Power Supply

 

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MODULE 2 POWER SUPPLY Lesson 1

Power Supply Types and Parts

Lesson 2

Rectifier Circuit

Lesson 3

Filter Circuit

Lesson 4

Voltage Regulator Circuit

EMTC 124 Electronic Products Assembly and Servicing  

Module II

 

2 MODULE II ELECTRONIC PRODUCTS ASSEMBLY AND SERVICING

 

INTRODUCTION

This module focuses on how a power supply operates. It explains the functions of the different blocks of power supply. It also discusses and shows the difference on the principle of operation of the various types of rectifier circuits.

OBJECTIVES

After studying the module, you should be able to: 1. under understand stand the bl block ock di diagram agram of po power wer ssupply upply 2. recog recognize nize the fu function nctionss of the diff different erent pa parts rts of powe powerr suppl supply y 3. make a PCB de design sign o off a reg regulate ulated d powe powerr supp supply ly

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DIRECTIONS/ MODULE ORGANIZER 

There are four lessons in the module. Read each lesson carefully then answer the exercises/activities to find out how much you have benefited from it. Work on these exercises carefully and submit your output to your instructor. Discuss with your instructor in case you encounter difficulty. Good luck and happy reading!!!

EMTC 124 Electronic Products Assembly and Servicing

Module II

 

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Lesson 1

Power Supply Types and Parts supply   is an usedfrequency to changeand thecurrent electrical currentPower from supply  a source intoelectrical accuratedevice voltage, to supply the load. It is sometimes called electrical power converters. Power is the backbone of any electronic system and the power supply is what feeds the system. Choosing the right supply can be the critical difference between a device working at optimum levels and one that may deliver inconsistent results.

Types of Power Supply The power supply circuits are classified into different types based on the power they utilize in providing for circuits or devices. 1. SMPS – Switch Mode Power Supply An SMPS power supply or computer power supply is a type of power supply that includes a switching regulator for converting electrical-power powerfully. Similar to other power supplies, it transmits the power from a DC source or AC source to DC loads, such as a PC (personal computer), while changing the characteristics of current and voltage.

2. UPS - Uninterruptible Power Supply A UPS is an electrical device that permits a PC to keep working for some time as the main power supply is lost. This device is also given protection from power flow. It includes a battery to store the energy when the device detects a power loss from the main source.

EMTC 124 Electronic Products Assembly and Servicing

Module II

 

4 3. AC Power Supply Typically, an AC power supply acquires the voltage from the mains supply and the voltage can be step-up or stepdown by using a transformer to the required voltage and some filtering may take place. The different types of AC power supplies are designed to offer an almost stable current and output voltage may change based on the load’s impedance.

4. DC Power Supply A DC power supply is one that provides a consistent DC voltage to its load. Based on its plan, a DC power supply might be controlled from a DC supply or from an AC supply like the power mains.

5. RPS – Regulated Power Supply An RPS is a fixed circuit used to change unregulated ac into stable dc. Here, a rectifier is used to convert ac into dc. The sort of stabilization can be controlled to ensure that the output remains in certain restrictions beneath various load conditions.

6. Programmable Power Supply This type of power supply permits remote control for its operation via analog input otherwise digital interfaces like GPIB or RS232.The controlled properties of this supply include current, voltage and frequency. These type of supplies are used in a wide range of applications like fabrication of semiconductors, X-ray generators, monitoring of crystal growth and automated apparatus testing.

EMTC 124 Electronic Products Assembly and Servicing

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Parts of Power Supply Virtually every piece of electronic equipment, e.g., computers and their peripherals, calculators, TV and hi-fi equipment, and instruments, is powered from a DC power source, be it a battery or a DC power supply. Most of this equipment requires not only DC voltage but voltage that is also well filtered and regulated. An ideal power supply would be characterized by supplying a smooth and constant output voltage Power supply electronic circuits can be split into a number of sections or building blocks. Each is important to the operation of the power supply as a whole, but each section of the power supply electronics is required to perform its function satisfactorily for the successful operation of the whole unit. Below is the block diagram of a regulated power supply.

The main parts of a power supply are the following: a. Tr Tran ansf sfor orme merr – The transformer serves two primary purposes; a) to step-up or step-down the input line voltage, and b) to couple this voltage to the rectifier section. A basic power supply the input power transformer has its primary winding connected to the mains (line) supply. A secondary winding, electro-magnetically coupled but electrically isolated from the primary is used to obtain an AC voltage of suitable amplitude, and after further processing by the PSU, to drive the electronics circuit it is to supply.

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The transformer stage must be able to supply the current needed. If too small a transformer is used, it is likely that the power supply's ability to maintain full output voltage at full output current will be impaired. With too small a transformer, the losses will increase dramatically as full load is placed on the transformer.

b. Re Rect ctif ifie ierr – This section converts the ac signal to pulsating dc voltage. There are several forms of rectifiers used but all are made up of diodes. Pulsating dc voltage is not desirable to use so the signal must be transferred to the next section which is the filter section. c. Filter – It is used to smooth out the pulsations/ripples of the rectified voltage to a level of dc voltage. It converts the pulsating dc voltage to filtered dc voltage. The most common filter is a large capacitor. The filter section contains a network of resistors, capacitors or inductors that controls the rise and fall time of the varying signal so that the signal remains at a more constant dc level. d. Regulator – It is used to control/deliver and maintain a fixed or constant voltage to the load. maintains thechanges output of the power supplyand at current a constant level in It spite of large in load current or in input line voltage. Depending upon the design of the equipment, the output of the regulator will maintain a constant dc voltage within certain limits.

EMTC 124 Electronic Products Assembly and Servicing

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Lesson 2

 

Rectifier Circuit

In electronics, Rectifier circuit is the most used circuit because almost every electronic appliances operate on DC but the availability of the DC sources are limited as such as electrical outlets in our home provides AC. The rectifier is the perfect candidate for this job in industries and at home to convert AC into DC. Even our cellphone chargers use rectifiers to convert the AC from our home outlets to DC. Different types of rectifiers are used for specific applications.

Rectifier is an electronic device that is made of one or more diodes that converts the alternating current into direct current. It is used for rectification – the process of conversion of the ac into dc. Based on the type of rectification circuit does, the rectifiers are classified into two categories: a. HalfHalf-wave wave rrectif ectifier ier – onl only y conve converts rts hal halff of the AC w wave ave into D DC C signal b. FullFull-wave wave rrectifi ectifier er – con converts verts c comple omplete te AC sig signal nal into D DC C

1. Half-Wave Rectifier Circuit rectifier is a type of rectifier which converts half of the A half-wave rectifier is AC input signal (positive half signal) into pulsating DC output signal and the remaining half signal (negative half diode. cycle) is blocked or lost. In half-wave rectifier circuit, we use only a single The half wave rectifier circuit can often be used with a transformer if it is to be used for powering equipment in any way. Normally in this application the input alternating waveform is provided via a transformer. This is used to provide the required input voltage. During the first half cycle (positive half cycle), point A is positive and point B is negative. The diode D1 is at forward bias causing it to allow the passage of current and producing a voltage drop across the load.

EMTC 124 Electronic Products Assembly and Servicing

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During the 2nd  half cycle (negative half cycle), point A becomes negative and point B is positive. D1 becomes reverse biased and no conduction takes place and no voltage drop across the load.

It is important to check the polarity when replacing diodes. If they are replaced in the wrong polarity, a reversal in output voltage will cause damage to circuits connected in the load.

This type of rectifier is cheap but is only suitable for fairly nondemanding uses. The DC voltage produced by the single diode is less than with the other systems, limiting the efficiency of the power supply, and the amount of AC ripple left on the DC supply is generally greater.

2. Full Wave Rectifier Circuit In half-wave rectifiers, half of the power provided by the source is not used. To solve this problem, we have to use full-wave rectifiers. The EMTC 124 Electronic Products Assembly and Servicing

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9 minimum full-wave rectifier is composed of two diodes, but it requires a center-tapped transformer. A bridge rectifier composed of four diodes that can use a “normal” transformer. The full wave rectifier circuit is one that is widely used for power supplies and many other areas where a full wave rectification is required.

Full-wave rectifier  rectifier  is a type of rectifier which converts the full AC input signal (positive half cycle and negative half cycle) to pulsating DC output signal. Unlike the half-wave rectifier, the input signal is not wasted in full-wave rectifier. The efficiency of full-wave rectifier is high as compared to the half-wave rectifier. There are two main forms of full wave rectifier circuit that can be used: a) Center-tapped full wave rectifier or Two Diode Full wave Rectifier, b) Full-wave Bridge Rectifier. Each type has its own features and is suited for different applications.

A. Center-tapped full wave rectifier or Two Diode Full wave Rectifier The two diode full wave rectifier circuit is not so widely used with semiconductor diodes as it requires the use of a center-tapped transformer. A transformer is used whose secondary winding is split equally into two halves with a common center tapped connection. Utilizing a center-tap in the transformer means only half the full voltage across the two halves of the wind together can be utilized. This transformer is more efficient which produces two anti-phase outputs, as shown in the figure below.

EMTC 124 Electronic Products Assembly and Servicing

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During the first half cycle, point A is positive and point B is negative. The diode D1 will allow passage of current because it establishes a forward bias. The diode D2 will not allow current flow since it is at reverse bias.

During the 2nd half cycle, the polarities at point A and point B reverse. Point A becomes negative, and Point B positive. The current path will be through D2 this time because D2 is at forward bias while D1 is at reverse bias. The concept of the full wave rectifier is that it utilizes both halves of the waveform to provide an output and this greatly improves its efficiency.

Comparison of full and half wave rectifier circuits A further advantage when used in a power supply is that the resulting output is much easier to smooth. When using a smoothing capacitor, the EMTC 124 Electronic Products Assembly and Servicing

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11 time between the peaks is much greater for a half wave rectifier than for a full wave rectifier. Below is the smoothed waveform from diode rectifier circuit.

It can be seen from the figure above, that the fundamental frequency within the rectified waveform is twice that of the source waveform - there are twice as many peaks in the rectified waveform. This can often be heard when there is a small amount of background hum on an audio circuit.

Full wave rectifier advantages a. Altho Although ugh the ful fulll wave recti rectifier fier cir circuit cuit req requires uires mo more re diode diodess than a half wave rectifier circuit, it has advantages in terms of utilizing both halves of the AC waveform to provide the output. b. The majo majorr advan advantage tage of a fullfull-wave wave re rectifie ctifierr is the delive delivery ry of twice as many peaks to the load. c. Full wav wave e devel develops ops an out output put cycle ffrequ requency ency of 12 120Hz, 0Hz, as co compar mpared ed to the 60Hz frequency of the half-wave rectifier. d. Easie Easierr to provi provide de smoot smoothing hing as a res result ult of ripp ripple le freq frequency uency

Full wave rectifier disadvantages a. Becau Because se a center ta tapped pped tra transfor nsformer mer is used used,, the voltag voltage e delive delivered red to the load will only be ½ of the secondary voltage. b. More compl complicate icated d than half wave recti rectifier. fier. c. The twi twice ce freq frequency uency h hum um on an au audio dio circ circuit uit may b be e more a audibl udible. e.

B. Full-Wave Bridge Rectifier Another type of circuit that produces the same output waveform as Rectifier. This the full wave rectifier circuit is that of the Full Wave Bridge Rectifier. EMTC 124 Electronic Products Assembly and Servicing

Module II

 

12 type of single phase rectifier uses four individual rectifying diodes connected in a closed loop "bridge" configuration to produce the desired output. The main advantage of this bridge circuit is that it does not require a special center tapped transformer, thereby reducing its size and cost.

Often four diode bridges can be bought as single items, making construction of the overall circuit much simpler. The image to the right shows a typical single phase bridge rectifier with one corner cut off. This cut-off corner indicates that the terminal nearest to the corner is the positive or +Ve output terminal or lead with the opposite (diagonal) lead being the negative or -Ve output lead. The other two connecting leads are for the input alternating voltage from a transformer secondary winding.

During the first half cycle, D1 and D3 is at forward bias; D2 and D4 at reverse bias. The current path is through D1 then to the load, the current flows to D3 and then returns to the positive part of the t he transformer.

During the next half-cycle, the polarities at points A and B reverse. Point A is negative and Point B is positive. Now, D2 and D4 are at forward bias; and D3 and D1 at rreverse everse bia bias. s. The importan importantt point he here re is that, during the both halves of the input cycle, current flows through the load.

EMTC 124 Electronic Products Assembly and Servicing

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Advantage over Center-tapped full wave rectifier It has a higher output voltage. This results because the bridge rectifier uses the whole secondary, rather than splitting the voltage in half as the full-wave rectifier does.

Lesson 3

Filter Circuit filter   is used in power supplies to eliminate the AC fluctuation A filter present in the rectification process of a Power Supply. It is used to smooth out the pulsations to a constant level of dc voltage. This filter may be a capacitor. We need to filter pulsating dc because amplifiers, oscillators and other load circuits require a smooth steady flow. The ideal waveform into which the pulsating dc should be converted is like the voltage from the EMTC 124 Electronic Products Assembly and Servicing

Module II

 

14 battery. To accomplish this task, capacitors and inductors are used to smooth out the pulsating dc look like smooth dc voltage at the output. This filtering.. is called filtering Half-wave Capacitive Filter

During the input cycle, the capacitor charges to the peak applied voltage. Here, the waveform has reached its peak and then begins to head toward negative current flow in the circuit (A). The energy that has been stored on the capacitor is discharged through the load. The discharge cycle is shown by the lines connected between the peaks (B). This discharge process supplies a steady current to the load. If a steady current flow is seen at the loop, a steady voltage is developed. Before the capacitor has a chance to fully discharge, a new input cycle starts. This new cycle begins to discharge the capacitor (C).

Full-wave Capacitive Filter EMTC 124 Electronic Products Assembly and Servicing

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Ripple  – it is the variation of voltage which is developed in any filtering Ripple  action.

A capacitor-input filter will charge and discharge such that it fills in the “gaps” between each peak. This reduces variations of voltage. This voltage variation is called ripple voltage. The advantage of a full-wave rectifier over a half-wave is quite clear. The capacitor can more effectively reduce the ripple when the time between peaks is shorter.

Factors/conditions of an Effective Capacitive Filter Note:  To be an effective filter, the capacitor must charge quickly, Note:  and then discharge slowly through the load. pulses (frequency of powersupply) a. Time between pulses (frequency

EMTC 124 Electronic Products Assembly and Servicing

Module II

 

16 In half-wave or full-wave circuits, there is a timespan between the peaks of a waveform.

For the half-wave circuit, the time between pulses is 16.6ms which is equivalent to 60Hz frequency. In the full-wave circuit, the pulses arrive twice as often. Therefore, the frequency for this circuit is 8.3 ms or 120 Hz. Because of this, the half-wave rectifier will have a larger ripple than the full-wave rectifier since its capacitor has a longer discharge period. b. Size of the capacitor The larger filter capacitor, the lower the ripple developed in the output of the powersupply. Because of the large capacitance needed for filtering, electrolytic capacitor are used. Thesecapacitors will charge quickly when the diodes are forward biased, and discharge through the load slowly. If the filter capacitor is too large, it may cause damage due to the large amounts of current required to charge the capacitor. Such large amount of current could cause the transformer or the rectifiers to over heat, resulting in failure of these components.

c. Size of the load If the load resistance decreases, the discharge time of the filter capacitor is reduced.

Lesson 4

 

Voltage Regulator Circuit

Voltage Regulators EMTC 124 Electronic Products Assembly and Servicing

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These are integrated circuits designed to regulate the voltage at their input to a constant, fixed voltage at their output, irrespective of changes in load current or input voltage. It is a system designed to automatically maintain a constant voltage level. Electronic designs are usually made up of diverse electronic components operate at of different voltage levels. As such, to reliably meetthat thesometimes power requirements a particular design or that of the different components within the design, voltage regulators are usually employed in the power supply unit to regulate the voltage at the main source to what is needed across different sections of the device

Types of Voltage Regulators Voltage regulators can be categorized based on different factors like their applications, voltages at which they operate, power conversion mechanisms, and many more. 1. Linear Voltage Regulators Linear voltage regulators use the principles of voltage dividers to transform the voltage at their input to the desired voltage at their output. They employ a feedback loop that automatically varies the resistance in the system to counter the effect of variations in the load impedance and input voltage, all to ensure the output voltage is kept constant. In situation where the voltage at the input or the load current at the output is too high, the regulators could generate heat levels that may lead to its breakdown. To mitigate this, designers usually employ heat sinks. Another point is the need for the voltage at the input to be greater than the voltage at the output by a minimum value called the drop-out voltage. This voltage value is usually around 2V and is sometimes a major source of concern for designers working on low power applications due to power loss. Some popular examples of linear voltage regulators include: 78xx (e.g. L7805 (5V), L7812 (12V) series of voltage regulators.

Advantages of using a Linear Voltage Regulator a. Simpl Simple, e, easy to d design esign and imple implement ment b. Gene Generates rates a lowe lowerr amo amount unt of EMI a and nd no noise ise c. Fast re respons sponse e time to c change hangess in load cu curren rrentt or inpu inputt voltag voltage e conditions d. Low rripp ipple le vo volta ltage ge at th the e outp output. ut. EMTC 124 Electronic Products Assembly and Servicing

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Dis-advantages of using a Linear Voltage Regulator a. Low effi efficienc ciency y as a large amo amount unt of electrica electricall energ energy y is wasted as heat b. DropDrop-out out volt voltage age req requirem uirement ent make makess them a bad choi choice ce for low power applications c. Occup Occupy y more space on PCB due to ttheir heir n need eed fo forr heat sinks.

2. Switching Voltage Regulators Although they feature a more complex design and require more components to function, switching voltage regulators are super-efficient regulators in scenarios where power loss cannot be condoned. The voltage regulation mechanism in switching voltage regulators involves rapidly switching an element connected in series with an energy EMTC 124 Electronic Products Assembly and Servicing

Module II

 

19 storage component (capacitor or inductor) to periodically interrupt the flow of current and transform the voltage from one value to the other. How this is done depends on the signal from a feedback mechanism like the one employed in the linear regulator. Unlike in linear voltage regulators, the switching element is either in a fully conducting or switched-off state. It dissipates no power and allows the regulator to attain a high level of efficiency beyond that of the linear voltage regulator.

Advantages of using a Switching Voltage Regulator a. Sm Smal alll siz size b. Hi High gh ef effi fici cien ency cy c. Fit Fitss low low pow power er app applic licati ation on d. They can pr provide ovide an ou output tput vol voltage tage tha thatt is greate greaterr than or les lesss than the input voltage

Dis-advantages of using a Switching Voltage Regulator a. b. c. d. e.

Co Comp mpli lica cate ted d de desi sign gn Ex Expe pen nsive sive Requi Requires res more elect electronic ronic compo components nents Hig High h out output put v volt oltage age rip ripple ple High EMI and n noise oise gener generation ation rate ratess

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MODULE SUMMARY

EMTC 124 Electronic Products Assembly and Servicing

Module II

 

21 In module 2, you have learned about what a power supply is. You have learned their functions, types, and the function of each part/block. You have also learned the different types of rectifier and voltage regulator. There are four lessons in module 2. Lesson 1 consists of two main parts focused on the types of power supply, as well as the block diagram or the parts of power supply. Lesson 2 tackles the types of rectifier circuit. Half-wave rectifier with one diode; and the two types of full-wave rectifier: Full-wave center-tapped with two diodes and full-wave bridge rectifier with four diodes. Lesson 3 deals with how a filter circuit works in a power supply. It also has two types, the half-wave and the full-wave capacitive filter. It also discusses the factors or conditions of an effective capacitive filter. Lesson 4 discusses the types of voltage regulator circuit. It includes the advantages and disadvantages of linear voltage regulator and switching voltage regulator. Congratulations! You have just studied Module 2. now you are ready to evaluate how much you have benefited from your reading by answering the summative test. Good Luck!!!

EMTC 124 Electronic Products Assembly and Servicing

Module II