1. Field of Invention
The present invention relates to a switching regulator eliminating beat oscillation and a method for eliminating beat oscillation; particularly, it relates to a switching regulator eliminating beat oscillation and a method for eliminating beat oscillation by means of an inductorless filter.
2. Description of Related Art
There are many types of switching regulators, including buck converter, boost converter, inverter converter, and mixing types such as buck-boost converter and inverter-boost converter. By way of example, FIG. 1 shows a conventional buck switching regulator 1 which comprises two transistors Q1 and Q2; the gates of the transistors Q1 and Q2 are controlled by up-gate and low-gate signals UG and LG, respectively. The switching of the transistors Q1 and Q2 controls the current amount and direction on the inductor L, so that power is transmitted to the output OUT. The signals UG and LG are generated according to a feedback signal FB obtained from the output OUT, for example by a PWM (Pulse Width Modulation) controller 10 or a PFM (Pulse Frequency Modulation, not shown), etc. The feedback control mechanism is well known to those skilled in this art and therefore is not redundantly explained here. FIG. 2 and FIG. 3 show boost switching regulator 2 and inverter switching regulator 3, respectively.
Referring to FIG. 4, in a switching regulator (buck converter as an example), a PWM comparator (PWM) compares a voltage signal Vc with a saw tooth wave to generate the signals for controlling the transistorsQ1 and Q2 (for simplicity in explaining the theory, only one PWM comparator is shown in the circuit in FIG. 4; in an actual circuit, the two transistors may not be controlled by the same PWM comparator). The saw tooth wave has a frequency fs, while the voltage signal Vc has a frequency fp. Referring to FIG. 5A-5C, when the voltage signal Vc carries a frequency fp, in the voltage Vd at the common node between the transistors Q1, Q2 and the inductor L, side bands (or called beat oscillation) will be generated at the frequencies −2fs+fp, −fp, −fs+fp, fs−fp, . . . , that is, at the frequency mfs±fp (wherein m is an integer). Because the inductor L and the output capacitor provide low-pass filtering effect, most of the side bands in the output voltage Vo will be filtered out. However when the frequency fp is close to the frequency fs, the frequencies −fs+fp and fs−fp are low frequencies, which can not be filtered out. Instead, the beat oscillation still exists in the output voltage Vo, as shown in FIG. 5C.
The problem mentioned above will become more serious in multiphase switching regulators. Referring to FIG. 6, the first-phase buck switching regulator circuit generates current io1, and the second-phase buck switching regulator circuit generates current io2, constituting the total output current io. The beat oscillation results in a serious imbalance between the output currents io1 and io2 of the two phases as shown in FIG. 5D, and impacts the stability of the circuit.
In view of the foregoing, this invention proposes a switching regulator eliminating beat oscillation, and a method for controlling a switching regulator, to remedy the defects in the conventional technology.