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

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 P TOTAL = 10W + 5W = 15W Efficiency = (5W of load power) / (15W total power) = = 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

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.

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 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.