■ PHOTO 5. The breadboard of the
constant current supply. Since the
MOSFET is acting as a variable
resistor, considerable heat is
generated. A heatsink is required.
■ PHOTO 4. The breadboard of the voltage doubler
(without filter). This is placed in a sturdy enclosure
for safety (not shown, see text). Use short wires
because the switching speeds are high.
and inductors (if used) can be
smaller, though, which saves
money and space. Additionally,
the higher the operating frequency,
the greater the likelihood of EMI
Clearly, when you are switching
kilowatts of power, you have to be
concerned with unintentional RF
(Radio Frequency) emissions. The
square waves — so essential for
efficiency — contain loads of higher frequency harmonics.
The Voltage Doubler Details
reasonable results (considering the input was a
square wave). Obviously, be sure your transformer is
rated properly for current and voltage. Output filtering is
required and is dependent on the frequency, transformer,
A motor can be directly connected to the H-bridge.
For optimal performance, you will want to be able to
independently control the legs of the bridge. This cannot
be done with a fixed oscillator. Instead, the Q and /Q
signals should be adjustable. By changing the phase and
pulse width, the motor's direction and speed can be
controlled with precision. As noted above, if you want to
use this H-bridge as a motor controller you should pay
close attention to possible voltage and current spikes.
You can always use the H-bridge as an audio amplifier
by attaching a loudspeaker to the output. This would be a
"Class D" design where pulses are applied to the speaker.
Again, the pulses would have to be precisely controlled,
typically by a microcontroller. The high frequency pulses
have to be filtered out to provide a low distortion audio
signal. For good audio, the switching frequency should
probably be increased to about 100 kHz. A Class D
amplifier using this H-bridge has the potential of providing
kilowatts of audio power (see the
sidebar on Bridge Power).
Lastly, the circuit shown here
operates at a nominal 35 kHz. By
changing the timing capacitor in
the 555 circuit, other frequencies
can be obtained. There is a
trade-off, however. The higher
the frequency, the greater the
switching losses. The capacitors
■ PHOTO 6. With a load of 0.1
ohms, this two volt signal means
that 20 amps are flowing. The
ringing is because the feedback
loop was opened (see text).
The design concept was to provide a simple voltage
doubler for basic DC power supplies. In particular, I had a
0-30 volt, three amp supply. There are occasions when I
needed a higher output voltage. Making the H-bridge
voltage doubler is simplicity itself. Just connect two
capacitors and a bridge rectifier as shown in Figure 3.
The output will be a pretty good DC signal as shown in
Photo 2. Only 165 ns switching glitches are present
(photograph 3). These glitches can be reduced to about
200 mV with the optional output filter.
Note that these glitches can be significantly reduced
or even eliminated by fine-tuning the switch timing. Some
driver chips allow this. Alternatively, you could use a
microcontroller or dedicated digital hardware for better
Photo 2 shows that there are 22 volts out of the
circuit with a 12 volt input. The circuit load was a 68 ohm
resistor which pulled about 1/3 of an amp and dissipated
over seven watts of heat. The MOSFETs weren't even
warm. However, the bridge rectifier got quite hot because
it was deliberately under-rated (will discuss shortly).
Photo 4 shows the breadboard of the voltage doubler.
It's important to keep things as
close together as practical. Long
leads with high frequencies can
result in poor performance. Note
that heatsinks for the MOSFETs
are not needed for this typical
operation of a few amps. I did not
include the fuse because I will
always be using it with a current-limited power supply. If you do not
use such a supply, you should use
a slow-blow fuse about 300%
above the highest current output
you expect. For example, if you
power a device that draws one