Overheating Diode in Power Supply with TOPSwitch

[2X]

Hello,
I have a power supply where on part of it has an SMPS trasformer with a TOPSwitch (TOP201YAI). The diode (D88) and the TOPSwitch were shorted. I replaced them but again the diode and the TOPSwitch were shorted. I used a variac with 36Vrms as a source instead of the mains  (230Vrms) and with a thermal camera I saw that the diode (D88) in 5 sec reachs 90Celcious and if I don't cut the supply the temperature will continue to increase. I disconnected the diode and soldered cables on PCB in order to measure the current that flows through the diode and the current it just 55mADC. I can't understand at all the reason that this diode is overheating. Also, when I disconnect the TOPSwitch the diode don't overheat. I replaced the TOPSwitch with a brand new (TOP201YN - the final letters of the part number was not exaclty the same) and the same thing again. I made a schematic design and I can't understand what causes the overheating of the diode.
At the picture with the cells are the measurements (with KEYSIGHT U1733C) of the SMPS transformer with all possible combinations between each pin.

[DavidAlfa]

1N4007 is too slow for 100KHz, was it the original part?
I've seen some schematics using a 1n4148 (1ns switching time).
I don't think the meter measures current very well at 100KHz?

Make sure the capacitors are not shorted, sometimes they show ok when measuring wih the DMM but conducts under higher voltages, I'd test them under 12V.

[2X]

I am not sure that the 1N4007 is the original component. Because the 1N4007 isn't ultra fast you believe is a reason for such overheating?

[DavidAlfa]

Absolutely, you will never see a 1n4007 in the high-speed section of switching power supplies!

Every diode has a turn-off time, when the input voltage goes negative the current will flow backwards for a small time.
For the 1N4007 it's about 1.5us, this is not a problem at 100-120Hz, but at 100KHz it will:

100K *1.5us = 150ms!

Power diodes meant for high speed power switching are much faster, for example the FR302 has 150ns switching time.

Try a 1n4148, or anything fast.

[fant]

Try with UF4007, superfast and with all the other characteristics similar

[2X]

Finally the problem was the speed of the diode as you all said. I placed the UF4007 and works fine. Thanks all of you for the replies and help.