I'm trying to switch the latching solenoid on and off with an H-bridge circuit (see figure).

H-bridge circuit

Components I use in this circuit: P-MOS , N-MOS , NPN

I've run this circuit a few times with no problems, I turn the solenoid on and off. However, after turning it on and off a few times, one of the P-MOS gets hot and breaks down. Since MOSFETs have body diodes in themselves, I didn't connect the diode again to the circuit.

Should I add 4 diodes to protect MOSFETs from inductive load? This is the only solution I can think of. If you have another suggestion, I'd like to explore it.

  • When I supply the solenoid with 12V from the power supply, it consumes a maximum of 2.6A current.

1 Answer 1


If you have another suggestion, I'd like to explore it.

  1. Make sure you have a bulk capacitor connected close to the H-bridge power terminals to catch the reverse energy when the solenoid is switched. It could be that your 12 volt power supply is incapable of soaking up this energy return and it's rising significantly above 12 volts and damaging the PMOS (PJA3405_R1_00001) devices.
  2. The PMOS is also a little light-weight on reverse diode current; the diode is only rated at 1.5 amps continuous current and this is less than half the rating for the MOSFET's forward conduction so, maybe add extra diodes or choose a better MOSFET.
  3. The maximum power dissipation for the MOSFET is 1.25 watts AND the data sheet has no SOA (safe operating area) curve. This concerns me because, when the reverse diode conducts (2.6 amps) it will have a forward volt drop of about 0.85 volts (see fig 6 in data sheet) and this is a peak power of 2.21 watts i.e. greater than the absolute max power rating.

The NMOS device appears to have a better rating in this critical areas: -

  • Continuous current of 5.6 amps
  • 1
    \$\begingroup\$ I'd like to add a few things about the max dissipation of the PMOS: Because of it's on-state dissipation it'll be hot (about 0.4W brings a delta-T of 40°C), so the max power dissipation should be de-rated (e.g. less than 1 Watt). Not to mention the increase of on-resistance under these conditions. \$\endgroup\$ Nov 24, 2022 at 11:01
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    \$\begingroup\$ @harmonica make sure that the connection between 4700 uF and H-bridge is short and, supplement the decoupling (close to the bridge) with ceramic capacitors of 1 uF and 10 nF to ensure minimal inductive reactance. \$\endgroup\$
    – Andy aka
    Nov 24, 2022 at 12:38
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    \$\begingroup\$ @Andyaka, After adding extra diodes, all MOSFETs (P-MOS and N-MOS) broken... I guess I must select new MOSFETs \$\endgroup\$
    – harmonica
    Nov 24, 2022 at 13:30
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    \$\begingroup\$ Yes, it sounds like you are just pushing them too hard. Of course, if you had more copper on the drain and source connections you would remove more heat. Try and pick MOSFETs that have a safe operating area curve in their data sheet too. \$\endgroup\$
    – Andy aka
    Nov 24, 2022 at 13:34
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    \$\begingroup\$ It can be tricky to get right. It's probably OK for the current if you have sufficient copper around the device on the PCB (see note 5 on page 2 of data sheet). However, if the device is only being switched intermittently you may get away with less copper. This size of package is looking borderline for your application I reckon. \$\endgroup\$
    – Andy aka
    Nov 24, 2022 at 14:49

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