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QMX SMPS issues, and a possible cure
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I'd like to know the firmware and hardware rev from anybody who has trouble with the QMX 3.3 and 5.0v power supplies.
Mostly, I'd like to know how often these failures occur with the most recent firmware and hardware.
If that gets noted here in the forum, the pressure increases to find a more robust solution.
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Symptoms might be a bad diode at D108 or D109, a fried processor, fails to work after jiggling the power connector,
fails to work after accidentally shorting a power supply with a probe, fails to work after multiple short power button presses.
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As I previously reported, I have some linear regulators on a one inch square scrap of vector board that plugs into JP105,
this seems to work well after some minor mods to the QMX.? Specifically, need to lift the drain lead of Q106 and Q107 to
disable the old SMPS's, need to add 200k in parallel with R112 and R113 so the firmware thinks it's seeing a bit more than
3.3v and 5v (it hangs if it sees a wee bit less) and short drain to source across Q111 so the 3.3v linear can drive ADC_3V3.
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I'd prefer that Hans change the firmware such that at power up it first checks if LIN_REG_EN is low, and if so skips all the
SMPS start up and voltage checks.? Then the only mod to the QMX would be to add a 10k resistor from LIN_REG_EN to ground.
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I don't encourage others to repeat my efforts at this time unless this is simply something you enjoy doing.
Odds of blowing up your QMX are greater with my mods and hacks than they are with the old SMPS's.
Success is not guaranteed.? Reports of SMPS failures are no longer very frequent with Hans' latest firmware,
especially given that there are?thousands of QMX's in operation.??
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As Hans has said, there are many different kinds of failures being reported in the forum.? The majority are due
to bad soldering, especially around attempts to burn the enamel off of the magnet wire.? Fortunately, most?
failures are easy to repair.? Unfortunately, an SMPS failure often blows the 100 pin STM32F446 processor.
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I feel that the primary issue with the old SMPS's is that the voltage gets checked by the processor only once
every millisecond or so.? That's fine if everything works as it normally does, but something unusual can?
upset things faster than the processor can react.? For example, if the 5v rail is momentarily shorted to ground
by a scope probe, the processor might increase current to maintain 5.0v but not react fast enough when the
short goes away.? Designing an SMPS from scratch (or in firmware) is a difficult exercise in control system theory.
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I just received my Kicad PCB fab for a one inch square board to replace my linear regulator vector board.
This design has a couple SOT23-6 SMPS IC's.? There's a huge number of different parts from different venders to choose from.
Max current varies from 500ma to 2A. They operate at frequencies varying from 0.5 MHz to 2.2 Mhz.? Cost can be as little at $0.10.
Provision is made to allow the board to be fully shielded using cheap copper foil, held up by L shaped header pins.
I am designing for the cheapest SMPS IC's which operate at an internally determined frequency, more expensive part
(still under $1) can be driven by an external clock source such as a processor PWM pin to eliminate any birdies.
The board can be separately built and fully checked out, including how it behaves with an intermittent or varying Vin, momentarily
shorted outputs, momentary low current demand, and changes in temperature.? It could be temporarily plugged into header pins
at JP105 of the QMX, or permanently soldered in place to those header pins.? If plugged in, I would recommend adding a
soldered wire connection for ground, since if that flakes out momentarily it could be disastrous for the QMX. ?
If it's successful, perhaps we of the forum can make it available as a kit.?
Or perhaps Hans would like to add it to his shop for purchase.
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Jerry, KE7ER
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I started this thread in response to this post ?/g/QRPLabs/message/139515
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Re: QMX+ shutdown problem
On Sat, Feb 22, 2025 at 04:10 AM, Kevin (KQ4DTX) wrote:
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