source:Industry News Popular:adapter release time:2021-06-21 10:03:30 Article author:sznbone
In order to prevent radio frequency current from flowing between the primary and secondary windings or between the primary and the grounded safety shield, the main switching transformer usually requires at least an electrostatic RFI shield on the primary winding. In some applications, a safety shield is required between the primary and secondary windings. Electrostatic RFI shielding and safety shielding mainly have different safety standards in terms of structure, location, and connection. Safety shielding loops to the grounding surface or chassis. Surface RF shielding generally returns to the input or output circuit. EMI shielding and wiring heads are made of very thin copper sheets. It only needs to transmit a small current. However, for safety reasons, the rated current of the safety shield must be at least three times the rated current of the power adapter fuse.
The figure shows a typical circuit of the safety shield and RF shield in the off-line switching transformer. In the fully shielded application shown in the figure, the two RF1 shields are close to the primary and secondary windings, and the safety shield is located at the two Between two RFI shields. If the secondary RFI shield is not required, then the safety shield should be located between the primary RF shield and any output windings. For further careful isolation, the primary side RF shield will be DC-isolated from the input power line by a series capacitor. This capacitor is generally sufficient at a rated isolation voltage of 0.01µF.
Only when maximum noise suppression is required or when the output voltage is high, use the secondary side RF shield shown in the figure, and this shield will return to the common end of the output line. Use transformer shielding only when necessary, because it increases the height of components and windings, which increases leakage inductance and degrades performance.
The loop current of the high-frequency shield may be quite large during switching transients. In order to prevent this current from being coupled to the secondary side through a general transformer action, the shielding connection point should be in the center and the surface should not be on the edge. In this way, the capacitively coupled shield loop current flows in the opposite direction of the respective half of the shield, eliminating all inductive coupling effects. It should be remembered that the ends of the shield must be insulated from each other to avoid a closed loop.
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