This is an N channel MOSFET and this is an P channel MOSFET
and most of the time, in my projects I control these transistors with an Arduino for example.
This particular configuration is called a MOSFET half bridge or inverter where we have the high
side and the low side and the output in the middle. But controlling these MOSFETs with an
Arduino, is not that easy as connecting digital pins to their gates. If you’re making projects
that include MOSFET control you must know what types of MSOFETs we have, what is a driver,
how power dissipation will affect your circuit, how a MOSFET gate opens, how to control the low
side and the high side and most important of all, when using a MOSFET bridge, what is bootstrap
and how to implement it. In this video we will learn all that, so guys, let’s get started.
What’s up my friends, welcome back.
All these are MOSFETs and there are thousands of models and a lot of types of MOSFETs.
But one of the main differences is their channel type. It could be N or P.
The main difference between an N-Channel and a P-Channel MOSFET is that N-Channel
usually goes to the ground or negative side of the load, and the P-Channel to the VCC
or the positive side and that might result into problems when using them in a circuit.
But before we go into those problems and the possible solution, let’s see some very common
MOSFET examples. This is the MOSFET I use most, the IRFZ44N. It is an N-channel MOSFET,
could withstand 49 amps, 55 volts and it has a RDS resistance of around 17 milli ohms. However,
this is not a logic level MOSFET so we can’t control it directly with the Arduino, because the
Arduino works at only 5V and with that voltage the gate of the IRFZ44N will not fully open.
On the other side we have the IRL Z44N and the L stands for logic level. That means it will
fully open with 5V at the gate so we could easily use it with signals from an Arduino for example.
Actually I’ve used this MOSFET with my electronic load project because in that way I could use the
DAC module with a maximum voltage of 5V to control the electronic load. Check that video below.
We could also have the IRL I 24 4N and the I would stand for insulated.
You would see that the back pad of the MOSFET is insulated with a plastic tab, so we could
connect it directly to a metal case or other parts without the danger of creating a short circuit.
With a metal pad you must use thermal insulators and plastic
screws if you want to have decent insulation. And here are some other
N channel MOSFETs I usually use with my projects and the main specs of each.
On the other side, we have the P channel MSOFETs. As mentioned before, N-channels
are better with the LOW side and P-channels are better with the high side. That’s why we
connect the N-channel MSOFETs with the source to ground and the load connected to the drain.
And we usually connect the P-channel with the source to VCC of the high side and the drain
is connected to the load on the low side. N-channels will open their gate when the
voltage between the gate and source which is called GS voltage is getting bigger.
That’s why we usually have pulldown resistor to ground at the gate of an N channel MOSFET.
In that way the voltage between gate and source is 0 so the MOSFET is OFF.
Then, just by applying a positive voltage at the gate, we start opening the gate and the
MOSFET will start conducting current towards ground and by that, we supply the load.
On the other side, the P-channel MOSFET will start opening the gate when the voltage at the
gate is getting lower than the voltage at the source. But in this case the gate has a pullup
resistor to Vcc, so the GS voltage is Vcc minus Vcc and that’s equal to 0, so the MOSFET is off.
In this case we must pull the gate to ground in order to turn on the MOSFET.
So you see the difference? We turn on an N-channel MOSFET with higher values than
the source voltage and we turn on a P-channel MOSFET with lower values than the source voltage.
That’s why in digital we say that we activate the N channel with a
1 and the P channel with a 0 where 0 stands for ground and 1 stands for Vcc.
The problems start when we add a powerful load to the P-channel configuration. Usually,
P channels have an ON resistance a lot higher than the N channels as for example these two MOSFETs.
A higher resistance will result into a lot more power dissipation
and that will heat up the MOSFET a lot with high current values.
For example my ESC project was working with current values of 10 to 20 amps.
If I were to use a P MOSFET like the IRF96 30 with a ON resistance of 800 milli ohms,
that will dissipate a lot of power and for sure the MOSFET will burn out.
So, instead of using P-MOSFETs for the high side, we sometimes use N-channel
MOSFETs as well because they could have ON resistance of only a few milli ohms.
But here comes the problem. Now the load is connected at the source of the N channel MOSFET.
We add a pulldown to the gate so the MOSFET is in the OFF position so the voltage between the source
and the load is 0. Let’s say is a logic level MOSFET and I apply 5V at the gate, the MOSFET
will turn on because the GS voltage is 5 minus 0 equal to 5V. But when the MOSFET turns ON it will
also apply VCC to the LOAD so now the voltage at the source is VCC which could be for example 12V.
So at this point the GS voltage is 5V minus 12V equal to -7 volts, so the gate voltage
is lower than the source voltage so the N-channel MOSFET will be turned OFF.
That’s the main problem of this configuration.
To solve this we use what is called bootstrap. This is the most-simple bootstrap schematic.
The N channel MOSFET is connected as before to 12V, the load is below but now we have a
diode and a capacitor connected between the drain and the source together with
two resistors and a NPN transistor. When the input signal is HIGH, the NPN transistor is ON.
That means that the gate of the MOSFET is connected to 0 V so the MOSFET is OFF.
But at the same time this capacitor will charge up through this diode. It will charge up at 12V.
So now, when I place a LOW signal at the NPN transistor base, it will turn it OFF.
So at this exact moment, the voltage between the source and the gate will be 12V of the
capacitor in series with 12V from VCC so 24V. So this time the GS voltage is 24 - 12 equal
to 12V so the MOSFET is still ON. That’s what bootstrap is doing, elevating the
gate voltage so the gate to source voltage is always higher so the MOSFET is still ON.
One thing to have in mind. If the ON and OFF switch process is very slow, the capacitor
might have time to discharge through the 10K resistor or leak through the reverse of the diode.
In that case the voltage in series won’t be 12V anymore and it might not work for slow switching.
Instead of using this crude circuit, you could directly use a MOSFET driver which already has a
bootstrap function inside. For example, for my ESC project I was using the IR 21 04 driver. If you
look at the schematic, this driver is connected to the gate of the high side, the low side but
also to the middle position where the load is connected to one of the phase of the motor.
We also have a diode and a capacitor connected to this middle position for the bootstrap process.
That’s how I was able to use N channel MOSFETs for both sides of the bridge.
The low side is controlled very easily because the source is connected to ground.
And the high side is controlled with the bootstrap process which adds up the capacitor voltage so the
gate voltage will be higher than the source, so the gate will open as well, and that’s it.
The driver does this automatically and with protection included.
A driver will also work at higher voltages, for example this driver works at 20V. It gets logic
level at the input but it could output 20V so we can forget about the logic level MOSFET problem.
That’s why it is a lot better to use a dedicated MOSFET driver.
I usually use the IR 21 04 and the IR 21 01. So guys, now you know a bit more about MOSFETs,
about voltages at the gate and how to control them, about what is bootstrap
and why we use this implementation and also why MOSFET drivers are the best solution.
I hope this will help you design better circuits for your projects and to learn
more check my ESC project where I use a triple bridge and 3 derivers at the same time. Also my
electronic load video where I use a logic level MOSFET and control its gate with a DAC module.
Another interesting project you might want to check is the portable soldering iron.
In that cased I’m using a P-channel MOSFET and the load is connected on the LOW side.
Check that schematic to see how to overcome some of the problems.
So guys, consider giving me a like or comment below. Thanks again and see you later guys.