Smps And Linear Power Supply Comparison

SMPS and linear power supply comparison There are two main types of regulated power supplies available: SMPS and linear. The reasons for choosing one type or the other can be summarized as: Comparison of a Linear power supply and a switched-mode power supply Linear power supply

Switching power supply

Notes

A transformer's power handling capacity of given If a transformer is used, size and weight increases Size large due to low operating Smaller due to higher with frequency provided that and frequency (mains power operating frequency (typically hysteresis losses can be kept weight frequency is at 50 or 60 Hz). 50 kHz - 1 MHz) down. Therefore, higher Small if transformerless. operating frequency means either higher capacity or smaller transformer. With transformer used, any voltages available; if Output transformerless, not Any voltages available. voltage exceeding input. If Voltage varies little with load. unregulated, voltage varies significantly with load.

A SMPS can usually cope with wider variation of input before the output voltage changes.

The only heat generated is in the non-ideal aspects of the components. Switching losses in the transistors, onresistance of the switching If regulated, output voltage Efficie transistors, equivalent series is regulated by dissipating Output is regulated using duty ncy, resistance in the inductor excess power as heat cycle control, which draws heat, and capacitors, core losses in resulting in a typical only the power required by the and the inductor, and rectifier efficiency of 30-40%[1]; if load. In all SMPS topologies, power voltage drop contribute to a unregulated, transformer the transistors are always dissipa typical efficiency of 60iron and copper losses switched fully on or fully off. tion 70%. However, by significant. optimizing SMPS design, the amount of power loss and heat can be minimized; a good design can have an efficiency of 95%. Compl Unregulated may be diode exity and capacitor; regulated has a voltage regulating IC or discrete circuit and a noise

Consists of a controller IC, one or several power transistors and diodes as well as a power transformer,

Multiple voltages can be generated by one transformer core. For this SMPSs have to use duty

filtering capacitor.

inductors, and filter capacitors.

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.

Mild high-frequency interference may be generated by AC rectifier EMI/RFI produced due to the Radio diodes under heavy current current being switched on and freque loading, while most other off sharply. Therefore, EMI ncy supply types produce no filters and RF shielding are interfer high-frequency interference. needed to reduce the ence Some mains hum induction disruptive interference. 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.

Unregulated PSUs may have a little AC ripple Electro superimposed upon the DC nic Noisier due to the switching component at twice mains noise frequency of the SMPS. An frequency (100-120 Hz). at the unfiltered output may cause Can cause audible mains output glitches in digital circuits or hum in audio equipment or termin noise in audio circuits. brightness ripples or banded als distortions in analog security cameras.

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)

Electro Causes harmonic distortion nic to the input AC, but noise relatively little or no high at the frequency noise.

This can be prevented if a (properly earthed) EMI/RFI filter is connected between the input terminals and the

Very low cost SMPS may couple electrical switching noise back onto the mains power line, causing

input termin als

interference with A/V equipment connected to the same phase. Non powerfactor-corrected SMPSs also cause harmonic distortion.

Faint, usually inaudible Acoust mains hum, usually due to Inaudible to humans, unless ic vibration of windings in the they have a fan or are noise transformer and/or unloaded/malfunctioning. magnetostriction.

bridge rectifier.

The operating frequency of an unloaded SMPS is sometimes in the audible human range.

Active/Passive power factor correction in the SMPS can Low for a regulated supply Ranging from low to medium offset this problem and are Power because current is drawn since a simple SMPS without even required by some factor from the mains at the peaks PFC draws current spikes at electric regulation of the voltage sinusoid. the peaks of the AC sinusoid. authorities, particularly in Europe. Due to regulations concerning EMI/RFI radiation, many SMPS contain EMI/RFI filtering at the input stage before the bridge rectifier consisting of Supplies with transformers capacitors and inductors. allow metalwork to be Two capacitors are grounded, safely. Dangerous connected in series with the if primary/secondary Common rail of equipment Live and Neutral rails with insulation breaks down, (including casing) is energised the Earth connection in Risk of unlikely with reasonable to half mains voltage, but at between the two capacitors. electric design. Transformerless high impedance, unless This forms a capacitive shock mains-operated supply equipment is earthed/grounded divider that energises the dangerous. In both linear or doesn't contain EMI/RFI common rail at half mains and SM the mains, and filtering at the input terminals. voltage. Its high impedance possibly the output voltages, current source can provide a are hazardous and must be tingling or a 'bite' to the well-isolated. operator or can be exploited to light an Earth Fault LED. However, this current may cause nuisance tripping on the most sensitive residualcurrent devices. Risk of Very low, unless a short Can fail so as to make output The floating voltage is equipm occurs between the primary voltage very high. Can in caused by capacitors

some cases destroy input stages in amplifiers if floating bridging the primary and voltage exceeds transistor secondary sides of the power base-emitter breakdown supply. A connection to an voltage, causing the earthed equipment will ent and secondary windings or transistor's gain to drop and cause a momentary (and damag the regulator fails by noise levels to increase.[2] potentially destructive) spike e shorting internally. Mitigated by good failsafe in current at the connector as design. Failure of a the voltage at the secondary component in the SMPS itself side of the capacitor can cause further damage to equalises to earth potential. other PSU components; can be difficult to troubleshoot.

Applications Switched mode mobile phone charger 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 rated frequencies from 50 Hz to 60 Hz and voltages from 100 V to 240 V (although a manual voltage range switch may be required). In practice they will operate from a much wider frequency range and often from a DC supply as well. In 2006, at an Intel Developers Forum, Google engineers proposed the use of a single 12 V supply inside PCs, due to the high efficiency of switch mode supplies directly on the PCB.[10] Most modern desktop and laptop computers already have a DC-DC converter on the motherboard, to step down the voltage from the power supply or the battery to the CPU core voltage, as low as 0.8 V for a low voltage CPU to 1.2-1.5 V for a desktop CPU as of 2007. Most laptop computers also have a DC-AC converter to step up the voltage from the battery to drive the backlight in the flat-screen, typically around 1000 Vrms.[11] 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 material (BOM) by removing secondary-side sensing components such as optocouplers.

Why Use a Switching Regulator? Switching regulators offer three main advantages compared to a linear regulators. First, switching efficiency can be much better than linear. Second, because less energy is lost in the transfer, smaller components and less thermal management are required. Third, the energy stored by an inductor in a switching regulator can be transformed to output

voltages that can be greater than the input (boost), negative (inverter), or can even be transferred through a transformer to provide electrical isolation with respect to the input Linear regulators provide lower noise and higher bandwidth; their simplicity can sometimes offer a less expensive solution. There are, admittedly, disadvantages with switching regulators. They can be noisy and require energy management in the form of a control loop. Fortunately the solution to these control problems is found integrated in modern switching-mode controller chips.

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