Jinx wrote...

>Hi all, could I have a little help please. I've got an application
>that amongst other things requires a variable voltage from
>10V to 20V. It's to charge a large cap for a solenoid pulser.
>I've got this voltager quadrupler circuit that I could drive with
>5V PWM from a PIC pin. The multiplier may not get to 20V,
>but 15V or more should hopefully be enough

>Questions -

>How do I select the capacitors for best efficiency at a
>particular frequency. Ripple is not especially important,
>neither is frequency ?

If you have a charge-pumping type of circuit like this, and the
voltages on the input capacitor, flying capacitor, and output
capacitor are all the same and are all nearly constant (e.g., as in an
ICL7660-type voltage inverter) you can get efficiencies approaching
100%.

But that's not the case here: you're discharging the solenoid
reservoir cap, then recharging it through this multiplier gadget
consisting of [I assume] capacitors and switching diodes.  The
voltages on all the caps are different, and are changing in time.  The
net effect is that as the multiplier string charges the reservoir cap,
it does so in a process that pretty much resembles an exponential RC
charging curve.  The PWM drive frequency, along with the size of the
multiplier capacitors, determines the effective "resistance" involved.

Bottom line is, the efficiency won't be anything to write home about.
But how fast you can replenish the reservoir cap will depend on the
drive frequency- the higher the frequency, the faster the charge
pumping- and the multiplier capacitor sizes.

>Will varying the M:S ratio of the signal produce a variable
>DC at the output of the multiplier ?

Nope, not unless you go so close to 0% or 100% duty cycle that the
pumping capacitors just don't have enough time to complete charge
transfer before the opposite half of the cycle starts.  50% duty cycle
is best, something close to 50% is usually adequate.

>Do I need a load R apart from the solenoid reservoir cap ?

Nope.  The voltage on the reservoir cap will asymptotically approach
the multiplier factor (4X?) times the peak-to-peak drive voltage,
minus diode drops.  It won't just keep rising, if that was your
concern.

>What possible down sides are there for the PIC ?

All I can see might be output pin loading.  This could be tricky, as
your circuit is going to tend to draw output pin current in short,
sharp spikes.  Offhand, without a data sheet in front of me, I can't
hazard a guess as to a safe limit.  Could you perchance use some kind
of buffer?  I've used a 74HC244 octal bus driver with all sections
paralleled to do this kind of thing (in a voltage doubler, at least)
and it did the job.

Hope this helps a bit...

DD

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