Stupid Boost Converter

Switching power supplies in general are thought to be likely to explode. This is true. They are also thought to be hugely complicated and kinda magic. This is not true. In fact, it's possible to build a switching power supply that is stupidly simple, and simply stupid.

I needed a cheap and easy way to convert 3.6V from lithium-ion cellphone batteries to 5V to drive some logic chips, and didn't have any cool all-in-one-switching-power-supply chips from National Semiconductor this time. I did have leftover parts, a resistor kit, and the kind of determination that can only be called "pigheaded".

The resulting hack will convert about 100ma of 3.6V DC power to about 50-60ma of current at 5V. There's no way to boost the current much more without replacing the 555 and probably the TIP120 with faster components.

The 555 is wired up to make very narrow negative pulses at 60KHz, which are converted to very narrow positive pulses by the 2n2222*, which rapidly switch the big TIP120 transistor on and off. The TIP120, when on, sucks power straight from +3.6V to ground through the coil. This is only not a dead short because coils take a little time to start conducting.

...And once they start letting power through, they KEEP power moving until something makes it stop. Which means, when the TIP120 switches off, the inductor keeps going for a bit and shoves power through the diode into the huge capacitor on the other side. This is where the voltage boost happens, the inductor gives a higher voltage than was put into it -- at less current. No free lunch.

This is stupid because it's inefficient -- the zener diode on the output wastes power to regulate it to 5V. A smarter design would just slow down the oscillator, but I was lazy.

A few final notes: The MBR745 is just a fancy really-efficient diode. Any "shottky" diode work if it can handle the power. That unlabelled resistor in front of the TIP120 is 10K. The inductor is actually 20uH, not 1.

* Just about any small NPN transistor will work.

Copyright(C) Tyler Montbriand, 2004. Full permission is given to use these schematics and plans for personal use. For commercial use or publishing, contact the author. Send comments, etc to Tyler Montbriand,