Current Sensing in HEVs

Consumers are embracing environmentally friendly "green cars" as a result of the rising cost of fossil fuels and a growing concern for the health of the environment. Sales forecasts have predicted that green car sales will comprise 20%-25% of all vehicle sales by the year 2015. The hybrid electric vehicle (HEV) is quickly becoming the most popular green car and by 2015 is expected to comprise approximately 12% of global vehicle sales. Hybrid electric vehicles employ complex power electronic circuitry to control the flow of electric energy throughout the vehicle. In a single motor HEV (see figure 1) the motor acts as a drive motor in parallel with the internal combustion engine, or as a generator to charge the battery during regenerative braking. A typical HEV contains various systems that require electrical current sensors for maximally efficient operation; including AC motor and DC-DC converter applications. This article focuses on recent advances in Hall effect current sensor technology and the use of unique, high bandwidth, enhanced resolution current sensors in HEV applications.

In the HEV power cycle the main battery voltage is inverted as shown in Figure 1 and the resulting AC voltage is applied to the motor which in turn drives the wheels. During regenerative braking the AC motor also serves as a generator. When the regeneration system is active, the output of the motor/generator is rectified and converted to a DC voltage necessary to charge the HEV battery cells, completing the power cycle. If the HEV is a plug-in vehicle, then line voltage can also be rectified and used to charge the battery. The regenerative braking process is a primary contributor to the improved fuel efficiency of an HEV, since braking energy which is normally wasted in the form of heat is partially recovered and used to charge the main battery. To power the low voltage infotainment and body control subsystems in the car a DC-DC converter is typically used to reduce the hybrid battery voltage (typically 300 to 500V) to a lower level DC voltage.

One drawback of conventional Hall effect sensors, when used in current sensing applications, has been a general limitation in both accuracy and output signal bandwidth. However, Allegro® MicroSystems, Inc. has developed a broad family of Hall effect current sensor integrated circuits (ICs) that are ideally suited for HEV applications.

• Signal processing and package design innovations enable > 120kHz output bandwidth
• The highest current resolution, lowest noise spectral density Hall sensors in the marketplace
• Proprietary, small footprint sensor packages with galvanic isolation
• Reduced power loss: through hole compliant and low-resistance integrated conductor packages
• Precise factory programming of sensor gain and offset

The 3-phase, full bridge driver in a typical inverter converts DC battery voltage to a 3-phase AC voltage that is required for efficient operation of the system motor (see Figure 3b). The inverter phase currents are measured and the resulting information is typically used to control the pulse-width modulated (PWM) inverter switches (typically IGBTs). The inverter control loop requires high bandwidth current sensors to improve accuracy, and to maximize motor torque and overall motor efficiency. High side current sensors with fast response times also enable over-current protection during a short circuit condition from a motor phase to the system ground node. The Allegro A1360 Hall linear device is tailor made to meet the voltage isolation, > 200 ampere (A) load current, and high bandwidth demands of HEV inverter applications. The A1360 linear Hall effect sensor is typically placed 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 standard single-in line package (SIP). The A1360 Hall transducer provides an output voltage proportional to this 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 more conventional IC packages. Allegro SIP-based current sensors have a typical output bandwidth as high as 120kHz and offer high resolution, high accuracy performance that allows for higher 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 solution is also much smaller than bulky current transformers. The Allegro SIP package easily provides the necessary galvanic isolation because the sensor output leads are not connected to the high voltage, current carrying conductor in each phase of the motor.

The current sensing range and the isolation voltage required determine the optimum Allegro current sensor package for use in DC-DC converters. Current sensors in DC-DC converters often need to sense current down to DC frequencies. This need precludes the use of current transformers in fully optimized systems. Using shunt resistors in these applications is also challenging (or impossible) since the high input or output DC voltages require expensive, high common-mode input operational amplifiers. As a result of the inherent galvanic isolation of Allegro Hall effect sensor ICs, and their ability to sense both DC current and high frequency current signals, they are a logical choice for HEV DC-DC converter applications. A simplified regenerative DC-DC converter is shown in Figures 1 and 3a. The regenerative converter utilizes a current sensor that can operate at battery voltage levels. Accurately sensing the converter output current is a critical function as HEV battery life is extended by limiting the charge current delivered to the battery. The ACS714 current sensor is ideal for many lower current, subsystem DC-DC converter applications and is a factory trimmed, galvanically isolated sensor that is available in an extremely small form factor SOIC8 package with an integrated 1.2m? conductor for low power loss. Additionally, the Allegro ACS758 device incorporates both a 100μ? conductor and a ferromagnetic core into a small form factor, galvanically isolated package capable of sensing 50 through 200A. Currents above 200A can be sensed with the SIP based toroid configuration mentioned earlier. All of these Allegro solutions offer industry leading high bandwidth output performance and exceptional current resolution capabilities.

Allegro's latest generation Hall effect current sensor IC technology offers significant advantages in sensing both AC and DC currents in HEVs. Hall current sensors have inherent galvanic isolation for high side current sensing, and offer low power loss in high efficiency HEV applications. Recent improvements in Hall IC technology by Allegro have resulted in the development of industry leading high bandwidth, high resolution current sensor ICs that are ideally suited for use in HEV Inverter and DC-DC converter applications. www.allegromicro.com