Datasheet

5
UNDERSTANDING HV DIFFERENTIAL PROBE RATINGS
AC line voltages and power electronics systems can operate at very high voltages, and all parts of the measurement
circuit are not necessarily connected to ground, requiring HV safety ratings. Additionally, a variety of terms may be used
to describe the same voltage. Below is a simple tutorial to help you understand the various types of voltage terms and
how they relate to each other. Understanding these voltage terms is necessary to ensure the correct probe selection.
Rectied AC (DC Bus/Link Voltage)
For three-phase inputs, the three Line-Neutral (L-N) AC
Line inputs are rectied and summed to provide a “stiff” DC
(bus/link) voltage for the input to an inverter subsection.
The example shown to the right is for a 600 V
rms
system.
The Line-Neutral RMS voltages are 600V/√3, and each
V
peak
(L-N) is calculated as √2*V
rms
. The DC bus voltage
after rectication and ltering is equal to √3*V
peak
(L-N), or
847 V
dc
in this case.
Single-phase AC Line Input
Ratings are provided in V
rms
(also referred to as V
ac
) referred
Line-Neutral. AC Line inputs are usually grounded and these
voltages can be assumed to have a 0V reference voltage.
V
peak
is calculated as √2*V
rms
and V
pk-pk
is calculated as
2*V
peak
. The example to the right is for a 120 V
rms
rating.
Three-phase AC Line Input
Ratings are provided in V
rms
(also referred to as V
ac
)
referred Line-Line (L-L), which can be converted in
magnitude to a Line-Neutral basis by dividing by √3.
V
peak
(L-L) is calculated √2*V
rms
and V
pk-pk
is calculated
as 2*V
peak
. The example on the left is for a 480 V
rms
three-phase rated system with signals shown as Line-Line.
Inverter or Drive H-Bridge or
Cascaded H-Bridge Outputs
The nominal peak voltage (+ or -) of the PWM signals
without accounting for signal overshoot is equal to the DC
bus voltage. The measured 0V differential voltage value is
not ground-referenced, and therefore a differential probe
suitably rated to the DC bus voltage is recommended. Note
that the +V
peak
and -V
peak
plus overshoot safety margin
(typically ~50%) is what the HV differential probe must
measure - V
pk-pk
(2*V
peak
) will never occur in the circuit.