# 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.

#### dl324

Joined Mar 30, 2015
12,792
but isn't it too high for the gate?
All that matters is the maximum voltage differential between gate and source. The absolute voltage isn't relevant for Vgs limits.

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
So what is the purpose of D15, D16, C40, and C41?
It depends somewhat on what the voltage is on the source of Q1. From the circuit fragment given, it D15/16 limit the maximum voltage across L4. There must be some high frequency component on HT+ that the designer wanted to filter.

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?
What about this?

#### dl324

Joined Mar 30, 2015
12,792
What about this?
Those calculations assume that the zener anode is at 0V. We can't see that part of the schematic.

Thread Starter

#### Xavier Pacheco Paulino

Joined Oct 21, 2015
728
See attached the complete schematic.

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#### 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.

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.

Last edited:

#### crutschow

Joined Mar 14, 2008
27,000
Aren't these currents too low for the zener?
You are only concerned about the minimum Zener current when it is used as a regulator to generate a specific voltage.
Here it's just to limit the Vgs voltage, so its current can be anything from zero the its maximum rating.

Thread Starter

#### Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Ok. I just wanted to understand the math behind the selection of the six 470k resistors.