POWER—CONTINUOUS dc
200
180
160
140
120
100
80
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
PS
Power from
Power Supply
Power
Power
Amplifier
Dissipation
Supply
Power
Delivered
to Load
PL
PD
=
PS
–
PL
Power Delivered
to Load
60
PD
40
Power Dissipation
of Amplifier
Worst
Case
20
0
0
10
20
30
40
50
60
70
80
90 100
DC Output Voltage (% of V+ Supply)
FIGURE 3. DC Power Dissipation, Resistive Load.
Example 2
PULSED OPERATION
An unbalanced power supply is often used with power
amplifiers to allow a large unipolar output voltage. A
+70V/–5V power supply is used with the OPA502 to
drive a 30Ω load connected to ground. What is the worst
case power dissipation and SOA requirement?
Some applications must handle pulses of current or varying
current waveforms with a low duty-cycle. The SOA plot
sometimes shows an ability to supply larger currents for
short duration pulses. In Figure 2, the SOA limits are labeled
for 5ms, 1ms and 0.5ms pulses. The duty-cycle must be low
(approximately 5% or less), so that heating in the output
transistor is given time to dissipate.
Answer—
The worst case occurs at half output, where VO = 35V,
and VCE = 35V. The output current at this point would
be 35V/30Ω = 1.17A which is within the SOA. Power
dissipation would be 35V • 1.17A = 41W.
Unusual current waveforms can be estimated with an ap-
proximation to a rectangular pulse as shown in Figure 4.
With a resistive load, the most stressful condition is when
the output voltage is approximately half the supply voltage
as shown. For other types of loads, evaluate any condition
that produces significant load current and high VCE. Appli-
cations which pulse currents beyond the dc SOA of the
amplifier should be evaluated very carefully since they are
pushing the limits of the device. Good reliability is achieved
by taking a conservative approach to SOA limits.
Other points to consider: The maximum output voltage
would be approximately 65V, and 65V/30Ω = 2.17A.
At this point, VCE = 5V, a safe value on the SOA.
If the current limit were set to accommodate the full
output of 2.17A, it would not be safe for short-circuits
to ground. With a short-circuit to ground, VCE = 70V
where the maximum safe current is 0.4A.
AC SIGNALS
Imagine a time-varying signal that rapidly transverses the
curves in Figure 3. The point of maximum dissipation is
passed only briefly. If the signal changes rapidly enough
(above 50Hz), the thermal time constant of the device causes
the junction temperature to be determined by the average
power dissipation. So, ac applications are generally less
demanding than dc applications of the same peak voltage
and current requirements.
Supply Voltage
V+
Output Voltage
Waveform
VCE = (V+)/2
Equivalent
If the signal is bipolar, such as a sine wave centered around
zero, each output transistor “rests” for a half-cycle. The total
amplifier dissipation is shared between the two output tran-
sistors, lowering the effective thermal resistance of the
package.
Rectangular Pulse
0
Equivalent Duration
If the instantaneous peak dissipation point is within the SOA
of the amplifier, the primary concern is providing a suffi-
cient heat sink to prevent overheating. Since this peak
FIGURE 4. Pulsed Loads.
3
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