SMPS-WORKING,TYPES AND FUNCTIONS.

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SMPS

                                                             The role of a power supply is convert the mains voltage into a lower DC voltage which the components can run on. While there are two types of PSUs out there (linear and switching/switch-mode), we’ll just look at switch-mode power supplies (SMPSs) in this article, since they are the only type of power supply used in computers. The SMPS convert the input AC voltage to a Set of fixed DC Voltage and supplies the necessary power to motherboard, disk drives and all expansion cards. The advantage of using switch mode power supply is that it operates through fewer components such as electronic switches which work in one of two states on or off. These switches dissipate very little power and power convention can be accomplished with minimal power loss which leads to high efficiency. modern PCs use mainly two types of SMPS Units – AT and ATX.

Fig: Interior view of an ATX SMPS

      Types of Power Supplies

• Ferro – resonant power Supplies
• Linear Power Supplies 
•  Switch mode power Supplies

Ferro resonant Power Supplies

Principle of operation: This is the simplest type of power supply, which uses a Ferro resonant transformer. The secondary of the transformer remains instauration, even when the primary is out of saturation.  By secondary saturation we mean the magnetic full density level in the transformer secondary remains more or less constant. Hence, the secondary output voltage will remain constant, regardless of input voltage variations.  The output of the Ferro –resonant transformer is a sine wave.

Linear Power Supplies

They are usually conventional AC to DC supplies and commonly known as “Regulated power supplies”. In Linear supplies regulation taken place after rectification.

Type of Linear Power supplies:-

•             Series regulator

•             Shunt regulator

Switch mode power supplies

In a SMPS the controlling device works like an ideal switch, which is either on or off. By controlling the ratio of the duration of ‘on’ time and ‘off ’ time of a power transistor, which is also known as duty cycle, the power flow can be controlled in a very efficient way.

         In a switching power supply the switching regulations taken place in the primary side, as the main power transformers in switching supplies a constant current flow which is regulated by chopping the input voltage. And controlling the average current by Means of the duty cycle. Then  a higher current is drawn by the load, the control circuit of the SMPS suitably increases the on-time duration to accommodate such change in load current since the switch always operate either in situation on in cut off mode except for a brief transition period between these two states, the switching regulator can achieve very good regulation despite large fluctuations in input voltage and load variations.

Types of SMPS

•             Fly back mode SMPS

•             Forward mode SMPS

•             Push-pull mode SMPS

Fly back mode SMPS:

     It is normally used in TVs and VCRs. It is relatively simple and cheep since it does not need any output choke. But  it has relatively poor regulation and high ripple context in output.

 Forward mode SMPS:

             Units are used in device requesting 100 to 200 watts of output power and good regulation, forward mode SMPS circuit are also not used in PCs because their power requirements may exceed 200 watts and they need even better regulation.

 Push pull mode SMPS:

                 It is used for output power requirement is high (exceeding 200 watt) and better regulation with low are required. They are preferred choice in PCs. Push-pull designs are relatively complex and costly.

The importance of power supply in a PC:

The SMPS unit in PCs in responsible for converting the standard AC utility power into a form that the computer can use. It is responsible for powering every device in the computer. The power supply plays an important role in the following Areas of a system.

1.  Stability: a high quality power supply with sufficient capacity to meet the demands of the computer will provide years do stable power for

2.  Cooling: the powerfully contains the main farm that controls the flow of air through the PC case. This fan is a major component in the PC’s cooling system.

3.  Expandability: the capacity of the power supply is one factor that determines the upgrade potential of system.

   

PC power Supply Functions

In a PC the SMPS Server three crucial functions

•Delivers Required DC Power: In a PC power supply converts the AC main supply voltage to a set of lower and smooth DC voltage +/-5V,+/-12V(and +3.3V in modern ATX system)which are required to operate the electronic components and circuits in the system.

•Cooling of the system inside: Modern SMPS units used in PCs are small, highly efficient and are equipped with fans to cool the system inside.

•Protection of system components: The SMPS also protects the pc by not switching on the PC of the power supply cannot deliver enough electrical power to the PC.

Output voltages

+5 Volt Supply: This is the basic supply voltage for nearly all electronic components. Motors now sometimes use the5 volt supply, but usually only in the smaller drives, such are the 2.5 inch and some 3.5 inch drives. The +5 volt supply wires should always be red in any standard PC system.

+12 volt Supply: the main function of the +12V power is to run disk drive motors as well as the higher output processor voltage requirements in some of the newer boards +12V supply is used by any cooling fans in the system.

-12Volt Supply: it is not used by motherboard logic and routed to pin B7 of the A bus slots. Although this voltage can be used by any add-on card plugged into the ISA bus slots, usually only the serial port driver/receiver circuits use it for converting TTL signals rather they use +/-5V. Thus, it may happen that in a modern PC the -12V is left unused.

-5Volt Supply: is the -5V signal is directly rounded to the ISA bus pin B5 for use by add on cables. In earlier floppy drive controller cards; the analog data separator circuit used-5V.

+3.3 Volt Supply: this Supply voltage does not appear on the original XT-AT or mini XT compatible power supplies starting with the ATX design in 1995 any system with ATX compatible power supplies include a 3.3 volt supply  line from the power supply to the motherboard. Most CPUs and DRAM use 3.3 volts, and used as some PCI adapter cards. All high speed PCI adapter cards to use 3.3 volts for all communications with the PCI Bus.

+5VB(+5V, standby mode): the +5 VSB line is a +5 volt supply line that is active any time the AC power input is active regardless of whether the system is turned on the purpose of their supply line is to power standby circuiting, such as the PS-ON circuit the wake on LAN and remote ring on circuitry.

+3.3 Sense: it is used to sense the actual voltage of the 3.3 volt supply after it has reached the motherboard, which allows the power supply to actively adjust the 303 volt output to compensate for line losses between the power supply and the motherboard.