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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Circuits like these are often designed by trial and error. If the MOSFETs run coolest with no snubber, then use no snubber. If the gate voltage spikes worry you, put 15V zeners on the gates.
Putting snubbers on a bridge-type inverter is getting into quasi-resonant (aka Class-DE) territory. You need to ensure an inductive load, and introduce deadtime to give the voltage time to swing over. If you don't do that, all the energy in the snubber caps (0.5 * C * Vbus^2) gets dumped into the MOSFETs at every switching instant, which is why they get hotter than they did previously.
For more details read the Class-DE paper if you haven't already.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Experiment's comment about them being zeners was not right, but got me thinking abou the schottky's. Normally, the FREDs will conduct when the reverse voltage starts to rise. The schottkys would never see more than a few volts of reverse voltage. However, it did appear he was right that the FREDs were doing nothing so I started to think.
I took the inverter apart and tested each chip. The schottky 30v/70A barrier diodes were blown. I am thinking that when the switch opended the snubber capacitor was charged to the full voltage (700v), which exceeded the 30v limit on the schottky, eventually destroying it. This must have been the source of the heat. If I do plan to go ahead with the snubber, I will have to use schottkys with the proper voltage rating.
Registered Member #941
Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497
We all learn from our mistakes. In my case the heat is most likely a high RDSon of the devices themselves combined with 60+ Amps flowing through them, possibly 4 fold overcurrent, considering the fets/igbts in smps/vfd below 5kW are typically 600/1200V 16A rated.
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mosfets getting too hot
