IEEE Power Electronics Magazine - September 2019 - 25

the test conditions. Initially, the testing is done without
on the design of the dc-dc isolation stage [19], [20]. The novconnecting the primary side shield to the ground. The
elty of the design structure hinges on the design and impleCM currents through the gate driver for a 7.5-kV dcmentation of a robust isolation transformer, which mainlink voltage are shown in Figure 8(a). A maximum peak
tains the required insulation requirement, while having a
CM current of roughly 90 mA is observed for the dc-link
small coupling capacitance and an optimized footprint [10].
In addition, the gate driver should make
certain that the clearance and creepage distances for the MV are met.
dc Bus
To achieve these stringent requireC
ments, the MV isolation transformer is
xmer
CM
Current (IHS_PS)
built using a PCB-based planar windQ1
LP 1
Measurement
ing. With a planar winding, the area
Point
enclosed between the winding and the
CPS
core decreases drastically, which helps
LP 2
Gate
Cshield Isolated
w it h reduci ng t he coupl i ng c a pa ci Driver
GDPS
t a n c e bet ween t he w i nd i ng a nd t he
Cxmer
core. For an MV transformer, the insulation requirement is met by fabricating
Q2
dc Power
the PCB with Arlon 85S and having the
Supply
windings enclosed in the same, as shown
LP 1
LP 2
in Figure 7(a). Arlon 85S has a dielecGate
Isolated
tric strength of 1,200 V/mil, which is
Driver
Cshield
GDPS
1.5-times more retardant than that of
(a)
conventional flame-retardant 4 material.
CM
The outer dimensions of the custom-made
CM
Cxmer L
Chokes
Current (IHS_PS)
ferrite core are 28 mm × 28 mm × 4 mm.
Lp1
p2
Measurement
The area of cross section is 16 mm 2 .
Point
Figure 7( b) shows the designed gate
IPS
Cshield
IHS
driver, the dimensions of which are 6 in
IHS_PS
IHS_S
× 2 in × 1.5 in. The clearance and creepage requirements are satisfied via proper
CPS
dv/dt
Cxmer L
designing of the MV isolation transformer
Lp1
p2
Source
terminals and by providing the required
creepage through the support structure,
Cshield
as shown in Figure 7(c). A clearance of
ILS_PS
ILS_S
50 mm is maintained between the open
terminals of the isolation transformer,
(b)
and the creepage distance is maximized
to approximately 90 mm by providing
grooves on the support structure.
Ch2: 5 kV/div
Ch1: 5 kV/div
These clea ra nces satisf y the IEC
61800 - 5 -1 sta ndards (i.e., a minimum
clearance and creepage distance of 25
and 32.5 mm, respectively, for a working
Ch4: 50 mA /div
Ch3: 50 mA /div
voltage of 7.2-7.6 kV) [21]. Note that the
ferrite core is supported via the support
Reduced Peak
structure and does not touch the PCB
Time: 2 µs/div
windings. This eliminates the creepage
path through the transformer core. In
(c)
addition, the PCB is attached to the support structure with nylon screws, which FIG 9 (a) The architecture of the system showing the gate driver circuit with
makes it a nonactive component for con- power supplies, (b) an equivalent high-frequency circuit showing the CM current
path and grounding considerations, and (c) a comparison of the CM currents flowsidering clearance requirements.
ing through the gate driver circuit for the two cases. Ch1: voltage across the botThe gate driver is tested using a DPT tom of the device; Ch2: voltage across the top of the device; Ch3: CM currents
setup with 10-kV SiC MOSFETs. The gate when the shield is not grounded to earth; Ch4: CM currents after shielding the
resistance is maintained at 20 Ω for all of primary side of the gate driver [10]. GDPS: gate driver power supply.
September 2019

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IEEE Power Electronics Magazine - September 2019

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