Why not use two jellybean complimentary emitter-follower BJTs driven by a CMOS gate?
I just did a test instrument board that used some P210x optoisolators to drive the high-side MOSFETs, but they are not cheap.
--sp
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Best regards,
Spehro Pefhany
Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48
The logic signal from the controller is level shifted to the 24V rail plus bootstrap vcc with a current source and high side resistor.
The CD4050 boost that signal, riding on the switch node to drive the powet N FET gate.
I could replace the 4050 with complementary BJTs as emitter followers, but then the voltage drop of the npn makes it difficult to drive the gate properly off
I'd consider a PMOS high side element, myself... But, why is the BJT an 'npn'? Oughtn't you use a PNP, pulling down the collector to the upper NMOS source with a suitable resistor?
You can connect a resistor from the CMOS buffer output (and bases) to the two emitters which will draw the gate down to almost exactly the source voltage over a bit of time, and uses no steady-state current.
There are lots of gate driver chips around.. Microchip has some that are advertised at 6A or more drive current, though the actual specs don't look quite that rosy.
If you are switching continously (or can arrange to do so) there are also the bootstrap driver chips- they provide the high side supply and level shifting, for prices starting at under 20 cents (1K). Eg. NCP81161 Parts are available to handle >>1kV for motor controls, and some switchmode chips might be suitable for your application.
--sp
--
Best regards,
Spehro Pefhany
Amazon link for AoE 3rd Edition: http://tinyurl.com/ntrpwu8
Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48
The other possibility is to eliminate the mosfets entirely and use a gate driver chip to do the whole job. UCC27517A or something like that. Or a dual.
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