Switching power supply is characterized by strong electromagnetic noise, if not strictly controlled, will produce great interference. The techniques described below help reduce switching power supply noise and can be used for high-sensitivity analog circuits.

　1 circuit and device selection
　A key point is to keep dv / dt and di / dt at a low level, and there are many circuits that reduce the radiation by reducing dv / dt and / or di / dt, which also reduces the pressure on the switch tube, which includes ZVS (ZCS), ZCS (zero current switch), resonant mode (one of ZCS), SEPIC (single-ended primary inductor converter), CK (a set of magnetic structures, named after its inventor).
　Reducing the switching time does not necessarily result in an increase in efficiency because the RF oscillations of the magnetic elements require a strong loss of buffering and ultimately a diminishing return can be observed. The use of soft-switching technology, although slightly reduced efficiency, but in cost savings and filtering / shielding space occupied by greater benefits.

　2 damping
　In order to protect the switch from the parasitic parameters and other factors caused by the oscillation peak voltage shock often need damping. The damper is connected to the problematic coil, which can also reduce the emission.
　There are many types of dampers: from the EMC point of view, RC dampers are usually the best on the EMC, but more than the other heat. Weigh the pros and cons of all aspects of the buffer should be used with caution inductive resistance.

　3 magnetic components related problems and solutions
　It is important to note that the magnetic circuit of the inductor and the transformer is closed. For example, with a ring or a seamless core, the ring iron core is suitable for storing magnetic energy. If the magnetic ring is slit, a complete short circuit is required to reduce the parasitic leakage magnetic field.
　The primary switching noise is injected into the secondary through the coil turn-to-turn capacitance of the isolating transformer, producing common-mode noise at the secondary level, which is difficult to filter, and the emission path is generated due to the long flow path.
　A very effective technique is to connect the secondary capacitor to the primary power line in a secondary way, providing a return path for these common-mode currents, but pay attention to safety, never exceeding the total leakage current , This capacitor also helps the secondary filter to work better.
　The coil turn-to-turn shielding (inside the isolation transformer) can more effectively suppress the primary switching noise induced in the secondary. Although there have been more than five layers of shielding, but the three-tier shield is more common. The shield near the primary coil is usually connected to the primary power supply line, and the shield near the secondary coil is often connected to the common output (if any), and the intermediate shield is generally connected to the housing. In the prototype stage it is best to repeat the experiment to find the best way to connect the coil turns between the shield.
　The above two technologies can also reduce the input side of the secondary switching noise induced. The appropriate size of the output inductor can be secondary AC waveform into a half sine wave, which can significantly reduce the transformer winding noise (DC ripple).

　4 radiator
　There is a capacitance of 50pF between the radiator and the drain of the collector or TO247 power device, thus producing a strong emission. Simply connecting the heatsink to the case, it is only the noise to the earth, probably not to reduce the overall launch level.
The best practice is to connect them to an appropriate circuit node - a rectified output, but pay attention to safety requirements. Insulated spacers with shields can be connected to the switch tubes, connect their shields to the primary rectifier, and the heatsinks either suspended or connected to the enclosure.
　The heatsink can also be connected to a dangerous voltage line by a capacitor, and the inductance of the capacitor leads and the PCB traces may be "resonated" with the capacitor, which can be particularly effective in solving problems at some particular frequency. Should be tested on the prototype several times, and finally find the best way to install the heat sink.

　5 rectifier devices
　Used for rectification on a primary power supply and a secondary rectifier because of its reverse current, which can cause a lot of noise, preferably using a fast soft switch model device.