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Registered Member #4992
Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108
Hi everyone! Can someone please try and explain why I get a voltage rise on my gate, this only happens to the first mosfet that is switched on, on a normal half bridge circuit with 2 dc blocking caps for induction heating. The circuit runs very smooth at 200volts ac through a variac which is rectified. Its only that the high side gets slightly warmer than the low side.
This is the signal on both mosfet with te variac on. If the variac is switched off than both signals rise and fall times are even, from the gdt of my pll driver.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Where are you connecting the oscilloscope probes? With a grounded oscilloscope you can directly measure the gate voltage of the lower transistor. To measure the gate-source voltage of the upper transistor you have to subtract the signals at the gate and at the source. (I don't recomment lifting the oscilloscope from the ground and measuring the gate-source voltage directly.)
Registered Member #4992
Joined: Wed May 23 2012, 03:57PM
Location:
Posts: 108
The scope is grounded through a cap and the probes are directly on the gate of the mosfets. But anyway this is not to be considered as this is not the source of my problem. The problem is that the voltage of my high side mosfet is rising on the gate or it seems to build up a charge.
The gate could have been blown open circuit, in which case the fet will never turn on. It's rare, as they usually fail closed circuit, but not impossible.
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Voltage rise on gate
