I'm designing an LED stroboscope, and I have some dual MOSFETs from hard drives (where they drove the platter motors) that I'd like to use for switching the LED rapidly. Because there are two MOSFETs in each package, and just one is sufficient for the current and voltage, I thought I might as well come up with a way to use both. In most of the ones I have, the two MOSFETs are unconnected to each other internally, so I can connect them in any way or use them for totally separate purposes. I came up with a couple of ideas that I haven't seen used before, so I'd like some input on how well they might work.
Circuit 1 is just the single-MOSFET base circuit, where there's one MOSFET doing regular hard switching on the low side of the LED, driven by a gate driver. This is presented only for context for the other circuits.
Circuit 2 has two MOSFETs in series. One of them still does hard switching, while the other operates in its linear region, acting as a variable resistor to regulate current through the LED. The current is measured across a shunt resistor (which, despite being in the same place, has a lower value than the resistor in circuit 1, because it's no longer ballasting the LED on its own). I have a few questions about this circuit. I put the current source MOSFET lower, against ground, so that it would have a constant ground reference even when the hard switching one is turned off, because I thought that might help avoid oscillation. Is that reasoning sound and well-founded? Would having them swapped be better for some reason? Should one of them even be on the high side of the LED in this circuit? (It's a 12 V LED, so that'll need a high-side driver or me to pick one of the devices with a P-channel MOSFET.)
Circuit 3 also has two MOSFETs in series, but this time, they're both doing hard switching. One of them turns on to provide the rising edge of the current pulse, and the other one turns off to provide the falling edge. This is intended to achieve shorter pulse lengths than one MOSFET could (because of the time it takes to move the gate charge). Is this a good idea? Which way around should the MOSFETs go? I tried to optimize their arrangement for stability when one is off, but I don't have much MOSFET design experience, so I'm not sure this is actually the best arrangement.
Circuit 2 is the one I'm more interested in using in this project.