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need help regarding MOSFET.

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Full Member level 3
Mar 18, 2002
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Im using BSP88. Vds = 2.4,Vgs=2.4.When there is low to high transition at the GATE, im getting sudden shoot of 1.5 V at source side. It'll be there for 10uS then it settles down. Why its behaving like this? And whats the solution?

O Vcc = 2.4
> 470K
--|<- BSP88
--- |
> 6.8K


Can you clarify a bit. It is not very obvious from your drawing how the gate is connected. Is it driven separately or it is connected to the drain - you say both drain and gate are at 2.4v? Do you also mean that Vcc can simply have a value somewhere in the range you specified or it is also changing - like a signal?

gate-source capacitance

This looks like the gate-soruce capacitance is putting part of the gate drive on the source. Try slowing down the gate drive rise and fall times to see if it goes away.

Actually the input will be at 2.4 V when Gate will be enabled.

It is the current that flows into the gate to charge the G-S capacitor. The current flows into the G, than capacitivly coupled to S, through your S resistor and back. You can calculate required Q from GS capacity of your MOSFET and voltage (2,4V in your case).

can you draw it with any schematic program ? & please tell us if these values are from simulation or from your testbench

I agree with the others that this must be caused by the g-s capacitance. Try lowering your resistance values or add a resistor in series with the gate to minimize this phenomenon. I don't know the rest of your schematic but maybe you are better off with a bipolar transistor here.

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