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I am designing a circuit that will gate single phase 120V AC through back-to-back N-channel MOSFETs connected as shown below. When passing AC, one MOSFET is on and the body diode in the other is forward biased. The active MOSFET changes for the positive and negative half cycles. When blocking AC, both MOSFETs are off.

I am considering putting transient voltage suppression across the drains on the MOSFETs to protect them from breakdown caused by high voltage spikes. I would use 200V breakdown MOSFETs with a 150V breakdown TVS.

Is using a TVS across the MOSFETs a reasonable design decision?

Are the above component voltages reasonable for a 120V circuit?

Is there any reliability/survivability advantage to using MOSFETs with a drain-source breakdown voltage significantly higher than the TVS breakdown voltage, versus just slightly higher?

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    \$\begingroup\$ What is the expected load? \$\endgroup\$ – Bruce Abbott Aug 27 '19 at 3:42
  • \$\begingroup\$ Have you considered to use avalanche rated MOSFETs? \$\endgroup\$ – Marko Buršič Aug 27 '19 at 8:56
  • \$\begingroup\$ 120 VAC RMS = 170V peak, so your 150V TVS would just blow the very first time. \$\endgroup\$ – Marko Buršič Aug 27 '19 at 9:02
  • \$\begingroup\$ Thanks for correcting my RMS vs peak voltage error \$\endgroup\$ – crj11 Aug 27 '19 at 10:37
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Using 200V rated MOSFETs for 120VAC are under rated, like walking on the edge. You should choose higher breakdown voltage. Further you could use avalanche rated MOSFETs, they can conduct some short pulse surge in the reverse polarity. You can place an additional TVS accross them with little lower breakdown voltage.

It is not clear from your question what current is expected and how would you drive the MOSFETs.

When passing AC, one MOSFET is on and the body diode in the other is forward biased.

That's true, except that when turned ON, both transistors will conduct, when OFF only one body diode will conduct the other transistor will not.

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