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I want to use the circuit below to detect undervoltage at the input. I don't have any other supply but the input voltage, so I should use the input voltage as supply voltage.

The problem is that the input varies from 36 to 110 V and the TL431 can only tolerate 36 V for its cathode voltage.

Is there anything I can do to use the input voltage for the supply?

enter image description here

ocrdu
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Erfan Mirhoseini
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  • The current flow will be low (e.g. less than 5mA). So you can build a reference voltage from Vin (e.g. using a Zener diode) and use it as Vsup. – Rohat Kılıç Feb 21 '22 at 06:47
  • Thanks. Let's say I used a 12v zener, I would have 0.5 watts power loss a the resistor, isn't it too much considering the converter I'm using this detection for is a 100W converter? – Erfan Mirhoseini Feb 21 '22 at 06:58
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    Yes, there'll be a loss but the definition of "too much" depends on your efficiency requirements (With extra 0.5W loss the eff should drop by 0.5 to 1 percent). Actually, for a converter with 100W output and different functionalities, it's always good to have a separate auxiliary supply for output-unrelated uses such as UVP or driving a fan. If the converter is an off-line converter with a power transformer then having an aux supply is quite easy. If there's no chance to have an aux supply from the transformer then you have a few choices: A linear (e.g. zener) regulator or a buck regulator. – Rohat Kılıç Feb 21 '22 at 07:08

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