MOSFET gate voltage in this circuit

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
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 12V zener? Is it okay to say that, for example, if DC bus voltage is 1000V, then the zener current would be (1000-12)/(470k*6) = ~350 uA? If the FET is off, the gate voltage is clamped by D15 and D16 to ~ 800V, but isn't it too high for the gate?
 

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danadak

Joined Mar 10, 2018
4,057
The Vgs is clamped to 12 V by the zener.

With respect to ground cannot tell because schematic cuts off where
the Source of the MOSFET is connected to.


Regards, Dana.
 

Thread Starter

Xavier Pacheco Paulino

Joined Oct 21, 2015
728
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

Aren't these currents too low for the zener?

All that matters is the maximum voltage differential between gate and source. The absolute voltage isn't relevant for Vgs limits.
So what is the purpose of D15, D16, C40, and C41?
 

dl324

Joined Mar 30, 2015
12,792
See attached the complete schematic.
A zener diode doesn't regulate well below Iz, but it would still limit Vgs.
upload_2019-9-6_10-30-30.png
upload_2019-9-6_10-32-23.png

At a current of 1uA, a 12V zener would have about 9V across it.

Izk is the minimum operating current to assure reasonable regulation; 250uA for a 12V zener.

I included other zener values to illustrate how the Ir current changes. It's 100uA for a 2.4V zener.
 
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