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Safe Operating Area
Improving Reliability By monitoring Device Stress Limits
LAB
in PDF format
Every switching device has a
maximum voltage, maximum current and maximum power specified by the device manufacturer,
displayed on its technical application note. Reliability of the power supply is dependent
on not exceeding these limits.
In figure 1 the upper trace shows
a MOSFET's drain-source voltage,
and the lower trace shows the MOSFET's drain current waveform. Amplitude of current
vs. amplitude of voltage is displayed as an XY trace known as a Safe Operating Area Plot
(SOA). The lower left corner of the XY plot shows zero current
and zero voltage. The upper right corner of the XY trace shows maximum power.
Figure 1 - A Safe Operating Area Plot for a single power supply switching
cycle
The SOA plot is a feature of
LeCroy's PMA 2 Power Measure Analysis software. Because of the long memories available in
LeCroy scopes it is possible to find SOA violations that occur
for only a few cycles after an event, such as a short circuit or
startup. Such low duty cycle events can be problematic. If they go undetected they can degrade
the device over time reducing system reliability. Figure 2 shows an SOA plot taken
over 20 ms. Note that the long acquisition memory maintains the same 1 GS/s sample rate used
in figure 1.
Figure 2 An SOA plot for a whole cycle of the AC line (mains)
The primary interest in the SOA
plot is to determine if the device exceeds its maximum voltage,
current, or power ratings. In figure 3 the steady state operation
of a switched mode supply yields the SOA plot shown. In addition,
parameters read the maximum current, voltage, and power for the whole waveform.
The SOA plot for the power supply startup, plotted with the same
voltage and current scaling as used in the steady state condition,
shows a much different characteristic. The shape of the SOA plot shows larger excursions
from the origin. This indicates a greater stress on the device.
Figure 3 Using the maximum parameter to measure the maximum voltage, current, and power
Note that the peak voltage
has increased from 252 to 292 Volts. Peak current is 1.3 A compared to 0.617 A during
steady state. The maximum power dissipated by the MOSFET increases from 57.92 to
105.24 W.
X- StreamTM scopes have a host
of features that can be combined with the PMA2 software to aid
analysis and troubleshooting. Automatic testing of the peak values can be enabled to automatically
monitor out of specification conditions. Autoscrolling of zoom traces allows
the user to play the long waveform through on a cycle by cycle
basis to see the effects on the SOA plot.
Figure 4 The SOA plot for power supply startup shows a more severe operational range of voltage, current and power.
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