Hi,
I’ve got a little LED driver board that I made that is based on a FAN5333 driver (which is a boost converter) that I want to use to run a couple of LEDs for a project that I’m working on. The driver has one limitation which is that the output (drive voltage) must be larger than the input voltage.
Unfortunately the voltage drop across my LEDs isn’t enough to meet this requirement, so I’ve hatched a plan to put a reversed zener diode inline with the LEDs so that the total voltage drop through the load is big enough. Is this an astoundingly bad idea because it seems to work? Have I missed something?
Hi,
I usually pick the LED driver to suit the project, not the other way around but let’s see:
What is the current through the LED string?
What is the pn for the diode? we need to check it’s power ratings to make sure it can dissipate that power safely.
It’s not a bad idea but you are dissipating the power in that zener. Can you stack some additional LEDs in series instead? Or reduce the input voltage?
How many leds are you driving? What kind, esp color? If more than 1 are they in series or parallel? What is the input voltage?
Also for the rectifying diode, do you use a schottky or regular diode. Regular may be helpful in this case due to higher forward voltage.
If you do not have enough voltage on the LED string to operate the boost converter, it seems you should be looking at a buck converter instead? (or buck/boost)
Thanks for the feedback. To answer some of the questions:
There’s 80mA going through the string, with a voltage drop of about 2.5V across the LEDs that are in the string. The Zener I’ve got has a pn voltage of 3V, and is capable of 3W dissipation, so I’m way inside its capacity.
How many leds are you driving? What kind, esp color? If more than 1 are they in series or parallel? What is the input voltage?
It’s driving a pair of IR leds in series for a wildlife camera. I don’t really want to add extras (especially not any of the visible ones that I have on hand).
If you do not have enough voltage on the LED string to operate the boost converter, it seems you should be looking at a buck converter instead? (or buck/boost)
Fully agree. I used this circuit for a LED project a few years ago and got the board ordered without checking out the design in maximum depth. I’ve got it now, and the kids are keen to hang it up.
I think that the summary is that it’s not the optimal solution for the device, but it is a way to wriggle out of the place I’ve found myself in 
If you know the current you are driving them at, you could use a resistor instead of the zener. This implies that you always drive at same current…