Re: QMX SMPS issues, and a possible cure
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Don,
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These are pretty dinky supplies, no need for your 3/8" copper bolts.
But the file certainly makes for lots of crud to study.
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I loaded up the supply with 50 ohms on 5v, 25 ohms on 3.3v (100ma and 132ma respectively).
When scraping the 12v source to the supply across a file, I saw an occasional triangular peak
on 5v that got up to about 5.7v, the base of that triangle was about 100us.
This was overshoot as the supply recovered from a quick dip in 12v, and only when the?
5v rail didn't go below about 3v.? Scope photo attached.
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This is exactly the case that was killing QMX's when 12v power was intermittent,?
though I can guarantee the overshoot then was far far worse.? That has mostly
been addressed through firmware changes, though I suspect issues remain.
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In my Readme I say:
> Next rev of this board will have:
>? Larger inductors than the now 3x3mm, with pads large enough to do rework. >? Inductors (1uH?) added at input to each switcher to reduce noise on supply. ?
Larger inductors, both physically larger and more uH.
Because the current through the inductors now rises and falls very fast,
it very nearly reaches 0 current in the valleys on VCC.? Planning to go from 4.7uH to 22uH.
That will slow the rate at which current rises, and I'd expect it to cure the spike.
Adding the 1uH up front of the switchers (cap still across the switchers Vin) should also
help, meant primarily to reduce noise on the incoming rail but may also help avoid
sudden dips on Vin to the IC's.
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I also tried using the file as a load on both VCC and VDD, going from no load to 0 ohms.
Again saw spikes, the worst was still around 5.7v peak on VCC.
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The 3.3v rail was relatively clean in all cases, in part because it is supplied from VCC.
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Anyways, if there's enough interest and I have a few of these auto-stuffed,?
I'll plan on shipping a couple off to you for further tests.?
Also to Jeff Moore and Hans if they are interested.
These inductors are short and robust, the boards are small and cheap.
I'm confident the boards can go out as first class letter mail here in the US.
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Attached is a scope photo of the VCC spikes when under load and?
running the 12v supply across a file.
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Switching at 2mhz and a 100ma load on Vcc, 130ma on Vdd (typical for a QMX),
the efficiency is around 84%.
I don't expect switcher noise to be an issue, if so we can reduce it to around 500khz
by using a different variant of these IC's and significantly improve efficiency.
We may not quite reach the efficiency of the old scheme which operates at 100khz
but I for one am more interested in a robust design than the last few percent in efficiency.
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Jerry, KE7ER
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On Thu, Apr 3, 2025 at 01:18 PM, Donald S Brant Jr wrote:
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