LNK420 driver problem

Posted by: nkgavali on 06/12/2013

Hi,

I've designed LED driver (60W to 70W) using LNK420. The schematics and PCB layout is attached here. This circuit is not taking load. The voltage is 50V at 1.3A. I have applied 120V (50Hz) through dimmerstat. At no load the voltage reaching upto 66V to 68V.

Then, I connected the load (33ohm/50W) resistor but I found that output LED blinking and not taking the load. All resistors used here are 1% rated. I've made trasformer using ETD39 core (primary inductance is 690uH and leakage is 19.2uH). The winding directions are verified and isas per the design

I am also having doubt on the LNK420 footprint which I made here. Can someonen guide me over this?

Regards

NKG

Files

50V_1.3A_LNK420_Schematics.PDF (202.59 KB)

50V_1.3A_LNK420_PCB_Layout_.PDF (168.48 KB)

Anybody plz reply. I'm stuck here. I've used PIexpert 9.0 for the design. The .uds file is also attached here.

Please use an actual LED load instead of resistor load. LNK-PH does not operate in CV mode and the operating voltage depends on the LED load. Putting a resistor load will not set the correct load voltage and it is possible that the unit is going to over-voltage and will auto-restart

Hi,

Thanks for the reply. Now, I've connected LED load of 16 nos 1W LEDs in series configuration. I found that the driver source output current at 200mA at around 80V AC. then I slowly increased input voltage, I found that LED current decreased once it reaches lower than 70mA, the driver went into oscillation (I think it sees as the open load and go into auto re-start). Now, my questionis that why it not regulating current?

I want at least 600mA per branch so that I can attach 2 such branches into parallel.

-- Regards

N. K. Gavali

Dear Friend,

I saw your PCB and found

1 V pin route is much closer to Drain pin should away from Drain pin

2 BP pin cap C11 should be as close as possible to BP pin ( no long route )

3 Source pin route should go direct to C9

Check all component value again.

Try this

Ravik

Please remove the OVP circuit by disconnecting Q1 to eliminate one potential causes of why the unit is not regulating current.

Measure the bias supply (across R7) and calculate IFB by using the formula : (Vbias-2.4V) / (R14+R15). Adjust IFB to get the desired output power but it should be within 100uA to 200uA to avoid pulse skipping or reduced duty cycle mode.

Make sure the device has sufficient heatsink to prevent hitting OTP.

Verify layout and check transformer orientation.

LNK420 driver problem

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