Author Topic: MOSFET gate voltage in this circuit  (Read 789 times)

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Offline XaviPachecoTopic starter

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MOSFET gate voltage in this circuit
« on: September 05, 2019, 07:25:41 pm »
The circuit attached below is from Texas Instruments: http://www.ti.com/tool/TIDA-00173

My question is: what is the voltage at the gate of the MOSFET Q1? It is defined by the string of resistors R45, R42, R39, R36, R34 and R32, and the TVS diodes D16 and D15. But how do I calculate the exact value? I know the 12V zener across gate-source is for overvoltage protection.
 

Offline T3sl4co1l

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Re: MOSFET gate voltage in this circuit
« Reply #1 on: September 05, 2019, 07:46:06 pm »
It's pulled down through D31 to ~12V (plus Vds of the first transistor) when turning on, and clamped at ~800V by D15-16 when off.

Tim
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Offline XaviPachecoTopic starter

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Re: MOSFET gate voltage in this circuit
« Reply #2 on: September 05, 2019, 08:53:38 pm »
The DC bus voltage ranges from 400V to 1200V, this means:

For 400V, Iz = (400-12)/(470k*6) = 137.58 uA
For 1200V, Iz = (1200-12)/(470k*6) = 421.27 uA

Iz = Zener current

Aren't these currents too low for the zener?
 


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