Low-Side Drive & High-Side Drive Techniques for General Power Applications
MOSFET, short for Metal-Oxide-Semiconductor Field-Effect Transistor , is a widely used semiconductor device in modern electronic circuits. In power electronics and switching control applications, MOSFETs are widely used for their fast switching speed, high efficiency, and low conduction losses. However, for better performance, MOSFETs must be driven properly according to their position in the circuit and system requirements.
Among various drive topologies, low-side and high-side configurations are the most widely used in general power control, such as DC-DC converters, LED dimmers, and battery-powered systems.
1. Low-Side Drive
In a low-side drive configuration, the N-MOSFET is placed between the load and ground (load on top, MOSFET at the bottom). The gate is controlled directly by the output voltage of an MCU or driver.
This topology is suitable for applications such as LED dimming, motor startup, switch control, power management, and most PWM control scenarios (e.g., Buck converters).
Advantages
- Simple drive circuitry that can be controlled directly by an MCU
- Fast switching speed
- Low cost and low on-resistance (RDS(on)) for N-MOSFET
Limitation
- Current spikes may occur at the ground side, affecting signal quality (ground separation and filtering are necessary).
- Not suitable for high-side power switching.
Design Considerations
When designing a low-side drive,
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First select an appropriate MOSFET
Ensure the MOSFET’s threshold voltage (Vth) is lower than the drive voltage to achieve full conduction and choose a MOSFET with low on-resistance (RDS(on)) to reduce power loss and heat generation.
For drive voltage and current considerations, ensure the drive voltage is sufficient to fully turn on the MOSFET. -
Gate resistor
Use an appropriate gate resistor to control the MOSFET’s switching speed, reduce electromagnetic interference (EMI), and prevent gate oscillation. -
Pull-down resistor
Include a pull-down resistor at the gate to prevent false triggering. When the driver is off, the pull-down resistor ensures the MOSFET remains off, avoiding unintended conduction due to floating voltage. -
PCB layout and grounding
Minimize loop area to reduce EMI and ensure good grounding connections for the driver and power MOSFET to avoid issues caused by ground potential differences.
Example
The TI UCC27324 high-speed dual MOSFET driver is used for high-speed switching applications, providing up to 4A peak source and sink currents.
The output stage can also be paralleled to deliver higher single-channel drive power.
To better understand effective layout practices, let’s look at the recommended PCB layout for the TI UCC27324DR, which demonstrates practical techniques for minimizing loop area.
- Top-to-bottom PCB trace layout, where the gate drive trace (red, including the series gate resistor) and the return trace (blue) are placed directly on top of each other but on opposite sides of a dual-layer PCB.
- Close placement of the DC bypass capacitor. Two capacitors are used in parallel to provide low impedance across a wide frequency spectrum. The smaller, closer capacitor handles high-frequency transients.
- Twin vias are used to provide a low-impedance path, minimizing parasitic inductance in the return loop.
In all cases, the loop area between the signal and return paths is kept as small as possible, which is essential for minimizing EMI in high-speed switching environments.
Tech Tip: The term loop area is closely related to antenna theory in RF design. A large loop area behaves like an efficient antenna, radiating EMI. Therefore, reducing loop area is one of the most effective ways to suppress radiated emissions in PCB layout.
2. High-side drive
In a high-side drive configuration, the MOSFET is placed between the power supply and the load (load at the bottom, MOSFET on top). This typically uses a P-MOSFET or an N-MOSFET with a driver (bootstrap).
It is suitable for controlling power switches (e.g., battery positive terminal switching, power management) and high-side scenarios in H-bridge half-bridge/full-bridge drives.
Advantages
- Suitability for positive power terminal control
- Allows load ground to float when off (enhanced safety),
- Useful in lithium battery switching and reverse connection protection
Limitations
- P-MOSFETs have higher Rds(on) and slightly inferior performance.
- For N-MOSFET high-side drives, a bootstrap circuit or charge pump is required
- More complex drive circuitry than low-side drive
Design Considerations
When designing a high-side drive,
-
first consider the drive voltage requirements.
The source voltage of a high-side N-channel MOSFET rises with the load voltage, so its gate must be about 10V higher than the source to ensure full conduction.
A bootstrap circuit is often used to provide the required high-side drive voltage. -
The bootstrap capacitor must charge during the MOSFET’s off time, making it unsuitable for applications requiring prolonged conduction.
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For applications requiring prolonged conduction or high reliability, isolated drive methods such as optocouplers, transformers, or capacitive isolation can provide stable high-side drive voltage. Additionally, it includes protection mechanisms such as overvoltage protection, undervoltage lockout (UVLO), and short-circuit protection to prevent device damage.
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For PCB layout, minimize parasitic inductance and capacitance to ensure signal integrity and avoid EMI issues caused by poor layout.
Example
The onsemi FAN73711 is a high-side gate driver capable of operating from 3.3V or 5V control signals. It includes a built-in bootstrap mechanism, undervoltage lockout (UVLO), and short-circuit protection.
Drive Topology Comparison Table
| Drive Topology | Applicable Scenarios | Advantages | Disadvantages | Common IC Solutions |
|---|---|---|---|---|
| Low-Side Drive | Single-channel switching, low-voltage applications (e.g., buck converters, LED dimming, power switches) | - Simple gate drive design- Low cost- Fast switching with N-MOSFETs- Logic-level compatible | - Cannot switch positive rail- Ground spikes can affect signal integrity | TI UCCx732x, Microchip MIC4422 |
| High-Side Drive | High-side load control, battery switching, motor drives (half-bridge/full-bridge) | - Enables positive rail control- Improved safety (load disconnected when off)- Suitable for high-voltage switching | - Requires bootstrap or isolated driver- P-MOS has higher RDS(on)- Complex drive voltage requirement | Infineon IRS2007, onsemi FAN73711 |
Conclusion
Choosing the correct MOSFET drive topology is essential for stable and efficient power control systems. Low side drivers and High side drivers can offer a practical balance between performance for many Low to mid power application. Low-side drives are great when cost, ease of implementation, and logic compatibility are priorities. On the other hand, high-side drives provide the flexibility to switch the positive supply rail, making them ideal for battery-powered systems or situations requiring safer load disconnection.
With the right components and design practices, these basic topologies can serve as solid foundations for a wide range of electronic systems.
Applicable Part Numbers
| DigiKey Part Number | Manufacturer Part Number |
|---|---|
| 296-31999-2-ND,296-31999-1-ND,296-31999-6-ND | UCC27324DGNR |
| 296-27156-2-ND,296-27156-1-ND,296-27156-6-ND | UCC27324DR |
| 296-23486-2-ND,296-23486-1-ND,296-23486-6-ND | UCC27324QDRQ1 |
| 296-12533-5-ND | UCC27324P |
| UCC27324DGNRG4-ND | UCC27324DGNRG4 |
| UCC27324DRG4-ND | UCC27324DRG4 |
| 296-UCC27324PE4-ND | UCC27324PE4 |
| UCC27324PG4-ND | UCC27324PG4 |
| UCC27324DG4-ND | UCC27324DG4 |
| UCC27324DGNG4-ND | UCC27324DGNG4 |
| 296-12531-5-ND | UCC27324D |
| 296-12532-5-ND | UCC27324DGN |
| MIC4422AYM-TR-ND,MIC4422AYM-CT-ND,MIC4422AYM-DKR-ND | MIC4422AYM-TR |
| 576-1194-2-ND,576-1194-1-ND,576-1194-6-ND | MIC4422YM-TR |
| 576-1195-ND | MIC4422YN |
| 576-2321-ND | MIC4422ZT |
| 576-2318-ND | MIC4422AYM |
| 576-1793-5-ND | MIC4422ZM |
| 576-2320-ND | MIC4422ZN |
| MIC4422AZM-ND | MIC4422AZM |
| MIC4422ZM-TR-ND,MIC4422ZM-CT-ND,MIC4422ZM-DKR-ND | MIC4422ZM-TR |
| 576-2319-ND | MIC4422AYN |
| MIC4422AZN-ND | MIC4422AZN |
| 576-4475-5-ND | MIC4422YM |
| MIC4422AZT-ND | MIC4422AZT |
| MIC4422CM-TR-ND | MIC4422CM-TR |
| MIC4422ACM-TR-ND | MIC4422ACM-TR |
| MIC4422CM-ND | MIC4422CM |
| MIC4422ABM-TR-ND | MIC4422ABM-TR |
| MIC4422BM-ND | MIC4422BM |
| MIC4422ACM-ND | MIC4422ACM |
| MIC4422ABM-ND | MIC4422ABM |
| MIC4422AAM-TR-ND | MIC4422AAM-TR |
| MIC4422AAM-ND | MIC4422AAM |
| MIC4422AZM-TR-ND | MIC4422AZM-TR |
| MIC4422CTL2-ND | MIC4422CTL2 |
| MIC4422CTL3-ND | MIC4422CTL3 |
| 576-1522-2-ND,576-1522-1-ND,576-1522-6-ND | MIC4422BM TR |
| MIC4422BN-ND | MIC4422BN |
| MIC4422ABN-ND | MIC4422ABN |
| MIC4422ACN-ND | MIC4422ACN |
| MIC4422ACT-ND | MIC4422ACT |
| MIC4422CN-ND | MIC4422CN |
| MIC4422CT-ND | MIC4422CT |
| IRS2007STRPBFTR-ND,IRS2007STRPBFCT-ND,IRS2007STRPBFDKR-ND | IRS2007STRPBF |
| IRS2007SPBF-ND | IRS2007SPBF |
| 448-IRS2007MTRPBFAUMA1TR-ND,448-IRS2007MTRPBFAUMA1CT-ND,448-IRS2007MTRPBFAUMA1DKR-ND | IRS2007MTRPBFAUMA1 |
| FAN73711MXTR-ND,2832-FAN73711MXTR-ND,FAN73711MXCT-ND,FAN73711MXDKR-ND | FAN73711MX |
| FAN73711M-ND | FAN73711M |