IEEE Power Electronics Magazine - June 2015 - 28

probes. Higher-voltage and loweran impact on the circuit performance
speed devices require a much lower
and reflect the in-circuit measured
As GaN technology
bandwidth. The example in Figure 2
rise time; however, the measurement
improves, the gap in
with an 8.7-ns current fall time is
results may be significantly influrepresentative of a 40-MHz bandenced by these parasitic elements,
the availability of
width. Applying the equation for the
masking the potential performance
probes will further
graph in Figure 3, the measurement
of the GaN switch.
bandwidth required for a 5% meaThe simulation results shown in
increase.
surement accuracy is approximateFigure 4 indicate that an infinitely
ly 120 MHz.
fast GaN switch results in a 1.16-ns
rise time. In this case, 60% of the inductance limiting this measurement is external to the GaN
Current Probes
switch. The simulation results also show that the voltage
Many instrument manufacturers offer current probes with
appearing across the GaN die at the COSS is significantly
a 100-MHz bandwidth, which is quite close to the required
greater than the voltage measured at the interface between
bandwidth for high-voltage GaN devices. Unfortunately,
the GaN switch and the PCB. The interdependent relationwhile the current probe bandwidth may be barely suffiship between the host PCB and the GaN switch has other
cient, such current probes still have many additional limitaramifications as well. To optimize the circuit performance,
tions that preclude their use in such applications. These
it is necessary to separate the dependencies of the PCB, delimitations include the following.
coupling capacitors, and GaN device. The PCB and decou■■The probes are physically large, while the high-density
pling capacitor characteristics can be extracted using SGaN converter has limited physical access.
parameter measurements at the location of the GaN device.
■■The GaN switch current often exceeds the maximum ratThese S-parameter measurements can then be removed in
ings of most hall-effect probes. This is further exacerbated by the 100- and 250-A GaN switches expected to be
simulation (de-embedded) to determine how much the cirintroduced this year. Higher-current probes are available
cuit performance is limited by the PCB plane design and
with a reduced bandwidth.
decoupling capacitors.
■■The insertion impedance of such probes is significant
compared with the circuit, and so the insertion of the curCurrent Measurements
rent probe will significantly alter the measurement result.
Switch current measurements are traditionally performed
In addition to the published insertion impedance, it is
using a dc probe. The voltage and current measurements
also necessary to add in the inductance of the wire loop
are deskewed using a deskew and calibration fixture availnecessary for the measurement probe to clamp around.
able from most instrument manufacturers.
With the current probe all but eliminated from the possible
Equation (1) can be used to determine the bandcurrent measurement solutions, there are a few alternatives.
width required to accurately capture the current signal.
Low-voltage GaN devices, with edge speeds faster than
1 ns, far exceed the bandwidth rating of all dc hall-efCurrent-Sensing Resistors
fect probes and all ac clamp-on passive and Rogowski
An alternative to the current probe is to monitor the current signal using a sensing resistor. There are two categories of current sensors that can be used in this frequency
range. One category is the coaxial current shunt, while the
other category is the planar resistor.
When measuring the switch voltage and current, a dc
ground loop exists since both the voltage and current probes
are connected at the instrument (oscilloscope) ground and
also at different locations on the PCB being tested. For
this reason, a coaxial common-mode transformer may be
required to obtain the best measurement fidelity. The measurement is deskewed by placing the voltage probe on the
current-sensing test points and adjusting the oscilloscope
channel skew offset to assure that the voltage and current
channel edges are both simultaneous on the oscilloscope.
The oscilloscope user manual generally provides detailed
documentation for the deskewing process.
FIG 5 The high-voltage probe is shown on the left, the gate
It is also important to note that the cables connecting
drive probe is shown on the right, and the red body of the
the
oscilloscope to the current-sensing test point must be
coaxial current sensor is shown in the center. (Photo courtesy
impedance
matched. For example, a 50-X coaxial cable
of GaN Systems.)

IEEE PowEr ElEctronIcs MagazInE

z June 2015

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