Electronics > Power/Renewable Energy/EV's

LLC Resonant Converter for HiFi use?

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dch53:
I’m trying one of these (https://www.aliexpress.com/item/32921995271.html) LLC resonant converters in a class A valve amp I’m building. It provides

•   HT: 200V to 520V 300W maximum.
•   DC1 and DC2: 2.5V-6.5V adjustable 15W peak 7.5A.
•   DC3: fixed output 2.5V-15V, customizable, maximum 10W (not sure how it can be both fixed and customisable!)
•   DC4: -10V to -120V 50mA maximum

My requirements are well within the supply’s specs even with a 50% derating and it bench tests fine with the loads I need.

I’ve reverse-engineered a schematic for everything but the controller which is on a daughter-board with an opaque coating (so I can’t identify the control IC or assess the control loop). The schematic is very similar to those found in application notes and evaluation boards. The power supply is well made on a nicely laid-out 2-layer PCB with lots of vias.

The results have been very disappointing with significant noise at 100Hz and harmonics and strange “humps” in the response at around 1.5kHz, 3kHz, 4.5kHz etc, decreasing in amplitude with frequency.

The attached image shows the high voltage output of the supply (1kHz input signal to the amp).

The supply rectifies the mains and filters the result with 3 150uF capacitors. I’m guessing that the residual ripple is going straight through the resonant tank to the output. I was expecting the regulator to filter all that out.

I’m planning on trying a linear regulator between the mains filtering and the converter itself.

Meanwhile, can anyone tell me whether this is par for the course with these converters?

Are they not suitable for HiFi use in general or is it just this particular implementation?

Faringdon:
If you are getting these noises in your audio then it could possibly be that the layout of the amp, the  pre-amp, and/or the smps is poor.
Also, it is not PFC'd so you will get high 100Hz current pulses into the circuitry...but good layout and cap decoupling etc would normally take care of that.
Yes your LLC is likely  poorly regulated.....

If i was you i would do a  PFC'd LLC converter.....and do nice decoupling etc........but if your amp circuitry is poorly layed out, then even a really good psu wont work well....all switchers are noisy.
But yes.....it sounds like you are seeing the noise from the mains input current and its harmonics...which for you is bad, because you have no PFC.

I mean, you could build your own...or buy a say, 60v output , 400W SMPS...and add a boost converter to its output to give you your high voltage output....what high voltage output do you need...presumably fixed at , say 200V would be fine?

jonpaul:
Bonjour DCH53: Bravo for the excellent questions. Veteran of  high end hifi / cinema sound, (Sequerra  Dolby, THX) 1971..1990s.

0/ Rather than a spectrum, it is wiserr to post the schematic of the amp and PSU, as well as scope shot of the PARD/AC ripple/noise on the PSU 300V bus.

Use AC coupling and take precautions with scope or analyzer against B+ transients.


1/ Class A and B linear audio amps have zero  PS rail noise/PARD  rejection.

Thus a highly regulated and filtered supply is used ib all class  A audio.

2/ Most high end Class A amps use a linear PSU with choke/multiple section filters or linear regs.



2A/ I can see no reason to use a cheap poorly documented noisy chinese switcher.
Concentional transformer/rect/linear reg  mains trsf, caps, and regs for 250...1000V are readliy available or easily made.

3/ Suggest to  repurpose the Chinese switcher and study some lineal Class a valve amps.

4/ If you are serious about good sound, Many high end /audiophine websites and DIY forms will be glad to assist. Also ask our old friends Nelson Pass or Mark Levinson (Daniel HERTZ.CH)

Bon Chance


Jon

dch53:
Thanks for your response.

Performance of the amp improves significantly with a high-voltage regulator between the LLC and the amp. The amp design itself is well-regarded and has been widely built in the diy community.

If the LLC is poorly regulated would that suggest issues with the design of the control loop? Design of these converters is very complex and I'm over-simplifying, but if the control loop was overly-damped the regulation would not respond quickly enough to eliminate the 100Hz and harmonics.

If I end up concluding the LLC is of no use as is I'll disassemble one (I have 2) to fill in the gaps of my reverse-engineered schematic and maybe improve it.

I've been considering building a PFC. There are plenty of design guides, spreadsheets and reference designs around for both PFCs and LLCs. The main problem is the magnetics.

dch53:
Thanks for your reply.

The amp is a well regarded design built by many people in the diy community. I'm reluctant to post the schematic of the LLC resonant converter since it's a commercial product. I'll look at posting AC-coupled scope shots of the 400V rail. I already have some of the switching noise.

While I've built, repaired and modified amps and other HiFi equipment for myself and others I vowed I'd never build a Class A valve amp due the weight and cost.

Then I came across these power supplies which would eliminate quite a lot weight and some of the cost (no heavy mains transformer and banks of capacitors to get the HT good enough for an amp with no PSR).

I've built enough amps and the world of SMPSs and LLC resonant converters in particular has been a very interesting diversion.

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