How to protect a P-channel MOSFET when driving a motor?

Question

I want to switch a 12 V DC automotive fuel pump in a bench test rig using a P-channel MOSFET, as follows:

^{simulate this circuit – Schematic created using CircuitLab}

This is a simple on/off switch, no PWM is used. The datasheet for the FQPF47P06 gives its maximum Id as 30 A continuous, 120 A pulsed. Vdss max is -60 V and Vgss is +/-25 V. The data for the pump shows a maximum current draw of 20 A, but the most we observe in the application is about 5 A.

In a previous version of this circuit I didn't include D2 and underspecified the fuse rating; the fuse failed and the MOSFET was destroyed (became short circuit source to drain). I've guessed that this was because the motor inductance created a large negative voltage spike from drain to ground on disconnection, so I've added D2 to handle this situation.

• Does this look like a suitable setup to switch this load reliably, or have I overlooked anything?
• What diodes are suitable for D1/2 - is a 1N4007 OK or should I look for something faster and/or with a higher current rating?

Answer 1

Your basic concept makes sense, but you missed the fact that the "12 V" line of a car will sometimes have significant voltage spikes on it. Any cicuit connected directly to this power needs to be able to withstand 50 at least for short periods.

When the pump is on, even a short spike will apply high voltage to the FET gate, which will blow out the oxide instantly. Overvoltaging the FET S-D isn't good either.

Use a 60 V FET, and do something to clamp the gate voltage to a safe level.

Added:

I forgot to mention this earlier since the main issue was about nasty voltage spikes on the vehicle power line. No, 1N4007 is a bad choice for the diodes. In this case, I'd use Schottkys rated for 20 or 30 V. Those are cheap and readily available. Since the current will only run in them for a short time as the stored inductive energy is dissipated, you can use the peak current rating instead of the continuous current rating of the diodes. The diodes need to be able to handle peak current of whatever the motor current is.

Answer 2

A decent-sized load dump can produce fast spikes of ±200V or more on a vehicle power bus. You need to protect your MOSFET against such events.

I would start by putting a 15V zener diode across R1, and adding a few hundred ohms of resistance between Q1 and M1 to limit the current when it conducts.

I would also consider some filtering for such pulses by connecting some inductance in series with F1 and a ceramic capacitor from the MOSFET source to ground. This will block most of the high-frequency energy and "spread out" whatever does get through.

A capacitor across the motor (i.e., in parallel with D2) wouldn't hurt, either.