This lecture slide describes in detail about the power supply unit in the computer system and its various types.

1. Repair & Maintenance
D.Com II/II
UNIT 6: POWER SUPPLY
LECTURE BY : ER.ASHISH K.C (KHATRI)

2. Introduction
Power supply is responsible for powering every devices on a PC.
Power supply of a computer has following functions:
a. It converts household power supply (220V) to dc power supply (5V, 12V).
b. It distributes power supply in different cables of computer system according to the
requirements.
In a PC , the power supply is the metal box usually found in the corner of the case.
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3. Role of power supply:
1. Stability :
A good power supply device can ensure a longer years of device performance. Whereas poor
quality results frequent failure of devices and glitches in power.
2. Cooling:
The power supply contains a fan that controls the flow of air in the PC case. This fan plays a
vital role in cooling system.
3. Energy efficiency:
PC power supply work with computer components and software to reduce the amount of
power when they are idle.
4. Expandability:
The capacity of power supply is one factor that determines the ability to add new
devices/drivers in our system.
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4. How does computer uses the voltages:
In the power supply the computer uses the 3V and 5V to run the digital circuits.
The 12V is used to run motors , like disk drive and the fan to keep the computer from over
heating.
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5. Power rating:
The overall power drawn on a power supply unit is limited by the fact that all of the supply rails
come through one transformer and any of its primary side circuitry like switching components.
Total power requirements for a PC may range from 250 watts to more than 1000 watts for a
high performance computer with multiple graphics card.
A personal computer usually requires 300 to 500 watts.
Power supplies are designed around 40% greater than the calculated system power
consumption.
This protects against system performance degradation and against power supply overloading.
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6. Efficiency of Power supply unit:
Generally a power supply is 70% - 80% efficient.
For a 75% efficient power supply to produce 75W of DC output it would require 100W of AC
input and dissipate the remaining 25W in heat.
High quality power supplies can be over 80% efficient.
Energy efficient power supply unit waste less energy in heat and requires less airflow to cool.
Google’s server power supplies are more than 90% efficient.
HP’s server power supplies have reached 94% efficiency.
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7. Types of power supply:
Power supply typically have two basic design format:
a. Linear power supply
b. Switch mode
Linear power supply is relatively simple in design and
Switch power supply is a bit complex but is capable of more efficient power supply.
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8. 1. Linear Power Supply:
Linear power supply does not have any feed back of power.
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9. AC supply :
- The AC supply given to the transformer is three phase 220V AC input.
Transformer:
- The AC voltage is connected to the primary side of the transformer which steps the AC voltage
down to a desired level of output generally 0 to 5V or 12V.
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10. Rectifier:
- It is the circuit which converts AC voltage to pulsating DC voltage.
- Generally, Bridge rectifier is used.
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11. Filter:
- A simple capacitor filter is used to smooth the pulsating DC to normal DC voltage.
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12. Regulator & Divider:
- A regulator circuit removes the repulse in the filter output and also remains the same DC value
if the input voltage varies.
- A divider circuit splits the voltages into different levels.
- A desired DC output is given to every component devices of a computer system.
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13. Advantages of LPS:
a. The power supply is continuous.
b. The circuitry is simple.
c. These are reliable systems.
d. This system dynamically responds to load changes.
e. The circuit resistances are changed to regulate the output voltage.
f. As the components operate in linear region, the noise is low.
g. The ripple is very low in the output voltage
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14. Disadvantages of LPS:
a. The transformers used are heavier and large.
b. The heat dissipation is more.
c. The efficiency of linear power supply is 40 to 50%
d. Power is wasted in the form of heat in LPS circuits.
e. Single output voltage is obtained.
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15. 2. Switch Mode Power Supply (SMPS):
SMPS works on a principle consuming only as much power needed by the load.
The main input is directly rectified without the use of transformer to obtain a dc voltage.
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16. Stages of SMPS:
Input Stage :
The AC input supply signal 50 Hz is given directly to the rectifier and filter circuit combination
without using any transformer.
This output will have many variations and the capacitance value of the capacitor should be
higher to handle the input fluctuations.
This unregulated dc is given to the central switching section of SMPS.
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17. Switching Section
A fast switching device such as a Power transistor or a MOSFET is employed in this section,
which switches ON and OFF according to the variations
This output is given to the primary of the transformer present in this section.
Output Stage
The output signal from the switching section is again rectified and filtered, to get the required
DC voltage.
This is a regulated output voltage which is then given to the control circuit, which is a feedback
circuit.
The final output is obtained after considering the feedback signal.
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18. Control Circuit :
The output sensor senses the signal and joins it to the control unit.
The signal is isolated from the other section so that any sudden spikes should not affect the
circuitry.
A reference voltage is given as one input along with the signal to the error amplifier which is a
comparator that compares the signal with the required signal level.
By controlling the chopping frequency the final voltage level is maintained.
This is controlled by comparing the inputs given to the error amplifier, whose output helps to
decide whether to increase or decrease the chopping frequency.
The PWM oscillator produces a standard PWM wave fixed frequency.
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19. Advantages of SMPS:
The efficiency is as high as 80 to 90%
Less heat generation; less power wastage.
Reduced harmonic feedback into the supply mains.
The device is compact and small in size.
The manufacturing cost is reduced.
Provision for providing the required number of voltages.
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20. Disadvantages of SMPS:
The noise is present due to high frequency switching.
The circuit is complex.
It produces electromagnetic interference.
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21. LPS Vs SMPS
Parameters Linear Power Supply Switch Mode Power Supply
Definition It completes the stepping down of AC
voltage first then it converts it into DC.
It converts the input signal into DC first
then it steps down the voltage up to
desired level.
Efficiency Low efficiency i.e. about 20-25% High Efficiency i.e. about 80%
Voltage Regulation Voltage regulation is done by voltage
regulator.
Voltage regulation is done by feedback
circuit.
Reliability More reliable in comparison to SMPS. its reliability depends on the transistors
used for switching
Complexity Less complex than SMPS. More complex than Linear power
supply.
Applications Used in Audio frequency applications
and RF applications.
Used in chargers of mobile phones, DC
motors etc.
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22. End of Unit
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