SMPS What is a switched-mode power supply?

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This article explains the meaning of a switched-mode power supply and takes a closer look on its working priciples.

A switched-mode power supply, sometimes known as a switch-mode power supply or ‘SMPS’, is an electronic power supply that integrates a switching regulator for efficient electrical power conversion. Like other supplies, an SMPS transfers power from a DC or AC source to DC loads while converting voltage and current.

Unlike in linear power supplies, the pass transistor of a switched-mode supply is constantly switching between low-dissipation, full-on, and full-off states. It spends very little time in high dissipation transitions, and this minimizes the amount of electricity that is wasted.

Basic concept of a switch-mode power supply

The core concept of a SMPS is that regulation is handled by using a switching regulator. This uses a series switching element that turns the current supply to a smoothing capacitor between on and off states. The time that the series element is turned on for is controlled by the voltage on the capacitor. If the voltage is higher than what’s needed, the switching element is turned off. If lower than required, it’s turned on.

In its most basic form, the operation of a SMPS involves unregulated input DC being fed to the inverter section which consists of fast-switching components such as MOSFETs and bipolar transistors. This leads to the input voltage to appear at the primary winding as pulses at 20 to 200 kHz switching frequency. The transformer output is then rectified and smoothed to produce the required DC voltages.

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Types of switched-mode power supply: Isolated converters

Switched-mode power supplies can be broadly categorized into two types based on their circuit topology: isolated and non-isolated converters.

Isolated converters are those where isolation is maintained between the input and output circuitry. Despite there being several types of isolated switched-mode power supply, the main two types are known as ‘flyback’ and ‘forward’. Both make use of a transformer to separate the switching from the output while the secondary winding of the transformer acts as the energy storage element.

Flyback converter

A flyback converter is a power supply topology that uses a mutually coupled inductor to store energy when current passes through it, which is released when the power source is removed.

While flyback converters are similar to boost converters in architecture and performance, the primary winding of the transformer replaces the inductor while the second provides the output.

Forward converter

A forward converter is a DC/DC converter that uses a transformer to increase or decrease the output voltage and provide galvanic isolation for the load. With multiple output windings, it is possible to provide both higher and lower voltage outputs simultaneously using a forward converter.

Although the forward converter looks similar to a flyback converter, it operates in a fundamentally different way and is generally more efficient. While the flyback converter stores energy in the magnetic field in the inductor air gap during the time the transistor is conducting, the forward converter does not. Instead, energy flows directly to the output of the forward converter by transformer action during conduction.

APPLICATION NOTE

Verification Methods Of Snubber Circuits in Flyback Converters

Types of switched-mode power supply: Non-isolated converters

In a non-isolated converter, the input source and output load share a common current path during operation, and energy is transferred via energy storage elements such as inductors and capacitors. The main types of non-isolated converter are the ‘buck’, ‘boost’, and ‘buck-boost’.

Buck converter

A buck converter, also known as a ‘step down’ converter, is a DC-DC power converter that steps down voltage from its input supply while stepping up current to its output load. This type of converter typically contains either a diode and a transistor or two transistors and at least one energy storage element, such as a capacitor or inductor.
Buck converters provide much greater power efficiency and are often above 90 %, making them useful in computer power supply applications where the main supply voltage needs to be stepped down to power other components such as DRAM, CPU, and USB, which require much lower voltages.

Boost converter

A boost converter, also known as a ‘step-up’ converter, is a DC-DC power converter that does the opposite of a buck converter: it steps up voltage from its input while stepping up current to its output load.

Again, this type of converter typically contains either a diode and a transistor or two transistors and at least one energy storage element, such as a capacitor or inductor.

Buck-boost converter

A buck-boost converter combines elements of both a buck and boost converter, enabling it to supply a regulated DC output from a power source that delivers a voltage either above or below the regulated output voltage.

Since it combines elements of both buck and boost converters, the buck-boost often has a larger footprint than either of the two when used in isolation.

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