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Passing Mains harmonic emissions with dimmable offline LED driver


Well-Known Member

As you know, its not difficult to design and build an offline SMPS LED driver that can run at 235W output, and also be dimmable down to 25W output. (eg, a Boost PFC followed by a 2 transistor forward converter).

However, its far more challenging to make it able to pass mains harmonic emissions at 235W, and also at 25W. This is because the EMC filter necessary to make it pass conducted emissions at 235W, will generally mean that the mains harmonics at 25W are not a pass. (the input stage filter capacitors will generally be too big to allow a mains harmonic pass at 25W)

Would you agree with this?

How do the regulations deal with dimmable LED drivers. At what power level are mains harmonics tested?


Active Member
I feel guilty coming on to this forum promoting my company's products, but have you looked at the LT3799? This part modulates the peak current in the primary sense resistor in sympathy with the mains, so the input current rises and falls with the mains frequency, thus keeping input current in tune with the mains.


Well-Known Member
Thanks yes, I know and love the LT3799.

But even the LT3799, if used to design an offline flyback led driver, would suffer the problem which I describe….and in fact, I attach two LT3799 simulations which highlight the situation.

I attach on LTspice sim of an LT3799 based, PFC’d flyback led driver of 97W. I also attach a LT3799 based, PFC’d flyback led driver of 20W….you can see that the harmonics are significantly worse for the 20W case.

I actually did a 3rd simulation at 25w…..and that fails mains harmonics (3rd harmonic too high)…whereas the 97W simulation attached…is seen to pass mains harmonics completely.

My original postulation, is that it is not economically feasible to design a PFC’d offline SMPS LED driver of power 235w, that can pass mains harmonics at 235w, and also pass mains harmonics when dimmed down to 25w. Would you agree with this?

I attach the two LTspice sims and the excel of the harmonics.



Well-Known Member
Thanks, yes, but the fact remains, that a real world, offline SMPS LED driver that passesd mains harmonics at 235W, will not pass mains harmonics if dimmed down to 25W.....wouldn't you agree?
...because its conducted EMC filter, being designed for 235W, would result in too much distortion at 25W for it to be able to pass mains harmonics at 25W.

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