9 Ways to Drive a MOSFET: Technical Breakdown

Driving a MOSFET seems simple, but choosing the right circuit depends on your switching speed, voltage levels, and whether you are switching the high side or low side. Below is a summary of the nine methods covered in the video.

April 28, 2024 : Viewers have discovered some errors in the video/schematics, be aware!

• 0:00 Direct drive
• 0:57 Low side driver IC
• 1:46 Push-pull
• 2:41 inverted level shifter (NOTE: Should be PNP transistors in schematic)
• 3:31 High side driver IC
• 4:32 Bootstrap circuit
• 5:42 Floating gate driver
• 6:17 Optocoupler
• 7:16 Charge pump

1. Direct Drive

The simplest method: connecting a logic signal directly from a microcontroller to the MOSFET gate.

• How it works: Uses signal voltage (3.3V or 5V) to charge the gate relative to ground. Works best for low-side switching where the source is grounded.
• Pros: Simple, supports PWM, can stay "always on."
• Cons: Limited to low-side switching; switching speed is limited by the microcontroller's current output.

2. Low-Side Driver IC

Using a dedicated integrated circuit designed to move charge into the gate quickly.

• How it works: The IC takes a logic signal and translates it into a high-current output using the driver's supply voltage.
• Pros: Handles logic levels easily, very fast switching.
• Cons: Requires an additional component (the IC).

3. Push-Pull (Bipolar Transistors)

A classic discrete circuit using an NPN and a PNP transistor to amplify gate current.

• How it works: The NPN "pushes" current from the supply to the gate to turn it on, while the PNP "pulls" current from the gate to ground to turn it off.
• Pros: Fast switching, supports PWM, cheap, protects your microcontroller from high current spikes.
• Cons: Requires a constant signal to maintain state.

4. Inverted Level Shifter

Technical Note: The schematic in the video should utilize PNP transistors for proper operation.

• How it works: This allows you to send a gate voltage higher than your logic signal. It uses two separate pulse signals to trigger the state.
• Pros: Provides higher voltage to the gate, supports "always on" state.
• Cons: Does not support PWM.

5. High-Side Driver IC

Specifically designed for N-Channel MOSFETs sitting on the high side of a load.

• How it works: Uses an internal bootstrap mechanism. When the signal is low, a capacitor charges. When high, that capacitor provides a voltage higher than the supply to the gate.
• Pros: Fast and includes safety features to prevent shoot-through.
• Cons: No "always on" capability; requires PWM to keep the bootstrap capacitor charged.

6. Bootstrap Circuit

A discrete version of the High-Side Driver IC logic without the built-in safety features.

• How it works: Similar to the IC, a capacitor stores charge while the MOSFET is off and "boosts" the gate voltage relative to the source when it needs to turn on.
• Pros: Fast and very inexpensive.
• Cons: No "always on" capability and no PWM support.

7. Floating Gate Driver

• How it works: Uses a completely isolated power supply for the gate drive. The ground of this isolated supply is tied directly to the MOSFET's source.
• Pros: Simple logic, supports PWM and "always on" states.
• Cons: Requires a secondary, isolated power supply.

8. Optocoupler Driver

• How it works: Uses light to bridge the gap between your control signal and a separate high-side power supply.
• Pros: Excellent isolation, can stay "always on."
• Cons: Requires a separate supply; generally does not support high-frequency PWM due to optocoupler speed limits.

9. Charge Pump

• How it works: A circuit that "pumps" a capacitor to a voltage roughly 8V above the main supply rail. This absolute voltage is then switched to the gate.
• Pros: Supports "always on" high-side switching without needing a separate isolated supply.
• Cons: Requires a relatively stable supply voltage to function correctly.

Comparison Table: Which Driver Should You Use?

Implementation Tips

Since the video/schematics have known errors regarding the PNP transistors in the level shifter, ensure you double-check your pinouts before breadboarding. If you need a deep dive into the math behind gate charge and switching times, check out the written article below.

Related Article: How to Use a MOSFET