Switching power supplies (UPS) are very common. The computer you are using now has a UPS with several output voltages (+12, -12, +5, -5 and + 3.3 V, at least). Almost all such units have a special PWM controller chip, as a rule, of the TL494CN type. Its analogue is the domestic microcircuit M1114EU4 (KR1114EU4).
Manufacturers
The chip under consideration 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 began the development of a line of these controllers, which led to the creation in the early 2000s. Chips TL494 Series. In addition to the UPSs already mentioned above, they can be found in DC voltage regulators, in controlled drives, in soft starters, - in a word, wherever PWM regulation 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
Analysis of the descriptions of the type of chip under consideration from different manufacturers shows the practical identity of its characteristics. The amount of information cited by different firms is almost the same. Moreover, TL494CN datasheet from brands such as Motorola, Inc and ON Semiconductor repeat each other in their structure, figures, tables and graphs. The presentation of the material by Texas Instruments is somewhat different from them, however, upon careful examination it becomes clear that this means an identical product.
Purpose of the TL494CN Chip
Traditionally, we will begin to describe it with the purpose and list of internal devices. It is a PWM controller with a fixed frequency, designed primarily for use in the UPS, and containing the following devices:
- sawtooth voltage generator (GPN);
- error amplifiers;
- source of reference (reference) voltage +5 V;
- dead time adjustment circuit;
- output transistor switches for current up to 500 mA;
- a circuit for selecting a one- or two-stroke mode of operation.
Limit parameters
As with any other microcircuit, the TL494CN description must necessarily contain a list of maximum permissible performance characteristics. Let's give them based on data from Motorola, Inc:
- Supply voltage: 42 V.
- The voltage at the collector of the output transistor: 42 V.
- Collector Current Output Transistor: 500 mA.
- Amplifier input voltage range: from - 0.3 V to +42 V.
- Power dissipation (at t <45 ° C): 1000 mW.
- Storage temperature range: from -55 to +125 ° .
- Range of working ambient temperatures: from 0 to +70 ° .
It should be noted that parameter 7 for the TL494IN chip is somewhat wider: from –25 to +85 ° .
Chip Design TL494CN
The description in Russian of the findings of her case is shown in the figure below.
The microcircuit is placed in a plastic (indicated by the letter N at the end of its designation) 16-pin package with pdp-type terminals.
Her 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 range of pulse durations, 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 outputs for the output signals, 4 of them (1, 2, 15, 16) are inputs of 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 allowable load current of 250 mA (in continuous operation, not more than 200 mA). They can be connected in pairs (9 s 10, and 8 s 11) to control powerful field effect transistors (MOSFETs) with a maximum permissible current of 500 mA (not more than 400 mA in continuous mode).
What is the internal device TL494CN? Its scheme is shown in the figure below.
The chip 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 output 13 of the choice of a one- or two-stroke mode of operation of the microcircuit: if it has +5 V, the second mode is selected , if there is a minus 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 chip has conclusions 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 presentation of the material.
Input Pin Functions
Like any other electronic device. the chip in question has its inputs and outputs. We will start from the first. A list of these findings of TL494CN has already been given. A description in Russian of their functional purpose will be given below with detailed explanations.
Conclusion 1
This is the positive (non-inverting) input of the error signal amplifier 1. If the voltage on it is lower than the voltage at terminal 2, the output of the error amplifier 1 will be low. If it is higher than on pin 2, the signal of error amplifier 1 will become high. The output of the amplifier essentially repeats the positive input using pin 2 as a reference. The error amplifier functions will be described in more detail below.
Conclusion 2
This is the negative (inverting) input of error signal amplifier 1. If this pin is higher than pin 1, the output of error amplifier 1 will be low. If the voltage at this terminal is lower than the voltage at terminal 1, the output of the amplifier will be high.
Conclusion 15
It works exactly the same as No. 2. Often, a second error amplifier is not used in the TL494CN. In this case, its inclusion circuit contains terminal 15 simply connected to the 14th (reference voltage +5 V).
Conclusion 16
It works the same as No. 1. It is usually attached to general No. 7 when a second error amplifier is not used. With pin 15 connected to +5 V and No. 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 contact and each TL494CN internal amplifier 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 3.3 V, the output pulses are turned off (zero duty cycle). When the voltage on it is close to 0 V, the pulse duration is maximum. In the interval between 0 and 3.3 V, the pulse duration is from 50% to 0% (for each of the outputs of the PWM controller - on pins 9 and 10 in most devices).
If necessary, pin 3 can be used as an input signal or can be used to provide damping of the rate of change of the pulse width. If the voltage on it is high (> ~ 3.5 V), there is no way to start the UPS on the PWM controller (there will be no pulses from it).
Conclusion 4
It controls the output duty cycle range (Dead-Time Control). If the voltage on it is close to 0 V, the microcircuit will be able to produce both the minimum possible and maximum pulse widths (which is set by other input signals). If a voltage of about 1.5 V is applied to this output, 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 (> ~ 3.5 V), there is no way to start the UPS on the TL494CN. Its inclusion circuit often contains No. 4 connected directly to the ground.
- It is important to remember ! The signal at pins 3 and 4 should be lower than ~ 3.3 V. What will happen if it is close, for example, to + 5 V? How then will TL494CN 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 time-setting capacitor Ct, and its second contact is connected to the ground. The capacitance values are usually from 0.01 μF to 0.1 μF. Changes in the magnitude 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 a very low temperature coefficient (with a very small change in capacitance with a change in temperature) are used here.
Conclusion 6
To connect a surge resistor Rt, and its second contact is connected to the ground. The values of Rt and Ct determine the frequency of the GPN.
Conclusion 7
It joins the common wire of the device circuit on the PWM controller.
Conclusion 12
It is marked with VCC letters. The "plus" of the TL494CN power supply is connected to it. Its switching circuit usually contains No. 12 connected to a power supply switch. Many UPSs use this pin to turn on the power (and the UPS itself) and turn it off. If it has +12 V and No. 7 is grounded, the GPN and ION chips will work.
Conclusion 13
This is the input of the operating mode. Its functioning has been described above.
Output Functions
Above, they were listed for TL494CN. A description in Russian of their functional purpose will be given below with detailed explanations.
Conclusion 8
There are 2 npn transistors on this chip, which are its output keys. This pin is the collector of transistor 1, usually connected to a constant voltage source (12 V). Nevertheless, in the circuits of some devices it is used as an output, and you can see the meander on it (as well as on No. 11).
Conclusion 9
This is the emitter of transistor 1. It controls a powerful UPS transistor (field-based in most cases) in a push-pull circuit, either directly or through an intermediate transistor.
Conclusion 10
This is the emitter of transistor 2. In the single-cycle operation mode, the signal on it is the same as on No. 9. In the two-cycle mode, the signals on No. 9 and 10 are out of phase, that is, when the signal level is high on one, then it is low on the other, and vice versa. In most devices, the signals from the emitters of the output transistor switches of the chip in question control powerful field-effect transistors, which are turned ON when the voltage at pins 9 and 10 is high (above ~ 3.5 V, but it does not relate to the 3.3 V level at no No. 3 and 4).
Conclusion 11
This is the collector of transistor 2, usually connected to a constant voltage source (+12 V).
- Note : In devices on TL494CN, its switching circuit may contain as collectors, emitters of transistors 1 and 2 as outputs of the PWM controller, although the second option is more common. However, there are options when exactly pins 8 and 11 are outputs. If you find a small transformer in the circuit between the microcircuit and field-effect transistors, the output signal is most likely taken from them (from the collectors).
Conclusion 14
This is an ION output, also described above.
Principle of operation
How does the TL494CN chip work? A description of its operation will be given based on materials from Motorola, Inc. Pulse output with latitudinal modulation is achieved by comparing a positive sawtooth signal from the capacitor Ct with any of the two control signals. The logic circuits OR NOT control the output transistors Q1 and Q2, open them only when the signal at the clock input (C1) of the trigger (see the functional diagram TL494CN) goes low.
Thus, if at the input C1 of the trigger the level is a logical unit, then the output transistors are closed in both operating modes: single-cycle and push-pull. If a clock signal is present at this input , then in push-pull mode, transistor switches open one after another upon arrival of a cut-off of a clock pulse on a 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 GPN 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 correspondingly linear increase or decrease in the width of the voltage pulses at the outputs of the microcircuit.
As control signals, voltage can be used from terminal 4 (dead time control), error amplifier inputs or feedback signal input from terminal 3.
The first steps to work with the chip
Before making any useful device, it is recommended that you study how TL494CN works. How to check its performance?
Take your breadboard, install a chip on it, and connect the wires according to the diagram below.
If everything is connected correctly, then the circuit will work. Leave pins 3 and 4 not free. Use your oscilloscope to check the operation of the GPN - on pin 6 you should see a sawtooth voltage. The outputs will be zero. How to determine their performance in TL494CN. Checking it can be performed as follows:
- Connect the feedback output (No. 3) and the dead time control output (No. 4) to the common terminal (No. 7).
- Now you should detect rectangular pulses at the outputs of the chip.
How to amplify the output signal?
The output of the TL494CN is quite low-current, and of course you want more power. Thus, we must add some powerful transistors. The most simple to use (and very easy to get - from an old computer motherboard) n-channel power MOS transistors. In this case, we must invert the output of TL494CN, because if we connect an n-channel MOS transistor to it, then in the absence of a pulse at the output of the microcircuit, it will be open for direct current flow. At the same time, the MOS transistor can simply burn out ... So we get the universal npn transistor and connect it according to the diagram below.
The powerful MOSFET in this 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 permissible 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 on pin 6 of the microcircuit with an oscilloscope, you will see a “saw”. At No. 8 (K1), you can still see rectangular pulses, and at the drain of the MOS transistor, the pulses are the same in shape, but of a larger magnitude.
But how to increase the voltage at the output?
Now let's get some voltage higher with TL494CN. The connection and wiring diagram is the same - on the breadboard. Of course, you can’t get a sufficiently high voltage on it, especially since there is no radiator on power MOS transistors. 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 transformation ratio is 10. If you supply 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 a 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 hit. Another danger - if you install a large capacitor 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 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 transformer output circuit is widely used in any UPS, including PC power supplies. In these devices, the first transformer connected through transistor switches to the outputs of the PWM controller serves for galvanic isolation of the low - voltage part of the circuit, including the TL494CN, from its high-voltage part, containing the mains voltage transformer.
Voltage regulator
Typically, in homemade small electronic devices, power is provided by a typical PC UPS based on TL494CN.The power supply circuit for PC power supply is well known, and the units themselves are easily accessible, since millions of old PCs are disposed of annually or sold for parts. But as a rule, these UPSs produce voltages no 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 would be difficult to find, and too much power would be dissipated at 5 V in the logic elements.
TL494 ( ) . - , - TL494CN. .
: npn- .
Main parts: T1, Q1, L1, D1. Bipolar T1 is used to control a powerful MOS transistor 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 inductor). 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 small capacitor, in order to prevent the influence of noise coming to the input of the error amplifier, the value of 0.01uF will be more or less normal. Larger values will slow down the installation of the required voltage.
C6 is an even smaller capacitor, it is used to filter high-frequency interference. Its capacity is up to several hundred picofarads.