chap04.pdf

Switch Realization

We have seen in previous chapters that the switching elements of the buck, boost, and several other dc-dc

converters can be implemented using a transistor and diode. One might wonder why this is so, and how

to realize semiconductor switches in general. These are worthwhile questions to ask, and switch imple-

mentation can depend on the power processing function being performed. The switches of inverters and

cycloconverters require more complicated implementations than those of dc-dc converters. Also, the way

in which a semiconductor switch is implemented can alter the behavior of a converter in ways not pre-

dicted by the ideal-switch analysis of the previous chapters—an example is the discontinuous conduction

mode treated in the next chapter. The realization of switches using transistors and diodes is the subject of

this chapter.

Semiconductor power devices behave as single-pole single-throw

(SPST) switches, represented ideally in Fig. 4.1. So, although we often draw

converter schematics using ideal single-pole double-throw (SPDT) switches as

in Fig. 4.2(a), the schematic of Fig. 4.2(b) containing SPST switches is more

realistic. The realization of a SPDT switch using two SPST switches is not as

trivial as it might at first seem, because Fig. 4.2(a) and 4.2(b) are not exactly

equivalent. It is possible for both SPST switches to be simultaneously in the on

state or in the off state, leading to behavior not predicted by the SPDT switch

of Fig. 4.2(a). In addition, it is possible for the switch state to depend on the

applied voltage or current waveforms—a familiar example is the diode. Indeed,

it is common for these phenomena to occur in converters operating at light

load, or occasionally at heavy load, leading to the discontinuous conduction

mode previously mentioned. The converter properties are then significantly modified.

How an ideal switch can be realized using semiconductor devices depends on the polarity of the

voltage that the devices must block in the off state, and on the polarity of the current that the devices

Switch Realization

must conduct in the on state. For example, in the dc–dc buck converter of Fig. 4.2(b), switch A must

block positive voltage when in the off state, and must conduct positive current when in the on state.

If, for all intended converter operating points, the current and blocking voltage lie in a single quadrant of

the plane as illustrated in Fig. 4.3, then the switch can be implemented in a simple manner using a tran-

sistor or a diode. Use of single-quadrant switches is common in dc–dc converters. Their operation is dis-

cussed briefly here.

In inverter circuits, two-quadrant switches are

required. The output current is ac, and hence is some-

times positive and sometimes negative. If this current

flows through the switch, then its current is ac, and the

semiconductor switch realization is more complicated.

A two-quadrant SPST switch can be realized using a

transistor and diode. The dual case also sometimes

occurs, in which the switch current is always positive,

but the blocking voltage is ac. This type of two-quadrant

switch can be constructed using a different arrangement

of a transistor and diode. Cycloconverters generally

require four-quadrant switches, which are capable of

blocking ac voltages and conducting ac currents. Real-

izations of these elements are also discussed in this

chapter.

Next, the synchronous rectifier is examined.

The reverse-conducting capability of the metal oxide

semiconductor field-effect transistor (MOSFET) allows it to be used where a diode would normally be

required. If the MOSFET on-resistance is sufficiently small, then its conduction loss is less than that

obtained using a diode. Synchronous rectifiers are sometimes used in low-voltage high-current applica-

tions to obtain improved efficiency. Several basic references treating single-, two-, and four-quadrant

4.1 Switch Applications

switches are listed at the end of this chapter [1–8].

Several power semiconductor devices are briefly discussed in Section 4.2. Majority-carrier

devices, including the MOSFET and Schottky diode, exhibit very fast switching times, and hence are

preferred when the off state voltage levels are not too high. Minority-carrier devices, including the bipo-

lar junction transistor (BJT), insulated-gate bipolar transistor (IGBT), and thyristors [gate turn-off (GTO)

and MOS-controlled thyristor (MCT)] exhibit high breakdown voltages with low forward voltage drops,

at the expense of reduced switching speed.

Having realized the switches using semiconductor devices, switching loss can next be dis-

cussed. There are a number of mechanisms that cause energy to be lost during the switching transitions

[11]. When a transistor drives a clamped inductive load, it experiences high instantaneous power loss

during the switching transitions. Diode stored charge further increases this loss, during the transistor

turn-on transition. Energy stored in certain parasitic capacitances and inductances is lost during switch-

ing. Parasitic ringing, which decays before the end of the switching period, also indicates the presence of

switching loss. Switching loss increases directly with switching frequency, and imposes a maximum

limit on the operating frequencies of practical converters.

4.1

SWITCH APPLICATIONS

4.1.1

Single-Quadrant Switches

The ideal SPST switch is illustrated in Fig. 4.1. The switch contains power terminals 1 and 0, with cur-

rent and voltage polarities defined as shown. In the on state, the voltage v is zero, while the current i is

zero in the off state. There is sometimes a third terminal C , where a control signal is applied. Distinguish-

ing features of the SPST switch include the control method (active vs. passive) and the region of the i–v

plane in which they can operate.

A passive switch does not contain a

control terminal C . The state of the switch is

determined by the waveforms i ( t ) and v ( t )

applied to terminals 0 and 1. The most common

example is the diode, illustrated in Fig. 4.4. The

ideal diode requires that and The

diode is off ( i = 0) when v < 0, and is on ( v = 0)

when i> 0. It can block negative voltage but not

positive voltage. A passive SPST switch can be

realized using a diode provided that the intended

operating points [i.e., the values of v ( t ) and i ( t )

when the switch is in the on and off states] lie on

the diode characteristic of Fig. 4.4(b).

The conducting state of an active

switch is determined by the signal applied to the control terminal C. The state does not directly depend

on the waveforms v ( t ) and i ( t ) applied to terminals 0 and 1. The BJT, MOSFET, IGBT, GTO, and MCT

are examples of active switches. Idealized characteristics i ( t ) vs. v ( t ) for the BJT and IGBT are sketched

in Fig. 4.5. When the control terminal causes the transistor to be in the off state, i= 0 and the device is

capable of blocking positive voltage:

When the control terminal causes the transistor to be in the on

state,

v = 0

and the device is capable of conducting positive current:

The reverse-conducting and

Switch Realization

reverse-blocking characteristics of the BJT and IGBT are poor or nonexistent, and have essentially no

application in the power converter area. The power MOSFET (Fig. 4.6) has similar characteristics,

except that it is able to conduct current in the reverse direction. With one notable exception (the synchro-

nous rectifier discussed later), the MOSFET is normally operated with in the same manner as the

BJT and IGBT. So an active SPST switch can be realized using a BJT, IGBT, or MOSFET, provided that

the intended operating points lie on the transistor characteristic of Fig. 4.5(b).

To determine how to implement an SPST switch using a transistor or diode, one compares the

switch operating points with the i–v characteristics of Figs. 4.4(b), 4.5(b), and 4.6(b). For example, when

it is intended that the SPOT switch of Fig. 4.2(a) be in position 1, SPST switch A of Fig. 4.2(b) is closed,

and SPST switch B is opened. Switch A then conducts the positive inductor current, and switch B

must block negative voltage, These switch operating points are illustrated in Fig. 4.7. Likewise,

when it is intended that the SPDT switch of Fig. 4.2(a) be in position 2, then SPST switch A is opened

and switch B is closed. Switch B then conducts the positive inductor current,

while switch A

blocks positive voltage,

By comparison of the switch A operating points of Fig. 4.7(a) with Figs. 4.5(b) and 4.6(b), it can

be seen that a transistor (BJT, IGBT, or MOSFET) could be used, since switch A must block positive

voltage and conduct positive current. Likewise, comparison of Fig. 4.7(b) with Fig. 4.4(b) reveals that

switch B can be implemented using a diode, since switch B must block negative voltage and conduct pos-

itive current. Hence a valid switch realization is given in Fig. 4.8.

4.1 Switch Applications

Figure 4.8 is an example of a single-quadrant switch realization: the devices are capable of con-

ducting current of only one polarity, and blocking voltage of only one polarity. When the controller turns

the transistor on, the diode becomes reverse-biased since It is required that be positive; oth-

erwise, the diode will be forward-biased. The transistor conducts current This current should also be

positive, so that the transistor conducts in the forward direction.

When the controller turns the transistor off, the diode must turn on so that the inductor current

can continue to flow. Turning the transistor off causes the inductor current to decrease. Since

the inductor voltage becomes sufficiently negative to forward-bias the diode, and the

diode turns on. Diodes that operate in this manner are sometimes called freewheeling diodes. It is

required that be positive; otherwise, the diode cannot be forward-biased since

The transistor

blocks voltage

this voltage should be positive to avoid operating the transistor in the reverse blocking

mode.

4.1.2

Current-Bidirectional Two-Quadrant Switches

In any number of applications such as dc-ac inverters and servo amplifiers, it is required that the switch-

ing elements conduct currents of both polarities, but block only positive voltages. A current-bidirectional

two-quadrant SPST switch of this type can be realized using a transistor and diode, connected in an anti-

parallel manner as in Fig. 4.9.

The MOSFET of Fig. 4.6 is also a two-quadrant switch. However, it should be noted here that

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