LED failure detection circuitry gives intermittent fault

[eem2am]

led failure detection

Hello,

Please could you examine the enclosed circuit which is a parallel LED driver with LED failure detection.

whole schematic

(http://i32.tinypic.com/1z67b45.jpg)

Close up of LED failure detection circuitry

(http://i32.tinypic.com/2lw7wra.jpg)


The LED current in each LED is 300mA .
Input volts is 13.5V

When a LED fails , capacitor C13 charges up, then switches Q6 ON, which turns off the PWM controller by taking the COMP pin low.

Unfortunately, every about 1 in 10 PCB’s with this circuit do not come ON when switched ON.
-the LEDs flash for about 400ms and then the LEDs just go OFF.
-Then, when switched ON at another time, it maybe works OK.

So , what is happening is that this circuit is giving intermittent faults

-that is , it is sometimes switching itself OFF when the LEDs have NOT actually failed.

Please – do you agree with this here conclusion about why these intermittent faults are occurring with this circuit………….

Q4 parts have variable hfe’s –indeed the hfe will vary with temperature also.
-So sometimes, a particular Q4 on a particular PCB will not be switched on by the current through R22 and so will not manage to stop C13 from charging up and turning Q6 ON , -which turns off the PWM controller even when a LED failure has not occurred.

Do you agree with this analysis?


(By the way, the actual circuit uses a ZXLD1360 but it’s the same for the purpose of analysing the LED failure detection circuitry.
Also, the failure detection signal is also passed on to an indicator LED etc but that’s not important to show here.)

Added after 1 hours 38 minutes:

BTW here is the .asc file

**broken link removed**


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