M3 has |Vds| > 300V |Vgs| = 10V
Protecting the load from V1 = -300V 100ms spike:
Does this diagram, with carefully picked components, look like it will do the job ? Hw do I select R1, D2 and D3 ?
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3 Answers
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Answer edited since I mis-read the original schematic (Thought it was an NFET).
It may work, but presumably when the FET is on and Vin is 12V the source is also about 12V and the gate is zero. If the max Vgs is 10V you're exceeding that spec. You could put some resistance in series with the gate and use your diodes to clamp the max Vgs to <10V.
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1\$\begingroup\$ Also, the source of the MOSFET will be at about -299V while the gate is at GND, so that will greatly exceed the Vgs maximum and the device will be destroyed. Can you just replace the MOSFET with a diode? \$\endgroup\$ Aug 2, 2022 at 22:06
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\$\begingroup\$ >>When V1 is +12V, the FET will be off, Care to explain why ? \$\endgroup\$– kellogsAug 2, 2022 at 23:27
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\$\begingroup\$ @kellogs, sorry, my glasses must have been dirty, I could have sworn you showed an N-channel FET when I first read your question. I'll update the answer. \$\endgroup\$– John DAug 3, 2022 at 1:20
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\$\begingroup\$ It is connected in an odd way, I must admit. Sorry, Vgs = 10V is not the max but the typical datasheet value. M3 would be of standard variety where Vgs_max is often 20V. \$\endgroup\$– kellogsAug 3, 2022 at 9:37
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I had also misread the MOSFET as NMOS. For the PMOS device, it looks like the circuit should work just fine:
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\$\begingroup\$ Thanks. These aren't really 6v2 Zeners, are they ? i would expect V(load) around 7 ~ 7.5V with such Zeners \$\endgroup\$– kellogsAug 3, 2022 at 9:29
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\$\begingroup\$ They are 6.2 volt zeners, with about 230 mA when supply is 12V, so about 1.5 watts. A 47 ohm resistor works as well, with 100 mA and 620 mW. It also works if you remove the zeners and R2. The gate voltage is then about 12 volts. \$\endgroup\$ Aug 3, 2022 at 20:38
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You need to move R2 to the vertical net joining the gate to GND. That will allow D2 to protect the gate < 10 V. You don't need D3.
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\$\begingroup\$ Would that be enough to also protect D2 from -300V spike ? I can't find any max Vf or Vz for these Zeners... how do I pick them ? \$\endgroup\$– kellogsAug 3, 2022 at 9:34
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\$\begingroup\$ The -300V won't (shouldn't) get farther than the drain of the MOSFET. Use a 4V7 - 10 V zener for D2; remove D1, and make R1 ~ 1 kΩ. If the -300 V spike has very fast edges (ns, us range), then the drain-gate capacitance of the FET will couple to the 1k and actually allow the FET to temporarily turn on. You may need to make R1 lower than 1k in that case. \$\endgroup\$– jp314Aug 3, 2022 at 15:38