CHAPTER 16: Power Circuits: Switching and Amplifying pptx

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 • Suppose a system draws Amp from a +10 Volt supply and 0.5 Amps from a –10 Volt supply It delivers Watts of signal to its load Calculate the efficiency P(+) = 10V  1A = 10 Watts P(–) = 10V  0.5A = Watts PTOTAL = 10W + 5W = 15W Efficiency = (5W of load power) / (15W total power) = 0.333 = 33.3% Heat QUESTION: In the previous problem, 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 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 of a Class-B Amp Crossover distortion is characteristic of Class-B amplifiers Example 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  ...Objectives Describe and Analyze: • Efficiency • MOSFET vs BJTs • Power switching circuits • Classes of amplifiers • Power amplifiers • Heat sinks Introduction • This chapter looks at circuits... 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... excessive temperature Efficiency power delivered to load η = efficiency = ×100% power drawn from supplies Calculation • Suppose a system draws Amp from a +10 Volt supply and 0.5 Amps from a –10 Volt