Siemens 24V-20A Power Supply repairing need help!

[Pham Huu Thien]

Hi everyone,
I am a newbie. Currently, I am repairing a Power Supply 24V 20A of Siement, I have a problem with 2 power BJT and a Mosfet. The BJT is BU508 and Mosfet is BUK543-50B.
The Mosfet is switching on for the BJTs, and the BJTs is switching on for tranformer. They are broken, I have replaced the Mosfet is IRFZ44 (I can't buy a BUK543-50B at my local) and the BJT is BU508. After replace, I powered on my Power Supply then the BJT is broken again and a zener. The IC drive for the Mosfet is UC3843.
I query that I replaced Mosfet not right. Can everyone tell me that the Mosfet is right or don't? If don't right, Can tell me the equivalent Mosfet and BJT? Again, I can't buy a BUK543-50B and a BU508 at my local.
Thank you so much.

 

[Nigel Goodwin]

You need to find the actual cause, not just the main symptoms (the dead power transistors).

There's almost always something which makes them fail, hence they just blow again.

The chip is one cause, various diodes are another, and electrolytics yet another. There's also the chance of duff resistors, either causing the fault, or been blown by the transistors failing.

I've repaired thousands of SMPSU's in TV's etc. and generally you have to replace the same components in the same models, I used to have all the parts for specific models in their own draws.

But if you don't replace ALL the faulty components, it WILL blow again.

One other thing which can cause it, is the reservoir capacitor - again model dependent - some models instantly and seriously destroy the PSU if it goes duff, where some continue working with very little ill effect.

 

[Pham Huu Thien]

thank you,
I don't know the reservoir capacitor that you tell. Could you explain it for me.
I don't have any BJT and Mosfet for my PSU. So, could you tell me other method to repair it?

 

[Nigel Goodwin]

SMPSU's are complicated to repair, you need to first have a good knowledge and understanding of how they work and Electronics in general.

It's not something you can learn in forum.

 

[pfofit]

In addition, the original devices were isolated in a plastic case. Your replacements have metal tabs that connect to the collector of the transistor and the drain of the mosfet.

If they were connected to a combined heat sink or ground case and you did not provide isolation tabs, then stuff will be doomed.

cheers

 

[Pham Huu Thien]

In addition, the original devices were isolated in a plastic case. Your replacements have metal tabs that connect to the collector of the transistor and the drain of the mosfet.

If they were connected to a combined heat sink or ground case and you did not provide isolation tabs, then stuff will be doomed.

cheers

Thank you very much!
I don't mount them to the Heat Sink Aluminum. I have checked diodes, they don't break. I have also tested the control circuit, the circuit is good. I think that the power circuit has a trip. I don't know that I replace the component not right or capacitors not good. because I don't test capacitors.

 

[KeepItSimpleStupid]

Many SMPS's require a minimum load. If they don't have that, they will fail. A "rule of thumb" is about 10-20% of the rated load.

 

[Pham Huu Thien]

Many SMPS's require a minimum load. If they don't have that, they will fail. A "rule of thumb" is about 10-20% of the rated load.

Thank you!

 

[KeepItSimpleStupid]

Some stuffto back up my comment: **broken link removed**