Hi

iam pulling apart smps to get a better understanding of how they fail. I have a simple brick psu which has died with shorted switching transistor and bad primary side cap.

it looks like the drive for the swiching transistor comes from another small npn transisor. The transformer has two primary coils. the 2nd primary side seems to be responsible for the chopping action somehow.

What iam struggling to understand is how during startup does the circuit get going?

Any explanations would be much appreciated.
thanks.

 

Well, for example we can use a start-up resistors.
http://www.smps.us/12volt-powersupply.html (R7 , R8)
http://www.sp9hvw.info/uploads/ATX-3 JNC Y-B200-.jpg (R3 and R5 in standby supply )

 

Thanks Jony130, I think I need to try and simulate the circuit somehow to get it straight in my head. It all seems a bit chicken'n'egg.