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15-1 Electronics Principles & Applications Eighth Edition Charles A. Schuler Chapter 15 Regulated Power Supplies (student version) ©2013 McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-2 INTRODUCTION • Open-Loop Voltage Regulation • Closed-Loop Voltage Regulation • Current and Voltage Limiting • Switch-Mode Regulators • Troubleshooting McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-3 Dear Student: This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow you to view that segment again, if you want to. McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-4 Concept Preview • Conducting zener diodes show a relatively constant voltage drop. • Zener shunt regulators are not practical for large load currents. • A zener can regulate the base voltage of a series pass transistor. • Adding an error amplifier provides much better voltage regulation (closed loop operation). • IC voltage regulators contain a pass transistor, a voltage reference, and an error amplifier. McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-5 Reverse bias in Volts 4 6 2 0 20 40 60 V I 80 Reverse current in mA 100 120 140 V McGraw-Hill The voltage across a conducting zener is relatively constant. © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-6 Using a zener diode as a voltage regulator Unregulated Supply Load The load is in parallel with the zener and will see a relatively constant voltage as long as the zener is conducting. McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-7 Series pass transistor VBE Unregulated Supply Load The amplified zener regulator is used when large load currents are required. The voltage regulation of this circuit is fair since VBE is relatively constant. McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-8 Closed-loop is required for demanding applications. VOUT Error Amplifier If VOUT decreases, the amplifier output goes more positive to increase the drive to the pass transistor. McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-9 The pass transistor, reference, and error amplifier are inside IC regulators. McGraw-Hill 7805 © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-10 Concept Review • Conducting zener diodes show a relatively constant voltage drop. • Zener shunt regulators are not practical for large load currents. • A zener can regulate the base voltage of a series pass transistor. • Adding an error amplifier provides much better voltage regulation (closed loop operation). • IC voltage regulators contain a pass transistor, a voltage reference, and an error amplifier. Repeat Segment McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-11 Concept Preview • The current capabilities of an IC regulator can be extended by adding a boost transistor. • A boost transistor can be protected from overcurrent by adding a current limit transistor. • Conventional current limiting won’t always protect a pass transistor. Foldback current limiting offers better protection. • Parallel pass transistors require emitter swamping resistors so they will share the load current. • A crowbar circuit provides overvoltage protection. McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. IC voltage regulators have modest current limits. This circuit extends their capabilities. 15-12 PNP current boost transistor R 7805 When the drop across R reaches 0.7 volts, the boost transistor turns on. McGraw-Hill Load © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-13 This current limit circuit protects the pass transistor. R2 Current-limit transistor 7812 R1 Load When the drop across R2 reaches 0.7 V, the current limit transistor turns on and shunts R1. McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. VOUT 15-14 Conventional current limiting Constant current region IL Short circuit McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. VOUT 15-15 Foldback current limiting Foldback current region IL Short circuit McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-16 Foldback current limiting Q1 +Vin R2 R3 R1 R4 R5 RL R7 Q2 R6 VZ McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. Parallel transistors need emitter swamping resistors to ensure current sharing. McGraw-Hill 15-17 © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-18 Fuse If the zener conducts, the SCR turns on and blows the fuse. 7812 Crowbar over-voltage protection circuit McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-19 Linear power supply quiz Pass transistors are connected in _______ with the load. series IC regulators can provide more current with the addition of a ______ transistor. boost The two types of current limiting are conventional and __________. foldback Emitter swamping resistors force parallel transistors to share ________. current Crowbar circuits are used to protect a load from excess __________. voltage McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-20 Concept Review • The current capabilities of an IC regulator can be extended by adding a boost transistor. • A boost transistor can be protected from overcurrent by adding a current limit transistor. • Conventional current limiting won’t always protect a pass transistor. Foldback current limiting offers better protection. • Parallel pass transistors require emitter swamping resistors so they will share the load current. • A crowbar circuit provides overvoltage protection. Repeat Segment McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-21 Concept Preview • Switch mode power supplies use pulse width modulation and achieve much better efficiency than linear power supplies. • Switchers operate at tens of kilohertz so that smaller inductors and capacitors can be used. • The three basic switching configurations are stepdown, step-up, and inverting. All three store energy in an inductor. • Converter type switchers use transformers. • Flyback type switchers use transformers and a critical conduction mode of operation. McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-22 Power supplies with pass transistors are linear and are not as efficient as switch-mode power supplies. PC = IC x VCE (The heat loss in a pass transistor can be significant.) Recall: Pulse width modulation (PWM) is one way to use a digital approach to an analog problem. PWM Average value McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-23 Switch-mode power supply VIN VLOAD PWM Step-down configuration VLOAD < VIN McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-24 Switch-mode power supply VIN CEMF VLOAD PWM Step-up configuration VLOAD > VIN McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-25 Switch-mode power supply VIN CEMF VLOAD PWM Inverting configuration McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-26 Converter type switch-mode supply OSC. & PWM McGraw-Hill VREF © 2013 The McGraw-Hill Companies, Inc. All rights reserved. Topology/Type Linear (step-down) Arrangement Strong points Limitations • Inexpensive • Can be very small • Low no-load current • Low-noise/low EMI • Usually the best solution for smaller loads • VOUT must be less than VIN • Inefficient at high-input voltages and/or large loads • Can require a large heat sink • Limited output power • Limited range of input/output voltage ratio 15-27 Charge pump (boost or invert polarity) • Inexpensive • Small • Can boost or invert Buck (step-down) • Lowest peak current • Efficient • Modest cost • Low-ripple current in output-filter capacitor • Simple inductor • Low switch-stress voltage • VOUT must be less than VIN • High-side switch • Low peak current • Low-side switch • Simple inductor • Low switch-stress voltage • VOUT must be greater than VIN • Output cannot be completely turned off by removing drive • No short-circuit protection • Simple inductor • Negative output only • High-side switch • High peak currents Boost (step-up) Buck-boost (inverts polarity) Flyback (step-down, step-up or inverts polarity) McGraw-Hill • Isolated output • Transformer instead of • Can offer multiple inductor outputs • High peak currents • Steps up/down, • High switch-stress inverts voltage • Low-side switch © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-28 Switch-mode quiz Switch-mode supplies have better _______ than linear supplies. efficiency Switch-mode supplies use pulse width _____________. modulation A higher voltage is provided by the __________ configuration. step-up The opposite polarity is provided by the __________ configuration. inverting A push-pull arrangement is common in ________ type power supplies. converter McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-29 Linear Supply Troubleshooting • A shorted pass transistor produces high output voltage. • An open pass transistor produces no output voltage. • Voltage error can be due to overloads, the reference voltage, the error amplifier, etc. • IC regulators can oscillate. Check bypass capacitors. • IC regulators are subject to RF fields. McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-30 Flyback switching power supply using critical conduction mode (CCM) McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-31 Flyback switching power supply waveforms The transformer current falls to zero before increasing again (CCM). McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-32 Switch-Mode Troubleshooting • May not function unless loaded (use a dummy load). • Use oscilloscope to view PWM waveforms. • Frequency can be critical. • May use an optoisolator in the feedback circuit. Disable and substitute for the feedback signal. • Check the reference voltage. • Check for overloads. McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-33 Concept Review • Switch mode power supplies use pulse width modulation and achieve much better efficiency than linear power supplies. • Switchers operate at tens of kilohertz so that smaller inductors and capacitors can be used. • The three basic switching configurations are stepdown, step-up, and inverting. All three store energy in an inductor. • Converter type switchers use transformers. • Flyback type switchers use transformers and a critical conduction mode of operation. Repeat Segment McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 15-34 REVIEW • Open-Loop Voltage Regulation • Closed-Loop Voltage Regulation • Current and Voltage Limiting • Switch-Mode Regulators • Troubleshooting McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved.