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'[OT] Doubling 12 volts...maybe use a 8-pin PIC!'
1998\05\28@210500 by

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.

I'm not an expert on this topic, but I've done similar voltage
doubling with a ICL7660 to double 5v to 10v, but with much
lower current (< 50ma).  Maybe the spec sheet for the 7660
would give you a starting point to think about as long as
it lists theory.

--Anil

> {Original Message removed}
-----Original Message-----
From: NCS Products <ncsWORLDNET.ATT.NET>
To: PICLISTMITVMA.MIT.EDU <PICLISTMITVMA.MIT.EDU>
Date: Thursday, May 28, 1998 8:10 PM
Subject: [OT] Doubling 12 volts...maybe use a 8-pin PIC!

>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.

This COULD work, but the heat dissipation in the switches would
be horrific.  What you want is a boost converter, VERY simple.

Look up a datasheet on the CS384X series from Cherry Semi.
(also made by Unitrode and others)

A chip, A fet, an inductor, a diode, a big cap, and a few small Rs and Cs.

I've done 60W designs solely with Digi-Key parts, no custom inductors.

On Thu, 28 May 1998 21:04:55 -0400 NCS Products <ncsWORLDNET.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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I remenber a 20 year old electronics magazine with a design for this
using only bipolar transistors, so MOSFETS are not necessarily
required.

Actually, the easiest way to do what you want is to charge a _single_
capacitor to 12V across the input supply, then switch both capacitor
poles so the -ve end is now connected to the +ve input supply, and the
+ve end of the cap is now at 24V.  Dump this voltage to another cap
and you have your 24V output.

Note that because of various circuit losses, at 1A you are unlikely to
get a full 24V output.

CIAO - Martin.

______________________________ Reply Separator _________________________________
Subject: [OT] Doubling 12 volts...maybe use a 8-pin PIC!
Author:  pic microcontroller discussion list <PICLISTMITVMA.MIT.EDU> at
Internet
Date:    5/28/98 9:04 PM

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.

>     Actually, the easiest way to do what you want is to charge a _single_
>     capacitor to 12V across the input supply, then switch both capacitor
>     poles so the -ve end is now connected to the +ve input supply, and the
>     +ve end of the cap is now at 24V.  Dump this voltage to another cap
>     and you have your 24V output.
>
>     Note that because of various circuit losses, at 1A you are unlikely to
>     get a full 24V output.

Because you are using a capacitor, the currents will be very high when the
cap is connected, and the resulting I^2R losses are rather high.

The output voltage can't get to 24 except at zero load.  If there's a
significant
load, then you get into a problem where you have to have a large ripple
in order to transfer any power. If the output cap only droops 1V, then the
max
charge you can transfer is .5 * C  (The V^2 term reduces to 1)

The C then has to be HUGE, and it has to be charged FAST, which means
lots of I, and the I^2R losses go through the roof.

An inductive boost switcher would be a lot more efficient. I've done a 60W
boost switcher from 16V - 32V in about 2sq in that isn't even hardly warm.

----------
> From: NCS Products <ncsWORLDNET.ATT.NET>
> To: PICLISTMITVMA.MIT.EDU
> Subject: [OT] Doubling 12 volts...maybe use a 8-pin PIC!
> Date: Friday, May 29, 1998 8:04 AM
>
> 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.

How about LM78S40? It is an universal switching regulator. It can be used
as step up, step down as well as inverter.
Max current is 1.5 Amps.
you may seach for 78s40 pdf file from National as well as it's application
note(how to design this chip).
goodluck

----------
> From: Anil K. Patel <anil.patelAUTODESK.COM>
> To: PICLISTMITVMA.MIT.EDU
> Subject: Re: [OT] Doubling 12 volts...maybe use a 8-pin PIC!
> Date: Thursday, May 28, 1998 8:09 PM
>
> I'm not an expert on this topic, but I've done similar voltage
> doubling with a ICL7660 to double 5v to 10v, but with much
> lower current (< 50ma).  Maybe the spec sheet for the 7660
> would give you a starting point to think about as long as
> it lists theory.
>
> --Anil
>
> > {Original Message removed}
----------
> From: Anil K. Patel <anil.patelAUTODESK.COM>
> To: PICLISTMITVMA.MIT.EDU
> Subject: Re: [OT] Doubling 12 volts...maybe use a 8-pin PIC!
> Date: Thursday, May 28, 1998 8:09 PM
>
> I'm not an expert on this topic, but I've done similar voltage
> doubling with a ICL7660 to double 5v to 10v, but with much
> lower current (< 50ma).  Maybe the spec sheet for the 7660
> would give you a starting point to think about as long as
> it lists theory.
>
> --Anil
>
> > {Original Message removed}

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