“An SMPS i.e Switched Mode Power-Supply is generally an electronic power-supply unit which provides the DC-Current to all the components present in the CPU Cabinet, such as “Motherboard”, “HDD” etc. ”

A switched-mode power supply (switching-mode power supply, switch-mode power supply,switched power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a DC or AC source (often mains power), to DC loads, such as a personal computer, while converting voltage and current characteristics. Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions, which minimizes wasted energy. Ideally, a switched-mode power supply dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time. In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor. This higher power conversion efficiency is an important advantage of a switched-mode power supply. Switched-mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight.

Switching regulators are used as replacements for linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated; their switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor power factor.

In Computer Industry, an SMPS is commonly accepted as a standard power-supply due to its common-advantages which are as follows:

  1. Therefore, SMPS is used inside the CPU Cabinet; it also reduces size & weightage of the CPU Cabinet.
  2. It also reduces the cost of Maintenance/Repair of the Particular Cabinet.
  3. Thus, it has a higher efficiency rating as compared to the Linear Power Supply.


  • A linear regulator provides the desired output voltage by dissipating excess power in ohmic losses (e.g., in a resistor or in the collector–emitter region of a pass transistor in its active mode). A linear regulator regulates either output voltage or current by dissipating the excess electric power in the form of heat, and hence its maximum power efficiency is voltage-out/voltage-in since the volt difference is wasted.
  • In contrast, a switched-mode power supply regulates either output voltage or current by switching ideal storage elements, like inductorsand capacitors, into and out of different electrical configurations. Ideal switching elements (e.g., transistors operated outside of their active mode) have no resistance when “closed” and carry no current when “open”, and so the converters can theoretically operate with 100% efficiency (i.e., all input power is delivered to the load; no power is wasted as dissipated heat).


The basic schematic of a boost converter.
For example, if a DC source, an inductor, a switch, and the corresponding electrical ground are placed in series and the switch is driven by a square wave, the peak-to-peak voltage of the waveform measured across the switch can exceed the input voltage from the DC source. This is because the inductor responds to changes in current by inducing its own voltage to counter the change in current, and this voltage adds to the source voltage while the switch is open. If a diode-and-capacitor combination is placed in parallel to the switch, the peak voltage can be stored in the capacitor, and the capacitor can be used as a DC source with an output voltage greater than the DC voltage driving the circuit. This boost converter acts like a step-up transformer for DC signals. A buck–boost converter works in a similar manner, but yields an output voltage which is opposite in polarity to the input voltage. Other buck circuits exist to boost the average output current with a reduction of voltage.
  • In a SMPS, the output current flow depends on the input power signal, the storage elements and circuit topologies used, and also on the pattern used (e.g., pulse-width modulation with an adjustable duty cycle) to drive the switching elements. The spectral density of these switching waveforms has energy concentrated at relatively high frequencies. As such, switching transients and ripple introduced onto the output waveforms can be filtered with a small LC filter.

Advantages and Disadvantages:


  1. The main advantage of the switching power supply is greater efficiency than linear regulators because the switching transistor dissipates little power when acting as a switch.
  2. Other advantages include smaller size and lighter weight from the elimination of heavy line-frequency transformers, and comparable heat generation.
  3. Standby power loss is often much less than transformers. However, transformers can achieve 98-99% conversion efficiency when in use, and can be significantly more efficient for power conversion.


  1. Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), a ripple voltage at the switching frequency and the harmonic frequencies thereof.
  2. Very low cost SMPSs may couple electrical switching noise back onto the mains power line, causing interference with A/V equipment connected to the same phase. Non-power-factor-corrected SMPSs also cause harmonic distortion.

Theory of Operation:

 Explained in Brief:smps
  1. In an SMPS, First an “AC Supply” is passed by the “AC Mains” To the First Block of SMPS i.e “Input Rectifier & Filter”.
  2. When this “AC Voltage” forms the AC Mains Goes to First Block of SMPS i.e Input Rectifier & Filter, it is first carried by the “Bridge-Rectifier”. This “Bridge-Rectifier” Consists of a full-wave diode which converts the obtained AC Voltage From AC Mains in a very high value producing an “Unregulated DC Voltage” which is passed to the Filter-Capacitor .When this “Unregulated DC Voltage” is received by Filter-Capacitor , then it removes the Ripples From the Obtained Unregulated DC Voltage & Produces a DC Voltage , which is passed further to the forward block i.e “Switching Transistor”.
  3. When the “DC Voltage” is received by “Switching Transistor”, then this “Switching-Transistor” Converts the obtained DC Voltage into Square Wave AC by Switching ON & OFF periodically at a very high frequency. This Switching-Process is generated by Pulse-Generator. Here, there is No Power-Loss due to Stabilization, because there is no Power-Dissipation across the transistor, when it is ON or OFF. & This Switching is controlled by the MOSFET Amplifier which sends the obtained Square Wave AC Pulses to the forward block i.e “Transformer”.
  4. When this “Square-Wave AC Voltage” is received by the “Transformer”, then the Obtained Square Wave AC Pulses are then received by the Primary-Winding of Transformer, which then passes this Square Wave AC Pulses to the Secondary Winding of Transformer .Here, it usually Steps-down the voltage & Sends this Step-down AC Voltage to the Further Block i.e “Full-Wave Rectifier”.
  5. When this “Step-Down Square-Wave AC Voltage” is received by the “Full-Wave Rectifier”, then, The “Obtained Step-Down AC Voltage” is then rectified by the Full-Wave Rectifier which produces a “DC Voltage”. This “DC Voltage” which consist some ripples which is passed from the rectifier is then sended to the forward block i.e “Filter”.
  6. Thus, This Obtained “DC Voltage” from the “Full-Wave Rectifier” contains some ripples, these ripples are filtered by the “Output-Filter” & this “Filtered DC Voltage” is passed as an “Output” on the “Output-Side” of the SMPS.
  7. Thus , to regulate the a Perfect Output , the Generated Filtered DC Voltage is then sended to the “Voltage-Comparator”.
  8. Here, the “Voltage-Comparator” Compares the “Obtained Filtered DC Voltage” with the “Reference-Voltage” by which it gets to know how many “Error-Voltage” are generated. This “Error-Voltage” generated is such that , whenever there is an increase in the Output Voltage , this “Error-Voltage” reduces the “ON” time of the MOSFET present in the “Switching-Transistor” & Hence whenever there is an decrease in the Output Voltage , this “Error-Voltage” reduces the “OFF” time of the MOSFET present in the “Switching-Transistor” . Thus, this generated “Error-Voltage” is then sended to the “Pulse-Width Generator”.
  9. Here, the “Pulse-Width Generator” chops the DC Voltage with the Error-Voltage by which it controls the “Switching-Transistor” to regulate the Exact Amount of Voltage Occurred. This makes the Output Stable in Both High or Low Cases.

This is How it Happens

 Explained in Detail:

Block diagram of a mains operated AC/DC SMPS with output voltage regulation

Input-Rectifier Stage:

AC, half-wave and full-wave rectified signals.

If the SMPS has an AC input, then the first stage is to convert the input to DC. This is called rectification. A SMPS with a DC input does not require this stage. In some power supplies (mostly computer ATX power supplies), the rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. This feature permits operation from power sources that are normally at 115 V or at 230 V. The rectifier produces an unregulated DC voltage which is then sent to a large filter capacitor. The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant high frequency energy which reduces the power factor. To correct for this, many newer SMPS will use a special PFC circuit to make the input current follow the sinusoidal shape of the AC input voltage, correcting the power factor. Power supplies that use Active PFC usually are auto-ranging, supporting input voltages from ~100 VAC – 250 VAC, with no input voltage selector switch.

An SMPS designed for AC input can usually be run from a DC supply, because the DC would pass through the rectifier unchanged.[20] If the power supply is designed for 115 VAC and has no voltage selector switch, the required DC voltage would be 163 VDC (115 × √2). This type of use may be harmful to the rectifier stage, however, as it will only use half of diodes in the rectifier for the full load. This could possibly result in overheating of these components, causing them to fail prematurely. On the other hand, if the power supply has a voltage selector switch, based on the Delon circuit, for 115/230V (computer ATX power supplies typically are in this category), the selector switch would have to be put in the 230 V position, and the required voltage would be 325 VDC (230 × √2). The diodes in this type of power supply will handle the DC current just fine because they are rated to handle double the nominal input current when operated in the 115 V mode, due to the operation of the voltage doubler. This is because the doubler, when in operation, uses only half of the bridge rectifier and runs twice as much current through it.[21] It is uncertain how an Auto-ranging/Active-PFC type power supply would react to being powered by DC.


The inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by running it through a power oscillator, whose output transformer is very small with few windings at a frequency of tens or hundreds of kilohertz. The frequency is usually chosen to be above 20 kHz, to make it inaudible to humans. The switching is implemented as a multistage (to achieve high gain) MOSFET amplifier. MOSFETs are a type of transistor with a low on-resistance and a high current-handling capacity.

Voltage-Converter and Output-Rectifier:

If the output is required to be isolated from the input, as is usually the case in mains power supplies, the inverted AC is used to drive the primary winding of a high-frequency transformer. This converts the voltage up or down to the required output level on its secondary winding. The output transformer in the block diagram serves this purpose.

If a DC output is required, the AC output from the transformer is rectified. For output voltages above ten volts or so, ordinary silicon diodes are commonly used. For lower voltages, Schottky diodes are commonly used as the rectifier elements; they have the advantages of faster recovery times than silicon diodes (allowing low-loss operation at higher frequencies) and a lower voltage drop when conducting. For even lower output voltages, MOSFETs may be used as synchronous rectifiers; compared to Schottky diodes, these have even lower conducting state voltage drops.

The rectified output is then smoothed by a filter consisting of inductors and capacitors. For higher switching frequencies, components with lower capacitance and inductance are needed.

Simpler, non-isolated power supplies contain an inductor instead of a transformer. This type includes boost converters, buck converters, and the buck-boost converters. These belong to the simplest class of single input, single output converters which use one inductor and one active switch. The buck converter reduces the input voltage in direct proportion to the ratio of conductive time to the total switching period, called the duty cycle. For example an ideal buck converter with a 10 V input operating at a 50% duty cycle will produce an average output voltage of 5 V. A feedback control loop is employed to regulate the output voltage by varying the duty cycle to compensate for variations in input voltage. The output voltage of a boost converter is always greater than the input voltage and the buck-boost output voltage is inverted but can be greater than, equal to, or less than the magnitude of its input voltage. There are many variations and extensions to this class of converters but these three form the basis of almost all isolated and non-isolated DC to DC converters. By adding a second inductor the Ćuk and SEPIC converters can be implemented, or, by adding additional active switches, various bridge converters can be realized.

Other types of SMPSs use a capacitordiode voltage multiplier instead of inductors and transformers. These are mostly used for generating high voltages at low currents (Cockcroft-Walton generator). The low voltage variant is called charge pump.


This charger for a small device such as a mobile phone is a simple off-line switching power supply with a European plug.

A feedback circuit monitors the output voltage and compares it with a reference voltage, as shown in the block diagram above. Depending on design and safety requirements, the controller may contain an isolation mechanism (such as an opto-coupler) to isolate it from the DC output. Switching supplies in computers, TVs and VCRs have these opto-couplers to tightly control the output voltage.

Open-loop regulators do not have a feedback circuit. Instead, they rely on feeding a constant voltage to the input of the transformer or inductor, and assume that the output will be correct. Regulated designs compensate for the impedance of the transformer or coil. Monopolar designs also compensate for the magnetic hysteresis of the core.

The feedback circuit needs power to run before it can generate power, so an additional non-switching power-supply for stand-by is added.