EKT214 - ANALOG ELECTRONIC CIRCUIT II Chapter 6 Voltage Regulator

Introduction Batteries are often shown on a schematic diagram as the source of DC voltage but usually the actual DC voltage source is a power supply. There are many types of power supply. Most are designed to convert high voltage AC mains electricity to a suitable low voltage supply for electronics circuits and other devices.

A more reliable method of obtaining DC power is to transform, rectify, filter and regulate an AC line voltage. A power supply can be broken down into a series of blocks, each of which performs a particular function.

Power Supplies For example a 5V regulated supply

Transformer

Transformers convert AC electricity from one voltage to another with little loss of power through inductively coupled electrical conductors. Transformers work only with AC voltage Step-up transformers increase voltage. Step-down transformers reduce voltage.

Full-wave bridge rectifier Electrical device that converts alternating current (AC) to direct current (DC) with a process known as rectification Full-wave bridge rectifier

Produces full-wave varying DC.

A bridge rectifier can be made using four individual diodes, but it is also available in special packages containing the four diodes required.

Smoothing

Regulator Voltage regulator ICs are available with fixed (typically 5, 12 and 15V) or variable output voltages.

Zener diode regulator For low current power supplies - a simple voltage regulator can be made with a resistor and a zener diode connected in reverse as shown in the diagram on the next page. Zener diodes are rated by their breakdown voltage Vz and maximum power Pz (typically 400mW or 1.3W).

Zener diode regulator

Voltage Regulation Line Regulation = Line regulation is the maintenance of a nearly constant output voltage when the input voltage varies. When the ac input (line) voltage of a power supply changes, an electronic circuit called a regulator maintains a nearly constant output voltage How well a regulator performs line regulation can be determined by the formula below. Line Regulation =

Voltage Regulation Load Regulation = Load regulation is the maintenance of a nearly constant output voltage when the load varies. When the amount of current through a load changes due to a varying load resistance, the voltage regulator must maintain a nearly constant output voltage across the load. How well a regulator performs load regulation can be determined by the formula below. Load Regulation =

Types of Regulator The fundamental classes of voltage regulators are linear regulator and switching regulators. Two basics types of linear regulator are : Series Regulator Shunt Regulator

Basic Linear Series Regulator In series regulation the control element is in series with the input and output. Simple series voltage regulator and block diagram

Basic op-amp series regulator The control element is a transistor in series with the load. The output sample circuit senses a change in the output voltage. The error detector compares the sample voltage with a reference voltage (zener diode) Control is achieved by controlling the voltage across Q1

Regulating Action The resistive voltage divider formed by R2 and R3 senses any changes in the output voltage. When the output voltage tries to decrease, a proportional voltage decrease is applied to the op-amp’s inverting input by the voltage divider. Zener diode sets the reference voltage for the non-inverting input of the op-amp. Any changes in the output are fed back to the inverting input of the op-amp. The difference voltage (error voltage) is amplified and the op-amp’s output voltage is increased. This increase is applied to the base of Q1 – causing the emitter voltage to increase until the voltage to the inverting input equals the reference voltage.

The regulated output voltage of the series regulator is :

Short-Circuit or Overload Protection Overload protection for a series regulator protects the control element in the case of a short. Also known as constant-current limiting. The load current through R4 produces a voltage from base to emitter of Q2 When IL reaches a predetermined maximum value, the voltage drop across R4 is sufficient to forward – biased the base – emitter junction of Q2 – causing it to conduct. When load current exceeds the predetermined level, Q2 diverts current from the base of Q1- reducing the load current through Q1 – preventing any additional of load current.

Regulator with Fold-Back Current Limiting Fold-back current limiting allows operation up to peak load current (high current regulator) With a shorted output the current is dropped to a lower value (folded back) to prevent the overheating the device. The voltage drop developed across R4 by the load current must not only overcome the base-emitter voltage – but VR5 + VBE must be overcome before Q2 conducts to limit current.

Basic Linear Shunt Regulator In shunt regulation the control element is in parallel with input and output. Simple shunt voltage regulator and block diagram

Basic op-amp shunt regulator with load resistor The control element is a transistor, Q1 parallel (shunt) with the load and a resistor R1 in series with the load. The operation same as series, except that regulation is achieved by controlling the current through Q1. Basic op-amp shunt regulator with load resistor

Regulating Action When the output voltage tries to decrease due to a change in input voltage or load current, the attempted decrease is sense by R3 and R4 - applied to the op-amp’s non-inverting input. The difference voltage increases the op-amp’s output voltage - - and increasing the collector voltage, Vc – keeping the output nearly constant.

Basic Switching Regulator The switching regulator is more efficient than the linear series or shunt type. This type regulator is ideal for high current applications since less power is dissipated. Basically, a switching regulator passes voltage to the load pulses, which are then filtered to provide a smooth dc voltage. With switching regulators 90% efficiencies can be achieved.

Basic step – down switching regulator Step – Down Configuration With the step-down (output voltage is less than the input voltage) configuration the control element Q1 is pulsed on and off at variable rate based on the load current. The pulsations are filtered out by the LC filter. Since Q1 is either on or off, the power lost in the control element is relatively small Basic step – down switching regulator

Switching regulator waveform When the on-time is increased, capacitor charges more – increasing the output voltage. When the on-time is decreased, the capacitor discharges more – decreasing the output voltage. The inductor smoothes the fluctuations of the output voltage caused by the charging and discharging. Switching regulator waveform

Step – Up Configuration The step-up configuration works much the same as the step-down. The difference is in the placement of the inductor and the fact that Q1 is shunt configured. During the time when Q1 is off the VL adds to VC stepping the voltage up by some amount.

Basic inverting switching regulator Voltage – Inverter Configuration With the voltage-inverter configuration the output voltage output is of opposite polarity of the input. This is achieved by VL forward-biasing reverse-biased diode during the off times producing current and charging the capacitor for voltage production during the off times. With switching regulators 90% efficiencies can be achieved. Basic inverting switching regulator

IC Regulator Regulation circuits in integrated circuit form are widely used. Their operation is no different but they are treated as a single device with associated components. These are generally three terminal devices that provide a positive or negative output. Some types are have variable voltage outputs. Connection of a three-terminal voltage regulator IC to a load

A typical 78XX series of IC regulators – three-terminal devices that provide a fixed positive output voltage. A typical 79XX series of IC regulators – three-terminal devices that provide a fixed negative output voltage Almost all applications of regulators require the device be secured to a heat sink to prevent thermal overload. The capacitors act as line filtration.

Adjustable Positive Linear Voltage Regulator Adjustable IC regulators are available with either positive or negative output. They can be set to produce a specific voltage by way of an external reference voltage divider network. Vout can be varied from 1.2V to 37V depending on the resistor values. The LM317 three-terminal adjustable positive voltage regulator

The External Pass Transistor To increase the current capability of an IC regulator an external pass transistor can be used in order to prevent thermal load condition. A 78XX-series three-terminal regulator with an external pass transistor

Regulator with current limiting The external transistor must be protected from excessive current. An additional current limiting circuit (Qlim and Rlim ) – to protect Qext from excessive current and possible burn out. Regulator with current limiting

Summary Voltage regulators keep a constant dc output despite input voltage or load changes. The two basic categories of voltage regulators are linear and switching. The two types of linear voltage regulators are series and shunt. The three types of switching are step-up, step-down, and inverting. Switching regulators are more efficient than linear making them ideal for low voltage high current applications.

IC regulators are available with fixed positive or negative output voltages or variable negative or positive output voltages. Both linear and switching type regulators are available in IC form. Current capacity of a voltage regulator can be increased with an external pass transistor.