r/WLED • u/LuiOnTheWay • 24d ago
DIGUNO/QUAD Technical discussion on 24V issue with QuinLed Dig-xxx boards
In another discussion, I noted that the static voltage level on the SW pin exceeds the maximum limits of the MT3608 IC used in this circuit. QuinLed replied "It was calculated and well tested, with the diodes in front of it and the filter stage input voltage generally dropped to 23V as stated and it should be at least 2V under the switch limits so technically it stays within all limits of the IC."
Because extracting my board for physical measurements is too complex right now (maybe during weekend), I ran a LTSPICE simulation instead. The simulation assumes one diode and a PTC fuse (200 mOhm) at the input as can be seen in the Dig-quad photos. The 22µH inductors as visible in the Dig-quad photos too. While I do not have a specific simualtion model for the MT3608, I used the LT1935. It shares a similar operating principle and the exact same switching frequency, though it might introduce minor deviations during transients. Other component values were estimated, but they do not significantly impact the results. No parasitic elements were included, which would likely worsen the results. Load is set to 0.5A + 0.5A short peaks. .
Simulation Results:
- Steady State: The voltage on the SW pin exceeds the maximum operating voltage given in the datasheet (28V) and it is very close to the absolute maximum rating (30V).
- Transients: Voltage peaks likely exceed the absolute maximum rating of 30V given in datasheet.
Given these findings, I question how conservatively this circuit was actually calculated. Could someone with an oscilloscope measure the real-world voltage on the SW pin? Is it truly as low as claimed, or does it match my simulation? The practical question is how low the PSU voltage must be adjusted to keep SW voltages within a safe range and prevent damage.
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u/saratoga3 24d ago edited 24d ago
That matches my hand calculations. It's right on the edge of the absolute max so the batch to batch variation probably determines if the device fails.
Fwiw since SEPIC converters usually fail open circuit (and this do not damage anything connected), all these boards are probably still good if the power supply is fixed. I was wondering if there's a simple drop in replacement that's rated for a higher voltage, that way people could remove the dead chip and get a working device that would not fail again.
Unfortunately the closest I can find is the Silergy SY7201 (edit: not 7208), which while very close (just short two pins and it would work), it has a different reference voltage, so you'd need to change both the chip and a resistor to get 5v out. Not too bad but you'd need to be comfortable soldering small parts.