IEEE Power Electronics Magazine - June 2019 - 35

can be smaller, potentially board-mounted as opposed to
grounded. In this case, they can be electrically connected to
the primary dc bus so that induced "displacement" currents
circulate locally (Figure 7). This arrangement also helps with
safety; a grounded heatsink would need agency-rated "basic"
insulation to a transistor tab, which can introduce high-thermal resistance.
If the heatsink must be grounded, current induced
through dV/dt can be reduced by using a thicker insulator,
such as a beryllium oxide pad, which causes capacitance
to reduce in proportion to the separation distance. Another
approach is to use an insulator pad with an electrostatic
screen embedded between layers that is connected to
the dc supply. However, this is an expensive solution that
degrades thermal resistance (Figure 8).
Capacitance from device-switching nodes to other components and the enclosure also allows ground-displacement current to flow; therefore, devices should be physically isolated as much as possible. Some topologies, such
as full bridges, have switching nodes that are inherently in
antiphase. A careful layout can potentially arrange parasitic capacitances so that the induced ground currents cancel to some extent.

Heatsink Connected
Grounded Heatsink: to HT+: Current
CM Current Flows
Flows Locally
to Ground
FIG 7 Grounded heatsinks are a source of CM interference current. Connecting to the high voltage node HT+ can prevent
this. HT: high tension.

There will always be significant capacitance across transformers in isolated converters allowing displacement current
from the primary to ground through outputs. This can be
blocked with an electrostatic screen between transformer
primaries and secondaries, which, as stated previously, are
connected to the primary dc bus. When transformers are
wound with split primaries for low-leakage inductance, voltage transients are reduced; however, multiple screens may
be needed, making transformer construction more complex
and costly and agency-rated insulation more difficult to
achieve. For this case, when using the popular triple-insulated wire for safety insulation, it is better to use this for secondaries rather than primaries; this is because the screens
would also require bulky triple insulation-either by tape or
something similar-if triple-insulated wires were used for
the primary windings.
CM and differential-mode filters are the "backstops" for
reducing EMI to acceptable levels; however, they have their
own design problems. It is always better to minimize the
effects at the source rather than to filter them later. The best
designs at low power require little or no additional filtering with the latest resonant converter topologies and techniques, such as frequency "dithering" to spread emissions
so that the level in the measurement bandwidth is reduced.
It is important to remember that main switching transistors are not the only source of EMI; rectifier diodes in
power factor correction (PFC) stages and on dc outputs
can also contribute at high frequencies because of reverse
bias charge recovery. For this reason, Schottky diodes or
SiC types are much preferred due to their absence of Q RR .

EMI Can Be Controlled
EMI is a fact of life for power designers and, with faster
switching speeds, the potential for problems can increase.
However, SiC devices can be "tuned" to acceptable levels by
using a combination of gate control, snubbers, good layout
practice, and an appreciation of the routes that EMI can take.

About the Author
Switching
Transistor

Insulated Copper Foil

Grounded
Heatsink

Anup Bhalla (abhalla@unitedsic.com) received his B.S.
degree from the Indian Institute of Technology, Delhi, in
1989, and his Ph.D. degree from Rensselaer Polytechnic
Institute, Troy, New York, in 1995, both in electrical engineering. Currently, he is the vice president of engineering at
UnitedSiC. Prior to joining UnitedSiC in 2012, he held various product development and marketing positions at Alpha
and Omega Semiconductor (AOS), where he was a cofounder of the company. He has authored or coauthored nearly
100 patents throughout his career at Harris, Vishay Siliconix, AOS, and UnitedSiC.

References
To Primary HT+

[1] Mike Zhu, "Switching fast SiC FETs with a snubber," UnitedSiC, Monmouth Junction, NJ, Rep. AN0018, 2018.

FIG 8 An electrostatic screen between the transistor tab and
heatsink blocks induced currents to ground.

[2] TI, "PCB design guidelines for reduced EMI," Texas Instruments, Dallas,
TX, Rep. SZZA009, 1999.

June 2019

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IEEE Power Electronics Magazine - June 2019

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