IEEE Electrification Magazine - September 2013 - 60

The components of the state-of-the-art IPT system are
shown in Figure 2. The characteristics of each block are
discussed in detail in the following sections. In addition to
the components described below, the primary and
secondary are equipped with all necessary sensors and
control circuits to generate the firing signals for the
switches and to control the transferred power. Additionally,
communication modules are used to add a further level of
intelligence and controllability to
the system and ensure safe and efficient power transfer.

compensation topologies include series compensation with
matching transformer, series-parallel inductor-capacitor
(LC) compensation, and series-parallel inductor-capacitor-
capacitor (LCC) compensation (Figure 3). The series compensation with matching transformer, shown in Figure 3(a),
makes use of the series capacitor to eliminate the reactive
power flow, and the transformer for galvanic isolation and
impedance matching. The main limitations of the topology
are that it fails to keep the track current constant in face of load variations and that the capacitor VA rating
is quite high. The series-parallel LC
compensation topology distributes
the VA rating over two elements,
reducing the stress on individual
components. In addition, the current
in the coil is controlled by the magnitude of the input voltage source,
making the coil current load independent. Another variant of this
topology, shown in Figure 3(c),
includes a series capacitor that can
be used as an additional degree of
freedom to control the VA rating of
the inverter or to ensure zero-current-switching in the inverter.

An alternative
hybridization method
to extend the utility
of PHEVs and EVs is
to enable a power
exchange between
the vehicle and the
grid while the vehicle
is moving.

Primary Converter and
Compensation Circuit

On the primary, a power supply
delivers high-frequency current and
voltage at its output by using modern switching elements and converter topologies. Although direct ac-ac
conversion from the grid input to the
high-frequency output is possible
through the use of matrix converters,
most topologies are based on the
well-known two-stage ac-dc-ac conversion. A unity power factor stage or
three-phase line filters might be considered at the input to reduce the
reactive power exchange and harmonic pollution of the
grid. Modern IPT systems make use of voltage-fed fullbridge resonant topologies, taking advantage of modern
metal-oxide-semiconductor field-effect transistor (MOSFET) and insulated-gate bipolar transistor (IGBT) switches.
Although IGBTs are more suitable for high-power systems,
paralleling MOSFET devices can provide higher operating
frequencies and lower losses but typically at a higher price.
Since the primary coil is dominantly inductive, the
increase in the signal frequency will linearly increase the
volt-ampere (VA) ratings required to drive the current into
the unloaded coil, increasing the VA ratings of the inverter.
As a result, a compensation circuit is placed between the
inverter and the primary coil. The compensation circuit
consists of one or more reactive elements (inductors and
capacitors) that are arranged in a particular formation to
achieve different design goals. The commonly used primary

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Inductively Coupled Coils
The design of the coupled coils has a profound impact
on system efficiency, and their design is therefore a critical
component of the IPT system. The coil conductors are
typically made using Litz wires because of their small
resistance at high frequencies. At very high frequencies
and for designs with special requirements, planar and
tubular conductors have also been considered. Ferromagnetic material is commonly used to improve the
coupling coefficient and to contain the magnetic flux. In
the case of stationary wireless chargers, a combination
of ferromagnetic material and aluminum is used to
maximize the coupling coefficient while ensuring that
the produced flux is fully contained underneath the
vehicle, even when there is a misalignment between
the source and the receiver. In the case of weakly coupled coils, the use of ferromagnetic materials to

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(a)

(b)

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(c)

Figure 3. The IPT primary compensation circuits: (a) a series compensation circuit with matching transformer, (b) a series parallel LC compensation circuit, and (c) an LCC compensation circuit.

I E E E E l e c t r i f i c atio n Magaz ine / september 2013

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2013