[SOLVED] A wierd problem with MOSFET

[kappa_am]

greetings,
To evaluate a MOSFET driver IC, I have implemented shown circuit. The upper switch work well. it means when it switch ON, upper lamp gets ON and the other lamp get OFF.
but when down switch is fired, the down lamp does not get OFF, however the upper lamp get ON, even when I provide continuous high gate signal
the gate resistor is 12 ohm.
when I increase the gate resistor to 120 ohm, the problem get solve in continuous fired state.
I think it's because of internal capacitor of MOSFET. I would be grateful if somebody explain this to me.

Thank you

 

[chuckey]

If the gate of the upper FET is held at a fixed DC, dropping its drain more negative when the lower FET comes switches on the top transistor.
Frank

 

[Orson Cart]

Yes, you need to take the gate of the top fet to gnd (0v) to ensure it is off when the lower fet is turned on, as, when the lower fet is on it pulls the source of the top fet to gnd, if you leave the top fet gate higher than Vgs threshold (relative to gnd) then you will turn the top fet on when you turn the bottom fet on.

 

[kappa_am]

Thank you for your replies
the switches always are in complementary states. Furthermore I use separate DC sources to fire switches.

 

[Orson Cart]

Do you accicentally connect the 0V of the separate Dc sources any where? and quick measure with a DVM will confirm or otherwise.

 

[kappa_am]

No, I am sure that I have not connect the ground of separate DC sources. the circuit is very simple. I have checked the circuit and found the error. the used optocoupler has different characteristics than what is indicated in datasheet!!!! there was not enough current to feed diriver IC. I have change operating point of optocoupler, and ok, it works.

Thanks All