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I tried to convert an old 12.09V LiteOn power supply to an adjustable voltage power supply by installing a pot to change the voltage divider for its TL1431C shunt regulator.

I was able to make it change output voltage between 5V and 13.84V (the capacitors on the output side are all rated 25V). Anything more than 13.84V will cause the power supply to shut down (output ~ 0V) due to OVP.

I found this information about OVP in the datasheet of the SMPS control IC used in this power supply. Can you please explain how this OVP works without a known reference voltage as in the case for TL1431/TL431?

From the datasheet:

7.6.2 OverVoltage Protection (OVP)
An accurate output OVP is implemented by measuring the voltage at the AUX pin during the secondary stroke. As the auxiliary winding voltage is a well-defined replica of the output voltage, the OVP level can be adjusted by the external resistor divider ratio \$R_{AUX2} / (R_{AUX1} + R_{AUX2} ) \$.

Source: https://www.nxp.com/docs/en/data-sheet/TEA18361T.pdf

enter image description here

On my actual power supply, there are only 2 resistors 15 KOhm (153) and 100 KOhm (1003) that are directly connected to the AUX pin. To add insult to injury, these resistors are also connected in parallel, resulting in a resistance of about 13 KOhm (as expected) at their two ends. I expected to see two resistors in series to form a voltage divider around the AUX pin. Even if these two resistors were in series, their actual values would still not make any sense. I could change the pot to increase the output voltage to about 13.84 V before OVP kicked in and shut it down. I wanted to increase the upper voltage limit using the formula:

$$ OVP_{limit} = 3.0 * (R_{aux1} + R_{aux2}) / (R_{aux2}) $$

Or is there a way to just disable OVP altogether?

I do not yet want to remove these tiny SMD resistors prior to being able to understand properly how their values are calculated to produce the upper voltage limit.

There's a post about OVP voltage divider using another IC LTC4364 for which its datasheet provides a very clear example:

Setting Over voltage and Under voltage limits for LTC4364

For my case, the NXP TEA18361T datasheet and the actual power supply parts are just totally confusing to me.

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  • \$\begingroup\$ Can you post a schematic? What if you change the AUX resistor divider? What is your goal? \$\endgroup\$
    – winny
    Dec 1, 2020 at 7:48
  • \$\begingroup\$ Hi winny, I don't have a schematic for this power supply. I've tried to check all the resistors around the NXP TEA18361 IC to see which ones play a role in the voltage divider for the AUX pin 9. So far, none of these resistor values make any sense to me based on the formula: 3 = max_voltage * R AUX2 / (R AUX1 + R AUX2 ) . Thanks. \$\endgroup\$ Dec 2, 2020 at 1:20
  • \$\begingroup\$ Post the schematic from the datasheet. What is your goal? What are you trying to accomplish? Have you tried to change the ratio between Raux1 and Raux2? \$\endgroup\$
    – winny
    Dec 2, 2020 at 8:48
  • \$\begingroup\$ Hi winny, I added some more information in the question. \$\endgroup\$ Dec 2, 2020 at 16:32

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That 3V at the inverting input of the comparator is an internal reference voltage:

TEA18361T OVP subcircuit from datasheet

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  • \$\begingroup\$ Thanks a lot Vir. I missed that opamp drawing in one of their diagrams. \$\endgroup\$ Dec 2, 2020 at 1:21

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