I've recently entered the wonderful world of power electronics design and I'm a bit overwhelmed with a design I'm working on.

I would like to step-up a 3.3v supply to approximately 100v. I have some wiggle room on the exact voltage, and what seems to be an above average voltage ripple (say ~0.5v) is acceptable for this project. The output current requirements are low, at approximately 2mA. The design will be laid out on a PCB, where cost requirements and design size are flexible.

My question is what is the right path for me to be begin designing this circuit and what is feasible?
1) A single boost converter seems incapable of providing the step-up ratio. Using the equation D = 1 - (Vin * η)/(Vout), where η is the efficiency of the converter (I'll guess 0.9 as a reasonable value) and D is the duty cycle, D would have to be 97.03%. To the best of my understanding, this is an impossible duty cycle to achieve. Am I correct in this? Additionally, is 0.9 for η a reasonable assumption?

2) Using two boost converters in series, perhaps the first stepping up to 16v and the second from 16v to 100v. This gives duty cycles of 81.43% and 85.6%, respectively. Are two boost converters placed in series a good design? Feasible? Possible? If so, any recommendations on a regulator?

3) Using a flyback converter. I am most baffled here, as with all of the searching I have done I can't seem to find any reliable examples of stepping up a low voltage to a much higher one (e.g., 3.3v -> ~100v). To the best of my understanding, a step-up transformer with an appropriate turn ratio (approx 1:30) should be able to achieve this conversion. However, when using the TI WEBENCH tool, it doesn't seem to have any solutions to 3.3v -> 100v, usually stating that Vin is too low for the part. Are there concerns here that I'm missing? What would be a good place to start with this?

Any and all advice and help would be much appreciated! Thanks in advance.

 

This is an example of one way of doing it:

I chose the Linear Tech part because it was available in LTspice. Other switching regulators will work but they must be able to run on 3 volts in and have to have a switch voltage of at leat 65 volts.
The "trick" to using a 65 volt part is having a voltage doubler on the output.
The diodes should be rated for at least 100 volts and the caps for at least 150 volts. The caps should be low ESR, high frequency types such as ceramic.

 

Flyback is the way to go. Vout~=Vin * Ton/Toff * N. Easy to control if you require a regulated output.