Automotive systems traditionally have had no need for the high-voltage performance of insulated gate bipolar transistors (IGBTs). However, new developments in conventional and electric vehicles are changing this situation and increasing the demand for IGBTs that are optimised for automotive applications.
For example, high-intensity discharge (HID) headlamps and super-accurate direct fuel-injection systems have become common in today’s cars and commercial vehicles such as buses and trucks. Their associated electrical drives and controls operate at voltages above 100 V.
These developments are drawing high-voltage performance IGBTs into automotive applications. In addition, hybrid/electric vehicles (HEVs), which require efficient power control at voltages close to 1000 V, will drive demands for new types of automotive IGBTs.
High-Voltage Automotive Apps
In an HID-lamp ballast, IGBTs are typically used in an H-bridge configuration to manage the operation of the lamp throughout the ignition, warm-up, and run modes. And in direct fuel-injection applications, IGBTs control the high-voltage actuators for piezo elements inside the precision injectors (Fig. 1).
The injectors deliver fuel typically up to seven times a second and at such highly optimised distribution and density that a direct-injected diesel engine can now return fuel efficiency equal to that of a gasoline hybrid-electric vehicle.
Optimum IGBT performance depends on suitable high-voltage driver ICs that are engineered for safety and reliability. This requirement can best be addressed using a combination of matching drivers and IGBT switches such as International Rectifier’s AUIRS2123, AUIRS2124, and AUIRS21811 driver ICs and associated AUIRGR4045D or AUIRGR4045D IGBTs optimised for automotive applications.
Into The Electrical Age
In addition to such high-voltage applications in conventional cars, coming generations of HEVs will introduce a much wider range of high-voltage switching tasks for IGBTs. These vehicles will typically feature a high-voltage power supply ranging from slightly above 100 V in so-called mild hybrids up to more than 800 V or 900 V batteries in other types of vehicles.
The IGBT breakdown-voltage requirement is around 600 V or 650 V for most hybrids, and it can go up to 1200 V for full hybrids, plug-in hybrids, or full-electric vehicles.
In these future generations of vehicles, the most challenging power-management task will be the main inverter. This bidirectional ac-dc system drives the main propulsion motors of the power train (Fig. 2).
The inverter drives the electric motor and may be required to provide more than 100 kW in the motor mode, propelling the car on battery power only. In the generator mode, the same inverter uses the motor as a generator/alternator to recover kinetic energy as the car is braking or decelerating to recharge the battery.
Besides the main motor-drive inverter, there are also some peripheral motor drives such as the air-conditioning compressor and the electric power steering motor. Since motor power is typically in the range of 1 to 5 kW, high-voltage drives can help to improve efficiency compared to direct 12-V operation at much higher current levels.
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Managing Energy Flow
Proper management of energy flow between the various systems, the high-voltage circuitry, and the 12-V power net requires sophisticated techniques. Therefore, besides powerful motor-drive inverters, IGBTs are also required as power switches in highly power-intensive dc-dc converter applications that transfer energy in one or either direction between the 12-V power system and the high-voltage battery (Fig. 3).
In addition, so called plug-in hybrids as well as full-electric vehicles are designed to carry large batteries capable of storing enough energy to drive the car for an extended range purely on battery power. Hence, these systems also require a charging system that can transfer a large quantity of energy from the domestic outlet or public charging station to the vehicle’s battery, within a relatively short period of time. Here, also, ac-dc converters are typically used as chargers to transfer the ac power into a dc power source, charging the vehicle’s battery.
As these new high-voltage and high-current automotive applications emerge, IGBT technologies must evolve to help vehicle manufacturers deliver energy-efficient and low-emission cars. Although robust electrical standards for HEVs are far from finalised, it is possible to define silicon process platforms for IGBTs that can support the main requirements for the applications that have so far been anticipated.
As a very sophisticated device, the IGBT can be optimised in a variety of ways to address application-specific tasks. For example, various parameters can be adjusted to optimise switching performance for a particular task. The HEV applications discussed earlier cover three general categories of switching requirements:
- Fuel injection/ignition/HID systems switching at significantly below 1 kHz
- Motor-drive inverters switching in the range of 5 to 10 kHz
- Power supplies, dc-dc converters, and chargers at frequencies above 10 kHz
In sub-1kHz switching applications, the ideal device should be chosen to minimise conduction losses. In fast switching power-supply applications, the IGBT’s switching efficiency has priority. For motor drives in the 5- to 10-kHz range, the IGBTs are typically designed to deliver an optimal compromise between switching and conduction losses in that frequency range.
International Rectifier’s newest trench-IGBT platform provides the flexibility to tune IGBT product families to fulfill a specific task. Low on-state voltage drop (VCEON), combined with low switching losses, significantly improves efficiency and enables high power density over a wide range of switching frequencies. The devices can also be optimised for applications at high switching frequencies.
The trench IGBTs boast good current sharing in parallel operation, due to tightly controlled parameter distribution and the positive temperature coefficient for VCEON. The devices also have rugged transient performance, delivering high reliability, by providing a square reverse-bias safe operating area (RBSOA), ≥10-µs short-circuit safe operating area (SOA), and pulsed current (ICM) and clamped inductive load current (ILM) up to four times the maximum rated current.
In addition, their high gate-emitter versus gate-collector capacitance ratio (CGE/CGC) prevents (dV/dt) retriggering in fast switching applications. Trench IGBTs also produce low generated electromagnetic interference (EMI), with the inclusion of an ultra-fast, soft-recovery co-pack diode and by maintaining low (dV/dt) and (di/dt) during switching.
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Moreover, bondwire-free packages offering low package parasitic (especially inductance) and wirebond packages offering high thermal-cycling reliability are available. These packages provide designers with a number of choices to optimise mechanical and electrical performance through package selection.
For any automotive application, ruggedness and reliability are very important attributes that must be designed into IGBT power switches. Since large quantities of energy, in the region of many kilowatts, are likely to flow through motor drives and power supplies, safety features are essential in case of a system failure.
Dealing With Motor Short Circuits
One important and serious failure mode is a short circuit in an electric motor, which can seriously damage the car’s entire electronic system. It is usually necessary to provide sufficient time for the motor control unit to detect a short circuit and safely shut down the motor, allowing the electrical energy stored in its windings (and the kinetic energy in its moving parts) to dissipate.
The software typically needs a few microseconds to initiate an emergency program and safely shut down the motor by applying a defined switch-off pattern to the inverter. In industrial motors, around 10 µs is usually sufficient, partly due to the long cable distance between the electronic control unit and the motor.
In automotive applications, the required short-circuit withstand time of IGBTs is typically in the region of 5 to 6 µs. That’s because the motor-control unit is closer to the motor itself, and it’s often directly attached. For power-supply applications, the IGBT performance is typically maximised for efficiency and no short-circuit rating is applied (see the table).
The IGBT is an ideal power switch for the increasing number of high-voltage applications and power-management devices now being incorporated in conventional cars and new HEVs. It offers high performance and supports application-specific optimization, enabling designers to select devices according to important parameters such as switching frequency and short-circuit rating.
It is also important to select the right high-voltage driver IC to maximise the IGBT’s inherent performance advantages. IR’s wide range of automotive gate-driver ICs combine with IGBTs optimised for automotive applications to provide system designers with effective chipset solutions for most high-voltage applications in standard combustion engine vehicles and HEVs.