IEEE Power Electronics Magazine - June 2022 - 24

Given the voltage limits of power semiconductors nowadays,
input series and output parallel (ISOP) is a popular
candidate as shown in Figure 12(b) [17]. Since such dc-dc
stage still engages the HF transformer, here " Transformerless "
actually means that no 50/60 Hz transformer is used,
but instead still incorporates a high frequency [>10 kHz]
transformer that is much more compact in size and weight.
One exemplary ISOP XFC is presented in Figure 12(c) [18],
proposed by the UTK team and Wolfspeed, under the sponsor
of U.S. Department of Energy. To transfer the power, the
ac voltage at the grid is first rectified and transferred to the
primary side dc link, such as 4.3 kVdc which allows 6 kV SiC
MOSFETs to be adopted. The number of series-connected
H-bridges on the grid side is determined by the ac-grid voltage
and the switch voltage rating. Then, the high-frequency
inverter converts the dc voltage to high-frequency ac voltage
and transfers power from the dc link to the resonant
network formed by compensation components (resonant
inductor and capacitors).
A MV HF transformer is needed for voltage isolation purposes.
If only one car is charged from the XFC, the transformer
secondary side only needs one winding. In this
particular case, the XFC is designed to charge two cars simultaneously,
so two secondary-side windings are used. The
secondary windings then induce a stepped-down ac voltage,
which will be rectified by the LV H-bridges with output voltage
being paralleled forming the LVDC bus (e.g., 1.3 kVdc).
To charge regular EV batteries of 200-450 Vdc, another buck
converter is needed to step-down the 1.3 kVdc to the battery
voltage. Note using XFC technology to charge the battery
still needs to secure high quality output voltage/current at
the battery side, for instance output voltage 200 - 920 V with
ripple of ±5% or ±5 V, and output current up to 500 Adc with
ripple <1.5 A and frequency <10 Hz, based on IEC 61851-23.
To alleviate the switching loss and transformer stress
thereby avoiding the scenario in FIG 4, a typical resonanttype
isolated MVDC-LVDC converter can be a good choice,
e.g., an LC type also called dc transformer (DCX) converter.
The resonant network only incorporates series-connected
capacitors C and transformer leakage inductance L on the
secondary side. The transformer mutual inductance is much
3,000
2,000
1,000
-1,000
-2,000
-3,000
0.79994
0.79996
0.79998
0.8
Time (s)
FIG 13 Voltage and current waveforms of the primary side of the DCX transformer [18].
24 IEEE POWER ELECTRONICS MAGAZINE z June 2022
0.80002
0.80004
larger than the leakage inductance. Therefore, it has little
effect on the resonance. At the resonance frequency, this
topology behaves as a constant voltage source. The transformer
turns ratio can be accurately designed to transform
the 4.3 kVdc bus to a 1.3 kVdc output. Essentially, such DCX
topology is made to eliminate the switching losses, given
the switching moments all happen around current zero
crossing points, which helps in natural ZVS turn-on and
zero-current switching turn-off for both primary side and
secondary side switches as shown in Figure 13. In addition,
the resonant topology avoids the transformer windings
from having to accommodate the high dv/dt of the switches,
further facilitating the transformer design.
The first major concern with this topology is its resonant
capacitors. High voltages at resonant frequency are
induced across the resonant capacitor. Such high voltage
stress, usually multiple times of the dc-bus voltage, requires
a large number of film capacitors in series and parallel,
resulting in large capacitor banks. Meanwhile, such highfrequency
voltage is also subject to significant EMI issues.
The second major concern lies in the MV high-frequency
transformer, which has issues of partial discharge. In recent
years, researchers are aware of the challenge of such MV
transformer design. The high dv/dt of SiC devices requires
the larger distance among turns, which enlarges the size of
the MV transformer.
Nevertheless, with the operating frequency of a transformer
increasing, reduction of the transformer size and
weight is still expected, given its core cross section is
reduced inversely proportionally to the frequency. Nanocrystalline
cores, for instance, can be produced with sheet
thicknesses as low as 13 μm, in contrast to the 350 μm
thickness of conventional grain-oriented electrical steel
used at the line frequency. However, for MV insulation, the
miniaturization of the transformer creates a direct challenge
for the dielectric design, given increasing frequency
does not reduce the clearance distance required for insulation.
Meanwhile, because of the MV ratings required, the
insulation material layer, which encapsulates the MV-winding
and isolates it from the LV-winding and the core, has to
be rather thick, which increases the transformer size again.
300
200
100
-100
-200
-300
0.80006
Voltage (V)
Current (A)
IEEE Power Electronics Magazine - June 2022

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