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):


simulate this circuit – Schematic created using CircuitLab

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?


simulate this circuit

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.

  • 1
    \$\begingroup\$ You might consider a gate resistor to slow down your switching times, so your current source doesn't have to react so fast to a changing load. I know this is not what you're asking, but if this is what's causing your transients it may eliminate them. \$\endgroup\$ May 22, 2019 at 20:26
  • \$\begingroup\$ yes, in fact there is something like that in the circuit, and I might look at it again and try to slow the switching down a bit more. It also occurs to me that this circuit doesn't do much about switch-on transients. There are several levels of DC-DC converter supplying the +12V, the last one being a Traco power one. That might also be an issue. \$\endgroup\$
    – danmcb
    May 22, 2019 at 20:29
  • 1
    \$\begingroup\$ I would take a second look at the AC output impedance of the LM317 as a current source, I suspect it's quite poor. \$\endgroup\$
    – sstobbe
    May 22, 2019 at 22:19
  • 2
    \$\begingroup\$ That's only a few hours of runtime which is pretty poor. Definitely avoid overshoot on current and avoid parasitic inductance of long traces and reverse voltage . Overvoltage? your laser is a faster than zener Just avoid overcurrent in >10 ns range \$\endgroup\$ May 23, 2019 at 0:23
  • \$\begingroup\$ So, you are using laser diodes that don't have a data sheet eh? Good luck. \$\endgroup\$
    – Andy aka
    May 23, 2019 at 7:31

2 Answers 2


Use a GROUND plane.

And make the enclosed area of M1, R1, D1 about 0.5 cm on a side. In other words, you should get the inductance, through which the switched-current flows, down down down to just a few nanoHenries.

Given Vspike = L * dI/dT,

with 100 nanoHenries inductance (4", 100mm, of wire NOT over a plane) and 0.2amps switching in 5 nanoSeconds, the

Vspike = 100nH * 0.2amp / 5nsec = (the 'nano' cancel) == 4 volt spike.

Notice the spike-generator is M1/R1/D1. You can place the gate-driver some distance away, but use twisted-pair or coax-cable to connect to the FET's gate. And explore using source-termination (approximately 100 ohms at input to the TwPair, or 50 ohm input to coax). If you choose to look at the gate signal, use a 1pF or 2pF active-probe, with the probe's GND connected only 1cm away to the GND plane.

  • \$\begingroup\$ yes, this is helpful. Indeed we probably need to get the driver electronics much closer to the diode. Also we can slow the rise/fall times quite a bit, which will also help. Thank you. \$\endgroup\$
    – danmcb
    May 23, 2019 at 10:16

Thanks to those who posted suggestions, and particularly analogsystemsrf. The solution turned out to be a redesign of the driver board. Instead of a MOSFET switch, I went to a discrete bipolar design, basically because it turned out to be easier to achieve a controlled ramp up and ramp down of laser current. The current source was also improved by using a discrete PNP transistor.

We also did some physical redesign to reduce lead length to less than 100mm.

With the redesign, the voltage/current waveform is very well behaved, with rise/fall times of about 100ns, and no detectable ringing when viewed with the scope. As the pulse width has a minimum value of around 500ns, this is quite OK.

I also added some TVS diodes at all input ports to give some protection against ESD from normal handling of the unit.

On a first test, we've been able to scan for some hours with no issues with the laser. Hopefully the problem is now solved.


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