On Mon, Oct 21, 2024 at 07:55 PM, Jerry Gaffke wrote:
Perhaps a bigger cap?
Sorry Jerry, just one more disagreement here, then I will back out of this conversation.
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Why should the design ever have to expect a sudden large step on Vin after the PWM 3.3V is stabilized (~1/4 second or so after power-on)?
It shouldn't.? The input power should be stable by then.? The QMX is spec'd to have a Vin of 7 - 12VDC, not a rapidly changing voltage with very high slew rates.? The input power should not instantaneously increase by a large delta-V from any input power source.? If it does, the problem is in the power source, and should be dealt with there.? As far as I am aware, only one reported failure has happened this way from thousands of units. And if the input voltage ramps up over several msec instead of a sudden step change, all is handled fine.? Or if a voltage change of modest delta-V happens, it is handled fine.? So when this sudden step happens, and fuses the protection diode - that is what that diode is there for, and it did its job.? Just replace it, and fix the power input to avoid step voltages.? If someone's PD supply just does that, choose a different one that does not (mine does not), or put an L-C in the cable to reduce the rise time of the power step.
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I also spent a 40 yr career in digital electronics and communications system design.? Over-design and solving problems in the wrong place are also big issues to deal with.
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So, in my opinion (which may have evolved over time, and is obviously different than yours), no design change needs to be made here.? These power supplies are reasonably robust as-is.? The? diode failures due to assembly issues which result in mechanical shorting would be hidden by the suggested fixes, and become more insidious than they already are.? And I'm suspicious that the 'trivial' fix of using an SCR to replace/augment the zener would have similar problems to the high-wattage low-voltage zeners - things are rarely as simple as they seem at first glance.?
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Design changes that really have an important impact on the vast majority of users are where the focus and energy should be placed (SSB and the other 100 things already on Hans's list).
