r 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, which is inefficient; 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, and rectifier voltage drop will lower SMPS efficiency. 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. 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||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.|
|Electronic noise at the input terminals||Causes harmonic distortion to the input AC, but 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.||