Interesting details about H-bridges and MOSFETS

As I said, I salvaged my MOSFETS from old CRT computer monitors.

The T in MOSFET stands for Transistor. FETs, or Field Effect Transistors,
work in a different way than the more familiar BJTs or Bipolar Junction Transistors.

It's more common to use BJTs in applications where you want to use the Linear Region of Operation.  FETs are different  in two important ways:

1)   The Gate terminal has a very high impedance, which means almost no current  flows through it--the voltage you apply to it is basically creating a "field" or an electrical charge, that controls the current flow between the Source terminal and the Drain.

2) FETs are optimized for operation in the Saturation and Cutoff Regions --  completely ON (max current flow) or OFF (zero current flow) between Source and Drain terminals.

MOSFETS DO have a Linear region, but we want to avoid that! We want our MOSFETS to be either all the way ON or all the way OFF.

Why, you ask? First of all, if the MOSFETS don't turn on completely, then
they are limiting the current flow to our motor. Motors need their full design current if they are to put out their full design torque.

Secondly, if the MOSFET isn't completely on, then it is acting like a resistor. It's limiting current flow and therefore wasting some of the power you want to send to the motor in the form of heat.

N-Channel and P-Channel

There are two basic types of semiconductor devices...N-channel and P-channel.

N-channel devices are more common. They are simpler (and therefore cheaper) to manufacture, and they (generally speaking) work better and last longer.

Here would be a good place for a link to some technical discussion of what that's all about. I know I read it somewhere....check back later.

Three major differences between N-channel and P-channel MOSFETS:

1. N-channel MOSFETS turn On with a positive voltage at the Gate terminal. P-channel MOSFETS turn On with a negative voltage at the Gate terminal.
2. N-channel devices switch from On to Off faster.
3. N-channel MOSFETS have a lower resistance when in the ON state.

The MOSFETS I was able to salvage were all of the N-Channel variety. And of that number, most were IRFS640A. Here's a link to a datasheet for them.

There are designs for MOSFET H-bridges that use a mixture of N-channel and P-channel MOSFETS. Said designs have their own particular blends of pros and cons.  One pro is that you don't need a separate, higher voltage to apply to the Gates. But P-channel MOSFETS have some cons both in terms of cost and performance (as noted in above).

You turn an N-channel MOSFET On by applying a positive voltage to the Gate, relative to the Source terminal.

The two MOSFETS located in the lower vertical legs of the H-bridge only require the voltage to be higher than zero (ground) by 4.5 to 7 Volts.

With an N-channel MOSFET H-bridge, the voltage required to turn either of the two top MOSFETS On will be higher than the power supply voltage you supply to the Drain terminals.

Why is that?

To turn the motor On, we need to turn one of those top MOSFETS completely ON.
That would raise the voltage at  its Source terminal, which is connected to the motor, to the value of our supply voltage. But to do that, we need a voltage at the Gate terminal that is 4.5 to 7 volts HIGHER than our supply!


Since my salvaged MOSFETS are all N-channel, that's what my H-bridge uses.  Therefore, I needed some way to provide that higher-than-supply voltage to the gates of those two upper MOSFETS.

After talking with my friend Gus about this, I decided to go with his suggestion to build a Cascade Voltage Multiplier circuit, to convert the power supply voltage (nominal 12 volts) to approximately 21 volts DC.
 
And notice that the gate of one upper MOSFET is wired in parallel with the gate of one lower MOSFET in the opposite leg. The datasheet for any MOSFET should tell you what the upper limit is for just about anything--here we are concerned with the upper limit for "source to gate voltage".

In the case of my N-channnel MOSFET H-bridge, the voltage would need to be high enough to turn that upper MOSFET completely on and yet not high enough to damage its associated lower MOSFET.

In the case of the MOSFETS I used (IRFS640A) the absolute max for Source to Gate voltage is +/-30 volts. So the 21 volts my little voltage multiplier produces is plenty to do the job, and yet well within that limit.