Re: [CAD_CAM_EDM_DRO] Why Bipolar?

Joel Jacobs wrote:

> ----- Original Message -----
> From: Jon Elson <jmelson@...>
> . Driving the
> > high side FETs is easy compared to driving high side bipolar transistors,
> > since no gate current is needed.
> >
> > Jon
>
> Hi Jon,
> Power MOSFETS certinly do require gate current! those gates are around 1500
> to 2500 picofarads and you have to charge and discharge that capacitance to
> turn them on and off.

Yes, of course, there is substantial current during the turn-on and turn-off.
But, moderate gain single bipolar transistors have really low gain, and so for a 5
Amp drive, you may have to supply 500 mA to the transistor the entire time it
is turned on. Supplying 500 mA referenced to the varying emitter voltage level
is a real challenge! Driving a power FET is much easier. A modest capacitor
can hold the charge to turn the transistor on, and then it takes no current to
keep it on. During the time between turn-ons you can recharge that capacitor.

> There is also miller capacitance between the gate and
> drain so if you are switching high voltages the inverted signal at the drain
> will fight the gate current. For high frequency switching as in a power
> supply or chopper drive your gate drive circuit needs to be able to drive
> about an amp (for a very short time) to get good switching speeds from the
> FETs. I usually use a bipolar totem pole transistor driver 2n3904/2n3906
> with a series resistor to limit the gate current to one amp.

The IR 2113 and similar drivers can source up to 5 A to get the FETs on quickly.
This is imperative for IGBTs, which will fail if you ever let them run in linear mode,
due to current hogging by one small area of the die. But, this all happens in
under 100 nS, and then it takes no power to keep the transistor on!

Jon