Boost LED driver failing - Page 1
Forum section: electronics / beginners

author: microwhat
author_rank: Contributor
timestamp: February 13, 2025, 05:26:31 am
content: I've assembled a boost LED driver with the AL8853AQS, and my design doesn't work.The assembly will run for a random amount of time, ranging from tens of seconds to tens of hours, before the controller will stop running. Neither the controller nor MOSFET nor diode are even warm to the touch when running. The inductor is somewhat warm, but cool enough I can keep a finger on it indefinitely. When the controller is running, I measure 3.5V at PWM (pin 8 ), 1.5V at OVP (pin 7), 2.1V at Comp (pin 6), 0.2V at FB (pin 5), and about 150mV at CS (pin 4). I see a lot of noise at these pins, which seems to spike at ~5V. When the controller stops, I measure 0.7V at PWM, 0.6V at OVP, and 0 at the others.I've tried R7 at 0 and 20Ω, and C4 at 0, 120pF and 100nF, all without apparent effect. When I designed the PC board, I tried to keep the switching current loop as small as I could, but I don't know if my layout is good enough.C3 is two capacitors in series, because I don't have any laying around with sufficient voltage rating by themselves. I measure 15-16V across each one when the controller is running.The LED string runs at 400mA, 31-34V depending on its temperature. The inductor is a Bourns SRN1060-330M which supposedly saturates at 2.8A.I don't know what else to check, and I see no point in frying more controllers until I understand why my design is deficient. How can I improve this design?

author: BennoG
author_rank: Frequent Contributor
timestamp: February 13, 2025, 06:44:24 am
content: my guess is your OVP kicks in.It will reset when the input is below 100mV, this is probably not happening.After reading what you already dit, try to put a 100n capacitor over R6Benno

author: rteodor
author_rank: Frequent Contributor
timestamp: February 13, 2025, 07:48:15 am
content: Not sure I could help but I'm interested in the chip.Just to clarify, by "controller will stop running" you mean U1 gets damaged, then you replace that part in the same circuit and the new controller runs for a while until it gets damaged too, right ?Can you check some cold resistance measurement on the damaged part compared to a good one ?How did you probed the noise ?

author: xvr
author_rank: Frequent Contributor
timestamp: February 13, 2025, 11:14:44 am
content: QuoteIt seems too much. This IC has a lot of protection circuits which can be triggered at these spikes.

author: benseno
author_rank: Regular Contributor
timestamp: February 13, 2025, 01:34:31 pm
content: From chip's user guide and example application - in place C3 on your schematics they have used couple of x7r ceramic capacitors instead of series connected electrolytic in your design. That could affect feedback signals as BennoG said.https://www.diodes.com/assets/Evaluation-Boards/AL8853AQEV1-User-Guide.pdf

author: microwhat
author_rank: Contributor
timestamp: February 14, 2025, 01:56:07 am
content: Quote from: rteodor on February 13, 2025, 07:48:15 amThat's correct.Quote from: rteodor on February 13, 2025, 07:48:15 amFrom where to where?Quote from: rteodor on February 13, 2025, 07:48:15 amOscilloscope channel ground attached to power supply common, measuring each input pin.Quote from: benseno on February 13, 2025, 01:34:31 pmI'll order some and try that, as well as the 100nF cap suggested by BennoG

author: rteodor
author_rank: Frequent Contributor
timestamp: February 14, 2025, 09:17:44 am
content: Quote from: microwhat on February 14, 2025, 01:56:07 amWith some luck this can hint on how the damage occurs. FB, CS and PWM look the most exposed but I would look also at GATE, CS and even OVP to GND. Also if you have more than one burned chip and you found something "interesting" then check if the measurements are consistent on another burned chip ("interesting" = one pin measured significantly different on the bad chip while the other pins remain the same).

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 14, 2025, 09:36:09 am
content: You may want to look into those two elcos in series...- I assume you considered the equivalent capacity here is half the original?- Without a balancing resistor voltage on one may rise over time, perhaps causing issue.Not saying this is your root problem, you still may want to check it though..

author: microwhat
author_rank: Contributor
timestamp: February 17, 2025, 04:57:18 am
content: I've measured the latest faulty chip and a new one:Old Gate-GND: 389KΩOld PWM-GND: 217KΩNew Gate-GND: 378KΩNew PWM-GND: 212KΩAll other pins to ground >2MΩ.I already threw out the other failed chips so I can't check them.

author: microwhat
author_rank: Contributor
timestamp: February 17, 2025, 05:03:29 am
content: Quote from: Ice-Tea on February 14, 2025, 09:36:09 amI think I will design a new board and order ceramics with the proper voltage rating as specified in the evaluation board datasheet. When I was looking, I observed the voltage on each capacitor to be half the output voltage, but I wasn't looking continuously.The bodge might work in theory but I want to eliminate the variable.

author: microwhat
author_rank: Contributor
timestamp: February 20, 2025, 04:24:10 am
content: I've reworked the schematic and board layout. Would anyone be willing to comment on my new design?

author: microwhat
author_rank: Contributor
timestamp: February 24, 2025, 04:29:37 am
content: I think I can reduce the area of the hot loop with the following layout.If I understand correctly, that includes Q1 - D1 - C10/C6/C7/C8 - R3/R3.1. Is this correct?Additionally, I've moved C4 and R4 closer to the hot loop, assuming that the current through them will be as noisy as the current through R3/R3.1, and I should try to minimize their loop area as well.