How to drive IR LED using a FET

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larsof54

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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
 

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

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
 

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.
 

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

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
 

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