LNK418

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I've developed an up to 33W isolated LED driver using an LNK418 controller (full power). The output is about 82V 0.4A

The driver works perfectly. It is very efficient (>90%) and the power factor is high (about 0.96).

My problem is the full compliance with the IEC61000-3-2 class C because the 5th and the 7th harmonics are out of the limits (@230Vac).

I think the solution is to increase the primary inductance-->bigger core but the space is limited.

At the moment I'm using an RM8 ferrite core with Lp=500uH (54 turns primary side/ 53 turns secondary side).
If I increase the Lp there could be saturation problems.
Is it possible in your opinion to use a smaller controller (ex. LNK416) with a smaller peak current or will it limit the output power?
With a smaller controller I could increase the input inductance and correct the input current waveform.

What other solutions do you suggest?

I've tried also to redesign the trasformer increasing the primary turns (from 54 turns 500uH to 120 turns 1.9mH)and increasing the reflected voltage(less secondary turns). In this case the harmonics are OK and the efficiency is still >90% (in this case the controller works mosty in continuous conduction mode), but the core coud saturate at the maximum current allowed by the controller.
Could this couse problems if I work anly around 230Vac?
I've looked at the startup current and it's quite far from saturation point, but I need to be sure about this topic.
Attached you can find the design sheet.

Thank you in advance

Best Regards

Nicola Zanforlin

Hi Nicola,
The bottom side of the spreadsheet will help you to calculate Class C limits for IEC61000-3-2 before building your transformer.

I think your approach is correct but you need to avoid exceeding your stress in a worst case condition. I am attaching the spreadsheet for you to adjust the number of turns in order to avoid saturation and exceeding the stress of the device. Since you have a limited space, you can slightly compromise the wire size in order to fit in the available bobbin area.

You can not use smaller device than LNK418 because your power is quite high.

Always consider operating at 265VAc in order to avoid field returns.

You have a challenging design because you have a limited space but I think you can do it.

Regards,
PI-Jono