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"Swinging" inductor for CCM PFC
TimNJ:
Hi everyone,
Here's the situation. I'm working on a CCM PFC stage, currently using an RM10-size ferrite core inductor. There is a stringent line harmonics requirement that we are trying to meet, which drove the decision to use CCM in the first place. Transition-mode (TM) is much more typical in this power range (~120W), for a "normal" application.
The efficiency is pretty good, and actually very much optimized at this point. I went through the whole switching loss vs Rds-on optimization thing, and we've tried many different MOSFETs to try to find the sweet spot.
But, I'd really like to get another 1% efficiency, if possible.
I've seen many commercial power supplies with CCM PFC stages use a Sendust or moly-permalloy (MPP) core, instead of ferrite. These converters can typically get away with a lower switching frequency, which gives way to lower switching loss. With a high enough magnetic field intensity (current through the inductor), the permeability begins to roll off, but gradually, unlike ferrite. Depending on the load, the permeability of these cores can "swing" from high to low, and back to high again, during one 60Hz mains cycle. This can be advantageous for CCM converters, but I still can't exactly wrap my head around why!?
I've seen several references to swinging inductors in CCM PFC controller application notes, but never a detailed design procedure, or at least a solid explanation of the theory surrounding them.
Here's one app note, for example: https://www.infineon.com/dgdl/Infineon-ApplicationNote_PFCCCMBoostConverterDesignGuide-AN-v02_00-EN.pdf?fileId=5546d4624a56eed8014a62c75a923b05
Can anyone point me to any good resources on swinging inductors for this application?
Thanks!
NiHaoMike:
At what load level are you trying to optimize? At light load, gating off the drive signal at low instantaneous input voltage will give the best results.
T3sl4co1l:
Big downside is the variation in loop speed and gain, which at least shouldn't be much (the swing is only ~3x for reasonable construction and materials?) but may contribute to distortion, who knows.
I never felt convinced that a swinging choke really makes any difference. AFAIK it's an old fashioned minor cost saving measure. You still need the turns*area to handle the waveform, and maintaining inductance is just a matter of a little more wire and gap.
Tim
TimNJ:
--- Quote from: NiHaoMike on July 03, 2019, 12:29:12 am ---At what load level are you trying to optimize? At light load, gating off the drive signal at low instantaneous input voltage will give the best results.
--- End quote ---
We are trying to get improvement at 50-100% load. (That's a pretty big range, I know. Maybe we can call it "75%".)
Do you mean gating off the transistor completely near the AC zero crossing? Or some sort of burst-mode?
Thanks!
TimNJ:
--- Quote from: T3sl4co1l on July 03, 2019, 02:00:39 am ---Big downside is the variation in loop speed and gain, which at least shouldn't be much (the swing is only ~3x for reasonable construction and materials?) but may contribute to distortion, who knows.
I never felt convinced that a swinging choke really makes any difference. AFAIK it's an old fashioned minor cost saving measure. You still need the turns*area to handle the waveform, and maintaining inductance is just a matter of a little more wire and gap.
Tim
--- End quote ---
That's an interesting perspective that is seemingly contrary to a few app notes and designs I've seen. Maybe it's an idea that's been passed down over time without much thought about why?
Still, let's suppose we have a 3mH inductor at no load. What advantage is there if this inductor looks like 1mH inductor at full load, peak of the AC wave? How and what does that help you with? Maybe you can "get away" with using a smaller inductor by allowing partial saturation for a portion of the AC wave. But, I would imagine harmonic generation will be worse while the inductance is down..
Thanks.
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