Running and Dimming 10-15A LEDs With PWM, SMPS, PIC, MOSFET—Drawbacks or Issues?

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#1 21684522 03 Jun 2020 17:10

Graham Rounce Graham Rounce

Anonymous

Post #1
21684522 03 Jun 2020 17:10

Hi all, hope you're still good & fit.
I want to run and dim a load of leds, up to 10 or 15 amps' worth, and in my simple-minded way I'm thinking of doing it with pwm, using a switch mode psu, a PIC to supply the waveform, followed by a big transistor (mosfet?), perhaps with an LMx stabiliser.
Would anyone like to point out any drawbacks with this plan, eg it not working, or broadcasting interference, and/or suggestions?Thanks,
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#2 21684523 03 Jun 2020 20:16

Giovanni Di Maria Giovanni Di Maria

Anonymous

Post #2
21684523 03 Jun 2020 20:16

HiIn you project the biggest problem is, obviously, the high current.I think that the best way is use DC/DC converter to supply the circuit of PIC.And the PWM to supply the LEDs.Remeber that switching tecniques are very efficiently BUT transmit interference to the line and to the closest circuits.
I suggest to try new SiC MOSFET, with a very low RDSon.I also, suggest you to protect the MCU circuit from interferences, using metallic box, many filter capacitor, filter on the line, etc.
In genera, you will not have problems.RegardsGiovanni Di Maria
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#3 21684524 03 Jun 2020 20:42

Graham Rounce Graham Rounce

Anonymous

Post #3
21684524 03 Jun 2020 20:42

Ok, thank-you. I can power the PIC semi-separately as you suggest.There will be three loops of leds, each about 50cm across. They may or may not be all on at once.Should I not worry too much, then, about interfering with radio, tv, phone, wifi, etc?G.
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#4 21684525 03 Jun 2020 20:54

Giovanni Di Maria Giovanni Di Maria

Anonymous

Post #4
21684525 03 Jun 2020 20:54

HiI think not intereferences !!!Choose well your PWM frequency.The PWM signal, as a square signal, contains a lot of harmonics. You can use a pass-bass filter to cut them.In every case, the PWM signal is at low frequency ( >= 20 Khz AND <= 400 Khz), and the signal of your device are very very high:- Radio AM: 500-1600 Khz- TV: > 500 Mhz (with Digital TV, TVI does not exist)- WiFi: > 2 Ghz.
New devices are very filtered and they have a very good filter to eliminate input interferences.
PS.Also Sic MOSFET is very cold since its RDSon is very low.
Regards.Giovanni Di Maria
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#5 21684526 03 Jun 2020 21:06

Graham Rounce Graham Rounce

Anonymous

Post #5
21684526 03 Jun 2020 21:06

Hi - "The PWM signal, as a square signal, contains a lot of harmonics" - Yes, that's what I was worried about
"You can use a pass-bass filter to cut them" That's an inductor in series with the fast-pulsing power, and a capacitance across it? What sort of values? I suppose I can test it with an old (non-DAB) radio?Thanks again,
#6 21684527 03 Jun 2020 21:22

Giovanni Di Maria Giovanni Di Maria

Anonymous

Post #6
21684527 03 Jun 2020 21:22

Hi.
Not an single inductor but a LC filter (Pi or T). You can use many online solvers and calculators online and simulate them with a good electronic software.
The values of the components of your filter depend from the frequency of PWM signal. But you can start to cut harmonics from, i think, 500-600 Khz to above.
You can test it with an old radio (AM). At these frequency it's very probable you can hear some interference. FM radio are NOT affected by this problem. WiFi is not affected too.
Modern devices are protected against interferences. More, if your house-circuit is well "grounded", the problems are very low.
CheersGiovanni Di Maria
#7 21684528 03 Jun 2020 22:20

Graham Rounce Graham Rounce

Anonymous

Post #7
21684528 03 Jun 2020 22:20

Ok, thanks v much again.Tc,
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Topic summary

The discussion addresses the design considerations for running and dimming high-current LED loads (10-15A) using PWM controlled by a PIC microcontroller, powered by a switch-mode power supply (SMPS), and switched via a MOSFET. Key challenges include managing high current and mitigating electromagnetic interference (EMI) generated by PWM switching. It is recommended to power the PIC separately using a DC/DC converter to isolate control circuitry from noise. Using SiC MOSFETs with low RDS(on) is advised for efficient switching and reduced heat dissipation. EMI concerns can be addressed by selecting an appropriate PWM frequency (typically between 20 kHz and 400 kHz) to avoid interference with AM radio, TV, and WiFi bands, and by implementing LC low-pass filters (Pi or T configurations) to attenuate high-frequency harmonics. Proper shielding, filtering capacitors, and good grounding practices further reduce interference risks. Testing interference with an AM radio is suggested to evaluate EMI presence. Overall, with careful design, PWM dimming of high-current LEDs using a PIC, SMPS, and MOSFET is feasible without significant interference issues.
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Running and Dimming 10-15A LEDs With PWM, SMPS, PIC, MOSFET—Drawbacks or Issues?

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