Author Topic: FET Driver (discrete)  (Read 6594 times)

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Offline izuaTopic starter

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FET Driver (discrete)
« on: September 22, 2008, 08:48:57 PM »
I want to build my discrete FET driver. I haven't found one around that is both simple to use and can drive high voltages.
What I basically think of is a push/pull stage with BC transistors. I plan using BC846 and BC856 (complementary, SOT-23 type, 80V, 100mA).
The FETs will be from the IRF family, most likely 740. Drain-source voltage should be under 30 volts. I'll drive them with at least 10 volts on their base (referenced to ground, it should be 40V, right?)

I assume that a configuration such as this one: http://www.izua.ro/DSC09517.JPG will directly short any BJT transistor. For example, if the logic says 1, the top (NPN) opens through the bias resistor, charging the FET's gate. Since the gate is a capacitance, it won't be a direct short, the gate will be just connected to VCC and allowed to draw current. If it's going to happen for a long time, it will probably blow this transistor.

Something tells me the top (pushing, sourcing) transistor should be a PNP instead of a NPN.
So, I want some ideas. If I'll put a limiting resistor on the collector of the push-pull (two transistor) stage, I'm coming back where I started, since the driving transistors won't switch fast enough, or give the required voltage to efficiently drive the FETs.
Maybe I should use smaller FETs instead? Any hints or ideas appreciated.

Thanks!
« Last Edit: September 22, 2008, 08:49:50 PM by izua »
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Offline ArcMan

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Re: FET Driver (discrete)
« Reply #1 on: September 23, 2008, 09:27:39 AM »
I highly recommend using a dedicated MOSFET driver chip.  I use the HIP4081A.  There are alot of considerations when building a high-power MOSFET drive, and the HIP4081A takes care of them.  It also uses charge pumps to drive N-channel MOSFETs on the high side - a desirable function since N-channel MOSFETs have a much lower Rds ON than P-channel.

I can post the circuit that I used if you'd like.  I'm out of town for a couple days, but I can post it when I get back.
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Offline izuaTopic starter

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Re: FET Driver (discrete)
« Reply #2 on: September 23, 2008, 05:51:27 PM »
The problem with dedicated chips is that they're pretty hard to find.
Do post a circuit, however. The chargepump is definiteyl a good thing to have.
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Offline ArcMan

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Re: FET Driver (discrete)
« Reply #3 on: September 24, 2008, 08:58:54 PM »
Actually, they are quite easy to find.  Newark Electronics has over 800 of the DIP versions for sale for $4.00 apiece.

I'll post the circuit after I get back home.
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Offline izuaTopic starter

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Re: FET Driver (discrete)
« Reply #4 on: September 25, 2008, 02:01:52 PM »
The closest newmark electronics store is on another continent for me :P
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