As demand for electric propulsion increases, power semiconductors for controlling and conditioning the electrical energy are being thrust into centre stage; some, such as high-power IGBTs and rectifier diodes, must toughen up to meet the challenges ahead.

HEV infrastructure

Broadly speaking, there are two main powertrain architectures for HEVs. In a parallel hybrid, a battery-driven electric motor and a small petrol engine both provide propulsion. The petrol engine also recharges the battery. Alternatively, a serial hybrid uses the petrol engine to drive a generator that provides power to an electric motor; only the electric motor provides propulsion. These two architectures can be combined, allowing either the electric motor or the combustion engine to drive the wheels so as to provide the speed and acceleration demanded by the driver. This is the approach used in the Prius.

Future generations of HEVs, now waiting in the wings, will be recharged from the mains and operate almost exclusively on electric power. The petrol engine will only be used to charge the batteries if recharging from the mains is not possible, for example on longer journeys.

Hence, depending on its architecture, a hybrid vehicle will have at least one motor-generator, which draws energy from the battery to provide propulsion and channels energy back to the battery during recharging. The battery is recharged from the petrol engine, or from the motor-generator during regenerative braking. This demands a number of power electronic subsystems to generate suitable waveforms to drive the unit as a motor, and to condition its output when the battery is being recharged.

Hybrid vehicle design also requires changes to systems such as air conditioning, which requires an electrically driven compressor instead of a conventional compressor driven by the petrol engine. This is necessary so that the air conditioning will continue to operate whether the petrol engine or the electric motor is providing propulsion.

Power conversion

Historically, hybrid cars have used NiMH battery technology but this is about to give way to Li-ion technology, which offers greater energy density. In any case, size and weight limitations restrict the battery voltage to around 200V. Hence a boost converter is implemented to step up the battery voltage to a suitable level to drive the motor. Depending on the output power of the motor, which must enable the vehicle to achieve targets for performance such as acceleration and top speed, this may be around 500V. Hence, for a 50kW motor the transistors for the boost converter must be rated in excess of 100A and 500V.

After the boost converter stage, a DC/AC inverter generates a three-phase AC waveform at the elevated voltage to drive the motor-generator. IGBTs are preferred as switching devices for both the converter and inverter functions.

During regenerative braking, the AC output of the motor-generator (acting as a generator) must be rectified, filtered and regulated to a suitable voltage to recharge the battery. The output of the generator is rectified using large freewheel diodes connected in parallel with the inverter IGBTs. Since the power generated during braking is proportional to the weight of the car, this must be considered when sizing the freewheel diodes. Following rectification, filtering produces a smooth DC waveform and the voltage is then stepped down to produce a stable DC supply for battery charging.

Hybrid-ready power semiconductors

The electronic content of cars has been increasing steadily in recent years. Already, this is thought to be as high as 25% in ordinary combustion-engine vehicles. Hence many types of components have already been engineered to operate at elevated temperatures and achieve the long-term reliability required for use in an automotive context. These include devices such as microcontrollers, op-amps and comparators that are used throughout the power-conditioning subsystems for a hybrid vehicle. However HEVs, with their requirement for multi-kW electric traction, now require high-power semiconductors to achieve similar improvements so as to meet the stringent demands of the automotive sector.

The power IGBTs being used in HEV converter and inverter circuits, for example, are significantly different to similar devices used in industrial applications. All IGBTs generate considerable heat during normal operation. When used in the HEV they may also be positioned within a confined space with little cooling, close to a heat source such as the petrol engine, and within ambient temperatures as high as 55°C. Hence exposure to high temperatures is a major hazard for these devices. They must also withstand repeated power cycling to satisfy car buyers’ expectations for high reliability and long lifetime.

To fulfil their role in the hybrid-electric future, suitable IGBTs must be able to withstand high levels of thermal stress that would cause solder cracks in conventionally designed devices. These cracks tend to occur in the internal solder layers between the IGBT chip, the insulating substrate and the device’s baseplate. Whereas industrial IGBTs tend to have a silicon-oxide substrate with a copper baseplate for heat removal, this cannot withstand the temperature cycling expected in an HEV application. Instead, the IGBTs now being positioned for HEV applications tend to use substrate and baseplate materials having closely matched thermal expansion properties, such as aluminium nitride (AlN) and aluminium silicon carbide (AlSiC) for the substrate and baseplate respectively. This can reduce the stresses that cause solder cracks to form.

HEV IGBTs also typically have smaller die sizes than equivalent parts for industrial use. Using several relatively small dice in parallel, rather than a single, large die, diminishes the change in die size due to thermal expansion, and hence reduces thermal stress on the connections to the die. Additionally, using several smaller dice provides freedom for the IGBT designer to position the dice so as to optimise the spread of heat across the device.

Since heat is the single greatest threat to the reliability of IGBTs in the HEV powertrain, the devices will typically include an on-chip temperature sensing diode as well as built-in current sensing. These capabilities combine with integrated features of the gate driver IC to protect against overheating, short-circuit and over-current conditions.

Of course, several further electrical control functions are necessary for successful operation of a HEV powertrain, including battery management, system supervision and safety mechanisms. Thermal management, including careful positioning of power-handling modules within the vehicle, cooling, heatsinking, and the use of thermally conductive gap fillers where necessary, is another important aspect of the HEV electrical designer’s responsibilities.

Electronic components have come a long way in helping to improve both the environmental performance and driving experience delivered by modern cars. As fossil-fuel engines give way to electric traction, new high-power semiconductors satisfying the extreme demands of the automotive industry will provide extra help to save the planet.

Alistair Winning is a Technical Editor at Farnell

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