10. Output Stages and Power Supplies
10. Output Stages and Power Supplies
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10.1 Thermal Considerations
Considerable power is dissipated as heat in power devices.
Unless provisions are for made for this heat to flow flow in
the air, the device can be destroyed by overheating.
Heat sinks are mounted together with power devices to
cool them.
Figure 10.1 Typical heat
sink for a power device.
Figure 10.4 Clip-on heat sinks are suitable for
power dissipations of a few watts.
10. Output Stages and Power Supplies
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Thermal Resistance
Most important is the temperature of the pn-junction.
The temperature difference is proportional to the thermal power:
T J − T A = θ JA PD
(10.1)
TJ is the temperature of the junction,
TA is the temperature of the ambient
PD is the power dissipated in the device
θJC is the called thermal resistance from junction to ambient.
For the case of the device
T J − T C = θ JC PD
(10.2)
TC is the temperature of the case
θJC is the called thermal resistance from junction to case
θ JA = θ JC + θ CS + θ SA
10. Output Stages and Power Supplies
TLT-8016 Basic Analog Circuits
(10.3)
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10.2 Power Devices
Power BJT’s
Table 10.1. Comparison of the parameters of a small signal BJT with those of a
power BJT
Parameters
Smallsignal
BJT
(2N2222)
Power BJT
(2N3055A)
Maximum power dissipation
PDmax
1.8W
115W
Maximum collector current
ICmax
0.8A
15A
VCEmax
40V
60V
35-100
5-20
6V
7V
300MHz
0.8MHz
8pF
60-600pF
βmin
VEBmax
ft
Output capacitance Cobo (≈Cµ)
10. Output Stages and Power Supplies
TLT-8016 Basic Analog Circuits
1. Power BJT have smaller values
of current gain β.
2. The device capacitances of
power BJT are higher because of
higher junction area.
3. The transition frequency tends
to be much lower for power BJT.
4. The reverse leakage current of
collector-base junction is larger for
power BJT.
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The Effect of Temperature on BJT Parameters
As the temperature increases:
1. β increases. For temperature variation from -55°C to +150°C β can triple in value.
2. The leakage current ICB0 increases. It doubles for each 10°C increase in temperature.
3. Base-emitter voltage VBE decreases by 2.5mV per 1°C.
10. Output Stages and Power Supplies
TLT-8016 Basic Analog Circuits
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Maximum Ratings and Safe Operating Area
Maximum power dissipation limit
I CVCE = PD max
(10.4)
Figure 10.6 Maximum power dissipation limit.
Figure 10.7 Safe operating area for a typical 10-A, 100-V, 50-W
power BJT. Notice the logarithmic scales for VCE and IC.
10. Output Stages and Power Supplies
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Other limits:
1. Maximum collector current ICmax. If it is
exceeded the bond-wires melt.
2. Maximum collector-emitter voltage VCE max.
Defined from avalanche breakdown or punchthrough.
3. Second breakdown limit. At high VCE the current
becomes concentrated in a small area of the
junction, in which an overheating appears.
Figure 10.7 Safe operating area for a typical 10-A, 100-V, 50-W
power BJT. Notice the logarithmic scales for VCE and IC.
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Power MOSFETS
Figure 10.8a Double-diffused power MOSFET.
10. Output Stages and Power Supplies
When a positive voltage is applied to the
gate, a channel of n-type semiconductor
appears in the p+ area below the gate,
which connects the drain and the source.
The drain current enters from the drain
electrode (at the bottom of the picture),
reaches the upper surface, where
laterally flows to the source. Since the
channel is short, its resistance is small,
which permits a large current. This
device is more appropriate for operating
as electronic switch.
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10.3 Class - A Output Stages
The Class - A Emitter - Follower Output Stages
Figure 10.11 Emitter-follower output stage.
Efficiency
η=
Po
Psupplies
10. Output Stages and Power Supplies
× 100%
(10.18)
For class A power amplifiers ηmax = 25%
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10.4 Class-B Amplifiers
Direct coupled circuit having complementary
symmetry.
During the positive half-wave conducts Q1
forming the positive half-wave at the output.
During the negative half-wave conducts Q2
forming the negative half wave at the output.
Figure 10.18 Basic class-B amplifier that exhibits serious
crossover distortion.
10. Output Stages and Power Supplies
Review of Crossover Distortion
In fact Q1 is conducting when vs > 0.6V.
Q2 is on when vs < -0.6V.
For –0.6 < vs < 0.6 no BJT conduct. In this way
large cross-over distortion are presenting.
Figure 9.10 Waveforms for the circuit of Figure 10.18.
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Biasing Circuits
Figure 10.19 Adding the bias voltage sources reduces
crossover distortion, but unless Vbias is reduced with
temperature, thermal runaway can occur.
Figure 10.20 By using diodes in the bias network, automatic
adjustment of the bias voltage with temperature is provided.
10. Output Stages and Power Supplies
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Power Calculations in Class–B Amplifiers
Since only one BJT is conducting during one half
period, the efficiency of class B amplifier is
better.
η=
Vmπ
×100%
4VCC
(10.18)
ηmax = 78% at Vm = VCC.
Figure 10.23 Output power, device power, and
efficiency versus peak output amplitude.
10. Output Stages and Power Supplies
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10.5 Linear Voltage Regulators
Overview of Power - Supply Design
Ripples of the output voltage of power
supplies. Due to
• Variation of the input ac voltage;
• Non-ideal rectifying;
• Variation of the load current.
Voltage regulator: circuit that automatically
adjusts the output voltage to maintain a
nearly constant value, regardless the input
voltage and the load current.
Linear voltage regulators: transistors are
operating in active region.
Switching regulators: transistors operate as
switches.
10. Output Stages and Power Supplies
Figure 10.29 Line-operated power-supply block diagram.
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Linear Voltage Regulators
The Voltage Regulators as a
Negative - Feedback System
Figure 10.30 Linear series regulator.
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Series versus Shunt Regulators
Series regulators: the output voltage is regulating
by the output of an amplifier, which is in series
with the load.
Shunt regulators: regulating element is in
parallel the the load.
Figure 10.31 Simple shunt regulator circuit.
10. Output Stages and Power Supplies
The Zener diode operates in reverse breakdown region.
The breakdown voltage is stable and depends very small
from the current through the diode.
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Integrated - Circuit Voltage Regulators
LM78Lxx – IC regulator for various positive voltages depending on the digits xx
(e.g. 7805 for 5V; 7809 for 9V; 7812 for 12V; etc.).
LM79Lxx – IC regulator for various negative voltages depending on the digits xx.
10. Output Stages and Power Supplies
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