LNK419/420 maximum duty cycle
Hello,
I've designed a trimpot dimmable LED driver using LNK419 ano/or LNK420 for 30pcs. LEDs in series. The output current is needed to be in range 300mA to 750mA and universal input voltage.
I experienced the problem with duty cycle limiting at cca 50% which exhibits at input voltages below some 120VAC which showed output current decreasing and high frequency noise from a transformer. Originally I used LNK419 which worked at its limit and I thought the limitation was given by overcurrent protection due transformer oversaturation and/or high drain peak current. So I replaced it by LNK420 (to try to found out a difference) but without any significant change. So I measured transformer current forms at various input voltages and found out no oversaturation problem but the maximum duty cycle was limited at some 50% (50% of cycle is ON and 50% is OFF) although LNK-PH series datasheet gives Dmax of 90% to 99%. The peak drain current is about 3.2A, so the overcurrent protection should be inactive. The BP capacitor is 100uF for full power capability.
What can be a reason of this limitation?
Many thanks for your help and best regards
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I have attached required files. The basic ideas originate from RDR-290 which I tested all modifications on, and from DER-289.
Two paralelled transformers according the spread sheed are used because of low profile design. I have used a FerroxCube EFD25 core which has a 0.57mm air gap and AL of 160. During prototype testing under- and over- voltage protection circuit (T1, T2, T3) is disabled by disassembling R22. Resistors R12, R13 and R15 have been changed against the schematics to achieve needed output current range. Diodes D6 and D7 are Schottky type ones (UF5408, MUR480 or MUR8100) which haven't been precised yet.
Assuming the opto-coupler is still not conducting, then the FB current already exceeds 220uA. R17 value is just fine for minimum IFB without going to auto-restart. Please increase the value of either R20 and R18 so that IFB max does not exceed 210uA.
You can also try to reduce the value of R21. Maybe the BP current is not enough. Try 3k.
Lastly, I don't know if you are able to get the intended output current at higher input voltage but please recheck the component values on the secondary side because I think the regulation current will be too high using the resistor values you used.
Please provide more info on how you implement the dimming circuit. Please note that if you are varying the feedback current IFB, it should not exceed >220uA or the unit will go to skip cycle mode and reduce the output current. Better provide schematic and design spreadsheet as well to aid in understanding what's causing the problem.