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Mains harmonics of dimmed down offline LED drivers

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treez

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If one has designed an offline 230VAC, SMPS based LED driver whose power is 150W maximum, then as you know, one has to design a mains input filter to allow it to pass mains conducted emissions at 150W.
However, one then finds that this relatively highly attenuative filter prevents the converter from being able to pass mains harmonic regulations when it is dimmed down to 25W.
Do you know if the EMC regulations (mains harmonics regs) of the EU and the USA have allowances for this? (ie excuses for 25W operation of a 150W rated LED driver)
 

Thanks, as follows...the attached shows mains input current and voltage for a Buckboost LED driver...one at 150W (max power), and the other at 25W (dimmed down). As you can see, the behviour at 25W is not so good in terms of mains harmonics.
 

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  • Mains input current and voltage__25W.jpg
    Mains input current and voltage__25W.jpg
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  • Mains input current and voltage__150W.jpg
    Mains input current and voltage__150W.jpg
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Search " EMI problems in triac dimmable led drivers".
 

Thanks, but truly i am not speaking of Triac dimming here.....Triac dimming as we both know definetely gives poor harmonics.
 

Yes, I only referenced it because in the beginning of the article it states that European emc and harmonics standards are only specified at full load and non-dimmed condition. I didn't think the statement only referred to triac dimming but after reading again I'm not so sure.
 
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It is not bad: you still have power factor ~95% (cos theta =0.95).
 
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I guess the reason for the harmonic distortion is a filter capacitor behind the mains rectifier. In a different filter topology, the capacitor would only cause displacement current but no harmonics.
 
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I'm not convinced that the EMI filter alone cab be blamed for that distortion. Limited dynamic range on the controller is likely a large contributor. What do the waveforms look like without the filter?
 

I'm not convinced that the EMI filter alone cab be blamed for that distortion...

But the overall feature broadly seen in post #3 simply suggests that the equivalent effect seen in the final waveform is that of a simple capacitor.

Of course the conclusions can be different if we see the effects of frequency dependence but with the information at hand, the simplest conclusion is perhaps the most appealing...
 

Thanks to Mtwieg…it emerges that as you suspected, the problem was not the filter after all….. it is that the controller used (LT1243) has two diodes at the output of its error amplifier..(the COMP pin). It is this fact that is meaning that when the lower dimming ranges are used, the error voltage gets near the voltage of those two diodes, and so the control signal goes a bit haywire…this causes the distortion. (as the attached schem and ltspice sim shows)
I suppose I will have to try and use the L6561 controller instead, because it can be setup in constant off time and doesn’t have the two diodes, but in any case, maybe the regulations state that if you pass mains harmonics at 150W (max power), then maybe you don’t have to pass mains harmonics so closely at low dimming levels.

Having said that about L6561 , I would have to find a way of bypassing the multiplier etc etc

LT1243 datasheet:-
http://cds.linear.com/docs/en/datasheet/1241fa.pdf

L6561 datasheet:-
http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00001174.pdf

L6561 in constant off time:-
http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00257417.pdf
 

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  • PFC buckboost 150W.txt
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  • PFC buckboost 150W.pdf
    71.1 KB · Views: 103

Actually it is better than i thought, the distorted mains input current waveform, (as attached) at the 25W power level, gives a power factor of 0.95, and passes all Mains harmonics.
 

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  • Mains input current and voltage__25W.jpg
    Mains input current and voltage__25W.jpg
    73 KB · Views: 154

Thanks to Mtwieg…it emerges that as you suspected, the problem was not the filter after all….. it is that the controller used (LT1243) has two diodes at the output of its error amplifier..(the COMP pin). It is this fact that is meaning that when the lower dimming ranges are used, the error voltage gets near the voltage of those two diodes, and so the control signal goes a bit haywire…this causes the distortion. (as the attached schem and ltspice sim shows)
This is definitely part of the problem. Near the zero crossings, the current setpoint is basically zero, which may cause the error amplifier to saturate on way or another, and it may take time to settle back into the range where it actually causes the PWM output to start firing. Those two diodes lengthen that time. But you've compensated for that somewhat by adding D1, which prevents COMP from saturating all the way to zero. Adding a second series diode should help more. But this can likely be better avoided by using an actual PFC controller which is designed to avoid such issues...

Having said that about L6561 , I would have to find a way of bypassing the multiplier etc etc
Why?
 
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