Tektronix IsoVu TIVM Series, User Manual

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Application examples
The following examples are provided to help you become familiar with the TIVM
Series IsoVu measurement system and to achieve the best performance for your
application.
Example 1: High-side V
GS
measurement
Advancements in the components used in switching power supplies have made
characterizing the performance of these power supplies increasingly dif
fi cult and
challenging. A particularly challenging measurement is measuring the high-side
V
GS
in a half bridge. To accurately make this measurement, an exceptionally
good CMRR is required from the test system. The following fi gure shows an
example of this circuit.
Figure 15: Half-bridge circuit showing the gate, source, and drain of the high-side
FET
In this type of circuit, the gate-source voltage is of interest because the rate at
which the device switches is determined by the gate drive characteristics. The
reference node for this measurement is the high-side source node, which switches
between the input supply voltage and the local PCB ground during operation. In a
measurement system without suf
fi cient CMRR, this rapidly changing common
mode voltage results in interference which obscures the measurement. It is
important to note that the CMRR for all measurement systems is frequency
dependent; however, the frequency that is critical for this measurement is not
the switching frequency, but the frequency corresponding to the edge rate. For
example, to accurately characterize a power supply with a switching frequency
of 100 kHz and an edge rate of 1 ns, a system with good CMRR at 350 Mhz is
necessary because of the edge speed.
TIVM Series Measurement System User Manual 23