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The solid state relay with MOSFETs connected back-to-back (source-to-source) is claimed to work by the mutual action of the body diode according to this website, and this TI paper.

However, I had been thinking that in the on-state, the body-diode is bypassed by the channel (see this question and answer), even if current is flowing from source to drain in reverse direction. In order to test this hypothesis, I have set up an LTSpice model where I chose the supply voltage of the current path so low (0.01 V) that it is clearly below the threshold voltage of the body diodes. And, surprise surprise, the SSR is still working perfectly, even though the body diodes can't possibly be active.

Could it be that a source as reputable as TI is actually wrong?

enter image description here

oliver
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  • Curious, is there a mechanism to self-close as duplicate? Well, I'll get that process going anyway: [AC MOSFET switching without any diodes? (Low-cost AC solid-state relay with MOSFETs)](https://electronics.stackexchange.com/questions/411909/ac-mosfet-switching-without-any-diodes-low-cost-ac-solid-state-relay-with-mosf) – Tim Williams Apr 06 '23 at 22:20
  • Also, added some info to that question if you're curious – Tim Williams Apr 06 '23 at 22:32

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After asking the question I have discovered that there is already the answer here on SEEE. In fact, TI is wrong. That's a pity because TI is otherwise a very rich source for valuable information. But since this wrong information is already being reproduced by other websites, this is no minor fault. It might severely misguide newbies like me about the MOSFETs functioning.

oliver
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