TL494CN: wiring diagram, description in English, converter circuit

Switching power supplies (UPS) are very common. The computer that you are using now has a UPS with several output voltages (+12, -12, +5, -5 and + 3.3V, at least). Almost all such units have a special PWM controller chip, usually of the type TL494CN. Its analogue is the domestic microcircuit M1114EU4 (KR1114EU4).

Manufacturers

This chip belongs to the list of the most common and widely used integrated electronic circuits. Its predecessor was a series of UC38xx PWM controllers from Unitrode. In 1999 this company was bought by Texas Instruments, and since then the development of the line of these controllers began, which led to the creation in the early 2000s. Microcircuit series TL494. In addition to the already mentioned UPS, they can be found in regulators of constant voltage, in controlled drives, in soft starters, in a word everywhere, where PWM control is used.

Among the companies that cloned this chip, there are such world famous brands as Motorola, Inc, International Rectifier, Fairchild Semiconductor, ON Semiconductor. All of them give a detailed description of their products, the so-called TL494CN datasheet.

Documentation

An analysis of the descriptions of the type of microchip under consideration from different manufacturers shows the practical identity of its characteristics. The volume of information given by different firms is almost the same. Moreover, TL494CN datasheet from such brands as Motorola, Inc and ON Semiconductor repeat each other in their structure, given figures, tables and graphs. Somewhat different from them is the presentation of the material from Texas Instruments, but if you carefully study it, it becomes clear that an identical product is meant.

Purpose of the TL494CN

Description of its traditionally begin with the appointment and the list of internal devices. It is a PWM controller with a fixed frequency, intended primarily for use in a UPS, and containing the following devices:

• Generator of sawtooth voltage (GPN);
• Error amplifiers;
• Source of the reference (reference) voltage +5 V;
• Dead time adjustment circuit;
• Output transistor switches for currents up to 500 mA;
• Scheme for selecting one- or two-stroke operation.

Limit Parameters

As with any other chip, the TL494CN description must contain a list of maximum permissible performance characteristics. Let's give them based on Motorola, Inc:

1. Power supply: 42 V.
2. Voltage at the collector of the output transistor: 42 V.
3. Collector current of the output transistor: 500 mA.
4. Amplifier input voltage range: from -0.3 V to +42 V.
5. Power dissipation (at t <45 ° C): 1000 mW.
6. Storage temperature range: -55 to + 125 ° C.
7. Operating ambient temperature range: 0 to +70 ° C.

It should be noted that parameter 7 for the TL494IN is somewhat wider: -25 to +85 ° C.

Design of the TL494CN

The description of the conclusions of her corps in Russian is shown in the figure below.

The chip is placed in a plastic (as indicated by the letter N at the end of its designation) 16-pin package with pdp-type terminals.

Its appearance is shown in the photo below.

TL494CN: Functional diagram

So, the task of this chip is Pulse Width Modulated (PWM) pulse voltage generated inside both regulated and unregulated UPS. 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 automotive audio amplifiers).

The TL494CN has a total of 6 outputs for the output signals, 4 of them (1, 2, 15, 16) are the inputs of the internal error amplifiers used to protect the UPS from current and potential overloads. Contact No. 4 is a signal input from 0 to 3 V for adjusting the duty cycle of the output rectangular pulses, and No. 3 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 permissible load current of 250 mA (in continuous mode no more than 200 mA). They can be connected in pairs (9 with 10, and 8 with 11) to control powerful MOSFETs with a maximum permissible current of 500 mA (no more than 400 mA in continuous mode).

What is the internal TL494CN device? The diagram is shown in the figure below.

The chip has a built-in reference voltage (ION) +5 V (No. 14). It is usually used as a reference voltage (with an accuracy of ± 1%) applied to the inputs of circuits consuming no more than 10 mA, for example, to pin 13 of selecting a single- or two-cycle operation mode of the microcircuit: if there is +5 V, selects the second mode , If there is a minus supply voltage on it - the first.

To adjust the frequency of the sawtooth voltage generator (GPN), use a capacitor and a resistor connected to pins 5 and 6, respectively. And, of course, the microcircuit has leads for connecting plus and minus the power supply (numbers 12 and 7 respectively) in the range from 7 to 42 V.

It can be seen from the diagram that there are a number of internal devices in the TL494CN. The description in Russian of their functional purpose will be given below in the course of the presentation of the material.

Output functions of input signals

Like any other electronic device. The considered microcircuit has its inputs and outputs. We will begin with the first. The list of these conclusions of TL494CN has already been given above. The description in Russian of their functional purpose will be further explained with detailed explanations.

Conclusion 1

This is a positive (non-inverting) input of the signal amplifier of error 1. If the voltage on it is lower than the voltage at pin 2, the output of error amplifier 1 will be low. If it is higher than on pin 2, the signal of the error amplifier 1 becomes high. The output of the amplifier essentially repeats 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 the signal amplifier of error 1. If this pin is higher than pin 1, the output of error amplifier 1 will be low. If the voltage at this pin is lower than the voltage at pin 1, the output of the amplifier will be high.

Conclusion 15

It works exactly the same as No. 2. Often, the second error amplifier is not used in the TL494CN. The circuit of its inclusion in this case contains a terminal 15 simply connected to the 14th (reference voltage +5 V).

Conclusion 16

It works in the same way as No. 1. It is usually connected to the common No. 7 when the second error amplifier is not used. With pin 15 connected to +5 V and No. 16 connected to the common, the output of the second amplifier is low and therefore has no effect on the operation of the chip.

Conclusion 3

This contact and each internal amplifier TL494CN are interconnected via diodes. If the signal at the output of any of them changes from low to high, then at No. 3 it also goes high. When the signal at this pin exceeds 3.3V, the output pulses are turned off (zero duty cycle). When the voltage on it is close to 0 V, the pulse duration is maximum. Between 0 and 3.3 V, the pulse width is between 50% and 0% (for each of the PWM controller outputs - on pins 9 and 10 in most devices).

If necessary, contact 3 can be used as an input signal or can be used to provide damping of the pulse width change rate. If the voltage 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 dead-time control range. If the voltage on it is close to 0 V, the chip 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 1.5 V is applied to this terminal, the width of the output pulse will be limited to 50% of its maximum width (or ~ 25% of the duty cycle for the push-pull mode of the PWM controller). If the voltage is high (> ~ 3.5V), there is no way to start the UPS on the TL494CN. The scheme of its inclusion often contains No. 4, connected directly to the ground.

• It is important to remember ! The signal on terminals 3 and 4 should be below ~ 3.3 V. And what if it is close, for example, to + 5V? How then will the TL494CN behave? The circuit of the voltage converter on it will not produce impulses, i.e. There will be no output voltage from the UPS.

Conclusion 5

It serves to connect the time-consuming capacitor Ct, with its second contact being connected to the ground. The capacitance values are usually from 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. Typically, high-quality capacitors with very low temperature coefficient (with very small capacitance change with temperature change) are used here.

Conclusion 6

To connect the time-consuming resistor Rt, with its second contact connected to the ground. The values of Rt and Ct determine the frequency of the GPN.

• F = 1.1: (Rt × Ct).

Conclusion 7

It is connected to the common wire of the device circuit on the PWM controller.

Conclusion 12

It is marked with VCC letters. To him joins the "plus" power supply TL494CN. The circuit of its inclusion usually contains No. 12, connected to the switch of the power supply. Many UPSs use this output to turn on the power (and the UPS itself) and turn it off. If it has +12 V and 7 on it grounded, the GPN and ION chips will work.

Conclusion 13

This is the mode input. Its functioning was described above.

Output functions of output signals

Above they were listed for TL494CN. The description in Russian of their functional purpose will be given below with detailed explanations.

Conclusion 8

On this chip there are 2 npn-transistors, which are its output keys. This output is the collector of transistor 1, usually connected to a constant voltage source (12 V). Nevertheless, in some devices, it is used as an output, and you can see a meander on it (as in No. 11).

Conclusion 9

This is the emitter of transistor 1. It controls a powerful transistor UPS (field in most cases) in a push-pull circuit either directly or via an intermediate transistor.

Conclusion 10

This is the emitter of the transistor 2. In the single-cycle operation mode, the signal on it is the same as at No. 9. In the two-stroke mode, the signals in Nos. 9 and 10 are out of phase, that is, when the signal level is high, and vice versa. In most devices, the signals from the emitters of the output transistor switches of the circuit under consideration control the powerful FETs, which are turned ON when the voltage at terminals 9 and 10 is high (above ~ 3.5 V, but it does not refer to 3.3 V at No. № 3 and 4).

Conclusion 11

This is the collector of transistor 2, usually connected to a source of direct voltage (+12 V).

• Note : In devices with the TL494CN, the wiring diagram may contain PWM controller outputs as collectors, but emitters of transistors 1 and 2, although the second variant is more common. There are, however, options when exactly contacts 8 and 11 are outputs. If you find a small transformer in the circuit between the chip and the FETs, the output signal is most likely taken from them (from the collectors).

Conclusion 14

This is the output of the ION, also described above.

Principle of operation

How does the TL494CN work? A description of the order of its work will be given on the materials of Motorola, Inc. Pulse output with latitudinal modulation is achieved by comparing the positive sawtooth signal from the capacitor Ct to either of the two control signals. Logic NOR circuits of the output transistors Q1 and Q2 open them only when the signal at the clock input (C1) of the trigger (see function block TL494CN) goes to the low level.

Thus, if at the input C1 of the trigger is a logical one, then the output transistors are closed in both modes of operation: single-cycle and two-stroke. If there is a clock signal at this input , then in the push-pull mode, the transistor switches are opened in turn by the arrival of the cutoff of the clock pulse to the trigger. In the single-cycle mode, the trigger is not used, and both output keys open synchronously.

This open state (in both modes) is only possible in that part of the GPN period, when the sawtooth voltage is greater than the control signals. Thus, increasing or decreasing the magnitude of the control signal causes a linear increase or decrease in the width of the voltage pulses at the outputs of the microchip, respectively.

As the control signals, the voltage from pin 4 (dead time control), the inputs of the error amplifiers or the input of the feedback signal from pin 3 can be used.

The first steps to work with the chip

Before doing any useful device, it is recommended to study how the TL494CN works. How to test its performance?

Take your development board, install the IC on it and connect the wires according to the diagram below.

If everything is connected correctly, the circuit will work. Leave conclusions 3 and 4 not free. Use your oscilloscope to check the operation of the GPN - at pin 6 you should see a sawtooth voltage. The outputs will be zero. How to determine their performance in the TL494CN. It can be checked as follows:

1. Connect the feedback output (No. 3) and the dead time control output (No. 4) to the common terminal (No. 7).
2. Now you should detect the rectangular pulses at the outputs of the chip.

How to strengthen the output signal?

The output of the TL494CN is quite low-current, and you, of course, want more power. So we need to add a few powerful transistors. The most simple to use (and very easy to get - from the old computer motherboard) n-channel power MOSFETs. We must in this case invert the output of TL494CN, since if we connect the n-channel MOSFET to it, then in the absence of a pulse at the output of the chip it will be open for the flow of direct current. In this case, the MOSFET can simply burn ... So get a universal npn-transistor and connect according to the below diagram.

A powerful MOSFET in this circuit is controlled in a passive mode. This is not very good, but for testing purposes and low power is quite suitable. R1 in the circuit is the load of the NPN transistor. Select it according to the maximum permissible current of its collector. R2 is the load of our power cascade. In the following experiments it will be replaced by a transformer.

If we now see the oscilloscope signal on pin 6 of the chip, you will see a "saw". At No. 8 (K1), you can still see the rectangular pulses, and on the MOS transistor drain the same pulse shape, but larger.

And how to raise the voltage at the output?

Now let's get some voltage higher with the TL494CN. The wiring and wiring diagram uses the same - on the breadboard. Of course, there is not enough high voltage on it, especially since there is not any radiator on power MOSFETs. And yet, connect a small transformer to the output stage, according to this scheme.

The primary winding of the transformer contains 10 turns. The secondary winding contains about 100 turns. Thus, the conversion ratio is 10. If you apply 10V to the primary winding, you should get about 100 V at the output. The core is made of ferrite. You can use some medium sized core from the PC power supply transformer.

Be careful, the output of the transformer is under high voltage. The current is very low and will not kill you. But you can get a good kick. Another danger - if you install a large capacitor at the output, it will accumulate a large charge. Therefore, after switching 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 operates on DC voltage, and it needs about 160 V to light up. (The power of the entire device is about 15 V - an order of magnitude lower.)

The circuit with transformer output 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 including the TL494CN from its high-voltage part containing the mains voltage transformer.

Voltage regulator

Typically, in self-made small electronic devices, the power supply is provided by a typical PC UPS running on the TL494CN. The scheme for switching on the PSU of the PC is well known, and the blocks themselves are easily accessible, since millions of old PCs are annually disposed of or sold for spare parts. But as a rule, these UPSs produce voltages no higher than 12 V. This is too small for a frequency-controlled drive. Of course, one could try and use a PC of increased voltage for 25 V, but it will be difficult to find, and too much power will dissipate at a voltage of 5 V in the logic elements.

However, on the TL494 (or analogs) it is possible to build any circuits with output to increased power and voltage. Using typical PC UPS parts and powerful MOSFETs from the motherboard, you can build a PWM voltage regulator on the TL494CN. The circuit of the converter is shown in the figure below.

On it you can see the circuit of the microcircuit switching and output stage on two transistors: universal npn- and powerful MOSFET.

The main parts: T1, Q1, L1, D1. Bipolar T1 is used to control a powerful MOSFET connected in a simplified manner, so called. "Passive". L1 is an inductance throttle from the old HP printer (about 50 turns, 1 cm height, 0.5 cm width with windings, open throttle). D1 is a Schottky diode from another device. TL494 is connected in an alternative way to the above, although any of them can be used.

C8 is a capacitor of low capacitance, in order to prevent the noise input to the input of the error amplifier, the value of 0.01uF will be more or less normal. Higher values will slow down the setting of the required voltage.

C6 is an even smaller capacitor, it is used to filter high-frequency noise. Its capacity is up to several hundred picofarads.