AC/DC Power Supply
Compared with linear supplies, switched-mode power supplies(SMPSs) not only provide a substantial boost to energy efficiency but are also smaller and lighter – advantages that have seen SMPSs completely replace their linear counterparts in many applications.
The offline AC/DC supply shown below is an example of a SMPS with power factor correction (PFC). Power factor is a unitless quantity that measures the ratio of AC power dissipated by the load (true power) to the total amount of AC power sent to the load (apparent power). A purely resistive, i.e. non-reactive, load dissipates 100% of the apparent power, which means the circuit has a unity power factor. In this case, the voltage and current are both sinusoidal and completely in phase. However, a reactive load causes a phase shift between the supplied AC voltage and current, effectively reducing power efficiency. This is undesirable, so circuits are designed to add power factor correction (PFC) to power supplies.
The power factor is “corrected” by a PFC control circuit which forces the current waveform to follow the voltage waveform as accurately as possible by pulse-width modulating the current with a power MOSFET or IGBT. In this way the PFC circuit drives the power factor much closer to 1, increasing overall energy efficiency. Designs using PFC can yield extremely efficient AC/DC supplies, while SMPSs without PFC may have a power factor of 0.6 or lower.
The secondary or output side of the power supply uses a flyback topology to achieve the appropriate output voltage. This voltage is regulated using an opto-isolated feedback loop to a pulse-width modulation (PWM) controller, which adjusts the supply’s switching duty cycle accordingly.
Filter
Diode
Controller
