Most electrical and electronic equipment will have periodic or intermittent voltage and current changes during operation.
For example, a switching power supply will have a fixed switching frequency. The turn-on and turn-off pulses of the MOS tube will be accompanied by its rising and falling edges. (di/dt), and bring very rich harmonics; in addition to AC-DC, DC-DC switching power supplies used on various occasions, there are also some inverters (photovoltaic, motor control, etc.) PWM signals. Are interference sources caused by EMI problems? These sources generate some electromagnetic energy concentrated in certain frequencies, which are radiated into the surrounding environment through corresponding circuits, antennas, or equivalent antennas.
Conduction test product power port, signal port external interference on the line, and radiation, the test product itself, including cables, interference to space emission.
The three major measures for EMC rectification are shielding, filtering, and grounding.
The nature of EMI mentioned above is actually a source of interference. The propagation path is also an extremely important entry point when actually solving problems. In the circuit, many inconspicuous links have actually been playing such an important role.
For example, when PCB wiring, why can’t it be routed at 90°? Only from the perspective of EMI, 90° wiring will cause distortion of the impedance of the line, forming an equivalent transmitting antenna, and high-frequency signals are easily transmitted through this path; in addition, the signal line, the high-level line and the return line of the power line Why is it necessary to route the wires as close as possible to reduce the loop area?
Because of such a loop, from the perspective of EMI, it is still an equivalent transmitting antenna, similar to a loop antenna, and its transmission efficiency is proportional to its area.
Regarding the sensitive source, the antenna simulates the sensitive source during the emission test, and the product itself is the sensitive source during the anti-interference test. As for the anti-interference test, this article will not start. So, when the EMI test fails, how should we rectify it, or what kind of thinking should we take to rectify it?
When the test is unqualified, first eliminate the reasons for the test itself, including the layout of the test harness, product placement, grounding, test equipment such as LISN grounding, measurement switches, 50Ω matching, etc. Secondly, according to the test items, a preliminary judgment can be made.
For example, the conduction test of the power port should start with the investigation of the power circuit, such as disconnecting the power supply of a certain function and then testing, compare the results, and whether the cause can be found. , The second is to check the coupling interference of the product or other lines in the system.
Through targeted and reasonable investigations and comparison tests, after finding the cause of the unqualified, using the three major EMC rectification measures and starting to prescribe the right remedy is the correct rectification idea.
EMI interference is generally divided into common-mode interference and differential mode interference, and most devices are also used according to these interference types. (Common mode current: the interference current flows in the loop formed between the wire and the ground at the same potential on the wire; differential mode current: the interference current flows between the signal wires or between the positive and negative lines of the power supply.