Datasheet

Application Note
www.tektronix.com/power8
Figure 13. Switching Loss Measurements on an IGBT.
Switching Device Analysis
The prevailing DC power supply architecture in most modern
systems is the SMPS because of its ability to efficiently
handle changing input voltages and loads. The SMPS
minimizes the use of lossy components such as resistors
and linear-mode transistors, and emphasizes components
that are (ideally) lossless. SMPS devices also include a
control section containing elements such as pulse-width-
modulated regulators, pulse-rate-modulated regulators,
and feedback loops.
SMPS technology rests on power semiconductor switching
devices such as Metal Oxide Semiconductor Field Effect
Transistors (MOSFET) and Insulated Gate Bipolar Transistors
(IGBT). These devices offer fast switching times and are able
to withstand erratic voltage spikes. Additionally, transistors
dissipate very little power in either the On or Off states,
achieving high efficiency with low heat dissipation. For the
most part, the switching device determines the overall
performance of an SMPS. Key measurements for switching
devices include:
Switching Loss
Safe Operating Area
Slew Rate
Switching Loss
Transistor switch circuits typically dissipate the most energy
during transitions because circuit parasitics prevent the
devices from switching instantaneously. The energy lost in a
switching device, such as MOSFET or IGBT as it transitions
from an OFF to ON state is defined as Turn-on loss. Similarly,
Turn-off loss is the energy lost when the switching device
transitions from an ON to OFF state. Transistor circuits lose
energy during switching due to dissipative elements in the
parasitic capacitance and inductance and charge stored in
the diode. A proper analysis of these losses is essential to
characterize the supply and gauge its efficiency.
The switching loss measurements as shown in Figure 13
are made on complete cycles within the selected region
of the acquisition (by default, the entire waveform) and the
statistics of those measurements are accumulated across the
acquisition, but not between acquisitions.
A major challenge in measuring Turn-on and Turn-off losses is
that the losses occur over very short time periods, while the
losses during the remainder of the switching cycle are minimal.
This requires that the timing between the voltage and current
waveforms is very precise, that measurement system offsets
are minimized, and that the measurement’s dynamic range
is adequate to accurately measure the On and Off voltages
and currents. As discussed earlier, the probe offsets must be
nulled out, the current probe must be degaussed to remove
any residual DC flux in the probe, and the skew between
channels must be minimized.