Flickering problem with LNK460VG
Hi,
I have designed a LED power supply for 12" traffic sign with lnk460.
everything is ok including PF and line regulation but it flickers when change in the mains voltage occurs.(even around 5v change).
I didn't notice the problem because the voltage was fine in the lab.
We have ordered 1000 set of board and components and now I face this problem.
I guess lnk460 is not fast enough and there is a large setteling time in the control loop.
Any help and suggestion will be appreciated.
input voltage:220VAC
output voltage:150VDC
output current:50mA
primary inductance:550uH
secondery inductance:550uH
output capacitor 560nF (with larger cap the flickering get worse)
Comments
Thanks for your response.
actually I tested larger caps up to 20uf but it got worse.
for example with a 20uf cap, control loop instability and output current oscillation occurred.
besides turn on/off time are important for us.(<100ms)
Line Transients are not too large and long and sometimes are invisible but there is a little -but not regular-flickering any way.
This oscillation is not too much in quantity , may be +/-20%, but its visible and annoying.
I think I have to go with a second stage current regulation for the LED string.
Am I right?
Too much or too small capacitance can cause instability. You should not see any flicker under normal operating conditions. If you measure the peak to peak output current ripple (on the LED string) with a current probe and adjust the output capacitance so that the line frequency (actually it is 2X line frequency) ripple is 15 to 20 mA, does the flicker go away?
Z. Cochrane
I changed the capacitor and over 3uf it get worse.(but ripple current is still high)
I tested different transformer turns 550uh/550uh,750uh/750uh,1300uh/1300uh and 1300uh seems to be the best one.
I will try larger inductance and different cap size and report the results as soon as possible.
Thanks for your help.
You should use PI Xls software to select the proper inductor value. You can download this software at our website. PI Xls is part of our free software included in the PI Expert software.
Z. Cochrane
As I remember it doesn't accept 150v output.
I calculated the primary inductance for about 10w output and 2us on time.
I will try it again.
Thank you.
I changed the configuration to CV by using voltage feedback instead of the current.(for 150v output)
for eliminating the flicker I added a linear current source with a safe voltage margin. Efficiency is not too important for us but the PFC is.
Now It is working fine and there is no flickering.
any suggestion?
LinkSwitch PL makes a loop correction every half line cycle. If a single transient of less than 1/2 line cycle is applied, a change in output current will occur for that 1/2 line cycle. If a constant stream of transients occur, the flicker will be more noticeable. What is the amplitude and duration of the transient that you apply to the input?
Under normal operating conditions, what is the peak to peak output current ripple? 560 nF output capacitor seems small. If you increase the output capacitance such that the output ripple current is 15 mA peak to peak, does that make the transient response worse or better?
Z. Cochrane