These are a couple of circuits that can drive a MOSFET. Each of them has their own pros and cons, so whether or not they are useful is highly dependent of what you want to do with them.
Table of Contents
Low side
1. Direct drive
This is the direct drive method. We can drive a MOSFET straight from our logic with 5 to 20 volts to the gate of the MOSFET.
You can control it manually by sending an ON or an OFF pulse.
Or send a PWM signal.
Since this is an low side MOSFET, the source of the MOSFET is always equal to ground.
When connecting the gate to our positive rail, the gate to source voltage is also the signal voltage, and if it is more than your MOSFET’s threshold, it will turn on.
And to turn it off you simply connect it to ground.
What are the pros?
- Simple
- Can be always on
- Can do PWM
What are the cons?
- Only low side switching
Images
2. Low side driver IC
This is a low side gate driver IC circuit. The TC427CPA for example is designed specifically to drive a MOSFET.
You can control it with a high or low signal.
Or you can give it a PWM signal.
It can take a 3.3 or 5 volt signal as an input. And depending on whether the signal is high or low…
..it will either send the drivers supply voltage to the gate to turn it on…
… or it will connect the gate to ground to turn it off.
What are the pros?
- Handle logic level
- Fast switching
What are the cons?
- Need an IC
Images
3. Push-pull
This is a push pull circuit.
You can not use a pulse to turn it on or off, it needs a constant signal.
Like a PWM signal.
This circuit amplifies the current send to the gate, this allows for faster switching. Your microcontroller will thank you for this.
When the signal coming in is high, the NPN transistor allows a larger current to come through from the supply, which is led to the gate.
Then when the incoming signal goes low, the PNP transistor starts to let current through. From the gate to the ground, and turns the MOSFET off.
What are the pros?
- Faster switching
- PWM
- Can be always on
- Cheap
What are the cons?
- Needs constant signal
Images
4. Inverted level shifter
This is the inverted level shifter circuit. It allows us to send a voltage higher than the signal is, to the gate.
It is controlled by two separate pulse signals, and it can not run on PWM
When we pull the first signal to ground, the second PNP transistor will start conducting…
…and discharges the gate into ground, turning the MOSFET off.
…if we pull the second signal to ground, the first transistor will start to conduct…
And the supply voltage is able to reach the gate of the MOSFET, turning it on.
What are to pros?
- Higher voltage to gate
- Can be always on
What are the cons?
- No PWM
Images
High side
5. High side driver IC
This is the high side driver IC circuit. The IR2125 used in this example is designed specifically two drive a MOSFET on the high side.
It can not be controlled by manual signals (unless you are really, really fast), only PWM will work. This is a safety feature.
When a low signal enters the input pin, it connect the gate to the source of the MOSFET, turning it off.
The source is then at ground level, and also the capacitor that is connected to it. This capacitor then gets charged by the supply voltage through the diode.
And when the incoming signal is high, the positive side of the capacitor gets connected to the gate of the MOSFET, and since its negative side is always connected to the source, the voltage applied to the gate is always relative to the source.
What are the pros?
- Fast
- Safe
What are the cons?
- No manual signaling
- No always on
Images
6. Bootstrap circuit
We can also drive a MOSFET on the high side without an IC using discrete components.
This is a bootstrap driver circuit, it works similar to the driver IC, but without the safety features.
It can accept manual signaling. But no PWM.
When the second signal is pulled low, the gate-transistor starts to conduct. And the gate pin of the MOSFET gets discharged to the source pin. This turns the MOSFET of and causes the source pin to be at ground level.
The capacitor negative pin connected to it now also is at ground level, The capacitor can now charge through the diode connected to its positive pin.
If the other signal is pulled to the ground, then the second PNP transistor conducts and the capacitor now charges the gate. Turning the MOSFET on. The capacitor is still connected to the source so the voltage applied to the gate stays relative to the source.
What are to pros?
- Cheap
- Fast
What are the cons?
- No always on
- No PWM
Images
7. Floating gate driver
This is a floating gate driver. It works by using an isolated supply for signaling and driving, and connecting its ground to the MOSFETs source pin.
It can do manual signaling.
And it can also do PWM.
What are the pros?
- Simple
- Can do always on
What are the cons?
- Needs a separate supply
Images
8. Opto-coupler
This is the opto-coupler driver. This circuit uses a second supply connected to the source of the MOSFET. The signals however can share the same ground with either supply.
It can do manual signaling, but no PWM.
When the first signal is pulled high, it powers the diode in the first opto-coupler, this causes the transistor to conduct and through the red jumper wires it connects the separate supply voltage to the gate. The ground of that supply is connected to the source pin, so the voltage is always relative and the MOSFET turns on.
The other signal, when high, activates the second opto-coupler. This then uses the two black jumper wires to connect the gate to the source, turning the MOSFET off.
What are the pros?
- Can do always on
What are the cons?
- Needs a separate supply
Images
9. Transformer
At this moment I do not have a transformer so I can’t test this circuit.
10. Charge pump
This is the charge-pump circuit, it works a lot like the bootstrap circuit.
It has a classic charge pump, that charges a capacitor eight volts above the MOSFETs supply voltage.
When the first signal is pulled low, a PNP transistor conducts and allows the capacitor to charge the gate to an absolute voltage of supply plus eight, turning it on.
the second signal, when pulled low, controls the gate-transistor we have seen before. When this PNP transistor conducts it discharges the gate to the source of the MOSFET.
What are the pros?
- Can do always on
What are the cons?
- Needs relatively stable supply voltage.
Images
Video
Summary
I made a table listing all the methods in short.
| Nr | Name | Description | Signaling | Always on | Pros | Cons |
|---|---|---|---|---|---|---|
| 1 | Direct drive | Straight from your logic to the gate. | Single signal line to pull up or pull down. | Yes | Simple, always on, PWM | Only low-side |
| 2 | ||||||
| 3 | ||||||
| 4 | ||||||
| 5 | ||||||
| 6 | Bootstrap circuit | Charges a capacitor with negative attached to the source pin when MOSFET is off. Discharge it into gate to switch the MOSFET on. | Two separate signal lines that are pulled to ground to signal. | No | Cheap, high-side, fast | No PWM, no always on |
| 7 | ||||||
| 8 | ||||||
| 9 | ||||||
| 10 | Charge-pump circuit | Uses PWM to charge a capacitor to a voltage above the supply. Two transistors either remove or apply this voltage to the gate of the MOSFET. | Two separate signal lines that are pulled to ground to signal. | Yes | Cheap, high-side, fast, always on | No PWM ,needs a steady supply |