using transistors to switch LED's with floating load buck regulator

I have a project I am trying to do where I use a Zetex ZXLD1360 constant current buck regulator to drive either one of two strings of three high power LED's.

I have string of blue and a string of red LED's. The output from the reguator is connected to two NPN transistors, which control which string of LED's is turned on (the LED and inductor are connected to the emitters of the transistors). I use a microcontroller to put the base of each transistor (through a resistor) either high or low to turn on or off the LED's. I am using PWM to control the brightness of the LED's.

The probelm I am having is when I turn on the red LED's, the blue LED's are still illuminated (and vise versa), but very very dimly (you can barely see them on, but they are still on).

I am trying to fix this problem... my guess as to what may be happenning is that since the buck regulator uses an inductor, then maybe it's dropping below ground with respect to the ground signal from the microcontroller, meaning that even though the base of the transistor is ground, the emitter is still lower, turning it on a little bit.

If I pull the microcontroller out of the circuit and just put 5V (through a resistor) on the base of the transistor, the string that isn't turned on stays completely off (to the eye), so I think the issue is that there is a very short period where the turned-off transistor turns on, and when I am using PWM, the rate this occurs is enough to make the turned off LED's glow visibly.

I am trying to fix this problem so that only the LED's I want are turned on.

Does anyone have any ideas? I may be totally wrong on why the turned- off LED's are lighting up dimly, so if anyone has any ideas on what may be causing this, I am all ears. Also, if anyone has any ideas on a fix, I am all ears. I was thinking maybe tying the base of the transistor to the emitter with a resistor would help?

If anyone has any suggestions on what to check to identify the probelm, or any suggestions on how to resolve it, I would love to hear about it.

Thanks, Chris

Firstly, are you turning them completely off, or using PWM? Pulse-width modulation schemes often have minimum duty cycle, not 0%.

Second, what does the current regulator do when its load is open circuited? Current regulators are intended to feed a SHORT CIRCUIT gracefully, but an open circuit is just going to unbalance the current feedback. It sounds like your problem could be a bit of AC sneaking through (for instance) the Miller capacitor, turning a transistor ON somehow; the current regulator, or the PWM source, or connecting wiring, could be the AC source.

Probably not your issue, but LEDs are known to take a while to turn off (due to charge storage); allow a few microseconds to drop the light output of an LED after removing power. Also, some (especially white) LEDs use phosphors that are slow to decay (maybe milliseconds to seconds).

using transistors to switch LED's with floating load buck regulator - problem