So I've been trying to make a H-bridge motor driver but for some reason my "logic level" mosfets have massive voltage drops when controlled with 5 volts.
I have tried 2 different types: IRL2203N and P70NF03L. Both are advertised as logic level, yet my 5v drain-to-source voltage drops to 3.3V when 5V is applied to the gate. I tried controlling 12 volts with it and it dropped to ~4V when controlled by 5V from arduino and to 10.6V when controlled with 12V from the same power source.
Does it make any sense to control my mosfets with yet another transistor that is controlled with 5V from arduino which then enables 12V to pass to the MOSFET gate?
This would be a very good place for a schematic (not a Fritzing breadboard mess).
Did you also tried to just control the load with a single (low side) mosfet?
Because you're probably in trouble with the high side of the H-bridge. If you also use a N-channel at the high side you need to pull the gate above the high rail to turn it on. Also, you probably may not pull it to GND to turn it off...
septillion:
This would be a very good place for a schematic (not a Fritzing breadboard mess).Did you also tried to just control the load with a singe (low side) mosfet?
Because you're probably in trouble with the high side of the H-bridge. If you also use a N-channel at the high side you need to pull the gate above the high rail to turn it on. Also, you probably may not pull it to GND to turn it off...
Wait... so I'm not supposed to use N-channels at the high side? I havent actually built the H bridge yet, I have just been testing the mosfets by themselves. I literally have nothing wired up except +5V at drain, +5V at gate and measuring 3.3V between source and ground. With +12V drain and +12V gate it respectively measures ~10.6V. Am I using these wrong?
Marsunpaisti:
Wait... so I'm not supposed to use N-channels at the high side?
Yes, but you need to drive the gate at least 5V higher than the supply voltage of the motor, and it cannot be lower than the supply voltage. You need a bootstrap gate driver.
Marsunpaisti:
Wait... so I'm not supposed to use N-channels at the high side? I havent actually built the H bridge yet, I have just been testing the mosfets by themselves. I literally have nothing wired up except +5V at drain, +5V at gate and measuring 3.3V between source and ground. With +12V drain and +12V gate it respectively measures ~10.6V. Am I using these wrong?
The way you describe that you have no load, motor or resistor etc, connected. So you're trying to pull infinite current through the MOSFET and the only reason you're reading 3.3V is because the power supply is collapsing.
You need a load of some sort between +5V and drain.
Steve
PieterP:
Yes, but you need to drive the gate at least 5V higher than the supply voltage of the motor, and it cannot be lower than the supply voltage. You need a bootstrap gate driver.
Oh right. So N-fets are for low-side switching. How should I switch the high side for the H-bridge then?
Use P-channel MOSFETs.
Or as mentioned above, a bootstrap gate driver (which basically includes a boost converter to bring up the voltage by 5-10V above the high value).
Do make sure you don't switch on both the high and low side of one half of the bridge at the same time or you get shoot-through. That normally leads to lots of heat, often followed by smoke, fire, or (for high powered ones) explosions.
wvmarle:
Use P-channel MOSFETs.
That doesn't work if the logic supply voltage is lower than the motor supply voltage. P-channel MOSFETs are not normally used for motor drivers because of their lower charge carrier mobility, which causes them to be less efficient. They're also more expensive.
As I said before, you need a gate driver. Those use bootstrapping capacitors to get a voltage higher than the supply voltage.
Read chapter 5.2 of this document: http://www.ti.com/lit/ml/slua618/slua618.pdf
PieterP:
That doesn't work if the logic supply voltage is lower than the motor supply voltage.
You just have to add a transistor to switch the gate in that case. For most motor driver applications (12-24V) you don't even need a logic level pmos, just pull the gate to GND and you have -12V or -24V on the gate (the latter may even be too much for some MOSFETs).
Whether to use a pmos or bootstrap an nmos high side depends also on how much you're switching. An nmos generally has a lower on resistance indeed and may be a bit cheaper, the more expensive driver circuit makes up for that.