A lot of calculators out there suggest values, but for higher voltages, these circuits fail.
When generating high voltages (step-up, boost converter) the calculated duty cycle is typically (much) larger than 85.7 %. With a "standard
configuration", the necessary duty-cycle may never be reached. An extension, as described in
Application Note 920
by Onsemi is necessary.
This is often forgotten to mention and/or the presented circuit gives no clue either.
This website shall deal with especially this aspect of forcing higher duty-cycles by increasing the dischargeing current of the timing capacitor -
and therefore reach higher voltages :-)
The Application Note calls this "Ratio Extender Circuit
". - Wow !
The circuit consists of a diode and a transistor. Both are originally germanium types, but a schottky diode and a silicium pnp transistor also do the job.
(We tested 1N34A, AA113 and 1N5711 and all worked fine.) The function of the transistor is to increase the current when dischargeing the capacitor.
With the shown con- figuration, the off-time could be lowered to a minimum of 750 ns. This corresponds to a duty cycle of ~ 99.5 %.
The left picture shows the output waveform at Pin 2 (Switch Emitter) with the "standard configuration", whilst the picture on the right side is taken with the
'Ratio Extender Circuit'. For both measurements, the Pin 5 (Comparator Inverting Input) was tied to GND. The load at Pin 2 was 100 Ω and the supply was 6 V.
The timing capacitor was a 10 nF MKS2.