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1.0 MEANING OF SOA GRAPH
SOA (Safe-Operating-Area) graphs dene the acceptable
limits of stresses to which power op amps can be subjected.
Figure 1 depicts a typical SOA graph.
The voltage value on the right of the graph denes the maxi-
mum with no regard to output current or device temperature.
Note that this voltage is related to the output voltage but is
different; these two voltages are NOT interchangeable. The
current value at the top of the graph denes maximum; again,
with no regard to temperature or voltage. Inside the graph
area will be one or more curves with a slope of -1 (as voltage
doubles, current drops to half) labeled with a case tempera-
ture. These are constant power lines dened by DC thermal
resistance and the rise from case temperature to maximum
junction temperature (2.6°C/W and 200°C in this case). Lines
with a steeper slope (about -1.5 in this case) are unique to
bipolar output transistors. The steady state second breakdown
reduces the amplier's ability to dissipate DC power as voltage
becomes more dominant in the power equation. Fortunately,
the lines with time labels indicate higher power stress levels
are allowed as long as duration of the power stress does not
exceed the time label.
Transient SOA limits shown on data sheets are based on
a 10% duty cycle pulse starting with junctions at 25°C. The
repetition rate then would logically be dened by the time re-
quired for the junction to return to 25°C between pulses. Some
ampliers such as PA85 allow transient currents beyond the
maximum continuous current rating. Most often though, the
transient ratings are based on power or second breakdown
restrictions.
2.0 ANALYTICAL METHODS
2.1 PLOTTING RESISTIVE LOAD LINES
Resistive load lines can be plotted quite easily. Keep in
mind that since SOA graphs are log-log graphs, the resistive
load line will have a curvature, so several points should be
calculated and plotted.
Output voltage and current will always have the same po-
larity with a resistive load, so calculations can be performed
at all times from 0 to 90° of the output cycle. Figure 2 depicts
an example of resistive load line. It is interesting to note that
this is a safe load which can require a 5 amp peak capability.
If the output of the PA12 is unintentionally shorted to ground
the voltage stress on the output will be 50 volts, which is not
safe at the 5 amp current limit required to drive the load. In
applications where the amplier output would not be subject
to abuse these operating conditions are acceptable. Foldback
current limiting can be used to improve on the safety of this
situation.
2.2 PLOTTING REACTIVE LOAD LINES
Reactive load lines of any phase angle can be plotted with
the methods shown here. A completely reactive load line almost
looks like an ellipse on the SOA graph since current stresses
will occur at two different levels of voltage stress.
The voltage output waveform will dene the reference phase
angles for all point-by-point stress calculations. The waveform
shown in Figure 3 (next page), starts at 90°, since current
in capacitive loads will lead in phase. The waveform ends at
270° since that corresponds to the maximum phase lag of the
current in an inductive load. All calculations will be within the
limits of these angles.
Calculations proceed according to the steps in Figure 4 (next
page). Currents will only need to be calculated over 180° since
the load line for each half of the amplier is a mirror image.






































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Capacitive loads will start at 90° and progress through +90°.
Inductive load calculations will start at +90° and progress to
+270°. Step 2 in the procedure denes the starting angle for
calculations based on the load phase angle. For example, a
45° load would start at 45° and continue to +135°. A 45°
load will start at 45° and continue to 225°
Example of a typical load calculation:
In the resistive load example the load line of a 9 ohm resistive
load was plotted and was quite safe. For this example lets use
the same impedance but with a 60° phase angle.
Figure 5 shows a PA12 driving an inductive load of 9 ohm
at 60°. From step 2 of our procedure we know we begin cal-
culating current at 60° and continue to 60+180 or 240°. For
this example we will use 15° increments. Actually, it is only
necessary to perform the step-by-step calculations for current
up to the point where peak current occurs, in this case 150°.
The increments back down to 240° will be a mirror image of
what was just calculated. However, dont get this lazy with the
voltage calculations we have yet to do.
Even though the point of maximum voltage stress occurring
at full negative output swings is not evident in the voltage stress
calculations, it is inconsequential since we have established
that no current ows through the output device at that time.
As long as the amplier itself is within its voltage ratings, all
will be well.
After all points are calculated, they may be plotted on the
SOA graph. In this particular case note there are signicant
excursions beyond the steady state 25°C SOA limit. Is this
acceptable? Consider the following factors to help make the
decision: 1. The area beyond the continuous SOA is quite size-
able. 2. A calculation of maximum average power dissipation
using the formula shown in Fig 5 (refer to General Operating
Considerations for background on this calculation) shows the
amplier dissipating 107 watts.
The 107 watt dissipation will certainly cause the amplier
to operate at signicantly elevated temperatures, even with
a generous heatsink. Realistically the SOA limits are being
reduced by the heating. We can even dene exactly how badly.
Assume a 25°C ambient (thats pretty optimistic). Using an
HS05 heatsink rated 0.85°C/watt results in an amplier case
temperature 91°C. It is evident that the PA12 is not well suited
for this application. The alternatives include either paralleling
PA12s or upgrading to a PA05.
2.3 SPECIAL CASES OF LOAD LINE PLOTTING

AND OTHER GENERALIZATIONS
For parallel connected ampliers, assume each amplier









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occurs at a voltage stress of Vs+(0.707Vs). A reactive load
line within that operating point will likely be within all operating
points and representative of minimum acceptable load.
Figure 6B depicts the worst case acceptable reactive load. It
really doesnt ll up much of the permissible operating region,
but such are the trade-offs involved in driving difcult loads.
3.0 BENCH TESTING SOA
An oscilloscope can be used to do real world plotting of load
lines on actual working circuits. The SOA limits can be drawn
in on the scope screen if necessary. First, the SOA limits
should be redrawn on linear-linear graph paper. This results in
drives a load rated at half the current of the total load. In es-
sence, double the load impedance. Do just the opposite for
a bridge circuit.
Some simple relationships to keep in mind: for totally reac-
tive loads maximum current occurs at a voltage stress corre-
sponding to Vs and maximum dissipation occurs at a voltage
stress of Vs+(0.707Vs), where current is also 0.707*Ipeak. In
a resistive load, stresses are much less with maximum current
occurring at maximum output voltage swing. This generally
corresponds to the maximum swing specication given in
every amplier data sheet.
Also keep in mind that there are many load lines which will
t well within an amplier SOA and be quite safe to drive. Yet
these same loads can demand current capa