Author:
Shaun Milano, Strategic Marketing Manager - Michael Doogue, Director, Linear and Current Sensor Business - Georges El Bacha, Systems Engineer, Allegro MicroSystems
Date
08/19/2012
PDF
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Figure 1: Typical HEV system block diagram
Consumers are embracing environmentally friendly green cars due to the rising cost of fossil fuels and a growing concern for the health of the environment. Sales forecasts predict that green cars will comprise 3% to 4% of all vehicle sales by the year 2016. After 2016, estimates call for a more rapid adoption of green cars, with green car market share ranging from 8% to 19% in 2021. The most popular green car, both today and in forecasts through 2021, is the HEV (hybrid electric vehicle). HEVs employ complex power electronic circuitry to control the flow of electric energy through the vehicle. In a single motor HEV, the motor acts as a traction drive in parallel with the internal-combustion engine, or as a generator to charge the battery during regenerative braking. Both HEVs and EVs (electric vehicles) contain various systems that require electrical current sensors for maximally efficient operation, including AC-motor and DC-DC-converter applications. The HEV Power Cycle In the HEV power cycle the power converter inverts the main battery voltage and applies the resulting AC voltage to the traction motor, which in turn drives the wheels (Figure 1). During regenerative braking, the AC motor also serves as a generator. When the regeneration system is active, the power converter rectifies the output of the motor-generator to a DC voltage sufficient to charge the HEV battery cells, completing the power cycle. For EVs and plug-in HEVs, the power converter also rectifies the line voltage to charge the battery. The regenerative braking process is a primary contributor to HEV's and EV's energy efficiency because the power converter partially recovers braking energy normally wasted in the form of heat, and uses it to charge the main battery. To power the low voltage infotainment and body-control subsystems in the car, a DC-DC converter reduces the battery voltageâ€"typically 300 to 500 Vâ€"to a lower-level DC voltage. One drawback of conventional Hall-effect sensors in current sensing applications has been a general limitation in both accuracy and output signal bandwidth. Signal-processing and package-design innovations enable new Hall-effect current sensors with > 120 kHz output bandwidths, low output phase shift, high current resolution, and low noise spectral density.
Current Sensing in Inverter Applications The driver in a typical inverter converts DC battery voltage to a 3? AC voltage that the traction motor requires for efficient operation (Figure 2). Current sensors measure the three inverter phase currents and, typically, a controller uses the resulting information to control the PWM inverter switchesâ€"usually IGBTs. The inverter control loop requires high-bandwidth current sensors to improve accuracy, maximize motor torque, and maximize overall motor efficiency. High-side current sensors with fast response times also enable over-current protection during short circuit conditions from a motor phase to the system-ground node. For example, the Allegro A1360 linear Hall device meets the voltage isolation, > 200 A load current, and high-bandwidth demands of HEV inverter applications. The Hall-effect sensor IC typically locates in the gap of a ferromagnetic toroid which surrounds each inverter phase conductor in the motor (Figure 2). As current flows in the conductor, the toroid concentrates the resulting magnetic field through the SIP (single inline package) device. The transducer provides an output voltage proportional to the current. The device is available in a proprietary, 1 mm thick package that reduces eddy current losses to improve sensor output bandwidth when compared to conventional IC packages. The A1360 and similar devices have a typical output bandwidth as high as 120 kHz and offer high-resolution high-accuracy performance that allow for high-speed control of the PWM switches in an inverter system. Additionally, these SIP sensors offer a 3 μs output response time for IGBT over-current protection applications. The form factor of this current-measurement circuit is also smaller than current transformers. Nonetheless, it provides the necessary galvanic isolation because the sensor IC output leads do not connect to the high-voltage current-carrying conductor in each of the motor's phases. Electric Motor Control A prominent trend for improving energy efficiency in HEVs and EVs (and, to a lesser extent, in internal-combustion engines with idle-stop capability) has been the conversion of belt-driven and hydraulic actuators to electrically-driven actuators. For instance, in traditional internal combustion engines a fan belt drives the cooling fan, which operates continuously while the engine is running. The same applies to power-steering pumps and other belt-driven loads. Replacing belt-driven actuators with electric motors improves energy efficiency and allows for greater control of the actuators. Precision, high-speed current-sensor ICs provide the bandwidth, response time, and accuracy performance necessary to optimize motor performance but must meet qualifications for the demanding under-hood environment. DC-DC Converters The current sensing range and the isolation voltage determine the optimum current-sensor IC package for use in DC-DC converters. Current sensors in DC-DC converters often must sense current in a spectrum that includes DC. This requirement precludes current transformers in fully optimized systems. Using shunt resistors in these applications is also challenging because the high input or output DC voltages require expensive, high-common-mode input operational amplifiers. A Hall-effect sensor with inherent galvanic isolation, wide current bandwidth, and a transduction response that extends to DC is a logical choice for DC-DC converter applications in HEVs. A regenerative converter uses a current-sensor IC that can operate at traction-battery voltages. Accurately sensing the converter output current is a critical function because correctly metering the charge current that the converter delivers to the HEV battery extends its operating life. The ACS714, for example, is a current-sensor IC suitable for many lower current, subsystem DC-DC converter applications. The ACS714 is a factory-trimmed, galvanically-isolated sensor IC available in an extremely small form factor SOIC-8 package with an integrated 1.2 m? conductor for low power loss. For higher-current applications, current sensor ICs such as the Allegro ACS758 incorporate a 100 μ? conductor and a ferromagnetic core into a small form factor galvanically-isolated package capable of sensing 50 through 200 A. For greater measurement ranges, the SIP based toroid configuration mentioned earlier can sense currents above 200 A. www.allegromicro.com Captions:
