on at two volts, or eight volts
below the source voltage.
If you are continuously
switching the load on and off in
less than a second or so, there is
another approach that can be used
with N-channel parts. This is called
bootstrapping and is shown in
Figure 6. This is a modified circuit
found in the Siliconix MOSPOWER
Applications Handbook. The
conceptual design of the circuit is
subtle but fairly simple. When the
transistor is on, the MOSFET's gate is pulled low and the
capacitor is charged to VCC ( 10 volts) through the
isolation diode. When the transistor is turned off, the gate
drive rises to VCC because of the charged capacitor
(mostly via R2 and R3). Since the MOSFET is off, the
source pin voltage is pulled low through the load. This
means that the gate voltage is well above the source pin
voltage and the MOSFET will start to conduct. As this
happens, the capacitor acts as a voltage source in series
with the source pin. So, any voltage on the source pin is
added through the capacitor to the gate. (The negative
side of the capacitor gets pushed up by the increasing
source pin voltage which pushes up the positive side by
an equal amount.) In effect, the part pulls the gate voltage
up by its own bootstraps.
Naturally, theory and practice are different. The
capacitor should be at least 10 times the gate capacitance.
In most cases, 0.1 µF will work. The diode is any power
type with an appropriate voltage rating. The resistors R1
and R2 are the tricky components.
The pullup resistor (R1) determines how big the
voltage increase is. This is because it connects the gate
drive to the five volt supply. Any voltage greater than five
volts will be pulled down to five volts through this resistor.
Note that this resistor may not always be visible. For
example, a 555 timer (connected to R2-R3) can supply
100 mA for an equivalent pullup resistance of about
50 ohms (at five volts). Obviously, a 10K resistor above
five volts by any significant amount.
As shown in Figure 6, the 10K value for R2 only
supplies 10 volts (referenced to ground) to the gate which
is not adequate if the source is also at 10 volts. If R2 is
increased to 100K ohms, over 17 volts is applied to
the gate which is probably okay for
most applications with the IRF540.
Note that the turn on time is also
controlled by R2 (the turn off time is
controlled by R3). Charging the 1,500
pF gate through 100K takes about 30
µs to turn on the device (measured).
So, you trade off speed versus
voltage. The general rule of thumb
is that R2 should be about 1/10 of
the equivalent pull up resistor, R1.
■ PHOTO 2. A CMOS 555 timer
(Texas Instruments TLC555) takes
about 800 ns to turn on the MOSFET
which is about 16 times longer than
the bipolar version. Turn off time is
about 50 ns.
Resistor R3 controls the turn
off time and is included mostly for
completeness. Oftentimes, a gate
series resistor is shown. It is not
necessary. As shown, the turn off
time is about 2 µs. If R3 is replaced
by a wire, the turn off time drops
to about 500 ns.
These problems can be eliminated by using an
open-collector transistor circuit which is shown in Figure 7.
In this case, there is no connection to the five volt supply
so there is no pulldown problem. This eliminates R1 and
allows the use of a much smaller resistor for R2. This
resistor is now chosen to limit the current into the
transistor to a safe level (100 mA as shown). Resistor R3
can also be eliminated. This circuit provides about
18 volts to the gate and switching time is about 4 µs
for turn on and 500 ns for turn off.
However, if you want to use a high-side N-channel
part you should really consider using a driver chip. The
LM5109B only costs about $1.60 and drives a high-side
and low-side MOSFET in a half-bridge configuration. It's
rated for 90 volts (to the MOSFET) and can turn them on
and off in 15 ns with 1,000 pF gate capacitance.
Considering the time and effort in designing your own
high-side driver, this is a good deal. There are lots of other
parts available, as well.
This month we've looked at power MOSFETs and
found that they have some very useful attributes. They're
cheap and powerful, and can be fairly easy to implement.
Naturally, there are practical considerations in addition to
Next time, we will build two projects. The first is a
transformerless, high current voltage doubler using a full-bridge design that has many other useful applications (like
motor control). The second is a linear, constant current
power supply capable of providing 20 amps or more. NV
■ TABLE 1. Low-side drive summary (times are measured at load).
Circuit Turn off time Turn on time Comments (10V D-S with 100 mA load)
TTL 'LS04 500 ns 100,000 ns Can't drive fully on. Logic-level parts available.
TTL/pullup 200 ns 3,000 nsS Speed limited by pullup 1K resistor. (74145)
CMOS 8,000 ns 1,000 ns Very slow but turns on all the way. Easy. (CD4069)
CMOS x 6 2,000 ns 400 ns Better than above, but still slow. (CD4069)
NE555 175 ns 60 ns Bipolar good, CMOS poor. (Photos 1 and 2)
Discreet 400 ns 2,500 ns Speed limited by 1K pullup resistor. (Figure 4)
Totem pole 175 ns 150 ns Very good. (Figure 5)
Driver 175 ns 50 ns Best speed (see text). (LM5109B)
January 2009 53