How to use a MOSFET

Where to put a MOSFET in a circuit and how to drive it?

There are many options depending on what it is you want to do, each with their own pros and cons. So I put together some examples that might give you an idea of which way you want to go.

Direct drive

The simplest solution to switch a load is by directly driving the MOSFET with a logic-signal, perhaps from a micro controller.

There is a resistor between the logic-input and the gate-pin of the MOSFET. It has two functions. First is to prevent ringing on the MOSFET's gate and second is to limit current in case the pin is driven by a micro controller.

I recommend a 470R resistor when driving from an Arduino and 10R when current limiting is already handled.

This does require the use of a logic-level MOSFET. They can switch to fully open when 3.3V or 5V is applied to the gate-pin of the MOSFET.

The MOSFET 'looks' at the gate-pin voltage relative to the source-pin. So to truly apply the logic voltage to the gate-pin, the source-pin needs to share the same ground as the logic-signal.

And that means we can not put the load between the MOSFET's source-pin and the circuit's ground. If we did that the source-pin's voltage would be well above ground and we can't get the gate-pin 3.3V or 5V above that voltage.

If that were to happen, the MOSFET gets stuck halfway open and starts producing lots of heat.

So the load needs to be between VCC and the MOSFET's drain-pin. Not really a problem if driving a single load. Cars used to do that many decades ago. The cars frames were at positive voltage, not ground, "the more you know :)"

Push-pull

What if we need more current for faster switching?

For that we can use a push-pull transistor circuit.

This circuit is also limited to using a logic-level MOSFET because the output voltage can not exceed the input voltage.

And because the output signal is tied to ground, this is also a low-side switching circuit.

Finally, because the use of transistors and their diode like internals, the output can not reach VCC or GROUND, there is a 0.1V-ish limit on that. But that should not cause any problems.

Besides the MOSFET, load and input signal, there are 1 resistor and 2 transistors. The transistors share the same base and emitter pins.

Q1: This is a NPN transistor. It will conduct when the voltage at the base-pin is HIGHER than the voltage at the emitter-pin. That makes the output voltage RISE until it is close to the input voltage.

Q2: This is a PNP transistor. It will conduct when the voltage at the base-pin is LOWER than the voltage at the emitter-pin. That makes the output voltage DROP until it is close to the input voltage.

Together that causes the output voltage to follow the input voltage.

R1: The resistor again is there to prevent spikes on the gate-pin of the MOSFET.

Inverted level shifter

If we need a higher voltage MOSFET, like 100V or more capable, then logic-level switching can become expensive. Especially if we also want more current capability and low resistance.

Those MOSFETs like to see 10V or more at the gate-pin (relative to the source-pin of course).

So we need to convert our logic input-signal to a higher voltage.

This circuit can do that, but with a catch. It inverts the signal! High becomes low and low becomes high. If an inverted signal unwanted, we can simply add another identical transistor setup before the first one, and the inverted signal gets inverted again.

Also, we need an extra power supply to give the circuit the 10V+ to send to the MOSFET.

R1: The transistor keeps the MOSFET's gate-pin at VCC while the transistor is not conducting. Thus turning the MOSFET on.

Q1: When the input is high. The base-pin of the transistor is at a higher voltage than the emitter-pin, because that is tied to ground. It draws all the current from VCC and the gate-pin, taking them close to GROUND turning the MOSFET off.

Please note: The resistor determines how fast the MOSFET can switch on, but when the transistor is conducting, that becomes a constant current draw to ground, limited only by that resistor.

Low side driver IC

Using a dedicated driver IC makes life a lot easier. We now can use a logic signal to drive the MOSFET with a higher voltage and higher current.

It does need a few extra components, a bootstrap capacitor for charging the gate-pin of the MOSFET and a diode to charge that bootstrap capacitor. It also need a 10V+ power supply and of course the resistor for the gate-pin of the MOSFET.

With a low side driver IC we are obviously still limited to low side switching.

These ICs are designed to source and sink large gate currents, which allows the MOSFET to switch much faster and spend less time in its high resistance (linear) region. That directly reduces heat production in the MOSFET.

Most low side driver ICs take a logic-level input and translate that into a higher gate voltage referenced to ground. Internally they often use a push-pull output stage, similar to a discrete transistor version, but optimized for speed and robustness.

Because everything is integrated, things like shoot-through current, timing, and gate discharge are handled much better than with a hand-built solution.

Some driver ICs also include extra features such as under-voltage lockout, thermal protection, or a current-sense input.

If the MOSFET supports current sensing, the driver can shut the gate off automatically when an over-current condition is detected. That can save us both the MOSFET or even the entire circuit.

So a low side driver IC is often the cleanest and most reliable option when you need fast switching, higher power, or better protection, without wanting to design the gate drive circuitry yourself.

Discrete capacitor bootstrap

After the previous circuits, which were all low-side drivers, this one can switch a load from the top. This offers several advantages::

1. We can switch power to another circuit while maintaining a common ground. This allows connections to other parts of that circuit.
2. We can easily measure voltage and current on the load.

How does it work?

From left to right, first we have the input signal, and a resistor to limit current from the logic output pin that most likely controls the switching.

Then there is R9 and Q12, forming a logic level inverter.

Now 2 things can happen.

Input signal HIGH:

• The inverted signal is low.
• Base of Q8 pulled low, Q8 starts conducting.
• Voltage in C2 now can get to the gate of MOSFET Q7.
• MOSFET turns on.

Input signal LOW:

• The inverted signal is high.
• Base of Q11 now also high, Q11 starts conducting.
• Base of Q10 now low, Q10 starts conducting.
• Gate of MOSFET Q7 now connected to ground.
• MOSFET turns off.

Diode D2 allows C2 to charge when the MOSFET’s source is at ground while the MOSFET is OFF. It also prevents C2 from discharging into VCC when the MOSFET is on.

Charge pump

Like the previous circuit, this one can switch a load from the top. This offers several advantages::

1. We can switch power to another circuit while maintaining a common ground. This allows connections to other parts of that circuit.
2. We can easily measure voltage and current on the load.

One aditional advantage is that this circuit can keep a load on 100% of the time, because the capacitor is continiously charged by the charge pump and doesn't rely on the down-cycle for that.

How does it work?

On the top, there is a classic charge pump. If the 2 PWM signals are out of phase (inverted) and 5V, we can create a voltage around 4 x 5V = 20V (minus a bit for the diodes). To drive the MOSFET's gate above VCC.

From left to right, first we have the input signal, and a resistor to limit current from the logic output pin that most likely controls the switching.

Then there is R9 and Q12, forming a logic level inverter.

Now 2 things can happen.

Input signal HIGH:

• The inverted signal is low.
• Base of Q8 pulled low, Q8 starts conducting.
• Voltage in C2 now can get to the gate of MOSFET Q7.
• MOSFET turns on.

Input signal LOW:

• The inverted signal is high.
• Base of Q11 now also high, Q11 starts conducting.
• Base of Q10 now low, Q10 starts conducting.
• Gate of MOSFET Q7 now connected to ground.
• MOSFET turns off.

Floating gate driver

Optocoupler

Transformer

High side driver IC