On Thu, 28 May 1998 21:04:55 -0400 NCS Products <ncs@WORLDNET.ATT.NET>
writes:
>I need to double a 12VDC input to get 24VDC at .5 - 1A.
>
>This can be done, I'm sure, but how?
>I think the basic concept is, you charge two capacitors in parallel,
>then discharge in series.
>
>What type of switches (MOSFETS?) do I need, and how many.
>Any pointers to analog info would be much appreciated.
>

        For this high a current, I'd suggest a "boost" converter.  Have a
look at chips from Maxim or National (and others) just for this purpose.
The boost converter is nice because you can use an N-channel FET with the
source grounded.  Incoming DC goes through an inductor.  The output of
the inductor goes through a diode to a capacitor to ground.  Output is
across the capacitor.  If you have just this circuit, the ouput is about
the same as the input (less diode drop and inductor DC resistance drop).
Now, the FET shorts the "output" side of the inductor to ground.  The
current in the inductor ramps up linearly.  Open the FET and the current
through the inductor remains the same, but now goes through the diode
into the capacitor and the load.  The inductor current then ramps down.
The net effect is to increase the capacitor voltage.  Feedback from the
output voltage adjusts the duty cycle of the FET short to ground to
adjust the output voltage.
        The "charge pump" voltage boosters (or inverters) using
capacitors without inductors seem fine for relatively low current levels
(such as for driving RS232 drivers).  To get much more current, the
capacitors get a little large.

Harold


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