The Switch Matrix

1.5.1 The Switch Matrix

i The most readily apparent difference between a power elec-

tronic circuit and other types of electronic circuits is the switch action. In contrast to a digital circuit, the switches do not indi-

V in

V out

cate a logic level. Control is effected by determining the times at

which switches should operate. Whether there is just one switch or a large group, there is a complexity limit: if a converter has m inputs and n outputs, even the densest possible collection

FIGURE 1.9 Switching converter Example 1.6. (From [2], c of switches would have a single switch between each input and Oxford University Press, Inc.; used by permission.)

output lines. The m × n switches in the circuit can be arranged

8 P. T. Krein

DC FIGURE 1.10 The general switch matrix.

n output lines

load FIGURE 1.12 Three-phase bridge rectifier circuit, a 3 × 2 switch matrix.

according to their connections. The pattern suggests a matrix, as shown in Fig. 1.10.

Power electronic circuits fall into two broad classes: switch matrix. In a computer power supply with five separate

1. Direct switch matrix circuits. In these circuits, energy

dc loads, the switch matrix could be 2 × 10. Very few practical storage elements are connected to the matrix only at converters have more than 24 switches, and most designs use

the input and output terminals. The storage elements fewer than 12. effectively become part of the source or the load. A

A switch matrix provides a way to organize devices for a rectifier with an external low-pass filter is an exam- given application. It also helps us focus on three major task

ple of a direct switch matrix circuit. In the litera- areas, which must be addressed individually and effectively in ture, ac–ac versions of these circuits are sometimes order to produce a useful power electronic system. called matrix converters.

• The “Hardware” Task – Build a switch matrix. This

2. Indirect switch matrix circuits, also termed embedded involves the selection of appropriate semiconductor converters. These circuits, like the polarity-reverser

switches and the auxiliary elements that drive and protect example, have energy storage elements connected

them.

within the matrix structure. Indirect switch matrix • The “Software” Task – Operate the matrix to achieve the circuits are most commonly analyzed as a cascade

desired conversion. All operational decisions are imple- connection of direct switch matrix circuits with storage

mented by adjusting switch timing. in between.

• The “Interface” Task – Add energy storage elements to The switch matrices in realistic applications are small. A 2 ×2

provide the filters or intermediate storage necessary to switch matrix, for example, covers all possible cases with a

meet the application requirements. Lossless filters with single-port input source and a two-terminal load. The matrix

simple structures are required.

is commonly drawn as the H-bridge shown in Fig. 1.11. In a rectifier or other converter, we must choose the electronic

A more complicated example is the three-phase bridge rectifier parts, how to operate them, and how best to filter the output to shown in Fig. 1.12. There are three possible inputs, and the two satisfy the needs of the load. terminals of the dc circuit provide outputs, which gives a 3 ×2