Part 2: Projects
The H-bridge
The voltage doubler is based on a standard H-bridge
design, shown in Figure 1. Basically, an H-bridge
functions as a DPDT switch which reverses the polarity to
a load. There are many applications for this circuit such as
switching power supplies and motor control. Without
getting into the details, switching at high speed gives good
motor control and very efficient power supplies. The high
frequency allows the use of smaller inductors and capacitors
which can significantly reduce size, cost, and weight.
Our H-bridge is shown in Figure 2. It consists of
several simple building blocks. The first is a regulated
power supply (U1 and associated
■ PHOTO 1. I made
a DIP adapter for the
components). This is needed
surface mount drivers because other circuits may not be
from a "component
able to function over the full input
carrier." The large
solder blobs on
range of the circuit. A three-
the carrier pins act terminal regulator is chosen to
as a heatsink when
provide a stable 10. 5 volts. Any
soldering to the
voltage from 10 to 12 volts is fine
driver chip. Once one
pin is soldered, the for the low-power circuits.
rest go fairly easily.
54
February 2009
by Gerard Fonte
Generally, you want to keep it over eight volts for proper
boot-strap operation (see Part 1). The other low voltage
parts like to see something at 15 volts or less. A fixed
12 volt regulator (LM7812) can also be used here.
The second part is the master clock (U2 and associated
parts) and is a simple 555 oscillator with a twist. As
shown, it provides a very precise 50% duty cycle with a
frequency of about 35 kHz. The twist is that the output
pin is NOT used for the charging and discharging of the
timing capacitor, as is the case for most other 50% duty
cycle designs. This makes the circuit completely insensitive
to output loading. Additionally, those other "50%" designs
are more typically 45%-55%, or worse. Note that I verified
proper operation with bipolar, CMOS, and low power
555 timer versions. A 50% duty cycle is not an absolute
requirement for this design, but it distributes the power
evenly through both halves of the bridge.
A DPDT switch makes and breaks contacts simultaneously (in theory). Since we are using N-channel parts for
both the high side and low side, we will have to use a two
phase clock. That is, both a "clock" signal and an "inverted
clock" signal must be available. A CMOS 4069 inverter (U3)
is used to invert the 555 signal. I chose to use separate
inverters because they were available. You could
run the Q signals directly from the 555 and the /Q
inverted signals from a single inverter if desired.
As noted in Part 1, getting the power
MOSFETs to switch quickly is not a trivial task.
For that reason, I used special driver chips for this
part of the circuit (U4 and U5). The LM5109 are
inexpensive and easy to use. They provide a peak
■ FIGURE 1. Conceptually, an H-bridge is just a
double-pole, double-throw switch that reverses
the polarity of the power to the load.
