I'm some way into a project which involves switching 0.5W laser diodes at 100kHz, with a pulse of around 1us in width. I'm having issues with the diodes (which I don't have a datasheet for) failing after about a billion switching cycles.
Although I see a pretty well behaved waveform with my 60MHz scope, I'm thinking that the issue might be transients, possibly too fast to be seen on it.
The circuit I use is as follows (a bit simplified):
To switch the diode on, the FET is switched off, obviously. The 10R resistor keeps about 2V across the laser in the off state - that may be superfluous, haven't tried without it. The voltage across the laser in the on state is about 5.5V.The current source is an LM317 with a 6R2 resistor.
Now I am remiss that there is no transient protection, but I am wondering how to do it. A diode to protect against negative voltage is a no-brainer. But what about a clamping diode against overvoltage? Zeners are not fast enough I think. But what about a something like this?
D3 and D4 would be Schottky I suppose. The idea is that the clamping voltage is always present, so the speed of clamping is only dependent on the diode, which should be very fast. It's a little bit wasteful of power and parts, but that is not really an issue here.
The leads from the driver PCB to the diode are a bit long (about 15cm), that might also be improved on a driver PCB redesign.
Thoughts? if anyone has direct experience of these circuits and real-world experience to offer, that would be much appreciated.