I'm wanting to research the theory of operation for this type of voltage booster circuit, does this design have a particular name? Early days for me, don't yet understand difference between buck boost, flyback etc. Any help appreciated.

This particular circuit is from an old home computer that only has a single line +9V (unregulated) supply, but also requires +12V to power a couple of video ICs. This circuit takes the +9 and provides +12.

Circuit diagram


It's a non-isolating self-oscillating flyback converter.

Pin 5 from the transformer provides positive feedback to make TR106 oscillate.

When the transistor stops conducting Pin 3 goes positive, D101 conducts and charges the output capacitor, C116. L104 and the following capacitors filter the output.

When the output voltage reaches the set point (12v), zener diode ZD101 starts conducting causing TR107 to divert base current from TR106 and either stop or reduce the level of oscillation.

It is surprising that a transformer would be used for such a small step-up.

A non-isolating booster would normally use just an inductor in the collector of the main switch transistor (TR106). It is not so easy to make it self-oscillating though and would require a different arrangement.

Alternatively with the same circuit D101 could be fed from the collector of TR106 and reduce the number of windings on the transformer.

One advantage of a separate winding for the output is that in the case of a short circuit on the output the circuit stops oscillating and the fault current is low. With the conventional step-up the short circuit current can flow directly from the incoming power supply (9V in this case) through the diode to the short.

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  • \$\begingroup\$ Thanks so much, Kevin, great stuff. Re your comment about transformer, is there an alternative way of stepping up from 9 to 12 that is simpler? Ta! \$\endgroup\$ – Chalky Sep 17 '17 at 19:02
  • \$\begingroup\$ Thanks, saw your edit, much appreciated. I'm off to look at a faulty one, will try a repair first, otherwise I might consider replacing with a modern DC-DC booster IC. \$\endgroup\$ – Chalky Sep 18 '17 at 0:12
  • \$\begingroup\$ Follow-up: I found +9V at base of TR106, but also +9V at collector. Ah ha. With emitter at earth, I should see collector pulled down. Replaced TR106. Fixed. Thanks again Kevin, your fast response and clear explanation of the circuit operation enabled me to fix a lovely piece of gear (Sega SC-3000H), which I'm donating to another enthusiast. \$\endgroup\$ – Chalky Sep 18 '17 at 11:56

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