PART1:
THEORY
Power MOSFET Basics
Power MOSFETs (HEXFETs are International Rectifier's
trademarked name for their products) are typically used in
power switching applications and are classified as transistors.
They have three leads like a transistor but are voltage in,
current out devices. This is because the gate is completely
isolated from the rest of the device. No significant DC current can flow from the gate to either the source or drain.
Instead, the charge on the gate affects the conductivity
between the drain and
source. This is like the
charge on one capacitor plate affecting the
other plate. Typically,
about eight volts will
turn on the device
completely. When on,
the part exhibits a low
resistance without any
non-linear forward
voltage drop as seen
in bipolar transistors.
Modern MOSFETs can
have on resistances of
less than 10 milliohms.
A little math shows
that this device can
handle 10 amps with
one watt converted
into waste heat (power = current2 x resistance). Since
many MOSFETs come in TO-220 packages, no heatsink is
needed in this instance. So, if the voltage is 100 volts at
10 amps, then 1,000 watts of power are switched with
only one watt lost. That's 99.9% power efficiency. The
IRFB-4410 has these specifications and costs about $4.50.
That's fairly expensive for a power MOSFET.
The resistance in the off state is so high that it is usually
not stated in the datasheet. Instead, they typically define the
50 January 2009
■ FIGURE 1. It takes virtually
nothing to turn a MOSFET on
and off. Here, reversed biased
diodes do the job with about
10 nA of current. The voltmeter
reads zero when the MOSFET
is on/conducting.
by Gerard Fonte
breakdown voltage as when 250 µA of current flows through
the part. For most practical purposes, the power MOSFET
can be considered a switch: either it's on or it's off. (Next
time, we'll look at linear/non-switching applications.)
However, the key point is that it takes some time to
go from a very high resistance to a very low resistance.
This switching time determines the efficiency of the
system and will be examined in more detail shortly.
MOSFETs come in two flavors: P-channel and N-channel.
However, because of the physics involved, the P-channel
types cannot match the low on resistance of the N-channel
type. For that reason,
there are many more N-channel parts available
and at a lower cost. Most
designs will use an N-channel device even if it
requires additional effort.
P-channel parts are not
often seen except in
special applications.
Unlike bipolar transistors, MOSFETs have a
positive temperature
coefficient. This means
that their resistance
increases with temperature.
This can be extremely
useful. As they heat up,
they impede current flow more which tends to stabilize
the system. Bipolar transistors allow more current to flow
as they get hot. This increased current heats them up more
so that they pass more current, which further increases
their heat and so forth and so on. This is also called thermal
runaway. When it happens, the transistor is usually lost —
possibly along with additional downstream damage.
The positive temperature coefficient means that paralleling identical MOSFETs for additional power is relatively
easy. If one device gets too hot, its increased resistance
effectively pushes excess current to the other parts auto-
■ FIGURE 2. Low-side drive
(Figure 2A, left) versus high-side
drive (Figure 2B, right) is
defined by which power supply
rail the MOSFET is connected to.
