SWITCHMODE. POWER SUPPLY. HANDBOOK. Keith Billings. Taylor Morey. Third Edition. New York Chicago San Francisco Lisbon London. (iii) Compare the merits and demerits of SMPS vis-à-vis linear power supplies mode and linear power supplies to gain some desired advantages of both the. A switched-mode power supply is an electronic power supply that incorporates a switching (download as PDF from backcocodembe.cf backcocodembe.cf); Pressman, Abraham I.; Billings, Keith; Morey, Taylor (), Switching.
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in the SWITCHMODE Power Supplies Reference Manual and Design Guide provides real SMPS examples, and identifies several application notes and. in the Switch−Mode Power Supply Reference Manual should prove useful. supplies. It also provides real SMPS examples, and identifies several application . Switch Mode Power Supplies: From Circuit Theory to the Workbench. Michael Tse. Power Electronics Research Centre. Department of Electronic & Information .
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. 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.
Switched-mode power supply
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.
Switched-mode power supply
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 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. One of the two switches must be active e.
regulated SMPS - 1 or 2-phase - 100.240 V AC- 48 V - 2.5 A
Sometimes, the topology can be changed simply by re-labeling the connections. The neutral point clamped NPC topology is used in power supplies and active filters and is mentioned here for completeness. Switchers become less efficient as duty cycles become extremely short. For large voltage changes, a transformer isolated topology may be better. All isolated topologies include a transformer , and thus can produce an output of higher or lower voltage than the input by adjusting the turns ratio.
Higher input voltage and synchronous rectification mode makes the conversion process more efficient. The power consumption of the controller also has to be taken into account. Higher switching frequency allows component sizes to be shrunk, but can produce more RFI.
A resonant forward converter produces the lowest EMI of any SMPS approach because it uses a soft-switching resonant waveform compared with conventional hard switching. For failure in switching components, circuit board and so on read the failure modes of electronics article. That may expose connected loads to the full input volt and current, and precipitate wild oscillations in output. Failure of the switching transistor is common. Due to the large switching voltages this transistor must handle around V for a V AC mains supply , these transistors often short out, in turn immediately blowing the main internal power fuse.
The main filter capacitor will often store up to volts long after the power cord has been removed from the wall. Not all power supplies contain a small "bleeder" resistor to slowly discharge this capacitor.
Any contact with this capacitor may result in a severe electrical shock. The primary and secondary sides may be connected with a capacitor to reduce EMI and compensate for various capacitive couplings in the converter circuit, where the transformer is one.
This may result in electric shock in some cases. Switched-mode power supply units PSUs in domestic products such as personal computers often have universal inputs, meaning that they can accept power from mains supplies throughout the world, although a manual voltage range switch may be required.
Switch-mode power supplies can tolerate a wide range of power frequencies and voltages. Due to their high volumes mobile phone chargers have always been particularly cost sensitive. The first chargers were linear power supplies , but they quickly moved to the cost effective ringing choke converter RCC SMPS topology, when new levels of efficiency were required.
Recently, the demand for even lower no-load power requirements in the application has meant that flyback topology is being used more widely; primary side sensing flyback controllers are also helping to cut the bill of materials BOM by removing secondary-side sensing components such as optocouplers. Switched-mode power supplies are used for DC to DC conversion as well. A common use for switched-mode power supplies is as extra-low-voltage sources for lighting, and for this application they are often called "electronic transformers".
From Wikipedia, the free encyclopedia. Main article: Copper loss.
See also: Power factor. Electronics portal Energy portal. Retrieved 21 March Archived from the original on 2 August Page 9 mydocs. Search the page for "doubler" for more info. Retrieved March Retrieved Archived from the original on Archived from the original PDF on APEC , pp.
Information and symptoms". Retrieved from " https: Power supplies Power electronics Electric power conversion Voltage regulation. Hidden categories: All articles with failed verification Articles with failed verification from December All articles with unsourced statements Articles with unsourced statements from November Commons category link is on Wikidata.
Smaller transformer if used; else inductor due to higher operating frequency typically 50 kHz — 1 MHz. Size and weight of adequate RF shielding may be significant.
A transformer's power handling capacity of given size and weight increases with frequency provided that hysteresis losses can be kept down. Therefore, higher operating frequency means either a higher capacity or smaller transformer. With transformer used, any voltages available; if transformerless, limited to what can be achieved with a voltage doubler.
If unregulated, voltage varies significantly with load. Any voltages available, limited only by transistor breakdown voltages in many circuits. Voltage varies little with load. Efficiency , heat, and power dissipation. If regulated: Output is regulated using duty cycle control; the transistors are switched fully on or fully off, so very little resistive losses between input and the load.
The only heat generated is in the non-ideal aspects of the components and quiescent current in the control circuitry. Unregulated may be simply a diode and capacitor; regulated has a voltage-regulating circuit and a noise-filtering capacitor; usually a simpler circuit and simpler feedback loop stability criteria than switched-mode circuits. Consists of a controller IC, one or several power transistors and diodes as well as a power transformer, inductors, and filter capacitors.
For this SMPSs have to use duty cycle control. One of the outputs has to be chosen to feed the voltage regulation feedback loop usually 3. The following table compares linear regulated and unregulated AC-to-DC supplies with switching regulators in general: Comparison of a linear power supply and a switched-mode power supply Linear power supply Notes Size and weight Heatsinks for high power linear regulators add size and weight. Smaller transformer if used; else inductor due to higher operating frequency typically 50 kHz — 1 MHz.
Size and weight of adequate RF shielding may be significant. A transformer's power handling capacity of given size and weight increases with frequency provided that hysteresis losses can be kept down. Therefore, higher operating frequency means either a higher capacity or smaller transformer. Output voltage With transformer used, any voltages available; if transformerless, limited to what can be achieved with a voltage doubler.
If unregulated, voltage varies significantly with load. Any voltages available, limited only by transistor breakdown voltages in many circuits. Voltage varies little with load. An SMPS can usually cope with wider variation of input before the output voltage changes. Output is regulated using duty cycle control; the transistors are switched fully on or fully off, so very little resistive losses between input and the load.
The only heat generated is in the non-ideal aspects of the components and quiescent current in the control circuitry. Complexity Unregulated may be simply a diode and capacitor; regulated has a voltage-regulating circuit and a noise-filtering capacitor; usually a simpler circuit and simpler feedback loop stability criteria than switched-mode circuits. Consists of a controller IC, one or several power transistors and diodes as well as a power transformer, inductors, and filter capacitors.
For this SMPSs have to use duty cycle control. One of the outputs has to be chosen to feed the voltage regulation feedback loop usually 3.
The other outputs usually track the regulated one pretty well. Both need a careful selection of their transformers. Due to the high operating frequencies in SMPSs, the stray inductance and capacitance of the printed circuit board traces become important.
Radio frequency interference Mild high-frequency interference may be generated by AC rectifier diodes under heavy current loading, while most other supply types produce no high-frequency interference.
Some mains hum induction into unshielded cables, problematical for low-signal audio. Long wires between the components may reduce the high frequency filter efficiency provided by the capacitors at the inlet and outlet.
Stable switching frequency may be important. It can cause audible mains hum in audio equipment, brightness ripples or banded distortions in analog security cameras. Noisier due to the switching frequency of the SMPS.
An unfiltered output may cause glitches in digital circuits or noise in audio circuits. This can be suppressed with capacitors and other filtering circuitry in the output stage. With a switched mode PSU the switching frequency can be chosen to keep the noise out of the circuits working frequency band e.
Non power-factor-corrected SMPSs also cause harmonic distortion. Acoustic noise Faint, usually inaudible mains hum, usually due to vibration of windings in the transformer or magnetostriction. The operating frequency of an unloaded SMPS is sometimes in the audible human range, and may sound subjectively quite loud for people whose hearing is very sensitive to the relevant frequency range.
Power factor Low for a regulated supply because current is drawn from the mains at the peaks of the voltage sinusoid , unless a choke-input or resistor-input circuit follows the rectifier now rare. The internal resistance of low-power transformers in linear power supplies usually limits the peak current each cycle and thus gives a better power factor than many switched-mode power supplies that directly rectify the mains with little series resistance.
Inrush current Large current when mains-powered linear power supply equipment is switched on until magnetic flux of transformer stabilises and capacitors charge completely, unless a slow-start circuit is used.
Extremely large peak "in-rush" surge current limited only by the impedance of the input supply and any series resistance to the filter capacitors. Empty filter capacitors initially draw large amounts of current as they charge up, with larger capacitors drawing larger amounts of peak current. Being many times above the normal operating current, this greatly stresses components subject to the surge, complicates fuse selection to avoid nuisance blowing and may cause problems with equipment employing overcurrent protection such as uninterruptible power supplies.
Mitigated by use of a suitable soft-start circuit or series resistor. Risk of electric shock Supplies with transformers isolate the incoming power supply from the powered device and so allow metalwork of the enclosure to be grounded safely. Transformerless mains-operated supply dangerous. In both linear and switch-mode the mains, and possibly the output voltages, are hazardous and must be well-isolated.
Two capacitors are connected in series with the Live and Neutral rails with the Earth connection in between the two capacitors. This forms a capacitive divider that energizes the common rail at half mains voltage. Its high impedance current source can provide a tingling or a 'bite' to the operator or can be exploited to light an Earth Fault LED. However, this current may cause nuisance tripping on the most sensitive residual-current devices. It can also provide some very mild tingling sensation but it's safe to the user .
Risk of equipment damage Very low, unless a short occurs between the primary and secondary windings or the regulator fails by shorting internally. Can fail so as to make output voltage very high. Stress on capacitors may cause them to explode. Can in some cases destroy input stages in amplifiers if floating voltage exceeds transistor base-emitter breakdown voltage, causing the transistor's gain to drop and noise levels to increase.
The floating voltage is caused by capacitors bridging the primary and secondary sides of the power supply. Connection to earthed equipment will cause a momentary and potentially destructive spike in current at the connector as the voltage at the secondary side of the capacitor equalizes to earth potential. 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.
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.
Voltage converter and output rectifier[ edit ] 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.
Simpler, non-isolated power supplies contain an inductor instead of a transformer.Please turn to the next chapter to read the full SMPS circuits operation in details. One thing Holt has to his credit is that he created the switching power supply that allowed us to do a very lightweight computer".
While every attempt has been made to verify the information provided here, the author cannot assume any responsibility for any loss, injury, errors, inaccuracies, omissions or inconvenience sustained by anyone resulting from this information.
These belong to the simplest class of single input, single output converters which use one inductor and one active switch. Monopolar designs also compensate for the magnetic hysteresis of the core. Recommended Electronic Repair E-Books…………………. Another option is to send the SMPT for rewinding. 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.
Other types of SMPSs use a capacitor — diode voltage multiplier instead of inductors and transformers.
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