A DC voltage is used as the power supply in many electronic devices. Generally, the DC voltage is derived from an AC power source. The AC voltage is rectified into an unregulated DC voltage by a rectifier bridge. The unregulated DC voltage is converted into the DC voltage as needed by a switching power supply.
A transformer or inductor is usually used as a tank element in the switching power supply. For example, a transformer is used in the flyback converter. A switch is electrically coupled to the primary winding of the transformer. The switch is turned on and off so as to alternately store energy in the transformer and transfer the stored energy to the secondary winding of the transformer. An output capacitor is electrically coupled to the secondary winding of the transformer and a rectified voltage is generated thereon. The rectified voltage provides the DC output voltage of the switching power supply. The DC output voltage increases and decreases inversely with the load. The heavier the load, which means the higher the output current, the lower the output voltage, and vice versa. Generally, the DC output voltage is fed back to control compensation for the variation of the load.
Under CCM (continuous current mode, which means the current flowing through the tank element is continuous), the output power of the switching power supply is
            P              out        ⁢        _        ⁢        CCM              =                  1        2            ⁢              L        ⁡                  (                                    I              peak              2                        -                          I              valley              2                                )                    ⁢      f      ⁢                          ⁢      η        ,while under DCM (discontinuous current mode, which means the current flowing through the tank element is discontinuous), the output power is
            P              out        ⁢        _        ⁢        DCM              =                  1        2            ⁢              LI        peak        2            ⁢      f      ⁢                          ⁢      η        ,wherein L is the inductance of the tank element, Ipeak is the peak value of the current flowing through the tank element, Ivalley is the valley value of the current flowing through the tank element, f is the switching frequency and η is the efficiency of the switching power supply.
Generally, the switching frequency of the switching power supply is very high, such as tens of kilohertz, to get high efficiency and small volume. The high-frequency switching will cause a serious EMI (electromagnetic interference) problem, which may not only reduce the quality of the power network, but also influence the electrical devices connected to or close by the switching power supply. EMI standards, such as EN55022, are established as a result. The EMI limit in these standards is often lower during high frequency, and higher during low frequency. So, it is difficult for a switching power supply with ultra-high switching frequency to pass the EMI standards. Furthermore, the higher the switching frequency, the larger the power loss.