AB-039 Datasheet, PDF (4/6 Pages) Burr-Brown (TI) – POWER AMPLIFIER STRESS AND POWER HANDLING LIMITATIONS

condition is passed only briefly during an ac cycle, ac
applications operate reliably, closer to the SOA limit.
Figure 5 shows the power curves for a power amplifier with
±40V supplies and an 8Ω resistive load. Again, powers are
plotted with respect to the percentage of maximum voltage
output. As with dc, the power delivered from the power
supply increases linearly with output voltage and the power
delivered to the load increases with the square of the output
voltage. The power dissipated by the amplifier, P_D, is the
difference of the first two curves. The shape of the P_D curve
is similar to the dc signal case, but does not approach zero
at 100% output voltage. This is because at full ac output
voltage, the output is rapidly transversing the whole curve (0
to 100%) of Figure 4. Figure 5 shows the average dissipation
of this dynamic condition.
Amplifier dissipation reaches a maximum when the peaks of
the ac output waveform are approximately 63% of the power
supply voltage. For this sine wave amplitude, the instanta-
neous output voltage hovers near the crucial half-supply-
voltage value for a large portion of the ac cycle.
The normalized values read from the right side of the curve
in Figure 5 can be scaled to any supply voltage and load
resistance. To find your amplifier dissipation at a given
signal level, multiply the reading taken from the right-side
scale by (V+)²/R_L.
AC applications rarely must endure continuous operation at
the maximum dissipation point of Figure 5. An audio ampli-
fier, for instance, with voice or music typically dissipates
much less than this worst-case value, regardless of the signal
amplitude. Yet, since a continuous sine wave signal of any
amplitude is conceivable, this worst-case condition is a
useful benchmark. Depending on the application, you might
want to design for this condition.
REACTIVE LOADS—AC SIGNALS
Figure 6 shows the relationship of voltage and current in
purely inductive load. The current lags the load voltage by
90°. At peak current, the load voltage is zero. This means
that the amplifier must deliver peak current with the full V+
across the conducting transistor (V– for negative half-cycle
peak current). The situation is equally severe for a capacitive
load. Check for this condition of voltage and current on the
SOA curve.
Once again, consider the curve in Figure 5. Power amplifier
dissipation is equal to the power from the power supply
minus the power delivered to the load. The power from the
power supply, P_S, is the same whether the load impedance is
resistive or reactive. But if the load is completely reactive
(inductive or capacitive), the power delivered to the load is
zero. So the power dissipated by the amplifier is equal to the
power from the power supply. At full output this is approxi-
mately three times the worst-case amplifier dissipation with
a resistive load!
A reactive load is a very demanding case, requiring a large
heat sink compared to a resistive load. Fortunately, purely
reactive loads are rare. An ac motor, for instance, could not
be purely inductive, or it would be incapable of performing
any mechanical work.
FINDING POWER DISSIPATION
Unusual loads and signals can be challenging to evaluate.
Use the principle that amplifier power dissipation is equal to
the power from the supplies minus the load power.
Power delivered from the power supplies can be measured
as shown in Figure 7. The power from each supply is equal
to the average current times its voltage. If the output wave-
form is asymmetrical, measure and calculate the positive
POWER—AVERAGE ac, RESISTIVE LOAD
140
0.7
P_L
120
Power Delivered
0.6
to Load
100
0.5
P_S
80
Power from
0.4
Power Supply
Worst
60
Case
0.3
40
0.2
20
P_D
0.1
Power Dissipation
0
of Amplifier
0
0 10 20 30 40 50 60 70 80 90 100
AC Peak Output Voltage (% of V+ Supply)
FIGURE 5. AC Power Dissipation, Resistive Load.
Amplifier
Dissipation
P_D
=
Power
Supply
Power
P_S
Power
Delivered
to Load
–
P_L
4