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Analog Circuits: Chapter 11 - Voltage Regulators Overview

Analog Circuits: Chapter 11 by Weiwei Xia covers voltage regulators in electronic systems. It discusses constant voltage supply, rectifiers, filters, series feedback regulators, and their components. The content delves into the operation and design of voltage regulators, showcasing circuit diagrams and examples.

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Analog Circuits: Chapter 11 - Voltage Regulators Overview

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  1. Teaching Materials of Analog Circuits Chap 11 Weiwei Xia College of Physics Science and Technology Yangzhou University Yangzhou, 225002 Email: wwxia@yzu.edu.cn

  2. Chapter 10 Voltage Regulators § §10.0 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 pulse circuit techniques to develop reliable constant– voltage (and constant-current) power suppliers. A voltage regulator is a circuit or device that

  3. § §10.0 Preview provide a constant voltage to a load. The output vol- age is controlled by the internal circuitry and is rel- atively independent of the load current supplied by the regulator. A basic diagram of a dc power supply is shown in Figure 10.1. Power transformer Voltage regulator AC Diode rectifier Filter Load Voltage source Figure 10.1

  4. § §10.0 Preview O O O O O t t t t t Figure 10.2 We briefly considered constant-voltage circuits or voltage regulators ,When we studied diode circuits in Chapter 1, In this Chapter , We will discuss examples of IC voltage regulators.

  5. § §10.1 Rectifier and Filter 一、 一、Single-Phase Bridge Rectifier v2 Tr D1 ωt O D4 v2 v1 iD1 iD3 iD2 iD4 iD1 iD3 iD2 iD4 iD1 iD3 iD2 iD4 + vL － D3 D2 iL RL ωt O vL VL=0.9V2 (10-1) ωt O D1√ D3 D2√ D4 D1√ D3 D2√ D4 D1√ D3 D2√ D4 0.9V2 RL IL= (10-2) Figure 10.3

  6. § §10.1 Rectifier and Filter 1 0.45V2 IL = (10-3) 2 RL (10-4) ID= VRM = 2 V2 4 4 4 π 3π 15π 35π 2 vL= 2 V2(—－—— cos2ωt－ —— cos4ωt－ —— cos6ωt．．．) VLν V22－VL2 Kν= —— = ————=0.483 (10-5) VL VL 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 Figure 10.4. 二、

  7. § §10.1 Rectifier and Filter T ( + ) (－ －) τd=RLC≥(3～5)—2 Tr D1 D4 v1 v2 (10-6) + vL － S D3 D2 (－ －) ( + ) C VL=(1.1 ～1.2) V2 RL (10-7) Figure 10.4 The diode in a filtered rectifier circuit conducts for a brief interval near the peak of the sinusoidal input signal.(Figure 10.5)

  8. § §10.1 Rectifier and Filter v2, vL, vC τc=(Rint∥ ∥RL)C ≈RintC ↓ ωt O D2 D4 √ D2 D4 √ D1 D3 √ D1 D3 √ T τd=RLC≥(3～ ～5)—↑2 τc=RintC↓ (11-6) iL, iD O ωt Figure 10.5

  9. § §10.1 Rectifier and Filter VL Tr 2 V2 L v1 v2 0.9V2 + vL － RL O IL Figure 10.7 Figure 10.6 三、 三、Inductance Filter Example 10.1 Yang P339 Example 11-1

  10. § §10.2 Series Feedback Regulators The fundamental classes of voltage regulators are linear regulators 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)

  11. § §10.2 Series Feedback Regulators      VO VI VO VO IO T △VO= ———△VI+ —— △ IO + —— △T  or △VO= KV△VI + Ro△ IO + ST△T △VO △VI KV= ——— 1. △IO =0 △T=0 Voltage Regulation △VO/VO △VI SV= —————×100% (%/V) (10-9) △IO =0 △T=0

  12. § §10.2 Series Feedback Regulators Voltage-Regulation Coefficient △VO/VO △VI /VI γ= ————— (10-10) △IO =0 △T=0 2. Output Resistance △VO △IO Ro= ——— (Ω) (10-11) △VI =0 △T=0 3. Temperature Coefficient △VO △T ST= ——— (mV/oC) (10-12) △VI =0 △IO=0

  13. § §10.2 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. Control element VI VO Reference voltage Error detector Sample circuit Figure 10.8

  14. § §10.2 Series Feedback Regulators Notice that the 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 basic op-amp series regulator circuit is shown in Figure 10.9. The resistive voltage divider formed by R1and R2senses any change in the output voltage.

  15. § §10.2 Series Feedback Regulators Reference voltage Error detector Control element VI↑( IL～)→ VO↑ → VI VF↑→ VB↓(IC↓) → R VREF Sample circuit c T VCE↑ → VO ↓ + － VB b A VF e R2 VF= ———VO R1+R2 + R1 DZ R1 VO RL R1 R2 VO≈(1+—)VREF R2 － － (10-13) Figure 10.9

  16. § §10.2 Series Feedback Regulators When the output tries to decrease because of a decrease in VIor 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) hold 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 is

  17. § §10.2 Series Feedback Regulators applied to the base of T, 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, is used with a heat sink because it must handle all of the load current. For highest accuracy, DZis replaced with an IC reference. As shown in Figure 10.10.

  18. § §10.2 Series Feedback Regulators + VREF= VBE3+IC2 RC2 + RC2 I0 RC1 VT RE2 IC2 IC1 T3 IC2 IC2= (——)ln ( — ) IC1 VRE F VI T2 T1 RE2 － － － － VT RC2 RE2 IC2 IC1 VREF=VBE3+ ———ln ( — ) Figure 10.10 CJ336, CJ329; MC1403, AD580. EG q VREF= ——=1.205V (10-14)

  19. § §10.3 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 the 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

  20. § §10.3 Three-Terminal Regulator 1 DZ3 VI R4 T1 T4 DZ4 T5 T10 T11 R14 T2 T3 R9 R10 T9 T12 R3 IO R15 R11 2 T14 T13 R2 VO R7 R12 VREF T7 T8 R8 VF R1 DZ1 D2 DZ2 T6 R5 R13 D1 R6 3 Figure 10.11

  21. § §10.3 Three-Terminal Regulator DZ2 provides the basic reference voltage. Transistor T1and 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 DZ2reaches the Zener vol- tage, transistor T2turns off, since the B-E voltage goes to zero ( DZ1and DZ2are identical ) and, the start-up circuit is then effectively disconnected from the refer-ence voltage circuit.

  22. § §10.3 Three-Terminal Regulator 2. Reference voltage The reference portion of the circuit is composed of Zener diode DZ2 and transistor T4、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 amplifier,

  23. § §10.3 Three-Terminal Regulator VZ2－3VBE R1+ R2+ R3 VREF= ————— R1+2VBE (10-15) 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, 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

  24. § §10.3 Three-Terminal Regulator voltage, and thus the output voltage are essentially independent of the supply voltage. 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 for the series-pass transistors. The series-pass output

  25. § §10.3 Three-Terminal Regulator transistors 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 inverting 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

  26. § §10.3 Three-Terminal Regulator current in T7 induces a larger current in T10, which in turn produces a larger current in T11and 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. Usually, the B-E voltage of T14 is approximatly

  27. § §10.3 Three-Terminal Regulator 330mV, which means that both T14and T13are 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 causes T13 to conduct. The current in T13 shunts current away from the output series-pass transistors and produces thermal shutdown.

  28. § §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 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.

  29. § §10.3 Three-Terminal Regulator VI－VZ3－VZ4－IOR11－VO R14+R15 VBE12= IOR11+ R15 (10-16) R14+R15 R15 IO=VBE12————－［(VI－VO)－VZ3－VZ4］——— R11R14 (10-17) R14 R11 (VI－VO)↑(VCE10-11)↑→ IO↓ → PC＜PCM 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.

  30. § §10.3 Three-Terminal Regulator VREF R1 VI VO=VREF+( —— +Iadj ) R2 Error detector I R2 R1 VREF =VREF(1+ — ) + Iadj R2 + － VB T A Control element VO Iadj ↓↓ Reference voltage 1.2V I1 R1 R2 R1 Iadj VO≈VREF(1+ — ) I2 R2 (10-17) Figure 10.12

  31. § §10.3 Three-Terminal Regulator The LM337 is the negative output counterpart of the LM317 is a good example of this type of IC regulator. Like the LM317, the LM337 requires two external resistors for output voltage adjustment. 三、Applications of Three-Terminal Regulator 三、 1. Fixed Three-Terminal Regulator Figure 10.13 shows the basic circuit configur- ation of a fixed three-terminal regulator. In some applications, capacitors may be inserted across the

  32. § §10.3 Three-Terminal Regulator input and output terminals. D 1 2 78LXX + + C2 C1 + C3 3 VI － VO 0.1μF 0.33μF 10μF － Figure 10.13

  33. § §10.3 Three-Terminal Regulator IC1 IO1 R 3 T1 R 1 IO=IO1+IO2 IO↑→ IO1↑→ T3 R 2 IO2 IO VR3↑→ T3√ → VI 1 2 78LXX T2 VO T2(saturation) → C1 C2 3 0.33μF 0.1μF VBE1↓→ IC1↓ Figure 10.14 2. Adjustable Three-Terminal Regulator

  34. § §10.3 Three-Terminal Regulator VO VI +(1.2～22V) +25V 2 LM317 R1 120Ω C1 adj1 1 0.1μF C2 R2 2kΩ 10μF R2′ 2kΩ C4 10μF C3 1 adj2 R1′ 120Ω 0.1μF 2 LM337 －VI －25V －VO －(1.2～22V) Figure 10.15

  35. § §10.3 Three-Terminal Regulator R1 IO1 I1 LM317-1 0.1Ω adj1 R3 2kΩ + －741 VAO VO R2 I2 IO2 (1.2～22V) VI LM317-2 0.1Ω 25V R4 120Ω C1 0.1μF adj2 C2 1μF R5 2kΩ Figure 10.16

  36. Summary 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. The 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.

  37. Summary 3. Zener diode operate in the reverse breakdown region. Since the breakdown voltage nearly constant over a wide 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 voltage regulators are linear and switching.

  38. Summary 6. Two basic types of linear regulators are series and 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 of positive or negative polarities. 9. An external pass transistor increases the current capability of a regulator.

  39. 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. 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.

  40. Problem Example 10.2 Yang P354 Problem 11-6 A VI1 B T3 + R1 + DZ2 R2 D3 D1 300Ω T2 R3 T1 Tr C + C2 － + C1 RP A VI 300Ω v1 VO v2 + E 2000μF D 1000μF DZ1 300Ω R5 R4 D4 D2 － － － － Figure 10.17

  41. Problem Example 10.3 Yang P353-354 Problem 11-4 Solution: (1) VI I VI↑(RL↑)→VREF↑→VB↑(VBE2↑) T1 VA IB1=I－ －IC2 IE1 →IE2↑(IC2↑) IB1↓→IE1↓ →IL IL DZ ↓ →VREF ↓ T2 VB VRE F RL R IE2 (2) VREF=VZ+VBE2=6.3+0.7=7V. Figure 10.18

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