Poor output current regulation

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Dear Sir,

 

I am using LNK417EG for output power of 17 Watts. (33 V, 0.53A)

I changed the value of RV1, RV2 & RFB1.....but the  ouput current changes from 0.50 to 0.58 in particular input voltage(140-170).

But the Output current is constant (0.53A)in rest of the input voltage conditions. 

What should be the value of BP pin cap in our case.....( 10uF or 100uF) ?.... 

I have followed the PCB layout as per the design guide. Please suggest any ideas to improve ouput current regulation....

 

Thanks

I.Jothi Ganesh 

This voltage range is somewhat in a gray area because low-line applications is usually below 140VAC while for high-line, the input is usually above 180VAC. 

 

If you need tighter regulation in the 140V to 170V range, you can fine-tune RV1 and RV2 (try using PIXLS Designer Fine tuning section). However, you said that regulation is already good below and above this range, so you might have a problem with output regulation within the normal operating range.Please try anyway.

 

You can try to increase the input capacitane as well and check if it helps or not. You can also try to use a lower device (LNK416EG) so that operating IFB is different and might provide better regulation.

 

BP pin capacitor choice depends on where you want to operate the device: For reduced mode, use 10uF. For full mode, 100uF. 

 

 

Dear Sir,

 

Thanks for your reply.

 

I trimmed RV1 to 3.44 M and RV2 to 1 M....... RFB1 to 109 K ..... The output current is very well regulated (0.52 to 0.53 A) @ Input voltage range of 120 - 265 VAC.........

 

But once the IC enters Input Over voltage shutdown (@ 295VAC) ......the same driver varies its output current in the range 0.50 A to 0.56 A  ( @120 -265VAC) after restart from Over voltage shutdown..... 

 

This malfunction restores to normal when the supply is turned OFF and turned ON again. Please advice.

 

Thanks 

 

Try to put a small capacitor (100nF to 330nF) from Vpin to Source pin and place it very close to the pin. The OVP should not affect the regulation. My guess is you could be having a noise problem. Can you send the schematic for quick check?

Dear Sir

 

I have attached the schematic of the LED Driver.

 

I found the Output current variation (0.50A to 0.57A) today even without entering to OVP shutdown. This malfunction also restored to normal when the power is turned OFF and turned ON again.

 

And the output current is 0.50A at the time of startup and slowly comes to 0.53 A within 10 Mins and stablizes after that. ( will it be due to temperature warm up of components ?...)

 

Shall I put a 100nF 50V 1206 Ceramic cap from V pin to source ?..

 

Thanks very much for your kindly support 

You are right. The output current may change at startup because of temperature coefficient of LNK-PH and other components, as well as the drop on LED voltage as it becomes hotter. Naturally, you can expect some variation from device to device, and from board to board.

 

However, please consider if it is really required to have such tight regulation even at 140 to 170VAC. You may try to put an 0805 ceramic cap from Vpin to Source or change BP cap to 10uF (and adjust the IFB higher) but if you need such tight regulation even at these voltage range, you need to directly sense the output current. Please refer to DER-296 for design implementation idea. If you decide to do this, please verify that the efficiency is still acceptable.