High power PFC - choice of topology and key components - Page 1
Forum section: electronics / projects

author: Kremmen
author_rank: Super Contributor
timestamp: November 09, 2012, 09:38:22 am
content: I am in the process of designing a ~2kW full bridge SMPS. Right now i am working on the PFC front end and one design goal is of course minimizing the dissipation of the switch components.I haven't locked any of the circuit parameters yet but kind of default assumption is that the rail would be 400VDC making the average current at nominal power to be 5 A + losses. With that in mind i have made some initial choices. The PFC topology would be boost as usual, and the switch maybe FCH74N60 or similar and rectifier a SiC Schottky diode, probably something like Infineon IDW10G65C5. So far so good.To reduce the voltage stress on the smoothing caps and ease component selection there, another possibility would be to lower the rail voltage to say 200 VDC. Now we get to the point; increasing the current will increase losses in the switches and my question is, if anyone has come across a) an optimized PFC topology for these power and voltage levels and b) does anyone have a clever suggestion for a synchronous rectifier circuit to replace the boost diode. I can come up with functional high side circuits myself but they all have the drawback of becoming a bit complex and cumbersome. So if anyone has done this with good results, i would be most interested to hear about it, or good ideas in general.

author: Kremmen
author_rank: Super Contributor
timestamp: November 09, 2012, 01:46:19 pm
content: Thanks AcHmed for a very useful reply.First: no, i am not really going for universal input - this is for EU single phase line power i.e. 230VAC 50Hz nominal.At these power levels only CCM mode in the PFC would make sense to me. Also, i will do all my own magnetics as well; it is too much of a pain in the ass to try and find suitable ready made components that will be slightly off anyway.For the 400 V rail i am definitely thinking SiC Schottky diodes, but just because of the increased losses for the lower voltage alternative, i wanted to ask about synchronous options for the rectifier. But it starts to look like i will go with the 400 volt rail anyway, probably makes life easier on the whole.I too have briefly thought about interleaving several boosters to reduce the loading of individual components. However, due to the nature of the thing i have not discovered how it is possible to separate the PFC phasing and thereby reduce the capacitor ripple. What i mean is, the input current is supposed to be in phase with the input voltage to maximize power factor; that is the reason of existence of a PFC. Now, since the supply is single phase, all "interleaved" sub-PFC's would have to actually be in phase, thereby defeating the interleaving idea. Or am i missing something here?I have oscillated between 65 and 133 kHz myself without firmly committing to either yet. The higher frequency would bring smaller magnetics but what experience do you have regarding relative switching losses and other tradeoffs between the two frequencies?

author: penfold
author_rank: Frequent Contributor
timestamp: November 09, 2012, 03:23:21 pm
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: November 09, 2012, 03:31:52 pm
content: 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.

author: Kremmen
author_rank: Super Contributor
timestamp: November 09, 2012, 06:04:27 pm
content: Again, thanks all for good commenting.Quote from: penfold on November 09, 2012, 03:23:21 pmYes, 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.Quote from: SeanB on November 09, 2012, 03:31:52 pmYes, 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.Quote from: AcHmed99 on November 09, 2012, 04:04:16 pmYou 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.QuoteI'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.QuoteYep, 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?QuoteJust the reasons why i won't be looking for ready made magnetics for this.

author: Kremmen
author_rank: Super Contributor
timestamp: November 09, 2012, 06:51:39 pm
content: Quote from: AcHmed99 on November 09, 2012, 06:32:21 pmThink cyclical. In power engineering everybody pictures the AC polyphase power as voltage and current vectors rotating in some coordinate system. Same thing here; your individual boosters are cyclically phased around the full 360 degrees ( or 2pi, if you think in radians). The only spacing that makes sense is an even one, otherwise you will have shorter and longer gaps between phases/pulses in the time domain. So for 3 phases the natural and only phasing is 120 degrees apart. For 2 phases, 180 degrees and for 4, 90 and so on and so on.Pulse overlap then just follows from the number of individual phases and the pulse duty cycles (D). I don't immediately see why the pulses couldn't overlap considerably. At least electrically there should not be a problem.Right now i am also thinking of generating the phase sequence using a microcontroller, possibly an ATXMega. Another possibility would be to use a CPLD like Xilinx CoolRunner II but those want all kinds of core and I/O voltages that i may not have the energy to bother with. So, we'll see. I have some of those left over from a previous project so maybe i will think about it though.

author: Kremmen
author_rank: Super Contributor
timestamp: November 09, 2012, 07:45:26 pm
content: Managed to find an actual chip from TIhttp://www.ti.com/lit/ds/symlink/ucc28070.pdffor multiphase PFC. What is even more interesting it has external clock syncing. This would indicate that you can cascade these controllers to create possibly arbitrarily many phases. I would prefer this to a software solution if only because the hardware cannot crash and of course the development cycle will be reduced significantly with ready to roll components.