For the MCM power supply, because it works in a high-frequency switching state of several hundred kHz, it is easy to become a source of interference. Judging from the reports and actual measures of similar foreign companies, solving the electromagnetic interference of DC/DC converters is mainly to meet the requirements of conducted emission from the power line of 10kHz～10MHz (ie CE102 in the national military standard GJB151A-97).

Key technology to solve

1. Circuit design technology

Through EDA simulation, use reliability optimization and reliability simplification technology to design circuit parameters, focusing on solving the following problems. ① Self-excited oscillation of the line: reasonably select the vibration elimination network to eliminate the oscillation caused by the unreasonable selection of the R, L, and C parameters of the DC/DC converter, and reduce the level of EMI. Because the DC/DC power supply works in a high-frequency switching state, it is easy to form high-frequency self-excitation. Sometimes the reaction is self-excitation when it is normal and light-load with full load, and sometimes it is self-excitation at normal high temperature or low temperature at room temperature, so the components The selection of the compensation network and the application of the compensation network are particularly important.

② Effective suppression of ripple and noise: The suppression methods can be roughly summarized into two categories, namely, reducing its own ripple and noise and designing filter circuits.

In order to suppress external high-frequency interference, and also to suppress the external conduction interference of the DC/DC converter, filter circuits are designed at the input and output ends of the DC/DC converter to suppress common mode and differential mode interference and reduce EMI levels. Among them, C1, C2, C3 are differential mode filter capacitors, C4 and C5 are common mode filter capacitors, L1 is a common mode choke coil, and L2 is a differential mode filter inductor.

In order to reduce the conduction of EMI through the input and output terminals of the DC/DC converter, in addition to LC filtering at the input and output terminals, high-frequency filter capacitors are added between the input ground of the power supply and the metal casing, and between the output ground and the metal casing. , In order to reduce the generation of common mode interference. However, it should be noted that the withstand voltage of the capacitor must be greater than 500V to meet the requirements of the product's isolation voltage.

In the figure, the input filter circuit composed of L1 and C1 and the output filter circuit composed of L2 and C2 can reduce the size of the ripple current, thereby reducing electromagnetic interference transmitted through radiation. The filter capacitors C1 and C2 use multiple capacitors in parallel to reduce the equivalent series resistance, thereby reducing the ripple voltage. C3, C4, C5, and C6 are used to filter common mode interference, and its value should not be large to avoid large leakage current.

2 Suppress interference source technology

The main interference sources of DC/DC converters are high-frequency transformers, power switch tubes and rectifier diodes. For this reason, measures are taken one by one.