POWER I

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ELECTRONICS PRIMER by Martin F. Schlecht

circuit. Most likely you chose one designed to process information. Computers, telecommunication equipment, TVs, and stereos - all components prevalent in our lives - are based on such circuits. These products process information that can be manipulated in the media of other engineer- ing disciplines. Those disciplines, however, cannot match the speed, density, and low cost of today’s electronic technology.

Some people now fmd electrical engineer- ing so synonymous with information process- ing that they sometimes forget the great value of the “electrical world” for the distribution and control of energy. But there are electronic circuits whose purpose is to process power rather than information. The digital or analog circuits in each of the aforementioned elec- tronic systems use electrical sources in a form (5 V ~ C , for example) that’s not readily avai- able from the wall plug. A “power circuit,’’ which in these cases is called a “power sup- ply,” is needed to convert the 6O-Hz, 1 lO-V,, sinusoidal input voltage waveform to a con- stant, regulated, and often electrically isolated output voltage waveform.

Power circuits also control motor speed in appliances, electric vehicles, and indus- trial processing equipment. Power circuits interface photovoltaic arrays, which create dc waveforms, with the ac power system, allowing the generated energy to be insert- ed into the utility grid. The ubiquitous household light dimmer is also a power cir- cuit. Power circuits have been made to han- dle power levels ranging from less than a watt to more than 10 megawatts.

If there is a universal definition of a power circuit, it would be one that “pro- cesses electrical energy in a way that would be 100-percent efficient if the circuit’s components were ideal.” A stereo amplifi- er is not a power circuit under this defini- tion; it may provide 500 W of power to its load, but it does so through a linear amplifi- er whose basic operation can never produce 100-percent efficiency. It is not the level of power that matters, but the approach.

The Ideal Circuit To understand how a power circuit func- tions, consider the need to supply a load with an ac voltage waveform when the

8755-3996/92/$3.000 1992 IEEE Circuits and Devices

source of power is a dc voltage source. (For simplicity, we will con- sider the load to be a resistor, although it could be, for example, the induction motor of a ventilating fan.) One way of performing the conversion is to provide a set of pos- itive switches, P1 and P2, and a set of negative switches, NI and N2, in a bridge configuration (Fig. la). If we alternately close the set of posi- tive switches and the set of negative switches, each for half the period of the fundamental frequency we are trying to create, an ac voltage wave- form will appear across the load.

The ac waveform created by the switches is a squarewave, and we might have preferred a fundamental sinusoid. If this is the case, we can add a filter at the out- put to attenuate the harmonic components of the squarewave before the voltage wave- form is presented to the load. In keeping with our desire to be 100 percent efficient. this filter will use inductors and capacitors, not resistors and capacitors. The filter might have a simple low-pass configuration (Fig. 1 b) or use a series resonant trap tuned to the fundamental frequency (Fig. IC) to provide greater selectivity between the fundamental and the harmonic components.

If the switches in our power converter have zero resistance when they are on, zero leakage current when they are off, and make the transition from one state to the other with infinite speed, they will never have any dissipation. With such ideal switches, and with pure inductors and capacitors, the conversion efficiency from dc to ac is 100 percent.

Changing the form of the electrical power is only part of what the power cir- cuit of Fig. I does. Just as important is the circuit’s ability to adjust the parameters of the output waveform. For instance, we can vary the output waveform’s frequency by making the switches’ conduction periods longer or shorter. Or, if we close switches PI and NI between the periods when both positive or both negative switches are con- ducting, we will create an ac waveform called a quasi-squarewave (Fig. Id). We can adjust this waveform’s fundamental component by varying the duration of the waveform’s zero voltage sections. If the load is a ventilating-fan motor, this ability to control both the frequency and ampli- tude of the drive waveform permits us to

Effects of Real Components Another example of a power cir- cuit’s possible form arises from the need to provide an automobile’s microprocessor control unit with a regulated 5 VdL from a battery whose voltage varies around 12 Vdi. An appropriate power circuit can provide this function much more efficiently than a linear regulator (Fig. 2a). The two ideal switches are alternately turned on and off, with one or the other conducting at all times. The voltage across switch S2 is shown in Fig. 2b. (We have assumed that the switches are oper- ated at a constant frequency and

nd-jtist the speed of rhe f an fi-om /em to maximum no matter M hat (he load.

that the percentage of the total period T that switch SI conducts is D, the duty ratio.)

P1

NI

. Converting power from dc to ac. (a) Four switches arranged in a ‘bridge” configuration create an ac squarewave from a dc voltage source. (b) A low-pass filter removes harmon- ics in the squarewave that would otherwise appear across the load. (c) A resonant filter tuned to the fundamental frequency provides greater attenuation of the harmonic compo- nents. (d) A “quasisquarewave, “ in which the fundamental component is controlled by varying the zero-volfage durafions.

January 1992 33

is negative, so we would use a diode in this location (Fig. 2c). Because the inductor maintains its current, the diode will turn on whenever the transistor is turned off.

Neither the transistor nor the diode are ideal switches. When they are on, they have a voltage drop across them that pro- duces a “conduction loss” when multiplied by the on-state current. When they are off, a leakage current flows that is negligible in most cases, but it can be significant at high temperatures or at applied voltages that exceed the device’s rated voltage. In addi- tion, a semiconductor device takes time to switch between its on-state and off-state. During this time, a large voltage and a large current exist simultaneously (Fig. 3). The energy dissipated during these transi- tions, called the switching loss, contributes to the average power dissipated in the device, and this contribution increases as the switching frequency is raised.

The energy storage components are not ideal, either. The inductor dissipates power because of the resistance in its conductors and the hysteresis loss in its magnetic material. The capacitor dissipates power because of the hysteresis loss in its dielectric material. The skin effect (in which current flows only near the surface of a conductor) and proximity effects (in which current flowing in one con- ductor induces eddy currents in a nearby con- ductor) produce higher conductor losses than would be expected from simple dc calcula- tions. There are a variety of magnetic and dielectric materials that trade off hysteresis loss at a given frequency for permeability or permittivity. In general, for a given amount of required peakenergy storage, the efficiency of an inductor or capacitor can be increased only by making the element physically bigger.

Besides loss, the other non-ideal charac- teristic of inductors and capacitors is their parasitic energy storage. Since the capaci- tors used in power circuits are often physi- cally quite large, they have a significant series inductance. Similarly, inductors have significant parallel capacitance. These par- asitic components keep a power circuit’s inductors and capacitors from performing their filtering function over as broad a range of frequencies as we would like.

Because a power circuit’s components are not ideal, we must make design tradeoffs. In our dc-dc converter, for instance, the inductor and capacitor typically make up a large fraction of the circuit’s size and weight. If we raise the switching frequency, we can

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reduce the inductance and capacitance proportionally. But any given semi- conductor-device technology has a certain maximum switching speed. If we raise the switching frequency without changing the switch technolo- gy, we will incur higher average dissi- pation in the switches and, consequently, a lower converter eff- ciency. Alternatively, we might choose to leave the switching frequen- cy alone, use smaller inductors and capacitors, and tolerate more ac ripple on the 5-V output waveform. In gen- eral, our job as power-circuit design- ers is to make tradeoffs in circuit size (or weight), conversion efficiency, and waveform quality (which involves parameters such as ac-ripple content).

The design space for power electronics actually has several more “dimensions.” There is always a need to increase manufacturing yield, increase reliability, and reduce cost. Although designers of all electronic circuits share these goals, the challenge in power elec- tronics is unique because of the typically small product runs, the custom nature of the products and some of their components, and the large amount of hand assembly required.

Supporting Technologies While power circuits are usually quite sim- ple in form, their design and fabrication requires vast knowledge and sophisticated (and continually improving) technologies. Because the nonideal nature of components prevents a power circuit from being “perfect,” there is a need for ever-better semiconductor power devices and energy-storage elements. The semiconductor power devices available in the 1960s, for example, included bipolar diodes (with a pin structure), bipolar junction transistors (having a lightly doped collector region), and a thyristor (in the form of a silicon controlled rectifier). In the 1970s, these devices, with improved power-handling capa- bility and switching speed, were joined by the newly developed power Schottky diode, power MOSFET, gate turn-off thyristor (GTO), and static induction transistor (SIT). These new device structures permitted operd- tion at higher frequencies and higher power levels with greater ease of control.

More recently, we have seen the devel- opment of the insulated-gate bipolar tran- sistor (IGBT) and the MOS-controlled thyristor (MCT). These devices combine MOS and bipolar processing technologies

January 1992

t o produce a device that exhibits the strengths of both device types, much as the BICMOS technology has improved com- bined analog and digital circuits. Also new to the scene are power integrated circuits (PICs), in which low-level analog, digital- control, and drive circuits are integrated on the same die with high-voltage, high-cur- rent devices to reduce the cost and failure rates associated with interconnections.

The control techniques required in power circuits are very complex. These circuits are highly nonlinear systems whose incremental

open-loop characteristics often include several lightly damped poles and a right-half plane zero. Incremental-analysis tools have been developed, but the large-signal behavior of the nonlinear system remains a concern because over- shoots caused by transients can destroy the circuit or its load. Under certain conditions, power circuits can also display chaotic behavior that is not well understood or easily corrected.

Considerable engineering effort and knowledge is required to take a power circuit from a laboratory pro- totype to a finished product. A

power electronic engineer must be an expert in areas as diverse as thermal design, circuit and system packaging, circuit protection, and safety and electromagnetic interference regulations.

High-Frequency, High-Density Power Supplies As one example of how power electronic technology has advanced over the years, consider the history of power supplies. In the early 1950s, semiconductor power diodes became a practical alternative to vac-

v5w I ‘ 5w

Energy lost during switch psw t transitions

t

1 Typical waveforms for a power transistor (a) Voltage and current waveforms showin! details of switch transitions. (b) Power dissipation within the switch showing switching loss and conduction loss.

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tributed power-supply systems, in which power is bused at a high unregulated volt- age (48 V, for example) and converted to the final voltage at the point of load. Third, with a converter this small, customers are likely to accept the power supply as a stan- dard “component” to be designed into their units rather than as a subsystem to be cus- tomized for their application. This should lead to higher product runs, lower produc- tion cost, and higher reliability.

Education in Power Electronics Most of today’s power electronic engineers “fell into their job,” often coming to the discipline with an analog-circuit back- ground rather than specific university train- ing. In the late 1970s, only a few U.S. universities such as Cal Tech, Duke, MIT, University of Missouri, and Virginia Tech, had substantial graduate-level research pro- grams. Today, there are over 30 U.S. uni- versities (and 45 professors) with power electronic programs, and the number is growing. Many of these universities also provide undergraduate courses in power electronics at the senior level, and power circuits are increasingly being used as examples in basic core courses such as net- work analysis and control. Perhaps, in time, more electrical engineers will find their way to the intriguing, challenging, and rewarding field of power electronics.

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M a r t i n F. Schlechf [MI is Associate Professor of Electrical Engineering and Computer Science at the Massachusetts Institute of Technology in Cambridge, MA, and is associated with the Institute’s Laboratory for Electromagnetic and Electronic Systems, and the Microsystems Technology Laboratories.

Recommended Reading 1. Kassakian, J . G., M. F. Schlecht, and G. C. Verghese, Principles of Power Electronics (Addison-Wesley, 199 I ) .

2. Mohan, N. , T. M. Undeland, and W. P. Robbins, Power Electronics: Converters, Applications, and Design (Wiley. 1989).

3 . Baliga, B. J. , Modem Power Devices (Wiley, 1987).

4. Heumann, K . , Basic Principles of Power Electronics (Springer-Verlag, 1986).

5 . Bedford, B. D. and R. G. Hoft, Principles of Inverter Circuits] (Wiley, 1964).

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