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.
SMPS can also be classified into four types according to the input and output waveforms, as follows.
- AC in, DC out: rectifier, off-line converter
- DC in, DC out: voltage converter, or current converter, or DC to DC converter
- AC in, AC out: frequency changer, cycloconverter
- DC in, AC out: inverter
AC and DC are abbreviations for alternating current and direct current.
Switched-mode PSUs in domestic products such as personal computers often have universal inputs, meaning that they can accept power from most mains supplies throughout the world, with frequencies from 50 Hz to 60 Hz and voltages from 100 V to 240 V (although a manual voltage "range" switch may be required).
SMPS compared with linear PSUs
Two main types of regulated power supply are available: SMPS and linear. The reasons for choosing one type or the other can be summarised as follows.
- Size and weight. Linear power supplies use a line-(mains-)frequency transformer (if they are isolating types), and line-frequency smoothing filters. These components are larger and heavier than the corresponding parts of a SMPS, which works at a higher frequency.
- Efficiency. Linear power supplies regulate their output by generating a higher voltage than needed at the output, then reducing it by converting some of the electrical power to heat. This loss is a necessary part of the operation of the circuit, and cannot be eliminated by improving the design. SMPS generate no more voltage than they require, and only a small amount of energy is wasted.
- Heat output. This is determined by the efficiency, above. Linear PSUs output much more heat than SMPS.
- Complexity. Linear PSUs are simple enough to be designed by beginners. SMPS are complicated, difficult to design well, and the higher number of components makes them more expensive to assemble and to repair.
- Noise. The switched currents in an SMPS contain much more energy at high frequencies than those in a linear PSU. This high-frequency energy can be transmitted by electromagnetic induction to other nearby equipment, or as radio waves over long distances, causing interference. Care is therefore needed to eliminate this energy at source, or to contain it by screening.
- Power factor. If the current drawn by a load such as a SMPS from an AC supply is nonsinusoidal and/or out of phase with the supply voltage waveform, the power factor will be less than unity and the efficiency, capacity and reliability of generating plants and the transmission grid can be substantially decreased. The simplest and most common SMPS designs have a power factor of about 0.6, and their use in increasingly popular personal computers and compact fluorescent lamps presents a growing problem. Power factor correction (PFC) circuits can reduce this problem, and are required in some countries by regulation. Notably, power factor correction is not yet widely required or used in North America.
How an SMPS works
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. An SMPS with a DC input does not require this stage. An 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!)
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.
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.
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.
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.
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.
Other types of SMPS use a capacitor-diode voltage multiplier instead of inductors and transformers. These are mostly used for generating high voltages at low currents.
A feedback circuits monitors the output voltage and compares it with a reference voltage, which is set manually or electronically to the desired output. If there is an error in the output voltage, the feedback circuit compensates by adjusts the timing with which the MOSFETs are switched on and off. This part of the power supply is called the switching regulator.
Open-loop regulators do not have a feedback circuit. Instead, they rely on feeding a constant voltage to the input of the transformer or inductor, and assume that the output will be correct.
Unlike most other appliances, switched mode power supplies tend to be constant power devices, drawing more current as the line voltage reduces. Also, in common with many static rectifiers, maximum current draw occurs at the peaks of the waveform cycle. This means that basic switched mode power supplies tend to produce more harmonics in the mains power line and have a worse power factor than other types of appliances. This may cause stability problems in some situations such as emergency generator systems or for very heavy loads on ordinary power mains (as it can lead to increased neutral current and increased heating of the utility transformers. However, higher-quality switched-mode power supplies with power-factor correction (PFC) are available; these are designed to present a near resistive load to the mains. European regulatory standards are now beginning to require power factor correction and harmonic reduction.
Switched-mode power supplies can classified according to the circuit topology.
- buck regulator (single inductor; output voltage < input voltage)
- boost regulator (single inductor; output voltage > input voltage)
- buckboost regulator (single inductor; output voltage can be more or less than the input voltage)
- flyback regulator (uses output transformer; allows multiple outputs and input-to-output isolation)
- forward regulator (uses output transformer; allows multiple outputs and input-to-output isolation)
- Cuk converter (uses a capacitor for energy storage; produces negative voltage for positive input)