In contrast, a linear power supply regulates the output voltage by continually dissipating power in the pass transistor. This higher power conversion efficiency is an important advantage of a switched-mode power supply. Switched-mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight. Switching regulators are used as replacements for linear regulators when higher efficiency, smaller size or lighter weight are required.
They are, however, more complicated; their switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor power factor. A linear regulator provides the desired output voltage by dissipating excess power in ohmic losses e. In contrast, a SMPS changes output voltage and current by switching ideally lossless storage elements, such as inductors and capacitors , between different electrical configurations. For example, if a DC source, an inductor, a switch, and the corresponding electrical ground are placed in series and the switch is driven by a square wave , the peak-to-peak voltage of the waveform measured across the switch can exceed the input voltage from the DC source.
This is because the inductor responds to changes in current by inducing its own voltage to counter the change in current, and this voltage adds to the source voltage while the switch is open.
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If a diode-and-capacitor combination is placed in parallel to the switch, the peak voltage can be stored in the capacitor, and the capacitor can be used as a DC source with an output voltage greater than the DC voltage driving the circuit. This boost converter acts like a step-up transformer for DC signals.
A buck—boost converter works in a similar manner, but yields an output voltage which is opposite in polarity to the input voltage. Other buck circuits exist to boost the average output current with a reduction of voltage.
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In a SMPS, the output current flow depends on the input power signal, the storage elements and circuit topologies used, and also on the pattern used e. The spectral density of these switching waveforms has energy concentrated at relatively high frequencies.
As such, switching transients and ripple introduced onto the output waveforms can be filtered with a small LC filter. The main advantage of the switching power supply is greater efficiency than linear regulators because the switching transistor dissipates little power when acting as a switch. Other advantages include smaller size, lower noise, and lighter weight from the elimination of heavy line-frequency transformers, and comparable heat generation. Standby power loss is often much less than transformers.
Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy that the low-pass filter must block to avoid electromagnetic interference EMI , a ripple voltage at the switching frequency and the harmonic frequencies thereof. Non- power-factor-corrected SMPSs also cause harmonic distortion.
There are two main types of regulated power supplies available: SMPS and linear. The following table compares linear regulated and unregulated AC-to-DC supplies with switching regulators in general:. This is called rectification. In some power supplies mostly computer ATX power supplies , the rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically.
The rectifier produces an unregulated DC voltage which is then sent to a large 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. To correct for this, many newer SMPS will use a special PFC circuit to make the input current follow the sinusoidal shape of the AC input voltage, correcting the power factor. This type of use may be harmful to the rectifier stage, however, as it will only use half of diodes in the rectifier for the full load. This could possibly result in overheating of these components, causing them to fail prematurely. The diodes in this type of power supply will handle the DC current just fine because they are rated to handle double the nominal input current when operated in the V mode, due to the operation of the voltage doubler.
DC–DC Power Converters - Erickson - - Major Reference Works - Wiley Online Library
This is because the doubler, when in operation, uses only half of the bridge rectifier and runs twice as much current through it. The inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by running it through a power oscillator, whose output transformer is very small with few windings at a frequency of tens or hundreds of kilohertz. 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. If a DC output is required, the AC output from the transformer is rectified. For output voltages above ten volts or so, ordinary silicon diodes are commonly used. For lower voltages, Schottky diodes are commonly used as the rectifier elements; they have the advantages of faster recovery times than silicon diodes allowing low-loss operation at higher frequencies and a lower voltage drop when conducting.
For even lower output voltages, MOSFETs may be used as synchronous rectifiers ; compared to Schottky diodes, these have even lower conducting state voltage drops. The rectified output is then smoothed by a filter consisting of inductors and capacitors. For higher switching frequencies, components with lower capacitance and inductance are needed.
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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 buck—boost converters. These belong to the simplest class of single input, single output converters which use one inductor and one active switch.
The buck converter reduces the input voltage in direct proportion to the ratio of conductive time to the total switching period, called the duty cycle. A feedback control loop is employed to regulate the output voltage by varying the duty cycle 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 basis of almost all isolated and non-isolated DC to DC converters. Other types of SMPSs use a capacitor — diode voltage multiplier instead of inductors and transformers. These are mostly used for generating high voltages at low currents Cockcroft-Walton generator.
The low voltage variant is called charge pump. A feedback circuit monitors the output voltage and compares it with a reference voltage. Depending on design and safety requirements, the controller may contain an isolation mechanism such as an opto-coupler to isolate it from the DC output.
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Switching supplies in computers, TVs and VCRs have these opto-couplers to tightly control the output voltage. 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. Regulated designs compensate for the impedance of the transformer or coil.
Monopolar designs also compensate for the magnetic hysteresis of the core.
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The feedback circuit needs power to run before it can generate power, so an additional non-switching power-supply for stand-by is added. Any switched-mode power supply that gets its power from an AC power line called an "off-line" converter  requires a transformer for galvanic isolation. Some DC-to-DC converters may also include a transformer, although isolation may not be critical in these cases. SMPS transformers run at high frequency. There are additional design tradeoffs.
The terminal voltage of a transformer is proportional to the product of the core area, magnetic flux, and frequency. By using a much higher frequency, the core area and so the mass of the core can be greatly reduced. However, core losses increase at higher frequencies. Cores generally use ferrite material which has a low loss at the high frequencies and high flux densities used.
Also, more energy is lost during transitions of the switching semiconductor at higher frequencies. Furthermore, more attention to the physical layout of the circuit board is required as parasitics become more significant, and the amount of electromagnetic interference will be more pronounced. For these frequencies, the skin effect is only significant when the conductors are large, more than 0. Switching power supplies must pay more attention to the skin effect because it is a source of power loss. The effective resistance of conductors increases, because current concentrates near the surface of the conductor and the inner portion carries less current than at low frequencies.
The skin effect is exacerbated by the harmonics present in the high speed pulse width modulation PWM switching waveforms. The appropriate skin depth is not just the depth at the fundamental, but also the skin depths at the harmonics. In addition to the skin effect, there is also a proximity effect , which is another source of power loss. Simple off-line switched mode power supplies incorporate a simple full-wave rectifier connected to a large energy storing capacitor.
Such SMPSs draw current from the AC line in short pulses when the mains instantaneous voltage exceeds the voltage across this capacitor. During the remaining portion of the AC cycle the capacitor provides energy to the power supply. As a result, the input current of such basic switched mode power supplies has high harmonic content and relatively low power factor.
This creates extra load on utility lines, increases heating of building wiring, the utility transformers , and standard AC electric motors, and may cause stability problems in some applications such as in emergency generator systems or aircraft generators. Harmonics can be removed by filtering, but the filters are expensive.
Unlike displacement power factor created by linear inductive or capacitive loads, this distortion cannot be corrected by addition of a single linear component. Additional circuits are required to counteract the effect of the brief current pulses. Putting a current regulated boost chopper stage after the off-line rectifier to charge the storage capacitor can correct the power factor, but increases the complexity and cost.
The standard defines four classes of equipment depending on its type and current waveform. The most rigorous limits class D are established for personal computers, computer monitors, and TV receivers. To comply with these requirements, modern switched-mode power supplies normally include an additional power factor correction PFC stage. Switched-mode power supplies can be classified according to the circuit topology.
The most important distinction is between isolated converters and non-isolated ones. Non-isolated converters are simplest, with the three basic types using a single inductor for energy storage. In the voltage relation column, D is the duty cycle of the converter, and can vary from 0 to 1. The input voltage V 1 is assumed to be greater than zero; if it is negative, for consistency, negate the output voltage V 2. The buck, boost, and buck—boost topologies are all strongly related.
Input, output and ground come together at one point. One of the three passes through an inductor on the way, while the other two pass through switches.