The Integrated Circuit (IC) used in an old computer switching supply is often a TL494 sometimes marked IR3M02. This is an analog IC that controls transistors which are connected to a transformer. The transformer can be use to step up or step down the voltage.
The IC can be divided up into several sections for easy understanding of its operation. They are: the oscillator, the reference, the error amplifier, and the output transistors.
THE OSCILLATOR
This determines the frequency of operation of the device and it normally runs in the range 10 to 100 khz. This is so that the transformer can be a ferrite type for small size and efficiency. Any type of transformer can be used. The frequency of oscillation can be set by one resistor and one capacitor, on pin 6 and pin 5 respectively. Values of 12 k ohms and 0.001 uf give about 100 khz.
THE REFERENCE
The Reference has a double function. It provides a stable voltage that the output can be referred or compared to, to enable the regulator to work. The Reference also acts like a voltage regulator to provide a stable voltage so that the internal control circuits are isolated from power supply variations and noise. The IC can operate from any power supply in the range from 7 to 40 volts DC, and the reference can be measured at pin 14, which is 5 volts DC.
ERROR AMPLIFIER
There are 2 independent error amplifiers and either or both can be used. One is typically used for voltage regulation, and the other for current regulation, but they are uncommitted. Connect the negative input on one amplifier to the reference, and connect the positive input to a voltage divider between the output voltage and earth. The IC will then adjust the output to make the voltage at the junction of the voltage divider match the reference voltage. If the voltage is too low, the pulses get wider, and if the voltage is too high the pulses get narrower. Connect the unused amplifier inputs, negative to reference and positive to earth.
OUTPUT TRANSISTORS
There are 2 output transistors that can be used to drive a transformer. I have used them, but they only can drive 200 mA and they overheat and shut down if you overload them. They are not short circuit proof! You need external transistors for any appreciable power output. They can be connected any way you like as the collectors and emitters are available. They can also be operated in push pull or parallel by using the Control pin (13). They have protection so that both are not ON at the same time (for push pull operation). This protection can be used as a Soft Start feature. Connect a resistor to earth from pin 4, and a capacitor between pin 4 and pin 13. The time constant of these is the slow turn on time. A resistor of 4700 ohms and a capacitor of 10 uf gives a turn on of about 200 mS. If soft start is not required, just join pin 4 to pin 13.
TESTING
The IC can be tested in sections. Reference: Connect 12 volts to the IC, and see if the reference is working. Oscillator: Connect a resistor and capacitor and see if the oscillator works. Use an oscilloscope to see if there is a sawtooth waveform on the capacitor pin. Output Transistors: Connect the emitters to earth and connect the collectors to the 12 volts via individual 1000 ohm resistors. Connect the two negative inputs to the reference. Connect the two positive inputs to earth. Use an oscilloscope to see if there is a square wave on each of the collectors. Error Amplifiers: After testing the output transistors, connect one of the positive inputs to 12 volts through a 1000 ohm resistor. Use an oscilloscope to see if the square wave has gone, and the output is at 12 volts. Test the other amplifier in the same way.
The real test is to build a circuit and see if it works.