IEEE Electrification Magazine - March 2017 - 33

variety of solutions are in mass production. Since each of
these solutions has certain advantages and disadvantages,
the designer must select the sensor that best matches
their requirements.
The most common technologies available include core
and coreless Hall effect, resistive shunt, and, more recently,
the giant magneto resistive (GMR) sensor. As the ac output
terminals have high-voltage square wave, the current sensor must provide voltage isolation between the currentcarrying power conductor and its output signal being fed
back to the controller, for which feedback signals could be
either analog or digital. EV inverters can have ac output currents from a few hundred to over 1,000 A rms. The maximum current occurs for a short duration under a heavy
acceleration event. Sensors with large dynamic range have
an advantage under this condition. By being located directly
on or very close to the output ac bus bar, temperature rise
on the bus bar will, in turn, raise the operating temperature
of the sensor. High-temperature capability, such as 125 °C, is
needed for compatibility with the surrounding environment. In addition, sensor bandwidth should be sufficiently
high to support the control requirements of the closed-loop
system. For most inverters, 30-kHz bandwidth or higher is
necessary. Phase shift should also be minimal and consistent up to the maximum fundamental frequency, which is
usually in the range of 500-1500 Hz.
Traditional Hall effect sensors use a gapped magnetic
core and a Hall effect sensor integrated circuit (IC) located
in the air gap [see Figure 5(a)]. These sensors have proven
performance and are widely employed in EV applications.
However, some potential drawbacks of the core-type Hall
effect sensor include physical size and weight as well as
limited measurement range due to finite core saturation
flux density. To overcome these drawbacks, a variety of
coreless sensors are available. Care must be taken to be
certain that cross coupling effects due to stray fields of
adjacent conductors do not corrupt the sensed signal.
Additionally, resistive-shunt-based solutions are viable options in the EV application. The shunt element
can be a discrete device bolted into the system, as seen
in Figure 5(b), or even integrated into the bus bar structure itself. The resistive element by necessity has a very
low ohm value. The small voltage signal is measured
across the shunt, amplified, processed, and converted to
a digital bit stream before being passed to the controller
using digital isolators. The associated controller must
then decode the digital signal. While the sensor has low
resistance, it will still contribute power dissipation to
the bus bar assembly, so careful consideration must be
taken to ensure acceptable temperature rise.
GMR devices (Bohn et al., Jogschies et al., and Ouyang
et al.) are another type of current sensor technology that
may be well suited for EV applications. In a GMR-type sensor element, alternating sandwiched layers of magnetic
and nonmagnetic material are utilized. The resistivity of
the stack changes as a function of the induced magnetic

field. Typical construction has the sensor mounted in close
proximity to the current carrying bus bar, and magnetic
shielding with a high-permeability material may be
employed to minimize the impact of stray external fields.
GMR devices promise to be lightweight, small volume, high
bandwidth, and low offset. However, they are not yet widely available from suppliers. These sensors will likely gain
market share in years to come.

Gate Drive
The gate drive circuit's primary function is to receive digital PWM-gate command signals from the controller, then
isolate and buffer them as needed to drive the power
semiconductor switches. A second, but equally important,
function is to provide fast hardware-based fault protection.
This section will examine these two functions and explore
the various solutions being used in the industry today.
The controller is most commonly referenced to as the
vehicle chassis, whereas the HVdc energy source (i.e., a
high-voltage battery pack) is isolated from the chassis for
safety reasons. Hence, the IGBT gate connections are all
floating with respect to the controller ground reference.
The gate driver circuit must transfer the controller digital
command signal from the chassis reference to the floating potential using some form of isolation. Three common forms of digital isolators are employed: optical,
magnetic, and capacitive. All of these are available from a
variety of qualified automotive part suppliers. One of the
most important parameters to consider is the CM transient immunity (CMTI) rating of the part. As the main
power switch is turned on or off, a huge voltage transient
is applied across the isolator. The CMTI rating is a measure of the voltage transient (specified in kV/μs) that the
part can withstand while preserving the signal integrity.
Not only must the proper part be selected, but careful
layout of the gate drive circuit card is required to minimize noise coupling and ensure glitch-free operation.
Beyond the normal operating mode requirements,
the gate driver is also required to protect the inverter
under abnormal conditions. Some of the protection features often found in EV inverters may include antishoot
through, desaturation, overcurrent, overtemperature,
and undervoltage.

(a)

(b)

Figure 5. Different sensor technologies available for ac current sensing: (a) a three-channel Hall effect (photo courtesy LEM) and (b) resistive shunt (photo courtesy Isabellenhuette).

IEEE Electrific ation Magazine / march 2 0 1 7

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2017