I have recently finished a similar design. We are now at the pre-production phase, having already validated the first prototypes, which operate perfectly under similar constraints as the ones you describe in your question. Because of IPR limitations, I can only give you some general guidelines.
Based on the following assumptions:
- You are trying to avoid the use of a transformer by all means (which may not be the case),
- The power requirements of the load is in the miliwatt range,
- You don't want to step-up the voltage supply first, but will use the 2VDC directly for powering your flyback design block.
- You need to keep the cost and PCB space to the absolute minimum.
I suggest the following approach:
- Rule out the MOSFET and use a fast switching high voltage BJT (rated at least to 200V, better yet to 300V).
- Select the most appropriate inductor for your constraints and depending on your requirements for maximum power delivered to the load.
- Calculate the maximum flyback voltage, and make sure you can get at more than 200V voltage flyback "peaks". The involved parameters here are: ILpk (inductor peak current), Cts (total sum of parasitic capacitances at the flyback node) and SWITCHING SPEED of the BJT.
- The last one is hugely important, and rarely mentioned in the technical literature. You may have a big ILpk and very little Cts, but if your switching (off) speed is not fast enough, the flyback voltage will suffer. Reminder: Vflyback = -L * dI/dt.
The above can be implemented with:
- A simple (single) inductor or
- A 1:1 coupled inductor.
Apparently, from the design equations point of view, there seems to be no need for the 1:1 coupled inductor, as the single inductor will work exactly as well, most probably with less resonance apparatus. However, using a 1:1 coupled inductor will help avoiding EMI generation. I suggest the use the 1:1 coupled inductor in case you have to EMI certify your final product.
** However the previous assumptions, should you want to explore the flyback transformer approach, there now exist very nice miniature (micro-power) SMD flyback transformers. **
For instance, this Coilcraft model allows up to 1:100 turns ratio with 300Vrms isolation among primary and secondary coils: