IEEE Power Electronics Magazine - June 2015 - 29

can be used but only if terminated
into a 50-X oscilloscope input [4]. The
coaxial common-mode transformer
also has an impact on the measurement bandwidth. A high-quality common-mode transformer, such as the
Picotest J2102A, has a flat response of
up to several hundred megahertz.

surements. One notable example is
the Vishay CSM3637 series, which
While GaN provides
can measure up to 40 Amps-rms with
many performance
a four-wire measurement and very
low (though unspecified) inductance.
benefits, this
In either case, the current-sensing
performance
resistor is often added to a special
version of the PCB used only for the
improvement brings
purpose of acquiring this measurewith
it
many
ment. This approach is less than
coaxial shunt
ideal, as the production design used
The coaxial current shunt is available
measurement
for extracting power measurements
in many resistance values and many
challenges.
is no longer identical.
styles with bandwidths up to 1 GHz or
The addition of a current-sensing
more. The significant benefits of the
resistor in the source lead can sigcoaxial shunt are that it is well shieldnificantly reduce the switching speed and also increase the
ed, making it less susceptible to external EMI pickup. The
common source or power loop inductance. In addition to
coaxial current shunt is available in many styles. The meadistorting the rise and fall time measurements, the added insurement shown in Figure 5 includes a 10-mX coaxial shunt
ductance can increase the voltage and current ringing. Comavailable from T&M Research, Inc. [3].
mon source inductance can cause false triggering and gate
The particular style of coaxial current shunt used in Figvoltage overshoot. Power loop inductance can cause ringing
ure 5 has an impact on the measurement due to the leaded
that leads to overvoltage. All ringing can add to the EMI.
connections of the shunt to the PCB. The exposed leads
visible in this photo add approximately 25 nH per lead per
inch. In this case, the coaxial current shunt is likely responIntegrated Magnetic Field Measurements
sible for the majority of the 13-nH total loop inductance disOne possible method of measuring the switch current is
cussed previously and, therefore, affects the measurement
capturing the magnetic field surrounding a trace connectaccuracy. A leadless device such as the T&M Research TTed to the drain of the GaN switch. Much like a Rogowski
SDN might yield a lower inductance.
coil, the current signal can be obtained by integrating the
magnetic field around the trace. The resulting signal is ac
coupled and does not include dc levels, but it could proMetal Foil Current-Sensing Resistor
vide information regarding the current amplitude as well
Several companies offer low-value two- and four-wire curas the rise and fall time. As with the other methods, it is
rent-sensing resistors that are suitable for such current meaimportant to deskew the voltage and current signals if
viewing them simultaneously.
This measurement setup is shown in Figure 6, and the
measurement results are shown in Figure 7. The fidelity of this measurement is determined by the size of the

FIG 6 The setup showing the use of a high-speed current injector applied to a 3.3-V power supply bus on the Picotest VRTS3
demonstration board. An H-field probe is placed in line with
and directly above the PCB trace connecting the current injector
to the power supply bus.

FIG 7 The measurement results showing the near-field voltage
signal in CH1 (yellow). The current monitor from the signal
injector is shown in CH2 (green), and the integrated result of
CH1 is shown in the Math1 (blue) trace.

June 2015

z IEEE PowEr ElEctronIcs MagazInE

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - June 2015