Effect of high-frequency transformer shielding copper-platinum on leakage inductance, parasitic capacitance and EMI

2019-06-01 11:07:39 JUKE CHINA ODM OEM Transformer factory Read

Effect of high-frequency transformer shielding copper-platinum on leakage inductance, parasitic capacitance and EMI

 The High frequency transformer shields the influence of copper and platinum on leakage inductance, parasitic capacitance and EMI; when the high-frequency transformer is wound, the winding or copper foil is added between the primary side and the secondary side, and one end is connected to the working side of the primary side, called the internal Shielding: The main purpose is to return the primary common mode interference signal to the disturbance source through the shielding layer. Without this shielding layer, a common mode interference signal is transmitted to the secondary side through the interlayer capacitance between the primary and secondary windings. , causing EMI problems at the output.
Effect of high-frequency transformer shielding copper-platinum on leakage inductance, parasitic capacitance and EMI.jpgOn the outside of the transformer, the core and the winding are covered with a layer of copper foil. This layer of copper foil is closed to form a loop circuit called external shield. The leakage inductance of the transformer is not closed inside the core, but leaks into the external space. In high frequency applications, a strong leakage magnetic field induces a voltage on the closed loop of the input and output ports, resulting in poor EMI test results. If there is an external shielding layer, according to the principle of electromagnetic induction, a current is induced in the shielding layer to form an opposite magnetic field, which counteracts the influence of the leakage magnetic field of the transformer.
 
The transformer plus shielded copper and platinum is mainly designed according to the EMI condition. According to the common mode noise and differential mode noise, it can be seen that the main effect is on common mode interference. . .
1. The common mode noise current Icm refers to the current component of the noise interference of the LINE, NEUTRAL two lines with respect to the ground line;
The common mode noise interference current is mainly caused by the parasitic capacitance of the power tube to the ground in the power supply circuit, the parasitic capacitance of the fast diode to the ground, and the parasitic capacitance and stray capacitance of the transformer;
2. Differential mode noise current Idm refers to the noise interference current component flowing directly through the L and N lines (not flowing through the ground line);
The differential mode noise interference current is caused by the discontinuous current at the primary side of the power supply circuit and the parasitic resistance and parasitic inductance on the input filter large capacitor (electrolytic capacitor)!
 
Add internal shielding layer, increase the interlayer distance, reduce the interlayer capacitance, reduce the primary and secondary coupling degree, and increase the leakage inductance!
The transformer is a source of noise, and the leakage inductance of the primary secondary and the primary interlayer capacitance, the secondary interlayer capacitance, and the coupling capacitance between the primary and secondary are the channels of noise. The primary or secondary inter-layer capacitance can be reduced by reducing the number of layers of the winding, and increasing the width of the transformer skeleton window can reduce the number of layers of the winding. Separate windings such as the primary sandwich winding method can reduce the primary leakage inductance, but increase the primary and secondary coupling capacitances by increasing the primary and secondary contact areas. A copper-shielded shield reduces the coupling capacitance between the primary and secondary. The shield is wound between the primary and secondary and is connected to primary or secondary static points such as primary and secondary. The shielding layer reduces the coupling coefficient of the primary and secondary, thereby increasing the leakage inductance.