IEEE Power Electronics Magazine - September 2016 - 60

White Hot

by Robert V. White

Driving Wide Bandgap Success

ight out of college, I went to
work in a power supply
design group at Digital
Equipment Corporation (DEC). My
first project was to design the
power supply for DEC's first desktop computer. I was given some
designs with similar specifications
as a starting point. The only choice
for the main power switch was a
bipolar transistor.
Power bipolar transistors could
work well but had difficult drive
requirements. To turn the transistor
on quickly required a pulse of current
essentially equal to the current being
switched. That is, if the collector was
10 A, then the peak base current had
to be about 10 A. The same rule of
thumb held for turn off. If the collector current being turned off was 10 A,
then one had to supply a 10-A negative pulse of base current. Furthermore, the current gain of these power
transistors was low, typically in the
range of 3-5. This meant that to keep
t he t r a n si s t or wh i le c a r r y i n g
10 A, the base had to be driven with a
current of 3-4 A. An additional re quirement was that the transistor
should not be driven too hard into
saturation (excessive base current)
as that would substantially slow the
turn off.
Fortunately, I had at my disposal
the Gregorich base-drive circuit,

Digital Object Identifier 10.1109/MPEL.2016.2585469
Date of publication: 7 September 2016

IEEE POWER ELECTRONICS MAGAZINE

for the quick adoption of the silicon
invented by Jim Gregorich, who was
power MOSFET.
the technical lead for all of DEC's
Today, we are seeing a constant
power supply groups. This circuit
stream of new power switching
used a proportional base drive-a
devices made with GaN and SiC comcurrent transformer that coupled a
ing to market. However, each of these
fixed proportion of the emitter curseems to require a more complicated
rent to the base. This provided the
drive circuit than the silicon MOSconstant current needed to keep the
FET. Efficient Power Conversion
transistor on. There was also a clever
(EPC) was probably the first compaconnection to auxiliary voltages
ny to commercially offer a powerto get the large turn-on and turnswitching transistor
off currents and a
made with GaN.
Baker clamp circuit
There were severto keep the transisFortunately, I had
a l adva ntages to
tor out of hard satuat my disposal the
these devices, includration. This circuit
Gregorich base-drive
i ng en ha ncement
worked so well that,
circuit, invented by
mode (e-mode, norin 1980, I could use
Jim Gregorich.
mally off) operation,
t he a stou nd i n g l y
favorable figures of
high switching fremerit (FOM) when
quency of 50 kHz.
compared to similarly rated silicon
Given this difficulty in driving
MOSFETs, and the nearly ideal flippower bipolar transistors, the relative
chip package. However, even with
simplicity of driving power MOSFETs
e-mode operation, the gate drive
was the power supply engineer's
requirements were quite particular.
dream. One could sometimes drive a
The gate-to-source voltage needed to
MOSFET from the output of a pulsebe 5 V to assure the device was fully
width modulation (PWM) controller
enhanced (on), but gate-to-source voltIC. If a bit more turn-on and turn-off
ages as low as 6 V could lead to device
current was needed to drive the input
damage. This meant these devices gencapacitance of the MOSFET, a simple
erally could not be driven by existing
NPN-PNP transistor voltage follower/
MOSFET drives. EPC developed effecbuffer was often all that was needed.
tive, but complicated and relatively
For the two transistor forward conhigh parts count, driver circuits using
verter and bridge topologies, simple
discrete components. Many engineers
and low-power pulse transformer
looked at this and said, "No thanks!"
circuits could drive the MOSFET. It is
my opinion that this simplicity of
drive was one of the major reasons
(continued on page 57)

z September 2016

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - September 2016