single cell dc/dc converter PCB BC327 PNP

this is a dc/dc converter PCB I designed a few years ago. One transistor, two coils, and some passive components. It is adjustable.


dcdc2 (82K)

prototype photo actual PCBs are infrared soldered.

New revision of the dc/dc converter PCB!

It has been tested from 1.2 volts to 5 volts input range.
Small PIC circuits can be powered directly from 1.2 volts battery with no further regulation.

The circuit also can generate higher voltages for instace small 12v currents, from 2 batteries.
I have successfully built it into a RS232 based PIC programmer (Vpp=12 volts)

It can power blue and white LEDs…or these 6v bubble LEDs which can not work correctly at 5 volts!

Here the SCHEMATIC!!!

dcdc_schematic (1K)

The coils are in the 10s uH range, while the oscillator coil typically should have
1/2 of the value of the storage coil.
capacitors are some nF, and parallel resistor some K Ohm.
C3 is not a capacitor- it is meant to be to give PCB pads for alternative diode!
So you should not built in this capacitor also the LED is not needed.
Storage capacitor is 1uF typically. If you use larger value, it could charge up,
and then discharge into your circuit at startup! Typically most PIC circuits
have a small capacitor on their own, so nothing harmful can happen.

Many PNP type transitors can be used, and virtually any diode.

I found that MPSA-55 gives slighly better results than the BC 327-25.
Also the 2N3906 can be used.

Without a parallel resistor the circuit won’t start up, however, in order to reduce
quitescent current, also the resistor can be increased, typically 20 or 30k are
possible for very low currents.
An extra trick is to add a very tiny (27pF) capacitor from the output capacitor
towards the supply ground- quitescent current can even be increased further!

This circuit is not so much effective as some chips however at low currents,
the differences are not that big, as also chips tend to have a low effiency at
low currents (some mA).

If you need 30mA, 50mA or more, at high efficieny, you should consider a specialized
dc/dc converter chip such as the MCP 1640 from Microchip!
It is a good IC, small SOT-23, and it can provide 3.3 volts or 5 volts from 1.2 volts!
The chip is just a little more choosy about inductors, while in the datasheet they write
it must be 4.7 uH, I built it with 22 uH, works, while 2.2 uH does not start up correctly.

Handheld devices most likely will use an IC not a discrete transistor circuit- however,
there is the case where you maybe need 12 volts from 2 batteries! And this circuit can do it.
It is not so much a voltage source, rather it provides a range of voltages, and it adapts
according to the load. If for instance LEDs light up, the voltage drops down slightly!
So it is pretty much current limited and somehow short-circuit proof.

here a demo video: