Hi,
I am looking for a circuit which limits the voltage but it should have a leakage current less than 1mA.

My requirement is the following:
1. I am getting 5.6V from the boost circuit, I need to use a super cap at R6. I can use a zener to limit the voltage but it is having a very high leakage current. The zener diode i used has a zener voltage of 3.9V. If I go for very high zener voltage of 5.6 or 10V it will have low reverse bias current. but it will not meet my certification requirements. He applies a fault condition of Vz*1.05 which will damage the super cap.

Any insights on circuits of it should have a over voltage protection of 3.9V yet the current leakage should limit to less than 1mA?

I tried below circuit. in terms of voltage it is good but has a very high leakage current of 142mA.

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Thanks,
Ashok

 

A TL431 Zener diode will work down to 1mA, but what are you trying to achieve?

 

Below is the sim of a circuit using a TLV431 programmable shunt reference (Zener):
The TLV431 is a lower current and voltage version of the TL431.

The output voltage (green trace) is clamped to 3.9V for the R1-R2 resistor values shown.
The quiescent current should be less than 100μA below its regulating point.

 

Any shunt arrangement to limit voltage will certainly have a high leakage current. That is the truth. To limit the voltage for a load a series voltage regulator will avoid leakage current, but it will still not be as efficient as it should be.
That is all the advice I can give as we have not enough information..

 

Any shunt arrangement to limit voltage will certainly have a high leakage current.

The circuit I posted has less than 100µA, which is well below the <1mA the TS mentioned in his first sentence.

 

The circuit I posted has less than 100µA, which is well below the <1mA the TS mentioned in his first sentence.

As the voltage rises a bit that current will also rise. How else can a shunt type voltage limiter function??

 

As the voltage rises a bit that current will also rise. How else can a shunt type voltage limiter function??

Of course.
I was referring to the quiescent current before the circuit limits the output.

 

use a 5V6 zener in a series regulator , then drop the voltage to 3.9(ish) with 2 diodes.
This is just a sim , you would have to build the circuit in real life to test.

 

or use the tlv431 as mentioned earlier , but in a series regulator configuration. This shows a normal tl431 , but I didn't have the model for the tlv. Same circuit. This is just the most basic circuit , in reality you may have to add some comp network on the ref pin of the tlv , but that won't affect the current.

 

or use the tlv431 as mentioned earlier , but in a series regulator configuration.

That will have a least a 1V minimum voltage drop.
Don't know if that's acceptable for the TS's application.

If he use's a series regulator, then it likely should be one with a dropout voltage of near zero volts.

 

It might work to use a mosfet forced into saturation under normal conditions.

Now a question: Is the rated voltage to be held during an over-voltage incident? Or is a droput to some lower level, or zero, acceptable.
Do we even know what the over voltage-protected device is??

 

The circuit I posted has less than 100µA, which is well below the <1mA the TS mentioned in his first sentence.

its working thanks!