To be clear, that spike to 5.7v under worst case 12v supply interruptions would not have damaged anything on the QMX+.
Also, the QMX+ has 470uF on VCC at C107, when I repeat the test with 470uF added the spike is pretty much gone (about 0.1v).
This board would work safely in the QMX+ as originally built.
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My board did not have any ringing on normal power up, only with extremely short and sharp peterbations on the +12v supply.
It could be that most SMPS designs have some minor trouble here, not a condition that is normally tested for.
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Just increasing the VCC inductor from 4.7uH to 10uH did not reduce the spike much, it needed the larger 470uF capacitor.
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Performing these tests where load or supply are interrupted by scraping a wire across a file would likely kill?
a stock QMX, even with the latest firmware.? Also, it would be a difficult test to conduct without risking the STM32F.
This new board has the advantage of being fully testable without being tied to a QMX.
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Designing a robust feedback loop on a switching power supply from scratch is not trivial.
There's a huge range of fault conditions to account for, and tradeoffs to be made between quick response and undesired ringing.
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Jerry, KE7ER
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On Thu, Apr 3, 2025 at 05:26 PM, Jerry Gaffke wrote:
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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