IEEE Power Electronics Magazine - March 2018 - 44

High-Voltage active, Differential
amplifier with Matched
Probe Pairs

Each type of highvoltage oscilloscope
probe has different
strengths and weaknesses that lean it
more toward some
power electronics
measurement applications than others.

A final option for high-voltage probing
is the active, differential amplifier with
matched probes. This type of system,
e.g., the Teledyne LeCroy DA1855A, is
ideally suited for conduction-loss testing. The active, differential amplifier
with matched probes features excellent overdrive recovery and offset
ad justment, and it is very good for
device switch ing performance measurements and measuring very small
voltages, such as drops across a shunt
or series resistor. Bandwidth is within
the 100-MHz range with an attenuation
of 1-1,000×. At 100 dB, these probe systems also have
excellent CMRR ratings.

Fitting High-Voltage Probes to Applications
Power electronics encompass a broad range of highvoltage probing applications, which fall into three primary categories:
■■power semiconductor devices, such as gate drive, conduction loss, and switching loss
■■sensing or discrete components, such as a series/shunt
resistor, sensor signals, and discrete components
■■system inputs/outputs, such as line-side (ac) input,
dc bus/link, and inverter/drive pulsewidth-modulation
(PWM) output.
Tables 1 and 2 show how the four high-voltage probe
types map into these application areas at varying voltage
levels. First, a note on the color coding in the charts:
■■Green denotes the perfect probe for a given application.
■■Yellow indicates that there will be some performance
compromises, but some users can overlook them.
■■Red suggests that the probe will deliver a result without
being damaged but will not work well.
■■Black specifies to never use the probe for this application
because it is potentially dangerous to the operator, the
oscilloscope, the probe, and/or the DUT.

Applications for a 170-1,500-Vdc Bus/Link
Table 1 breaks down the probes and applications at the
170-1,500-Vdc bus/link voltage level. This is typical for
devices in the 120/240-600-Vac class as well as for grid-tied
solar photovoltaic inverters at the 1,500-Vdc level. The
entire low-voltage class of probes is entirely unsuitable,
although experienced engineers may be able to recognize
some possible exceptions. However, in such cases, practical
and liability considerations usually restrict the use of lowvoltage-class probes when probing high-voltage circuits.
From Table 1, we can determine that only one probe
is ideal for any given application. The active, singleended fiber-optic probe is wel l su ited fo r gate-dr ive

IEEE PowEr ElEctronIcs MagazInE

z March 2018

measurements and for sensor signals. The active, differential amplifier with matched probes works best
in conduction and switching loss
measurements and for series/shunt
resistor measurements. For discrete
components and all system input/output-related applications, it is important to choose the active, differential
probes for the best combination of
performance and cost.

Applications at >1,500-Vdc Bus/
Link Voltage (Medium-Voltage
5-kV Class Equipment)

Moving up to a 5-kV-class apparatus,
such as a 4,160-V motor drive, requires
the presence of a high-voltage bus, perhaps 6 kVdc. At
these voltage levels, an active, differential amplifier with
matched probes will struggle with measurements of
device switching and conduction losses due to the amplifier's CM voltage limitations (Table 2). Some tasks are
possible with active, differential probes, although even
good-quality ones such as the HVD series cannot measure
line-to-line ac signals with a 4,160-V input. It can, however, manage line-to-neutral measurements at such voltages. An active, single-ended, fiber-optic probe handles
tasks such as gate-drive and sensor-signal measurements
with ease.

Conclusions
Each type of high-voltage oscilloscope probe has different
strengths and weaknesses that lean it more toward some
power electronics measurement applications than others.
Being aware of the limitations of each can help users be better equipped to obtain the best measurement result for the
application at hand.

About the Authors
Ken Johnson (kenneth.johnson@teledyne.com) received
his B.S.E.E. degree from Rensselaer Polytechnic Institute,
Troy, New York, in 1987. He has been a product manager with Teledyne LeCroy, Chestnut Ridge, New York,
for more than 15 years. Previously, he spent ten years at
Hipotronics, Brewster, New York, working in high-voltage
testing and measurement.
David Maliniak (david.maliniak@teledyne.com)
received his B.A. degree in journalism from New York
University in 1980. He has been with Teledyne LeCroy,
Chestnut Ridge, New York, for five years in a technical
marketing communications role, following more than
30 years of working as a writer/editor in the businessto-business original equipment manufacturer technical press.

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2018