SMPS using TL494

[walker1]

I want to modify an old PC supply for my needs (+12 V and -5V only)

I have most of it up and running, but the circuit used for driving the high voltage switches somwhat baffles me.

I have analysed 4 or 5 different supplies using the 494. All use the same Half-Bridge method.

Most of the circuits differs (using transistors, op-ams etc.) but the they all have one thing in common.
The duty-cycle of the chip is connected backwards, so a too high voltage on the output results in a higher duty-cycle out of the chip. These outputs are then inverted in an x-tra set of transistors driving the pulse tranformer.

Why do they do that ?
Has it something to do with the self-oscilating startup coupling, where the current to the primary coil on the main transformer is looped through the pulse transformer ?

 

[m3]

Hello Walker,

This pulse transformer have three functions:

1- Self-oscilating when power supply is turned on, this function generate a initial voltage to supply a TL494 locate in secundary side. A auxiliar power supply is not necessary

2- Isolate a trigger pulse from TL494 to power transistor

3- Over current protection, when a current in primary side pass in looped through the pulse transformer a proportional voltage is generated in secundary side, this voltage is rectified by small diodes (1N4148) and conected a protecion circuit (SCR type) , in short-circuit or overload the voltge rise and trig the protection circuit.

To modify a PC power supply , remove a feedback resistor from +5V output (conected in pin 2 or 15 of TL494), most of the circuits have a dual feedback (+5V and +12V). Modify a value of feedback resistor in +12V output and remove a power diode from +5V output.

M3

 

[walker1]

I sort of figured that one with the overcurrent out, but why use the same pulse driving circuit, if the primary current is sensed by a separate current transformer ?
(Usually in circuits using comparators in addition to the 494)

I have only seen the 494 directly drive the pulse transformer, if there is a small power transformer supplying working voltage to the secondary side. (usually in older constructions, and the primary current is normally sensed by an x-tra transformer)

It could be, however, that the peak voltage on the transformer can be higher, than the 42 V, the 494 output can take.
I have the datasheet for the transistors used, but I have not yet checked that parameter out.

As I mentined in the 1st entry, I know how to do most of the modifications needed for my purpose. It was just why nearly all couplings with the 494 used that output circuit.

 

[m3]

Walker,

Simple answer: production cost.

 

[walker1]

More correctly, one might expect:

Skip the devellopment !
Use what others have already made, even though it might be like shooting sparrows with missiles.