Unstable / flickering output from LED driver circuit - Page 1
Forum section: electronics / projects

author: Ara
author_rank: Newbie
timestamp: August 26, 2021, 12:06:20 am
content: Hi everyone,I am redesigning the layout and replaced some parts with equivalents on the PCB of one of my projects. There are 4 LED driver circuits on the PCB. I used LM3421 as the driver IC and it's configured in the Boost application.The issue with this design appears when I apply the load (JR-M28) to each circuit. All LED driver circuits are performing differently even though they are the same design with the same enable input, and PWM signal. The only difference is the positioning on the board.Information about the issues (input: PWM at 4kHz with 100% duty cycle):- Every circuits have the same input and PWM signal.- Only 1 circuit performing correctly and give the output to the LED module as what is expected (output current is approximately 0.8A-0.9A).- There is one circuit give the lower output current to the LED module (output current is approximately 0.6A). The duty cycle of the output appears to be around 80%.Compare the output between the working circuit, and the 20% power loss circuit.- The other two circuits have the most interference from the current loop around the ICs. There seems to be more power loss and less duty cycle compare to the other two circuits.Compare the output between the working circuit, and the high power loss circuit.Also, this design has been created and tested before and it performed OK. The only differences between the one that I redesigned and the old design are the resistor and capacitor size (from 0603 to 0805), the different N-channel MOSFET, and little bit of the layout, and the clearance between components. I also changed the layers of the PCB where 1st layer with signal tracks, and polygon GND plane, 2nd layer with 24V power plane, 3rd layer with GND plane and 4th layer with only signal tracks.I already follow the layout guidelines from the datasheet, and tried to improve the performance of the LED driver circuits by replacing some components on the spare board (but still has the same issue). I am quite puzzled as these four driver circuits are exactly the same design with the only difference being their positioning on the PCB. I've probed around my board and I suspect that my issues are caused by interference from the switching of the circuit. Even if I just populate one driver circuit, I still get the same flickering output from the lamps. There aren't any non-optimal layouts or routes in my redesigned board that stand out to me.Does anyone have any ideas on what could be causing the unstable/flickering outputs from the drivers?Thanks.

author: moffy
author_rank: Super Contributor
timestamp: August 26, 2021, 02:08:54 am
content: The LEDs you mentioned are either 110Vac or 240Vac according to the manufacturer website. Have you modified them?According to the datasheet (page 51,52) for the LM3421 C17 or CFS is DNP or Do Not Populate.

author: Ara
author_rank: Newbie
timestamp: August 26, 2021, 04:33:06 am
content: Hi Moffy,Thanks for your suggestions. The forward voltage of the LED array is actually DC44-48V. I think the manufacturer also sell an AC mains driver which is what they're referring to on the web page. For my application, I am designing my own driver instead of using theirs.For the LM3421, I actually tried to remove Cfsbefore according to the LM3421 example (I also changed Rfsto 0 and 10.2 ohms as well). However, the power supply presented that the output was over the current limit (>6A) which was actually blew up the track on my PCB and the diode (the expected output current 0.9A). FYI: I tested the LED module with "their" driver before and the LED module is only draw 1.7A at 24V from power supply.

author: moffy
author_rank: Super Contributor
timestamp: August 26, 2021, 04:51:48 am
content: As a test for output resonance I would be tempted to put a resistor in series with the LED load, say 1-2 ohms, just to dampen any output resonance. Those 4.7uf/50v ceramic caps have a high Q and will tend to ring unless dampened. It's easy to try.Another thing to try might be to lower the number of LEDs in the output string to drop the output voltage and see if that has any effect.

author: Ara
author_rank: Newbie
timestamp: August 26, 2021, 05:36:00 am
content: Hi Moffy,I have parallel 2x 2.4 ohms resistor, and in series with the LED module. It flickers/strobes at much lower rate instead of giving a steady output.Unfortunately the LED module is a unit so it's not really feasible to drop the number of LEDs in the string.

author: moffy
author_rank: Super Contributor
timestamp: August 26, 2021, 06:52:44 am
content: The slower flickering does sound a bit like a resonance/feedback issue.You could get several 5.1v/5w zener diodes (they are cheap) and put them in series as a load. Each diode provides a tap voltage, very similar to the LEDs. Just don't run them too long as they will get very hot.

author: Ara
author_rank: Newbie
timestamp: August 26, 2021, 07:33:37 am
content: Thanks for that, I will give it a shot and see what happens. It could be a resonance issue since it is a constant current driver. However I just don't understand why some drivers are seemingly working OK and others aren't.