Addressing the need for MOSFETs in BLDC motors

With as much as 70% of industrial electricity usage attributed to motors, combined with rising level of automation, this pushes the need for an overall cost-effective motor. This is further disrupted, with the move away from low-cost brushed DC motors to the more efficient brushless DC motors (BLDCs). As the name suggests, BLDC does not use brushes for energising the coils; thereby it eliminates all the mechanical wear for greater reliability whilst reducing electrical noise as there is no arcing. BLDCs are smaller, lighter and offer a better power-to-weight ratio, with a wider dynamic response and improved torque. These factors are significant in bringing down the total cost of ownership as more motors are deployed to automate processes.

Migrating to BLDC motors

Moving to BLDCs requires more complex drive solutions than brushed DC motors; Diodes Inc understands the challenges and, as shown in the BLDC diagram of Figure 1, has developed products including power management, gate drivers, MOSFETs, IGBTs and Hall effect sensors optimised for BLDC. The control algorithm for the BLDC motor is handled by a microcontroller (MCU), which offer the additional benefit of providing relatively simple integration to a wider system.

In a BLDC, the rotor is a permanent magnet, whilst the stator applies a rotating electro-magnetic field to induce the rotor to spin. This means that the rotor position and the timing of the current in the stator coils is critical for the control. Moving to BLDCs is, arguably, more challenging to control compared to a brushed motor as it lacks the switching electro-mechanical contact. In Figure 1, the MCU controls the power by monitoring the rotor's position via the Hall sensors and then the BLDC motor's stator coils are energised by switching the current flow in the correct sequence. When the position sensing is not critical, then further Bill of Material savings can be made by replacing the Hall Sensors with sensorless field oriented control.

Improved drive

The switching element in the BLDC is a power transistor, typically a MOSFET (or IGBT), that will switch the drive current to create and collapse electromagnetic fields in the stator coils, rotating around the rotor formed of a permanent magnet. Detecting the position of the rotor in the stator coils is fundamental to generating the correct energizing fields in the coils. In BLDCs that employ sensors it is the magnetic field that is detected, while in sensorless versions the control circuit measures back-EMF to determine the stator position.

Either way, the coils are energized through MOSFETs (or IGBTs) arranged in a half-bridge topology. The selection of the switching element is a major factor in the overall efficiency and performance of a BLDC; figures provided in datasheets are for use under specific conditions, which may or may not coincide with the operating conditions of the actual application. For this reason, it is essential to understand the application before selecting the most suitable switching element whether that is a MOSFET (or IGBT).

Similarly, the operating parameters of the MOSFETs (or IGBTs) chosen will have a direct and significant impact on the total solution. Careful consideration of these parameters will ensure the MOSFETs (or IGBTs) selected best meet the requirements. This article will focus on selecting MOSFETs for BLDC motors.

Key MOSFET Parameters for BLDC

In general, there are three main areas that should be considered: reliability, efficiency and design related parameters. Reliability relates to the extreme limits of a device, and ensuring these limits are never tested during normal operation. Specifically, this relates to selecting a device with a breakdown voltage that provides sufficient protection against transients that may be introduced through other design choices. For example, for a BLDC operating from a 12V supply, a breakdown voltage of 40V would suffice. Similarly, in a 24V or 48V system, a MOSFET with a breakdown voltage of 60V or 100V, respectively, would provide sufficient protection. It is also important to consider the drain current ratings, specifically under pulse conditions. In a BLDC application, a start-up or stall current could exceed the full load current by as much as three times, so a device with suitable drain pulse current capabilities is advised.

For high-power motor drive circuits - typically in excess of 50W - the channel on-resistance, R_DS(ON) is an important parameter in terms of reliability and efficiency; a lower R_DS(ON) will help maximise efficiency (depending also switching frequency), minimise heat dissipation and therefore increase reliability. The 'right' R_DS(ON) is also dependent on the operating voltage, for example a 400W (0.5HP) motor operating at 12V DC will draw over 30A, in which case a power MOSFET with an R_DS(ON) of <2mΩ (such as the 40V DMTH41M8SPS) would be appropriate. The same motor running from a 24V battery would draw around 16A, so a power MOSFET with an R_DS(ON) of <8mΩ would be more appropriate (such as the 60V DMTH6004SPS). These figures are calculated on <1.5W power dissipation in each half-bridge, split across two MOSFETs, and not exceeding the maximum junction temperature of the MOSFET which is typically 150 or 175°C. Further thermal management measures such as heatsinks or forced air flow may be necessary. The R_DS(ON) has a critical behaviour on the power level that can be achieved, For example, a power MOSFET with an R_DS(ON) of 1mΩ (such as the DMTH4001SPS) running from a 12V supply could drive a motor of >500W. However, it should also be noted that the R_DS(ON) has a significant impact on the total system cost, particularly as a 3-Phase BLDC motor requires at least six power MOSFETs, so optimisation should cover both cost and efficiency.

Switching Performance

In relation to MOSFETs, efficiency is generally an indication of how well a device manages heat dissipation, particularly at the junction. Good thermal design will always be necessary, but there are several other parameters that should be considered when selecting a MOSFET. As well as the R_DS(ON), this includes the gate charge (Q_G). These two parameters are interrelated; as a larger MOSFET of the same cell-pitch structure will have a lower R_DS(ON), but it will also have a higher Q_G, due to the increasing capacitance of a larger MOSFET structure. This higher gate charge can have a significant impact on switching applications like BLDC drivers.

Avoiding Shoot-Through

Driving a BLDC with 3-phases (coils) is typically achieved by a PWM (pulse width modulated) signal generated by the MCU for energising each of the phases. Figure 2 shows a typical half-bridge circuit to one coil (1-phase) of a BLDC. If both the MOSFETs are turned on at the same time, it results in a shoot-through from the power supply Vcc to ground return, which will have catastrophic effects on the MOSFETs leading to device failure. To address this, a time period will be designed into the PWM signals, known as dead-time, which ensures only one MOSFET is conducting at any given time. The MOSFETs' switching time will influence the length of dead-time required, a parameter that is also affected by the Q_G of the device. During the dead-time, the body diode of the MOSFET provides a commutation path, this is again not ideal due to the higher power losses of the body diode's I-V characteristic. Hence a good design works with the minimum possible dead-time while avoiding any possibility for shoot-through.

Each of the MOSFETs will exhibit a Miller capacitance between the drain and gate, see C_rss (C_gd) in Figure 2; this is a parameter that could result in a shoot-through. This Miller capacitance, combined with the gate series resistance (R_g), the MOSFETs inherent V_gs(th) level and switch node dV/dt could result in charge coupling onto the MOSFET gate causing the MOSFET to falsely turn-on, leading to a shoot-through event.

Adequate MOSFET gate drive voltage

Another important parameter is the level of gate-source drive voltage (V_GS) that is being applied on the MOSFET gate and how this relates to the zero temperature coefficient (ZTC) point. To ensure a MOSFET is adequately turned on, sufficient V_GS needs to be applied and typically this is 5V (logic) or 10V (standard) MOSFET, depending on the type of MOSFET, logic or standard level V_gs(th). Without sufficient V_GS, then the R_DS(ON) can rapidly increase and it can significantly vary from device-to-device and be highly temperature dependent. Graph's for Diodes DMTH6004SPS are used to illustrate this in figure 3.

The extreme case with a low V_GS would be if it goes below the ZTC and into the MOSFET's positive temperature coefficient region causing drain current crowding in the MOSFET cells, leading to thermal runaway as a hot spot forms and then the device fails.

MOSFETs in a full-bridge configuration

For a given size, an N-channel MOSFET will typically feature an R_DS(ON) half that of the equivalent P-channel device and for this reason it is common to specify N-channel MOSFETs in motor drive applications. Figure 4 shows five stages of a full bridge motor drive circuit using N-channel MOSFETs. It is important to note, also, that such circuits are subject to the effects of reverse current flow due to the body diode of the MOSFETs. PWM algorithms that are able to minimise dead time can reduce these effects, while specifying MOSFETs with a low Vf fast recovery parallel diode is also advisable.

Conclusion

Brushless DC motors are increasingly being specified for industrial and automotive applications. They offer greater efficiency, higher reliability and increased control in a widening number of functions, including replacing mechanical pumps, fans and to automate industrial processes.

Driving a BLDC requires a combination of an MCU for control, coupled with suitably specified MOSFETs to deliver the power. Thermal management lies at the heart of good design, and this extends to understanding how the unique requirements of BLDC drive circuits can be best met using the right MOSFET design.

By understanding and appreciating the pertinent parameters, engineers can select the right MOSFETs for the task, ensuring the highest reliability and efficiency in even the harshest environments.

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