MOSFET controlled HP LED circuit

[lzhong1]

I have designed a NMOS controlled high power LED circuit as attached.
Purpose of this circuit:
1. when one of the LEDs degraded (Vf decrease), the current flow through it will be decreased as well.
2. the rest LEDs which working in normal condition will get more current since the total current is supplied by a constant current driver.
3. LM358 will amplifier the voltage across LED then supply to the Gate of corresponding MOSFET.
4. MOSFET works like a voltage controlled resistor.

But the problem is:
1. It's hard for me to set the corresponding resistance value to each resistors
2. When I set the R1:R2 of LM358 as 1.5:1, the result is correct, however, if adjust to 2:1, the result is wrong.

Hope anyone can help me to find out the circuit problem or how to set the values of the resistors!
Thanks in advance!!!

 

[schmitt trigger]

From your description, the circuit is not stable.

If a LED decreases its Vf, your circuit will attempt to drive less current thru it.
This in turn will further drop Vf, which will further decrease the current, further decreasing Vf......etc.

 

[lzhong1]

Thanks for your reply!
Actually, I am going to design a current splitting circuit for parallel high power LEDs with constant current source.
Right now, the LED is under normal current operation range (100 mA to 1000 mA).
As I known, the LED will only be degraded (Vf drop) when over current or over heated.
Or do you have any idea to design such a circuit?