IEEE Electrification - June 2021 - 27

chargers. Thus, for nonisolated chargers,
it is essential to properly manage
the CM current so that the GFCI
can be connected during charging
without nuisance tripping. In practice,
the touch current test, as defined
in UL 2202, is considered the gold
standard and the most effective indicator
of personnel safety. The human
body impedance network, as defined
in UL 2202, is provided in Figure 5.
According to the standard, the
touch current level
is considered
With minimum add-on
hardware, an onboard
level 2 or level 3
battery charger could
meet the peak power
consumption of a
typical American
household.
safe to end users when the output
voltage (Vbodyout) is less than 0.25
root-mean-square (RMS) volts.
EMI-related considerations for integrated chargers
often involve the following:
x the CM EMI to the grid
x the CM EMI to the battery bus
x the leakage current through the vehicle chassis.
CM EMI standards are often open-formed to protect
power sources. In the case of bidirectional chargers, EMI
standards are imposed on both the grid side and the
battery side. A larger CM noise current is expected in
nonisolated chargers, as the equivalent CM impedance
is significantly lower than in the isolated chargers. Part
of the noise current would become an undesired emission
to the grid and the battery bus, and the rest would
circulate inside the vehicle chassis. This is potentially
dangerous since the noise current could interfere with
the other circuits, as the vehicle chassis serves as the
common ground for all onboard circuitry. Bulky CM filters
could be inevitable in the charger design to limit the
CM current flow.
It is important to note that depending on the current
paths and the frequency range of the CM leakage current,
there are different impacts. CM current that emits
to the grid and goes through the ground is considered
the CM EMI noise and regulated by noise spectrum limits,
typically starting from 150 kHz. The very same current,
below 150 kHz, causes potential electric shocks and
GFCI/RCD nuisance tripping. CM current that circulates
inside the vehicle could interfere with other onboard
circuitry. The battery side EMI standard
puts spectral limits on the measured
noise inside the vehicle, as well.
CM Challenges and Nonisolated
Charger Mitigation Techniques
As identified in the preceding section,
the CM leakage current in nonisolated
charging systems needs proper
mitigation so that the only available
protection, the GFCI, will not be lost.
This section explores the potential
CM leakage current issues by first
reviewing the grounding systems and
then looking at circuit modeling and
the potential risks of leakage current in nonisolated
chargers. Possible mitigation techniques are discussed at
the end.
Overview of Grounding Systems
Different grounding systems have a major impact on system/personnel
safety and noise propagation paths. Earth-
earth (TT), earth-neutral (TN), and isolated earth (IT) are
the three main types of grounding systems. The TT
grounding system is widely adopted in Japan, France, Italy,
and Egypt. It is characterized by a high-impedance return
path for fault and noise currents. As shown in Figure 6, the
grid is connected to the utility ground through an electrode
impedance RN (usually around 10 X), and the vehicle
chassis is connected to the local ground through another
grounding impedance RG (it is usually lower than 100 X,
but it could go up to several hundreds of ohms.). In this
Generator or
Transformer
Onboard Electronics
RN Rsoil
RG
Vehicle
Chassis
Figure 6. The TT grounding system.
0.22 µF
1
2
500 Ω
1.5 kΩ
10 kΩ
22 nF
Generator or
Transformer
Vbodyout
Onboard Electronics
Protective
Earth
RN Rsoil
RG
Vehicle
Chassis
Figure 5. The human body impedance network (Zbody).
Figure 7. The TN grounding system.
IEEE Electrification Magazine / JUNE 2021
27
IEEE Electrification - June 2021

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