Problem with SMPS similar to ATX SMPS.

[omaxtech]

Dear Sir,
I am having problem with a medical equipment SMPS which is more similar with the 200W ATX SMPS schematc also.
The fuses are not blowing but the Switching transistors - 2SC2625 ( 02 no's) get shorted with the turn on ( CE gets shorted). I have also replaced the TL 494 etc.
Please guide.

Thanks
_________________
Thanks,

Santosh Navale

[vtech]

Not seeing the schematic, as a general rule with any smps, you need to have some sort of a feedback loop to control or else it will immediately destruct. Any driver txistor or diodes can also cause it. ..if you can, power on slowly as you monitor waveforms/current draw

[torbjorn]

Make sure that there are no shorted turns or any other defects in the transformer. Of cours, there are probably no specifications available for it, but you could de-solder the transformer and measure the inductance of the primary winding, then compare that to the transformer of an ATX power supply of roughly the same wattage.

If you have a LCR meter with the option of measuring at different frequencies, then a sure sign of shorted turns is that you get very different inductance readings when changing the frequency from, say 1 kHz to 10 kHz.

Also, check out any snubber networks (resistors, capacitors, diodes) that might be fitted across the primary winding or the switching transistors.
Make sure that any current measurement shunts (low-R resistors in series with the emitter of any of the switching transistors) are ok.

Try running upp the TL494 without applying mains power to the unit (supply it from a bench power supply) and make sure that the clock
oscillator is running at the correct frequency, that there comes a square wave at both driver outputs and so on.

[omaxtech]

Thanks Torbjorn for your valuable suggestions. I mean time just changed the 2no's of 2SC2625 Switching transistors of different make & now the transistor burn / short problem is no more. The lot of transistors with the supplier was probly not good ( but the transistors would show correct diode conditions on multimeter).
But now the o/p of 24V DC is not contineous but it comes & goes at regular interval. I replaced the capacitors, TL494, Checked the diodes etc but still could not get the defective component. But I will try out your method of running the TL 494 from a desk supply..

Thanks,
_________________
Thanks,

Santosh Navale

[SZASZ]

You should check some resistor near the two transisitors, some kind of resistors with the value 1Ohm, 2,2Ohm.
Do you have a green wire on the output the power suplly?