Mosfet swtich

I have a mosfet (irf3205), i want to switch [40 volt] positive supply using it. I can able to switch [40 volt] negative supply. How can able to switch positive supply using n channel mosfet. I have attached a circuit , but its not working. Can anyone guide me, advance thanks.

Hi,
try googling high side switching N-CH MOSFET
or
googling high side switching driver N-CH MOSFET

You may need a high side driver IC to do the job.

Tom... :slight_smile:

Thanks tom. But I want to switch using transistor, I have a doubt. If want to switch (40 to 50 volt) high side with n channel , what would be the gate voltage of the MOSFET. Is 20 volt in gate is enough to switch.

Nihaludeen:
Thanks tom. But I want to switch using transistor, I have a doubt. If want to switch (40 to 50 volt) high side with n channel , what would be the gate voltage of the MOSFET. Is 20 volt in gate is enough to switch.

That is what a High Side Driver is going to do, you don't need the full 20V gate, that is an Absolute Maximum rating.
A P-ch MOSFET would make things easier.
If you want to use a BJT then you need to use a PNP transistor.
Tom... :slight_smile:

Is it possible with the combination ( pnp transistor and a n channel mosfet ), can able to switch high volt (say 50v).

Nihaludeen:
Is it possible with the combination ( pnp transistor and a n channel mosfet ), can able to switch high volt (say 50v).

You need +60volt on the gate of this n-channel mosfet to switch +50volt high-side.
A voltage difference of +10volt between gate and source.
Because you have drawn an n-channel source follower.
N-channel fets can only be used high-side with a suitable mosfet driver chip.

Is it related to to this thread.
Leo..

Leo, i thought that gate voltage of n mosfet should not exceed 24 volt. But as per your suggestion if i give a 50 volt for a gate to switch 40 volt , the mosfet can handle such large volt on gate??

Voltage between gate and source must not exceed +/- 20volt.
Gate must be 10volt higher than source for this n-channel mosfet to fully conduct.
And equal or less than the source to stop conducting.
Drain must be higher than source, otherwise the body diode will conduct.
With these motor currents, the gate should be switched with several amps, to keep switching losses (heat) at bay.
So a special mosfet driver chip.
Shoot-through in a h-bridge is a problem too (instant death).
So a special mosfet driver chip.

I doubt you can build a h-bridge for a 360watt motor without solid knowledge of semiconductors.
Read a meter of books first before trying that.
In the mean time, buy a suitable h-bridge from ebay to drive this motor.
Leo..

Thanks leo

In fast switching using irf3205 (on and off delay 14 and 50 ns as per datasheet ), i have tried to switch 20 volt
with a delay of 10 ms. But my output shows 10-12 volt. How can i get exact supply volt with 10 ms.
Sorry if i asked basic question.

Nihaludeen:
In fast switching using irf3205 (on and off delay 14 and 50 ns as per datasheet ), i have tried to switch 20 volt
with a delay of 10 ms. But my output shows 10-12 volt. How can i get exact supply volt with 10 ms.
Sorry if i asked basic question.

I don't understand anything of that story without a circuit diagram.
Leo..

Hi,
What are you trying to high side switch, and why do you need 10mS switch time?

As @Wawa asks, can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Thanks.. Tom... :slight_smile:

Thanks guys for your replies, i am learning about to switch most faster (with every possibility).
I attached my circuit for high side switch. How to switch the mosfet faster.
Mosfet= irf3205
transistor npn and pnp = mje3055/2955

Hi,
Ops pic.

Tom.. :slight_smile:

Hi,
The problem with your circuit is that for the MOSFET to switch ON, you must have a positive gate to source voltage.

When you apply signal to your circuit, your MOSFET will turn ON
But as the voltage across your load approaches Vcc, the gate to source voltage will drop and the MOSFET will not fully conduct.
Hence a balance occurs and you get 12V.

What is your application, what is the load you are switching?

Tom.. :slight_smile:

I am learning completely about mosfet switching, in order to make a h bridge circuit.
Can you guide me, how could i fully conduct my mosfet in my circuit.

Hi,
First what is your Load, what are you switching Vcc to?
This may help with a suitable circuit.

Thanks.. Tom.. :slight_smile:

bldc motor (360watts)

What Tom said.

You are drawing a source follower.
Source is always Vgs(th) lower than the gate.
For the source to reach the same level as the drain (VCC), you must have at least VCC+Vgs(th) at the gate.
More (10volt) if you want to draw significant current from the source.

If the "switch" is not fully "closed", then voltage across and current through the mosfet will heat it up quickly.

To use an n-channel fet like this you must use a chip with a bootstrap circuit (to kick gate drive voltage above VCC).
Leo..

Thankyou leo…