GaN based BLDC motor control and electrification of multi-voltage vehicle systems
An introduction to GaN (gallium nitride) for power conversion and control applications
The benefits of gallium nitride semiconductors stem from its unique material and electronic properties. GaN devices offer five key characteristics compared to silicon based devices:
- High dielectric strength
- High current density
- High switching speeds
- Low on-resistance
- Ability to withstand higher operating temperature
Compared to silicon, gallium nitride has ten times higher electrical breakdown characteristics, three times the band gap, and exceptional carrier mobility.
Taking advantage of these properties, our partner, GaN Systems has successfully developed transistors with an on-resistance lower than that attainable with silicon, and even better than a mechanical relay contact. This together with GaN’s inherent negligible charge storage permits the design of power switching circuits with formerly unheard of efficiencies, small size and very low heat losses.
In applications where efficiency, and therefore size and weight, is important, GaN is an important enabling technology.
Advantages over existing solutions
At medium voltages (48 – 72 V), shown in Figure 1, the power losses in gallium nitride are compared with that of MOSFETs. As the switching frequency increases above 10 kHz there are up to 6-fold reductions in losses using GaN. The knock-on benefit to size, cost and power dissipation gets more impressive as the switching frequency rises further. The ultimate benefit of GaN is to reduce the size and weight of switching power converters by increasing the switching frequency.
Figure 1 – Medium Voltage Switching Power Loss vs. Frequency
The benefits at higher voltages (300 – 500 V) are even more significant. At higher voltages the transition losses of semiconductors become more problematic for power electronic designs. Figure 2 clearly shows how GaN semiconductors have clear advantages over conventional MOSFET and IGBT technologies.
Figure 2 – High Voltage Switching Power Loss vs. Frequency
GaN provides another advantage at higher voltages. Due to its high band-gap combined with high electron mobility, it has lower conduction losses as compared to existing technologies. As shown in Figure 3 GaN’s conduction losses are lower than both MOSFET and IGBT technology. This means that GaN is a drop-in replacement to increase efficiency and power density even in existing low frequency high voltage applications.
Figure 3 – Conduction Power Losses vs. Current at 25 degC
Previously it was not achievable to have high voltage high frequency designs. With gallium nitride it is now possible to achieve high voltage, high current, and high frequency switching designs. GaN can be useful to incrementally improve existing designs. It is more groundbreaking to consider what could not be done before but can be achieved now due to GaN technology.
Figure 4 – GaN Benefits Spider Diagram (courtesy of GaN Systems)
Advantages of GaN in motor control applications
Hybrid and electric vehicles have a substantial power conversion requirement; a drive train can be operated up to 100s of kW. A typical silicon-based converter will be no more than 95% efficient, therefore having at least a 5% loss. These losses, as heat, have to be dissipated by water-cooling in separate radiator systems.
Gallium nitride semiconductor based converters can achieve 98 to 99% efficiency – a threefold reduction in losses at typical operating voltages. GaN can operate at higher junction temperatures and can be air-cooled rather than water cooled. Moreover, silicon efficiency of 95% is only achieved at optimum full load and can drop to as low as 70% at lower, more common driving loads. Gallium nitride devices can maintain an efficiency of 90% even at low loads due to the smaller drive circuit losses.
GaN circuits can easily be driven in parallel for high-current operation. With higher switching frequency, there is opportunity for more efficient operation at high motor speeds even with high pole count. A wide temperature range allows co-packaging with the motor and a wide input voltage range can be achieved with very similar electrical architecture.
Dana experience using GaN for 12V to 300V BLDC motor controllers
Dana have designed and implemented custom motor control electronics to take advantage of the benefits of GaN semiconductors in applications with a wide range of input voltages from 12V to 300V, making the design ideally suited to the electrification of auxiliary vehicle systems on multi-voltage conventional, hybrid-electric and pure electric vehicles. This controller design provides a functional starting-point for the development of 48V and above high-speed motor-driven vehicle systems.
Our approach was to design one motor controller for all voltages using GaN switching devices with a simple interface of power, communication and ground. Communication is via open drain PWM or LIN with robust internal diagnostics. This includes detailed parametric performance measurements in real time to enable prognostics for critical applications or for high inspection cost applications.
The GaN Demo PCB shown above is a prototype designed to be a technology demonstrator for gallium nitride semiconductors in BLDC motor design. The prototype has voltage and current feedback for each of the 3 motor phase control half bridges. It is circular in shape to depict an application in which the controller PCB would be mounted onto the end of the motor assembly. The GaN Demo PCB is capable of driving motors up to 1.5 kW at 300 V while having a diameter of only 4”. It has a power density 5x that of existing 12 V BLDC motor controllers.