LNK306' based LED driver twinkles; please help me with a choke!

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Hi PI !

Here is my design' file and circuit. It blinks and clicks instead of normal operation. Where is my fault? What kind of (configuration) choke should I use to get normal driving process?

The output power could be too high and the device goes into auto-restart because it doesn't skip cycles anymore.

You can check the switching waveforms to confirm that there is no skip cycle.

I.e. choke' replacement won't help? Is the spreadsheet file attached just above correct one? I have the ancient oscilloscope that can't offer me the kind of information you suggests above due to poor sichronization. It's a pity.

The chain of < 4 x 1W LEDs is operated almost properly (my digital multimeter goes "280 mA DC"). "Almost" because I can hear a choke quietly whistles. I think there is still problem to control even such a short LED' chain, but without restarting...

I've encountered in my practice with LNK304 - there wasn't ANY problem to build MANY nice and cute little power supplies. Now is just a kind of hard luck for me... Thanks for your kind response. 

> The output power could be too high ...

Should I try to reduce choke' value? How much?

The inductor size looks small and most likely saturates based on Lp=6.8mH, Ip = 570mA, Ae=28.7mm2, and N = 50 (if this is what you used).

The flux density is way above the rating of this core size. Please use a larger core and make sure that the flux density is below saturation value.


Thank you PI-Crumb!

Should I keep trying two-halves core (pot type for instance) to get non-magnetic gap between them in order to increase core saturation' capability? I've used the tiny gap (laquer layer) in the try described in order mostly not to shift away the saturation but to tune-up the result inductance of fully loaded bobbin inside. 

I'll try to twice the pot' size and see what will happen. Thanks again PI-Crumb!

I've just tried 21,5 mm pot; 7 mH; 1,2 Ohm; 5 um gap (a piece of paper); 20 mH without gap.
Any difference. (((( Four leds' chain works virtually well but inductor whistles; five leds' chain twinkles and inductor clicks. 
I'm afraid LNK306 just doesn't fit to run 25 x 1W LEDs. But I don't understand why PI Expert allows me to calculate it without any cautions or comments.

The spreadsheet does provide a warning that the power dissipation of the device is too high. Please calculate the flux density of your inductor and make sure it does not saturate. Better if you can probe the inductor current. If you are sure that the inductor design is sound (no saturation), try operating at the rated output voltage (85V), and reduce the output current, say down to 100 mA or set to a value that won't give a caution about power dissipation of the device.

Yes, actually PIExpert does provide a caution about IC's extra power dissipation, but I thought I could cure it with a sized heatsink put into airflow. I didn't suspect that caution could affect the inductor somehow. Now I'm aware of core saturation problem and appreciate your help very much. There must be some kind of online calculators on the web like this one: http://www.daycounter.com/Calculators/Max-Flux-Density-Calculator.phtml
Thanks again!