How a Switching Mode Power Supply Works

A switching mode power supply (SMPS) is an electronic circuit that efficiently converts electrical power from one form to another, typically from mains AC voltage to a regulated DC output. Let's break down the key components and processes involved in a typical SMPS:

Block Diagram of a Switch Mode Power Supply electrobuff.blogspot.com

Input Filter:

The SMPS begins with an input filter that reduces electromagnetic interference (EMI) and radio frequency interference (RFI) from the incoming AC power. This filter typically includes inductors and capacitors.

Rectification and Smoothing:

The filtered AC power is then rectified to convert it into pulsating DC using a diode bridge. This pulsating DC is then smoothed using a capacitor to reduce voltage fluctuations.

Power Factor Correction (PFC):

In order to improve the power factor and make the power supply more efficient, a power factor correction stage is often employed. This corrects the phase relationship between the input voltage and current, minimizing wasted power.

Switching Transistor:

The heart of the SMPS is the switching transistor, which rapidly switches the DC voltage on and off. This process enables high-frequency AC voltage to be generated, reducing the size of the transformer and improving efficiency.

Controller IC:

A controller IC (Integrated Circuit) is responsible for regulating the output voltage. It monitors the output voltage and adjusts the switching frequency of the transistor to maintain a stable output.

Ferrite Power Transformer:

The high-frequency AC voltage generated by the switching transistor is then passed through a ferrite power transformer. The transformer steps down or steps up the voltage as required, and its compact design is made possible by the high frequency of operation.

Output Rectification and Smoothing:

The transformer output is rectified again using diodes to convert the AC voltage back to DC. Another smoothing capacitor further reduces voltage ripples, resulting in a relatively stable DC voltage.

Voltage Feedback and Regulation using Voltage Reference IC:

A portion of the output voltage is fed back to the controller IC through a voltage feedback loop. The controller compares this feedback voltage with a reference voltage from a precision voltage reference IC. Any deviations trigger adjustments in the switching frequency to maintain the desired output voltage.

Optocoupler:

An optocoupler is often employed to provide electrical isolation between the input and output sides of the SMPS. It uses a combination of an LED and a photodetector to transmit the feedback signal while maintaining isolation.

This combination of components and processes allows the switching mode power supply to efficiently regulate the output voltage while minimizing size and maximizing energy efficiency.

Types of Switching Mode Power Supplies

Block Diagram of a Switch Mode Power Supply electrobuff.blogspot.com

Switching Mode Power Supply Practical Circuit Diagram

12v 5A SMPS Circuit 1

12Volts 5Amps Switching Mode Power Supply Explained Practical Circuit electrobuff.blogspot.co

12V 1Amp SMPS Circuit 2

Switching Mode Power Supply Practical Circuit electrobuff.blogspot.com

Shown above is a straightforward yet working switch mode power supply. The heart of this circuit is the TNY267 switching regulator. This one IC performs the functions of the PWM generator, MOSFET chopper switch, error amp, and control circuit.

The final output of this circuit is 12V, and it can deliver 1A:

Vin is a 100-300V AC mains connector.
MOV is a metal oxide varistor, used to protect the circuit from high voltage spikes.
D3 is a full-wave bridge rectifier, and the DC output appears across capacitor C2. With a 220V AC input, this would be about 220V * 1.4 = 308V DC, so be careful!
D2 is a 180V transient suppression diode to handle back EMF spikes.
The TNY267 runs at about 132kHz.
Diode D1 (Schottky) on the secondary rectifies the 132kHz AC, and capacitor C1 smooths out the ripple.
Capacitor C3 is a required bypass capacitor.
Resistors R1, R2, and diode D5 provide a feedback circuit to the TNY267 via an optoisolator to ensure there is electrical isolation from the mains at all points.
The primary of transistor T1 is 157t, and the secondary is 14t. The core is an E19 type ferrite transformer with a central core of about 4.5×4.5mm.

Parts of the circuit above contain very high AC and DC voltages. Make absolutely certain that the components you use are rated to handle such voltages. And never work on high voltage devices unless you are fully trained and competent to do so.