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switch mode power supply full report
Post: #1

The report is an overview of Switch Mode Power Supply abbreviated as SMPS. The purpose of report is to provide a general understanding of SMPS and its applications. There is a brief discussion of historical development of SMPS, followed by the common topologies of the same. The principle of operation is described which is almost same for all the topologies. Then the steady state analysis is discussed to have an insight of the operation under ideal and real conditions. The details of operation and output of common SMPS topologies is presented. The comparison of SMPS and Linear Regulators is shown which justifies the continuously increasing use of the SMPS. Everything has a dark side too. SMPSs are not the exception. They too have drawbacks and are discussed. Finally the wide areas of application of SMPS in present scenario are discussed.
Submitted by Raj Rakesh Roll no: U06EE542
B.Tech IV
Year 2009 -2010
Switch Mode Power Supplies i.e. SMPSs are the consequence of the never ending urge of smaller and lighter yet efficient power supply to our electrical and electronic devices. The majority of electronic DC loads are supplied from standard power sources. Unfortunately, standard source voltages may not match the levels required by microprocessors, motors, LEDs, or other loads, especially when the source voltage is not regulated. Battery-powered devices are prime examples of the problem: the typical voltage of a standard Li+ cell or NiMH stack is either too high/low or drops too far during discharge to be used in conventional applications. Considering the multiple DC voltage levels required by many electronic devices, we need a way to convert standard power-source potentials into the voltages dictated by the load. Voltage conversion must be a versatile, efficient, reliable process. Switch-mode power supplies (SMPSs) are frequently used to provide the various levels of DC output power needed for modern applications, and are indispensable in achieving highly efficient, reliable DC-DC power-conversion systems. The versatility of SMPSs solves the problem of converting a standard source voltage into a usable, specified output voltage. There are numerous SMPS topologies, which are classified into fundamental categories”these power supplies step up, step down, invert, or even step up and down the input voltage. Unlike linear regulators, which can only step down an input, SMPS are attractive because a topology can be selected to fit nearly any output voltage. Switch Mode power supplies use the principle of quantized power transfer to implement voltage regulation. Through the control of transistors operating as switches (on or off) with energy storage components such as inductors and capacitors, a switch mode power supply transfers just enough energy from the input to the output to achieve the desired output voltage and currents. High efficiency is the principal advantage with SMPS designs, other benefits naturally occur as a direct result of minimizing power loss. For example, a reduced thermal footprint is observed in the SMPS when compared to its less efficient counterparts. This benefit equates to reduced thermal-management requirements. Also, lifetime increases due to improved reliability, because components are not subjected to excessive heat, as they would be in a less efficient system.
The high efficiency afforded by SMPSs is not without its penalties. The most often cited issue regarding switch-mode converters is their propensity to radiate electromagnetic interference (EMI) and conduct noise. They can be quite complex and require additional external components too, both of which can equate to an increase in overall cost of the power supply. Despite these issues, SMPSs are widely used in numerous applications. The disadvantages can be managed, and the efficiency and versatility gained from their use is very desirable, and often required. They are the answer to our never ending urge of smaller and efficient power supply requirements.
Earlier developed models of SMPSs were highly ineffective. In the mid 1960s, it was popular to say that the switch mode power supplies were one microsecond away from disaster. And there were plenty of evidences to support this cynical comment. Even the manufacturers did not completely understand the various failure mechanisms of their new bipolar power transistors. And the users tend to worsen the problem by doing things like connecting these devices in parallel for increased power handling capacity. Designs that seem to be perfect in lab failed in field. On the other hand if the design worked well in the field, the designers have no idea why it did. In fact they may not have been so called power supply designers at all, but rather general purpose engineers who have to design their own power supplies as a necessary evil along with their other more important modules. Or they may have been young engineers who were handed power supply designs as learning experiences. Therefore, despite the apparent size, weight and efficiency advantages of SMPSs, it took many years for them to be generally accepted. By contrast, today the high reliability of SMPSs is taken for granted. The main factors working behind the evolution are: Materials used for the manufacturing are better. The devices are better and their general understanding has improved too. The manufacturing techniques are far superior and precise. Integrated circuit controllers, well supported by excellent application notes are available today. An overall improved design i.e. well electrical, mechanical and thermal design. Now the designers are more equipped with several simulation softwares and have a sound background with specialization in power electronics devices. Presently, SMPSs are extensively used in electrical and electronic devices which have application in industries as well as in our day to day life. They have became more reliable and efficient with the recent evolution of SMPS.
Fig. 2.1.An SMPS used for computer
Fig. 2.2.Internal circuit of an SMPS
SMPSs can convert a DC input voltage into a different DC output voltage, depending on the circuit topology. There are numerous SMPS topologies used in the engineering world but three are fundamental and seen most often. These topologies are classified according to their conversion function: Step-down (Buck). Step-up (Boost). Step-up/down (Buck-boost or Inverter).
Fig. 3.1.Common SMPS topologies showing charge-discharge paths
All three fundamental topologies include a MOSFET switch, a diode, an output capacitor, and an inductor. The MOSFET, which is the actively controlled component in the circuit, is interfaced to a controller (not shown). This controller applies a pulse width- modulated (PWM) square-wave signal to the MOSFET's gate, thereby switching the device on and off. Doing so it varies the duty cycle D of the square wave signal which directly affects the output voltage of the SMPS. 12
¦. (3.1)
Fig.3.2 Different sections of a commercially used SMPS
To maintain a constant output voltage, the controller senses the SMPS output voltage and varies the duty cycle (D) of the square-wave signal, dictating how long the MOSFET is on during each switching period (TS). The value of D, which is the ratio of the square wave's on time to its switching period (TON/TS), directly affects the voltage observed at the SMPS output. The on and off states of the MOSFET divide the SMPS circuit into two phases: A charge phase and a discharge phase, both of which describe the energy transfer of the inductor (path loops are shown in Figure 3.1). Energy stored in the inductor during the charging phase is transferred to the output load and capacitor during the discharge phase. The capacitor supports the load while the inductor is charging and sustains the output voltage. This cyclical transfer of energy between the circuit elements maintains the output voltage at the proper value, in accordance with its topology.
Fig 4.1.Voltage and current characteristics for a steady-state inductor
The inductor is central to the energy transfer from source to load during each switching cycle. Without it, the SMPS would not function when the MOSFET is switched. The energy (E) stored in an inductor (L) is dependent upon its current (I): E = 0.5 L*I2 ¦. (4.1) Therefore, energy change in the inductor is gauged by the change in its current (IL), which is due to the voltage applied across it (VL) over a specific time period (T): IL = VL*T/ L ¦.(4.2) The (IL) is a linear ramp, as a constant voltage is applied across the inductor during each switching phase (Figure 4.1). The inductor voltage during the switching phase is determined by performing a Kirchhoff™s voltage loop, paying careful attention to polarities and VIN/VOUT relationships. For example, inductor voltage for the step-up converter during the discharge phase is -(VOUT - VIN). Because VOUT > VIN, the inductor voltage is negative. During the charge phase, the MOSFET is on, the diode is reverse biased, and energy is transferred from the voltage source to the inductor (Figure 3.1). Inductor current ramps up because VL is positive. Also, the output capacitance transfers the energy it stored from the previous cycle to the load in order to maintain a constant output voltage. During the discharge phase, the MOSFET turns off, and the diode becomes forward biased and, therefore, conducts. Because the source is no longer charging the inductor, the inductor's terminals swap polarity as it discharges energy to the load and replenishes the output capacitor (Figure 3.1). The inductor current ramp down as it imparts energy, according to the same transfer relationship given previously. The charge/discharge cycles repeat and maintain a steady-state switching condition. During the circuit's progression to a steady state, inductor current builds up to its final level, which is a superposition of DC current and the ramped AC current (or inductor ripple current) developed during the two circuit phases (Figure 4.1). The DC current level is related to output current, but depends on the position of the inductor in the SMPS circuit. The ripple current must be filtered out by the SMPS in order to deliver true DC current to the output. This filtering action is accomplished by the output capacitor, which offers little opposition to the high-frequency AC current. The unwanted output ripple current passes through the output capacitor, and maintains the capacitor's charge as the current passes to ground. Thus, the output capacitor also stabilizes the output voltage. In non 15
ideal applications, however, equivalent series resistance (ESR) of the output capacitor causes output-voltage ripple proportional to the ripple current that flows through it. So, in summary, energy is shuttled between the source, the inductor, and the output capacitor to maintain a constant output voltage and to supply the load.
To be in a steady state, a variable that repeats with period T S must be equal at the beginning and end of each period. As the inductor current is periodic, due to the charge and discharge phases described previously, the inductor current at the beginning of the PWM period must equal inductor current at the end. This means that the change in inductor current during the charge phase (ICHARGE) must equal the change in inductor current during the discharge phase (IDISCHARGE). Equating the change in inductor current for the charge and discharge phases, an interesting result is achieved, which is also referred to as the volt second rule: |ICHARGE| = |IDISCHARGE| |VCHARGE*D* TS/L| = |VDISCHARGE *(1 “ D)*TS/L| |VCHARGE|*D* TS = |VDISCHARGE | *(1 “ D)*TS Thus the voltage-time product during each circuit phase is equal. This means that, by observing the SMPS circuits of Figure 3.1, the ideal steady-state voltage-/currentconversion ratios can be found. For the step-down circuit, a Kirchhoff's voltage loop around the charge phase circuit reveals that inductor voltage is the difference between VIN and VOUT. Likewise, inductor voltage during the discharge phase circuit is -VOUT. Using the volt-second rule from equation 5.1, voltage-conversion ratio is determined. VCHARGE = VIN - VOUT. VDISHARGE = -VOUT. Thus we have, |VIN - VOUT | * D = |-VOUT | * (1 “ D) VOUT /VIN = D ¦.(5.2) ¦.(5.1)
Further, input power (PIN) equals output power (POUT) in an ideal circuit. Thus, the current-conversion ratio can be determined. PIN = POUT VIN *IIN = VOUT *IOUT IIN /IOUT = D ¦.(5.3)
Topology Buck Boost Inverter
VCR D 1/(1-D) D/(1-D)
CCR D 1/(1-D) D/(1-D)
Table 5.1. VCR and CCR of common topologies
The Buck converter is a step-down converter that changes a higher input voltage to a lower output voltage.
Fig.6.1 A Buck converter in switch on mode
In the presented figure transistor Q1 is turned on by the PWM signal. When Q1 is turned on, the current begin to flow from V IN through the transistor, through the inductor L1, and into the output capacitor and the load. The inductor L1 controls the current flow. The applied voltage across the inductor causes the current to increase linearly with time. This process is "Charging Up the Inductor". The inductor current flow charges the output 18
capacitor which raises the output voltage. A control circuit (not shown in the figure) monitors the output voltage, and when the output voltage reaches the desired value, the PWM signal is deasserted. It should be taken care that the frequency of the PWM signal must be high enough to insure than the current through the inductor L1 does not become too large.
Fig.6.2 A Buck converter in switch off mode
This figure shows the Buck converter after the PWM signal is deasserted and the transistor Q1 is turned off. The current no longer can flow from the V IN source, but the inductor current must continue to flow. As Q1 turns off, the inductor pulls current from the ground through the diode (D1). The voltage across the inductor is now reversed, so the current begins to decrease linearly with time. The inductance value must be larger enough for the given PWM frequency to insure that the inductor current does not drop to zero before the start of the next PWM cycle. If the current was to drop to zero, then the control mode is called "Discontinuous". The discontinuous mode of operation can be more difficult to control than the "Continuous" 19
mode where the current through the inductor is always greater than zero. The choice of the inductor value relative to the PWM frequency is important. A larger inductance value makes L1 physically larger and heavier, but it will reduce the current ripple that flow into and out of the output capacitor, reducing the resultant ripple voltage, and reducing the heat dissipation by the output capacitor. The output voltage level of the converter is given by: VOUT= VIN*D ¦. (6.1)
The "Boost" converter is similar to a Buck converter but instead of stepping down the input voltage, the output voltage is higher than the input voltage.
Fig.7.1 A Boost converter in switch on mode
When the transistor Q1 is turn on, the VIN voltage is applied across the inductor which 20
causes the inductor current to increase. While the current is flowing through L 1 and Q1, the inductor is being "charged up". While Q1 is turned on, the diode D1 is reversed biased so no current flow through the diode. The output capacitor COUT supplies the current to the load. As compared to a Buck converter, a Boost converter places more ripple current on the output capacitor. The output capacitor must be sized large enough to supply all of the load current while the transistor is turned on and still meet the output voltage ripple requirements.
Fig.7.2 A Boost converter in switch off mode
In the Boost converter after the transistor is turned off, the inductor's current will continue to flow. The inductor current will forward bias the diode and the current flows into the output capacitor and the load. The inductor's current flows "Up-Hill", charging the output capacitor, and raises the output voltage. The output voltage level of the converter is given by: VOUT= VIN/ (1-D) ¦. (7.1) 21
The output voltage will be less than the ideal equation because of voltage drops across the inductor and the diode. Boost converters typically operate in a Discontinuous mode where the inductor current drops to zero before the start of the next PWM cycle as compared to Buck converters that usually operate in a Continuous mode. Operation of a Boost converter in Continuous mode may experience oscillations unless the bandwidth of the control loop is greatly reduced, and the inductor current is properly limited.
The Buck-Boost converter is similar to the Boost converter except that a negative output voltage is generated. The Buck-Boost converter, as compared to the Buck and the Boost converters, is the only converter where there is no direct current flow from the input supply (VIN) to the output load. All of the energy transfer is via the inductor L1.
Fig.8.1 A Buck-Boost converter in switch on mode
When Q1 turns on, the current flows through transistor and the inductor, charging up the 22
stored energy in the inductor. While Q1 is turned on, the diode D1 is reverse biased and no current flows through D1. The output capacitor COUT must supply all of the current to the load at this time.
Fig.8.2 A Buck-Boost converter in switch off mode
When the transistor Q1 turns off, the inductor current flow will forward bias the diode D1 causing it to conduct. The current flows from the load, through the diode, and then through the inductor. The output voltage level of the converter is given by: VOUT= VIN*D/ (1-D) ¦. (8.1) 23
Buck-Boost converters typically operate in a Discontinuous mode where the inductor current drops to zero before the start of the next PWM cycle as compared to Buck converters that usually operate in a Continuous mode. Operation of a Buck-Boost converter in Continuous mode may experience oscillations unless the bandwidth of the control loop is greatly reduced, and the inductor current is properly limited.
Apart from the three basic topologies, there is one more very important topology: PushPull Converters. The "Push-Pull" converter is a transformer based converter that is typically used for higher power applications. By using a transformer, any combination of input to output voltages and polarities is achievable. The Push-Pull converter's transformer enables any input to output voltage ratio to be obtained. The transformer also provides isolation between the input and output terminals, so any polarity between the input and output terminals is possible. The Push-Pull converter has two transistors that operate in an alternating fashion. One transistor cycles on and off for one PWM period, and then the other transistor cycles on and off on the next PWM cycle. The current flow through alternating windings in the transformer builds the magnetic flux in one direction and then in the other direction. This action resets the magnetic flux on every cycle so that very high duty cycles can be used.
Fig.9.1 A Push-Pull converter in when Q1 is on
When Q1 is turned on, the current flow through the transformer transfers energy to the secondary winding of the transformer. The current flows from the secondary winding through the diode D1 and into the inductor, COUT, and the load. When Q1 is turned off, the inductor current will continue to flow through the transformer winding and diode D1 until the inductor current drops to zero, or the diode become reversed biased by the action of Q2 turning on.
Fig.9.2 A Push-Pull converter in when Q2 is on
When Q2 is turned on, the current flow through the transformer transfers energy to the secondary winding of the transformer. The current flows from the secondary winding through the diode D2 and into the inductor, COUT, and the load. When Q2 is turned off, the inductor current will continue to flow through the transformer winding and diode D2 until the inductor current drops to zero, or the diode become reversed biased by the action of Q1 turning on. The output voltage level of the converter is given by: VOUT= VIN*D* (NS/NP) ¦. (9.1) Where, NS = Number of secondary winding turns NP = Number of primary winding turns The transformer provides several advantages: The transformer allows any input to output voltage ratio to be obtained. This is very useful in situations where the input voltage may vary higher and lower than the output voltage such as in battery powered applications. Buck and Boost converters do not provide this capability.
The transformer turns ratio also enables the designer to optimize power efficiency in MOSFET based applications by maximizing the duty cycle and minimize I2R losses. The transformer provides isolation between the input and output terminals. The isolation permits output polarity independence of the input terminals. The transformer isolation capability can provide safety isolation between low voltage user accessible circuitry and high voltage circuitry.
The disadvantage of the transformer is that it increases the cost, weight, and size of a product while decreasing the efficiency of the power converter.
Switching and linear regulators use fundamentally different techniques to produce a regulated output voltage from an unregulated input. Each technique has advantages and disadvantages, so the application will determine the most suitable choice. Linear power supplies can only step-down an input voltage to produce a lower output voltage. This is done by operating a bipolar transistor or MOSFET pass unit in its linear operating mode; that is, the drive to the pass unit is proportionally changed to maintain the required output voltage. Operating in this mode means that there is always a headroom voltage, VDROP, between the input and the output. Consequently the regulator dissipates a considerable amount of power, given by (VDROP *ILOAD). This headroom loss causes the linear regulator to only be 35 to 65 percent efficient. For example, if a 5.0 V regulator has a 12 V input and is supplying 100 mA, it must dissipate 700 mW in the regulator in order to deliver 500 mW to the load, an efficiency of only 42 percent. The cost of the heat sink actually makes the linear regulator uneconomical above 10 watts for small applications. Below that point, however, linear regulators are cost effective in step down applications A low drop-out (LDO) regulator uses an improved output stage that can reduce VDROP to considerably less than 1.0 V. This increases the efficiency and allows the linear regulator to be used in higher power applications. Designing with a linear regulator is simple and cheap, requiring few external components. A linear design is considerably quieter than a switcher since there is no high-frequency switching noise. Switching power supplies operate by rapidly switching the pass units between two efficient operating states: cutoff, where there is a high voltage across the pass unit but no current flow; and saturation, where there is a high current through the pass unit but at a very small voltage drop. Essentially, the semiconductor power switch creates an AC voltage from the input DC voltage. This AC voltage can then be stepped-up or down by transformers and then finally filtered back to DC at its output. Switching power supplies are much more efficient, ranging from 65 to 95 percent. The downside of a switching design is that it is considerably more complex. In addition, the output voltage contains switching noise, which must be removed for many applications. Although there are clear differences between linear and switching regulators, many applications require both types to be used. For example, a switching regulator may 28
provide the initial regulation, then a linear regulator may provide post-regulation for a noise-sensitive part of the design, such as a sensor interface circuit.
Chapter 11: Advantages of SMPS
The major advantages of SMPS are: Higher Efficiency. SMPS has higher efficiency, almost 90% which is too high as compared to 50% efficiency of linear regulators. While a linear regulator maintains the desired output voltage by dissipating excess power in a pass power transistor, the SMPS switches a power transistor between saturation and cut off region. Thus saving a lot of power as transistor dissipates very little power when it is outside the active region. Compactness and Light Weight. SMPS switches at a much higher frequency (tens to hundreds of kHz) than that of the AC line (mains). This means that the transformer that it feeds can be much smaller than one connected directly to the line/mains. So the low frequency transformers which are bulky and heavy weight are eliminated, reducing the size of SMPS. Less Thermal Management Requirement. Thermal management requirements of SMPS are comparatively lesser due to the low power loss. Reduced thermal footprint is observed in the SMPS when compared to its less efficient counterparts. Easier PFC support. Power Factor Correction (PFC) is becoming a government mandated requirement for power supplies in countries around the world. PFC is the process that insures that the input voltages and currents from the AC power line into a power supply are in phase to achieve a Unity Power Factor. PFC is very costly to achieve in a linear power Supply. Enhanced Lifetime and Reliability. All these factors like less loss, higher efficiency, lesser thermal footprints, considered together make the SMPS much reliable and increases their lifetime.
Chapter 12: Drawbacks of SMPS
Every coin has two sides; same is the case with SMPS too. The high efficiency afforded by SMPSs is not without its penalties. EMI radiation and noise conduction. Electromagnetic radiation is caused by the fast transitions of current- and voltageswitching waveforms that exist in SMPS circuits. Rapidly changing voltages at the inductor node cause radiated electric fields, while fast-switching currents of the charge/discharge loops produce magnetic fields. Conducted noise, however, is propagated to input and output circuits when SMPS input/output capacitances and PCB parasitics present higher impedances to switching currents. Complexity in circuit design. SMPSs can be quite complex and require additional external components, both of which can equate to an increase in overall cost of the power supply. But good component placement and PCB layout techniques take good care of the EMI and noise problems. Choosing correct components according to the datasheet of the SMPS ICs may keep the complexities away.
SMPSâ„¢s are having wide range of applications. Some of them are: Machine tool industries. Security systems (Close Circuit Cameras). In computers and other electronic accessories. Support supplies with PLCâ„¢s. ESPs of power plants.
1. An introduction to Switch Mode Power Supply Maxim journal dated September 27, 2007; application note 4087. 2. An introduction to Switch Mode Power Supply A journal from Microchip Technology. 3. Switch Mode Power Supply Rev. 3A, July-2002 reference manual from ON Semiconductor. 4. Switch Mode Power Supply A book by A. K. Maini. 5. From Wikipedia. 6. id=vUNb3PCRRu0C&printsec=frontcover&dq=switch+mode+power+supply#v= onepage&q=&f=true From Google Books

2.1 2.2 3.1 3.2 4.1 6.1 6.2 7.1 7.2 8.1 8.2 9.1 9.2 Internal circuit of an SMPS. Common SMPS topologies showing charge discharge paths. Different sections of a commercially used SMPS. Voltage and current characteristics for a steady state inductor. A Buck converter in switch on mode A Buck converter in switch off mode A Boost converter in switch on mode A Boost converter in switch off mode A Buck Boost converter in switch on mode A Buck Boost converter in switch off mode A Push Pull converter with Q1 on A Push Pull converter with Q2 on
An SMPS used for computer.
Page No.
11 11 12 13 14 18 19 20 21 22 23 24 25
SMPS PWM MOSFET IC LED PCB PLC ESP ESR VCR CCR PFC Switch Mode Power Supply Pulse Width Modulation Metal Oxide Semiconductor Field Effect Transistor Integrated Circuit Light Emitting Diode Printed Circuit Board Programmable Logic Circuit Electrostatic Precipitator Equivalent Series Resistance Voltage Conversion Ratio Current Conversion Ratio Power Factor Control

Under the Guidance of MR. M. A. MULLA
Department of Electrical Engineering Sardar Vallabhbhai National Institute of Technology, Surat.
This is to certify that the seminars report titled Switch Mode Power Supply submitted by Mr. Raj Rakesh Roll No. U06EE542, is record of bonafide work carried out by him, in partial fulfillment of the requirement for the award of the Degree of Bachelor of Technology (Electrical Engineering).
Date: -
Examiner 1 : ____________
Examiner 2 : ____________
Examiner 3 : ____________
Examiner 4 : ____________
GUIDE (Mr. M. A. Mulla)
HOD (Prof. Mrs. V. A. Shah)
I must acknowledge the strength, energy and patience that almighty GOD bestowed upon me to start & accomplish this work with the support of all concerned, a few of them I am trying to name hereunder. I would like to express my deep sense of gratitude to my guide Mr. M. A. Mulla (Lecturer, EED, SVNIT, SURAT) for his valuable guidance and motivation and for his extreme cooperation to complete my seminars work successfully. I would like to express my sincere respect and profound gratitude to Prof. V.A. Shah, Head of Electrical Engineering Department for supporting me and providing the facilities for my seminars work. I appreciate all my colleagues whose direct and indirect contribution helped me a lot to accomplish this seminars work. I would also like to thank all the teaching and non teaching staff for cooperating with me and providing valuable advice which helped me in the completion of this seminars.
Abstract List Of Figures List Of Abbreviations Used References
Page No. 5 6 7 31
Chapter No.
1 2 3 4 5 6 7 8 9 10 11 12 13 Introduction
A Brief History Of Development Common Topologies Principle Of Operation Steady State Analysis Buck Converter Boost Converter Buck Boost Converter Push Pull Converter SMPS Vs Linear Regulators Advantages Of SMPS Drawbacks Of SMPS Areas Of Application
Page No.
8-9 10-11 12-13 14-15 16-17 18-19 20-21 22-23 24-26 27 28 29 30
Post: #2

NAVNEET T – 3523036


The electrical power that is supplied by your power company is what is known as Alternating Current or AC. This current is constantly changing amplitude and polarity . The electrons (current) move back and forth at a rate of 60 times per second. This type of current is not suitable for most electronic circuits. It must be converted into a Direct Current or DC which moves in one direction only. Your Multi- Purpose Power Supply does exactly that. It converts AC into DC.

Power supply is a reference to a source of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU.
A power supply is an important element of any electronic kit. Most electronic equipment require a smooth constant dc supply for flowless functioning.
The a.c. voltage from mains is fed to a step down transformer . the lowered voltage is input to a rectifier which gives a fluctuating dc output. This ripple is removed by filter circuit giving an unregulated dc output.
This filter output is input to a regulator circuit. This removes the ripple and gives a smooth and ripple free DC output. It also keeps the voltage fluctuating. The output of a regulator is a regulated dc voltage.

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