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Today, a power supply must deliver performance and reliability, and
equally important, compliance with national and regional power quality
standards (such as the IEEE 519-1992 specification in the United
States). The engineer must characterize power levels, output purity,
and harmonic feedback into the power line and be prepared to measure high-frequency switching device outputs, noise levels, power characteristics, and more.
The oscilloscope has become a favored tool for power supply design
and troubleshooting because it easily meets these diverse requirements. The TDS3000B oscilloscope:
Measures voltage, current, and power with equal ease
Measures floating voltages safely
Provides harmonic measurements
Makes the user’s job easy and safe
The TDS3000B Digital Phosphor
Oscilloscope Advantage
The TDS3000B Series digital phosphor oscilloscopes (DPOs) equipped
with the Fast Fourier Transform Application Module (TDS3FFT) has the
bandwidth, waveform capture rate, and advanced analysis capability to
meet today’s power measurement, design, troubleshooting, and testing
requirements. In addition, they are compatible with a wide range of
probe types, including passive (voltage), active FET, current, differential, and high-voltage probes. This latter type of probe provides a safe
means of connecting to ungrounded (floating) circuits. The built-in
TekProbe® interface simplifies power measurement by providing correctly-scaled readings automatically, in proper units (volts, milliamperes, etc.).
TDS3000B DPO
Solves Today’s
Power Measurement
Problems
Due to the industry-wide shift to switching power supplies, the definition of “power measurements” has
changed, and with it, the number of design applications that require such measurements.
Meets Power Supply Design, Troubleshooting, and Testing Needs
Application Note
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Harmonic measurement and display
Power harmonic measurements require a tool that can display the signal’s various spectral components. The TDS3000B’s harmonic display
reveals the relative magnitude of the harmonics to the fundamental frequency. The optional Fast Fourier Transform Application Module (FFT)
equips the TDS3000B oscilloscopes for this type of reading. It provides
advanced analysis features that convert a time-domain acquisition into
a true harmonic display, which is essential for harmonic measurements
on 50 and 60 Hz lines.
Bandwidth to handle the fastest frequencies
Switching transistor and IGBT circuits, noise, and transients all make
demands on an oscilloscope’s ability to capture power supply signals
accurately and reliably. Switching frequencies are increasing with each
new generation of power supplies. Viewing these signals calls for
ample bandwidth. TDS3000B DPO models range in bandwidth from
100 MHz to 500 MHz, sufficient for even the fastest power supply
switching frequencies.
DPO dependably captures and displays
transients
Along with bandwidth, an oscilloscope’s waveform capture rate – the
number of times per second the instrument can trigger, compile the
waveform, draw a fresh display, and re-arm for the next trigger –
determines how reliably transients are acquired. The TDS3000B instruments offer a waveform capture rate that far surpasses digital storage
oscilloscopes, which means that transients are much more likely to be
acquired. It is easy to distinguish an occasional transient from the
characteristics of the background waveform because the DPO’s intensity-graded digital phosphor display highlights the most frequentlyoccurring areas of the signal.
Measuring Instantaneous Power
with the TDS3000B
Characterizing the instantaneous power dissipation in switching transistors is part of almost every power supply design project. It’s key to
choosing a component (such as the power MOSFET in Figure 1) that is
both cost-effective and reliable under the stresses of worst-case operation. The procedure involves making a floating measurement simultaneously with a current measurement. The TDS3000B’s TekProbe interface is compatible with the P5205 High-Voltage Differential Probe and
the TCP202 Current Probe; other current probes are available for higher current measurements. This pairing provides exceptionally accurate
results.
The high-voltage differential probe is necessary because the voltage of
interest (V
ds
on the MOSFET circuit) is across the drain-to-source terminals of the transistor, and neither is grounded. The TDS3000B, like
most oscilloscopes, is not designed to measure floating signals directly.
A differential probe is required for making safe floating measurements
with the TDS3000B. The P5205 accepts the ungrounded signal and
delivers a single-ended, grounded signal to the scope input.
Before making power measurements, it may be necessary to equalize
the delay between the voltage and current probes using a procedure
known as “deskewing.” The P5205 and the TCP202 are inherently
matched to within ±2 ns, minimizing delay errors, but other probe
combinations will need to be deskewed. This step is of critical importance, since a small offset in the timing of the voltage and current
traces can cause a large error in the instantaneous power reading.
TDS3000B DPO Solves Today’s Power Measurement Problems
Application Note
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Figure 1. Connecting a differential probe to a power MOSFET.
Connect to CH2
Connect to CH1
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