AB-039 Datasheet, PDF (5/6 Pages) Burr-Brown (TI) – POWER AMPLIFIER STRESS AND POWER HANDLING LIMITATIONS

Load
Voltage
Load
Current
V+ Supply
V_CE
V– Supply
At this instant, V_O = 0, but I_O at maximum.
FIGURE 6. Voltage and Current Waveforms for Inductive
Load.
and negative supplies separately and add the powers. If the
waveform is symmetrical, you can measure one and multi-
ply by two. Use an average-responding meter to measure the
current. A simple D’Arsonval type meter movement with a
current shunt works well. Do not use an rms-responding
meter.
For sinusoids, finding the load power is easy—
P_LOAD = (I_O rms) • (V_O rms) • cos(θ)
Where θ is the phase angle between load voltage and
current. (See Figure 8 for measurement methods.)
For complex waveforms, the load power is more difficult to
measure. You may know something about your load which
allows you to determine load power. If not, you can build a
circuit that measures load power using a multiplier IC to
V+
P_S = (V+) • I₁ + |V–| • I₂
I₁
P_D = P_S – P_L
continuously multiply load voltage and current. The average
dc output of the multiplier is proportional to the average load
power. See the MPY100 data sheet for a circuit to measure
power with a multiplier.
UNUSUAL LOADS
Usually an op amp sources current to the load (Q₁ conduct-
ing, Figure 1), when the output voltage is positive. But
depending on the type of load and the voltage to which it is
referenced, an op amp might have to sink current (Q₂
conducting) with positive output voltage. Or, it could be
required to source current with negative output voltage. In
these cases, the voltage across the conducting transistor is
larger than V+ or V–.
An example of this situation is a power op amp connected as
a current source. The output of a current source might be
connected to any voltage potential within its compliance
range. Sourcing high current to a negative potential node
would produce high dissipation and require good SOA.
MOTOR LOADS
Motor loads can be tricky to evaluate. They are like a
reactive load since stored energy (mechanical) can be deliv-
ered back to the amplifier. Motor and load inertia can cause
the amplifier to dissipate very high power when speed is
changed.
Electro-mechanical systems can be modeled with electric
circuits. This is a science in itself—beyond the scope of this
discussion.
You can, however, measure the V-I demand of a motor (or
any other load) under actual load conditions. Figure 8 shows
a current sense resistor placed in series with the load. With
load voltage and current displayed on separate oscilloscope
traces, you can find the conditions of maximum stress. Be
sure to consider the voltage across the conducting transistor,
(V_CE), not the amplifier output voltage. The most stressful
conditions may occur with moderate current, but low load
voltage.
An X-Y type display of voltage and current (Figure 8B) may
also help identify troublesome conditions. More demanding
combinations of voltage and current are those that deviate
from a straight-line resistive load.
P_D
I₂
Load
V–
NOTE: I₁ and I₂ are readings from D'Arsonval-type average-responding
meters. RMS-responding meters will not provide accurate results.
FIGURE 7. Measuring Power Supply Power.
5