LNK420 application in 60WLED driver

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Dear sir, We have built a 60V/1A( 60W) LED CC driver using LNK420. based on PI Expert. We have built the transformer using the PQ3220 Core. The Output is not coming upto 1A.It is around .78A/64V. Also the THD is around 25%.Please suggest ways to fine tune and get the full current of 1A.

Also ways to reduce THD to less than 15%. I have seen some circuits employing a RC input from the Tank Capacitor into the feed back PIN on LNK.We dont seem to have seen any such recommendation in PIliterature on LNK-PH family of ICs though! Please throw some light on this and suggest ways to improve THD and Power Factor without losing the Output current regulation.  



The output current can be tuned by adjusting the Iv and IFB current fed into the IC. Iout is proportional to IFB and inversely proportional to Iv. You can measure the actual IFB and Iv on the board and make sure it matches the spreadsheet. IFB is set by the RFB resistor and normally the bias voltage used in the spreadsheet may not always be the same with the actual bias on the board causing a different IFB. Same with the Iv.

For improving THD, you can increase the KP and/or reduce VOR of the design. Other method is to add RC network between FB pin and input bus. This technique makes the IFB current a function of the rectified input voltage and as a result makes the input current more sinusoidal. To implement this make sure that the limits of IFB (IFBskip and IFBAR) is taken into design consideration. However, this technique is not yet incorporated on the design software and will require the user to iterate RFB resistor to center the output current. If you need detailed support on this, I suggest you contact the local FAE on your area for you to understand better and optimize the design using this technique.


dear sir,
i was developed 75w led driver as per the RDR290 reference. but the driver not working. the voltage in pinV(2) is 15v. the voltage across the C3 is 330v and voltage across the Bridge rectifier output is 270V. the all output pin show the line voltage. the voltage across the pin D and S is 270V.output voltage is 0. please help me

Voltage on Vpin should only be 3V and voltage on C3 should be equal to the peak input voltage. You probably get the input voltage at the drain pin because the IC is not switching yet. Make sure that Q1 and Q2 is off initially. To isolate Q1 and Q2, you can open R20 first and see whether the IC wakes up. If the IC wakes up, then check the network of Q1 and Q2 and make sure it behaves at it should be.

Dear sir,

Thanks for your replay. i have remove the R20 resister from this circuit. but we get 15v volt on the pin V and 2 volt in pin BP and voltage across the D and S was 275 volt. voltage across the C3 is 330v. in the reference design PDF there is no resistance( in Lower V Pin Resistor Value is 0) connected to V and S. in transformer design we wound the turn in 1 to 3 and 3 to 2 , but in schematic we connect the primary between 1 and 3 . and in BOM some material are missing( C4,D8 and R4). we are test the circuit in 230v input with NO LOAD and LED load (36V,20W) but we don't get any voltage or current .please help me sir
with regards,

C4, D8 and R4 is not on schematic that's why its not on the BOM. If you copy the gerber file for the board, don't populate these 3 components.

Primary should be connected between pins 1 and 2. I'll feedback the mistake in the schematic.

For the IC to start, BP must reach ~6V. If you start it with open load, ovp will kick-in and latched shutdown the IC. You should start it with an led load if you want to see output come-up.

BP is initially charged from the drain, (make sure D4 is in correct mounting polarity). BP will not charge to 6V if something external is pulling it down. make sure D5 polarity is correct and Q1 and Q2 is off. D8 should not be mounted.

Dear Sir,
Thank you for your replay. The LED driver was working with LED load(36V,20W). but the led is ON after some time(2 sec) ,that led is OFF.can i test the 75W ,36V led driver using 20w,36V led?.
Please help me
With regards,

Good to know that its kinda working already.

If this is exactly the same as RDR290, if you load it with 36V LED, it will deliver 2.1Amperes because its a CC supply.
You need to make sure that the LED can handle this amount of current.

One possible reason for it to turn-on and off is autorestart. Pls refer to datasheet for further details. If the current on FB pin goes below IFBAR, the unit will go into AR mode in which you will see the LED turning on and off. To verify this, make sure that the bias voltage is at ~20V when you load it with the 36V LED. In normal operation, bias~20V, IFB is above IFBAR.

Hope this will help you make your unit go.


Dear sir;

Thank for your valuable replay, we are remove the resister R20. then the driver is working with 36v,20W led.but transformer producing humming noise. please help
With regards,

please help me sir. how to reduce the humming sound.

Make sure the unit has stable operation and transformer is varnished.

I would like to request you kindly do needful in my critical following problem.

I have designed 75W led driver as per RDR-290.

Load of LED LUMINAIRE is: 32V/2.1A.

All the parameters are showing up to the mark.(i.e:PF>0.98,THD<9.5%, Effi n=88%)
(To improve THD performance I have added RC network)
I am facing flickering problem while using driver in Industrial area, especially whom soever having motorized equipment (i.e:drill machines/cutter machines).

During switch on-off-on-off.......Our luminary gets flickered.

I have tried so many possible remedial actions (with respect to EMI) to get rid of this problem

I have spared so many days in trial and error, unfortunately I have not got any solution.

the mark. Due to this problem my commitments have been stuck up.

Your prompt response in this regard will be highly appreciated.

Kindly help me to get out from this mess.


Mukesh Israni,

To get to the bottom of this, you will need to find out what causes it to flicker. is it auto restarting? or regulation just changes with sudden line changes. Can you provide the plot of output current, drain current and input voltage when flicker is happening. We need to understand what causes the flicker before we can addressed it.

This might be difficult problem to troubleshoot using the forum. Another option that I can suggest to you is to give a call to the nearest sales office of power integrations and ask help from the local field application engineer.

dear sir ,

Thank you for your replay. i have test this led driver with 90w led it consume 2.5A,36 volt and test with 20w led it consume 0.49A and 54volt. In application note has specified that the RDR290 design for 2.1A CC operation. but the load varied the output current also varied. please give an answer , why the system was not operating in 2.1A CC mode.
and also what are the modification to do for cc mode operation.
NOTE: we are removing the R20 resister in circuit(other wise system will on and off immediately). the system will work perfectly other than the current change.
please help me sir. we waiting for valuable replay.
with regards,

RDR 290 is designed to be CC from 29V to 36V. The CC region of LinkSwitch-PH is described in figure 5 of the datasheet.

In summary, the CC operation is from IFB(DCMAX) to IFB(SKIP). IFB is proportional to the output voltage. If you connect a higher voltage LED, IFB will increase and if it increase beyond IFBSKIP, cycles will be skipped and output power will be reduced. On the other side, if you go below IFB(DCMAXR) with lower LED voltage, maximum duty is limited which will also cause reduction in power.

The network connected to R20 is for OVP. If you don't put back R20, you will lose the OVP function. Its probably shutting down because the bias voltage is higher causing Q2 to turn-on.

dear sir,
thanks for your replay. sir i have one doubt, can i adjust the output power the this driver to 60w,80w,50w etc. if yes, how to adjust the output power of this driver.
with regards,

One way is to design the deriver at maximum power of 80W using PIXls and adjust IFB to 200uA for 80W output power for the maximum LED load voltage your using. You will have CC upto ~100uA for led load less than the max led voltage at 80W.
I guess this is also related to this forum that you may like to watch