Reduce the design experience of switching power supply electromagnetic noise - Database & Sql Blog Articles

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The characteristic of the switching power supply is that it generates strong electromagnetic noise. If it is not strictly controlled, it will cause great interference. The techniques presented in this article help to reduce switching power supply noise and can be used in high sensitivity analog circuits.

Circuit and device selection

A key point is to keep dv/dt and di/dt at a low level. There are many circuits that reduce the radiation by reducing dv/dt and/or di/dt, which also reduces the stress on the switching tubes. These circuits include ZVS (Zero Voltage Switch). ZCS (Zero Current Switch). Resonance mode. (a type 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 lead to an increase in efficiency, because the RF oscillation of the magnetic component requires a strong loss of buffering, and finally a continuously weakened return stroke can be observed. The use of soft-switching technology, while slightly reducing efficiency, has greater benefits in terms of cost savings and space for filtering/shielding.

Damping

In order to protect the switching tube from the impact of the oscillating spike voltage due to factors such as parasitic parameters, damping is often required, and the damper is connected to the coil in question, which also reduces the emission.

There are many types of dampers: From an EMC perspective, RC dampers are usually the best at EMC, but more than others. Weigh the pros and cons of all aspects, and use inductive resistors carefully in the buffer.

heat sink

There is a 50pF capacitor between the heat sink and the collector or the drain of the TO247 power device, so a strong emission can be produced. Simply attaching the heat sink directly to the case, this simply directs the noise to the ground and may not reduce the overall emission level.

A good practice is to connect them to an appropriate circuit node - a rectified output, but pay attention to safety requirements. The insulating spacers with shielding can be connected to the switch tubes, and the shielded inner layers are connected to the primary rectifying ends, and the heat sinks are either suspended or connected to the casing.

The heat sink can also be connected to a line with dangerous voltage through a capacitor. The inductance of the capacitor lead and the PCB trace may “resonate” with the capacitor, which is especially effective for solving problems at certain special frequencies. It should be tested several times on the prototype to find the best installation method for the heat sink.

Rectifier

Rectifiers and secondary rectifiers used on primary power supplies can cause a lot of noise because of their reverse current. It is best to use devices with fast soft switching models.

Magnetic components related issues and solutions

It is important to note that the magnetic circuit of the inductor and transformer is to be closed. For example, with a toroidal or seamless core, the toroidal iron powder core is suitable for storing magnetic energy. If the magnetic ring is slit, a complete short-circuit ring is needed to reduce parasitic leakage.

The primary switching noise is injected into the secondary through the inter-turn capacitor of the isolation transformer, producing common-mode noise at the secondary, which is difficult to filter out, and due to the long flow path, the emission phenomenon occurs.

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 pay attention to safety and never exceed the total leakage current indicated by the safety standard. This capacitor also helps the secondary filter to work better.

The inter-turn coil shield (within the isolation transformer) can more effectively suppress the primary switching noise induced on the secondary. Although there have been more than five layers of shielding, three layers of shielding are more common. The shield near the primary coil is typically connected to the primary power line, and the shield near the secondary coil is often connected to the common output ground (if any), and the intermediate shield is typically attached to the chassis. It is best to experiment repeatedly in the prototype phase to find the best way to connect the coil turns.

Both of these techniques also reduce the secondary switching noise induced at the input. An appropriately sized output inductor can turn the secondary AC waveform into a half sine wave, which can significantly reduce the noise between the transformer windings (DC ripple).