BCM Boost PFC is so very poor...Switching frequency increase at lighter loads - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: June 19, 2020, 09:20:59 pm
content: HiDo you know when the BCM Boost PFC is going to be shown up for its great poorness?The big problem, not discussed in the datasheets, is the increase in switching frequency as load lightens. The attached shows the frequency at mains peak for 200W load, and 25W load…39kHz to 314kHz !!!At the higher frequencies, the BCM Boost PFC wont be able to valley switch as well as at max load, so it will suffer high turn off losses at these high frequencies.So the BCM Boost PFC is OK if you are going to be on near maximum load most of the time…otherwise…..bad news.None of the OffTheShelf BCM Boost PFC controllers limit the switching frequency below 400kHz.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 19, 2020, 11:04:19 pm
content: What's with the 'so very poor' and 'great poorness' and 'big problem' and '!!!' and 'it will suffer high turn off losses' and 'bad news' ??Tell us what you really think, or better still - don't.If you can manage to appreciate that the switching frequency changes, then try and also manage to do an actual design for an actual application and do actual measurements and present all of that to start a thread, then we would have something of interest to read and comment on.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 20, 2020, 01:57:46 am
content: Switch into burst mode at light load, especially below the point where the minimum power factor requirement applies.

author: ocset
author_rank: Super Contributor
timestamp: June 20, 2020, 10:43:33 am
content: QuoteThanks, some of them do this, some dont.But all allow up to at least 400khz switching frequency......which due to the switching losses, presents a very bad solution.Many of them do not limit the switching frequency at all, other than the obvious LEB, and stray capacitances in the circuit which will result in some delays.That range of frequency shown in the top post, is way too wide...and its even worse than this, because as you know BCM Boost PFC frequency gets worse as you move away from mains peak.I mean seriously......if youre not at maximum load or therabouts all the time, stick to CCM Boost PFC and use SiC diodes i would have thought?If you want to reduce noise due to CCM "snap-off" of the Boost diode, then just arrange for the current to come nearly down to zero before switch on of the FET.The L6564 for example, has no frequency limitation, but does have burst mode, but by the time Burst has kicked in, the frequency could be well high.https://www.st.com/resource/en/datasheet/l6564.pdfQuoteThanks, youve hit the nail on the head, the excel i show in the top post shows exactly why i wouldnt start a design with these BCM chips.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 20, 2020, 01:05:48 pm
content: Perhaps if you dare to dream.ZVS switchmode happily operated above 1MHz 35 years ago.Learn to harness what you have available.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 20, 2020, 01:34:56 pm
content: Quote from: treez on June 20, 2020, 10:43:33 amThe HP server PSUs I have worked with do burst mode using a circuit to effectively gate off the PFC during low instantaneous input voltage. It did that by injecting a signal into the controller to change the current setpoint to 0 and the phase angles it did so at depended on various conditions including the load and input voltage.

author: uer166
author_rank: Super Contributor
timestamp: June 23, 2020, 06:45:52 pm
content: Quote from: treez on June 20, 2020, 10:43:33 amIsn't that uh.. the definition of BCM (or close to it)?

author: ocset
author_rank: Super Contributor
timestamp: June 23, 2020, 08:46:00 pm
content: Yes, but without using the BCM boost methodology, its better, since you dont get the ridiculously high f(sw) at light load.I notice the Hiper PFC range by power integrations does pfc controllers for 70w, and does them in CCM, not BCM...i think finally the BCM marketing bubble has burst.By "bcm methodology", i mean the method of sensing the discharge of a bias coil so you know exactly when the boost inductor has fully discharged.