Power Supplies

Smps Circuit Diagram Power Supply

What is Power Supply?

Power-supply is an electronic circuit that is used for the purpose of providing the electrical-power to appliances or loads such as machines, computers, & so on. These electrical & electronic appliances require various types of power at different ranges & also with different characteristics. Therefore, for this reason the power is converted into the recommended forms (with desired qualities) by using some electronic power converters or power converters. In this project we’ll show you a circuit diagram of smps power supply alongwith description.

Electrical & electronic loads work with various forms of power supplies, such as AC power supply, High-voltage power supply, AC- to-DC power supply, Uninterruptable power supply (UPS) , Programmable power supply & Switch-mode power supply.

What is Switch-mode-power supply?

It is actually the electronic power supply integrated with the switching regulator for converting the electrical power efficiently from one form to another form with desired characteristics & is called as switch-mode power supply (in short smps). It is used to attain regulated DC output voltage from an unregulated AC or DC input voltage.

Circuit Diagram Of Smps Power Supply
Circuit Diagram Of Smps Power Supply

Like other power supplies, switch-mode power supply is a complicated circuit that supplies the power from a source to loads. Switch-mode power supply is necessary for power consuming electrical & electronic appliances & even for preparing electrical & electronic projects.

Circuit Diagram Of Smps Power Supply

Topologies of Switch Mode Power Supply

There are several types of topologies for SMPS, among those, a few are as follows

  • DC to DC converter
  • AC to DC converter
  • Fly back converter
  • Forward-converter

Switch Mode Power Supply’s Working Principle

Here is the working of a few types of topologies of switch mode power supply:

1. DC to DC Converter SMPS Working Principle

Primarily a high-voltage DC power is directly obtained from a DC power source in a DC to DC converter. Then, this hig- voltage DC-power is switched at an extremely high switching speed usually in the range of 15 KHz to 50 KHz.

And then it’s fed to a step-down transformer that is comparable to the weight & size of a transformer unit of 50Hz. The output of the step down transformer is then further provided to the rectifier. This filtered & rectified output DC power is utilized as a source for loads & a sample of this output power is used as a feedback for controlling the output voltage.The ON time of the oscillator is controlled with this feedback voltage, & a closed-loop regulator is formed.

Circuit Diagram Of Smps Power Supply
Circuit Diagram Of Smps Power Supply DC-DC

The output of the smps is regulated by means of PWM (Pulse-Width-Modulation). As given in the circuit above, the switch can be driven by the PWM-oscillator, such that the power delivered to the step-down transformer is controlled indirectly, & hence, the output is controlled by the pulse-width-modulation, as this pulse-width signal & the output-voltage are inversely related to each other.

If the duty cycle is 50%, the maximum power is transported through the step-down transformer & if duty cycle decreases, the power transferred will also decrease by decreasing the dissipation of power.

2. Working Principle of AC to DC Converter SMPS:

There is an AC input in an AC to DC converter SMPS. It is converted into DC by rectification process using a rectifier & filter. This unregulated DC voltage is fed to the large-filter capacitor or Power Factor Correction (PFC) circuits for correction of power factor as it is affected. This is because around voltage peaks, a short current is drawn through the rectifier, these current pulses have appreciably high-frequency energy which causes the power factor to decrease.

Circuit Diagram Of Smps Power Supply
Circuit Diagram Of Smps Power Supply AC-DC

It is somewhat similar to the above explained DC to DC converter, but instead of direct DC power supply, here AC input is used. So, the combination of the rectifier & filter, shown in the block diagram is used to convert the AC into DC & switching is done by using a power ‘MOSFET’ amplifier with which very high gain can be obtained. This MOSFET transistor has low on-resistance & can withstand  high current. The switching-frequency is chosen such that it must be placed inaudible to normal human beings (mostly above 20KHz) & switching action is controlled by a feedback using the PWM-oscillator.

The AC voltage is again fed to the output transformer. The output of this transformer is then rectified & smoothed by using the output rectifier & filter.In order to control the output voltage a feedback circuit is used  by comparing it with the reference voltage.

3. Fly back Converter type SMPS Working Principle

The switch mode power supply circuit with very low output power of less than 100W (watts) is commonly a type of Fly back converter SMPS & it is very simple & low cost circuit compared to other SMPS circuits. Hence, it is used for low-power applications.

Circuit Diagram Of Smps Power Supply
Circuit Diagram Of Smps Power Supply Fly-Back Converter

The unregulated input-voltage with a constant value is converted into  a required output voltage by fast switching with the help of a ‘MOSFET’; the switching frequency is around 100 kHz. The isolation of voltage can be achieved by means of a transformer. The switch operation can be controlled by means of a PWM control while implementing a practical fly back converter.

Fly back transformer displays different characteristics as compared to general transformer. The two windings of the fly back transformer come into play as magnetically coupled inductors. The output of this transformer is passed through a diode & a capacitor for filtering & rectification. As shown in the figure, the voltage across this filter capacitor is taken as the output voltage of the SMPS.

4. Forward-Converter type SMPS Working

Forward-converter type SMPS is almost same as the Fly back converter type SMPS, but in the forward-converter type, a control is connected for controlling the switch & at the output of transformer’s secondary winding & the rectification & filtering circuit is complex  as compared to the fly back converter.

It can be called as a DC to DC buck converter, alongwith a transformer used for isolation & scaling. In addition to the diode D1 & capacitor C, a diode D2 & an inductor L are interfaced at the output end. If switch S is switched ON, then the input is given to the primary winding of the transformer & hence, a scaled-voltage is produced at transformer’s secondary winding.

Circuit Diagram Of Smps Power Supply
Circuit Diagram Of Smps Power Supply Forward Converter

This is how, the diode D1 gets forward-biased & the scaled-voltage is moved through the low pass filter preceding the loads. If the switch S is turned off then currents from the primary & secondary winding reach to zero, but the current through the inductive filter & load can not be changed at once, & a path is provided to this current by the freewheeling diode D2. By means of the filter inductor, the desired voltage across the diode D2 & to maintain the EMF needed for maintaining the continuity of the current at inductive filter.

Different types of topologies are there in which SMPS can be realized such as Buck converter, Buck-boost converter, Boost converter: Self oscillating fly back converter, Boost-buck, Cuk, Sepic. But only a few are discussed in this article, namely DC to DC converter, AC to DC converter, Fly back converter & Forward-converter.

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