Push/Pull N-Channel MOSFET

[Flunze]

Let's pretend I got a Voltage of 50V, GND, -50V and want to put it into something with PWM, using Mosfets. The problem is, that the Mosfet has a capacity and will not instantly turn off, if there is no more voltage supplied. If I would put a negative Voltage to the Gate/Source, would the MOSFET turn and stay of, or would a negative Voltage applied also turn the MOSFET on? I will use a trigger Voltage of about 10-12 Volts. Not sure exactly.

[AndyC_772]

A negative Vgs will turn an N-channel MOSFET off, and will turn a P-channel MOSFET on.

If you've any doubts about how these devices work, download yourself a copy of LTSpice (here) and try simulating what you have in mind first.

[Psi]

Be sure you don't exceed the max gate-source voltage on the datasheet, it will be given in the format 15V+/-  or  20V+/-   etc.

[M. András]

its the little note on a few hundred volts fet datasheet to put a 1 megohm resistor between the gate and the source  to ensure its not exceeded right?

[PSR B1257]

to put a 1 megohm resistor between the gate and the source  to ensure its not exceeded right?

Where will you have read this?

That's wrong. Just a resistor from gate to source will not limit the GS-volgate of course. But you can use such a resistor to make sure, the MOSFET is turned off when the gate is floating.

To limit the GS-voltage, you have to put a zener diode parallel to gate and source. Kathode to gate and anode to source, in case of an N-channel. And you have to make sure also, that the value of the gate resistor is high enough, to limit the current trough the zener diode to less then its maximum.

[M. András]

to put a 1 megohm resistor between the gate and the source  to ensure its not exceeded right?

Where will you have read this?

That's wrong. Just a resistor from gate to source will not limit the GS-volgate of course. But you can use such a resistor to make sure, the MOSFET is turned off when the gate is floating.

To limit the GS-voltage, you have to put a zener diode parallel to gate and source. Kathode to gate and anode to source, in case of an N-channel. And you have to make sure also, that the value of the gate resistor is high enough, to limit the current trough the zener diode to less then its maximum.

VDGR TJ = 25°C to 150°C, RGS = 1MOhm 100 V its from an ixys fet datasheet. if i understood it incorrectly then thank you for the clarification

[PSR B1257]

Could you paste a link to that specific MOSFET?
I'm curious to see it.

[Psi]

VDGR TJ = 25°C to 150°C, RGS = 1MOhm 100 V its from an ixys fet datasheet. if i understood it incorrectly then thank you for the clarification

1MOhm? i assume you meant to type mOhm but even then 1mOhm is a crazy high spec'ed fet.
The only thing i can see on digikey that even comes close is 1.35mOhms / 100V and it costs NZ$250

Could it be 10mR? that would be much more normal.

[DaveW]

VDGR TJ = 25°C to 150°C, RGS = 1MOhm 100 V its from an ixys fet datasheet. if i understood it incorrectly then thank you for the clarification

1MOhm? i assume you meant to type mOhm but even then 1mOhm is a crazy high spec'ed fet.
The only thing i can see on digikey that even comes close is 1.35mOhms / 100V and it costs NZ$250

Could it be 10mR? that would be much more normal.

That's gate to source resistance, for which 1 meg would be typical. Drain to source would hopefully be in the milliohms range....

[Psi]

Oh true, i totally missed that.

[M. András]

http://ixapps.ixys.com/DataSheet/DS100238%28IXTN200N10L2%29.pdf
sure here is it

[PSR B1257]

Found it. But there is nowhere the information, that these resistor will limit the gate-source voltage.
The resistor is part of the test circuit to specify the drain-gate avalanche voltage hence it is indexed with "DGR": Drain Gate Resistive

[M. András]

what is the usage on this fet/casestyle that second source terminal? it says in the datasheet kelvin source or gate return terminal as usage. but whats the difference from a 3 pin/terminal casestyle?

[PSR B1257]

For this particular MOSFET there is no real need for the second source terminal. They would have chose it because of the high current (and therefor the high powerdissipation) this MOSFET is capable to drive. This case has a very low thermal resistance which causes low heating during operation.

You should also read this http://www.nxp.com/documents/application_note/AN10322.pdf for more information about MOSFETs with current sense terminals.