High-frequency transformers and EMI have to tell the story

- May 01, 2019-

When r & d engineers solve the thorny EMI problems, they often fail to seriously study the high frequency transformer design. The relationship between high frequency transformer and EMI is as follows.

  1. As the coil of the high frequency transformer contains high-frequency current, the transformer has actually become an antenna to receive H field. These H fields will impact nearby wires and conduct or radiate the H field beyond the sealed range through these wires.

  2. 2.Because some coils have oscillating voltages, they actually become antennas that receive electromagnetic field.

  3. 3.

Parasitic capacitance between primary and secondary coil can be transferred to the noise insulation outside as often secondary coil grounding with bottom together, so the noise will pass the ground send back again, become the common mode noise accordingly in order to reduce the leakage inductance, it is best to primary and secondary coil tightly together, but it also can increase the mutual inductance coil, thus increasing the common-mode noise.

Technology to prevent the occurrence of the above interference:

Meet safety specifications of the transformer between the primary and the secondary coil have three layers of meet safety specifications of the polyester tape (tool) in addition to the three layers of polyester adhesive tape, may also be additional inserts a Faraday shield plate, so that will bring together on insulating boundary noise current collection together, and the noise current shunt elsewhere (usually sent to the primary coil grounding) that are worth noting is that should use a very thin copper as a block, in order to avoid the loss due to a vortex, and make sure that can reduce the leakage inductance of the copper thick commonly 2 ~ 4Mil (mil), only a week around the central disk and a near the center of the copper wire welding, while the other end connected to the primary coil earthing end here to note that the ends of the copper shield should not have conductivity 

performance connection, because for transformer, it will short-circuit the circle can also be on the secondary coil (i.e., with three layers of insulation) and then adding a Faraday shield, and the shielding and the secondary coil grounding together. By the sunshine of cracks.


The periphery of the high frequency transformer will usually have a layer of copper shield (i.e., magnetic passband) surrounded by the radiation shielding mainly to keep out the design of the low cost usually let the shielding floating, but if necessary, the screen can also be used with the secondary coil grounding together together if according to this way, then you need to consider some security problems, such as strengthening of primary and secondary coil insulation effect between the provisions of the problem, and how the regulation between the primary and secondary coil peristalsis (along the distance on the surface of the insulation) and clearance of the shortest distance (space)If transformer outside dish is equipped with air gap, derived from peripheral air-gap flux can produce serious eddy current loss in flux so the magnetic flux with thickness is usually only 2 ~ 4 mil to note is that the magnetic flux with both ends of the can and should be welded together, because this is the outer shield, no matter how to also won't let transformer winding short-circuit condition but as rafael benitez's first screen, if the good winding technology.

From the point of view of electromagnetic interference, it is better to adopt the design with clearance in the center, that is, the outer disk of the HF transformer has no clearance. Unshielded air gaps create electromagnetic fields around them, in other words, a lot of EMI signals. In addition to causing large eddy current losses in the magnetic flux belt, these disturbances can also be powerful sources of radiation.