Datasheet
www.tektronix.com/power 9
Power Supply Measurement and Analysis with Bench Oscilloscopes
Figure 14. SOA Mask Testing. Figure 15. Slew Rate Measurements.
The other major challenge is the high dynamic range required
for accurate switching loss measurements. The voltage
across the switching device changes dramatically between
the On and Off states, making it difficult to accurately measure
both states in a single acquisition. There are three ways to
determine the correct values with the MDO/MSO/DPO4000
and MDO/MSO/DPO3000 Series:
Measure the voltage drop across the switching device
during conduction. Because this voltage is typically very
small compared with the voltage across the switching
device when it is not conducting, it is generally not possible
to accurately measure both voltages at the same vertical
setting on the oscilloscope.
Provide the RDS(on) value (best model for MOSFETs)
based on the device data sheet. This value is the expected
on-resistance between the drain and source of the device
when it is conducting.
Provide the VCE(sat) value (best model for BJTs and IGBTs)
based on the device data sheet. This is the expected
saturation voltage from the collector to the emitter of the
device when it is saturated.
Safe Operating Area (SOA)
The Safe Operating Area (SOA) of a transistor defines the
conditions over which the device can operate without
damage; specifically how much current can run through the
transistor at a given voltage. Exceeding these limits may cause
the transistor to fail. The SOA is a graphical test technique
which accounts for limitations of the switching device such as
maximum voltage, maximum current, and maximum power,
and assures that the switching device is operating within
specified limits.
The switching device manufacturer’s data sheet summarizes
certain constraints on the switching device. The object is to
ensure that the switching device will tolerate the operational
boundaries that the power supply must deal with in its end-
user environment. SOA test variables may include various load
scenarios, operating temperature variations, high and low line
input voltages, and more. As depicted in Figure 14, a user-
definable mask is created to ensure that the switching device
adheres to defined tolerances in regard to voltage, current,
and power. Mask violations are reported as failures in the
power application.
Slew Rate
To verify that the switching device is operating at maximum
efficiency, the slew rate of the voltage and current signals
is measured to verify that the circuit is operating within
specifications. As shown in Figure 15, the oscilloscope is used
to determine the slew rate of the switching signals by using
measurement cursors, simplifying gate drive characterization
and switch dv/dt or di/dt calculations.