Switching power supplies are characterised by the generation of strong electromagnetic noise which, if not strictly controlled, can be extremely disruptive. The techniques described below help to reduce switching power supply noise and can be used in highly sensitive analogue circuits.
1. Circuit and device selection
A key point is to keep dv/dt and di/dt at low levels. There are many circuits that reduce radiation by reducing dv/dt and/or di/dt, which also reduces the stress on the switching tubes, these include ZVS (zero voltage switching), ZCS (zero current switching), resonant mode. (a type of ZCS), SEPIC (Single-Ended Primary Inductance Converter), CK (a set of magnetic structures, named after their inventor), etc.
Reducing the switching time does not necessarily cause an increase in efficiency, as the RF oscillations of the magnetic components need to be buffered by strong losses and eventually a constantly weakening backhaul can be observed. The use of soft-switching techniques, although slightly reducing efficiency, has greater benefits in terms of cost savings and space taken up by filtering/shielding.
2. Damping
Damping is often required to protect switching tubes from oscillation spikes due to parasitic parameters etc. Dampers are connected to problematic coils, which can also reduce emission.
There are various types of dampers: from an EMC point of view, RC dampers are usually the best in terms of EMC, but generate more heat than others. Weighing up the pros and cons, inductive resistors should be used with caution in buffers.
3. Problems related to magnetic components and solutions
Particular attention needs to be paid to the magnetic circuit closure of inductors and transformers. For example, with toroidal or seamless cores, toroidal powdered iron cores are suitable for storing magnetic energy. If slits are made in the magnetic ring, a completely shorted ring is required to reduce the parasitic leakage field.
Primary switching noise is injected into the secondary via the coil-to-turn capacitance of the isolation transformer, generating common mode noise in the secondary, which is difficult to filter out and can cause emissions due to the long flow path.
A very effective technique is to connect the secondary ground to the primary power line with a small capacitor to provide a return path for these common mode currents, but be careful not to exceed the total leakage ground current indicated by the safety standards, this capacitor also helps the secondary filter to work better.
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