Download Fundamentals of Linear Electronics Integrated & Discrete

CHAPTER 16
Power Circuits:
Switching and
Amplifying
Objectives
Describe and Analyze:
• Efficiency
• MOSFET vs. BJTs
• Power switching circuits
• Classes of amplifiers
• Power amplifiers
• Heat sinks
Introduction
• This chapter looks at circuits designed to deliver
large amounts of power to loads.
• Efficiency is major concern in power circuits.
• Switching circuits are more efficient than the
equivalent linear circuits.
• Different class amplifiers differ in efficiency
• Heat sinks are required to prevent the failure of
semiconductors from excessive temperature.
Efficiency
power delivered to load
η = efficiency =
×100%
power drawn from supplies
Calculation 1
• Suppose a system draws 1 Amp from a +10 Volt
supply and 0.5 Amps from a –10 Volt supply. It
delivers 5 Watts of signal to its load. Calculate the
efficiency.
P(+) = 10V  1A = 10 Watts
P(–) = 10V  0.5A = 5 Watts
PTOTAL = 10W + 5W = 15W
Efficiency = (5W of load power) / (15W total power)
= 0.333 = 33.3%
Heat
QUESTION:
In the previous problem, 5 Watts out of 15 Watts were
delivered to the load. What happened to the other 10
Watts?
ANSWER:
It turned into heat in the resistors and
semiconductors of the system.
Calculation 2
QUESTION:
Suppose a transistor is used to control the flow of
power to a load. When the transistor is off, there are
100 Volts across it. When the transistor is on, there
are 10 Amps flowing through it. How much power
does the transistor dissipate if we assume no
voltage drop across it when it is on?
ANSWER:
None, as in zero.
MOSFETs vs. BJTs
MOSFETs vs. BJTs
• Power MOSFETs have become the device of choice in
many power switching circuits.
• The gate is voltage activated, and requires essentially no
power.
• MOSFETs switch quickly. Power is consumed during the
time it takes to switch, so the faster the better.
• For low to moderate currents, the VDS drop across a
MOSFET is lower than the VCE drop across a BJT.
• For high currents, the VCE drop in a BJT is lower than the
VDS drop in a MOSFET.
IGBTs
Insulated Gate Bipolar Transistors (IGBTs) combine
the best characteristics of MOSFETs and BJTs.
SMPS
• Switch-Mode Power Supplies (SMPS) applications
commonly use MOSFETs and IGBTs. Some BJTs are
still used.
• Practically all modern electronic equipment, such as
PCs, uses switch-mode power supplies.
• The transistors in SMPS circuits switch inductive
loads and must be protected from inductive “kicks”,
the high-voltage transients that occur when current in
an inductor is turned off abruptly.
Amplifier Classes
• Class-A amplifiers continuously conduct current in the
transistors. A common-emitter amplifier typically is Class-A.
The maximum efficiency of Class-A is 25%.
• Class-B amplifiers require pairs of transistors operating in
“push-pull”. Each transistor conducts half the time. When
one is off, the other is on. The maximum efficiency of ClassB is 78%.
• Class-C amplifiers use transistors as switches to pulse a
resonant LC circuit. The efficiency is 90%, but its use is
limited to RF amplifiers.
• Class-D amplifiers use transistor switches to pulse-widthmodulate a signal. The efficiency is over 90%.
Example 1 of a Class-B Amp
Crossover distortion is characteristic of Class-B
amplifiers.
Example 2 of a Class-B Amp
This circuit is sometimes called Class-AB
IC Power Amplifiers
Delivers 2.5W of signal with a good heat sink.
IC Power Amps
A bridge circuit can double the power to the load.
Class-D Amplifiers
Sounds like a linear amp, but it is switched.
Power Packages
Power semiconductor packages are designed to
transfer heat out of the chip.
Thermal Derating
As its temperature rises, a semiconductor is able to
dissipate less power without damaging the chip.
Heat Sinks
• A heat sink is any piece of metal that you can bolt the case
of a semiconductor to. It could be a metal chassis, or a
finned aluminum extrusion designed for the purpose. It
could be a few square inches of copper on a pc board.
• The job of a heat sink is to keep the semiconductor cool by
conducting heat away from its package.
• The key parameter of a heat sink is its surface area. The
more the better.
• Sometimes a fan is needed to move air across the heat sink
to help dissipate the heat.