Principles & Applications Electronics Principles & Applications Fifth Edition Charles A. Schuler Chapter 15 Regulated Power Supplies ©1999 Glencoe/McGraw-Hill

Open-Loop Voltage Regulation Closed-Loop Voltage Regulation INTRODUCTION Open-Loop Voltage Regulation Closed-Loop Voltage Regulation Current and Voltage Limiting Switch-Mode Regulators Troubleshooting

The voltage across a conducting zener is relatively constant. Reverse bias in Volts 6 4 2 20 I V 40 Reverse current in mA 60 80 100 120 140 The voltage across a conducting zener is relatively constant.

Using a zener diode as a voltage regulator Load Unregulated Supply The load is in parallel with the zener and will see a relatively constant voltage as long as the zener is conducting.

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.

Closed-loop is required for demanding applications. VOUT Error Amplifier If VOUT decreases, the amplifier output goes more positive to turn on the pass transistor harder.

7805 The pass transistor, reference, and error amplifier are inside IC regulators.

PNP Current boost transistor IC voltage regulators have modest current limits. This circuit extends their capabilities. PNP Current boost transistor R 7805 When the drop across R reaches 0.7 volts, the boost transistor turns on. Load

When the drop across R2 reaches 0.7 V, the current limit transistor 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.

Conventional current limiting VOUT Conventional current limiting Constant current region IL Short circuit

Foldback current limiting VOUT Foldback current limiting Foldback current region IL Short circuit

Foldback current limiting Q1 R2 +Vin R3 R5 R1 R4 RL Q2 R7 R6 VZ

Parallel transistors need emitter swamping resistors to ensure current sharing.

Crowbar over-voltage protection circuit Fuse If the zener conducts, the SCR comes on and blows the fuse. 7812 Crowbar over-voltage protection circuit

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

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

Switch-mode power supply VIN VLOAD PWM Step-down configuration VLOAD < VIN

Switch-mode power supply CEMF VIN VLOAD PWM Step-up configuration VLOAD > VIN

Switch-mode power supply VIN CEMF VLOAD PWM Inverting configuration

Converter type switch-mode supply OSC. & PWM VREF

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

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.

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.

REVIEW Open-Loop Voltage Regulation Closed-Loop Voltage Regulation Current and Voltage Limiting Switch-Mode Regulators Troubleshooting