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COMPUTER SMPS POWER SUPPLY
A power supply is the heart of the heart of the computer system. As a human being needs proper circulation of the blood for the human body to stay alive and work, a computer need a well regulated 5 volt and 12 volt DC power supply for the proper functioning of its various parts. These DC power are supplied to the computer by the power supply. As, without the proper functioning of a heart the human body can not work, without a good and proper power supply the computer system can not do any work.
The power from the mains is 240 volt AC current, with a lot of interference, so it can not be directly fed to the computer. A power supply is used in between the computer and the mains to convert the 240v AC current to a well regulated `12 volt and 5 volt DC current.
Usually the electronic components and circuits in the system (mother board, adapter cards and disk driver logic boards etc.) use the 5 volt power and the motors in the disk drive and the hard disk use the 12 volt power.
The power supply of the computer must ensure a proper, good and steady supply of both types of current to the system, so that the complete computer system can operate properly.
REGULATED POWER SUPPLY
A DC power supply which maintains a constant output voltage irrespective of the changing AC main supply or varying load is known as a regulated DC power supply.
In other words, an ideal regulated power supply is a electronic circuit designed to provide predetermined DC voltage as output, which is independent of
The current drawn by the load, any variations in the input AC line voltage, and any changes in the temperatures of its different parts etc. The basic power supply for a regulated power is the linear power supply, which is explained next.
A TYPICAL LINEAR POWER SUPPLY
The basic components of a typical linear power supply is contains a transformer, rectifier, filter capacitor and a regulator.
The transformer isolates the output from the main supply and also reduces the voltage level from 220V to 5V or 12 V as per the requirement
The rectifier is used for converting the AC voltage at the transformer output into DC voltage and the filter is used for removing the ripples present in the transformer output.
The regulator is used for maintaining a steady output voltage irrespective to any variation in the input voltage ,load currents and functioning of any components due to any change in the operating temperature.
The main disadvantage of a linear power supply is its large size and poor efficiency. In a linear power supply the output voltage is maintained steady by controlling the amount of extra power that is being consumed. This results in considerable amount of power loss and a very poor efficiency..
Because of these disadvantages of the linear power supply, the computer system use a completely different types of power supply, which is called SMPS or Switch Mode Power Supply
In the computer industry a SMPS has been commonly accepted as a standard power supply for the last decade and a half. The main advantages of using a SMPS as the power supply inside the computers are its reduced size, less cost and high efficiency compared to an ordinary linear power supply.
The main difference between the linear power supply and the switch mode power supply is the way the output voltage generated and regulated.
BLOCK DIAGRAM SMPS POWER
WORKING OF SMPS
In a SMPS, the main AC supply is first sent to a rectifier/filter unit, which converts the AC supply into a very high value DC voltage and filters it.
This high voltage DC supply is then given to a switching transistor circuit. This switching transistor circuit is switched on and off at a very high speed by a control circuit which generates very high frequency square wave pulses.
The switching tr circuit switches sth4e given high voltage DC, on &off at the same high frequency and gives square wave pulses as the output, These square wavepulses are then given to the primary winding of a SM transformer, These pulses induce a voltage at the primary winding of the transformer which will generate voltage at the secondary winding.
This voltage at the secondary winding is then rectified and filtered to produce the required output. To regulate the output, the generated output voltage is send back to the switching section. This voltage is first send to a sense amplifier which compares it with a reference voltage and generates an error voltage.
This error voltage is such that
(a) When there is an increase in the output voltage the error voltage reduces the “on” time of the switching transistor, which reduces the output voltage.
(b) When there is a reduction in the output voltage the error voltage increases the “on” time of the switching transistor, which increases the output voltage.
This error voltage is send to the control circuit which controls the switching transistor to regulate the output voltage. This makes the output stable in both the high or low output conditions.
Other than these basic operation, most of the SMPS have facility to protect themselves from overload and short circuit in the output section.
DISADVANTAGES OF SMPS
SMPS has some drawbacks as well. They are
1- Electromagnetic interference (EMI) is a natural by product ON-OFF switching of the switching circuit. This interference can be conducted to the load (resulting in higher output ripple and noise) it can be conducted back to the AC line and it can be radiated into surrounding atmosphere
2- The rapid switching of the chopping current generates strong interfering signals in the circuit.
3- Higher harmonics or oscillatory currents due to trapped energy lie in the radio frequency range and can cause interference on the display screen.
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4- Strong induced voltages get fed back to the mains, thus polluting the main supply to the other devices in use nearby.
5- Strong EMI interference generated by the SMPS, without proper shielding can cause the computer or other devices to generate intermittent problems.
A SMPS inside the computer is properly shielded with metal case to avoid the EMI interference to the computer and other nearby objects.
COMPUTER POWER SUPPLY’s DUTIES
The power supply has three main duties.
(a) It supplies the PC with power.
(b) It cools the system.
© It protects the internal components.
The power supply cools the computer system with its built in fan, Which is why PCs make noise when it is turned on, The fan draws in the air through the openings in the PC’s Case and the disk drives. This air flows over and cools the motherboards and the expansion cards and is finally drawn in through the power supply and out of the exhaust port behind the PC.
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The power supply also protects the PC by not switching on the PC if the power supply cannot give the enough electricity. (This might occur during a “brownout” or as the result of damage to the power supply or wires).
Also, the power supply is designed in such a way that if it fails because of any reason it won’t take the rest of the system with it.
POWER SUPPLY OF A PC
As a PC contains many electronic components, a lot of memory adapter cards and a hard disk etc, It require a lot of high quality power, the early PC had just 65 watt power supply which was upgraded to a 135 watt supply in the XT machines.
PC-AT require even more power, a 200 watt or even 250 watt power supply is required with these type of machines, which will allow you for any future expansion, such as adding a CD-ROM drive or a network adapter etc.
The SMPS is a part of the main system box. It’s position in a TOWER model computer and a DESKTOP computer .
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In a DESKTOP type computer model lthe power supply is located at the right back side. Inside the main system unit, The power switch in the model of computers is located on the right side or on the back of the computer.
In a TOWER model the power supply is located at the top back side, inside the main system unit and in most of the models the power switch is located on the front panel of the computer
Modern SMPS power supplies used in computers are small and efficient and are also equipped with a fan for the cooling purpose.
of concentric air holes, through which the power supply fan can suck the hot air inside the main system unit. This hot air is pushed out through the fan exhaust outlet at the back of the power supply.
Depending on the strength of power supply and the amount of air that is sucked there may be one or two fans and inlet holes. This sucking of the hot air, pulls in and circulates the cool air from the outside, keeping the systems temperature down.
The main purpose of a power supply is to convert the mains voltage to much lower voltages used by the motherboards and disk drives, as well as the different adapter cards used inside the computer.
The monitor usually plugs into a power socked provided on the computer’s power supply and is controlled by the power switch of the computer. Althou7gh there is no fixed standard for power supplies, most of them will fit into any normal standard sized cabinet.
The back of the power supply contains foru things. They are-
1- The computer’s power connector, This power connector is a three-pronged connector into which you plug the power cable, other end of the cable goes to the wall socket.
The monitor’s power connector, The monitor’s power connector is a matching three-holed connector, Some monitors have special power cables that plug into this connector. If not, then the monitor can be plugged directly in the wall socket.
The fan exhaust port the fan exhaust port is the hole through which SMPS blows all the hot air and dust that it has sucked through the PC.
A power switch the power switch is provided on the side or the back to the power supply in a DESKTOP model cabinet, in a TOWER model computer it is provided on the back of the power supply or in the front panel of the computer.