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Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
furnace wrote ...
My probe is directly on the gate and the ground of my probe is connected to earth ground through a 1k resistor.
I'm not sure if I'm missing something (the circuit diagram you posted was a linear DC supply), but you cannot scope the high side FET with the probe ground clip connected (well, not without extra effort). The ground clip is connected to mains earth and with rectified 230V on the bus the high side gate will be bouncing back and forth between the 325VDC rails (slightly more with negative drive from the GDT). Most scopes don't appreciate more than 40V or so, unless you're using 10x probes. Grounding either of the DC rails will instantly trip your breaker/RCD due to the massive fault current. Sorry if this part is already obvious, or has already been said.
Using an isolating step-down transformer to power the DC bus at a low voltage can help with troubleshooting, as there is no risk of shorting out mains power through your scope.
Registered Member #4992
Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108
Thanks Grantx! I'm only scoping low side mosfet. I just fixed the problem by removing the earth ground and connected the variac only on live and neutral. Now the scope signal is clear. Another question! Can anyone tell me how to choose the right mosfet or Igbt according to their volt and current raiting, because I keep on blowing irfp460 when I reach 400v at 10 amps. I'd like to use mains full power which is 230v.
Registered Member #4992
Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108
Can someone please tel me why my low side mosfet is getting hotter than my high side mosfet on this signal. Are they even suppose to get hot because I figured that the coupling transformer should anticipate all the current and voltage?
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Voltage rise on gate
