| 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 3.3V or 5V loads are more fussy about their supply voltages than the 12V 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 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 |
