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SMPS for Hi-Fi?
SMPS for Hi-Fi?
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Old 2nd January 2019, 08:12 AM   #11
Norman Tracy is offline Norman Tracy  United States
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Nice high signal to noise ratio on this thread.

Eva, you should write textbooks. And suzyj said so much in so few words.

I clicked this thread expecting the typical dogma based proofs by vigorous assertion and instead find great illuminations into the issues and tradeoffs. Think about those issues and it becomes clear why it is the companies with engineering depth that succeed in using SMPS with high-end amps.
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Old 2nd January 2019, 04:19 PM   #12
CBS240 is offline CBS240  United States
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I am almost finished with the first build of my new amp and the SMPS supply. It is my first real go at offline SM supply. Anyway, very interesting what Suzyj is stating about the mains ripple leaking into the output, my VAS uses a small current, higher tier voltage supply and it is an unregulated auxiliary output. I suspected it could be modulated from the primary side. Nice to hear more knowledgeable people to confirm these issues. It certainly shows a 120Hz ripple (60 cycles here). Extra low frequency filtering is needed.

I am using a PQ32/30 core, half bridge assymetric. I like the POT-like shielding of the windings.

I also suspect that using the offline supply to power 12V, and then use toroid and push pull step up converter would eliminate the mains ripple components.....at the expense of complexity and a bit of efficiency. Lol
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Old 2nd January 2019, 04:30 PM   #13
voltwide is offline voltwide  Ireland
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Quote:
Originally Posted by suzyj View Post
There are three gotchas from my experience using off-the-shelf SMPS modules.

First is that they aren’t immune to 50Hz. They rectify the mains, filter it, then chop that at above audio frequency. The filtering on the rectified mains is only enough to meet their ripple specs, which are generally pretty bad. So the 50Hz is present at the transformer primary, and makes it all the way through to the output. You can often use an external rectifier and your own filter cap (though this will be at 450V), to reduce the 50Hz. Or even a capacitor multiplier on the input.

Second is that they can have components of their switching at audio frequency when under low load. Many SMPS “burp” when lightly loaded. They switch on briefly to charge their output cap, then shut down. This greatly increases efficiency, but is very audible. Often you’ll need to ensure the load is high enough to prevent that.

Third is you can’t just throw capacitance at the output to shut them up. That will just result in a supply that refuses to start, due to hitting current limit when trying to charge the capacitance. They usually have a output capacitance limit in their specs.

Best way to shut up a SMPS is to put a linear regulator or capacitance multiplier after it, with high ripple rejection through the whole audio band.

All this is true if you consider an unregulated smps, like a fixed frequency LLC converter without preceeding PFC. The 100Hz ripple is in the same ballpark as with conventional transformer - bridge rectifier supply, which worked for decades decently with decent amps.

But the off-the shelf meanwell smps and the likes are well regulated so there is quite little 100Hz ripple at the output, no need for a linear reg or other fancy stuff.
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Last edited by voltwide; 2nd January 2019 at 04:32 PM.
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Old 2nd January 2019, 06:11 PM   #14
adason is offline adason  United States
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Would this ripple filter work?
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Old 6th January 2019, 12:20 PM   #15
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by adason View Post
Would this ripple filter work?
Yes, that ripple filter would work. The current sharing of the parallel transistors should be improved with emitter resistors and the ripple tolerance is in the order of some 600mVpp only. It is a low power supply as each transistor can handle a maximum of 150mA.
The circuit needs modification for higher power.


NB: Evidently, the filter is not without a voltage drop (and power loss). 24V in will leave some 23.2V out.

Last edited by FauxFrench; 6th January 2019 at 12:23 PM.
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Old 9th October 2019, 05:53 PM   #16
Bigun is offline Bigun  Canada
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SMPS for Hi-Fi?
Quote:
Originally Posted by Eva View Post
Using bipolar transistors or IGBTs causes a dramatic reduction of ringing since they show capacitances an order of magnitude smaller than MOSFETs and have inherently limited switching rise and fall speeds as opposed to MOSFETs, reducing dI/dt and dV/dt
Hi Eva,

Is this still true of current production IGBT's and MOSFETs ?
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Old 9th October 2019, 07:19 PM   #17
jan.didden is offline jan.didden  Europe
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Have you guys seen this:

Audio SMPS units for sale

Jan
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Old 9th October 2019, 10:51 PM   #18
Eva is offline Eva  Spain
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Quote:
Originally Posted by mige0 View Post
Any advice as to how measure / verify such increased ringing?
Approach #1 is network analysis.

Low cost network analysis may be achieved with prototype geometrically representative of final design, square wave generator of suitable rise time, square wave injection through suitable impedance [such as to make injection inductance negligible], and voltage waveform analysis through oscilloscope of suitable bandwidth and resolution as to overcome attenuation due to injection impedance. If square wave with rise times exceeding speed required by source and load looks smooth, filter is good.

High cost network analysis involves equipment specifically designed for doing so. Often five digit price.

Approach #2 is network heuristics.

Each component package has characteristic inductance. PCB traces, wires or any conductor geometry has characteristic inductance and capacitance. Each capacitor type and size has characteristic ESR to capacitance / size ratio. This data can be derived from approach #1 applied to a representative set of components and annotated in proper Engineering Notes, allowing to make an accurate first guess for new designs or modifications. There is a wide selection of software to simulate RLC networks. From latest LTSpice to old Electronics Workbench.

Initial rule of thumb, which can and should be verified by every experimenter independently, is that 1nH/mm is an accurate first guess for component lead inductance and PCB track or wire inductance (no ground plane or shield). This figure can improve to 1nH per few cm with close ground plane as in 4+ layer PCB.
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Last edited by Eva; 9th October 2019 at 10:57 PM.
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