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Overvoltage in vout of LNK3209 buck

Posted by: treez on

Hi, Page 29 of AN-70 shows a dangerously high voltage on vout of LNK3209 in buck.

Will this still happen if we use opto feedback?

What size do you recomend for the zener and 100R resistor , that AN70 recomends , to mitigate this?

This is worrying to us, because there may be a temporary brownout which leads to sub 50v on vin.

In page 29, why isnt the voltage 15v, even when vin is 200V or so?..after all, it says it has the 24k pre-load.

 

回應

Submitted by PI-Chloe on 05/17/2023

Hi Treez, 

Good Day!  Thanks for your inquiry about LNK T2. 

Now AN70 Page29 will happen if you will operate LNK TN2 in a very low input voltage and no load condition. 

Please read page 28

"For Buck topology, it is not recommend operating the power supply at no-load and low bulk voltage (below 50 V). This will result to a high output voltage that can possibly damage the output capacitor. It is recommended to add a Resistor-Zener circuit in the output to avoid exceeding voltage rating of the output capacitor (Figure 32). The Resistor-Zener circuit will act as active pre-load."

If the application will not operate at this condition, This will not happen. 

The suggested Resistor and Zener in the output will act as an active pre-load. 

The Resistor and Zener will be active during no load and depending to the designer desired output voltage during no load. 

When activated, it will act as a load to bring down the Vo into a safe level. 

Thanks 

Regards

Patrick