A simple step-up converter (6V to 12V)


This step-up converter is intended for use in a
’67 Citroen
2CV. This car, and I use the word loosely, has a 6V battery and
won’t support a modern radio that needs 12V. The circuit described
here converts 6V to 12V at 1A sustained load current.

It works something like this:

L in series with (switch // C)

When the switch is closed an extra current flows
through the inductance and stores energy there. The capacitor
supplies the load with current during this time.

After the switch closes the capacitor is charged
by the energy stored in the inductance and an extra current starts
flowing through the load, causing the output voltage to rise
(energy is supplied directly from the input source also as long as
the diode is forward biased). During this time, the system behaves
like a RLC-circuit, so, after a while, the current decreases. The
switch is then closed again and the cycle repeats. One could say
that charge is pumped from input to output, increasing the output
voltage up to the point where there is an equilibrium between the
discharging of the capacitor while the switch is closed and the
charging by the inductor while the switch is open.

The output voltage equals (ton / toff
+ 1)×Uin and is controlled by PWM of the switching action.

To implement this, I have used the LM2577T-ADJ
from National Semiconductor. It
operates conform the given discription and is connected like so:

A simple step up converter (6V to 12V) #2

IC
Switcher/regulator
LM2577T-ADJ (National
Semiconductor)

R1 and R2
Voltage devider for monitoring
output voltage
20Kohms pot. (Bourns)

Cin
Decoupling
0.1µF, 63V MKS condensator
(WIMA)

L
Use a good quality coil!
160µH toroïd (2.5A, 70mohms,
nickel-iron core)

D
Current higher than output
current!
FR603 60V reverse breakdown, 3A
Schottky-diode

Rc and Cc
Pole-zero compensation network
2200ohms, 5% and 1µF, 63V elco
(Philips)

Cout
Get a low ESR type!
2200µF, 16V elco (Telecon)

A simple step up converter (6V to 12V) #3

You can download the PCB
design here (only 4Kb). It’s in CorelDraw 3.0 format (zip
compressed).

by Oscar den Uijl,
odu@xs4all.nl


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