avings) in "off-line" power supplies come from the fact that a high frequency transformer is a lot smaller than the 50/60 Hz transformers used before SMPS.
There are several differences in the design of transformers for 50 Hz vs 500 kHz. Firstly a low frequency transformer usually transfers energy through its core (soft iron), while the (usually ferrite) core of a high frequency transformer limits leakage. Since the waveforms in a SMPS are generally high speed (PWM square waves), the wiring must be capable of supporting high harmonics of the base frequency due to the skin effect, which is a major source of power loss.
Simple "off-line" switched mode power supplies incorporate a simple full wave rectifier connected to a large energy storing capacitor. Such SMPSs draw current from the AC line in short pulses when the mains instantaneous voltage exceeds the voltage across this capacitor. During the remaining portion of the AC cycle the capacitor provides energy to the power supply. As a result, the input current of such basic switched mode power supplies has high harmonic content and relatively low power factor. This creates extra load on utility lines, increases heating of the utility transformers and standard AC electric motors, and may cause stability problems in some applications such as in emergency generator systems or aircraft generators. Harmonics can be removed through the use of filter banks but the filtering is expensive, and the power utility may require a business with a very low power factor to purchase and install the filtering onsite.
In 2001 the European Union put into effect the standard IEC/EN61000-3-2 to set limits on the harmonics of the AC input current up to the 40th harmonic for equipment above 75 W. The standard defines four classes of equipment depending on its type and current waveform. The most rigorous limits (class D) are established for personal computers, computer monitors, and TV receivers. In order to comply with these requirements modern switched-mode power supplies normally include an additional power factor correction (PFC) stage.
Putting a current regulated boost chopper stage after the off-line rectifier (to charge the storage capacitor) can help correct the power factor, but increases the complexity (and cost).
|Type||Power [Watts]||Typical Efficiency||Relative cost||Input range [Volts]||Isolation||Energy storage||Voltage relation||Features|
|Buck||0–1000||75%||1.0||5–1000*||N||Single inductor||Out < In|
|Boost||0–150||78%||1.0||5–600*||N||Single inductor||Out > In|
|Buck-boost||0–150||78%||1.0||5–600*||N||Single inductor||Up or down||Inverted output voltage|
|Split-Pi (Boost-Buck)||0-2000||95%||>2.0||10-100||N||Two inductors + three capacitors||Up or down||Bidirectional power control In or Out|
|Flyback||0–150||78%||1.0||5–600||Y||Transformer||Up or down||Multiple outputs|
|Half-Forward||0–250||75%||1.2||5-500||Y||Transformer + inductor|
|Forward||78%||60-200||Y||Transformer + inductor||Multiple outputs|
|Resonant, zero voltage switched||>1000||>2.0|
|Ćuk||N||Capacitor + two inductors||Up or down||Negative voltage for positive input.|
|Ćuk isolated||Y||One (pure AC) transformer + two capacitors + two inductors||Up or down||Negative or positive voltage output.|
|Inverting charge-pump (Modified Ćuk)||N||Single inductor||Output voltage negative and higher-magnitude than positive input voltage.|
|SEPIC||N||Two inductors||Up or down|
|Charge pump||N||Capacitors only||Charge pumps used to generate very high voltages are usually called voltage multipliers.|
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, Intel proposed the use of a single 12 V supply inside PCs, due to the high efficiency of switch mode supplies directly o