Practical No-3

Aim: Study Of Power Supply and its types called as Switching-mode Power Supply and Linear- mode Power Supply.

Introduction: A power supply is a buffer circuit that provides power with the characteristics required by the load from a primary power source with characteristics incompatible with the load. It makes the load compatible with its power source. Example: A power source might be the 60 Hz, single phase, 120 V ac power found in a home in the United States or the 50 Hz, single phase, 220 V ac found in the United Kingdom. The load might be a logic circuit in a personal computer that requires regulated 5 V dc power. The power supply is the circuit that makes the 120 V ac or 220 V ac power source and 5 V dc load compatible. A power supply is sometimes called a power converter and the process is called power conversion. It is also sometimes called a power conditioner and the process is called power conditioning. The Power Sources Manufacturers Association's (PSMA) Handbook of Standardized Terminology for the Power Sources Industry gives this definition of a power supply. Power supplies belong to the field of power electronics, the use of electronics for the control and conversion of electrical power. The IEEE Power Electronics Society provides a more formal definition of power electronics in their constitution. Power Electronics -- This technology encompasses the effective use of electronic components, the application of circuit theory and design techniques, and the development of analytical tools toward efficient electronic conversion, control, and conditioning of electric power. Personal computers, AV equipment, and other common electronic devices incorporate a number of electronic components in order to realize the functions of those devices. "Power supply" is the name given to the functional block that supplies the voltage or current required to operate the electronic components.

Types Of Power Supply: Power supplies for electronic devices can be broadly divided into linear and switching power supplies. The linear supply is a relatively simple design that becomes increasingly bulky and heavy for high current devices; voltage regulation in a linear supply can result in low efficiency. A switched-mode supply of the same rating as a linear supply will be smaller, is usually more efficient, but will be more complex. So we will explain both types one by one as follows:

(1) Linear-mode Power Supply: An AC powered linear power supply usually uses a transformer to convert the voltage from the wall outlet (mains) to a different, usually a lower voltage. If it is used to produce DC, a rectifier is used. A capacitor is used to smooth the pulsating current from the rectifier. Some small periodic deviations from smooth direct current will remain, which is known as ripple. These pulsations occur at a frequency related to the AC power frequency (for example, a multiple of 50 or 60 Hz). The voltage produced by an unregulated power supply will vary depending on the load and on variations in the AC supply voltage. For critical electronics applications a linear regulator will be used to stabilize and adjust the voltage. This

regulator will also greatly reduce the ripple and noise in the output DC current. Linear regulators often provide current limiting, protecting the power supply and attached circuit from over current. The simplest DC power supply circuit consists of a single diode and resistor in series with the AC supply. This circuit is common in rechargeable flashlights.

A home-made linear power supply

Function: To continuously control the voltage between the input and output so that an unstable input voltage is lowered and converted into a stable voltage output. Type: Series regulators (such as 3-terminal regulators) and shunt regulators are available. A series regulator that operates even if the difference between the input voltage and output voltage is approximately 1 V is called low-dropout type (LDO type). Features: Simple because it's almost integrated into one chip as a power supply.

A linear power supply regulates the output by measuring that output and changing some parameter a tiny tiny amount so that it looks like the output is either constant or changing a very tiny tiny amount. The output transistor is operated in its linear region which often consumes a lot of power that goes to waste as heat.

(2) Switch-mode Power Supply: A switched-mode power supply (SMPS) works on a different principle. AC mains input is directly rectified without the use of a transformer, to obtain a DC voltage. This voltage is then sliced into small pieces by a high-speed electronic switch. The size of these slices grows larger as power output requirements increase. The input power slicing occurs at a very high speed (typically 10 kHz — 1 MHz). High frequency and high voltages in this first stage permit much smaller step down transformers than are in a linear power supply. After the transformer secondary, the AC is again rectified to DC. To keep output voltage constant, the power supply needs a sophisticated feedback controller to monitor current draw by the load. Modern switched-mode power supplies often include additional safety features such as the crowbar circuit to help protect the device and the user from harm.[4] In the event that an abnormal high current power draw is detected, the switched-mode supply can assume this is a direct short and will shut itself down before damage is done. For decades PC computer power supplies have also provided a power good

signal to the motherboard which prevents operation when abnormal supply voltages are present. Switched mode power supplies have an absolute limit on their minimum current output.They are only able to output above a certain wattage and cannot function below that point. By slicing up the sinousoidal AC wave into very small discrete pieces, the portion of the AC current not used stays in the power line as very small spikes of power that cannot be utilized by AC motors and results in waste heating of power line transformers. Hundreds of switched mode power supplies in a building can result in poor power quality for other customers surrounding that building, and high electric bills for the company if they are billed according to their power factor in addition to the kilowatts used. Filtering capacitor banks may be needed on the building power mains to suppress and absorb these negative power factor effects.

A switching-mode power supply is a power supply that provides the power supply function through low loss components such as capacitors, inductors, and transformers -- and the use of switches that are in one of two states, on or off. The advantage is that the switch dissipates very little power in either of these two states and power conversion can be accomplished with minimal power loss, which equates to high efficiency. A switching power supply regulates the output by measuring the output and changing some parameter a tiny tiny amount too, but instead of changing the output a tiny amount too it simply regulates the pulse width of the output transistor. A filter circuit at the output makes the output look like it only changes a small amount, but a good scope on the output will show some ripple that is the result of the constantly switching output transistor. The output transistor is operated as either fully on or fully off which does not waste power because when a transistor is fully on it doesn’t waste that much power and when it is fully off it does not waste much power either. Function: Controls ON/OFF of input and outputs a constant voltage by using a smoothing circuit. Type: Insulated type (AC-DC converter) and non-insulated type (DC-DC converter) are available. Application: Used as an AC-DC converter for stepping up, stepping down, or inverting the voltage, or as a DC-DC converter. Features: Little heat emission and high efficiency. Suitable for a large power supply.

A computer's switched mode power supply unit.

Power Supply Applications: There are a number of applications of Power Supply as follows:  Computer power supply: A modern computer power supply is a switchedmode supply designed to convert 110-240 V AC power from the mains supply, to several output both positive (and historically negative) DC voltages in the range 12V to 3.3V. The first computer power supplies were linear devices, but as cost became a driving factor, and weight became important, switched mode supplies are almost universal. The diverse collection of output voltages also have widely varying current draw requirements, which are difficult to all be supplied from the same switched-mode source. Consequently most modern computer power supplies actually consist of several different switched mode supplies, each producing just one voltage component and each able to vary its output based on component power requirements, and all are linked together to shut down as a group in the event of a fault condition. The most common modern computer power supplies are built to conform to the ATX form factor. The power rating of a PC power supply is not officially certified and is self-claimed by each manufacturer. A common way to reach the power figure for PC PSUs is by adding the power available on each rail, which will not give a true power figure. The more reputable makers advertise "True Wattage Rated" to give consumers the idea that they can trust the power advertised.  Welding power supply: Arc welding uses electricity to melt the surfaces of the metals in order to join them together through coalescence. The electricity is provided by a welding power supply, and can either be AC or DC. Arc welding typically requires high currents typically between 100 and 350 amps. Some types of welding can use as few as 10 amps, while some applications of spot welding employ currents as high as 60000 amps for an extremely short time. Older welding power supplies consisted of transformers or engines driving generators, while modern ones implement semiconductors and even microprocessors, greatly reducing their size and weight.  AC adapter: A linear or switched-mode power supply (or in some cases just a transformer) that is built into the top of a plug is known as a "wall wart", "power brick", "plug pack", "plug-in adapter", "adapter block", "domestic mains adapter" or just "power adapter". They are even more diverse than their names; often with either the same kind of DC plug offering different voltage or polarity, or a different plug offering the same voltage. "Universal" adapters attempt to replace missing or damaged ones, using multiple plugs and selectors for different voltages and polarities. Replacement power supplies must match the voltage of, and supply at least as much current as, the original power supply. Because they consume standby power, they are sometimes known as "electricity vampires" and may be plugged into a power strip to allow turning them off. Expensive switched-mode power supplies can cut off leaky electrolyte-capacitors, use powerless MOSFETs, and reduce their working

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frequency to get a gulp of energy once in a while to power, for example, a clock, which would otherwise need a battery. This type of power supply is popular among manufacturers of low cost electrical items because: Devices sold in the global marketplace don't need to be individually configured for 120 volt or 230 volt operation, just sold with the appropriate AC adapter. The device itself doesn't need to be tested for compliance with electrical safety regulations. Only the adapter needs to be tested. Product development becomes faster and cheaper, because the heat produced by the power supply is outside of the product. The device itself can be smaller and lighter, since it does not contain the power supply.

Power Supply Troubleshooting: The power supply can partially fail, completely fail, or even fail intermittently. The PSU can cause a myriad of problems and can frustrate computer users who are not use to dealing with this temperamental component. So some basic Power Supply Troubleshooting are:  To begin, is the power supply working? Just because the computer isn't powering up, doesn't mean the power supply isn't doing its job. Is the fan on the back spinning?  When troubleshooting power supply issues, you first make sure you have the correct voltage set on the rear of the unit. You should see a red switch that can be set to 115V or 230V. Depending on the country you live, this switch will need to be set appropriately. For instance, the Unites States uses 115V. Secondly, make sure you are not plugging into a power strip. Power strips can fail, so be sure to try plugging the power cable directly into a working, properly grounded wall outlet.  If you've been working inside the case, check the thin power wires running from the front of the case to the motherboard. These generally plug into the motherboard at the lower right of the motherboard and may be labeled something like "power sw" or "pw switch." If this wire has popped off, the PSU will not work. To test the switch, you can remove the "power sw" wire and short the pins by touching a flathead screwdrive to the two pins. If the switch is bad, the computer should power up right away. There is also the main motherboard connector, which is the long 20 pin connector. Make sure this is properly secured. Some graphics cards or motherboards also require an addition 4-pin wire connector. If you motherboard requires it, be sure this is plugged in to the appropriate socket.  Use a multi-meter or stand alone power supply tester to check the PSU. To test with a multi-meter, make sure the power supply is plugged in to a power source and that the 20-pin connector is plugged into the motherboard. It's best if you lay the case on it's side so you can look directly down on the motherboard. Next, you'll jam the black lead down into any of the black (ground) wires. Then you can probe each colored wire for the appropriate voltage. If voltage is seriously out of range or does not exist, then the power supply is bad.