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

 

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.

 

but there is 500ohms resistor on the gate during the on time

 

but that is reistance in parallel, not in series with the gate.

 

Do you have power supply decoupling caps on +30V?
Do you have a load on the drains that is not shown?

 

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?

 

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?

 

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?

 

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?

Can you show us a picture of your hardware, i.e. breadboard or PC board?

 

hello all
sorry for late respond (exams )
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