 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.||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 PSU 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.|
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 volt or 240 volt 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.