1、 Chap 11Teaching Materials of Analog Circuits Chapter 10 Voltage Regulators10.0 Preview The requirement for a reliable source of constant voltage in virtually all electronic systems has led to many advances in power supply design.Designers have used feedback and operational amplifiers,as well as pul
2、se circuit techniques to develop reliable constantvoltage(and constant-current)power suppliers.A voltage regulator is a circuit or device that 10.0 Previewprovide a constant voltage to a load.The output vol-age is controlled by the internal circuitry and is rel-atively independent of the load curren
3、t supplied by the regulator.A basic diagram of a dc power supply is shown in Figure 10.1.DioderectifierFilter VoltageregulatorLoad Powertransformer ACVoltage sourceFigure 10.110.0 PreviewtOtOtOtOtO We briefly considered constant-voltage circuits or voltage regulators,When we studied diode circuits i
4、n Chapter 1,In this Chapter,We will discuss examples of IC voltage regulators.Figure 10.210.1 Rectifier and Filter一、一、Single-Phase Bridge RectifierVL=0.9V2 (10-1)0.9V2 IL=(10-2)RLFigure 10.3D2 D4D1 D3iD2 iD4iD2 iD4iD2 iD4iD1 iD3v2tOiLtOvLtOiD1 iD3iD1 iD3D1 D3D2 D4D2 D4D1 D3+vL v2RLD1 v1D2D3D4Tr10.1
5、Rectifier and Filter 1 0.45V2 ID=IL=(10-3)2 RLVRM=2 V2(10-4)2 4 4 4vL=2 V2(cos2t cos4t cos6t)3 15 35 VL V22VL2 K=0.483 (10-5)VL VL二、二、Filters,Ripple Voltage,and Diode Current If a capacitor is added in parallel with the load resistor of a bridge rectifier to form a simple filter circuit,as shown in
6、Figure 10.4.10.1 Rectifier and FilterFigure 10.4 Td=RLC(35)2 (10-6)VL=(1.1 1.2)V2 (10-7)The diode in a filtered rectifier circuit conducts for a brief interval near the peak of the sinusoidal input signal.(Figure 10.5)SCTr v2RLD1 v1D2+vLD3D4(+)()()(+)10.1 Rectifier and FilterFigure 10.5v2,vL,vCtOtOi
7、L,iDD1 D3D2 D4D2 D4D1 D3 c=RintC Td=RLC(35)2 (11-6)c=(RintRL)C RintC 10.1 Rectifier and FilterFigure 10.6三、三、Inductance FilterExample 10.1 Yang P339 Example 11-1 2 V20.9V2VLOILFigure 10.7RL v1+vLTrL v210.2 Series Feedback Regulators The fundamental classes of voltage regulators are linear regulators
8、 and switching regulators.Both of these are available in integrated circuit form.There are two basic types of linear regulator.One is the series regulator and the other is the shunt regulator.In this section,we will look at the series regulator.一、一、Quality Index VO=f(VI,IO,T)(10-8)10.2 Series Feedba
9、ck Regulatorsor VO=KVVI+Ro IO+STT 1.VO KV=VI IO=0T=0 VO/VO SV=100%(%/V)(10-9)VI IO=0T=0Voltage Regulation VO VO VOVO=VI+IO+T VI IO T10.2 Series Feedback Regulators2.Output Resistance Voltage-Regulation Coefficient VO/VO=(10-10)VI/VI IO=0T=0 VO Ro=()(10-11)IO VI=0T=03.Temperature Coefficient VO ST=(m
10、V/oC)(10-12)T VI=0IO=010.2 Series Feedback Regulators二、Series Feedback Regulators The basic components of series feedback regula-tors are shown in the block diagram in Figure 10.8.Controlelement ErrordetectorSamplecircuitReference voltageVIVOFigure 10.810.2 Series Feedback Regulators Notice that the
11、 control element is in series with the load between input and output.The output sample cir-cuit senses a change in the output voltage.The error detector compares the sample voltage with a reference voltage and causes the control element to compensate in order to maintain a constant output voltage.A
12、basic op-amp series regulator circuit is shown in Figure 10.9.The resistive voltage divider formed by R1 and R2 senses any change in the output voltage.10.2 Series Feedback RegulatorsVI(IL)VO VF VB(IC)VCE VO R2 VF=VO R1+R2 R1 VO(1+)VREF R2(10-13)RLFigure 10.9+ARR1R1VB +VO VREFVFVIR2DZReference volta
13、ge ErrordetectorbceTControlelementSample circuit10.2 Series Feedback Regulators When the output tries to decrease because of a decrease in VI or because of a change in load current.A proportional voltage decrease is applied to the op-amp,s inverting input by the divider.Since the zener diode (DZ)hol
14、d the other op-amp input at a nearly fixed reference voltage,VREF,a small difference volt-age (error voltage)is increased across the op-amp,s inputs.This difference voltage is amplified,and the op-amp,s output voltage increases.This increase is10.2 Series Feedback Regulatorsapplied to the base of T,
15、causing the collector-emitter voltage VCE to decrease,and the output voltage VO to increase until the voltage to the inverting input again equals the reference(zener)voltage.This action offsets the attempted decrease in output voltage,thus keeping it nearly constant.The power transistor,T,isused wit
16、h a heat sink because it must handle all of the load current.For highest accuracy,DZ is replaced with an IC reference.As shown in Figure 10.10.10.2 Series Feedback RegulatorsCJ336,CJ329;MC1403,AD580.VREF=VBE3+IC2 RC2 VT IC1 IC2=()ln()RE2 IC2 VT RC2 IC1VREF=VBE3+ln()RE2 IC2 EG VREF=1.205V(10-14)q RC1
17、RC2IC1IC2I0T1T2T3RE2+VI +VREF Figure 10.1010.3 Three-Terminal Regulator一、一、Fixed Three-Terminal Regulator In this section,we will analyze an example of a three-terminal positive voltage regulator fabricated as an IC.The equivalent circuit,shown in Figure 10.11,is part of the 78LXX series,in which th
18、e XX design-ation indicates the output voltage of the regulator.For example,an 78L05 is an 5V regulator.1.Start-up Circuit Once the bias current is established,Zener diode 10.3 Three-Terminal RegulatorFigure 10.11VOR31T1DZ1DZ2D2D1T4T14T2T3T5T13R4R1R2T7R6R9T8T9R7R14T12DZ3DZ4R15T10T6VFT11R8R11R12R13VI
19、23R5VREFIOR1010.3 Three-Terminal RegulatorDZ2 provides the basic reference voltage.Transistor T1 and T2 and diode DZ1 form a start-up circuit that applies the initial bias to the reference voltage circuit.VI VDZ1 T1 T2,T3,T4 T5 VDZ2 As the voltage across DZ2 reaches the Zener vol-tage,transistor T2
20、turns off,since the B-E voltage goes to zero(DZ1 and DZ2 are identical)and,the start-up circuit is then effectively disconnected from the refer-ence voltage circuit.10.3 Three-Terminal Regulator2.Reference voltage The reference portion of the circuit is composed of Zener diode DZ2 and transistor T4、
21、T3、D2、D1、and resistance R1、R2、R3.T3、D2 and D1,which are used for temperature compensation.Zener diode DZ2 is biased by the current-source transistor T4.The temperature-compensated portion of the reference vol-tage at the node between R1 and R2 is applied to the base of T7,which is part of the error
22、amplifier,10.3 Three-Terminal Regulator The bias current in T4 is established by the cur-rent in T5,which is a multiple-collector transistor.Tr-ansistor T5 is biased by the current in T3 ,which is controlled by the Zener voltage across DZ2 and the B-E junction voltages of T3 ,D2 and D1,Consequently,
23、the bias currents in the reference portion of the cir-cuit become almost independent of the input supply voltage.This is in turn means that the reference VZ23VBEVREF=R1+2VBE (10-15)R1+R2+R310.3 Three-Terminal Regulatorvoltage,and thus the output voltage are essentially independent of the supply volt
24、age.The overall result is very good line regulation.3.The Error Amplifier and The Sample circuit The error amplifier is the differential pair T7 and T8,biased by T6 and R6,The error amplifier output is the input to the base of T9 ,which is connected as an emitter follower and forms part of the drive
25、 for the series-pass transistors.The series-pass output10.3 Three-Terminal Regulatortransistors T10 and T11 are connected in a Darlington emitter-follower configuration.A fraction of the output voltage,determined by the voltage R12 and R13,is fed back to the base of T8,which is the error-amplifier i
26、nverting terminal.If the output voltahe is slightly below its normal value,then the base voltage at T8 is smaller than that at T7,and the current in T7 becomes a larger fraction of the total diff-amp bias current.The increased 10.3 Three-Terminal Regulatorcurrent in T7 induces a larger current in T1
27、0,which in turn produces a larger current in T11 and increases the output voltage to the proper value.The opposite process occurs if the output voltage is above its normal value.4.Protection Devices Transistors T13 and T14 and resistor R3 in the re-gulator in Figure 10.11 provide thermal protection.
28、Usually,the B-E voltage of T14 is approximatly10.3 Three-Terminal Regulator330mV,which means that both T14 and T13 are eff-ectively cut off.As the temperature increases,the combination of a negative B-E temperature coeffi-cient and an increase in IC3 causes T14 to begin conducting,which in turn caus
29、es T13 to conduct.The current in T13 shunts current away from the output series-pass transistors and produces thermal shutdown.10.3 Three-Terminal Regulator Output current limiting is provide by transistor T12 and resistor R11,as we saw previously in op-amp output stages.The combination of resistors
30、 R14 and R15 and diode DZ3 and DZ4 produces what is called a foldback characteristic.The vast majority of the power dissipated in the regulator is usually due to the output current.The output current limit,to prevent power dissi-pation from reaching its maximum value PCM.10.3 Three-Terminal Regulato
31、r R14+R15 R15IO=VBE12(VIVO)VZ3VZ4 (10-17)R11R14 R14 R11二、二、Adjustable Three-Terminal Regulator The LM317 is an example of a three-terminal positive regulator with an adjustable output voltage.The standard configuration is shown in Figure 10.12.VIVZ3VZ4IOR11VOVBE12=IOR11+R15 (10-16)R14+R15(VIVO)(VCE1
32、0-11)IO PCPCM10.3 Three-Terminal RegulatorFigure 10.12+AR1VB VO VREFVIR2I1 ErrordetectorTControlelementIReference voltage 1.2VIadjI2 VREFVO=VREF+(+Iadj)R2 R1 R2=VREF(1+)+Iadj R2 R1 Iadj R2VO VREF(1+)R1 (10-17)10.3 Three-Terminal Regulator The LM337 is the negative output counterpart of the LM317 is
33、a good example of this type of IC regulator.Like the LM317,the LM337 requires two external resistors for output voltage adjustment.三、三、Applications of Three-Terminal Regulator 1.Fixed Three-Terminal Regulator Figure 10.13 shows the basic circuit configur-ation of a fixed three-terminal regulator.In
34、some applications,capacitors may be inserted across the10.3 Three-Terminal RegulatorFigure 10.13input and output terminals.+C20.1F78LXXDC10.33FC310F+VI+VO12310.3 Three-Terminal RegulatorFigure 10.14IO=IO1+IO2IO IO1VR3 T3 T2(saturation)VBE1 IC12.Adjustable Three-Terminal Regulator C20.1F78LXXT3 C10.3
35、3FVIVO123R2T2R3R1T1IO1IO2IOIC110.3 Three-Terminal RegulatorFigure 10.15 C210FLM317 C10.1F VI+25V VO+(1.222V)12LM33721 R1120R22kR22kR1120 C410F VO(1.222V)VI25V C30.1Fadj1adj210.3 Three-Terminal Regulator R10.1Figure 10.16C21FLM317-1 C10.1F VI25V VO(1.222V)I1LM317-2 R52kR32k R20.1R4120adj1adj2VAO+741I
36、2IO1IO2Summary 1.Bridge rectifier circuit convert a sinusoidal signal to an approximate dc signal.A dc power supply,which is used to bias electronic circuits and systems,utilize these types of circuits.A capacitor can be connected to the output of the rectifier circuit to reduce the rip-ple effect.T
37、he ripple voltage is caused by the charg-ing and discharging of the filter capacitor.2.Voltage regulators keep a constant dc output vol-tage when the input or load varies within limits.Summary 3.Zener diode operate in the reverse breakdown region.Since the breakdown voltage nearly constant over a wi
38、de range of currents,these devices are useful in voltage reference or regulator circuits.4.A basic voltage regulator consists of a refer-ence voltage source,an error detector,a sampling element,and a control device.Protection circuitry is also found in most regulators.5.Two basic categories of volta
39、ge regulators are linear and switching.Summary 6.Two basic types of linear regulators are seriesand shunt.7.In a series linear regulator,the control element is a transistor in series with the load.8.Three-terminal linear IC regulators are available for either fixed output or variable output voltages
40、 of positive or negative polarities.9.An external pass transistor increases the current capability of a regulator.Summary 10.The 78LXX series are three-terminal IC regula-tors with fixed positive output voltage.11.The 79LXX series are three-terminal IC regula-tors with fixed negative output voltage.
41、12.The LM317 is a three-terminal IC regulator with a positive variable output voltage.13.The LM337 is a three-terminal IC regulator with a negative variable output voltage.ProblemFigure 10.17+A300R1A +VO VI1R2DZ2DC v1Trv2D3D2D1D4 +C12000F+VI R5T1T2T3EDZ1R3R4RP +C2B 300 1000F300Example 10.2 Yang P354 Problem 11-6
