POWER SUPPLY CIRCUIT

Block Diagram Of A Complete Power Supply

Transformer Types

• Core Type • Shell Type

Typical Power Transformers

• Shell Method - provides maximum coupling between windings

- creates considerable interwinding capacitance - can not handle very much voltage

• Core Method - the coupling occurs by means of magnetic flux in the core - the interwinding capacitance is lower - can handle higher voltages

Parts Of A Transformer

• Core – provides a path for the magnetic lines of flux• Primary Winding – receives the energy from the AC

source• Secondary Winding – receives the energy from the

primary winding and delivers it to the load• Enclosure – protects the above components from dirt,

moisture and mechanical damage

Main Functions Of A Transformer

• Stepping up of Voltage• Stepping down of Voltage• Provides isolation between primary and secondary

windings• Impedance Matching

Transformer Formulas

• Turns Ratio, a : a = Ns/Np

• Voltage Ratio : Es/Ep = Ns/Np

• Current Ratio : Is/Ip = Np/Ns

• Impedance Ratio : Ns/Np = √Zs/Zp

• Transformers are rated according to the output voltage and current (VA capacity)

Transformer LossesCopper Loss or I²R loss – it is the losses due to the primary

and secondary windings of the transformer

Core Loss or Iron Loss

1. Eddy Current Loss – due to the changing magnetic field and the iron core conductivity. This current is produced in the iron core that does not aid transformer output. It can be minimized through laminating the core

2. Hysteresis Loss – since AC currents constantly change in magnitude and direction, the tiny molecular agents within the core are constantly being arranged and this process requires energy that causes some loss

Transformer Efficiency, ɳ

ɳ = Po/Pi x 100% , where

ɳ is the efficiency of the transformer

Po is the power output drawn by the load,

Po = Vʟ Iʟ (P.F.)

Pi is the power developed by the transformer itself

Pi = Po + Pʟosses

Rectifier

• An electronic circuit that converts AC voltage into pulsating DC

• It converts a signal having zero average value into one that has a nonzero average value

Rectifier Circuits

• Half-Wave Rectifier – the simplest rectifier circuit which uses only one diode

• Full-Wave Rectifier – most commonly used in DC power supplies and can either be : a. Center-tap b. Bridge type

Half-Wave Rectifier Circuits

Half-Wave Rectifier

• Advantages 1.cost less because circuit is less complicated 2.adequate only for power supplies not required to deliver much current

• Disadvantages 1. the output is difficult to filter 2. the output voltage can drop considerably when the supply is required to deliver high current 3. HW rectification puts a strain on the transformer and the diode

Bridge Type Rectifier

Positive And Negative Half Cycles

• Positive Half Cycle • Negative Half Cycle

Bridge Type Rectifier

• Advantages 1. the output voltage is easier to filter as compared to half-wave rectifiers 2. there are less ripple voltages 3. more efficient use of diodes

• Disadvantages 1. more complicated circuitry 2. more expensive

Formulas

• Average Value of Half-Wave Rectified Voltage Vdc=Vp/π = 0.318 Vp

• Effective Value of a Half-Wave Rectified Voltage Vrms=Vp/2=0.5Vp

• Average Value of a FW Rectified Signal Vdc=2Vp/π=0.636Vp

• Effective Value of a FW Rectified Signal Vrms=Vp/√2

Ripple Factor

• The pulsating dc output of a rectifier contains ripple voltages

• The amount of ripple can be measured by means of a ripple factor which is defined as r = ripple voltage in AC rms voltage / DC voltage

r = AC voltage / DC voltage

r = ripple voltage in rms / average dc voltage

= Vr (rms) / Vdc • The ripple factor is an indication of the effectiveness of a

filter, wherein, for a power supply, the lesser the value of the ripple, the better is the performance

Ripple Factor of Rectified Voltages

Half Wave Rectifier

Vr (rms) = 0.385Vm

r=Vr (rms)/Vdc x 100%

r=0.385Vm/0.318Vm x 100%

r = 121%

Full Wave Rectifier

Vr(rms) = 0.308Vm

r = Vr(rms)/Vdc x 100%

r = 0.308Vm/0.636Vm x 100%

r = 48%

Power Supply Filter

• The output of a rectifier is a pulsating dc and is not yet suitable for most dc powered devices

• These pulsations are known as ripple voltages • Ripples can be eliminated by means of filters• A filter circuit is necessary to provide a steadier dc

voltage; it is used to keep the ripple component from appearing in the output

• Filters are implemented with capacitors• When the rectified signal undergoes filtering, the output is

nearly smooth dc voltage• The small fluctuations in the filter output is called the ripple

Effects of Filter

How effective is a filter?

• The effectiveness of a filter is measured by the ripple factor. Generally, the ripple is undesirable, thus, the smaller the ripple, the better is the filtering action

How can a filter be made more efficient?

• The lower the ripple, the better the filter. Ripple factor can be lowered by increasing the value of the filter capacitor or increasing the load resistance

Two Basic Types of Filters

1. Capacitance Filters (C-filter)

2. RC filter

Capacitor Filter

• The simplest and most economical filter• A capacitor is connected across the output of a rectifier

and a dc output voltage is obtained across the capacitor• When the diode is forward biased, the capacitor charges

and when the diode is reverse biased, the capacitor discharges

• Ripple voltages is caused by the charging and discharging of the filter capacitor

The C-filter Equations

Vr (rms) = Idc / 4√3fC

Vdc = Vm – Vm/ T/4

%r = Vr (rms) / Vdc x 100%

With f=60 Hz, Idc in mA, C in μF and Rʟ in kΩ

Vr (rms) = 2.4 Idc / C = 2.4 Vdc / Rʟ C

Vdc = Vm – 4.16 Idc / C

% r = 2.4 Idc / C Vdc x 100%

% r = 2.4 / C Rʟ x 100%

RC Filters

• The ripple of the filtered voltage can be reduced by connecting an additional RC filter section

• The purpose of the added RC section is to pass most of the dc component and attenuate the ac component as much as possible

Full Wave C - Filter circuit

FW Capacitor Filter

The simple capacitor filter shown in figure 3-20 consists of a single-filter element.

This capacitor (C1) is connected across the output of the rectifier in parallel with the load.

The RC charge time of the filter capacitor (C1) must be short and the RC discharge time must be long to eliminate ripple action when using this filter.

In other words, the capacitor must charge up fast with preferably no discharge at all.

Better filtering also results when the frequency is high; therefore, the full-wave rectifier output is easier to filter than the half-wave rectifier because of its higher frequency.

Full Wave Rectifier W/ and W/o Capacitor Filters

RC Filters

• The ripple of the filtered voltage can be reduced by connecting an additional RC filter section

• The purpose of the added RC section is to pass most of the dc component and attenuate the ac component as much as possible

Low Pass Filter

By definition, a low-pass filter is a circuit offering easy passage to low-frequency signals and difficult passage to high-frequency signals.

There are two basic kinds of circuits capable of accomplishing this objective, and many variations of each one: The inductive low-pass filter and the capacitive low-pass filter.

Low Pass Filters

Inductive vs. Capacitive

Inductive low-pass filter

The inductor's impedance increases with increasing frequency. This high impedance in series tends to block high-frequency signals from getting to the load.

Capacitive low-pass filter.

The capacitor's impedance decreases with increasing frequency. This low impedance in parallel with the load resistance tends to short out high-frequency signals, dropping most of the voltage across series resistor R1.

High Pass Filter

A high-pass filter's task is just the opposite of a low-pass filter:

to offer easy passage of a high-frequency signal and difficult passage to a low-frequency signal.

As one might expect, the inductive and capacitive versions of the high-pass filter are just the opposite of their respective low-pass filter designs.

High Pass Filter

Inductive vs. Capacitive

Inductive high-pass filter.The inductor's impedance

decreases with decreasing frequency.

This low impedance in parallel tends to short out low-frequency signals from getting to the load resistor.

As a consequence, most of the voltage gets dropped across series resistor R1.

Capacitive high-pass filter.

The capacitor's impedance increases with decreasing frequency. This high impedance in series tends to block low-frequency signals from getting to load.

Applications of HP and LP Filters

RC Filters

Voltage Regulators

• A voltage regulator is an electronic circuit that provides a constant dc output voltage that is essentially independent of the input voltage, output load current and temperature

• Voltage regulators fall under two categories, the linear regulators and the switching regulators

• Voltage regulators can be connected from discrete transistors and other components, but most regulators are now available in IC types

• The input to a regulator comes from a filtered output of a rectifier

Sample PCB Design

Zener Diode Regulator

What is Regulation?

• Regulation that is expressed in percentage is a figure of merit used to specify the performance of a voltage regulator. It can be expressed in terms of input (line) regulation or load regulation

• Line regulation specifies how much change occurs in the output voltage for a given change in the input voltage. It is defined mathematically as,

LR= ∆Vout/∆Vin x 100%• Load Regulation specifies how much change occurs in

the output voltage from the no-load to full-load condition

LR=Vɴʟ-Vғʟ / Vғʟ x 100%

How Regulated is a Regulator?

• The operation of a regulator can be evaluated using percent regulation

• The smaller the percentage, the better is the performance of the power supply

• An ideal power supply has a regulation of 0%, meaning, there is no change in the output voltage even the line voltage or the line current drawn by the load changes

Basic Series Regulator

Basic Series Regulator Circuit

• The series element controls the amount of input voltage that goes to the output

• The zener-follower is the simplest series voltage regulator

• Op-amps can be used to achieve better regulation• Any attempted change in the output voltage is offset by

the negative feedback

Basic Series Regulator Circuit

Basic Shunt Regulator

• The control element which is usually a transistor is in parallel with the load

• The simplest form is the zener diode circuit• Improved regulation can be achieved by using two

transistors or employing an op amp with negative feedback

Basic Shunt Regulator Diagram

Series vs. Shunt

Shunt Regulator

• Has built-in short circuit protection

• Has low efficiency• When efficiency is not

important, shunt regulators are used because of advantage of simplicity

Series Regulator

• More efficient for most applications

• Requires current limiting because it has no short circuit protection

• When larger load currents are needed, efficiency becomes more important and the series regulator is preferred

Short Circuit Protection and Foldback Current Limiting

Short Circuit Protection• The pass transistor of a series

regulator as well as the rectifier diodes can be damaged if the load terminals have been shorted or an excessive amount of load current is drawn

• To protect against accidental short across the load, series regulators employ some form of current limiting

• Current limiting reduces the load voltage when the current becomes larger than the limiting value

Foldback Current Limiting• Reduces both the output voltage

and the output current, protecting the load from overcurrent as well as protecting the regulator

• In effect, there is a reduced power dissipation in the pass transistor when the load terminals are shorted

Assignment

Research on :1. Switching Regulators

- Operation & Circuit

- Topologies

2. IC Voltage Regulators

- Fixed Positive Voltage Regulators

- Fixed Negative Voltage Regulators

- Adjustable Voltage Regulators

3. Comparison between Linear (series & shunt) vs. Switching regulators in terms of efficiency, noise and design