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Electronic Devices Ninth Edition Floyd Chapter 17 Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Power Supply Regulation An ideal power supply provides a constant dc voltage despite changes to the input voltage or load conditions. The output voltage of a real power supply changes under load as shown in the second plot. The output is also sensitive to input voltage changes. Voltage Voltage VNL VNL VFL Ideal power supply 0 Real power supply Current 0 Electronic Devices, 9th edition Thomas L. Floyd 0 Current 0 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Line Regulation Line regulation is a measure of how well a power supply is able to maintain the dc output voltage for a change in the ac input line voltage. The formula for line regulation is VOUT Line Regulation = 100% VIN Line regulation can also be expressed in terms of percent change in VOUT per volt change on the VIN (%/V). VOUT / VOUT 100% Line Regulation = VIN Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Load Regulation Load regulation is a measure of how well a power supply is able to maintain the dc output voltage between no load and full load with the input voltage constant. It can be expressed as a percentage change in load voltage: VNL VFL Load Regulation = 100% VFL Load regulation can also be expressed in terms of percent change in the output per mA change in load current (%/mA). Sometimes a maximum error voltage is given in the specification as illustrated in the next slide for a commercial power supply. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Load Regulation Commercial power supplies, such as you have in lab, have excellent line and load regulation specifications. The BK Precision 1651A is an example of a triple output supply (two 0-24 V outputs and a fixed 5 V output). Voltage regulation specifications for this power supply are: Line regulation: Load regulation: Electronic Devices, 9th edition Thomas L. Floyd ≤0.01% +3 mV (Main supply) ≤5 mV (Fixed 5 V supply) ≤0.01%+3 mV (Main supply) ≤5 mV (Fixed 5 V supply) © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Load Regulation Sometimes the equivalent Thevenin resistance of a supply is specified in place of a load regulation specification. In this case, VOUT can be found by applying the voltage divider rule: RL VOUT VNL R R L OUT In terms of resistances, load regulation can be expressed as: Power Supply RTH = ROUT VOUT VTH = VNL RL ROUT Load regulation 100% RFL Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Load Regulation A power supply has an output resistance of 25 mW and a full load current of 0.50 A to a 10.0 W load. (a) What is the load regulation? (b) What is the no load output voltage? ROUT 0.025 W Load regulation 100% (a) 100% = 0.25% 10.0 W RFL (b) By Ohm’s law, VOUT = 5.0 V. VNL Electronic Devices, 9th edition Thomas L. Floyd VOUT RL R R L OUT 5.0 V = 5.013 V 10.0 W 0.025 W + 10.0 W © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Series Regulators Series Regulator block diagram: Control element VIN Reference voltage Error detector VOUT Sample circuit Basic series regulator circuit: Control element VIN VOUT Q1 R1 + VREF The control element maintains a constant output voltage by varying the collector-emitter voltage across the transistor. Electronic Devices, 9th edition Thomas L. Floyd – D1 Error detector R2 Sample circuit R3 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Series Regulators R The output voltage for the series regulator circuit is: VOUT 1 2 VREF R3 (a) What is the output voltage for the series regulator? (b) If the load current is 200 mA, what is the power dissipated by Q1? R (a) VOUT 1 2 VREF R3 100 kW 1+ 3.9 V 47 kW = 12.2 V (b) P = VI = (18 V – 12.2 V)(0.2 A) = 1.16 W Electronic Devices, 9th edition Thomas L. Floyd VIN VOUT 18 V Q1 R1 4.7 kW VREF + – 3.9 V D1 R2 100 kW R3 47 kW © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Series Regulators Current limiting prevents excessive load current. Q2 will conduct when the current through R4 develops 0.7 V across Q2’s VBE. This reduces base current to Q1, limiting the load current. The current limit is: I L(max) 0.7 V R4 For example, a 1.4 W resistor, limits current to about 0.5 A. Electronic Devices, 9th edition Thomas L. Floyd Q1 R4 VOUT VIN R1 Q2 + R2 Current limiter – R3 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Series Regulators Fold-back current limiting drops the load current well below the peak during overload conditions. Q2 conducts when VR5 +VBE = VR4 and begins current limiting. VR5 is found by applying the voltage-divider rule: R5 VR5 VOUT R R 6 5 An overload causes VR5 to drop because VOUT drops. This means that less current is needed to maintain conduction in Q2 and the load current drops. Electronic Devices, 9th edition Thomas L. Floyd Q1 R4 VOUT +VIN R5 R1 + – R2 R6 Q2 D1 R3 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Shunt Regulators Shunt Regulator block diagram: R1 VIN VOUT Reference voltage Error detector Control element (shunt) Basic shunt regulator circuit: Sample circuit VOUT VIN R1 R2 – VREF The control element maintains a constant output voltage by varying the collector current in the transistor. Electronic Devices, 9th edition Thomas L. Floyd Error detector Control element Q1 + RL R3 D1 Sample circuit R4 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Shunt Regulators Shunt regulators use a parallel transistor for the control element. If the output voltage changes, the op-amp senses the change and corrects the bias on Q1 to follow. For example, a decrease in output voltage causes a decrease in V VB and an increase in VC. V OUT IN R1 Although it is less efficient than the series regulator, the shunt regulator has inherent short-circuit protection. The maximum current when the output is shorted is VIN/R1. Electronic Devices, 9th edition Thomas L. Floyd R2 Error detector – VREF Control element Q1 + RL R3 D1 Sample circuit R4 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Switching Regulators All switching regulators control the output voltage by rapidly switching the input voltage on and off with a duty cycle that depends on the load. Because they use high frequency switching, they tend to be electrically noisy. Andecrease A increaseininthe theduty dutycycle cycledecreases increasesthe theoutput outputvoltage. voltage. on/off control tonton ton tofftoff toff tonton ton tofftoff toff tonton ton tofftoff toff tonton ton VC VC VC VOUT Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Switching Regulators A step-down switching regulator controls the output voltage by controlling the duty cycle to a series transistor. The duty cycle changes depending on the load requirement. Because the transistor is either ON or OFF on all switching regulators, the power dissipated in the transistor is very small and the regulator is very efficient. The pulses are smoothed by an LC filter. Electronic Devices, 9th edition Thomas L. Floyd Q1 VIN C Lcharges reversesLpolarity off on + + RL C D1 R1 VOUT Variable pulse-width oscillator R2 – + R3 D2 VREF © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Switching Regulators In a step-up switching regulator, the control element operates as a rapidly pulsing switch to ground. The switch on and off times are controlled by the output voltage. Step-up action is due to the fact the inductor changes polarity during switching and adds to VIN. Thus, the output voltage is larger than the input voltage. VIN + L field L field collapses builds OUT + R1 L + Variable pulse-width oscillator – D2 Electronic Devices, 9th edition Thomas L. Floyd charges discharges on off C C +V D1 C on off Q1 RL R2 + R3 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary Switching Regulators In a voltage-inverter switching regulator, the output is the opposite polarity of the input. It can be used in conjunction with a positive regulator from the same input source. Inversion occurs because the inductor reverses polarity when the diode conducts, charging the capacitor with the opposite polarity of the input. Electronic Devices, 9th edition Thomas L. Floyd off on Q1 +VIN off on D1 C discharges + L R1 Variable pulse-width oscillator –VOUT C C charges + L field collapses builds R2 RL + – R3 D2 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary IC Voltage Regulators Integrated circuit voltage regulators are available as series regulators or as switching regulators. The popular three-terminal regulators are often used on separate pc boards within a system because they are inexpensive and avoid problems associated with large power distribution 78XX Regulators systems (such as noise pickup). The 78XX series is a fixed positive output regulator available in various packages and with standard voltage outputs. Electronic Devices, 9th edition Thomas L. Floyd TO-220 case D-PAK 4 1 2 1 2 3 3 Type number Output voltage 7805 7806 7808 7809 7812 7815 7818 7824 +5.0 V +6.0 V +8.0 V +9.0 V +12.0 V +15.0 V +18.0 V +24.0 V © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary IC Voltage Regulators The only external components required with the 78XX series are input and output capacitors and some form of heat sink. These IC’s include thermal shutdown protection and internal current limiting. The 78XX series are primarily used for fixed output voltages, but with additional components, they can be set up for variable voltages or currents. Electronic Devices, 9th edition Thomas L. Floyd Positive input (1) Heat-sink surface 4 1 2 1 2 3 (3) 78XX 3 Positive output (2) Gnd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary IC Voltage Regulators The 79XX series is the negative output counterpart to the 78XX series, however the pin assignments are different on this series. Other specifications are basically the same. Negative input (2) (3) 79XX (1) Gnd Electronic Devices, 9th edition Thomas L. Floyd Negative output Type number Output voltage 7905 7905.2 7906 7908 7912 7915 7918 7924 –5.0 V –5.2 V –6.0 V –8.0 V –12.0 V –15.0 V –18.0 V –24.0 V © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary IC Voltage Regulators The LM317 is a adjustable positive output IC regulator. There is a fixed reference voltage of +1.25 V between the output and adjustment terminals. There is no ground pin. R The output voltage is calculated by: VOUT VREF 1 2 I ADJ R2 R1 What is VOUT? (Assume IADJ = 50 mA.) +20 V Positive input (3) LM317 (1) Adjustment 2 kΩ VOUT 1.25 V 1 50 μA 2 kW 150 W +16.8 V Positive (2) output R11 150 W C11 C3 C22 R22 2 kW = 16.8 V Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary IC Voltage Regulators IC regulators are limited to a maximum allowable current before shutting down. The circuit shown is uses an external pass transistor to increase the maximum available load current. Rext sets the point where Qext begins to conduct: Rext 0.7 V I max Qext VIN Rext C1 VOUT 78XX C2 For example, if Imax is 1.0 A, Rext = 0.7 W RL What minimum power rating is required for Rext? P = I2R = (1 A)2(0.7 W) = 0.7 W Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary IC Voltage Regulators The 78S40 is an IC containing all of the elements needed to configure a switching regulator, using a few external parts. It is a universal switching regulator subsystem because it can be configured as a step-down, step-up, or inverting regulator by the user. The data sheet shows typical circuits for these configurations. Here is the step-down configuration. Electronic Devices, 9th edition Thomas L. Floyd RCS VIN Noninvert Invert input input 9 10 Gnd CT Timing cap 11 12 Ipk Driver Switch VCC VCC sense collector collector 13 14 S Oscillator 15 16 16 Q1 Q Flip-flop – Comp. ++ 1.25 V V reference – D1 + 8 7 66 5 4 Reference Invert Noninvert VCC Output Rvoltage input input op-amp 2 R1 Q2 R 3 Switch emitter 2 1 Anode Cathode L VOUT CO © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Selected Key Terms Regulator A electronic circuit that maintains an essentially constant output voltage with changing input voltage or load current. Line regulation The percentage change in output voltage for a given change in input (line) voltage. Load regulation The percentage change in output voltage for a given change in load current. Linear regulator A voltage regulator in which the control element operates in the linear region. Switching A voltage regulator in which the control regulator element operates as a switch. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 1. The load regulation of an ideal power supply is a. 0% b. 25% c. 50% d. 100% Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 2. A correct formula for load regulation is a. V Load Regulation = OUT 100% VIN b. VNL VFL Load Regulation = 100% VFL c. VFL Load Regulation = 100% VNL VFL d. Load Regulation = Electronic Devices, 9th edition Thomas L. Floyd VOUT / VOUT 100% VIN © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 3. An alternate way to express load regulation is in terms of the a. output resistance and the full-load resistance b. output resistance and the shorted-load resistance c. input resistance and the full-load resistance d. input resistance and the shorted-load resistance Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 4. In the circuit shown, R4 = 0.7 W. The output current will be limited to a. 0.5 A b. 0.7 A c. 1.0 A d. 1.4 A Electronic Devices, 9th edition Thomas L. Floyd Q1 VIN R4 VOUT 0.7 W R1 Q2 + R2 Current limiter – R3 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 5. The block diagram for a series voltage regulator is shown. The yellow box represents a a. control element VIN VOUT b. sample circuit c. error detector ? d. reference voltage Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 6. The block diagram for a shunt voltage regulator is shown. The yellow box represents a a. control element R1 VIN VOUT b. sample circuit c. error detector ? d. reference voltage Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 7. The circuit in the blue shaded area is a a. high speed switching circuit b. fold-back current limiter c. reference source d. shunt regulator Q1 R4 VOUT +VIN R5 R1 + – R2 R6 Q2 D1 R3 Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 8. A major advantage of all switching regulators is a. low noise b. high output impedance c. high efficiency d. all of the above Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 9. The type of regulator circuit shown is a a. series linear b. series switching c. shunt switching onoffD Q1 +VIN 1 offon + R1 C discharges L –VOUT C C Variable + charges pulse-widthL L field field collapses builds R2 RL oscillator + d. none of the above – R3 D2 Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 10. The output voltage from a 7912 is a regulated a. +5 V b. +12 V c. 5 V d. 12 V Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz Answers: Electronic Devices, 9th edition Thomas L. Floyd 1. a 6. b 2. b 7. b 3. a 8. c 4. c 9. d 5. d 10. d © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.