IEEE Power Electronics Magazine - June 2019 - 34

Figure 1, our 100 V/ns would induce 1 A into just 10 pF.
As this current finds a return path, transient voltages are
generated across connection inductances and resistances,
thereby increasing the risk of control circuit malfunctions
and conducted and radiated emissions. Capacitances seen
by switches often result from insulation to ground, e.g.,
an attached heatsink. Current circulating to ground may
seem benign, but this common-mode (CM) noise can circulate externally and is specifically limited by international
EM compatibility (EMC) standards. The limits in Figure 2 relate to emissions measured with a standard lineimpedance stabilization network configured to indicate a

VGS
+
0 to +12 V -

Cascode
UJC1206K

DON
RG_ON
2.3 Ω 1 A, 40 V Ferrite
Bead
RG_OFF
20 Ω

FIG 5 A simple RC snubber can address both turn-on and -off
issues in WBG devices.

Wrong
* Thin Tracks
* Stubs
* Separated Tracks

Right
* Thin Tracks
* Direct Connections
* Close Tracking

FIG 6 An example of optimum tracking for a filter.

IEEE POWER ELECTRONICS MAGAZINE

EMI can be reduced by simply slowing down switching
edges. With SiC FETs, this can be achieved by adding series
gate resistance. It is common for devices to have an internal
minimum resistance to prevent parametric oscillation. Turnon and -off edges can be separately controlled by two resistors (with an isolation diode), as shown in Figure 4 for an
SiC cascode. A ferrite bead can be used to suppress highfrequency gate ringing.
This process works well but can defeat the purpose of
using SiC for its speed, which reduces EMI at the expense
of efficiency. Slowing gate-drive edges also restricts the
minimum useable pulsewidth from the controller, which
reduces the pulsewidth modulator adjustment range and
response time to cycle-by-cycle control.

Although a gate resistor is a practical way to slow-turn "on,"
slowing the "turn-off" drain dV/dt can also be achieved with
RC snubbers, which divert current into a capacitance as a
FET turns off, thereby slowing dV/dt (Figure 5). The resistor, however, dissipates power, which reduces efficiency.
Complex snubbers, which are nondissipative, may be used
to return energy to the supply or to generate auxiliary
power rails. However, recent studies by UnitedSiC [1] demonstrate that by using their fastest devices, only a small
snubber is necessary to keep voltage overshoots within
bounds, while dissipating less than the penalty incurred by
slowing turnoff with a gate resistor.
Good layout practice minimizes EMI [2]; all of the highcurrent, high-frequency paths should be as short as possible and have decoupling capacitors very close by. Connections are of lower inductance and resistance when they
are thicker. High-frequency "go" and "return" paths should
be routed so that they are adjacent to one another, thereby
ensuring radiation cancellation (Figure 6).

EM Fields

Slowing Edges Helps

Snubbers Can Work Well

FIG 4 An example of using gate resistors to control switch speed.

combination of CM (i.e., line and neutral-to-ground) and
differential-mode noise (measured from line to neutral).

z June 2019

A common source of EM radiation is high-current secondaries from transformers, which may be hanging wires used
for mechanical convenience (these should certainly be
twisted). Magnetic components themselves are a source of
EMI, and transformers will often have a "belly band,"
which acts as a shorted turn to leakage fields. Occasionally, there will be various magnetic component styles from
which to choose, which may affect EMI; gapped E core
ferrites will have lower losses than, e.g., powdered-iron
toroids but will have more field leakage.

Displacement Currents
Minimizing stray capacitances is difficult in layouts, especially at high power with large heatsinks. However, the latest
WBG devices can have such high efficiency that heatsinks

IEEE Power Electronics Magazine - June 2019

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