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I am using a MOSFET to read a square wave, which usually ranges between -1 and 16 V. However, on occasion, the signal can reach values of -30 to 45 V. My current setup uses a Zener-protected STN3N45K3 MOSFET (max Vgs +/- 30 V). The circuit is shown below. Note that the RPM_DI signal is connected to a microcontroller input, which is pulled-up to 3V3 via a 1K resistor. The input driver signal goes in through pin 3 in P20. Note also that D15 is the TVS diode I am planning on installing.

schematic

My questions are:

  1. Would it be a good idea to add a TVS diode (in the position of D15), with a Vcl < 30 V, such that I am protecting the MOSFET (additional to the included Zener)?
  2. If so, could the TVS diode become damaged due to the higher voltages? Note that when the signal reaches -30 to 45 V, these are not transient voltages, rather the square wave will stay at these voltages permanently. I don't, however, expect large currents.
  3. Should the TVS diode be placed before of after the gate resistor (R22)? If placed after, I understand the resistor would limit the current through the diode. If placed before (as in the current schematic above), I'm not sure what the current through the diode would be (?)

I guess my question is somewhat similar to what was asked here: Mosfet Gate-Source Zener. In terms of this other question, I would ask: Could the Zener become damaged after continuous application of the 30 V signal? Does this depend on the power dissipated by the Zener? If so, how would I calculate the power?

If anyone can help, I'd appreciate it!

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  • \$\begingroup\$ Where/why does this overvoltage occur? Line or prove ground inductance? It’ not clear how much energy is in this pulse. Can you get a flat line if you probe ground? Is it flyback thru Miller capacitance. Then reduce R22 to match FET gate resistance, Rg \$\endgroup\$ Apr 16, 2021 at 1:27
  • \$\begingroup\$ The signal coming in through pin 3 in P20 is a square wave coming from a motor alternator, which is used to calculate the RPM of the motor. That signal is usually from -1 to 16 V, but we have seen signals with amplitudes of -30 to 45 V. That is the "overvoltage". We still wish to be able to read those signals, but since they surpass the max Vgs of the FET, I was thinking of adding the TVS diode for protection (or clamping). With this in mind, would the TVS diode help? Any suggestions you could make towards the 3 questions asked in the post? \$\endgroup\$ Apr 16, 2021 at 2:26
  • \$\begingroup\$ You don’t show a load, but I suspect it is just 10k pullup to Vdd. I think it is better to just use a transistor with a voltage divider for Ib=5mA max Zener and turn on at a min (RPM) say 2 to 5V with 0.6 =Vbe and assume Ic/Ib=10. There no need for high current . You could also use a 1shot and average the Voltage like a tach unless you want no latency RPM \$\endgroup\$ Apr 16, 2021 at 2:36
  • \$\begingroup\$ Yes, the load is just a pullup to Vdd via a 1k resistor. Well, I think the reason I'd also like to use something like a TVS instead of a voltage divider is because, since this is a signal coming from an alternator which I cannot control, it may contain (from time to time) unwanted voltage spikes (possibly higher than the 45 V I've seen in "standard" signals). So I'd like to cut-off any large voltage/current that may damage my FET. Also, pardon for asking, but why are you referring to Ic and Ib when it's a MOSFET and not a BJT? May just be that I lack knowledge. \$\endgroup\$ Apr 16, 2021 at 3:02
  • \$\begingroup\$ Because I suggested you don’t need a FET or a TVS just a BJT, 2R’s and a Zener \$\endgroup\$ Apr 16, 2021 at 5:05

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Move the TVS labelled as D15 after resistor R22.

In this way, the protection becomes really effective.

Make sure R22 can stand 50 VDC. For example, a 0805 package will do.

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  • \$\begingroup\$ Thank you. Quick question: the current MOSFET has an included Zener protection with Vbr = +/- 30 V. So, when adding the TVS, should I select one with a |Vcl| < 30 V? Or could I go higher? \$\endgroup\$ Apr 16, 2021 at 13:26
  • \$\begingroup\$ The external TVS device is much more effective than the MOSFET's internal one. Pick a TVS that has a breakdown voltage lower than that of the MOSFET. I would select a 24 Volt bidirectional TVS diode. \$\endgroup\$ Apr 16, 2021 at 13:37

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