output voltage?
Now let's get some voltage up with the TL494CN. The switching and wiring diagram is the same - on the breadboard. Of course, you cannot get a sufficiently high voltage on it, especially since there is no heat sink on the power MOSFETs. However, connect a small transformer to the output stage according to this diagram.
The primary winding of the transformer contains 10 turns. The secondary winding contains about 100 turns. Thus, the transformation ratio is 10. If you apply 10V to the primary, you should get about 100V at the output. The core is made of ferrite. You can use some medium sized core from a PC power supply transformer.
Be careful, the output of the transformer is high voltage. The current is very low and will not kill you. But you can get a good hit. Another danger is if you install a largecapacitor at the output, it will accumulate a large charge. Therefore, after turning off the circuit, it should be discharged.
At the output of the circuit, you can turn on any indicator like a light bulb, as in the photo below.
It runs on DC voltage and needs about 160V to light up. (The power supply of the entire device is about 15 V - an order of magnitude lower.)
The transformer output circuit is widely used in any UPS, including PC power supplies. In these devices, the first transformer, connected via transistor switches to the outputs of the PWM controller, serves to galvanically isolate the low-voltage part of the circuit, which includes the TL494CN, from its high-voltage part, which contains the mains voltage transformer.
Voltage regulator
As a rule, in home-made small electronic devices, power is provided by a typical PC UPS, made on TL494CN. The power supply circuit of a PC is well known, and the blocks themselves are easily accessible, since millions of old PCs are disposed of annually or sold for spare parts. But as a rule, these UPSs do not produce voltages higher than 12 V. This is too little for a variable frequency drive. Of course, one could try and use an overvoltage PC UPS for 25 V, but it will be difficult to find, and too much power will be dissipated at 5 V in the logic elements.
However, on TL494 (or analogues) you can build any circuits with output to increased power and voltage. Using typical parts from PC UPS and high power MOStransistors from the motherboard, you can build a PWM voltage regulator on the TL494CN. The converter circuit is shown in the figure below.
On it you can see the switching circuit of the microcircuit and the output stage on two transistors: a universal npn- and a powerful MOS.
Main parts: T1, Q1, L1, D1. The bipolar T1 is used to drive a power MOSFET connected in a simplified way, the so-called. "passive". L1 is an inductor from an old HP printer (about 50 turns, 1 cm high, 0.5 cm wide with windings, open choke). D1 is a Schottky diode from another device. TL494 is wired in an alternative way to the above, although either can be used.
C8 is a small capacitance to prevent the effect of noise entering the input of the error amplifier, a value of 0.01uF will be more or less normal. Larger values will slow down the setting of the desired voltage.
C6 is an even smaller capacitor, it is used to filter high frequency noise. Its capacity is up to several hundred picofarads.
