I have used this circuit for years 3843 fed from the output with a
bootstrap startup circuit its input voltage span is from 4v to just
under 400v gives 24v output at about 30W the only thing you have to
remember is that the fet carries both the primary and secondary current
so must be sized for high Vds and a very low on resistance so it doesn't
get too hot when its stepping up from 4-5v. basic circuit (very poor
ascii art

------------L1------------C1-----D1>|------- Vout
                    |          |         |
                    D          L2        |
                   G           |         |
                    S          |        C2
                    |          |         |
                    R1         |         |
                    |          |         |
--------------------------------------------- 0v

L1=L2 can be almost any value we use 680uh ETD25 3f3 core
FET IRF740 AFI
R1 0r33
C1 mkp22 0.22 / 400v
D1 D4L40U
C2 220/35 LXFVB

Input cap (not shown 47/400 and .22 mkp22
Set the 3843 to operate at about 100khz and feed it from the output
(look up UC38xx there are 4 versions there is a low voltage start (8v)
and a higher voltage one (12v) and use the one that works with your
output voltage if there is a very high voltage input >150 the start
circuit needs special attention chip needs about 1mA to get it going
then it self powers from the output

Regards Steve...

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of John N. Power
Sent: 16 February 2004 20:14
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [EE]: Boost & Buck regulator in one?

> From:         Peter Mcalpine[SMTP:petemca1@BIGPOND.NET.AU]
> Sent:         Sunday, February 15, 2004 10:29 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE]: Boost & Buck regulator in one?

> Hi All,
> Might be stumbling down the wrong track, but worth investigating
> anyway...

> Can anyone recommend a switch mode regulator that will do boost
> and buck regulation?

> Basic usage is for a system that runs in a car, but requires
> backup battery. It also requires good 3.6V regulation.
> Current consumption at 3.6V is around 200ma max. backup battery
> is presently 3xAAA NiMh 650mA

> My idea is the battery can also be feed into the input of
> the switch mode so to maintain good regulation..

> Any help much appreciated as always :)

> Regards,
> Peter Mcalpine
> Sydney, Australia

A circuit that will do exactly what you want was published in the
October 2003 issue of "Nuts & Volts" magazine on page 33. It
uses a novel circuit topology called SEPIC (Single Ended Primary
Inductance Converter). To quote from the article: "The SEPIC
regulator permits both higher and lower voltages than Vin through
a unique arrangement of two coupled inductors." The last phrase
means "transformer".
A sample circuit is given. It is based on an LT1370 chip, and is
designed to convert the output of two Li-ion cells to 5 volts.
The one drawback is the transformer. The circuit calls for a
"BH Electronics 501-0725" but no further information is given.
If you are in a position to wind your own switching transformer,
this would be an interesting circuit to experiment with.

John Power

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