High frequency spikes and output diodes getting hot.

I have designed and built a flyback SMPS circuit for a high temperature motor controller board with the following spec:

40v - 75v input
5v @ 300mA
10v @ 10mA
5v @ 50mA (with isolated gnd)

I'm using a LM5020MM-2 controller, Wurth Elektronik power over ethernet tranformer part number 749119133, FCD4N60 MOSFET for the power switch. The circuit is attached.

The circuit powers up ok, with stable 5.0v regulated, 10.4v, 5.2v outputs with a 10mA loading. This works fine without any heating of components but when I increase the load to 150mA on the regulated 5v, both diodes on the outputs for the 5v and 10v get hot.
I have probed the outputs on a scope and found that there is a clear spike that corresponds to the switching from the power transistor. I think the spikes are what is causing the heating in the diodes, but don't have a solution.

Any ideas?

Thanks.

It's because you're using 1N4006 and 1N4001 diodes. They are nowhere NEAR fast enough. Replace them with ultrafast or schottky diodes and they will cool off.

Thanks, that solves it!
But I'm still getting the switching spikes on the output, although it doesn't seem to interfere with the DSP and motor controller IC. Is there a way to get rid of the spikes? Some kind of low pass filter?

THat's inherent with SMPS power supplies. Just better filtering, etc.

Those huge spikes are probably more a scope measurement issue and in reality are not close to that large in amplitude. Measuring fast transition signals requires an exceptionally good scope probe ground. Just the 6" length from the alligator clip to probe body can cause exaggerated waveforms. With fast transition signals, ground planes become important for return currents.

Make sure the scope probe ground wire is making prefect contact with the alligator clip (soldered to it). And the connection to the probe body is clean.

Try grounding the probe to a different ground point.

If you have a spare ground wire/clip, shorten it as much as possible.

Tektronix also makes electrical ground leads that connect right next to the probe tip so you don't use the 6" grounding wire/clip. Here's an example.
Tektronix 214 4125 00 > Ground Lead 214 4125 00 Tektronix Test Equipment

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