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I made a prototype circuit to detect the loss of individual 12V lithium batteries in a larger parallel bank due to BMS cutoff (12V continuous signal from an individual battery suddenly becomes open circuit). It's just a 10k/5k voltage divider on the Vin side, and a 10K pullup resistor on Vout.

I got the idea from this forum when someone answered my overall logic question by suggesting a mosfet inverter, but I found the 1K resistor they suggested on the input was too low to provide enough voltage to close the gate, so I had to increase it to 5K. I'm using IRF510N models, so maybe that was the difference although IRF530N appear similar.

But the circuit relies on the gate being held closed indefinitely so Vout remains at 0V. This could continue for months, or even years. Will the gate reliably open after all that time when Vin goes to 0V? It works fine now, but I can't find much about mosfets being held closed like this.

I'm not exceeding the max Vgs value, so there's no apparent stress being applied based on the datasheet's parameters. But even so, continuous operation like this is a problem with devices like side stable relays, even ones rated for continuous operation, which were my backup plan, so I wondered if mosfets have some similar issue. I hope not!

Thanks in advance.

VanGogh66
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  • Unless you’re operating the FETs outside their design spec I don’t believe you’ll have any problems. A gate voltage of 8V or even 12 shouldn’t put the device under any abnormal stress. BTW, open and closed is unusual terminology for FETs, but by I think your meaning is clear. – Frog May 08 '21 at 03:31
  • Thanks! What's the usual terminology? – VanGogh66 May 08 '21 at 03:37
  • Rather than ‘open’ I’d usually say ‘switched off’ and ‘saturated’ if it’s fully switched on, or ‘closed’ as you put it. Then in between there’s the linear region, although terminology does vary somewhat. – Frog May 08 '21 at 04:56

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Can an n-channel mosfet gate be held closed for months at a time and still reliably open when Vin falls below threshold?

Yes, provided the mosfet never leaves it's safe operation conditions. That is, the current, voltages, power, temperature, etc. never exceed their rated (functional) operating limits.

Math Keeps Me Busy
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  • Thanks! Do you have any sources to substantiate that? – VanGogh66 May 08 '21 at 03:32
  • @BeachInMexico Sorry, I don't, but perhaps someone else does. – Math Keeps Me Busy May 08 '21 at 03:33
  • It’s a tricky one, it’s not often you’ll see a paper that describes how something doesn’t happen. The nearest thing I can think of is charge carrier walkout in zener diodes. In real-world applications there are lots of situations where FETs are left in the saturated state for long periods without any reported issues. – Frog May 08 '21 at 05:01
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    @Frog I think that is the key. If there were failures associated with being "on" for long periods, someone would have noticed, and it would have been investigated and reported. – Math Keeps Me Busy May 08 '21 at 05:12
  • Thanks everyone! – VanGogh66 May 08 '21 at 22:07
  • There are some things that can happen if a transistor is on rather than off for a long period of time. You can have electromigration in surrounding metal traces, or hot electrons implanting themselves in your gate oxide changing the threshold. – Matt May 08 '21 at 22:50