04-09-2012, 04:57 PM
Hall Effect Current Sensing in Hybrid Electric Vehicle (HEV) Applications
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Introduction
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
predict that green car sales will comprise 20%–25%
of all vehicle sales by the year 2015.[1] 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.[1] Hybrid electric vehicles employ
complex power electronic circuitry to control the flow of
electric energy through 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.
The HEV Power Cycle
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
Current Sensing in Inverter Applications
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 3). 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 overcurrent
protection during a short circuit condition from a motor phase to
the system ground node.
DC-DC Converters
The current sensing range and the isolation voltage required
determine the optimum Allegro current sensor IC package for use
in DC-DC converters.
Current sensors in DC-DC converters are often required to sense
currents down to DC frequencies. This requirement 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 HEVs in DC-DC
converter applications.
A simplified regenerative DC-DC converter is shown in figures 1
and 3. 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.
Summary
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.