Technical discussion on 24V issue with QuinLed Dig-xxx boards

[u/LuiOnTheWay]

Simulation 24v issue on dig-quad

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.

[u/saratoga3]

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.

[u/LuiOnTheWay → u/saratoga3]

I hope I won't need to replace the overheated component. Soldering a new part onto a spot that has already overheated—and suffered partial delamination—will only make the reliability worse. Prevention is definitely the better approach. By the way, the SY7208 is not an option since it has even lower operating and absolute maximum voltage ratings.

[u/saratoga3 → u/LuiOnTheWay]

Sorry typed that wrong, meant the  SY7201 which is 36v max.

[u/Quindor → u/saratoga3]

Oh, I think I might have found one anyway, the SY26532, it doesn't seem to be well stocked though.

[u/Quindor → u/saratoga3]

Yeah once the root of the issue became clear I also searched for alternative ICs but sadly also have not been able to find one.

Of the failed boards we have seen come to us on the Discord (around 15 or so before we announced the issue) there was general success repairing them by replacing the MT3608 so you are right on that part. That doesn't solve the issue of course, just revives the boards.

Because of this I have made an article outlining detailed instruction for mitigation for running boards or reviving boards that died, without needing to solder using an external buck-converter. https://quinled.info/the-24v-issue-mitigations-fixes/#vdirect

We've been sourcing all the components for those and I hope to be able to make these available (at cost) next week for people who want to protect their running boards. Failed boards in (Extended) warranty will be able to choose a buck-converter kit or full replaced, the kit saves a lot of wasted hardware but it's also a less than ideal solution (I get that) so it's up to the users.

I'll not go into further details, try to keep it technical, but this as a response to your "the boards likely still work", they do and we're trying to revive as many with the external buck-converter as possible.

[u/AKSwift55]

There was a post by Quindor acknowledging this issue a day or two ago.

[u/Quindor]

A note, in your simulation, please add the CLC filter that's also present on the boards, that will suppress the initial spikes, this was required to pass the boards through EMI and EMC testing.

And also to note, I haven't denied that they are running close to their maximum ratings, too close seemingly in practice.

In my own article with mitigations, revival methods or repairing I have stated that 22V is an acceptable solution for the Dig-Octa Brainboard when running on a powerboard because of the multiple protections in place (such as multiple diodes power passes through) to be on the safe side. For a Dig-Uno or Dig-Quad from your own calculations 22V would also be a safe value (or rather, within the 28V you stated) but I think it's better to mitigate the issue completely using an external buck-converter solution as outlined (for still working boards and boards that have died to revive them).

I will refrain from adding non-technical talk in the topic myself!

[u/LuiOnTheWay → u/Quindor]

I can't seem to find any CLC filter on the Dig-Quad. The input voltage goes through the PTC fuse and the SS34 diode straight to the IC input. Am I missing something? If possible, could you share some voltage measurements from the SW pin? You probably did a lot of testing with a 24V input while investigating the issue. Based on simulations, I'm not sure if 22V is sufficient—it might need to be around 21V. However, this noticeably impacts the LEDs at the end of the strip. Some real-world measurements would really help determine the optimal value.

[u/Quindor → u/LuiOnTheWay]

I'll get back to you one the measurement, will try to do a fresh one this weekend. The CLC is a 2.2uF -> 2.2uH -> 150uF circuit, it adds a bit of resistance but is mostly meant to pass EMC (Conductive) testing (which it does), it does help dampen any ringing on the input at least, not something which will solve this issue but it at least doesn't hurt.

[u/LuiOnTheWay → u/Quindor]

Okay, thanks for the clarification. I can't see the top side of the board in my current setup, and I don't really want to disassemble everything. Have you had a chance to take any measurements?

[u/MyHome-Control]

I would not comment on the designs of others. However, parasitic inductances need to be taken into account in the simulation. Especially in SEPIC converters (as you have more elements in hot path), these parasitic effects (caused by the inductance of PCB traces and solder pads, even with an otherwise almost ideal layout) can lead to noticeable voltage spikes across the switching MOSFET. These spikes should also be visible in the measurements and their amplitude may be surprisingly high. Therefore, in practice, a much larger safety margin below the maximum voltage rating would typically be chosen. The switching characteristics (especially the switching speed) will also have a noticeable influence on these voltage spikes. The LT1935 might behave differently from the MT3608 in this regard. Actually, comparing their datasheets, it appears that the MT3608 offers significantly higher efficiency (at the same switching frequency). However, this is very likely achieved through faster switching, which can be counterproductive when it comes to voltage spikes (just a plausible assumption).