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> What Is Mosfet Driver? How Does It Work?
keathow87
Posted: March 05, 2011 04:05 pm
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1) Anyone please explain to me why we can't just drive the gate of the Mosfet by using DC voltage instead of using Mosfet driver??

2) How does Mosfet driver work actually?

3) What input should we give to the Mosfet driver to get the desired output?? Please explain using this Mosfet driver for example, if I want to have a 13.56MHz 15Vp output to be used for Mosfet switching http://www.farnell.com/datasheets/32537.pdf

Thanks in advanced and your help are very needed and appreciated =)
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tekwiz
Posted: March 05, 2011 11:09 pm
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Switching MOSFETs are generally driven with DC voltages or pulse trains. That chip is simply a convenient way of getting the proper drive signal without building a separate discrete circuit to do it.
But a MOSFET can be driven by just about any circuit capable of providing the right gate voltages & that is fast enough to ensure that the MOSFET spends the absolute minimum amount of time in it's linear gate voltage region. This is important, for it's when the MOSFET is in it's linear region that it acts like a resistor & produces a lot of heat.


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Ice-Tea
Posted: March 06, 2011 07:01 pm
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The gate of a FET is the equivalent of a capacitor. If you are switching the FET with a source with a high equivalent series resistor it will charge that cap slowly and during a part of that time the FET will be already conducting and exhibit high ohmiv values, the ohmic or linear region. Lots of power lost. A mosfet driver can deliver a lot of charge fast to the equivalent cap and make sure the FET switches fast.
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zero.vishnu
Posted: March 06, 2011 08:21 pm
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Typical logic systems such as microcontrollers use TTL 0-5V. They cant supply enough current to switch high power Mosfets on and off.

Most Mosfet turn on at 10-30V, and need a high current if you plan to do high speed switching. Therefore you need a mosfet driver as an interface between your logic system and Mosfet.

Just feed you logic (on 5V, off 0V) into your mosfet driver input pin, your mosfet driver output must be connected to the gate of the mosfet. The mosfet driver's VCC and GND must be connected to a 15V supply (or whatever voltage you want to drive the Mosfet).
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MikeGyver
Posted: March 07, 2011 01:25 am
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A mosfet gate is essentially a capacitor that holds a charge. Mosfet drivers are circuits that can sink and source relatively large amounts of power to move this charge back and forth. As the switching freq goes up, so does the average current flow required to slam the gate on and off.

Your typical power mosfet wont run anywhere near 13.56 MHz. You'd have to run a special RF transistor (and driver). Small signal mosfets might run that fast though.
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johansen
Posted: March 07, 2011 02:13 am
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it is doable, even with discrete components.
you just won't get good efficiency.. tubes might be better...

also, half the heat disappointed in the fets might be from the internal gate resistance alone.

its not *that* hard to get the fet to turn on in 10 nano seconds, but you'll need something more powerful than a 1 amp driver chip

Sch3mat1c will probably suggest using zetex transistors to drive the fets.
YMMV
i built a push-pull transformer using two 11 amp 30 volt surface mount fets that could switch 4 irfp450 fets on in 10 ns
and they used a few *watts* at 100Khz just for the gate drive.

http://johansense.com/induction_heater/prototype3.1_W.JPG
I can't recall if that image was taken with 7 ohm or 15 ohm resistors.


what voltage are you looking to switch at 13 Mhz?


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tekwiz
Posted: March 07, 2011 09:15 pm
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QUOTE (keathow87 @ March 05, 2011 07:05 am)

3) What input should we give to the Mosfet driver to get the desired output?? Please explain using this Mosfet driver for example, if I want to have a 13.56MHz 15Vp output to be used for Mosfet switching http://www.farnell.com/datasheets/32537.pdf


Your input would have to be in the form of a clean 13.56mhz square wave with fast rise & fall times. Minimum voltage would be 2.5V, but for proper operation, the voltage should be considerably over the minimum. 5V would be a good choice.
For a 15V output, you will need a supply voltage higher than 15.3V.
All of this information is in the datasheet. wink.gif


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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