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Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615
Like this. part numbers are just defaults - this is to show you the topology, you can play with all the values to get what you want. In a real circuit I would use a 12V TVS or Zener across the gate/source of the P-fet for safety.
Note that when simulating an unstable circuit like this you may need to specify some initial conditions - most probably on the capacitor. A transient sim starts with a DC operating point calculation, and with these sorts of circuits the simulator often needs a little help.
Also - please look at the way I and other members draw circuits, and try to emulate the logical flow you see. Some of your drawings have been very difficult to interpret.
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268
Thanks guys, I placed a p, mosfet at the top driven by a second p, fet and a voltage divider sending the signal to a noninverting comparator. It's working like I want it. Cheers
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268
My problem with the sim, I changed it to two Nmosfet with the top one driving a P. I'm still crunching data but I think this circuit is ready to test with the genny. Will update the other thread, just not making the same ground solved it. Thanks,
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268
I don't really understand the p mosfet, if I switch off ground then both p and n won't work, if I have ground, at drain for n and source for p, then a plus voltage will open n, but if the drain on p is open circuit then how will it open. Sorry for the basic questions but the book doesn't explain it right or its me, that part doesn't matter. Thanks
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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your Pch mosfet is upside down. Source should be connected to positive rail. The graph is meaningless to us unless you also tell us which node is being graphed. Use net labels on your schematic.
Show us Pch gate, Nch gate and capacitor voltage all on one graph. With appropriate labels.
Also its highly unlikely that a comparator will accept having its output pulled up to a voltage higher than its supply rails - thats why I put an Nch buffer in the schematic I showed you.
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268
The circuit works good, but I'm trying to replace the mosfet with igbt, I've downloaded a Fairchild sub for ltspice, but I'm not sure, of the overview design with igbt, any help appreciate, thanks.
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Problem with comparator and switching
