TOP Switch shuts down

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Hi,

I'm testing a power supply prototype, that takes 110v ac input, 24V,10A output, using topswitch.

Initially the clamp resistor was getting hot(more than 115c), after reducing the resitance and increasing its wattage this stopped and  the temperature stayed at 65c. But at 5A load, the damping resistor got burnt after 5 min . I increased its wattage also. It is not burning out now. But TOP261 shuts down, when i monitored its drain current, the leading edge spike starts after 3 mins, and it starts growing till topswitch shutsdown.

The measured clamp voltage and VOR are matching with the PIExpert s/w.

I measured the primary leakage inductance as 133uh and primary capacitance as110pf. The transformer spec says 116uh.

I have also attached the drain current and voltage waveform at 5A load.

Kindly suggest , how to proceed

After looking at waveform,spikes do not look excessive. There seems to be some instability  as the adjacent switch cycles have significantly different duty cycles. At this output power level, heatsinking is critically important. Place thermo-couple wire on tab off TOPSwitch and monitor tempersture. If tab is over 110c you will need better heatsinking

Hi Surak,

Thanks for your reply . Yes the waveform i posted earlier was with a poor heatsink connected. I had taken a new waveform shot, which shows a spike in the current waveform. At 5A load this spike starts increasing and after littewhile damping resistor gets burnt.

Growing spike suggests that there is still something wrong with your primary clamp ckt.  It might be the rectifying didoe in your clamp circuit is getting too hot and breaking down -- try replacing with  a higher current rated ultra fast diode