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:

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)ΧUin 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:

 

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 toroοd (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)

 

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