How to choose the right MOSFET for a motor drive

The driver circuit of a BLDC usually includes MOSFETs to generate and destroy the electromagnetic field generated by the stator coils, which rotate around the rotor formed by permanent magnets. Detecting the position of the stator is essential to generate the correct excitation field in the coil. In a BLDC using a sensor, it is the magnetic field that is detected, while in the sensorless version, the control circuit measures the back EMF to determine the stator position.

Automotive OEMs are migrating to BLDC to maximize efficiency and reliability. This article looks at important parameters that engineers should consider during the design process to achieve these goals.

The discovery of electromagnetic induction (EMI) changed the world and heralded a new era. Today, it touches every sector, market and industry. In many ways, the ability to generate electricity at will and convert that energy into motion with precise control and regularity is the hallmark of a developed society.

Generators and motors are by far the most common and widely deployed EMI implementations. With the exception of solar energy, most of the available electricity is generated this way, either by large turbines in power stations or by small generators in renewable energy solutions such as wind or waves.

In response to this abundance of energy, the counterpart of the generator, the electric motor, has successfully and inseparably replaced the purely mechanical form of power. The internal combustion engine may be the latest step in this journey as electric vehicles begin to spread on our roads.

However, there is an intermediate step in the transition of the auto industry to electricity, which is to replace mechanical equipment with electric motors.

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From a consumer perspective, perhaps the most obvious use of electric motors in cars is to drive power windows and seats. Central locking is another application that can be cited. Under the hood, more changes have taken place. Electric motors are increasingly specified as purely mechanical options for functions such as fans, pumps (water, oil, fuel), power steering and anti-lock braking, and automatic transmissions.

The reasons are clear; electric motors offer better control, higher efficiency, and higher reliability than mechanical alternatives. At the beginning of the transition, OEMs turned to stepper motors and brush-commutated motors, but more recently the automotive industry—like many others—has moved to brushless DC motors (BLDCs), and for good reason.

BLDC offers higher levels of efficiency, better control, wider dynamic range, and more torque. Since the technology is brushless — from an electrical standpoint, it’s actually non-contact — it eliminates all the electrical disturbances common to brushed DC motors. This helps reduce electromagnetic interference, which can cause problems for more sensitive components in the engine control unit (ECU). It also avoids the arcing and subsequent wear that is common with brush commutation, which can lead to degraded performance and eventual failure of brushed DC motors.

Of course, replacing a mechanical motor with an electrical alternative does require additional control electronics. In the case of BLDC, arguably, the lack of electrical contact exacerbates the situation. BLDCs are sometimes controlled by using Hall-effect switches, which provide the necessary feedback to the control loop. Recently, however, sensorless BLDCs have become popular as removing the sensor further reduces the bill of materials.

The control algorithms developed to drive the BLDC (with and without sensors) are handled by a microcontroller (MCU), which provides the added benefit of relatively simple vehicle network integration using CAN or LIN. MCUs designed for motor drive in automotive applications also feature a pre-driver stage to control the MOSFETs required to provide high drive currents through the motor coils. The final stage is critical in defining the efficiency of the overall motor drive solution, as described below.

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The driver circuit of a BLDC usually includes MOSFETs to generate and destroy the electromagnetic field generated by the stator coils, which rotate around the rotor formed by permanent magnets. Detecting the position of the stator is essential to generate the correct excitation field in the coil. In a BLDC using a sensor, it is the magnetic field that is detected, while in the sensorless version, the control circuit measures the back EMF to determine the stator position.

Either way, the coil is powered through MOSFETs arranged in a bridge topology. The choice of MOSFET is a major factor affecting the overall efficiency and performance of the BLDC. The data provided in the data sheet was used under specific conditions, which may or may not correspond to the operating conditions of the actual application. Therefore, the application must be understood before selecting the most suitable MOSFET.

Likewise, the operating parameters of the selected MOSFET will have a direct and significant impact on the overall solution. Careful consideration of these parameters will ensure that the MOSFET is selected to best meet the requirements.

In general, three main aspects should be considered: reliability, efficiency, and design. Reliability is about the limits of the equipment and ensuring that these limits are never tested during normal operation. Specifically, this involves choosing a device with a breakdown voltage that provides adequate protection against transients that might be introduced through other design choices. For example, for a BLDC running from a 12V supply, a breakdown voltage of 40V is sufficient. Likewise, in a 24V system, a MOSFET with a breakdown voltage of 60V will provide adequate protection. It is also important to consider drain-source current ratings, especially during surge or pulse conditions. In BLDC applications, the starting or stall current may exceed three times the full load current,

In the case of MOSFETs, efficiency generally refers to the device’s ability to manage heat dissipation, especially at the junction. Good thermal design is always necessary, especially in high ambient temperature environments such as automobiles, but there are several parameters that should be considered when selecting a MOSFET. These include on-resistance, Rds(on) and gate charge (Qg). These two parameters are interrelated; larger MOSFETs result in lower on-resistance, but also higher gate charge. This can have a major impact on switching applications such as BLDC drivers.

Temperature Coefficient

Driving a BLDC with three phases (coils) is usually accomplished with a PWM (pulse width modulation) signal generated by the MCU to power each phase. Figure 1 shows a typical bridge circuit for BLDC phases. If both MOSFETs are turned on at the same time, it will cause breakdown, which can have catastrophic effects. To solve this problem, a period, called dead time, will be designed into the PWM signal to ensure that only the expected MOSFET is on at any given time. The switching time of the MOSFET will affect the length of dead time required, and this parameter is also affected by the device gate charge. During dead time, the body diode of the MOSFET provides a commutation path, which again is not ideal due to higher power losses when the diode is conducting.

Every MOSFET exhibits dynamic capacitance (Crss in Figure 1); this is a parameter that can lead to breakdown. This parameter, combined with Rg, can cause the gate charge of the low-side MOSFET to rise high enough to turn it on during switching.

How to choose the right MOSFET for a motor drive
Figure 1. Typical Bridge Circuit for Driving a BLDC Motor Phase

Another important parameter to consider for switching applications such as BLDC drives is the zero temperature coefficient (ZTC) point. As shown in Figure 2, this is a point on the transfer curve (drain current,[ID]and gate-source voltage,[VGS]). Operating the device below this point results in a positive temperature coefficient of drain current, while operating the device above this point results in a negative temperature coefficient of drain current. Figure 2a shows the transfer characteristics of a low density planar MOSFET (ZXM61N03F) and Figure 2b shows the transfer characteristics of a high density planar MOSFET (ZXMN3A01E6). Generally, it is recommended to operate the equipment in a negative temperature coefficient area. The device in Figure 2b utilizes greater trench density to increase the number of vertical current flow paths in the channel. This has the positive effect of lowering Rds(on), although it also results in a higher ZTC point.

How to choose the right MOSFET for a motor drive
Figure 2a (left). Low Density Planar MOSFET ZXM61N03F
Figure 2b (right). High Density Trench MOSFET ZXMN3A01E6

For a given size, N-channel MOSFETs typically have half the Rds(on) of an equivalent P-channel device, so N-channel MOSFETs are often specified in motor drive applications. Figure 3 shows the five stages of a full-bridge motor drive circuit using N-channel MOSFETs. It is also important to note that such circuits are subject to reverse recovery currents due to the body diode of the MOSFET. A PWM algorithm that minimizes dead time can reduce these effects, and it is also recommended to specify MOSFETs with fast-recovery shunt diodes.

How to choose the right MOSFET for a motor drive
Figure 3. Circuit showing commutation sequence and body diode recovery related breakdown

in conclusion

Automotive OEMs are increasingly specifying brushless DC motors. They offer greater efficiency, greater reliability and control of more features, including replacement of mechanical pumps and fans.

Driving a BLDC requires a gao-class MCU for control combined with appropriately specified MOSFETs to provide power. Thermal management is at the heart of good design, and this extends to understanding how to use the correct MOSFET design to best meet the unique requirements of a BLDC drive circuit.

By understanding and evaluating the relevant parameters, engineers can select the right MOSFET for the task, ensuring the highest reliability and efficiency even in the harshest environments.

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