Quick intuitive way to find VPKPK ripple on Cout of Boost PFC? - Page 1
Forum section: electronics / general

author: opampsmoker
author_rank: Regular Contributor
timestamp: November 28, 2020, 01:48:47 pm
content: HiThe attached gives a calculation method for finding the amount of peak to peak voltage ripple on the output cap of a Boost PFC stage.It’s a bit round-the-houses, and I wondered if there are some alternative , quick, intuitive rule-of-thumb type calculations for this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 28, 2020, 08:51:58 pm
content: The waveform shown suggests a power factor much less than 1.0.Think about it: what is the power waveform drawn from the source, what is the power waveform drawn from the capacitor, and what is the power waveform in the capacitor?Converting this power, to energy, to voltage, gives your answer. Finally, you may find it convenient to approximate Vpp << Vout(DC), which is a good idea in most cases anyway (i.e. for purposes of meeting ripple ratings of electrolytics, and a conveniently stable output). (The general case for large Vpp just adds harmonics, a Fourier series approach will do.)Tim

author: opampsmoker
author_rank: Regular Contributor
timestamp: November 29, 2020, 09:13:01 am
content: QuoteThanks...well, the current waveform in the output cap is as in the middle waveform i show in the pdf of the top post.Basically, for the purposes of calculating 100Hz (or 120Hz in USA) Vpkpk ripple in Cout, i assume that the current flowing out of the boost pfc diode is a train of half sines. (as in the top waveform of the pdf of the top post) Are you saying that this isnt a good assumption? (if so, what is the calculation method chosen to find Vpkpk at 100Hz on the Boost PFC output cap?)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2020, 10:55:09 am
content: In a normally functioning active PFC, the diagram shows the input current (different scale), not the output current.Tim

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 03, 2020, 08:16:34 pm
content: QuoteThanks, yes thats right.And i have assumed that the "averaged" Boost PFC diode current is also a train of half sinusoids at 100Hz.....in order to calculate the 100Hz ripple current in the output cap.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 04, 2020, 02:45:07 am
content: Don't assume, prove it instead.Tim

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 12, 2020, 10:43:22 pm
content: Thanks,......AND8123 page 16 (equation 88) tells that the Vout ripple peak value of a 200W Boost PFC with VOUT(AV) = 400V and Cout = 22uF would be 33.2V (ie 66.4V pkpk).However, the actual simulation of a 200W Boost PFC with Vout(Av) = 400V and Cout = 22uF gives the Vout ripple as 72Vpkpk. This simulation is attached, and also the PDF picture of it.Equation 88 is derived by assuming that the average output current through the Boost diode is a train of Half sinusoids. However, the attached LTspice simulation shows that if the current really were a train of half sinusoids, then the Vout pkpk ripple would be 49V.So how do you really assess what is the Vout(pkpk) ripple of a Boost PFC? Clearly there is an element of dead-time at the mains zero crossing which means that the analogy with the train of half sinusoids is not fully accurate, would you agree?I would say that the LTspice simulation is the most accurate means. Would you agree? Equation 88 clearly does not take into account the dead-time at the mains zero cross.AND8123https://www.onsemi.com/pub/Collateral/AND8123-D.PDF

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2020, 11:25:52 pm
content: Three problems:1. That's way too much for most electrolytic capacitors. Poor thing will burn up in no time.2. Is that enough capacitance to stabilize the control loop? PFC usually needs quite slow response (~several mains cycles), so the step response of such a stage will be terrible (maybe a 100%-nominal-load step will brown it out all the way to zero?).3. Eq.88 is not the peak-to-peak ripple, instead you need to evaluate it at t = +/- pi/(4ω) and take the difference. You probably also missed the line immediately below, telling how to approximate it, i.e. making a small-signal (linearized) version. Which works when the ripple is small, but you're asking about large ripple, which violates the assumption stated.Anyway, the result is still within 10%, which sounds pretty damn good to me. If your application is so critical that you can't afford another 10% of ripple (let alone considering the natural 20% variation in capacitor value), something is seriously wrong. Tack that on as a 4th problem, I guess...The function generally looks like:https://www.wolframalpha.com/input/?i=sqrt%281+-+0.5+sin+t%29The 0.5 is effectively the ripple fraction RF. Adjust it to see different values. Notice for RF ~= 0, it looks sinusoidal, indeed the harmonics go to zero in the limit, as well they should. For RF --> 1, the waveform becomes increasingly peaky, until at RF=1 it makes full cusps. Which is to say, you aren't doing any damn PFC at all, the capacitor isn't even there. Also, clearly these conditions are incommensurate with a boost converter, but a SEPIC or flyback could approach this condition. Not that you have any reason to.Tim

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 13, 2020, 01:34:39 pm
content: Thanks , thats interesting, Equn 88 of AND8123 calculates the "instantaneous ripple voltage".As you know, Equn 88 is a maximum when sin(2wt) is 1. Doubling this max value gives the Vpkpk ripple which is also gotten from equn 91.i can see i made a mistake in not correcting my P(in) for the efficiency. When i do this, i get ripple values which make equn 88 correspond to the simulation.However, the value doesnt correspond to that of the simple pure half sine circuit shown to the RHS of the Boost PFC simulation.The value of Cout and the feedback loop is interesting. I appreciate Cout is a feedback loop parameter. I had always thought that as long as the feedback compensation components overall are set accordingly , then the Cout value can be whatever you want within reason (ie as long as its not overvoltaged or over-ripple currented). Also, as inferred, the vout shouldnt ripple down so much that it becomes equal to the post rectifier half sine at some points.As you know, Vout in a PFC can never go much (0.7v) below the post rectifier bus level.As you know, absolute minimisation of Cout in a Boost PFC is a hot topic because its wanted when film capacitors are used at Cout for long life performance. So thanks as we are trying to get to the bottom of this.