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

schematic

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?
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I don't think D1 is necessary. When M1 shuts off, the current through D2 will balance the current entering the coil preventing a negative spike. There should be no positive spike even if you ignore the body diode in M1. –  Tut Sep 4 at 15:42

2 Answers 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.

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Thanks - I didn't think to mention that this setup is on the bench, not in a vehicle. Is the capacitor suggestion still worthwhile and if so what sort of value and type would you suggest? –  nekomatic Sep 4 at 13:29

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.

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Thanks - I omitted to mention that this is on the bench, not in a vehicle. I've edited my question. –  nekomatic Sep 4 at 13:27
    
@neko: If this is only ever to be used powered from a lab supply or straight 12 V lead-acid battery where there will be no large spikes on the power, then your circuit is fine. Don't ever power it from car 12 V when the car is running, though. –  Olin Lathrop Sep 4 at 13:55

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