Electronics > Beginners
Is my MOSFET switch design sane?
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Oaklander:
I'm designing a controller for LED module operating at 24 volts. The module will use about an amp of current and is controlled by Atmega32.
After research I found that N-channel MOSFET used as low side switch is the best approach. I also learned that two resistors are recommended on the gate. One is pull down resistor and the other is in series with the gate to reduce some oscillation.
This was clear but the selection of the FET was not. My requirements were at least one amp of drain-source current, easy to solder SMD-package and logic level. There were hundreds of parts matching this criteria on Digikey and I settled on DMN6140LQ for now.

The circuit I designed is in the attached picture. The label Y is connected to AVR output and the connector on the top is connected to the cathode the LED.

My questions are the following?
1. Is my design sane or is there some error?
2. Are there some rules to select components like the FET in this example? There are so many nearly identical options. Is the best way to just select the cheapest?
JustMeHere:
You need to figure out the power draw of your LEDs.  Also, I would lean toward a FET that has a heatsink.
digsys:
FET selection is usually by critical use requirements.
IF you are not switching faster than say 10Hz, then it is primary just a power switch. The main spec I'd be looking at is Rds(on).
140mR is quite high, at 1A that is ~140mW, a lot for that package. It means you now have to worry about heatsinking.
Look for the lowest Rds(on) and if needed a slightly larger package. They may even be cheaper.
Lt_Flash:
You need to put pull-down resistor before gate resistor, otherwise you are creating a voltage divider. The idea of that pull-down is to keep transistor closed when no input is connected to its gate, so it sinks current when you disconnect input and it doesn't matter if its doing it after gate resistor (current flows from MOSFET's gate to ground via these two resistors when you disconnect input). I know that because of your choice of resistors it doesn't really matter cause final voltage would still be almost the same, but if you put 1k-2k resistor in series with gate - the voltage would start to drop and that's not what you really want as many transistors require at least 4.5V to open and you don't want to lose any voltage. So just put that pull-down before series gate resistor and it should be fine :)
IanMacdonald:
If you're driving from 5v logic you need to ensure that the threshold voltage is low enough to ensure full conduction. The device you've chosen is OK in this respect.
The gate resistors don't really do anything useful. If you want gate overvoltage protection, you need a series resistor and a ~12v zener to ground. Shouldn't be necessary if the logic supply is a guaranteed 5v though.
Always use a device that's a bit beefier than the current suggests. I'd suggest a TO220 device for this kind of application. Remember it may have to stand abuse such as shorts, as well as normal working. On which subject, don't overlook the need for fusing or current limiting.
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