Switch Mode Power Supply(SMPS) BY: Arijit Acharya NETAJI SUBHASH ENGINEERING COLLEGE M.tech(P.S.) Roll No - 1

 Contents:  Introduction  Theory  Types  Application  Advantage  Disadvantage  Future Scope

What is SMPS? A Switch Mode 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 source, like mains power, to a load, such as a personal computer, while converting voltage and current characteristics. An SMPS is usually employed to efficiently provide a regulated output voltage, typically at a level different from the input voltage.

Circuit Diagram of a typical SMPS

THEORY OF OPERATION: Block Diagram of SMPS

Input rectifier stage: The rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. The rectifier produces an unregulated DC voltage which is then sent to a large filter capacitor. Voltage converter and output rectifier: 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. Inverter stage: 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.

Regulation: A feedback circuit monitors the output voltage and compares it with a reference voltage, which shown in the block diagram serves this purpose. Depending on design/safety requirements, the controller may contain an isolation mechanism (such as opto couplers) to isolate it from the DC output. Switching supplies in computers, TVs and VCRs have these opto-couplers to tightly control the output voltage Precaution: The main filter capacitor will often store high voltage long after the power cord has been removed from the wall. Not all power supplies contain a small "bleeder" resistor to slowly discharge this capacitor. Any contact with this capacitor may result in a severe electrical shock.

TYPES of SMPS: SMPS can be classified according to the circuit topology. i) Isolated converters ii)Non-isolated converters a. Buck b. Boost c. Buck-boost d. Split-pi (or, boost-buck) e. Ćuk f. SEPIC g. Zeta h. Charge pump a. Flyback b. Ringing choke converter (RCC) c. Half-forward d. Forward e. Resonant forward f. Push-pull g. Half-bridge h. Full-bridge i. Resonant, zero voltage j. switched k. Isolated Ćuk

APPLICATIONS:  Personal computers  Battery chargers  Househeld appliances  Central power distribution  Vehicles  Consumer electronics  Space station

ADVANTAGES:  High efficiency: The switching action means the series regulator element is either on or off and therefore little energy is dissipated as heat and very high efficiency levels can be achieved.  Compact: As a result of the high efficiency and low levels of heat dissipation, the switch mode power supplies can be made more compact.  Flexible technology: Switch mode power supply technology can be used to provide high efficiency voltage conversions in voltage step up or "Boost" applications or step down "Buck" applications.

DISADVANTAGE:  Noise: The spikes can migrate into all areas of the circuits that the SMPS’s power if the spikes are not properly filtered. Additionally the spikes or transients can cause electromagnetic or RF interference which can affect other nearby items of electronic equipment, particularly if they receive radio signals.  Expert design required: It is often possible to put together a switch mode power supply that works. To ensure that it performs to the required specification can be more difficult. Ensuring the ripple and interference levels are maintained can be particularly tricky.  External components: external components are typically required. The most obvious is the reservoir capacitor, but filter components are also needed. These components all require space, and add to the cost.

FUTURE SCOPE:  Most commercial switch mode power supplies in the market today operate in the range 10 KHz to 50 KHz. There is now growing trend in research work and new power supply designs in increasing the switching frequencies upwards to 100 KHz and above.  MOSFETs are now increasingly replacing BJTs due to it inherit lack of storage and fall time affects when turned off.  Another new device likely to displace the BJT is the Insulated Gate Transistor (IGT). This device combines the low power drive characteristic of MOSFET with the low conduction losses and high blocking voltage characteristic of the BJT.  more and more integrated power devices will be introduced so simplifying board layout and reducing component count.Any new device or topology will have to justify its implementation based on mainly commercial criteria.

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