Conversion of ATX power supply (heat dissipation problem at low voltage high current)

Silvio

Well-known member
Dear All, lately I converted a computer ATX power supply rated at 400 watts. it is intended to work as a dual variable power supply from 1-30vdc at 5 amps
1) The topology is half bridge.
2) The chip is the TL494.
3) The ferrite transformer is re wound to give 2 secondary windings of 34 volts
4) the windings have separate full wave rectifier diodes
The power supply works fine and is strong and puts a hefty output of 28volts 5 amps on each winding also with current control from 0.1 to 6 amps

My Problem is that when loading the supply at around half the voltage say 15volts at 5 amps I am noticing quick heat build up in the switching transistors. When I load at say 24volts 5 amps on each winding the switching transistors heat at around body temperature and can run all day. I am loading the output with two 12v 60w car headlamps in series for 24 volts and only one when loading on 15volts. I have a cooling fan blowing on the heatsink which is temperature controlled.

Can anyone explain to me why is this all heat dissipation at lower wattage and how can it be cured.

The link of my project on youtube https://www.youtube.com/watch?v=uXzrmLrwPcg
 
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