> I *thought* (correct me if I'm wrong) that an IGBT needed 5V-15V,
depending
> on the device, gate-emitter to switch on. In the on condition it should
have
> less than 2V emitter-collector, unless I'm missing something (I usually am
> ;o)  ) that would necessitates a gate drive 3V-12V higher than Vdd.
> (assuming standard H-bridge with the high side collectors connected to
Vdd)

That is essentially correct. The following ramblings essentially reinforce
your assumptions:-)

For practical purposes an IGBT appears as a FET input driving a bipolar
output.
The FET input requires a specificed Vgson to operate and this will typically
be, as you note, in the 5 to 15 volt range.
The output is a bipolar transistor and will have Vsat(uration) voltage
appear across it when fully tuned on. Unlike a FET which is essentially an
ohmic channel when fully enhanced and has an on resistance Rdson and
therefore a voltage which varies with current, IGBTs have a saturation
voltage which is somewhat constant across a current range. IGBTs are usually
more appropriate at higher voltages and lower currents, and the on voltage
may be rather higher than 2 volts.

To turn the IGBT fully on the applied gate-emmitter voltage needs to be the
requisite Vge above the emitter. When fully on the emitter is vsat = a few
volts below top rail so the gate will need to be most of Vge above emitter
voltage.
Depending on how high Vge is the gate will PROBABLY have to be driven above
the high rail.

In some cases you MAY be able to design suich that the gate is driven only
to high rail and still have enough drive, but this is liable to be the
exception. Taking a psudo randomly selected IGBT (data sheet was to hand)
HGTP10N40C1D, 10A, 400v,. Vceonmax = 2.5v. Vgeth =2v typical, 4.5v max BUT
at Ic = 1ma
ie by connecting gate to high rail you MAY get turn on but only with minimal
current flow.
For full performance a Vge of 7v is required - in this case 4.5v above top
rail.

A topside FET driver with an opto coupler may indeed be an adequate solution
in some cases BUT a high side supply above positive rail will be needed :-(.



        Russell McMahon

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