Class D Mains supply - Page 1
Forum section: electronics / projects

author: moffy
author_rank: Super Contributor
timestamp: September 23, 2025, 12:41:33 am
content: I was recently able to repair a 1kW powered speaker, Bose F1 812, and I began considering how important the power supply is for the Class D amplifier. Bose use a PFC followed by a resonant supply, very nice but necessarily complicated. Since noise in the audio band on the power supply will make it into the output signal, apart from rejection by feedback, what would be a good and simple topology for say a 200W, 24V/10A mains supply? Not linear, preferably a switching supply without the need for a post regulator. Quasi Resonant Flyback looks interesting and one manufacturer is using it for audio supplies in this power range. Looking forward to your thoughts.

author: moffy
author_rank: Super Contributor
timestamp: September 23, 2025, 10:53:23 pm
content: Found an excellent article by TI:https://www.ti.com/lit/ml/slup409c/slup409c.pdf?ts=1758667940775&ref_url=https%253A%252F%252Fduckduckgo.com%252F

author: moffy
author_rank: Super Contributor
timestamp: September 24, 2025, 02:13:39 am
content: This link:https://www.power.com/design-support/downloads/simplis-model-tinyswitch-5Provides a very helpfull custom design program that very nicely contains a very detailed specification of the transformer needed, its structure, windings insulation, core etc. very nice.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 24, 2025, 12:20:57 pm
content: Are you looking at replacing the supply with one you design yourself, or is this just for curiosity's sake?The choice of the isolated DCDC topology is probably made based on the following:1. EMC (conducted emissions, mainly)2. Cost3. Density (i.e. power efficiency)None of these are directly related to the class D amplifier at all. Any topology could work, so long as the switching frequency is well outside of the audio band (like >60kHz), and is sufficiently shielded/filtered such that its EMI doesn't contaminate any sensitive analog systems.

author: moffy
author_rank: Super Contributor
timestamp: September 24, 2025, 12:49:54 pm
content: Quote from: mtwieg on September 24, 2025, 12:20:57 pmAt present it is a matter of curiosity stired by my recent experience with repairing the Bose speaker. Seeing the waveforms and looking at the schematics gelled the idea that the PSU is almost as important as the Class D amplifier design, because Class D unlike analogue doesn't have any inherent PSU rejection properties apart from the feedback.Your point about any PSU with switching well above the audio band is well taken, but you also have the modulation of the digital amp which will mix with any components in the supply rail and could end up back in the audio band. Just not sure either way, hence the post. My prefernece is for a relatively low noise switching supply.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 24, 2025, 01:24:53 pm
content: Quote from: moffy on September 24, 2025, 12:49:54 pmVery true, intermodulation should be considered. Would want to know the switching/sampling rate of the amplifier itself, and choose a switching frequency for the power supply which doesn't intermodulate with that into the audio band. One of the easiest ways of doing so is to make those frequencies the same (or harmonics of each other). But this rules out DCDC topologies with variable switching frequency.QuoteSure, but "relatively low noise" doesn't translate to an actual design. It's not a straightforward thing to quantify, unfortunately.

author: moffy
author_rank: Super Contributor
timestamp: September 24, 2025, 01:30:03 pm
content: Quote from: mtwieg on September 24, 2025, 01:24:53 pmTrue enough, it starts with a topology which then has to be implemented by numerous choices and details. Bose, for 1kW, use resonant supply, I am suggesting quasi resonant for lower power, 200W but am open to suggestions.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 24, 2025, 01:45:34 pm
content: With regards to suitability for use with an audio amplifier, the most important aspects are probably going to be the output ripple (it's magnitude, and the variation in frequency over line and load). I don't anticipate the choice between QR flyback or LLC to have a huge impact on that. Each has a lot of "knobs" to turn which alter output ripple. The venn diagrams mostly overlap. And if one tended to be a bit better (LLC, I'd bet), then you could probably make up the difference by adding a very modest LC filter to the other.

author: moffy
author_rank: Super Contributor
timestamp: September 24, 2025, 01:54:15 pm
content: LLC is definitely the preference for high power as the Venn diagram would show, 200W for quasi resonant is pushing it a little, but the calcs I've done so far show it's doable, the major concern is the very fast switching spikes that I hope the quasi resonant topology might mitigate. Choice of the output diode type, Schottky(silicon or SiC) or a fast soft recovery diode as well as the transformer construction will be crucial.

author: petemate
author_rank: Regular Contributor
timestamp: September 24, 2025, 02:14:46 pm
content: Rule of thumb says flyback for anything below approx. 100W. For a bit more, something like a forward converter could be a simple option up to maybe a few hundred watts. For anything above, LLCs become the most attractive solution due to efficiency, density and EMI.In class D, the amplifier gain s directly proportional to the supply voltage, so voltage drops introduces gain. So you'll need feedback to compensate. Also keep in mind that typically, music crest factor means that you'll need about 1/4 to 1/8 the PSU RMS power that you'll need for a given power rating of an amplifier(note that this is a different discussion from the "music power" or "peak music power" or whatever they call it) There is simply no need to make a larger power supply, as the additional capability would be wasted. The peaks are handled by power supply output capacitance anyway, so you only need to drive the rms signal of the amplifier, plus the overhead to cover the peaks.

author: moffy
author_rank: Super Contributor
timestamp: September 25, 2025, 07:53:17 am
content: Infineon have a fairly impressive little 300W(12V@25A) LLC demonstrator board:https://au.mouser.com/ProductDetail/Infineon-Technologies/EVAL2HS01G300W1TOBO1?qs=W0yvOO0ixfFYxKqXFLbyaw%3D%3Dcompact and not overly complicated a tempting proposition.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 25, 2025, 12:22:48 pm
content: One should also consider the phase shifted full bridge (PSFB). Has the benefits of zero voltage switching like LLC, but also can be used with a fixed switching frequency. Control is also relatively straightforward compared to LLC, and has better transient response and line/load range. Only downside is the need for a full bridge of FETs.

author: moffy
author_rank: Super Contributor
timestamp: September 25, 2025, 01:04:57 pm
content: Quote from: mtwieg on September 25, 2025, 12:22:48 pmHaven't heard of that previously, I will look it up, thanks.

author: moffy
author_rank: Super Contributor
timestamp: September 25, 2025, 01:17:50 pm
content: For those also that might not have heard of PSFB:https://www.ti.com/lit/ta/slup414/slup414.pdf?ts=1758723044761Looks interesting, also a roll your own solution, not a dedicated controller.P.S. Also:https://www.infineon.com/assets/row/public/documents/24/42/infineon-referencedesign-ref-xdp-1000w-gan-psfb-applicationnotes-en.pdf?fileId=5546d462766cbe86017684fe578d548f

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 25, 2025, 01:55:55 pm
content: Quote from: moffy on September 25, 2025, 01:17:50 pmThere are many PSFB controllers. A few examples:https://www.ti.com/product/UCC3895https://www.ti.com/product/LM5046https://www.ti.com/product/UCC28950https://www.analog.com/en/products/ltc1922-1.html

author: makebit
author_rank: Unknown
timestamp: September 27, 2025, 08:27:09 pm
content: For 240W, Would use a PFC stage, giving you a 400V output....then a Two Transistor forward.You could go fancy by doing synchronous FETs on the output if you wanted.Or you could just use a bigger diode heatsink.Or put a couple of UF diodes in parallel and right next to each other on the heatsink.Opto Feedback will give you a quite slow loop.....so youd get a bit of undershoot on a full load transient.....so maybe you could add big output capacitance...Or do a 36V isolated output, and then use a synch Buck to go from 36V to 24V, 10A...and being non isolated, that would give you a nice fast transient response.LLC isnt difficult....AN-4151 does all the LLC math for youhttps://www.resonant-converters.eu/fairchild-an.pdf..just use it to produce a graph of vout vs frequency at your max load and go from there.The thing to remember about LLC...is that if you switch at the upper resonant frequency, then your vout = (vin/2) *ns/npIts "vin/2" for a half bridge LLC 9ie with 2 fets)The point is, you must be aware of LLC failure modes. Ideally an LLC woudl be done with GaN FETs or SiC FETs with antipllel SiCi diode so no chance if damaging reverse recovoery.There are ways of doing LLC with normal Si FETs, but they involve a bit of faff if you want to avoid the failure modes. (The attached details some LLC failure modes)___ ___ ___ ___The following shows an HB LLC failure mode at startup...The "digital control" method shown at 16:26 of the following video...******* VIDEO ******https://assets.infineon.com/is/content/infineon/coolmos-hard-commutation-stress-module-2(This video is probably only find-able by typing "Infineon coolmos-hard-commutation-stress video" into the google bar)

author: moffy
author_rank: Super Contributor
timestamp: September 27, 2025, 10:23:30 pm
content: Quote from: makebit on September 27, 2025, 08:27:09 pmThank you that is a lot of usefull info. The idea of the low noise is to try and avoid hard switching, hence the resonant LLC ,quasi resonant flyback or phase shift that was mentioned. But having thought about it a little longer, as some have suggested, filtering might well be the answer. The advantage of driving a speaker is that it is a true floating load which solves any ground induced noise with the load, it doesn't care about common mode only differential.

author: David Hess
author_rank: Super Contributor
timestamp: September 30, 2025, 12:00:48 am
content: Quote from: makebit on September 27, 2025, 08:27:09 pmFast AC feedback through a transformer could be used in parallel with optocoupler feedback.Alternatively the control loop can be entirely on the secondary side with an isolation transformer to drive the primary side switches.

author: moffy
author_rank: Super Contributor
timestamp: September 30, 2025, 12:44:31 am
content: Quote from: David Hess on September 30, 2025, 12:00:48 amThat would be nice especially if the 100Hz rectified ripple could be removed because that is definitely in the audio band.

author: moffy
author_rank: Super Contributor
timestamp: September 30, 2025, 12:54:02 am
content: A really nice video on Quasi Resonant Flyback with links to some nice QSpice simulations:The QSpice sims:https://github.com/marcosalonsoelectronics/PE-82

author: Andy Chee
author_rank: Super Contributor
timestamp: September 30, 2025, 07:30:08 am
content: Given this thread is mostly about curiosity, then I'd suggest the single conversion amplifier topology of Class-D amps to be worth mentioning, PDF attached.A single/direct conversion amplifier is more realistic to design these days, thanks to the progress of GaN semiconductors. However as with all novel ideas, such a design is up against the tried and proven.

author: HumbleDeer
author_rank: Contributor
timestamp: October 01, 2025, 09:58:46 pm
content: Quote from: petemate on September 24, 2025, 02:14:46 pm1/4 to 1/8 of the rated power? I can beat that. Subwoofer amplifiers need honest ratings.

author: petemate
author_rank: Regular Contributor
timestamp: October 02, 2025, 10:44:03 pm
content: Quote from: HumbleDeer on October 01, 2025, 09:58:46 pmIts not a question about honesty, as I wrote. This is different from the 10kW of "instantaneous power" or whatever they come up with. This is about not having a need for a PSU with an RMS power rating equal to that of the amplifier that it supplies. This is no different for subwoofers, unless you spend all day listening to sine waves.

author: moffy
author_rank: Super Contributor
timestamp: October 02, 2025, 11:26:07 pm
content: Quote from: petemate on October 02, 2025, 10:44:03 pmIt is an interesting point that you both raise, but for instantaneous power and undersizing the PSU one would have to define the period and then provide non drooping storage for that period. I think sizing the PSU to the instantaneous power might in the end be more economical and a safer/conservative approach, it's not based on estimates which might be proven wrong or even be genre dependant.

author: petemate
author_rank: Regular Contributor
timestamp: October 03, 2025, 12:14:50 am
content: Quote from: moffy on October 02, 2025, 11:26:07 pmYou obviously need to have a rail high enough to support the peak power of the amp without clipping, but your PSU won't be fast enough to deal with the peaks, so you need a capacitor bank to prove the hold-up. We did a study once(can't remember specifics, but something like Spotify top 200) and found the 1-to-4 rule. Sure, you can build a 1:1 PSU:amp solution, but its wasteful and not really needed.The "usefulness" of this rule scales with output power, of course. There is little difference between e.g. a 25W and a 50W supply, but there is a big difference between a 250W and a 500W supply.

author: moffy
author_rank: Super Contributor
timestamp: October 03, 2025, 12:36:05 am
content: Quote from: petemate on October 03, 2025, 12:14:50 amDid you also note in the study, duration and variation with genre? I imagine they all play a part, for a design one needs worst case. You need enough capacitance on a PSU to hold the voltage during the response time, which is of the order of ms for the output caps on a fully sized supply, for an undersized supply I would imagine one would need storage of the order of 100s of ms which is a different and expensive proposition. Between 50W and 250W you probably go from flyback to half H bride and maybe PFC, 500W would be similar just some upgraded components above 500W maybe LLC makes a bit more sense, but one can shift the limits a bit.

author: moffy
author_rank: Super Contributor
timestamp: October 09, 2025, 12:10:05 am
content: Ran across this LLC supply on Aliexpress had to buy one to try:https://www.aliexpress.com/item/1005009637344268.html?spm=a2g0o.order_list.order_list_main.5.21ef1802UyjLOH