It is known for those skilled in the art that all electric system have radiation of one kind or another. Noise is typically an unwanted pest that interferes with clean power supply. There are several types of radiation, wherein the common types are electromagnetic interference (EMI) and radio frequency interference (RFI).
Generally, EMI is caused by the generation of radiation that is induced by the charge difference between three electrical wires from an AC power source, i.e. live (or hot) supply line, neutral line and ground line. EMI noise is classified into two common types. One is common-mode noise, wherein this is the radiation that is caused by the charge difference between live and neutral lines with respect to ground line. Another one is differential mode noise, wherein this is the radiation that is caused by the charge difference between live supply line and neutral line.
EMI has been proved to be an unintentional by-product that can seriously deteriorate the quality factor of power supply device and bring harmful effect to the power supply system. To address these undesired disturbances incurred due to electromagnetic radiation during the period of power transmission, S. Okochi and T. Takoda in their U.S. Pat. No. 5,179,362 describes a power line filter to attenuate the normal mode noise appearing between power lines and common mode noise appearing between a power line and a ground line, which is incorporated herein for reference. The EMI filter disclosed in this example includes a plurality of first inductors interposed between the live supply line and the neutral line, a first X-capacitor interposed between the live supply line and the neutral line on the load side relative to the first inductors, a plurality of second inductors interposed between the live line and the neutral line on the load side relative to the first X-capacitor, and a Y-capacitor interposed between the live line and the ground line as well as between the neutral line and the ground line on the load side relative to the second inductors, wherein the Y-capacitor constituting a common mode filter between the first inductors and the second inductors in response to common mode noise.
Although the EMI filter taught in this prior art reference is well suited for the suppression of EMI noise, the EMI filter given in this prior art reference is encompassed with numerous inductors and capacitors. For a power supply engineer, the use of electronic device having inductive or capacitive characteristic usually indicates high cost and space-consuming issues. If these bulky elements are to be mounted on a circuit board, it is unpractical in space utilization and its economic benefit is greatly downscaled.
It is therefore urged to provide a switching mode power supply including a compact power line filter being implemented in a costless and space-saving way to attenuate the differential mode noise occurring between the live supply line and the neutral line.