Six-channel Analog Pulse Width/Frequency Power Modulator
(Anticipated) Frequently-asked Questions (FAQ)
- You must be kidding! A dozen 555 timers?
I think I'm going to be sick!
- Although despised by many,
these chips are cheap, easy to find, rugged, reliable, flexible,
stable over wide ranges of temperatures and voltages,
fairly easy to work with,
and extremely well-documented on the web.
- How do you choose between using pulse-width modulation (PWM) and pulse-frequency modulation (PFM)?
- I would use pulse-width modulation (PWM) for most applications with the power fed
more-or-less directly to the device with little conditioning.
I would choose PWM for those devices with relatively narrow power frequency specifications.
Other things sensitive to the frequency at which they are driven,
such as muffin fans with resonant frequencies to avoid,
are likely to respond better to PWM at a fixed frequency.
- I would choose pulse-frequency modulation (PFM) when the output is heavily filtered,
nearly direct current (DC), before it gets to the device.
- What about a digital power modulator?
- I had originally looked into doing this digitally,
but the ICs needed (4-bit or better counters, accumulators, and comparators)
are increasingly difficult to find.
Resources permitting, I would be happy to program a CPLD or FPGA as the controller if someone can point me to a
generic open circuit design I may use to take care of uploading the device configuration.
Also, although I imagine using digital (quadrature) thumbwheels
to manually control frequency and pulse width,
I suppose one or more digital interfaces should also be included,
and this means choosing and programming
a protocol or three: CAN, some kind of RS-232 protocol, USB, etc.
- I can almost understand the 10Ω resistors, but One-Ohm resistors?
That's just silly.
- Hey, it's not my idea.
I used the schematic of Intersil's evaluation board as a guide to laying out the gate driver circuitry.
The relevant portion is on page 13 of their application note.
I presume these are here to keep the traces from ringing
as they are hit by relatively high-amperage impulses.
Without this damping it might be possible to find just the right/wrong
frequency to drive one or more traces at a resonance frequency
and emit loads of electromagnetic radiation or otherwise overjuice the circuit somehow.
This is not likely to be an issue unless you are pushing the pulse frequency up fairly high,
so you may be able to simply substitute plain wire for these low-value resistors in many applications.
You can also try more common low-value resistors if you are working with lower frequencies
and your MOSFETs don't mind a bit less gate current.
Or, you could try making some calibrated bad solder joints.
©2006-2007 David E. Wagner II.