Bridgeless totem pole power factor correction is not common...but why? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: July 18, 2018, 08:30:49 pm
content: HelloThe boss has asked me to evaluate a Bridgeless Totem Pole PFC (BTP-PFC) stage for our 1kW Offline Battery chargers.However, recently I had to reverse engineer loads of Offtheshelf offline battery chargers to 3Kw , and none contained a BTP-PFC.I am convinced there’s something of a trick question going on here, …its like the PFC’d resonant CUK converter that was supposed to do Vout regulation and PFC in a single stage, but it never took off.What is the snag with the BTP-PFC?For a start there’s the expensive GAN FETs that are needed. But what’s the gotcha?(This by the way, wasn’t one of the questions asked to me about it, so I am not cheating.)

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2018, 09:51:01 pm
content: You pretty much summed it up. It requires two high performance switches plus a pair of low performance switches where a standard boost PFC only requires one switch, a diode, and a bridge rectifier. Plus the drive electronics are more complicated because of two of the switches are on the high side and require level shifting.So what *are* they using for higher power active PFC other than a boost converter? The bridgeless totem pole PFC does not even have an advantage for higher power except lower losses and those could be applied to the boost PFC if desired with synchronous rectification and a better high frequency switch and diode.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 18, 2018, 10:20:23 pm
content: A 1200W HP server PSU I tore down used interleaved boost stages plus some digital trickery to boost efficiency at light load. And although I have yet to get a chance to tear down a recent Dell server PSU, they do allow disabling PFC through the BIOS or BMC.

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2018, 12:24:06 am
content: Interleaving PFC boost stages is pretty obvious for increasing power but I assumed at some point they use a different topology. On the other hand, boost converters are pretty hard to beat.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 19, 2018, 01:11:21 am
content: There are bridgeless PFCs, but they're just a pair of boost converters that work on alternate cycles. Theoretically eliminating losses in the bridge rectifier, but at the cost of requiring each half to handle the full power every alternate cycle. It's more economical to upsize the components a little instead of a lot and then use some digital logic to gate off the PFC during the low parts of the cycle, boosting efficiency while still meeting the power factor specification. The 80 Plus power factor requirements do not apply below 20% load so that can be a huge opportunity for energy saving on machines that idle at less than that.

author: Giaime
author_rank: Regular Contributor
timestamp: July 19, 2018, 08:02:14 am
content: This video clarifies some of the issues:

author: ocset
author_rank: Super Contributor
timestamp: July 19, 2018, 10:52:44 am
content: Thanks, that’s a great video.Bridgeless Totem pole PFC…https://www.youtube.com/watch?v=Jv2JVorAeiE&feature=youtu.beThe above youtube video on BTP-PFC certainly mentions a number of problems with the BTP-PFC.Interestingly, at 10:00 onwards, they state that overall diode losses for a conventional Boost PFC amount to around 1.3% at 240VAC.So for a 3kW supply , you are saving around 40W with a BTP-PFC…..so that’s certainly welcome to take that dissipation away from inside the enclosure.Quote….Interestingly, the above video does not mention the particular EMI problem involving those nodes which jump up and down at high frequency with respect to the neutral and/or the 0V line. The video does however, mention the common mode EMC problem involving the neutral flipping from PFC_POS to PFC_NEG instantly forcing the entire PFC bus to make a common mode leap.….at 34:16 of the video, it mentions this problem, and says that it can be mitigated by implementing a special soft-start in the slow FETs every time the supply crosses through zero.The video goes on to talk about reducing switching losses in the BTP-PFC by operating it in Boundary conduction mode and using ZVS…..the problem is, this reduces turn-on switching losses, but increases turn-off switching losses…so its not all good.Also, the video states that the BTP-PFC offers no size reduction compared to conventional Boost PFC.There are also the more obvious problems with sensing of current and line voltage. Along with the high side drives etc. The high-side slow FET obviously need an isolated high side supply for the driving of its gate.

author: ocset
author_rank: Super Contributor
timestamp: July 20, 2018, 08:42:36 pm
content: I spoke to a consultancy who want to design a BTP-PFC . They said they wish to have the controller ground at the live line just downstream of the EMC filter.(as in the attached schem and LTspice sim)This is based on the fact that they have the current sense resistor there, and so they will be able to directly read it with a diff amp there. This, they believe , will facilitate having an all analog controller.But I believe that this place is going to be a noisy place to have the controller ground, do you agree?

author: jbb
author_rank: Super Contributor
timestamp: July 20, 2018, 09:31:25 pm
content: Quote from: treez on July 20, 2018, 08:42:36 pmWell, that's a nonstandard approach. It might be a good idea or a bad idea. Certainly I would be a little worried about a) noise (probably not a big deal), b) how to connect it to everything else in the circuit and c) how to supply it with DC power.If the consultancy has done a few of these before, they may be right. If not...The classic placement for the current sense resistor (shown below) is after the rectifier, so that the control electronics can share a ground with the bus capacitor, low side MOSFET and whatever your output stage is. It will measure +ve current for the +ve line cycle and +ve current for the -ve line cycle, so some care will be required.

author: JohnG
author_rank: Frequent Contributor
timestamp: July 20, 2018, 11:28:31 pm
content: Quote from: David Hess on July 18, 2018, 09:51:01 pmFor a single inductor bridgeless totem pole boost PFC in continuous conduction mode, you need GaN or SiC MOSFETs for the half-bridge to minimize or eliminate reverse recovery in the fast switches. The reverse recovery losses basically make the circuit unusable if you use silicon MOSFETs. A synchronous rectifier on the boost basically makes it the fast switch half bridge part of the totem pole, unless you are using discontinuous or critical conduction mode operation, in which case you will have a challenging control problem.If it's your wall wart/PC/appliance supply, the extra efficiency is usually not worth the cost, but if you are telecom or data center 240VAC-48VDC rectifier, then the extra efficiency is worth a lot. They would like better than 98% over most of the output current range.

author: ocset
author_rank: Super Contributor
timestamp: July 21, 2018, 10:04:47 am
content: Thanks,I am wondering why not one single Application Note on the entire internet actually mentions the very bad EMC problem of the BTP-PFC?……why this “cover-up” over the BTP-PFC?The BTP-PFC brings with it a very bad common mode EMC problem due to the fact that the controller will inevitably be referenced to the DC Bus negative net. This net is by its nature, a very extensive net, going widely over the PCB (since it’s the reference point for much of the BTP-PFC circuitry) …the problem is, that in BTP-PFC, this net has a very high dv/dt with respect to earth ground….and this is the absolute recipe for bad common mode EMC issues. The attached (red) waveform shows the BTP-PFC problem (LTspice sim also attached)With conventional Boost PFC, If you probe the Neutral input with respect to “DC Bus ground”, then the dv/dt with respect to the DC Bus ground isn’t so high. (as the green waveform shows)However, with the BTP-PFC, the dv/dt between DC Bus ground and the Neutral is very high….and this means terrible common mode EMC problems. This situation is shown in the attached waveform diagram, which shows voltage between Neutral input and DC Bus ground for a BTP-PFC. (Red waveform)And the key point is that DC Bus ground, is , by its very nature, a very extensive net, probably running over a great area of the PCB…so what we have , is this wide area of PCB copper which has a very high dv/dt with respect to Earth ground (since neutral is ultimately connected to earth ground). –This is the absolute perfect recipe for disastrous common mode EMC issues…the high dv/dt of a wide area of pcb copper with respect to earth ground.The LTspice simulation attached actually shows the problem. I have never seen a real product in the market which actually uses BTP-PFC……and certainly on the entire internet you cannot find a single application note giving a conducted EMC scan of a BTP-PFC design.I am beginning to wonder if this BTP-PFC is going to end up getting consigned to the scrap-heap, due to the impractically extensive EMC solutions that are needed to accommodate it?

author: ocset
author_rank: Super Contributor
timestamp: July 22, 2018, 12:49:26 pm
content: Hello,The attached is a BTP-PFC with pure analog control. I cannot see any reason why this would not work…can you?Why are there no Pure Analog control solutions on the internet anywhere for BTP-PFC?Why do all solutions involve software controllers?

author: dmills
author_rank: Super Contributor
timestamp: July 22, 2018, 01:20:50 pm
content: That analog multiplier is going to wind up costing more then the micro would, and the micro gets you all the extra stuff like current limits, over voltage protection, under voltage lockout, soft start & power good indication, for effectively free in production quantities.You could feed the output of your digital isolator into a counter/compare block directly in the digital domain, or even simplify further with careful consideration of what to reference the control processor to.Why would anyone want to do this analog?Regards, Dan.

author: Cerebus
author_rank: Super Contributor
timestamp: July 22, 2018, 01:30:33 pm
content: Quote from: dmills on July 22, 2018, 01:20:50 pm(Thanks for the straight line Dan.)

author: dmills
author_rank: Super Contributor
timestamp: July 22, 2018, 01:52:26 pm
content: While at heart I agree with the sentiment, (And besides Bob was awesome) there really are things for which the easy way is digital.If you were designing a LT style controller chip you would probably go analog, but in the absence of such an all in one solution being available throwing a micro at it is usually the quick and low BOM cost way to market.I found myself using a little 8 pin micro in place of a 556 dual 555 timer the other day, it was cheaper and more compact.Regards, Dan.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 22, 2018, 05:05:33 pm
content: One simple trick I have found to boost PFC efficiency is to add a comparator to gate it off when the instantaneous input voltage is below a certain level. That's easy to do in analog if you're running the PSU at way below its rating (very common in hobbyist projects) and don't care too much about power factor, but if you do care about those, it's way easier to implement cut in point adjustment in digital logic.

author: JohnG
author_rank: Frequent Contributor
timestamp: July 22, 2018, 06:39:10 pm
content: Controllers, sensors and the like all depend on the end user and power level. Low-cost laptop adapters over 75W generally use a PFC with a inexpensive analog controller, although even there, digital is making inroads.A 3 kW telecom rectifier often uses dual interleaved CCM boost converters followed by an LLC. Many have TWO full DSP-based power controllers, isolators, etc. They are very efficient across most of the load range, robust, with high reliability, and high fault tolerance. If you tried to do all of it with analog, you would never get the product out in time.John