How to use BLDC motors to overcome design challenges in compact motor control systems

High power density, high efficiency, three-phase brushless DC (BLDC) motors powered by lithium-ion are used to develop cordless power tools, vacuum cleaners and e-bikes. However, to save space for more compact mechatronic products, designers are under pressure to further shrink motor control electronics.

The task is no small one. Besides the obvious challenge of squeezing the driver components into a small space, there are also thermal management issues caused by packing everything closer together, and of course, there’s the issue of electromagnetic interference (EMI).

Motor control circuit designers can achieve slimmer designs using a new generation of highly integrated gate drivers, the most critical element of a motor control system.

This article will first explore the operation of a BLDC motor before introducing suitable gate drivers and how they can be used to overcome the design challenges of compact motor control systems.

Building a better motor

Due to the dual pressures of energy efficiency and space saving in business, the design of electric motors has been developing rapidly. Digitally controlled BLDC motors represent one branch of this development. The popularity of this motor is due to the use of electronic commutation technology. With the help of this technology, the efficiency of BLDC motors is much higher than that of traditional (brush commutated) DC motors. If both motors are operated at the same speed and load, the efficiency of BLDC motors is 20% - 30% higher than that of traditional motors.

This improvement has enabled BLDC motors to become smaller, lighter, and quieter for a given power output. BLDC motors also offer a variety of other advantages, including better speed-to-torque characteristics, faster dynamic response, quiet operation, and a higher speed range. At the same time, engineers are also pushing designs toward higher voltages and higher frequencies, which allow compact motors to perform the same functions as larger conventional motors.

The key to the success of the BLDC motor lies in its electronic switch-mode power supply and motor control circuitry, which generates a three-phase input that produces a rotating magnetic field that pulls the motor's rotor to turn. Because the magnetic field and rotor rotate at the same frequency, this motor is classified as a "synchronous" motor. Hall effect sensors convey the relative position of the stator and rotor, ensuring that the controller can switch the magnetic field at the appropriate moment. It also uses "sensorless" technology to determine the position of the stator and rotor by monitoring the back electromotive force (EMF).

In a three-phase BLDC motor, the most common configuration for applying current sequentially is to arrange three pairs of power MOSFETs in a bridge configuration. Each pair of power MOSFETs acts as an inverter to convert the DC voltage from the power supply to the AC voltage required to drive the motor windings (Figure 1). In high-voltage applications, insulated gate bipolar transistors (IGBTs) are often used instead of MOSFETs.

Digitally controlled three-phase BLDC motors are usually controlled using three pairs of MOSFETs, with one pair of MOSFETs providing AC voltage to one motor winding.

The transistor pair consists of a low-side device (source connected to ground) and a high-side device (source floating between ground and the high-voltage supply rail).

In a typical layout, the input DC voltage is effectively converted to a modulated drive voltage using pulse width modulation (PWM) to control the MOSFET gate. A PWM frequency at least an order of magnitude higher than the expected maximum motor speed should be used. A pair of MOSFETs can control the magnetic field of one motor phase. For more information on driving BLDCs, see the library article, “How to Power and Control Brushless DC Motors.”

Motor control system

A complete motor control system includes a power supply, a host microcontroller, a gate driver, and MOSFETs in a half-bridge topology (Figure). The microcontroller sets the PWM duty cycle and is responsible for open-loop control. In low-voltage designs, the gate driver and MOSFET bridge are sometimes integrated into one unit. However, for high-power units, the gate driver and MOSFET bridge are separated for thermal management, which allows different process technologies for the gate driver and bridge and minimizes EMI.

Figure: Schematic diagram of BLDC motor control based on TI MSP 430 microcontroller. (Image source: Texas Instruments)

The MOSFET bridge can be made up of discrete devices or integrated chips. The key advantage of integrating the low-side and high-side MOSFETs into the same package is that it allows for a natural thermal balance between the upper and lower MOSFETs, even though the two MOSFETs have different power dissipations. Whether integrated or discrete, each pair of transistors requires independent gate drivers to control the switching timing and drive current.

Alternatively, the gate driver circuit can be designed using discrete components. The advantage of this approach is that engineers can precisely tune the gate driver to the MOSFET characteristics and optimize performance. However, this approach also has the disadvantage that it requires a high level of motor design experience and the space required to accommodate a discrete solution.

Modular motor control solutions offer another option, with a wide variety of integrated gate drivers available on the market. Good modular gate drive solutions include:

Highly integrated solution to minimize device space required

High drive current solution to reduce switching losses and improve efficiency

High gate drive voltage solution to ensure MOSFET is turned on with minimum internal resistance (“RDS(ON)”)

High-level over-current, over-voltage and over-temperature protection solutions ensure that the system can operate reliably under the worst-case conditions

Devices like the DRV8323x three-phase gate driver family from Texas Instruments not only meet the requirements of energy-efficient BLDC motors, but also reduce the system's component count while lowering cost and complexity.

The DRV8323x family has three models. Each model integrates three independent gate drivers capable of driving high-side and low-side MOSFET pairs. The gate driver consists of a charge pump to generate high gate voltages for the high-side transistors (supporting up to 100% duty cycle) and a linear regulator to power the low-side transistors.

TI gate drivers include sense amplifiers. If needed, the amplifier can be configured to amplify the voltage across the low-side MOSFET. These devices can source up to 1 A and sink 2 A of peak gate drive current, operate from a single supply, and have an ultra-wide input supply range of 6 V to 60 V.

For example, the DRV8323R version of the driver integrates three bidirectional current sense amplifiers to monitor the current level through each MOSFET bridge using low-side shunt resistors. The gain settings of the current sense amplifiers can be adjusted through the SPI or hardware interface. The microcontroller is connected to the EN_GATE of the DRV8323R so that the gate drive output can be enabled or disabled.

The DRV8323R driver also integrates a 600 mA buck regulator to power an external controller. This regulator can use either the gate driver supply or a separate supply (Figure).

Figure: Highly integrated gate drivers such as TI’s DRV8323R can reduce system component count, lower cost and complexity, and save space. (Image source: Texas Instruments)

These gate drivers include multiple protection features such as supply undervoltage lockout, charge pump undervoltage lockout, overcurrent monitoring, gate driver short-circuit detection, and thermal shutdown.

Each DRV832x is packaged in a chip measuring just 5 x 5 - 7 x 7 mm (depending on options). These products save the space required for more than 24 discrete components.

Designing with Integrated Gate Drivers

To help designers get started quickly, TI provides the reference design TIDA-01485. The TIDA-01485 is a 99% efficient, 1 kilowatt (kW) power level reference design for a three-phase 36-volt BLDC motor for various applications, such as power tools powered by a 10-cell Li-ion battery.

This reference design shows how to use highly integrated gate drivers, such as the DRV8323R, to save space in motor control designs by building one of the smallest motor control circuits in this power class. The reference design implements sensor-based control. (See the library article "Why and How to Sinusoidally Control a Three-Phase Brushless DC Motor.")

The key elements of the reference design include the MSP430F5132 microcontroller, the DRV8323R gate driver, and three CSD88599 60 V half-bridge MOSFET power modules (Figure).

Figure: The TIDA-01485 is a 99% efficient, 1 kW power stage reference design for a three-phase, 36 V BLDC motor powered by a 10-cell Li-ion battery. (Image source: Texas Instruments)

While gate drivers are highly integrated, modular solutions that remove many of the complexities of discrete designs, some design is still required to create a system that can fully utilize them. This reference design demonstrates a comprehensive solution for designers to use in their prototypes.

For example, the gate driver requires several decoupling capacitors for proper operation. In the reference design, a 1 microfarad (μF) capacitor (C13) decouples the low-side MOSFET drive voltage (DVDD), which comes from the DRV8323R's internal linear regulator (Figure). This capacitor must be placed as close to the gate driver as possible to minimize loop impedance. In addition, a second 4.7 μF capacitor (C10) is required to decouple the DC power input (PVDD) from the 36 V battery.

Figure: DRV8323R gate driver application circuit. Trace lengths should be minimized to limit EMI. (Image source: Texas Instruments)