Again, thanks all for good commenting.
At that kind of power rating you would be a lot better off considering an interleaved boost converter topology based PFC.
You would be putting that single switch through a lot of stress. By reducing the power per magnetic component you can probably make them a slightly more realistic size for a lower switching frequency.
Yes, my thoughts as well. In my previous post i worried about the method of interleaving. For a 3 phase supply you would have a natural 3 way interleaving but that does not work for 1 phase supply, so the interleaving must be "artificially" phased by the controller. That will be interesting - maybe a 3 phase inverter control scheme can be adapted to serve here. I will look into that.
Interleaving is to have the PFC subunits each switching not at the same time as the other, if you have 2 then as the one is starting a pulse the other is ending a pulse. With 3 then you have a 6x high frequency master clock divided by 3 and then divided by 2 to feed each converter, so that the 3 clocks are 120 degrees apart, so that each subunit handles only 1/3 of the full power, but all sum together draw and output wise to give correction. Capacitor ripple current then is 1/3, and at 3 times the switching rate, with a lowering of the need for current handling in it.
Yes, i guess i'll have to accept the requirement to create a sync circuit like this. Fortunately 3 phase power and the associated control strategies is familiar territory so as i noted above, this will be my plan A at this time.
I have no experience with interleaved PFC I just ran across it while researching my own PFC. It seems to be doable though how precisely I’m not sure. If I ever need a multi kW pfc it may be worth looking into.
You create a sequencer with evenly spaced phase progression; as many phases as your interleaving scheme calls for. A master controller then governs the switch D for all phases same as in a single phase system. This far it is easy, but the devil will be found in the details, as always.
www.ti.com/download/trng/docs/seminar/Topic5MO.pdf
For me going from 65kHz to 133kHz was mainly necessity to use the magnetics on hand it was the best power to size ratio for what I have. This doubles the driver switching losses naturally for the same fet but still only about 140mW, of course you will be needing some larger fets (possibly paralleled) probably 2 to 3 times the gate charge as mine. Actual losses from commutation I didn’t really evaluate (just thermocoupled the cases) but the use of SIC should greatly reduce fet switching losses as the fet doesn’t have to eat the reverse recovery current.
I'll have to do the math to see if it is worth going to 133 kHz if i end up interleaving the switches. ANd then there is the question if suitable controllers are to be found in the first place.
The main difficulty for me is the magnetics. Operating at 133kHz means for full skin penetration I need to use AWG about 26. Given the RMS current 4.7A 120Hz and then about 1.6A pk-pk 133kHz ripple this is a little impractical, litz is to expensive for a one off. I’m operating under the assumption that if the HF ripple current is kept relatively small and I wind 24AWG in bundles so as to provide a wide surface area for the HF current it should minimize the losses. The other thing I’m going to be trying is spacing my windings above the gap to keep them out of the high intensity fringing flux (1.5mm gap) this is a high loss area (hot spot). Of course when you do this it causes more stray flux but hopefully a bellyband will contain most of it.
Yep, exact same questions i am working with right now. It is starting to look like the higher frequency magnetics is not going to be worth the extra effort. I even looked into hybrid litz-solid wire winding strategies but that will be impractical in real life. Can't be bothered with such tricks. Fringing fields are turning out to be a pain, i still haven't fixed the core geometry to my satisfaction. And this phased booster of course will take everything back to the starting line. But it wouldn't be fun if it was easy, eh?
The original coil craft core was designed for 300W 133kHz. I was running it at 65kHz but reduced output Voltage but deep CCM winding losses dominate so really the inductor should have been operating cooler. The appnote for that inductor shows temperature rise of 105C for the inductor windings at 300W 133kHz. I think the coilcraft inductor has too few parallel windings and possibly excessive gap.
Just the reasons why i won't be looking for ready made magnetics for this.