Switched-mode power supply

A switched-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator — an internal control circuit that switches the load current rapidly on and off in order to stabilise the output voltage. Switching regulators are used as replacements for simpler linear regulators when higher efficiency, smaller size or lighter weight are required. They are, however, more complicated and more expensive, their switching currents can cause noise problems if not carefully suppressed, and simple designs can have a poor power factor.

How an SMPS works
Rectifier stage
If the SMPS has an AC input, then its first job is to convert the input to DC. This is called rectification. The rectifier circuit is often the same as that in a linear power supply, and produces an unregulated DC voltage which is then smoothed by a 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. Special control techniques can be employed by the following SMPS to force the average input current to follow the sinusoidal shape of the AC input voltage thus correcting the power factor. A SMPS with a DC input does not require this stage. A SMPS designed for AC input can often be run from a DC supply, as the DC passes through the rectifier stage unchanged. (The user should check the manual before trying this!)

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If an input range switch is used, the rectifier stage is usually configured to operate as a voltage doubler when operating on the low voltage (~120 VAC) range and as a straight rectifier when operating on the high voltage (~240 VAC) range. If an input range switch is not used, then a full-wave rectifier is usually used and the downstream inverter stage is simply designed to be flexible enough to accept the wide range of dc voltages that will be produced by the rectifier stage. In higher-power SMPSs, some form of automatic range switching may be used.

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Inverter stage
The inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by switching it on and off ('chopping') at a frequency of tens or hundreds of kilohertz (kHz). The frequency is usually chosen to be above 20 kHz, to make it inaudible to humans. The switching is done by MOSFETs, which are a type of transistor with a low on-resistance and a high current-handling capacity. This section refers to the block marked "Chopper" in the block diagram.

Related Topics:
Kilohertz - MOSFET - Transistor - Resistance

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Voltage converter
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.

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If a DC output is required, the AC output from the transformer is rectified and smoothed by a filter consisting of inductors and capacitors. The higher the switching frequency, the smaller these components can be made.

Related Topics:
Inductor - Capacitor
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Simpler, non-isolated power supplies contain an inductor instead of a transformer. This type includes boost converters, buck converters, and the so called "buck-boost converter". These belong to the simplest class of single input, single output converters which utilise one inductor and one active switch (MOSFET). The buck converter reduces the input voltage, in direct proportion, to the ratio of the active switch "on" time to the total switching period, called the Duty Ratio. For example an ideal buck converter with a 10V input operating at a duty ratio of 50% will produce an average output voltage of 5V. A feedback control loop is usually employed to maintain (regulate) the output voltage by varing the duty ratio 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 bases of almost all isolated and non-isolated DC to DC conveters. By adding a second inductor the Cuk and SEPIC converters can be implemented or by adding additional active switches various bridge converters can be realised.