Since I didn't want to continuously run 20 amps
while testing and for photography, I used a quick and
dirty method of turning off the circuit. I simply used a
transistor to pull the MOSFET gate to ground. This worked,
but opened the feedback loop. As a result, when the
transistor was turned off and normal linear operation
was restored, there was considerable ringing as the loop
re-stabilized. (If you don't want the disable control, you
can eliminate R9 and Q2, and replace R6 with a wire.)
Photo 6 shows the circuit in operation. It is providing
just about two volts into a 0.1 ohm load using a 14. 4
volt portable tool battery. Thus, the current is about 20
amps. Not bad for a handful of components. In this 5%
duty-cycle application, the heatsink was barely warm.
Conclusion
Power MOSFETS can control a lot of power easily.
An H-bridge voltage doubler circuit was seen to be
suitable for this design, as well as other applications.
An analog constant current project was able to easily
provide 20 amps into a 0.1 ohm load. A nice feature
of power MOSFETS is that if even higher currents are
needed, they can be paralleled very easily. In short,
power MOSFETs are an inexpensive and easy way of
controlling significant power.
NV
Switching Capacitor Ratings
It is extremely important to pay close attention to
capacitor ratings in power switching circuits. These designs
place a severe strain on the capacitor and it must be rated for
this type of operation. Using the wrong type of capacitor can
result in the capacitor exploding. Often, switching capacitors
are chosen because of these other ratings rather than its
capacitance. The two most important ratings to consider are:
ESR and ripple current (typically at a switching frequency of
100 kHz). The ESR (Effective Series Resistance) says how much
resistance the capacitor has. Ideally, a charge on one capacitor
plate should freely interact with the other plate. But, capacitors
1 to 0.01 ohms. Everything else being equal, usually the larger
the capacitor value is, the lower the ESR will be. So, if you are
driving a heavy load (say a two ohm loudspeaker), a one ohm
ESR will waste a lot of power and heat up the capacitor.
The second rating is the ripple current. This says how
much RMS current can safely pass through the capacitor
continuously. Again, typically, the larger the capacitor value,
the higher current it can handle.
These capacitors usually fail because heat degrades the
capacitor and increases the ESR. This causes a larger voltage
drop across the capacitor and additional heating. Eventually,
the capacitor gets too hot and bursts. This bursting can be
mild or explosive. Most capacitors have relief points that
reduce the chances of a violent explosion, but care should
always be taken. Always house these types of circuits in a
sturdy cabinet.
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February 2009 59
