How to drive IR LED using a FET

[larsof54]

I wish to pulse an IR LED at 2 MHz and 20% duty cycle. I have an AVR programmed to deliver that frequency and duty cycle. I tried using a 2N3904 but it maxes out around 1MHz. Would a FET be a better way to go? I was looking at a BS170. Any thoughts as to it's suitability? I have never used FETs before so selection is kind of foreign to me. Oh ya, I want to drive the LED with about 150mA and my supply voltage is 3.7 Volts.

Any help with this will be greatly appreciated.

Larry

 

[larsof54]

I'm hoping this schematic will work. Any thoughts?

 

[IanP]

You are expecting to much from this poor MOSFET ..
It is not designed to switch at 2MHz (20%) pulses of the duration of 0.1us ..
Also, to switch the MOSFET’s gate you need a proper driver (for example complementary emitter follower) and a microcontroller pin is definitely not the one to do so ..
Take a look at:
**broken link removed**

At this low voltage and relatively high frequency it looks like a BJT is just a better option ..
Try something like these ZTXnnn-s ..

Rgds,
IanP

 

[FvM]

I was tempted to post a similar comment on saturday. But I realized, that a small MOSFET as BS170 could fairly work, also
without a gate driver. A slow transistor as 2N3904 has a considerable switch-off delay as saturated switch. I assume, that
a simulation or a real circuit test prove both variants to work for the given specification. BS170 is resulting in a minimum
part count, however. (A gate resistor may be supplied at will).

P.S.: Another minimal solution is a 2N3904 as emitter follower. It avoids saturated switching and respective delays, but the
additional Vbe voltage drop may be unpleasant.

 

[larsof54]

I wired up an emitter follower test and it works like a charm. Thanks all for your advice and input
LW

 

[m-n]

Please if I use this BS170 in a first quadrant chopper what will be the appropriate diode can i use?

Also ,please more information about driving this MOSFET directly from a microcontroller.

Thank You