EV POWER ELECTRONICS Industry special – power electronics in cars
Despite some negative market factors, the number of electric vehicles (EVs) – especially full battery types – is expected to grow significantly over the next decade. However, to fulfil their potential, EVs must have the right power electronics design in place. This article looks at the issues involved for the complete power train, from battery to motor.
Predicting the electric vehicle (EV) market’s growth rate is difficult, as so many factors, both for and against growth, are involved. Positive drivers include increasing worldwide legislation to curb and eventually eliminate IC engines, social consciousness and the need to be greener, and enthusiasm about new technology. Against this there is range anxiety, higher purchase costs, lack of charging infrastructure, and limited expertise in servicing and repair. Covid and the current chip shortage are two further, but hopefully temporary, damping factors.
Nevertheless, Deloitte Insights’ global EV market forecast for the next decade is for a CAGR of 29 %, reaching 31.1 million vehicles in 2030 – of which, 25.3 million vehicles or 81 % of these would be BEVs – purely battery EVs with no hybrid or IC component.
Accordingly, an understanding of the power electronics on which BEVs depend – and which will play an increasing role in our lives – is valuable; therefore, we explore it below.
BEV power electronics break down into several key components: the motor, the inverter (sometimes called a controller), DC-DC converters, a charger, and the battery. We will start with the motor, as its technology influences the rest of the EV’s electronics.
BEV electric motors
Electric motors can be DC or AC, but in practice most modern EV motors are AC type. They offer better efficiency, lower maintenance, higher reliability, and, importantly, a regenerative braking ability. This means that the motor becomes a generator during braking, converting kinetic energy that would otherwise be lost in tyre and brake pad heating into electrical energy for recharging the battery.
EV AC motors are commonly implemented as induction motors (IMs), as they are relatively low cost and can operate in different environmental conditions. Maximum torque can be delivered during starting, which is desired for traction applications. However, because they work on AC power, an inverter is necessary to convert the battery’s DC output into a suitable three-phase AC current.
However, the inverter is sometimes called a controller because it does more than just DC-AC conversion. Its AC output power can be delivered in a wide range of frequencies; as motor speed depends on frequency, this can be used for vehicle speed control. The inverter also facilitates regenerative braking. When the accelerator pedal is released, the motor becomes a generator and produces three-phase sinusoidal AC power. The inverter accepts this, and converts it into a single DC output at a higher voltage than the battery’s, so charging becomes possible.
The inverter’s functionality is managed by software embedded in the device. This includes the ability to change motor rotation direction, so the vehicle can travel in reverse.
The true importance of DC-DC conversion
However, the battery does not just work with the inverter. The EV also has legacy 12 V loads such as lighting, indicator, and infotainment systems, often with increased power requirements, but also a growing number of high power loads operating at 48 V – examples include new drive, steer and brake by wire systems. While in an ICE vehicle these circuits would be supplied by a low voltage battery, the EV uses its traction battery as the primary power source. As this battery is typically rated at 400 V or 800 V, DC to DC conversion stages are needed. Loads supported by such a distribution system include the water pump, AC compressor, rear window defroster, electronic power steering, catalytic converter, heated steering wheel and seats, infotainment system, indicators, and headlights.
DC-DC conversion is also needed for vehicle charging. While EV batteries can be 400 V or 800 V, so can charging stations. Therefore, efficient DC-DC conversion at kW levels is needed to ensure compatibility between all cars and all charging stations.
While DC-DC conversion is essential for delivering the right voltages where they are needed, it also fulfils another, increasingly critical function. As electric motors and other devices distributed around the EV steadily improve in performance, they tend to demand more power - which, in a legacy system means more current. This can create heating problems and reduced efficiency due to I2R losses in the vehicle’s cables. These losses can be reduced by using cables with larger cross-sectional areas and lower resistance per metre, but such cables would be bulky, heavy, difficult to manoeuvre, and expensive – all highly undesirable attributes in a modern EV.
A much better approach is to use DC-DC conversion more strategically, transmitting power around the vehicle at higher voltages – and therefore lower currents - and only converting to lower voltages at the point of load. Vicor, for example, offers a range of modular DC-DC converters and regulators that allows such an approach, as shown in Fig.1.
The battery is the last component in the power chain, as well as being the largest and heaviest. It often comprises a battery pack with many hundreds of small, individual cells arranged in a series-parallel configuration to achieve the desired voltage and capacity. BEV batteries are typically 400 V or 800 V; for example, a 400 V nominal pack will often have around 96 series blocks, as in the Tesla Model 3. BEV battery capacities typically range from 30 kWh to 100 kWh or more.
The batteries, which are usually lithium-ion or a variant, operate at these high voltages to reduce I2R losses as previously described. However, this, together with the Li-ion chemistry, present multiple challenges for safety. Fuses and contactors with safety features are designed into both the positive and negative sides of the battery.
The battery pack also includes monitoring circuit boards that continuously check the voltage and temperature of each cell, as well as block interconnect temperatures. They also perform balancing, to ensure that the blocks of cells are kept within a few millivolts of one another, to allow maximum power to be transferred into and out of the pack. If a point temperature does start to rise, the monitoring boards can report on this before a damaged cell causes a more serious fault or even a fire.
Lithium-ion type batteries also require a battery management system (BMS) to monitor all aspects of the battery pack. This includes total charge transferred in and out, voltage measurements at different points within the battery, and system and pack isolation. It also manages functions like contactor control and economizer circuits, and other potentially redundant safety features can be incorporated.
The BMS is in constant communication with the monitoring boards, and runs a communications interface with the rest of the vehicle – often over either automotive ethernet or CANbus – where it communicates with the inverter, charger, and other systems. It is instrumental in managing and monitoring battery pack state of health and state of charge.