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How to deal with EMI and wire inductance on a LED PWM application?

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Faranight:

--- Quote from: MrAl on December 08, 2022, 07:18:54 pm ---I forgot to mention the most modern way to reduce EMI in converter circuits.   That's using resonant switching.
--- End quote ---
Oh? I thought using a spread spectrum was the preferred way to reduce switching EMI. It's used with some switching buck/boost PMIC's.
Not sure, if I'd be able to implement it with a PCA9685, maybe, if I continuously update the duty cycles with random deviations?


As for T3sl4co1l, you've given me a lot to think about.


--- Quote from: T3sl4co1l on December 08, 2022, 09:23:56 pm ---I have no idea what a 5Q2364EE5 is, it doesn't turn up anywhere, maybe it's rated enough, maybe not, no idea.
--- End quote ---
It's a SQ2364EES. No idea why the KiCad's font makes an S look like a 5. Those RC networks sound like an interesting proposal to try out. I'll make a test PCB with RC footprints from DG, GS and DS to try out with  various components and optimize the waveform, and I can easily measure the rise/fall times on the scope. I'll remove the D1 and C2 like you suggested.


--- Quote from: T3sl4co1l on December 08, 2022, 09:23:56 pm ---There's also no short circuit protection, which, meh, maybe that's not too important either
--- End quote ---
What kind of short circuit protection you were referring to? There is a barrel fuse present at the 24V power input of the board (not drawn in the OP schematics). Its current rating will be adjusted depending of the cumulative power of the LED's when driven at 100% duty cycle. Let's say 8 channels of 1A gives 8A total, so a 10A fuse would be installed. Or did you have something else in mind?

Best regards,
-F


jonpaul:
EMI ...ferrite bead, eg clamp on

wire inductance ...use twisted pair.   not an issue anyway, as the LED gets DC feed.

j

MrAl:

--- Quote from: Faranight on December 09, 2022, 08:57:47 am ---
--- Quote from: MrAl on December 08, 2022, 07:18:54 pm ---I forgot to mention the most modern way to reduce EMI in converter circuits.   That's using resonant switching.
--- End quote ---
Oh? I thought using a spread spectrum was the preferred way to reduce switching EMI. It's used with some switching buck/boost PMIC's.
Not sure, if I'd be able to implement it with a PCA9685, maybe, if I continuously update the duty cycles with random deviations?

--- End quote ---
Well i think it depends on the basic switching frequency and resonate converters are not that common.  But if you think about it, if you are only generating a sine wave at 1500Hz and only switching when the current or voltage is zero, how much can that really radiate?
I suppose once you get up into the 100's of kilohertz it may become more significant.

T3sl4co1l:

--- Quote from: Faranight on December 09, 2022, 08:57:47 am ---What kind of short circuit protection you were referring to? There is a barrel fuse present at the 24V power input of the board (not drawn in the OP schematics). Its current rating will be adjusted depending of the cumulative power of the LED's when driven at 100% duty cycle. Let's say 8 channels of 1A gives 8A total, so a 10A fuse would be installed. Or did you have something else in mind?

--- End quote ---

Namely, current limiting of the output, suitable to protect the device.  The MOSFET is long since a lump of charcoal before that 10A fuse even thinks anything's happened.  The simplest example would probably be a foldback current limiter, which takes another transistor and a couple resistors, per MOSFET.  Although, there are some catches with that, which make it kind of tricky to use with LEDs.  An alternative would be a flip-flop and comparator, which turns it off when peak current rises above a threshold, and the PWM input simply sets and resets the flip-flop (edge triggered 'on', level triggered 'off').  That will operate fast enough (~┬Ás) to protect the FET while repeatedly triggering at ~1kHz.  Of course if PWM comes from an MCU, you could potentially route a lot of functions through that, too (depending on how many GPIOs you can monitor with quick response, or have event inputs or analog comparators available; perhaps a combination of these even).

At the very least, it doubles the pin count between controller and switches (i.e. some kind of current sense signal to MCU), or multiplies the components needed per switch channel.  Meanwhile, LEDs should be a pretty stable load -- it's not unreasonable to choose the "charcoal" option -- the consequence might simply never happen.

Not to forget: there's the option of self-protected MOSFETs, which integrate some combination of the above features, among others; they're very popular in automotive applications for instance, as they can drive lamps, relays, solenoids, or short circuits, without destruction (or, at least a greatly reduced risk thereof).  They do cost more, but being automotive, they're surprisingly competitive.

Tim

inse:
Did you already calculate the power dissipation if you were doing linear dimming?
Put an R/C network at the PWM output and convert it into an analog voltage.
Thus you would get rid of all the EMC issues you are so afraid of.

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