Switching power adapter electrostatic shielding

Issue Time:2006-11-25
One of the most difficult problems in the design of a switching power adapter is to reduce the common mode conducted RF current to an acceptable range. This conductive electrical noise problem is mainly caused by parasitic static electricity and electromagnetic coupling between the switching elements and the ground plane. The ground plane may be the cabinet, cabinet, and grounding wire, depending on the type of equipment.

The designer should examine the entire layout, identify areas where these problems may exist, and introduce appropriate shielding methods during the design phase. In the later stages, it will be difficult to correct products with poor RFI design.

In most applications, where high frequency, high voltage switching waveforms may be capacitively coupled to the ground plane or secondary output, electrostatic shielding is required. A switch transistor is placed on the heat sink to rent a rectifier diode and the heat sink is in contact with the main housing, which is a typical position. In addition, magnetic fields and capacitive coupling can generate noise on components or lines that have large switching currents flowing through them. Other potentially problem areas include the output rectifier, the output capacitor mounted on the chassis, the primary side on the main switching transformer, the secondary side, and the capacitive coupling between the core and other drive or control transformers.

Electrostatic shielding applied to switchgear
When the part is mounted on a heat sink and the heat sink is thermally coupled to the housing, the conventional method of eliminating undesirable capacitive coupling is to place an electrostatic shield between the interfering component and the heat sink. This type of shielding is generally exemplified and must be insulated from the heat sink and the transistor or diode to shield the AC current that can pick up the capacitive coupling and return it to a convenient reference point for the input circuit. The reference point is generally the common negative terminal of the DC power adapter line, which is near the switching device. For secondary components, the reference point is typically the current flowing back to the common side of the secondary side of the transformer.

The figure shows an example of electrostatic shielding of a TO3 transistor. This primary switching transistor has high voltage and high frequency switching waveforms, unless there is proper shielding between the transistor case and the case, a large noise current will be coupled through the capacitance between them. In the shielded mounting arrangement shown, the copper shield will cause parasitic noise currents to flow back to the input circuit so that the resulting current loop does not direct current to the ground plane. This shield does not inject any significant current into the heat sink through the capacitor, which has a relatively small high frequency AC voltage relative to the chassis or ground plane. Designers will identify other areas where problems may occur, and similar shields can be used in this area.

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