will cause a signifi-
cant amount of heat
to be generated and
a simultaneous drop
in efficiency. High
speed switching —
power supplies — is
it allows the use of
smaller value capaci-
tors and inductors.
This saves money,
space, and weight.
■ FIGURE 6. This bootstrap design
increases the gate voltage but
is very slow ... about 30 µs turn
on time. Voltage increase is
determined by the ratio of R1
and R2. R1 tends to pull any gate
voltage down to five volts.
The simplest and
most common circuit
is called "low-side" driving (see Figure 2A). (Note, we
will mostly limit the discussion to N-channel devices for
brevity and simplicity.) In this case, the MOSFET is
connected directly to ground. High-side driving (Figure 2B)
places the load at ground and the MOSFET connects to
the power source. To turn on a low-side MOSFET, all you
have to do it raise the gate about eight volts above
ground. Grounding the gate turns it off.
It is possible to use five volt TTL logic level signals (a.k.a.,
microprocessor) to directly drive a MOSFET. However, this
won't turn the device completely on. Nevertheless, oftentimes this is sufficient. For the IRF540, a five volt gate drive
will allow about 10 amps to be switched (typically) instead
of the 28 amps specified. So, if your application doesn't
require the full power of the part, TTL signals may work. You
can always use an "open-collector" part that allows you to
pull up the logic output above five volts. Also, there are
special MOSFETs that are specified to operate with a five
volt gate drive. Naturally, they're more expensive, but they
may be worthwhile considering the additional expense and
difficulty of designing a higher voltage gate drive circuit.
Typically, these are identified as "logic-level" devices.
CMOS logic has the advantage of being able to
operate on eight volts or more without a problem.
However, they are terrible when it comes to drive current
— even with paralleled outputs.
Typically, they only provide a
couple of mA or so per output.
So, it's difficult to drive a MOSFET
gate at high speed. Many
applications don't need high
speed switching, however.
A 555 timer works quite well
as a driver. Be sure to use a
bipolar part (NE555) rather than
a CMOS part
Photos 1 and 2).
There are also
many discrete transistor circuits you
can use. They can
be single transistor
drivers as in Figure
3. The drawback
with this design is
that the pullup resistor limits the current
■ FIGURE 7. Using an open-collector
so the turn on speed
design eliminates pullup resistor
and gate resistor. Switching speed is slower than the
is much faster than Figure 6. Turn-on
turn off speed. A
time is about 4 µs.
totem pole design
(Figure 4) can be very effective. You can also use a
NPN/PNP design to eliminate the need for the inverter.
Lastly, there are special chips usually called "low-side
gate drivers" that provide a high current for very fast
switching. If you feel the need for speed, this is probably
the way to go. They cost a buck or so but eliminate the
practical problems of circuit design and testing. Naturally,
there are high-side gate drivers, half bridge gate drivers,
and full bridge gate drivers, as well. Table 1 provides a
summary of typical drive speeds as measured at the
load rather than at the gate. (Note that the gate driver
measurement of 175 ns is suspiciously slow. Perhaps
that was due to my simple test jig. Measurements of
the voltage doubler circuit showed symmetrical 50 ns
switching speeds. It's rated at 15 ns/1,000 pF.)
High Side Driving
■ PHOTO 1. A bipolar 555 timer
(Texas Instruments NE555) turns the
MOSFET on and off in about 50 ns.
52 January 2009
High side driving of an N-channel part can be tricky (see
Figure 2). The gate has to be about eight volts above the
source voltage to turn it on. However, because of the very
low resistance when it is on, there is very little voltage
drop between the drain and source. Thus, the source pin
voltage is often very close to VCC. So, to turn on the
device you may need a gate voltage greater than VCC.
There are some ways around this problem. The first
is to build a voltage multiplier. Obviously, this is not an
elegant solution. A P-channel part
might be the easy solution here
despite the higher cost and poorer
performance. Figure 5 shows the
typical hook-up. Note that the
source is connected to the positive
voltage. In this configuration, the
P-channel device will turn on with
a gate voltage eight volts below
the source pin. So, if VCC/source
is 10 volts, the part will start to
conduct when the gate drops to
about seven volts and will be fully