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Insight - How SMPS works

Written By: 

Abhimanyu Mathur

A computer is an assembly of several essential and optional electronic parts such CD drive, motherboard, hard disk etc. . Every peripheral has its own power requirement and to serve the same regulated power supplies such as linear power supplies, ferro resonant power supplies and switched mode power supplies, are used.

Switched mode power supplies are unanimously preferred as computer power supply units for their small size and high efficiency.  SMPS units dissipate less power by employing a transformer that regulates input voltage and provides constant current to the peripherals attached to the computer.
 
The input voltage is chopped to different levels or is switched through different levels using a chopping process. This is done at very high frequency which enhances the efficiency of the circuit.  A normal linear voltage supply can works at 30% efficiency, while an SMPS based power supply unit can provide 70-80% of efficiency. Also, using a high switching frequency usage makes the process quicker. An SMPS also has the features of being light-weight and economical as comparedto other power supplies. Also, the circuit topology of it can be varied depending upon the power requirements of the device to which it is attached.
 
Introduced in 1960s with the Apple-II, SMPS are a light weight, economic and power efficient solution in regulating power supply to a computer. This insight will detail with the internal parts, their working and wiring of SMPS.
 
Image of 450W Computer SMPS
 
Fig. 1: Image of 450W Computer SMPS
 
Various types of SMPS architectures are popular on the basis of their power handling capacities and applications. The one used in computer is push-pull type which can handle input voltage of more than 200V.
 
The image above shows a conventional 450W computer SMPS. From the outer casing, the SMPS fan and the power input plug can be seen. The purpose of installing a fan is to remove the heat produced by the SMPS. The outer casing is made from light weight metal casing, usually aluminum, so as to protect the internal structure. The casing is made by screwing upper and lower section and has segments cut to allow heat dissipation during the time a computer is in use.
 
Grilled Metal Casing of SMPS
 
Fig. 2: Grilled Metal Casing of SMPS
 
The image shows a grilled metal casing which provides better ventilation to the heat generated in the power supply unit.
As the power supply unit is responsible to fulfill electrical requirements of multiple devices, an SMPS can have various types of connectors. Depending on the connections, there can be AT (Advanced Technology) and ATX (Advanced Technology extended) power supplies. The one shown in the images is an ATX one.  In ATX power supplies, user can switch on and off the peripheral through software commands that are conveyed to the device by motherboard. On the other hand, in AT supplies, the user has to manually switch on and off the peripherals.
Wiring in both types of power supply units is also quite different as ATX doesn’t has a -5V biasing voltage but features efficient remote sensing 3.3V voltage and a few more connector wires which have been discussed later in this insight. Various versions of ATX based connectors are also widely popular as there have been phenomenal developments in the hardware of computer system too.

Inside the Metal Casing

 
PCB of SMPS
 
Fig. 3: PCB of SMPS
 
On unscrewing the metal casing we can see the PCB which forms the core of the SMPS. From regulating power for all the devices to powering the exhaust fan, PCB manages all the major and minor power needs of the computer. It can be divided into various sections on the basis of the function that each performs.
 
Filter, Rectifier, and Heat Sink Regions of SMPS
 
Fig. 4: Filter, Rectifier, and Heat Sink Regions of SMPS
 
The image above shows the filter, rectifier and heat sink regions of the SMPS. This section gets the input power supply, which is in AC form, converts and rectifies it into DC. A fuse is added to the PCB so that sudden high current impulses don’t harm and the circuit.
The fuse in the SMPS unit aids in forming a crowbar circuit. A crowbar circuit compares the input voltage value to the standard voltage requirements of the respective circuit. If the input values match the output ones, then only it allows it to flow to the circuit (SMPS in this case), otherwise, it stops the power supply. Crowbar circuit makes sure that the components of the circuit don’t get damaged as it is very difficult to remove or repair the components on a PCB.
 
Crowbar Circuit Area of SMPS
 
Fig. 5: Crowbar Circuit Area of SMPS 

 Contd

The central section of the SMPS houses an array of transformers which drop down the input voltage to match the requirements of peripherals attached.These transformers are designed to work at high frequencies as the input is a high frequency DC voltage. The transformer cores are made of ferrite and are lighter and small so that they can function at high frequencies.
 In order to accommodate the feature of high frequencies, precautions are taken so as to reduce the electromagnetic interference. Hence, shape and design of the PCB becomes a critical factor too.
 
Apart from the voltage down transformers, a power MOSFET forms one of the most important parts of the SMPS. This MOSFET works as a switch, taking the unregulated voltage from the voltage down transformers and chopping it down to control the output voltage level.
 
The control logic to the chopper is sent via a PWM modulation circuitry, which, in this SMPS has been done by IC 2958.It generates a high frequency feedback pulse which is sent to MOSFET so that width of output DC pulses can match the requirement of theperipheral.
 
It can also be said that IC regulates the duty cycle of the MOSFET switch. Duty cycle can be defined as the ratio time for whichthe MOSFET switch is ON to the total time it is under consideration.
 
The MOSFET switch works at very high frequency, thus providing highest of efficiency levels to the SMPS.  Compared to the frequency of input AC level of 50 or 60Hz, the MOSFET chops at a rate of 50KHz or more, which is at least 1000times of the input levels. The feature of high efficiency provided by the MOSFET makes SMPS an in-demand regulated power supply. Operation at high frequency increases the noise produced by the whole system.
 
Output Filter Coil of SMPS
 
Fig. 6: Output Filter Coil of SMPS
 
Output of the MOSFET is separated from the input using an isolation transformer and is passed through a low pass filter which smoothens out the output DC signal.

Output power supply voltages

An SMPS delivers medium output voltages such as 3.3V, 5V, 12V and biasing voltages like -12V.
To ease the process of mounting peripherals through combination of different wires, the wiring is followed by a color coding. The standard for color coding in a SMPS is as follows:
·         Orange wire represents  3.3 volt 
·         Yellow wire represents+12 volt 
·         Red wire represents +5 volt 
·         Blue wire represents -12 volt 
·         Black wire denotes ground 
·         Gray wire is a 5V supply that checks whether the peripheral is in working condition or not. It is of foremost importance that this wire signals correct, otherwise the supply to that peripheral would be reset until it receives a genuine signal.
·         Green wire serves for power.
 
A few connectors that formed by the combination of wires include SATA hard disk connector, DVD drive connector, motherboard connector.
 
As the hardware configurations of computers are increasing, power supplies are being asked to deliver more and more power. SMPS have now elevated from 450W levels to 650W levels and more. Constant changes in the PCB are taking place in order to reduce the heat produced or exhaust it off. Some SMPS even include a feature for an extra fan.  With usage of better switching circuitry, the EMI reduces and output is more refined.

Comments

very nice..

 

"high frequency DC voltage"  is very misleading.....dear admin please change thise.

 verygood

thankyou for ur information

 

Thankyou ...Engineers Garage

 

good...one..

 

what are general faults occur? How to know them n solve?????????

very nice

 

not badcheeky