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series mosfet as an AC switch

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carmusic

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Please check my electric plan, this will be use as a 100V 100A switch (about 0.5 hz) with no polarity check (i will switch pulsated dc (170V peak) but wire can be inverted so thats why i tried it with ac voltage), mosfet will be irfp4468pbf (100V 2.5Mohm)

do i really need the zener diodes i put? Is there one included in the mosfet?

gates will be driven by 2 small dc dc converter.
 

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Since your Vgs is floating no need for the zener , Actually the zener is mounted not to exceed Vgs rating . The zener seems to be doing a pretty dangerous thing in your schematic omit it.
 
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What's going to keep the current through all MOSFETs the same? They're not all identical and then one will pop first followed by the second, third, etc...
 
Ive just read an article from IRF and it is possible to parallel mosfet with minor unbalance
anyway the mosfet i will use can handle about 10 times the current i need, i just use multiple to get lower rds
 
Actually that's pretty true. Parallel mosfets is not as bad as parallel bjts.

That was my understanding also, as I thought one advantage of mosfets was their positive temperature coefficient where as most bjt have negative temp co? One shouldn't be paralleling power BJTs without emitter balance resistors for that reason.

Lefty
 
HeHeHeHe......he's gonna make smoke!:D

Oh well....I shouldn't make comments......Lord knows I have blown up a lot of stuff myself!
 
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Please check my electric plan, this will be use as a 100V 100A switch (about 0.5 hz) with no polarity check (i will switch pulsated dc (170V peak) but wire can be inverted so thats why i tried it with ac voltage), mosfet will be irfp4468pbf (100V 2.5Mohm)

do i really need the zener diodes i put? Is there one included in the mosfet?

gates will be driven by 2 small dc dc converter.


The lower mostfets (even numbered), although they have a positive Vgs, they are n-channel, and you have the current going through them from source to drain, which is backwards. This won't work. What you want to do is to emulate an analog switch, which is a paralleled pair of mosfets, one N-channel, one p-channel, which allows current to flow bi-directionally. There is much information on this at the analog devices website.
 
Here is what I mean:
 

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The lower mostfets (even numbered), although they have a positive Vgs, they are n-channel, and you have the current going through them from source to drain, which is backwards. This won't work. What you want to do is to emulate an analog switch, which is a paralleled pair of mosfets, one N-channel, one p-channel, which allows current to flow bi-directionally. There is much information on this at the analog devices website.

Hi Analog , The circuit will work this way untill he keeps providing isolated gate drives . The current is going bidirectional in the said schematic. Or I didnt get you?

-Adi
 
What you want to do is to emulate an analog switch, which is a paralleled pair of mosfets, one N-channel, one p-channel, which allows current to flow bi-directionally.

What if one don't have both n-ch and p-ch MOSFETs?

One can get the same blocking feature in both direction by placing two N-Ch MOSFETs in series and drive them with a common gate voltage.

It is a common practice in with all kinds of application.
 
Yes it is pretty standard in fact some AC (bidirectional )SSR uses two N-Ch Fets in series. I don’t know why he has two Bidirectional switches in series; parallel yes but why the series pair? What are you using to drive these, what is your load? Are you switching these at frequency or is it just an on/off switch. Mosfets also have leakage current I dont know if this is a factor for you. You know depending on your load your initial current can be substantially more then steady state (inrush). You also should know that if your gate drive isn’t adequate you could smoke all those FETS. I’m not just talking voltage you should have a gate drive that can pound out several amps that will turn those FETs on quickly,A large heatsink will also probably be needed, or your fets will operate out of there SOA at start-up then poof there goes $$$.

I’m not sure what you’re trying to do but there must be a better way. SSR’s switching these currents would be a challenge even for an experienced designer that’s why there so expensive. Here is an app note from Vishay on using there photovoltaic drivers you will definitely need a more robust driver but it may give you some ideas.

https://www.electro-tech-online.com/custompdfs/2008/12/ssrvo126.pdf

Good luck
 
i finally have found 200V mosfet my schematics will have only 2 mosfet back to back and 10 of each group in parallel.
 
I would think an SCR Switch with commutaion might be better to use. Many proven designs out there. SCR's have low on voltage. The freq you need is with in limets. 100amp SCR's are common.
 
no scr will have a dissipation of minimum of 70-120 watt (1V loss on SCR) ( and they don't work on DC (i am working with DC volatge, but i needed polarity protection), i have made my swith with 20 mosfets and it works perfectly, only 20 w of heat at 100 amp, my pcb board that have all of the mosfet will mesure only 7.4 x 3 inch and about 80$ for all materials a lot cheaper than a 100 amp scr with very big heatsink
 
yes I know you said DC that is why I Mentioned commutaion of the SCR. If
Low power dissipation is a concern,I can see why you would want low on Resistance devices. Still wondering what kind of Load you are switching.
 
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