Things you don’t know about high-frequency transformers and EMI

Do you understand high-frequency transformers and EMI? What is the connection between them? Many R&D engineers often complain that EMI problems are difficult to solve. In fact, if you study carefully, you will find that many times it is because of the failure to seriously study the design of the transformer.

The relationship between transformers and EMI is as follows:

1. Since the coil of the transformer carries high-frequency current, the transformer actually becomes an antenna for receiving the H field. These H fields can impinge on nearby traces and conduct or radiate the H fields beyond the enclosure through these traces.

2. Since some coils have swing voltages, they actually become antennas for receiving electromagnetic fields.

3. The parasitic capacitance between the primary and secondary coils can transmit noise outside the insulation layer. Since the ground of the secondary coil is usually connected to the base plate, the noise will be transmitted back through this ground plane and become common mode noise. Therefore, in order to reduce leakage inductance, it is best to place the primary and secondary coils close together, but this will also increase the mutual inductance of the coils, thereby increasing common mode noise.

Technologies to prevent the above interference situations:

1. Transformers that meet safety standards have three layers of polyester (Mylar) tape that meet safety standards between the primary and secondary coils. In addition to these three layers of polyester tape, an additional piece of Faraday shielding copper may be inserted to collect the noise currents collected at the insulation boundary and shunt these noise currents elsewhere (usually to primary coil ground).

It is worth noting that very thin copper sheets should be used as shields to avoid losses due to eddy currents and to ensure that leakage inductance is reduced. This piece of copper is generally 2 to 4 mils thick and only surrounds the central disk once. There is also a wire soldered near the center of the copper piece, and the other end is connected to the ground terminal of the primary coil. It should be noted here that there should be no conductive connection at both ends of the copper shield, because for the transformer, this will short-circuit the winding. You can also add a Faraday shield to the secondary coil (that is, after adding three layers of insulation), and this shield is connected to the ground of the secondary coil.

2. Usually the transformer is surrounded by a layer of copper shielding (i.e. "magnetic flux band"). This shield is mainly used to block radiation. Low-cost designs often leave this shield floating, but it can be tied to secondary coil ground if necessary. If connected in this way, some safety issues need to be considered, such as regulations to strengthen the insulation effect between the primary and secondary coils, and how to regulate the "creep" between the primary and secondary coils (along the insulation). a distance from a surface) and "gap" (the shortest distance in space). If the outer disk of the transformer is provided with an air gap, the surrounding magnetic flux originating from the air gap will cause severe eddy current losses in the flux band. Therefore, the thickness of this magnetic flux band is usually only 2 to 4 mils. It should be noted that the two ends of this flux strip can and should be welded together, because this is the outer shield and will not cause a short circuit in the windings of the transformer. But like the Lafarday shield, this outer shield can be eliminated if good winding techniques are used.

3. From the perspective of electromagnetic interference, it is best to adopt a design with a gap in the center of the flyback transformer, that is, there is no gap in the outer disk of the transformer. An unshielded air gap will generate electromagnetic fields in the surrounding area. In other words, a large number of EMI signals will be generated. In addition to causing a large amount of eddy current losses in the magnetic flux band, these disturbances will also become powerful radiation sources. The above is some analysis between high-frequency transformers and EMI. I hope it can help you.