mosfet gate oscillation problem

Thread Starter

mobydick

Joined Nov 19, 2010
27
hello
i am trying to build half bridg using p and n mosfets
when p type mosfet turn on oscillation appeare at the gate
and when i disconnect drains of the p and n mosfet it disappears
any help pleas
 

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Audioguru

Joined Dec 20, 2007
11,248
The gate-source of a Mosfet is a capacitor. The wire feeding the gate is a series inductor. The capacitance and inductance oscillate or ring.
That is why most Mosfets have a low value resistor (10 ohms to 100 ohms) at the gate pin in series with it to kill the inductance.
 

Thread Starter

mobydick

Joined Nov 19, 2010
27
but that is reistance in parallel, not in series with the gate.
during off time resistor is in parallel , but i have no problem at off time of the p mosfet
at on time of the p mosfet the resistor is in series (right !!)

Do you have power supply decoupling caps on +30V?
no i didnot
after adding 470 n oscillatin decreased (new screen shot attached ,note NG ch1 is 1v/div while PG ch2 is 5 v/div)
Do you have a load on the drains that is not shown?
no i donot . will that make diffrance?
 

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Ron H

Joined Apr 14, 2005
7,063
during off time resistor is in parallel , but i have no problem at off time of the p mosfet
at on time of the p mosfet the resistor is in series (right !!)


no i didnot
after adding 470 n oscillatin decreased (new screen shot attached ,note NG ch1 is 1v/div while PG ch2 is 5 v/div)

no i donot . will that make diffrance?
It will probably make a difference. Why don't you try it?
 

Thread Starter

mobydick

Joined Nov 19, 2010
27
when i put 50 ohms from drain to GND no change
and when i put 50 ohms from drain to +30 it changes( picture attached)
what is the reason for that oscillation?
 

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Thread Starter

mobydick

Joined Nov 19, 2010
27
hello all
sorry for late respond (exams :mad:)
any way , i did some simulations and put random inductors every where . i found the exact same noise happens when i put an inductor between source of the mosfet and ground . bad nois also from source to vcc inductor .
so i redesigned the circut with huge vcc and ground tracks and problem disappeared :)
thanks
 
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