I'm trying to figure out the best method to get around 200V @ 15mA from a LiPo battery, and had a few questions.
- Should I be converting down to some voltage (eg. my microcontroller supply rail) with a buck converter and then boosting up?
- Should I go with a boost converter or a flyback converter?
- What good off the shelf ICs exist for this type of design?
- What kind of losses would I expect in conversion?
My application is a wearable device, and these are the design concerns I'm juggling:
- Minimize power dissipation from losses (do I need heatsinking? assuming 75% efficiency, I'd have to drop 750mW somewhere)
- Needs to be small form factor (how much will this decrease efficiency?)
- How should I implement short circuit protection/current limiting?
Sorry if I missed anything and thanks for the help.
Best Answer
You are talking a big boost ratio.
Orthodox thinking would suggest flyback BUT that means a transformer that you will probably have to wind/design yourself.
One way to handle big boost ratios is to use a diode pump on the end of a boost convertor. The diode pump approach is good for high frequencies and low currents which is where you are.
The number of stages in the diode pump can be your choice. More stages makes normal fixed frequency, hard switched current mode perform efficiently, but it uses more pcb realestate and raises component count and reduces the peak drain volts on the N channel switching FET, which makes a low on resistance device easier and cheaper.
OK go for 4 stages if you must use the chip out of the book. 2 stages would work on a S TRAP or other switching loss reduction scheme. A big advantage of the diode pump approach is that you can use a cheap SMD off the shelf coil.
If you need 90% efficiency at a normal operating frequency you will have to start dealing to switching losses. Current limit is intrinsic on any cycle by cycle scheme.
Speaking from experience the boost diode pump is much more efficient than flyback.