DALI standard limits the maximum current as 250mA. I have a 12V source that is capable of supplying more current. I implemented from one of Motorola reference design the below schematics that limits the 12V source to 250mA.
DALI has 2 wires which are short circuited. so TX and RX can be thought as a loop. when 250mA is driven by R13 (2.7), it turns on Q3 which switches off the Q2 transistor, so that the power supply. It usually works fine but I experience that Q3 transistor burns. When Q3 burns it makes the all circuit to be useless. It happens rarely but it is till not a reliable solution. As it happens rarely I don't know how I can debug it. I assume a spike may cause the problem.
How can I improve the circuit so that Q3 would not burn and it would still limits the current supply to 250mA? Or would there be a better solution instead of the below circuit?