Power Supply

By

Mohd Zuhaimi Zolkifli

Inside PC Power Supply

Connector

Introduction

• A power supply unit (PSU) is the component that supplies power to the other components in a computer.

• Convert general-purpose alternating current (AC) electric power from the mains (220-240V) to usable low-voltage DC power for the internal components of the computer.

PC Power Supply• In a personal computer (PC), the power supply

is the metal box usually found in a corner of the case.

• The power supply is visible from the back of many systems because it contains the power-cord receptacle and the cooling fan.

• Power supplies, often referred to as "switching power supplies", use switcher technology to convert the AC input to lower DC voltages. The typical voltages supplied are: 3.3V, 5V and 12V.

• The 3.3V and 5V are typically used by digital circuits, while the 12V is used to run motors in disk drives and fans.

Types of Power Supply

• The first generation of computers power supplies were linear devices, but as cost became a driving factor, and weight became important, switched mode supplies are almost universal.

• So that, two(2) types of PC Power Supply are:– Linear Power Supply– Switching Power Supply

Overview

Linear Power Supply

Switching Power Supply

Linear 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.

• A rectifier is used to produce DC. • 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.

Further information, please refer to E1002.

You had learned it.You had learned it.

Switching Power Supply

• Prior to 1980 or so, power supplies tended to be heavy and bulky. They used large, heavy transformers and huge capacitors (some as large as soda cans) to convert line voltage at 120 volts and 60 hertz into 5 volts and 12 volts DC.

How it work? • The AC input is rectified and filtered to

give a steady DC voltage.• This DC input to the voltage switching

block is either blocked or allowed to pass through depending on the state of the switch.

• The resultant waveform which is a square wave is rectified and filtered (This waveform is sampled).

• If the current required at the load is high, then the switching control ensures that the voltage switch is on for longer periods.

• The sampling is done by a comparator and DC amplifier along with a DC reference.

• The output voltage is compared with a DC reference.

• If the output voltage reduces due to increase in load current, the comparator sends a positive signal to the pulse width modulator and increase the on time of the pulse.

Fault Testing

• We need correct load resistor to test the power supply.

• Also need isolation and auto transformer.– Isolation transformer : for safety– Auto transformer : provides adjustable AC output.

Power Supply Form Factors

• Technically, the power supply of PC’s described as a constant voltage switching power supply.– Constant Voltage: the power supply puts out

the same voltage to the computer’s internal components, no matter what the voltage of AC current running it or the capacity of the power supply.

– Switching: the design and power regulation technique that most supplier use.

Power Supply Form Factors

AT-styleAT-style vs. ATX-styleATX-style

AT-Style vs. ATX-Style• There are two basic differences between

AT and ATX power supplies: – The connectors – The soft switch.

• On older AT power supplies, the Power-on switch wire from the front of the computer is connected directly to the power supply.

• On newer ATX power supplies, the power switch on the front of the computer goes to the motherboard over a connector labeled something like; PS ON, Power SW, SW Power, etc

AT Power Connector

AT Power Connector

• Two main power connectors (P8 and P9, also

called P1 and P2), each with six pins that attach the power supply to the motherboard.

• These are rated at 5A per pin, at up to 250V.

• P8 and P9 connectors have them installed end to end so that the two black wires on both power cables are next to each other.

ATX Main Power Connector

ATX Main Power Connector

• The industry-standard ATX power supply-to-motherboard main connector is the Molex Mini-Fit, Jr. connector number 39-29-9202 (20 pins ATX connector).

• Molex rates each pin to handle 6A, at up to 600V.

ATX Auxiliary Power Connector

ATX Auxiliary Power Connector• In particular, chipsets and DIMMs were

designed to run on 3.3V, increasing the current demand at that voltage.

• This type of connector are rated for 5A per pin at up to 250V.

• The connector is normally keyed to prevent a misaligned connection.

• The additional +5V wire allows a total of 29A of +5V to be available to the motherboard, and the additional two +3.3V wires allow a total of 28A of +3.3V power to be available to the motherboard.

Peripheral Power Connector

Floppy Power Connector

RED +5V

BLACK GND

BLACK GND

YELLOW +12V

Y-Adapter Power Cable

Common Power Problem• Power failure: total loss of input voltage. • Surge: momentary or sustained increase in the

mains voltage. • Sag: momentary or sustained reduction in input

voltage. • Spikes; brief high voltage excursion. • Noise; high frequency transient or oscillation,

usually injected into the line by nearby equipment. • Frequency instability: temporary changes in the

mains frequency. • Harmonic distortion: departure from the ideal

sinusoidal waveform expected on the line.

Power Protection System

• Sometimes power supply by AC outlet not in stable condition (up to 280V).

• It called ‘Power Surge’.

• To overcome this situation, there is few power supply supporting peripherals such as:– Uninterrupted Power Supply (UPS)– Automatic Voltage Regulator (AVR)– Power Line Conditioner (PLC)

UPS

A small free-standing UPS A large datacenter-scale UPS

UPS• An electrical apparatus that provides emergency

power to a load when the input power source fails. • It will provide instantaneous or near-instantaneous

protection from input power interruptions by means of one or more attached batteries and associated electronic circuitry for low power users.

AVR

AVR• A device intended to regulate voltage

automatically: that is to take a varying voltage level and turn it into a constant voltage level.

• An automatic voltage regulator combined with one or more other power-quality capabilities such as:– Surge suppression,– Short circuit protection (circuit breaker),– Line noise reduction,– Phase-to-phase voltage balancing,– Harmonic filtering, etc.

PLC

PLC

• A device intended to improve the quality of the power that is delivered to electrical load equipment.

• A device that acts in one or more ways to deliver a voltage of the proper level and characteristics to enable load equipment to function properly.

Power Supply Specification

Power Supply

Wattage

PC Item Watts

Accelerated Graphics Port (AGP) card

20 to 30W

Peripheral Component Interconnect (PCI) card

5W

small computer system interface (SCSI) PCI card

20 to 25W

floppy disk drive 5W

network interface card 4W

50X CD-ROM drive 10 to 25W

RAM 10W per 128M

5200 RPM Integrated Drive Electronics (IDE) hard disk drive

5 to 11W

7200 RPM IDE hard disk drive 5 to 15W

Motherboard (without CPU or RAM)

20 to 30W

550 MHz Pentium III 30W

733 MHz Pentium III 23.5W

300 MHz Celeron 18W

600 MHz Athlon 45W

Power Use Calculation

• One way to see whether your system is capable of expansion is to calculate the levels of power drain in the various systems components and deduct the total from the maximum power supplied by the power supply.

• Example:>>>>>>>

• 200-watt power supply rated for 20A at +5V and 8A at +12V.

• Power Consumption Calculation– Available 5V Power : 20.0 A

• Motherboard - 5.0 A• 4 slots filled at 2.0 - 8.0 A• 3 ½” floppy drive logic - 0.5 A• 3 ½” hard disk drive logic - 0.5 A• CD-ROM/DVD drive logic - 1.0 A

– Remaining Power: 5.0 A

– Available 12V Power: 8.0 A• 4 slots filled at 0.175 each - 0.7 A• 3 ½” hard disk drive motor - 1.0 A• 3 ½” floppy drive motor - 1.0 A• Cooling fan motor - 0.1 A• CD ROM/DVD drive motor - 1.0 A

– Remaining Power: 4.2 A

• With half of slot filled, a floppy drive, and one hard disk, the system still has room for more.

Maximum Power Consumption Per Bus Slot (Amps)

Bus type +5V Power +12V Power

+3.3V Power

ISA 2.0 0.175 n/a

EISA 4.5 1.5 n/a

VL-bus 2.0 n/a n/a

16-bit MCA 1.6 0.175 n/a

32-bit MCA 2.0 0.175 n/a

PCI 5 0.5 7.6

Power Supply Troubleshooting

• List of PC problems that often are related to the power supply:– Any power-on startup failure– Spontaneous rebooting– Intermittent parity check– Hard disk and fan failing to spin (no +12V)– Overheating due to fan failure.– Electric shocks felt on the system case– Slight static discharges that disrupt system operation– Erratic recognition of bus-powered USB peripherals.

Power Supply Troubleshooting

• Fairly obvious symptoms point right to the power supply as a possible cause:– System that is completely dead (no fan, no

cursor)– Smoke– Blown circuit breakers

Zero in on common power supply-related problem

1. Check the AC power input. Make sure the cord is firmly seated in the wall socket and in the power supply socket. Try a difference cord.

2. Check the DC power connections. Make sure the motherboard and disk drive power connectors are firmly seated and making good contact. Check for loose screw.

3. Check the DC power output. Use digital multimeter to check for proper voltages. If it’s below spec, replace the power supply.

4. Check the installed peripherals. Remove all boards and drive and retest the system. If it works, add items back in one at a time until the system fails again. The last item added before the failure returns is likely defective

Repairing the Power Supply

• Simply replacing the supply with a new one is usually cheaper. Even high-quality power supplies are not that expensive when compared to the labor required to repair them.