McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Electronics Principles & Applications Seventh Edition Chapter 15 Regulated Power Supplies (student version) Charles A. Schuler McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved.

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Open-Loop Voltage Regulation Closed-Loop Voltage Regulation Current and Voltage Limiting Switch-Mode Regulators Troubleshooting INTRODUCTION

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved Reverse bias in Volts Reverse current in mA I V V The voltage across a conducting zener is relatively constant.

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Load The load is in parallel with the zener and will see a relatively constant voltage as long as the zener is conducting. Using a zener diode as a voltage regulator Unregulated Supply

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Load Unregulated Supply Series pass transistor The amplified zener regulator is used when large load currents are required. The voltage regulation of this circuit is fair since V BE is relatively constant. V BE

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Error Amplifier Closed-loop is required for demanding applications. V OUT If V OUT decreases, the amplifier output goes more positive to increase the drive to the pass transistor.

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. The pass transistor, reference, and error amplifier are inside IC regulators. 7805

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved PNP current boost transistor Load R When the drop across R reaches 0.7 volts, the boost transistor turns on. IC voltage regulators have modest current limits. This circuit extends their capabilities.

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved Load This current limit circuit protects the pass transistor. R2R2 When the drop across R 2 reaches 0.7 V, the current limit transistor turns on and shunts R 1. R1R1 Current-limit transistor

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. V OUT ILIL Conventional current limiting Short circuit Constant current region

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. V OUT ILIL Foldback current limiting Short circuit Foldback current region

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. R1R1 Foldback current limiting VZVZ R5R5 R6R6 RLRL +V in R2R2 R7R7 Q1Q1 Q2Q2 R3R3 R4R4

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Parallel transistors need emitter swamping resistors to ensure current sharing.

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved Crowbar over-voltage protection circuit Fuse If the zener conducts, the SCR turns on and blows the fuse.

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 step- down, 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved. Power supplies with pass transistors are linear and are not as efficient as switch-mode power supplies. Recall: Pulse width modulation (PWM) is one way to use a digital approach to an analog problem. PWM Average value P C = I C x V CE (The heat loss in a pass transistor can be significant.)

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Step-down configuration Switch-mode power supply PWM V IN V LOAD V LOAD < V IN

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Step-up configuration Switch-mode power supply PWM V IN V LOAD V LOAD > V IN CEMF

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Inverting configuration Switch-mode power supply PWM V IN V LOAD CEMF

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Converter type switch-mode supply OSC. & PWM V REF

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved. Flyback switching power supply

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. Flyback supply waveforms

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved. 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 step- down, 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 © 2008 The McGraw-Hill Companies Inc. All rights reserved. Open-Loop Voltage Regulation Closed-Loop Voltage Regulation Current and Voltage Limiting Switch-Mode Regulators Troubleshooting REVIEW