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I'm designing a MOSFET (FQP30N06L) circuit to drive a (50V, 3.4ohm) pinball solenoid controlled by a PWM pin on an Arduino DUE:

schematic

simulate this circuit – Schematic created using CircuitLab

The MOSFET gate and solenoid are tied to the terminal block to make external alterations easier. My concern is that, while altering the circuit I will forget to ground myself, and fry the MOSFET. My understanding is that when MOSFETs fail for this reason, they often become shorts which would fry the solenoid coil very quickly.

My question is, what can I do in-circuit between the terminal block and the MOSFET to protect the MOSFET from ESD?

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  • \$\begingroup\$ Why would it be exposed to ESD? \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 9 '14 at 20:39
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    \$\begingroup\$ A series resistor and appropriately spec'd TVS would likely be sufficient there. \$\endgroup\$ – Matt Young Dec 9 '14 at 20:44
  • \$\begingroup\$ Like Ignacio said, how do you know it needs ESD protection? If it does, how fast are you PWM'ing? The resistor will form an RC with gate capacitance and TVS capacitance. This will limit the rise and fall times at the gate, which has the potential will increase dissipation in the MOSFET. Whether it increases it significantly or not is another question, and the answer depends on the details. \$\endgroup\$ – mkeith Dec 9 '14 at 21:43
  • \$\begingroup\$ A fast switching diode is probably a useful improvement over the 1N4007. \$\endgroup\$ – Brian Drummond Dec 9 '14 at 22:38
  • \$\begingroup\$ @mkeith: I don't know the PWM frequency of the Arduino DUE, but the UNO and Mega use <1kHz (arduino.cc/en/Reference/AnalogWrite) so my guess is that it would be similar for the DUE. \$\endgroup\$ – user2557313 Dec 10 '14 at 14:43
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If the gate is truly connected to a terminal block, it is vulnerable and needs to be protected. A series resistor and TVS diode would be my first stop at protecting it.

schematic

simulate this circuit – Schematic created using CircuitLab

A cap in parallel with the TVS will help slow the ESD event down, if you can afford the added capacitance on the gate, and give the TVS a little more time to turn on. Make sure the traces are short as possible and the TVS has a solid ground connection. A regular Zener diode is just too slow for ESD protection. It's better than nothing, but at that point, your primary means of protection will be the MOSFET's internal clamp, if it has one.

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  • \$\begingroup\$ I can get hold of P6KE6.8s. Would they work in place of the SMF5.0A? And what size capacitor? Thanks! \$\endgroup\$ – user2557313 Dec 14 '14 at 2:29
  • \$\begingroup\$ That TVS looks like it will work. You need to think about it carefully if you went to put a cap in parallel with the TVS. Make sure the RC combination it forms with the seris resistor doesn't slow down your PWM rise times. \$\endgroup\$ – Matt Young Dec 14 '14 at 3:57
  • \$\begingroup\$ Also consider a pull-down resistor from G-S so that if the input becomes disconnected at the terminal block it is pulled off. The diode may have enough leakage to do that, but the resistor will make sure. Something like 20K is okay. \$\endgroup\$ – Spehro Pefhany Mar 10 '15 at 15:33
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    \$\begingroup\$ @SpehroPefhany I was working on the assumption it would already be there. Added for clarity. \$\endgroup\$ – Matt Young Mar 10 '15 at 15:39

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