# Low side switch

Discussion in 'General Electronics Discussion' started by Junior20, Mar 19, 2017.

1. ### Junior20

2
0
Mar 19, 2017
Hello everyone.
I'm trying to do low-side switch with N-MOSFET.

I don't want to connect something into it. My load should be resistor in order to see how this transistor works (turning on/off, power dissipation). I chose 600V transistor, 11A drain current, 0,3Ω Rdson. I know, it's big, but my goal is, to compare him to SiC transistor, so they need to be almost the same. Max Vdd is 30V. So, from Ohm's law i calculated that my load(resistor) should be 2,7Ω in order to achieve 11A drain current. But then i'll have above 300W power, which none resistor 2,7Ω can hold out. What can i do, to achieve 11A drain current? I heard, that i can connect more resistors in series and parralell and it helps me to spread power on single resistors, but i don't know how to do that.
Sorry if there are mistakes in english, but i'm from Poland and my english is not as good, as i want it to be

2. ### BobK

7,682
1,688
Jan 5, 2010
Use automotive (12V) bulbs. Two in series should be fine with 30V since they actually operate at 14V+ in a car. Use as many strings of 2 in parallel to take 11A.

And you better have a massive heat sink for that transistor and probably a fan.

11A at 0.3Ω is 36.3W.

If it is your intention to switch 30V at 11A there are much better mosfets. You don't need 600V and you can get on resistance of 10mΩ or less.

For comparison, this one would dissipate less than 1W and would not even need a heat sink:

60V 80A 7 mΩ

http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/FDP070AN06A0/?qs=sGAEpiMZZMshyDBzk1/Wi6u8DDnmpUwteajxSYi/CjQ=

Bob

hevans1944 and Junior20 like this.
3. ### Junior20

2
0
Mar 19, 2017
Thank You
Now i see it's some work with that, so maybe i will make lower drain current and supply voltage. Anyway thanks, now i have more possibilities

4. ### Audioguru

3,148
698
Sep 24, 2016
A cool incandescent light bulb resistance is about 1/10th its hot resistance so it will probably burn out your Mosfets and power supply when you first turn it on. Using a toaster or a heater are a little better but their resistance also change with temperature.

The output voltage from a micro is too low to fully turn on most Mosfets that need gate to source voltage to be 10V like the one in post #2. Look for a "logic level" Mosfet that turns on pretty well when gate to source is 4.5V.

5. ### BobK

7,682
1,688
Jan 5, 2010
I don't think 11 Amps worth of light bulbs would destroy a MOSFET rated for 80A continuous.

You do make a good point about the gate voltage though. You would need a level shifter to use this from a micro.

Bob