As there was a large amount of MC34064 in the lab (since the high voltage project :-) there arose the wish to have also a step down version with this versatile circuit. Of course it should be somehow different than those which already exist in the web. We therefore added an external switching transistor to be able to handle larger currents.

Happily, this device is that good-natured, that it will not stop completely if one component is slightly off the calculated value.

The addition of a switching transistor degrades the efficiency (right side), as the base driving current is lost due to the use of a pnp transistor. Choosing a npn type would have been the wiser choice. (For this circuit, we had in mind to go for higher input voltages). Both Power Supplies delivered + 12 V.



The input voltage was + 16 V (BLUE), + 22 V (RED) and + 28 V (GREEN).

Even if the needed current is well within the specification, the MC34063 may get really hot, wheras with the external switching transistor, it remains at room temperature.

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