Solution Finder


Posted by: Pramod Natu on Thu, 03/22/2012

I have designed 30W led driver using DER 286 as ref.
I have followed fig 4 not fig 3 in DER 286.
everything works fine for 20 min. no heating stable voltage & current at output.
After 20 min latch activates for unknown reason.
I have checked layout many times and it mostly resembles the pcb given in der286.
If I check the the voltage at the base of Q2 it rises very slowly and when it reaches 600 mv latch activets.
Voltage across Vr3 is also within limit.
can you tell me where to look for such kind of fault.


Submitted by PI-Jono on Thu, 03/22/2012

Maybe the zener used has more leakage current than the design sample. As such you need to
1. Add a resistor across the base to emitter of Q2.Use 100kOhms to offset the leakage.

2. If it does not work,then maybe the zener voltage is operating near the knee curve. Use slightly higher zener voltage in that case.


Submitted by Pramod Natu on Fri, 03/23/2012

Thank you very much for you reply.
while waiting for the reply what I did was , exactly the same thing that you have suggested . Only the value I used is 2.2M instead of 100K as suggested by you.
Now my driver is working very fine
thank you once again

Submitted by PI-Jono on Fri, 03/23/2012

We are glad your board is working fine and thank you very much for your feedback. Enjoy your design.