"When R & D engineers of high-frequency transformer solve difficult EMI problems, they often fail to seriously study the design of high-frequency transformer. The relationship between high frequency transformer and EMI is as follows.
1、 Because the coil of the high-frequency transformer carries high-frequency current, the high-frequency transformer has actually become an antenna for receiving H field. These h fields will impact the nearby routing and conduct or radiate the H field outside the sealed range through these routing.
2、 Because some coils have swing voltage, they actually become antennas for receiving electromagnetic fields.
3、 The parasitic capacitance between the primary and secondary coils can transmit the noise outside the insulating layer. Since the grounding of the secondary coil is usually connected with the base plate, these noises will be transmitted back through the grounding plate and become common mode noise. Therefore, in order to reduce the leakage inductance, it is best to close the primary and secondary coils together, but this will also increase the mutual inductance of the coil and increase the common mode noise.
Techniques to prevent the above interference:
1、 Three layers of Mylar tape meeting the safety specifications are pasted between the primary and secondary coils of high-frequency transformers meeting the safety specifications. In addition to these three layers of polyester tape, another Faraday shielding copper sheet may be inserted to collect the noise current collected at the insulation boundary and shunt the noise current elsewhere (usually transmitted to the grounding of the primary coil). It is worth noting that extremely thin copper sheets should be used as shielding to avoid loss due to eddy current and ensure that leakage inductance can be reduced.
This piece of copper is generally 2 ~ 4 mils thick and only surrounds the central plate. Another wire is welded near the center of the copper sheet, and the other end is connected to the grounding terminal of the primary coil. It should be noted here that the two ends of the copper shield should not be connected on conductivity performance, because for high-frequency transformers, this will make this winding short circuit. A Faraday shield can also be added to the secondary coil (i.e. after adding three layers of insulation), and this shield is connected with the grounding of the secondary coil.
2、 Usually, the periphery of high-frequency transformer will be surrounded by a layer of copper shield (i.e. "flux band"). This shield is mainly used to block radiation. Low cost designs usually allow the shield to float, but the shield can also be connected to the secondary coil ground if necessary. If they are connected together in this way, some safety problems need to be considered, such as the regulation of strengthening the insulation effect between primary and secondary coils, and how to regulate the "creep" (a distance along the insulation surface) and "gap" (the shortest distance of space) between primary and secondary coils.
If the outer disk of the high frequency transformer is provided with an air gap, the peripheral magnetic flux from the air gap will produce serious eddy current loss in the magnetic flux band. Therefore, the thickness of this magnetic flux band is usually only 2 ~ 4 mils. It should be noted that both ends of the magnetic flux band can and should be welded together, because it is an outer shield, which will not short circuit the winding of the high-frequency transformer anyway. However, like Lafayette shielding, if good winding technology is adopted, this outer shielding can also not be used.
3、 From the point of view of electromagnetic interference, the flyback high-frequency transformer is best designed with a gap in the center, that is, the outer disk of the high-frequency transformer has no gap. Unshielded air gap will generate electromagnetic field around, in other words, it will generate a large number of EMI signals. These disturbances will not only cause a large amount of eddy current loss in the magnetic flux band, but also become a powerful radiation source.
Fqj, read the full text“
Our other product:
