1v5 to 12V aux power supply ...

Sometimes you just need a simple effective step-up converter as part of an auxiliary supply.

This can be tricky when the minimum input volts are 1.5 as in this case.

The classic push-pull shown above is an adaptation of the classic Royer converter, with extra transistors Q3, Q4 to limit the ON time of the main transistors thereby fixing the frequency - to 70kHz in this case.

Extra windings are added to the collector points and add voltage to the 3V on the collector at turn off to provide the extra 9 volts required to give 12V out in total,

the startup Vout profile into 240 ohms is shown here:

taking just over 400uS to reach 12V across the load.

Suitable npn transistors are available with low Vce-sat and are fast enough for efficient operation. The zeners clamp the Vce to 10V max and provide a path for reverse current which the main transistors should not really carry.

The 10k resistor, Rstart, to start the converter in LT-spice, would ideally be copied to the base of Q2 for completely balanced operation ( or taken to the mid point of L3 - tap required ).

The beauty of this converter is it's simplicity and ruggedness, low core losses, pulse load capability and the ability to scale up to higher voltages and currents if required.

Efficiency can be maximised by using suitable schottky diodes for D7, D8, and added across the zeners if so desired.

The main magnetic can be wound on a ferrite toroid, or small EE core with bobbin, the entire circuit suitable for very high volume mass production.

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