Switching power feature is generating strong electromagnetic noise, if strict controls, will have a significant interference. Technology described below can help reduce the switching power supply noise, high sensitivity can be used analog circuits.
         Choose a circuit and devices
A key point is to keep the dv / dt and di / dt at a lower level, there are many circuits by reducing dv / dt and / or di / dt to reduce the radiation, which also reduces the pressure on the switch. These circuits include ZVS (zero voltage switching), ZCS (zero current switching), the resonance mode. (ZCS a), SEPIC (single-ended primary inductance converter), CK (a magnetic structure, named after its inventor), etc.
Reduce the switching time is not necessarily caused by efficiency, because the magnetic element needs a strong loss of the RF oscillation buffer, eventually weakening can be observed backhaul. The use of soft-switching technology, although slightly less efficient, but there are greater benefits in terms of cost savings and filter / screen space occupied.
2 damping
In order to protect the switch from oscillating voltage spikes due to the impact of parasitic and other factors often require damping. A damper connected to the coil in question, which can also reduce the transmission.
There are many types of dampers: From the perspective of EMC, RC damper on EMC is usually the best, but some more than others fever. Weigh the pros and cons of various aspects, should be used with caution in the buffer inductive resistor.
         Three magnetic components related problems and solutions
Special attention is required to the magnetic circuit inductors and transformers to be closed. For example, with a circular or seamless core, toroidal iron core suitable for storing magnetic energy of the occasion, if the slotted ring, you need a complete short-circuit loop to reduce parasitic leakage magnetic field.
Primary switching noise will be injected through an isolation transformer between the capacitor to the secondary coil turns in the secondary common mode noise, which is difficult to filter out noise currents and flows due to the longer path will generate emission phenomena.
A very effective technique is to use a small capacitor connected secondary to the primary power line to provide a return path for the common mode current, but pay attention to safety, do not exceed safety standards to indicate the total leakage current this also helps the secondary filter capacitor better job.
Shield between the coils (isolation transformer) can more effectively inhibit the induction of secondary primary switching noise. Although have had five or more shielding, but three more common shield. Usually near the primary coil is connected to the primary shield the power line, often close to the secondary winding is connected to a common output shield ground (if any), the intermediate shield is generally connected to the chassis. In repeated experiments with the prototype stage is best to find the best connection between the coils shield.
These two technologies can reduce the input sensor on the end of secondary switching noise. An appropriate size of the output inductor may become the secondary AC waveform half sine wave, it is possible to significantly reduce the noise between the transformer winding (DC ripple).
4 radiator
There 50pF capacitor between the radiator and TO247 collector or drain of the power device, it is possible to produce a strong emission. Only the heatsink directly connected to the case, this is just the noise toward the ground, it may not reduce the overall emission levels.
A better approach is: to connect them to an appropriate circuit node - a rectifier output, but pay attention to safety requirements. An insulating spacer having a shielding effect can be connected to the switch, to which the inner shield is connected to one end of the rectifier, the fins connected to either the suspension or chassis.
Inductance capacitor fins can also be connected to the line voltage is dangerous, capacitor leads and PCB traces and capacitance may constitute "resonance", which can solve the problem of some special frequencies are particularly effective. Several tests on the prototype should eventually find the best method to install the heatsink.
5 rectifier
A rectifier and secondary rectifiers for power supply because of its reverse current can cause a lot of noise, it is best to use fast and soft switching device models.