Simple isolated mosfet driver
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From: Jon Slaughter on 22 Dec 2009 21:36 John Larkin wrote: > On Tue, 22 Dec 2009 10:18:04 -0600, "Jon Slaughter" > <Jon_Slaughter(a)Hotmail.com> wrote: > >> Is it possible to drive the gate of a high side n-ch mosfet using an >> optocoupler to isolate and float the gate w.r.t to the source and >> still be effective? >> >> I want to continuously control the gate of a high side n-ch mosfet >> to prove a variable resistance for a high voltage load. >> >> >> G = Gate, S = Source, D = Drain >> OE = Opto Emitter, OC = Opto Collector >> >> Vcc--D >> >> >> OC--R2--D >> OE--G >> >> G--R1--S >> | >> Load >> | >> Gnd >> >> >> The idea is simple, a resistor(R1) connects the gate to the source. >> When no current is flowing the gate is then held at the same voltage >> as the source and the mosfet is turned off. >> >> An optocoupler is added to control current through that gate/source >> resistor which will "bias" the gate relative to the source and allow >> turning on the mosfet. A simple simulation shows this works but I'm >> not use how useful it is. Some protection mechanism for the opto >> would be needed as well as driving the gate too high. >> >> The isolation is necessary because of the high voltage used. I'm not >> worried about the "speed" as this isn't used for switching. >> >> > > If I understand your circuit, it puts a lot of voltage across the opto > output. A shunt circuit wouldn't. But then there *are* optos with 400 > volt phototransistors. [1] > > A photovoltaic coupler would be nice, as it can drive the gate > directly and doesn't need a high-side supply. They tend to be wimpy, > 10s of uA output, so they would be pretty slow. An advantage is that > there's no sneak current path to the load. > > > Or do this maybe: > > > V+-------+---------+ > | | > | | > Q | > | | > | d > +--------g > | s > c | > b | > e | > | | > | | > +---------+ > | > | > | > load > > where Q is a Supertex depletion fet current limiter, cbe is the opto > phototransistor, and there should be a zener g-s which I'm too lazy to > draw. It does leak a litle current into the load. > My original idea was somewhat similar. The problem is that the initial state is with the load recieving the full voltage. Hammy gave a datasheet where they use a jfet to discharge the gate and keep it at 0V relative to the source when no current. It should work well and also removes any need for protection.
From: Hammy on 23 Dec 2009 06:43
On Tue, 22 Dec 2009 20:32:05 -0600, "Jon Slaughter" <Jon_Slaughter(a)Hotmail.com> wrote: > >Thanks, That might do the trick. A bit expensive though for something that >seems relatively simple. The method they use with the fet for discharging >the gate might be all I need with any old optocoupler method. > Yes but it saves you from having to make an isolated supply. Page 18 of this pdf shows how to use a HS driver for continuous on time. It also shows some of IR's HS drivers. http://www.irf.com/technical-info/appnotes/an-978.pdf Some of Fairchild's HS drivers. FAN7371 4A, FAN7382 HI/LOW , FAN7361 |