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SMPS

SMPS

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Category: Computers
Published on Monday, 01 March 2010 16:03
Written by Administrator
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A Switched-mode power supply (also Switching-mode power supply, SMPS, or simply Switcher) is an electronic Power Supply Unit (PSU) that incorporates a switching regulator in order to provide the required output voltage. An SMPS is actually a power converter that transmits power from a source (e.g., a battery or the electrical power grid) to a load (e.g., a personal computer) with ideally no loss. The function of the converter is to provide a reliable output voltage often at a different level than the input voltage.
Like a linear power supply, the switched mode power supply too converts the available unregulated ac or dc input voltage to a regulated dc output voltage. However in case of SMPS with input supply drawn from the ac mains, the input voltage is first rectified and filtered using a capacitor at the rectifier output. The unregulated dc voltage across the capacitor is then fed to a high frequency dc-to-dc converter. Most of the dc-to-dc converters used in SMPS circuits have an intermediate high frequency ac conversion stage to facilitate the use of a high frequency transformer for voltage scaling and isolation. In contrast, in linear power supplies with input voltage drawn from ac mains, the mains voltage is first stepped down (and isolated) to the desired magnitude using a mains frequency transformer, followed by rectification and filtering. The high frequency transformer used in a SMPS circuit is much smaller in size and weight compared to the low frequency transformer of the linear power supply circuit.

A - bridge rectifier
B - input filter capacitors
between B and C - Heatsink of high-voltage transistors
C - transformer
between C and D - Heatsink of low-voltage, high-current rectifiers
D - output filter coil
E - output filter capacitors


The ‘Switched Mode Power Supply’ owes its name to the dc-to-dc switching converter for conversion from unregulated dc input voltage to regulated dc output voltage. The switch employed is turned ‘ON’ and ‘OFF’ (referred as switching) at a high frequency. During ‘ON’ mode the switch is in saturation mode with negligible voltage drop across the collector and emitter terminals of the switch where as in ‘OFF’ mode the switch is in cut-off mode with negligible current through the collector and emitter terminals. On the contrary the voltage-regulating switch, in a linear regulator circuit, always remains in the active region.
For example, the DC component (i.e., the time average) at one terminal of an inductor will match the DC component at the other terminal. 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 voltage waveform measured across the switch will also be a square wave. Because the inductor ensures that the average value of the output waveform matches the DC source voltage, the peak amplitude of the voltage across the switch will be twice the voltage of the input. If a diode-and-capacitor combination are 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 voltage higher than the DC voltage driving the circuit. This so-called boost converter acts like a step-up transformer for DC signals.

In an 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.g., PWM with an adjustable duty cycle) to drive the switching elements. Typically, the spectral density of these switching waveforms has energy concentrated at relatively high frequencies. As such, switching transients, like ripple, introduced onto the output waveforms can be filtered with small LC filters.

If the SMPS has an AC input, then the first stage is to convert the input to DC. This is called rectification. The rectifier circuit can be configured as a voltage doubler by the addition of a switch operated either manually or automatically. This is a feature of larger supplies to permit operation from nominally 120 V or 240 V supplies. 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. 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 the designer should try 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 (for 230 V AC this would be 330 V DC), as the DC passes through the rectifier stage unchanged. It's however advisable to consult the manual before trying this, though most supplies are quite capable of such operation even though nothing is mentioned in the documentation. However, this type of use may be harmful to the rectifier stage as it will only use half of diodes in the rectifier for the full load. This may result in overheating of these components, and cause them to fail prematurely.


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 V AC) range and as a straight rectifier when operating on the high voltage (~240 V AC) 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.

 

Comparison of a Linear power supply and a switched-mode power supply
Linear power supply Switching power supply Notes
Size and weight If a transformer is used, large due to low operating frequency (mains power frequency is at 50 or 60 Hz). Small if transformerless.

 Smaller due to higher operating frequency (typically 50 kHz - 1 MHz) 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 higher capacity or smaller transformer.
Output voltage With transformer used, any voltages available; if transformerless, not exceeding input. If unregulated, voltage varies significantly with load.

 Any voltages available. Voltage varies little with load. A SMPS can usually cope with wider variation of input before the output voltage changes.
Efficiency, heat, and power dissipation If regulated, output voltage is regulated by dissipating excess power as heat resulting in a typical efficiency of 30-40%; if unregulated, transformer iron and copper losses significant. Output is regulated using duty cycle control, which draws only the power required by the load. In all SMPS topologies, the transistors are always switched fully on or fully off. The only heat generated is in the non-ideal aspects of the components. Switching losses in the transistors, on-resistance of the switching transistors, equivalent series resistance in the inductor and capacitors, core losses in the inductor, and rectifier voltage drop contribute to a typical efficiency of 60-70%. However, by optimizing SMPS design, the amount of power loss and heat can be minimized; a good design can have an efficiency of 95%.
Complexity Unregulated may be diode and capacitor; regulated has a voltage regulating IC or discrete circuit and a noise filtering capacitor. Consists of a controller IC, one or several power transistors and diodes as well as a power transformer, inductors, and filter capacitors. Multiple voltages can be generated by one transformer core. 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 3.3 V or 5 V loads are more fussy about their supply voltages than the 12 V loads, so this drives the decision as to which feeds the feedback loop. 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.

 EMI/RFI produced due to the current being switched on and off sharply. Therefore, EMI filters and RF shielding are needed to reduce the disruptive interference. Long wires between the components may reduce the high frequency filter efficiency provided by the capacitors at the inlet and outlet.
Electronic noise at the output terminals Unregulated PSUs may have a little AC ripple superimposed upon the DC component at twice mains frequency (100-120 Hz). Can cause audible mains hum in audio equipment or 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.g. for audio systems above the range of human hearing)
Electronic noise at the input terminals Causes harmonic distortion to the input AC, but relatively little or no high frequency noise. Very low cost SMPS may couple electrical switching noise back onto the mains power line, causing interference with A/V equipment connected to the same phase. Non power-factor-corrected SMPSs also cause harmonic distortion.

 This can be prevented if a (properly earthed) EMI/RFI filter is connected between the input terminals and the bridge rectifier.
Acoustic noise Faint, usually inaudible mains hum, usually due to vibration of windings in the transformer and/or magnetostriction.

 Inaudible to humans, unless they have a fan or are unloaded/malfunctioning. The operating frequency of an unloaded SMPS is sometimes in the audible human range.
Power factor Low for a regulated supply because current is drawn from the mains at the peaks of the voltage sinusoid.

 Ranging from low to medium since a simple SMPS without PFC draws current spikes at the peaks of the AC sinusoid. Active/Passive power factor correction in the SMPS can offset this problem and are even required by some electric regulation authorities, particularly in Europe.
Risk of electric shock Supplies with transformers allow metalwork to be grounded, safely. Dangerous if primary/secondary insulation breaks down, unlikely with reasonable design. Transformerless mains-operated supply dangerous. In both linear and SM the mains, and possibly the output voltages, are hazardous and must be well-isolated. Common rail of equipment (including casing) is energised to half mains voltage, but at high impedance, unless equipment is earthed/grounded or doesn't contain EMI/RFI filtering at the input terminals. Due to regulations concerning EMI/RFI radiation, many SMPS contain EMI/RFI filtering at the input stage before the bridge rectifier consisting of capacitors and inductors. 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 energises 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.
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. 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. Mitigated by good failsafe design. Failure of a component in the SMPS itself can cause further damage to other CPU components; can be difficult to troubleshoot.
 The floating voltage is caused by capacitors bridging the primary and secondary sides of the power supply. A connection to an 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 equalises to earth potential.