With the integration and modularization of circuits, circuit analysis and design can be said to be systematic analysis and design. EMI solution research will have a significant impact on improving the performance of electronic products in the future. The increasing popularity of electronic products and the growing awareness of electromagnetic hazards have made reducing electromagnetic interference EMI an important topic in the current electronic science community. Below is an analysis of how to reduce EMI in power modules.
Modular power supply products are typically designed to comply with the International Radio Interference Special Committee or CISPR and Federal Communications Commission (FCC) standards. The CISPR standard usually only involves electromagnetic compatibility (EMC) emission testing methods and limitations. Generally, power modules have five sided shielding, effectively containing radiation emissions from adjacent components. However, the sixth side facing the printed circuit board (PC) is not shielded. It is recommended to place the grounding plane below the converter and connect it to the housing to control the converter to emit EMI.
For example, the power module adopts a metal shielding structure, and the manufacturer can provide CE and RE data sheet curves. The base electroplating converter can provide better near-field B-field radiation protection. At most frequencies, the base electroplated converter is about 10 dB quieter than an open frame design/ μ M.
Analysis of EMI reduction solutions:
A voltage regulator attenuates conduction and radiation energy at the source, allowing power designers to rest assured. Another method to reduce CE is to make the voltage paths of the modules adjacent and parallel to each other, and symmetry is always a good form of CE and EMI reduction. There is a ground plane below it, and multiple paths can also be stacked together. This is similar to running two lines in a twisted pair configuration, suitable for eliminating common mode noise. Avoid the circuit path running in the large loop, which will act as an antenna. Keep close to the power cord, which will greatly reduce the loop area and maintain a decrease in RE.
External input or output filters may also be required. If that's the case, then it's necessary to avoid the adverse effects of stray inductance and capacitance in the filter, otherwise it may lead to instability or performance degradation of the entire power system.
The input of the DC/DC power module is a constant power at low frequencies. As the voltage decreases and the current increases, it will exhibit negative impedance at the input source. When the combination of the impedance of the input filter and the impedance of the power module becomes negative, the converter will oscillate, resulting in a mismatch. One way to prevent this situation is to ensure that the output impedance of the filter is much smaller than the input impedance of the power module at all frequencies.
The resonant frequency of the filter is displayed as ω f. Its peak value is directly proportional to the damping ratio of the filter. Therefore, if its high impedance is close to the impedance of the power module, the underdamped filter may cause oscillation. The resonant frequency of the converter output filter is displayed as ω o. Any external output filter will change this. A robust and stable design combination will be to design a filter that reduces its peak output impedance (resonant frequency of the filter) by ten times or more compared to the input impedance of the power module (resonant frequency of the power). The output filter of the module will be combined with any external output filter.
The X capacitor is connected between the phases of the line, which can effectively resist symmetric interference (differential mode). Y capacitors are EMI capacitors that are fed from the input power supply to the chassis ground, effectively resisting asymmetric interference (common mode). Sometimes they are also connected to the chassis ground from the power output terminal of each converter.
The advantage of a synchronous power module is that it can eliminate the beat frequency generated by two or more devices operating at frequencies close to each other. If we can run multiple power modules at the same frequency, any EMC radiation generated will have similar spectral density, making it easier to filter out that specific frequency. Of course, the module power supply must have a SYNCH pin to apply external frequencies. Some modules can access the internal oscillator, which can then be used to drive the SYNCH pins of other modules in the master/slave configuration.
Sometimes rotating the power module or other magnetic components by 90 degrees (such as transformers and inductors) can improve the EMI performance of power supply design. Even subtle design changes can cause the EMI of the power supply to be higher than necessary, and designers need to understand where the noise comes from
