CHAPTER 16: Power Circuits: Switching and Amplifying pptx

• This chapter looks at circuits designed to deliver large amounts of power to loads.. • Efficiency is major concern in power circuits.. Calculation 2QUESTION: Suppose a transistor is

CHAPTER 16

Power Circuits: Switching and

Amplifying

Describe and Analyze:

• Efficiency

• MOSFET vs BJTs

• Power switching circuits

• Classes of amplifiers

• Power amplifiers

• Heat sinks

• 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

%

×

=

supplies from

drawn power

load to

delivered

power efficiency

η

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%

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 V DS drop across a

MOSFET is lower than the V CE drop across a BJT

• For high currents, the V CE drop in a BJT is lower than the

V DS drop in a MOSFET

Insulated Gate Bipolar Transistors (IGBTs) combine the best characteristics of MOSFETs and BJTs

• 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

Class-B 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-width-modulate 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