Hi,

long time reader first time poster.

I'm working on a project that turns off/on a couple of bulbs in a automobile. It does that through a AVR-cpu and a nmos/pmos (see attached picture). As you all know the enviroment in a automobile is quite hostile so overvoltage protections is needed. For the AVR I have chosen a automobile graded voltage regulator together with a series resistor. But for the transistors I'm a bit unsure on how to do it. The nmos is a FDV301N capable of 25V drain source voltage and the pmos is a fdd4685 capable of 40V drain-source and 20V gate-source. I would like to protect those against voltage-spikes up to 40V at least. Is a varistor in parallell a good idea? Should it have a series-resistor? The circuit will of course also be protected through a fuse.

 

Hi there,
Well, seems like you're sort of in the soup with the 25v rating on the N-ch MOSFET. If you get more than 25v in, the body diode will break down and turn on the P-ch MOSFET. If you used, say, a 20v Zener diode across the N-ch MOSFET to protect it, you'd be in the soup with your P-ch MOSFET and the Vgs; a 40v spike would give you a Vgs of 20v; right at the maximum.

However, you could use an LC filter to knock down the spikes. Radio Shack sells (or used to) an RF choke, catalog# 273-102; the couple I have are wound with AWG 24 magnet wire. I wouldn't want to run more than a couple of amps through them, but we don't know how many amps your project needs.

A couple of 220uF or so 50v electrolytic caps on either side of the choke to ground will take care of most of the spikes.

If your load needs more than a couple of amperes, you'll need a more capable inductor.

 

Hmm Yes I realize that the max rating on the NMOS was chosen to low. The circuit will be unnecessary complex due to that ( the circuit will use around 5A). I think I will go for another NMOS and only protect the gate on the PMOS with a zener. Thanks

 

I don’t believe protecting underrated parts is a sound way to things.
For a start you might want to ditch those MOSFETS and get some higher voltage ones >100V preferably internally protected for the harsh electronic environment of a vehicle!

 

That is of course the best way but there is a cost issue also. Besides, I haven't seen any PMOS-transistors rated for >100V on the gate (to source). Can you give a suggestion?

 

If you go with MOSFETs rated for over Vdss=40v, you should be OK.

Use a MOSFET rated for at least twice your expected average current. Don't forget that when incandescent lamps are first turned on, they have a much heavier initial current.

If you're planning on using PWM for intensity control, you'll need a more capable way of driving your MOSFET gate than you're showing.