Download Power-Derating Curve

Output Transistors
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Current gain / input impedance is
a vital parameter of a power
amplifier.
In the class A analysis, the load
impedance is scaled by a factor
of b.
High power transistors often
have a value of b of the order of
tens rather than hundreds.
Multiple Transistor Units
To realise a high current gain with
low b transistors, use two
cascaded emitter follower stages.
I C 2  b 2 I B 2  b 2 I C1  b1b 2 I B1
i.e. bTotal  b1b 2
Darlington Pair
Darlington Pair
configuration is
available in a single
integrated package.
VBE is double that of a
normal transistor.
b is typically between
1000 & 10000.
Efficiency
The efficiency of a power amplifier is
the ratio between the power delivered
to the load and the power drawn from
the supplies.
Power supply requirements and
transistor power dissipation ratings
depend on the efficiency.
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Power Dissipation
All power which is not delivered to the
load must be dissipated by the output
device(s) in the form of heat.

As a result, the temperature of the
device rises.

Temperature rise changes the
properties of a transistor and may, in
extreme cases, destroy it.
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Transistor Power Dissipation
Ambience / Air, TA
Case, TC
Junction, TJ
Power
dissipation
Heat travels only from a hot to a cold body
 TJ > TC > TA
Thermal Resistance
Temperature difference between the junction
and ambience depends on the power dissipation
and the thermal resistance between them.
TJ  TA  PD JA
JA is the thermal resistance between the
junction and ambience measured in C per Watt.
Power-Derating Curve
Junction temperature must not exceed Tjmax. Also,
power dissipation must not exceed PD0. Combining
these limitations gives the power derating curve.
PDmax
PD0
PD max 
0
TA0
= -1/Slope
TA
TJ max  TA
TJmax
 JA
TA
Using the Power Derating
Curve
Example : TIP 120
PDmax [W]
2
TA  25  PD max  2 W
TA  50  PD max  1.6 W
1.6
0
25 50
150
TA [C]
Find:
Increasing Power
Dissipation
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The only fixed point on the
power-derating curve is TJmax.
To increase power dissipation,
slope of power-derating curve
must be steeper.
Thermal resistance must be
lowered.
Heat-Sinks
JA can be broken down into a pair of series resistances.
 JA   JC   CA
JC = thermal resistance between junction and case
(fixed)
CA = thermal resistance between case and
ambience.
CA can be lowered by increasing the surface area of
the transistor case, i.e. by adding a heat-sink.
 CA   CS   SA
Power-Derating Curve
with respect to Case Temp
Maximum allowable power dissipation
Versus transistor case temperature.
PDmax
Pdmax(TCO)
PD max 
0
TC0
TC
TJ max  TC
 JC
TJmax
TC
EXAMPLE 9.5
Solution:
With a heat sink max. allowable power dissipation increases
From 1.6W to 13.1W.