I'm attempting to design a ~200V, relatively low-current (10mA expected, I'm allowing a bit more though) power supply for some Nixie tubes, and I've decided on using an MC34063 step-up converter. It is my understanding that to reach >40V some kind of external switch is necessary. By looking at specs and at several other designs like this, I have chosen the following components:
I should add that I am relatively inexperienced in this area, and I'm trying to use this as a learning opportunity.
Looking at the formulae on the MC34063 datasheet, the very first calculation for ton / toff (related to duty cycle) is:
V + V - V OUT F IN (min) --------------------- V - V IN (min) sat
Assuming these calculations are even valid for an external-switch configuration, I'm stuck with how to determine Vsat.
If the saturation voltage from the datasheet (1V typical, 1.3V max) adds with the gate-source threshold voltage of the IRF720 (2V min, 4V max), the worst case scenario well exceeds my minimum input voltage (4.75V, I'm allowing 5V ± 5%). This immediately smells like it's not going to work.
Upon further examination, all the designs I've been referencing have either a higher input voltage (~12V) or use another external transistor to drive the MOSFET.
Does anyone have any advice on how to choose a more suitable external switch, or how to calculate these parameters correctly for an external switch configuration?