Due to thermal considerations I am trying to replace D2 in this answer with a P-MOS FET. Came up with this diagram:

enter image description here

The two D1 symbols are just one bidirectional TVS diode (missing symbol in circuitlab) with Vcl_max = 24.4V and M1 has Vgs_th = -4V Vgs_max = -20V. Vds_max - should I worry about it ? Will this protect fine against -220V reverse voltage spikes ? What about +150V spikes ?


1 Answer 1


Modified diagram

Move things around like this:

  • TVS clamps forward overvoltage, protecting regulator
  • Diode blocks TVS in reverse (TVS can be -A or -CA type) allowing very negative voltage
  • MOV clamps excessive negative voltage for transient protection (optional?)
  • 9V zener + 10k limits Vgs(on) (optional if "13.8" maximum + transient is within Vgs(max))
  • Diode across C2 prevents reverse recovery from reversing U1 excessively (M1 will take some time to turn off, due to gate capacitance).

What environment are you working on anyway? 200V static is ridiculous. Is this like an industrial thing that needs to withstand cross-wiring faults (exposing 120/240VAC or other to control wires)?

The 13.8V sounds an awful lot like automotive standards, which you've severely misunderstood if that is the case -- the high voltages are transient only, and rather short in duration and high in impedance. They are easily filtered with LC filtering, and clamped with TVS. Only the slow stuff (cranking, jumping, reverse and load dump) are significant.

Overvoltage (whether load dump or otherwise) can be handled with a depletion MOS at this level.

  • \$\begingroup\$ Indeed, I am asking about the automotive environment and its transients, not all of which are such high impedance. sft-lab.com.cn/uploads/ISO%207637-2-2011.pdf 1. the "very negative" voltage for that specific TVS is -24.4V at most. Why do I need to worry about it ? 2. "(optional if "13.8" maximum + transient is within Vgs(max))" - that would be Vf_es3j + 24.4V i believe due to TVS action without any limiting resistor (!) as per your diagram. Does the MOV do the limiting through ES3J & SCMJ15CA ? 3. can the ES1J be eliminated if the Zener limiting resistor was made 100 - 1k ohm ? \$\endgroup\$
    – kellogs
    Aug 5 at 19:26
  • \$\begingroup\$ @kellogs Oh it is ISO 7637 after all? Yeah, don't worry about it, worst case 10 ohm, short pulses, clamp it with a diode, who cares. That's precisely what TVS are made for! 2. Correct; MOV is to limit negative excursion permitted by ES3J. Ignore both and just use SMCJ15CA (maybe even SMA will do) since you don't need high voltage withstand. 3. Maybe, I would want to test it first. \$\endgroup\$ Aug 5 at 22:36

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