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I need advice for a power transistor

Hi all,

Since DanM posted his laser blog I was hooked to the idea of pulse driving laser diodes.

I have created my own pulsing hardware using an ATMega328 to generate a 62.5 ns pulse at 16 Mhz, driving the diode trough an ULN2803 (Toshiba made) which seems to be able to respond well to such short pulses. 

However the ULN2803 is limited in power output so the highest current I could sink is about 700mA (I know the ULN2803 is limited to 500mA yes) by parralelling outputs. At this level the ULN2803 is way over its limits and starts overheating.

So to proceed with higher currents (I'm positive the diode can take at least 1A in pulsed mode) I'm looking for a NPN transistor with at least 1A/12V rating, faster then 60 ns on/off switching times which can be driven with 5V TTL logic .

Has anyone suggestions? I'm sure there are lots of options out there but I have a hard time finding the "right" one, I need an expert :)


Later edit: Thank you all for the input, I'm settling on a MOSFET, I'm sure I can locate some in the electronic junk I have at home to do some testing. If the laser diode still holds up at 1 A I'll mount it on the Phoenix - see if cutting is possible which such a low power diode in pulsed mode :D

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Aren't Darlington transistors slower than single transistors? I'm not sure he'll get the nanosecond on/off times he's looking for with that.

You and OddBot both mentioned FETs, which may be a better approach.

Yes, you are quite correct. As a general rule they are slower. Same thing as cascading 2 or 3 transistors. You have the 'turn on', 'turn off', etc. times of both devices to contend with. As with the other types of semiconductors mentioned, some can be found that will be fast enough.


At this point I would suggest you look at high voltage FET's such as an IRF820 which is rated for 2.5A @ 500V and has raise / fall times of about 8nS.

There are many factors that determine the switching speed of the FET such as how much current is being switched and the voltage you apply to the gate. The higher voltage FETs seem to switch quicker.

You might find better luck using a FET?


I searched around a bit, but the 60 ns on/off time is pretty tough to find!

I think I should have mention that my initial searches did not get me anything conclusive or parseable, hence I have asked LMR. I did stumble on the above also ... but thanks anyway:)