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Low-SideBuck-Boost on LNK3202

Posted by: rokali on Tue, 05/18/2021

Good day. 
I am trying to make a converter based on LNK3202, but it does not produce the required voltage level. I use the Low-SideBuck-Boost -Optocoupler Feedback topology page 3 AH70. If you replace LNK3202 with LNK3206, everything works as it should.
I use an LTV-816 optocoupler, a US1M diode, a BZV55С3V9 zener diode, a 2200 μH choke. 

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Submitted by PI-Dumbo on Wed, 05/19/2021


May I know what the required output voltage and current is? What is the produced output voltage when LNK3202 is used?

Thank you.

Required output voltage 5V, output current 40mA.

Output voltage is:

- about 2.5V with a resistor Rz = 470 Ohm

- about 4V without Rz

Submitted by PI-Dumbo on Thu, 05/20/2021


The power limit might have been reached. Can you send the schematic with component parts, and input specifications? Does the same result happen when tested using higher input voltage?


Submitted by rokali on Fri, 05/21/2021

Good day, thanks for your answers.


1. Input voltage AC 85-265 V 50 Hz.

2. The input is a half-wave rectifier.

3. The converter is currently without load.

4. Increasing the input voltage does not give positive results.

Attachment Size
LNK3202.pdf 16.33 KB
Submitted by PI-Dumbo on Sun, 05/23/2021


There might be a problem in the feedback. You can try reducing R42 to 220 ohms to supply 5mA to the optocoupler. 


Submitted by rokali on Mon, 05/24/2021


With a resistor R42 100 Ohm also does not work. The output voltage is 2.8 V.

Earlier, I conducted an experiment and reduced the value of the R42 resistor from 2 kOhm to 100 Ohm. Then I removed it completely. The result is described in the post above.

Submitted by PI-Dumbo on Wed, 05/26/2021


Have you tried testing it at no-load? 

Also, can you send the waveform during start-up at no-load and full-load so that see if auto-restart is observed?

Thank you.

Submitted by rokali on Wed, 05/26/2021


I wrote earlier that I am testing the converter without load (no-load mode).

Attached to the message are 2 photos waveform of the output voltage.


Attachment Size
load 50 mA.jpg 744.96 KB
no-load.jpg 759.12 KB
Submitted by PI-Chloe on Wed, 06/16/2021

Hi Rokali, 

Good day! 

To summarize the specification 

85Vac - 265Vac 

5V / 40mA  Load 

Input is Half Bridge  Thanks 


The concern is:

         LNK3202 is used Output is not regulating 

        LNK3206 is used Output is OK 


With the above experiment using LNK3202 and LNK3206, this is my perspective.    

     LNK3206 has a high current limit (ILIM) which can charge the output within 50msec.   

     LNK3202 has a low current limit (ILIM) which cannot charge the output within 50msec. 

     At start up Vo should be in regulation within 50msec. If not, Output will shutdown and retry again after 1.5sec. 

     Since the output capacitor is quite high and ILIM  and ILIM is low,   

Please try to reduce the Output capacitor 100uF to 10uF. 




Good day, PI-Chloe!

Thanks for your answer.

I tried the LNK3202 with a 10 uF capacitor. The converter still does not supply the required voltage.

I also tried the LNK3204 microcircuit on the same board. Everything works as it should.