Full-Bridge MOSFET Driver Selection and Design Guide

Created: February 11, 2022
Updated: June 25, 2023

Power delivery with stable DC voltage and current is critical for power regulators/converters, motor drivers, and other applications like lighting and pulse generation circuits. Many designers that work on lower power systems may be tempted to try and scale a lower power converter topology to a higher power system, but this does not work out in the designer’s favor. Trying to supply power with a standard converter/regulator topology inevitably leads to component failure, insufficient power delivery, or an oversized system with low efficiency.

For moderate to high DC power delivery, a bridge topology is a better choice, specifically a full-bridge topology with MOSFETs. The MOSFETs used as switching elements provide low on-state resistance, low power dissipation, high breakdown voltage, and high saturation current compared to comparable IGBTs. Using MOSFETs in this topology requires a full-bridge MOSFET driver to switch the transistors with a PWM signal. These components can be highly integrated with small footprint for driving high current loads, giving designers a compact option for their products.

Understanding Full-Bridge MOSFET Drivers

The standard topology used for power delivery in commercial-grade motors and power converters is one of many switching topologies. Switching converter designers are likely familiar with the standard buck, boost, buck-boost, and flyback converter topologies, where one or more switching transistors are used to regulate power output and set the output voltage level. These components are highly integrated as these systems are not generally used for high power DC-DC conversion or power delivery in motors.

To see why these components are important and how they are used, it helps to look at the two most common applications of full-bridge MOSFET drivers: LLC resonant converters and motor drives.

Example: Full-bridge LLC Resonant Converter

Instead, high power converters and motor drivers use a half-bridge or full-bridge configuration of switching transistors. Driver components for motors and power converters are designed to switch each side of the full-bridge or half-bridge circuit with a PWM signal, providing power delivery to the load. Full-bridge gate drivers for high power MOSFETs essentially integrate multiple gate driver IC’s for an individual transistor onto a single chip. The circuit diagram below shows a full-bridge driver IC with four MOSFETs used as the switching element in a LLC resonant converter.

In this system, the full-bridge driver’s job is to amplify a PWM signal and use this to switch the four transistors ON and OFF; only two transistors are on at a given time. In this topology, on the primary coil side, the driver IC switches the left high side and right low side transistors ON at the same time, while the other transistors remain OFF. In the next cycle, the states of all 4 transistors switch. Finally, on the secondary side of the coil, the output voltage is rectified with diodes to give a stable DC voltage.

Just like any other converter topology, the output current can be sensed and fed back into the input, which can then be used to adjust the PWM signal to ensure stable regulation. Any control functions, including any enable functionality on the full-bridge MOSFET driver IC, is implemented in a MCU or specialty logic. A very similar topology can be used to provide stable power regulation in large DC motors.

In the above power conversion application, we could implement a pure half-bridge (e.g., half-bridge LLC resonant converter). Half-bridge and full-bridge drivers can be used in motor driver circuits, as we’ll see below.

Example: Single Full-bridge MOSFET Driver vs. Dual Half-bridge for Motor Drives

Two common implementations of motor driving with stable, adjustable DC power are with full-bridge and half-bridge drivers. Two examples with half-bridge and full-bridge driving for a motor are shown in the diagrams below.

In this example, dual half-bridge drivers need to be carefully selected so that the driving signal passes through to the secondary driver IC. Certain components enable this by default, such as MAX14871 by Maxim Integrated. If this is not enabled in the driver component, you’ll need to send the PWM signal out to the secondary IC in parallel. Also, be careful to note the states of the MOSFETs in the half-bridge configuration; the top and bottom MOSFET states on each side are reversed to provide the required current flow in the motor.

In contrast, the panel on the right shows the same implementation but with a full-bridge MOSFET driver. In this circuit, the output from the driver IC switches the MOSFETs in pairs using a single PWM signal from an MCU. This highly integrated option reduces required component count and can still be used with precise feedback for speed or power control.

Full-bridge Driver vs. Gate Driver

A gate driver IC provides much the same function as a full-bridge driver: it switches a MOSFET between ON and OFF states. There are some differences in terms of how these components are implemented in a design. While a full-bridge driver is specifically designed for a fixed configuration with four MOSFETs, a gate driver can switch individual MOSFETs without requiring synchronization with any other gate driver. Note that you could create a full-bridge MOSFET driver circuit using four MOSFET gate drivers. Which you should choose depends on the supply voltage you need to switch the MOSFETs, and the level of integration needed in the component.

Selecting Full-bridge MOSFET Driver Components

Output current is probably the most important specification you’ll need to look at, and this specification should be compared to your transistors specifications. Here are some other important specifications you should examine when selecting a gate driver IC:

Integration. Some full-bridge MOSFET drivers will include the MOSFETs on-die. These components will dissipate power directly on the chip and will carry some saturation current rating.
Supply voltage. The supply voltage specification determines to what depth the MOSFETs can be switched between ON and OFF states.
Dual half-bridge option. Some full-bridge gate drivers can be used as two independent half-bridge drivers (see the motor control application above).
PWM frequency range and duty cycle. These parameters are important in any switching converter or driver. In particular, the duty cycle will determine the average power delivered, while the frequency can influence gain and impedance in the load component, particularly in inductive loads found in motor control and power products.
Temperature rating. These components might operate at high power, or they may operate near other components that dissipate high power. Consider the temperature rating and any cooling strategy you might need to implement in your system. If the driver includes integrated MOSFETs, then you’ll likely need some level of thermal cooling in your design to keep the component from overheating and failing.

Be sure to carefully match up your driver component, external MOSFETs (if not integrated on the driver die), PWM generator, and any components used in the feedback loop.

A Comparison of Two Full-bridge MOSFET Drivers

The L6203 full-bridge MOSFET gate driver from STMicroelectronics shows the kind of integration involved in these components and how they provide high power. This component is designed for driving small motors and includes an integrated H-bridge MOSFET arrangement with up to 48 V output voltage at moderately high current (5 A peak, 4 A RMS). The L6203 includes an internal voltage reference for precise regulation, thermal shutdown circuit, and enable pin from an external controller. A sense resistor can be connected to provide feedback for motor control. The input and enable pins can also be modulated to provide one-phase or two-phase chopping for an external motor.

A comparable component is the TLE7181EMXUMA1 from Infineon. From the block diagram below, we see that this component can be configured for dual half-bridge or full-bridge driving with 2 or 4 MOSFETs, respectively. These external MOSFETs are used for high current DC motor drives in 12 V power nets (up to 34 V supply voltage) at high current. To ensure reliability and prevent damage to downstream components, there is a comprehensive protection circuit that provides under/overvoltage, overcurrent, overtemperature, and short circuit protection. In addition, there is an integrated regulator for ensuring stable output.

These two components have varying levels of integration and on-chip features, but they are good examples of what you can expect from typical full-bridge MOSFET driver components. The L6203 integrates everything onto the die and provides a small-footprint solution, but the power output is limited by the on-die MOSFETs. Heat dissipation happens directly in the component, so cooling measures might be needed to prevent overheating.

In contrast, the TLE7181EMXUMA1 can be used with a range of powers, which will be limited by the pull from the motor, the external MOSFET current limits, and the power supply used with the external MOSFETs. Overall, the driver plus its external bridge circuit takes up more space, but you can get more power.

Other Components for Motor Drivers and Power Systems

The components shown above are highly integrated motor driver components, but there are always other components you’ll need to provide stable power delivery to a motor or power converter. Components for controlling/enabling the drive circuit, providing the PWM signal, sensing and regulating the power output, and providing filtering to ensure clean, noise-free power is delivered to the load in the system.

When you’re looking for full-bridge MOSFET drivers with any level of integration, try using the advanced search and filtration features in Octopart. You’ll have access to an extensive search engine with distributor data and parts specifications, all of which is accessible in a user-friendly interface. Take a look at our power management integrated circuits page to find the components you need for power conversion, conditioning, and management.

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