Research on EMC of MCM power supply module

① Planar conversion design specifications For high-frequency switching devices inside the power supply, such as power VMOS tubes, high-frequency transformers, rectifier tubes, etc., the loop area of ​​the circuit current should be reduced as much as possible, and should not be distributed parallel to other conduction strips for a long distance. The positive input terminal and the ground wire of the power supply should be as close as possible to reduce the loop area of ​​differential mode radiation. When designing the wiring, the routing should have as few turns as possible, and the turns are generally arc-shaped, because right angles or angles will produce current mutations and generate EMI interference. The line width on the conduction strip should not change suddenly, and there should be no sharp edges.

Figure 1 Schematic diagram of the filter In order to reduce the influence of transformer leakage inductance, the primary and secondary cross winding method is adopted and closely coupled. Pot-type magnetic cores are used as much as possible. Since the pot-type magnetic core can seal all coil windings inside the magnetic core, it has a good self-shielding effect and can effectively reduce EMI.

Figure 2 Input and output filter circuit is to absorb the overshoot generated by the rising and falling edges, and may cause self-excited oscillation. R and C absorption networks are added in the primary and secondary circuits to reduce spike interference. During debugging, the parameters of R and C must be carefully adjusted to ensure that the value of resistor R1 is between 30 and 200Ω and the value of capacitor C1 is between 100 and 1000P, so as not to affect the efficiency of the transformer.

② The power switch tube will generate electromagnetic interference EMI because the power tube works in a high-frequency on-off switching state. When a large pulse current flows through the switch tube, a rectangular wave is generally formed, which contains many high-frequency components. Due to the component parameters used in the switching power supply (such as the storage time of the switch tube, the large current of the output stage, and the reverse recovery time of the switch rectifier tube), the circuit will be short-circuited instantly, generating a large short-circuit current. Any wire with short-circuit current and the electromagnetic field generated by the transformer and inductor through which this pulse current flows can form a noise source.

The load of the switch tube is a high-frequency transformer. At the moment the switch tube is turned on, a large inrush current appears in the primary of the transformer, causing spike noise. This spike noise is actually a sharp pulse, which can cause interference in mild cases and may break down the switch tube in severe cases.

Therefore, the following measures must be taken: Optimize the design of the power tube drive circuit. Through the buffer circuit, the on-off process of the power switch tube can be delayed. Use R and C absorption circuits to maintain the circuit performance while reducing the EMI level of its electromagnetic interference.

③ Rectifier diodes During the turn-off period, the rectifier diodes will cause spike interference due to the reverse recovery time. In order to reduce this electromagnetic interference, it is necessary to select diodes with soft recovery characteristics, small reverse recovery current and short reverse recovery time. Schottky barrier diodes are majority carrier conduction diodes, and there is no storage and recombination effect of minority carriers, so they will also produce very small voltage spike interference, so the following measures are taken.

●Use R1 and C1 to form a bypass absorption network.

● Use multiple Schottky diodes in parallel to share the load current, effectively suppressing the EMI level formed by the rectifier diode. 3 EMC design technology during product plane conversion There are many aspects that affect product EMC. In addition to optimizing the design on the line, how to reasonably arrange the position of components and the wiring of the conduction band within the limited space of the substrate will also directly affect the anti-interference of each component in the circuit and the electromagnetic compatibility EMC index of the product.