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LNK460 led driver 35V 350 mA

Posted by: mth on Tue, 02/28/2012


our company try to use linkswitc-pl family to develop LED driver for 12 leds. I used RDR251 as based solution to develop schematics and PCB. Now i solder PCB and components and used transformer maded by myself which was calculated in PIXLs Designer software. Transformer is EFD20, Nprimary=87, Nsecondary=30 turns, air gap 0.86 mm (i used two parts of isolation in ferrite for both sides 0.4 mm thickness). Output voltage range is 20.7-40V.

First of all - i disconnect dumper, bleeder and other circuits for dimming. And tried to switch it on. It's started and stabilized current. PFC works very well - i maded FFT and see that there 50 Hz harmonic and other below 3-5% - so it's very good for me.
I leaved it for about all night to check overheating at full load - it's ok. I got efficiency about 76.54% now.

But there two problems which have to be solved now:

1. when i changing input voltage - there is no stability in output current and leds are flickering. Transformer is clickin (like a mouse) while i changind input voltage. Output current depend of input voltage too much - 0.29-0.37 A for all input voltage range. I want to increase transformer's number of turns in primary and secondary winding and increase air gap. I saw few messages where you recomend to use calculated value. But...
2. there are flickering with frequency 100 Hz at output with Pk-Pk value about 6.6 V. It's too much for lighting - flicker is bad for eyes and brain. I tried to increase output capacitance about 2 times and got 4.4 V Pk-Pk voltage ripple. As i understand 100Hz wave ripple associated with input voltage (there is no input capacitance).
3. I want to increase efiiciency to 85% (but understand that it'l be very hard)

Can you advice for me someting to solve problems which i described?


Submitted by mth on Tue, 02/28/2012

I tried to change some transformer parameters (i wind it as was described in designer. First transformer was wind - half of primary, full secondary, half primary. Also air gap was added in only central part of ferrite). Efficeincy increased to 80%. And driver get stability during changing input voltage.

For flickering problem - i tried to instal capacitor 1500 uF x 35 V but still have flicker - it's bad. I think that installing capacitor about 2000 uF will solve problem, but it's too big capacitance i think. need another solution.

Dimming part still not working good - i switch it off at this moment.


Thanks for using Power Integrations product on your design. I am currently reviewing your problem and I will get back to you once I have analyzed your circuit based on the inputs you provided. It's unusual to have flickering especially since you have not implemented the dimming section. I guess it's safe to assume that you have not connected a triac while troubleshooting.
I will reply again to this thread soon.

Shure, there is no triac connected at this moment - i'm using laboratory autotransformer to change input voltage and galvanic isolation - transformer with Ktr=1:1.
I can provide oscillograms for my design...

Submitted by PI-Crumb on Wed, 02/29/2012

When you say flicker, do you mean the LED is lighting ON and OFF? Or is it just a sudden change in the output current that cause the shimmer? We usually call this shimmer if the LED is not turning OFF totally.
I don't think adding more output capacitor will help.However, please review your layout and make sure you follow the design guideline. You may refer to AN-50 and look at PCB guideline section. This is a possible cause since you said you are seeing the 100 Hz line ripple on your output.
One more thing, if you consider designing for low-line only application or high-line only applications, we have other reference designs you can look up to which has higher efficiency (some >85%). In particular, these designs use buck topology.

Submitted by mth on Wed, 02/29/2012

Thank you for answer.

Flicker in this case - only sinus wave shape form of output voltage (or current) without going down to zero value - i have minimum voltage about 29 V and maximum about 35 V. When i looking through the camera - i see running lines caused by changing bright of LEDs with low frequency.
So "shimmer" is more correct word for this phenomenon. But in our standarts there is no difference between shimmer and flicker.

Abpout buck topology:
1. During selecting MCU for SMPS control i overview all RDK and DER files for PL family. And select flyback topology, because it have better input current characteristics - shape more smooth.
2. Output current in flyback design is stabilized by current control, not by average power in primary coil. Buck topology working by average power too, as i understand.
Average power method of control very bad for power LEDs (1 W and more) - Vf drop decrease with temperature and average power should be decreased too.
3. All designs on the site (flyback, buck/boost) have one problem - PkPk values for output voltage in LNK-PL have very big values. And it's have 100 Hz frequency. It's shoudn't be more than 1-3%. That why i want to increase output capacitance. Because it's only one way to solve problem.
4. Using inductance calculated by PIXLs designer also have some problems. As i said - i have two transformers at this moment. First - i designed by myself calculations and experience, and second maded by PIXls designer calculations. Second transformer gave much more value of overheating for transformer and SMPS controller.

Sorry for my english.

Submitted by PI-Crumb on Thu, 03/01/2012

I want to correct my comment earlier about the 100 Hz ripple since the output really does have the 100 Hz ripple. However, you are talking about shimmer.

First, there is NO problem with all the reference designs on the website. LED drivers do not operate in constant voltage mode since it's hard to control because as you said Vf varies in production and temperature. That's why we use current control. For this reason, it's not critical to have some output voltage ripple as long as the average output current which actually determine LED light intensity is stable. If you see instability in the current waveform, then the light intensity will change and this may cause shimmer.

Second, I don't know what type of camera you are using but it seems to me that it is just the refresh rate of the camera. This is not flickering cause by your design. You can see similar lines on TVs, flourescents, and these are all normal.
Please clarify that you actually see with your eyes the flickering/shimmering, and check that the output current is stable or not.

Third, there is no problem using buck topology using LNK-PL, since the device senses the output current in order to control LED light intensity. (e.g. DER-302).

If possible, please provide me your contact info, company, location and I will find out if there's a local FAE who can support you.

Submitted by mth on Thu, 03/01/2012

Ou! Sorry for my words - i want to say - voltage and current ripple with 100 Hz frequency is present on all devices, but it's normally if you don't use special equipment. Of course you don't see it by eyes.
About camera and refresh rate - of course i see lines because camera have refresh rate near the working frequency and thereis interference between light source and camera.

About method of control output current. I mean that for example in der 302 there are Rsense resistor, but current can be changed from 184 mA to 210 mA in voltage range. 26mA in all range. more than 10%... So if in supply power network will be big variations of voltages - we can see bright changes... in russia - it's normal to have 180-240 volts in network...

I can send contact info into private messages or email...

Based on your description, I don't think there's flickering/shimmering problem.
The fun part is when you start dimming and without proper damper/ bleeder, the real flickering problem will be observed.

I suggested exploring other topology because you mentioned about higher efficiency. If you are getting 80% now, then it will still go down once you implement the dimming circuit. You can implement the dimming circuit and find out if the efficiency is acceptable based on your requirements. I just presented other possible solutions. Usually, tight current regulation vs line voltage is not an issue especially for dimmer applications, since it's dimming anyway, and it's not like you expect to get huge line voltage fluctuations every time, that will annoy the end user.

Please let me know if you need further support. Thanks.