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This question is about an alternative path for relay flyback diodes.

I plan to drive panasonic 12VDC CP1-12V relay with a NUD3160LT1G power MOSFET and the NUD3160LT1G datasheet says because the MOSFET comes has protection diodes, there is no need for a "free−wheeling diode" to drive an inductive load.

My question is, is it good enough to have this topology of flyback path through high voltage zener diodes and ground and +12 ground rails or should I connect a reverse diode directly on the relay's coil anyway.

This is for a non critical one-of audio application in my car. Cost is not an issue but I would prefer avoiding an additional diode if possible. The NUD3160LT1G will be driven by a 5V ATMEGA328P MCU if it maters.

enter image description here

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  • \$\begingroup\$ Why would you connect zeners to the gate? That will cause sporadious turn on after turn-offs due to the inductive kickback and you have an oscillation amd/or broken MOSFET. Add a diode across the relay. \$\endgroup\$
    – winny
    Dec 30, 2016 at 16:58
  • \$\begingroup\$ How about a link to the data sheet that works? \$\endgroup\$
    – Andy aka
    Dec 30, 2016 at 16:58
  • \$\begingroup\$ @winny, the NUD3160 already come with those drain to source diodes but this is an interesting point. Is it possible that the negative feedback from the flyback is part of the protection? That is, it slows down the turnoff and thus reduces flyback voltage. \$\endgroup\$
    – user1139880
    Dec 30, 2016 at 17:26
  • \$\begingroup\$ That's a mighty strange situation. Still, diodes cost cents. Clamp that relay with a diode. \$\endgroup\$
    – winny
    Dec 30, 2016 at 19:35

3 Answers 3

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This is good the zener from the drain to gate turns on when Vds starts to rise due to the inductive load when the fet turns off. This partially turns the fet back on dissipating the inductive spike in the fet safely. This is a recognized method of flyback protection.

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  • \$\begingroup\$ *flyback gate oscillation and destruction. \$\endgroup\$
    – winny
    Dec 30, 2016 at 19:36
  • \$\begingroup\$ @winny I am interested reference or link please. \$\endgroup\$
    – RoyC
    Dec 30, 2016 at 21:45
  • \$\begingroup\$ Leared the hard way. Didn't publish it. In very obscure series stacked configurations you may save a device momentarily by turning it on this way. In normal situations, you don't want to allow your miller capacitance nor external zener diodes to win over your gate command. \$\endgroup\$
    – winny
    Dec 30, 2016 at 21:54
  • \$\begingroup\$ @winny my interest is that I am using this method to protect a remote shorting mosfet switch for a wind generator, it has worked well for a couple of years now, granted the situation in terms of 'load' is a little unusual and I could not use a flyback diode. These devices are not unusual example here digikey.com/en/product-highlight/d/diodes/…, they would not be sold if they did not work. Was this a high frequency switching case? \$\endgroup\$
    – RoyC
    Dec 30, 2016 at 22:20
  • \$\begingroup\$ Yes indeed! For very slow/DC cases I can see that is might work. \$\endgroup\$
    – winny
    Dec 30, 2016 at 22:48
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Since the driver is rated for such relay coils with 200mJ avalanche for coils with Rs>80Ω. It will work as is but the wiring loop area may spew some EM noise impulse affected by AM Radio for example. In this case I would use a relay with reverse diode built-in or added-on.

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It depends on whether the MOSFET in question can endure the peak (drain-source) voltage induced by the relay inductor when the MOSFET shuts off (under load).

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  • \$\begingroup\$ The NUD3160 comes with protection zeners per the diagram above. \$\endgroup\$
    – user1139880
    Dec 30, 2016 at 17:22

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