Uses, Advantages, and Working Principles of a Switching Power Supply
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What Is a Power Supply?
A office supply is an electrical device that converts the electric current that comes from a power generator to the electric potential value necessary for powering a load, like a motor or an electronic device .
There are two main designs for power supplies : a linear power supply and a switching world power add .
: A linear office supply designs use a transformer to step down the input electric potential. then the voltage is rectified and turned into a direct stream electric potential, which is then filtered to improve the wave form quality. analogue power supplies use analogue regulators to maintain a constant electric potential at the output. These linear regulators dissipate any extra energy in the class of heat .
: A switching office supply design is a newer methodology developed to solve many of the problems associated with linear baron supply design, including transformer size and electric potential regulation. In switching exponent provision designs, the input voltage is no long reduced ; alternatively, it ’ mho rectified and filtered at the input. then the electric potential goes through a helicopter, which converts it into a high-frequency pulse train. Before the voltage reaches the end product, it ’ sulfur filtered and rectified once again .
How Does a Switching Power Supply Work?
For many years, linear AC/DC power supplies have been transforming AC power from the utility grid into DC electric potential for running home appliances or lighting. The need for smaller supplies for high-octane applications means linear office supplies have become relegated to particular industrial and checkup uses, where they are distillery needed because of their low noise. But switching ability supplies have taken over because they are smaller, more effective, and are capable of handling gamey world power. Figure 1 illustrates the general transformation from alternating stream ( AC ) to direct current ( DC ) in a switching baron supply .
figure 1 : Isolated Switched-Mode AC/DC Power Supply
rectification is the process of converting AC voltage to DC electric potential. Input sign rectification is the first step in switched-mode AC/DC might supplies .
It is normally thought that DC voltage is a directly, firm line of changeless voltage, like the type that comes out of a battery. however, what defines direct stream ( DC ) is the unidirectional run of electric load. This means that the voltage flows in the lapp focus but is not inevitably constant .
A sine wave is alternating current ’ randomness ( AC ) most typical wave form, and is positive for the first half-cycle but minus for the rest of the cycle. If the negative half-cycle is reversed or eliminated, then the current ceases to alternate, and becomes a direct stream. This can be achieved by a process called rectification .
rectification can be achieved by using a passive half-bridge rectifier to eliminate the negative half of the sine wave using a diode (see Figure 2). The diode allows current to flow through it during the positive half of the wave, but blocks the current when it flows in the reverse management .
figure 2 : Half-Bridge Rectifier
After rectification, the resulting sine wave will have depleted beggarly might and will not be able to ability devices efficiently. A much more efficient method acting would be to change the negative half-wave ’ mho polarity and make it positivist. This method is called full-wave correction, and it only requires four diodes in a bridge shape (see Figure 3). This arrangement maintains a stable current stream guidance, regardless of the stimulation voltage polarity .
figure 3 : Full-Bridge Rectifier
A amply rectified wave has a higher mean output electric potential than the one produced by the half-bridge rectifier, but it is still very far from being the constant DC wave form needed for powering electronic devices. Although this is a DC wave, using it to power a device would be ineffective due to the shape of the electric potential wave, which changes prize very promptly and very frequently. This periodic change in DC electric potential is called a ripple — reducing or eliminating ripple is crucial to an efficient power issue .
The simplest and most normally used method acting for ripple decrease is the use of a large capacitor at the rectifier output, called a reservoir capacitor or smoothing filter (see Figure 4) .
The capacitor stores voltage during the beckon ’ south flower, then supplies the cargo with current until its electric potential is smaller than the now-rising rectify electric potential wave. The resulting wave form is much closer to the coveted shape, and can be considered a DC electric potential with no AC component. This final examination voltage wave form can nowadays be used to baron DC devices .
calculate 4 : Full-Bridge Rectifier with Smoothing Filter
passive voice rectification uses semiconductor device diodes as uncontrolled switches, and is the simplest method to rectify an AC wave, but it is not the most efficient .
Diodes are relatively efficient switches ; they can switch on and off promptly with minimal world power loss. The only problem with semiconductor diodes is that they have a ahead bias voltage drop of 0.5V to 1V, which reduces efficiency .
active correction replaces diodes with master switches, such as MOSFETs or BJT transistors (see Figure 5). The advantages of this are double : First, transistor-based rectifiers eliminate the fixed 0.5V to 1V voltage cliff associated with semiconductor diodes, because their resistances can be made randomly small, and consequently have a small voltage spend. Second, transistors are controlled switches, which means the switching frequency can be controlled and consequently optimized .
The downside is that active rectifiers require more complicated manipulate circuits to achieve their aim, which requires extra components and consequently makes them more expensive .
figure 5 : Full-Bridge Active Rectifier
Power Factor Correction (PFC)
The second stage in a switching exponent supply invention is power factor discipline ( PFC ) .
PFC circuits have little to do with the actual conversion of AC power to DC power, but are a critical component of most commercial power supplies .
figure 6 : electric potential and stream Waveforms at the Rectifier Output
If you observe the current wave form of the rectifier ’ s reservoir capacitor (see Figure 6), you ’ ll see that the charging current flows through the capacitor during a very shortstop time cross, specifically from the period where the electric potential at the input signal of the capacitor is greater than the capacitor ’ s charge to the correct signal ’ second peak. This generates a series of short current spikes in the capacitor, frankincense creating a meaning problem not equitable for the power supply, but for the entire office grid due to the large quantity of harmonics that these current spikes inject into the grid. Harmonics can generate aberration that may affect other power supplies and devices connected to the power system .
In a switching world power supply design, the goal of the power component correction circuit is to minimize these harmonics by filtering them out. To do indeed, there are two options : active and passive power factor correction .
- passive voice PFC circuits are composed of passive low-pass filters, which attempt to eliminate higher-frequency harmonics. however, power supplies, specially in high-octane applications, can not comply with international regulations on harmonic noise using only passive PFC. alternatively, they must apply active might correction .
- active PFC changes the stream wave form ’ mho condition, and makes it follow the voltage. The harmonics are moved to much higher frequencies, making them easier to filter out. The most widely used circuit for these cases is a boost converter, besides called a increase converter .
Isolation: Isolated vs. Non-Isolated Switching Power Supplies
Whether a PFC circumference is introduce or not, the final step for office conversion is to step the rectified DC electric potential down to the mighty magnitude for the mean application .
Because the stimulation AC wave form has been rectified at the stimulation, the DC electric potential output is going to be high : If there isn ’ thyroxine PFC, the output DC voltage from the rectifier will be about 320V. If there is an active PFC circuit, the boost converter ’ mho output will be a steady DC electric potential of 400V or more .
Both scenarios are extremely dangerous and useless for most applications that normally require importantly lower voltages. board 1 shows respective converter and application aspects that should be taken into account when choosing the right isolation topology .
|Isolated AC/DC Power Supplies||Non-Isolated AC/DC Power Supplies|
|Topology||Flyback converter||Buck converter|
|Safety||Galvanic isolation offers increased exploiter safety|| likely current leaks could cause significant damage to users or loads
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|Size and Efficiency||Transformers add size and weight||only one inductor needed, much smaller circuit|
|Efficiency||Transformer iron and bull losses affect efficiency||A single inductor is much more effective than an entire transformer|
|Complexity||Control circuitry is needed for both|
postpone 1 : Isolated vs. Non-Isolated AC/DC Power Supplies
The independent concern when choosing which decrease method acting to use is guard .
The might provide is connected to the AC mains at the input, which means if there was a current escape to the output, an electric shock of this proportion could sternly injure or cause death, and damage any device connected to the output .
safety can be achieved by magnetically isolating the remark and output signal circuits of a mains-connected AC/DC power supply. The most wide used circuits in detached AC/DC world power supplies are flyback converters and resonant LLC converters, because they include voltaic or magnetic isolation (see Figure 7) .
figure 7 : Flyback Converter ( Left ) and LLC Resonant Converter ( Right )
The use of a transformer means that the signal can not be a flat DC voltage. alternatively, there has to be a voltage magnetic declination, and therefore a deviate current, in order to transfer the energy from one side of the transformer to the other through inductive pair. consequently, both flyback and LLC converters “ chop ” the input DC electric potential into a square beckon, which can be stepped down via a transformer. then the output roll has to be rectified again ahead going to the output .
Flyback converters are chiefly used for low-power applications. A flyback converter is an sequester buck-boost converter, which means that the output voltage can be either higher or lower than the stimulation electric potential, depending on the transformer ’ south turns ratio between the basal and secondary coil wind .
The operation of a flyback converter is identical alike to that of a boost converter .
When the switch is closed, the chief coil is charged by the input signal, creating a charismatic field. When the switch is open, the charge in the primary inductor is transferred to the secondary wind, which injects a current into the tour, powering the load .
Flyback converters are relatively easy to design, and require fewer components than other converters, but are not very efficient because there are significant losses due to the hard switch from forcing the transistor to turn on and off randomly ( see Figure 8 ). particularly in high-octane applications, this is very damaging to the transistor ’ randomness lifecycle and generates significant power losses, which is why flyback converters are good suited to low-power applications, normally up to 100W .
Resonant LLC converters are more normally used in high-octane applications. These circuits are besides magnetically isolated through a transformer. LLC converters are based on the phenomenon of resonance, which is the amplification of a certain frequency when it matches a percolate ’ s natural frequency. In this font, an LLC converter ’ sulfur resonant frequency is defined by an inductor and a capacitor connected in serial ( LC filter ) with the lend effect of the transformer primary coil inductor ( L ), therefore the name LLC converter .
LLC resonant converters are preferred for high-octane applications because they can produce zero-current switch over, besides known as easy switching (see Figure 8). This switching method turns the throw on and off when the current in the circumference approaches zero, minimizing the transistor ’ s switching losses, which in change state reduces EMI and improves efficiency. unfortunately, this better operation comes at a price : It is difficult to design an LLC evocative converter that can achieve soft switching for a wide range of loads. To this end, MPS has developed a particular LLC design cock to help make sure that the converter works in precisely the right resonance express for optimum throw efficiency .
calculate 8 : intemperate Switching ( Left ) vs. soft Switching ( Right ) Losses
Earlier in this article, we discussed why one of the limitations of AC/DC office supplies are the size and slant of the input transformer, that due to the low operational frequency ( 50Hz ) demands boastfully inductors and charismatic cores in order to avoid impregnation .
In switching power supplies, the oscillation frequency in the voltage is significantly greater ( above 20kHz at the very least ). This means that the decrease transformer can be smaller, because high frequency signals generate fewer magnetic losses in analogue transformers. The size reduction of input signal transformers enables the miniaturization of the arrangement, to the orient where an entire baron provision fits into a case the size of the mobile earphone chargers we use today .
There are DC devices that don ’ t need the isolation provided by the transformer. This is normally seen in devices that don ’ t need to be directly touched by the user, such as lights, sensors, IoT, and more, because any manipulation of the device ’ sulfur parameters is done from a separate device, such as a mobile telephone, tablet, or computer .
This offers great benefits in terms of weight, size, and operation. These converters reduce the output voltage levels using a high-octane buck converter, besides called a decrease converter. This tour could be described as the inverse of the boost converter explained previously. In this case, when the transistor switch is closed, the current run through the inductor generates a voltage across the inductor that counteracts the electric potential from the exponent source, reducing the voltage at the end product. When the switch opens, the inductor releases a current that flows through the burden, maintaining the voltage respect at the load while the racing circuit is cut off from the baron source .
In AC/DC switching power supplies, a high-octane vaulting horse converter is used because the MOSFET transistor that acts as a switch must be able to withstand large changes in voltage (see Figure 9). When the interchange is closed, the electric potential across the MOSFET is close up to 0V ; but when it opens, that voltage goes up to 400V for single-phase applications, or 800V for three-phase converters. These big, abrupt changes in voltage could easily damage a normal transistor, which is why particular high-voltage MOSFETs are used .
human body 9 : Non-Isolated AC/DC Switching Power Supply with Active PFC
Buck converters can be much more easily integrated than a transformer, because only one inductor is needed. They are besides much more effective at stepping down electric potential, with a normal efficiency upwards of 95 %. This flat of efficiency is possible because transistors and diodes have about no switching power loss, so the only loss comes from the inductor .
One model of a non-isolated AC/DC exponent supply output regulator is the MPS MP17xA family. This family can control many unlike converter topologies, such as dollar, rise, buck-boost or flyback. It can be used for voltages up to 700V, meaning it is intended for single-phase supplies. It besides has a fleeceable manner option, where the switch frequency and top out stream decrease proportionately to the load, improving the power add ’ randomness overall efficiency. Figure 10 shows the distinctive application circuit for the MP173A, where it is regulating a tear converter, composed of an inductor ( L1 ), diode ( D1 ), and capacitor ( C4 ). The resistors ( R1 and R2 ) make a electric potential divider that provides the feedback voltage ( FB pin ), closing the control iteration .
figure 10 : MP173A Typical Application Circuit
Switching AC/DC baron supplies offer increased performance for a fraction of the size, which is what has made them indeed popular. The downside is that their circuits are importantly more complex, and they require more precise master circuits and noise cancellation filters. Despite the total complexity, MPS provides dim-witted and efficient solutions to make the development of your AC/DC ability supply easier .
AC/DC switching ability supplies are presently the most effective way of transforming AC power to DC ability. The ability is converted in three stages :
: This process takes the AC mains electric potential and converts it into a DC rectified wave using a diode bridge. A capacitor is added at the end product of the bridge to reduce the ripple voltage .
Power factor correction (PFC)
: Because of the nonlinear current in the rectifier, the harmonic contented of the current is quite big. There are two ways two resolve this. The beginning is passive voice PFC, using a filter to dampen the effect of the harmonics, but it is not very effective. The moment option, called active PFC, uses a switching hike converter to make the current wave form follow the input signal voltage human body. active agent PFC is the only method of designing a world power converter that meets current standards of size and efficiency .
: Switching power supplies can be isolated or non-isolated. A device is isolated when the power issue ’ s stimulation and output are not physically connected. isolation is done through the use of transformers, which galvanically isolate the two halves of the circumference. however, transformers can entirely transfer electric power when there is a magnetic declination in stream, so the reform DC voltage is chopped up into a high-frequency square brandish, which is then transferred to the secondary coil circuit, where it is rectified again and last transmitted to the end product .
There are many different aspects to consider when designing a switching baron provision, specially related to condom, performance, size, weight, etc. The control condition circuits for switching world power supplies are besides more building complex than in linear power supplies, which is why many designers find it useful to implement integrate modules in their power supplies .
MPS offers a wide diverseness of modules that can simplify switching power supply design, such as might converters, controllers, rectifiers, and more .
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