Switching Power Supplies

You too can design switching power supplies! In this article we present you withthe theoretical background and several experimental circuits.

S W I T C H I N G P O W E R - S U P P L Y D E S I G N H A S

long been regarded as something of ablack art practiced only by magicians andwizards. However, recent innovations inelectronics have provided us with compo-nents that simplify such designs consider-ably: high-density IC’s, high-per-meability ferrite inductor cores, and high-power, high-speed switching transistors.We will review fundamental power-supplyoperation. discuss underlying switching-supply theory, and then present the-oretical and practical information on howto design switching supplies using twopopular LSI IC’s. To simplify your designtasks, we include complete schematics,parts lists and PC artwork for several func-tional switching power supplies.

Regulator basicsAny regulated DC power supply-

switching or linear-is governed by afeedback mechanism that senses changesin the output voltage and generates a con-trol signal to compensate for thosechanges. As shown in Fig. 1, the basiclineur voltage regulator has four majorcomponents:0 1) A voltage referencel 2) An error amplifierl 3) A feedback mechanisml 4) A control element

VAUGHN D. MARTIN

AMPLFIER

FIG. I-THE FOUR BASIC COMPONENTS of alinear regulator are shown here. The control ele-ment is a transistor operated in the common-base mode.

The control element is usually a tran-sistor operated in the common-basemode. Its base current determines the cur-rent flowing through its emitter-collectorcircuit, and thus, indirectly, the outputvoltage of the regulator. That base currentis supplied by the error amplifier, usuallyan op-amp. The op-amp compares an in-ternal reference voltage to a portion of thecontrol element’s output voltage. The op-amp’s output is proportional to the dif-ference between those voltages, and thatis what controls the base current, andhence the collector current, of the controlelement.

Not shown in Fig. 1 is additional cir-

cuitry that protects the regulator fromover-current and over-heat conditions.Like the error amplifier, the protectioncircuit samples a portion of the outputvoltage; the output of the protection cir-cuit would be connected to the base of theseries pass transistor, and would force thetransistor into cutoff when it sensed anoverload condition.

In a linear regulator the series-passtransistor operates in the linear region;that is, current flows through the tran-sistor continuously as long as the inputvoltage exceeds the minimum necessaryto keep the regulator working. (Outputmay also be cut off, as we discussedabove, because of thermal or over-currentshutdown.) But a regulator does not nec-essarily have to operate in such a fashion.It is possible for it to switch rapidly be-tween saturation and cutoff, remaining inthe linear region only for the short periodof time required to perform the switchingaction. That is the basis of the switchingregulator.

Fight or switch?Although it might seem odd to operate

a power supply in such a manner, withproper circuit design much greater effi-ciency can be obtained than with linearoperation. Output voltage in the switch-

1CONTROL CONTROL

LOGIC ELEMENT

AMPLIFIER

OUTPUT

FIG. P-SWITCHING REGULATORS differ fromlinear regulators by the addition of control logicand an oscillator; the latter allow control of fre-quency, duty cycle or both, and those param-eters determine the regulator’s final output volt-age.

+vlN ’ S

Sl LlT+ Vour

01 Co

a

+v,N -L, _R

Ll T D1 1O+ VO"T

Sl -0 s icoa------

b

+%N o44 - o-VO”lnl

FIG. 3-THREE BASIC switching regulators areshown here. Step-down, step-up, and invertingregulators are shown at a, b, and c, respectively.

ing mode is controlled by varying the dutycycle. the frequency, or both, ofthe signalthat turns the transistor on and off. Asshown in Fig. 2, the switching regulator isconceptually similar to the linear reg-ulator. The blocks labeled “control logic“and “oscillator” are what allow the dutycycle or frequency to be controlled.

Switching supplies allow greater effi-ciency than linear supplies. but that is nottheir only advantage. As shown in Fig. 3,by rearranging a few components. thesame basic circuit can be made to step theinput voltage down ((I), to step it up (h), orto invert its polarity (c). And inversionmay happen simultaneously with the step-up and step-down functions.

In each of the circuits shown in Fig. 3,SI is reprcsentcd as a mechanical switch,but in the circuits we will be concernedwith, it is actually an electronic multi-

vibrator whose duty cycle, frequency, orboth may be varied by connecting appro-priate components. In the past (that is,before the invention of high-voltage,high-power semiconductors. and LSIICY’s). that switching action was per-formed by rotating machinery.

In Fig. 3-r!. the coil-capacitor networkmay be thought of as an energy reservoirthat is fed by the power source each timethe switch is closed. The diode serves twofunctions. First, it provides a dischargepath for the coil so that when the switchopens, there will be no arcing. Second, italso provides a path through which thecurrent that is stored in the coil and thecapacitor may flow while the switch isopen. That helps smooth the roughsawtooth that is the normal product ofsuch switching into a fairly “flat” trianglewave. Thus, less noise is generated, andthe average DC value of the output voltageis higher than it would be without thereturn diode. That type of circuit is knownas a wric~s switching supply. because theswitching device is in series with the inputvoltage source.

By contrast. the step-up circuit in Fig.3-h is called a sl~rrzt switching supply,because the switching device is in parallelwith the input voltage. When the switchopens, energy stored in the magnetic fieldof the coil is released as a large spike thatserves to charge capacitor C,,. Diode Dlhere serves to prevent the capacitor fromdischarging through the switch while it isclosed.

The inverter scheme in Fig. 3-c com-bines elements of both the step-up andstep-down circuits. Again the switching

device IS connected&. W,’ .hies with t e volt-age source; the coil a&u2 dimps Its storedenergy to the capacitor when the switch isopened. Here the diode ensures that thedischarged energy flows “backward”through the load.

Modern switching regulator IC’s usu-ally contain the voltage reference, switch-ing mechanism, protection circuitry and alow- to medium-current series-pass ele-ment. The circuit designer usually sup-plies discrete components to set theoscillating frequency of the switchingsupply, an appropriate inductor, and addi-tional series-pass elements to increase thecircuit’s current output.

The three most popular switching volt-age regulator IC’s today are probably Sig-netics’ NE556ON, National’s LM3524and Fairchild’s pA78S40. Robert Frost-holm explained the Signetics IC in theFebruary 1980 issue of Radio-Elec-tronics, so for the remainder of this art-cle, we will concentrate on the latter IC’s.

The LM3524A Pulse Width Modulator (PWM) is a

circuit that varies the duty cycle of a pulsetrain while keeping its frequency con-stant. The LM3524 has a built-in PWMthat is used to vary the “on” time of theseries-pass elements. As shown in Fig. 4,the output of the oscillator drives a flip-flop that in turn drives two NOR gates .Note that they are driven out of phase bythe flip-flop’s Q and 0 outputs. Also notethat the IC’s output is disabled by thecomparator whenever its output goeshigh. That will happen whenever the erroramplifier determines that the internal ref-

~___----____--------------------------7

1

IvREF ,‘,a’,

‘T’w 5V T6 ALL

INTERNAL CIRCUITRY II

*I\ GNOIr

ca/kLII

l- Fl..____________________---___-----_---_-_dFIG. 4-NATIONAL SEMICONDUCTOR’S LM3524 can tolerate a maximum input of forty volts, and canprovide as much as 100 ma of output current.