Three elements of switching power supplies and digital devices

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Three elements of switching power supplies and digital devices [Copy link]

EMC (Electromagnetic CompaTIbility) is a technology that studies electromagnetic interference. Electromagnetic interference is the electromagnetic energy around us that causes the operation of electronic equipment to produce inappropriate responses.

The technical purpose of EMC is to ensure that electrical devices or systems are neither affected by the electromagnetic environment nor interfere with the environment under common electromagnetic environment conditions.

EMC is a difficult issue in the industry. The article introduces the three laws of EMC, the three elements of EMC problems, the characteristics of electromagnetic disturbances, and the five-level EMC design method. It provides suggestions for companies on how to deal with EMC. We believe that EMC improvement should be treated symptomatically, just like diagnosing and treating a disease. We advocate adhering to the laws of EMC, considering and solving EMC problems as early as possible - conducting EMC design.

Below we understand the following three elements and three important laws in the field of EMC:

1. Three elements of EMC problems

Switching power supplies and digital devices generate strong radiation because the pulse current and voltage have rich high-frequency harmonics.

Electromagnetic interference includes radiated (high frequency) EMI and conducted (low frequency) EMI. That is, EMC problems are mainly caused by two ways: one is in the form of spatial electromagnetic wave interference; the other is in the form of conduction. In other words, the three elements that cause EMC problems are: electromagnetic interference source, coupling path, and sensitive equipment.

Radiated interference mainly pollutes the electromagnetic environment in space in the form of electromagnetic waves through the casing and connecting wires; conducted interference disturbs the public power grid through power lines or affects connected devices through other terminals (such as RF terminals, input terminals).

How to know EMC problems in advance to carry out EMC design:

Conduction, radiation

Disturbance source - (pathway) - sensitive receptor

Near Field Coupling

Possible sources of interference for IT and AV equipment

A) The local oscillation of FM receiver and TV receiver, the fundamental wave and harmonics are generated by the high-frequency head and the local oscillation circuit;

B) Switching pulses and high-order harmonics of the switching power supply, square waves and high-frequency harmonics of the synchronization signal, and line and field signals and high-frequency harmonics generated by the line scanning imaging circuit;

C) Various clock signals and high-frequency harmonics required for the operation of digital circuits, as well as their combinations, various clocks such as CPU chip working clock, MPEG decoder working clock, video synchronization clock (27MHz, 16.9344MHz, 40.5MHz), etc.;

D) Useless signals and spurious signals caused by digital signal square waves and high-frequency harmonics, high-order harmonics generated by crystal oscillators, and nonlinear circuit phenomena (nonlinear distortion, intermodulation, saturation distortion, cutoff distortion), etc.;

E) Non-sinusoidal waveforms, waveform glitches, overshoot, ringing, and parasitic frequency points in circuit design;

F) External disturbances received by sensitive receptors through coupling pathways include surges, fast pulse groups, static electricity, voltage drops, voltage changes and various electromagnetic fields.

Characteristics of electromagnetic disturbance:

① The frequency spectrum of the unit pulse is the widest;

② The low-frequency content in the spectrum depends on the area of the pulse, and the high-frequency component depends on the steepness of the front and back edges of the pulse;

③ The crystal oscillation level must meet a certain amplitude so that the digital circuit can work according to a certain timing, so that the interference generated by the crystal oscillator has the characteristics of covering bandwidth and high interference level;

④ When the polarization and directional characteristics of the transmitting and receiving antennas are the same, EMI radiation and reception are the most serious; the larger the area of the transmitting and receiving antennas, the greater the EMI damage;

⑤ Disturbance paths: radiation, conduction, coupling and a combination of radiation, conduction and coupling.

⑥ The power line conducted interference is mainly generated by common mode current;

⑦ Radiated interference is mainly caused by the loop formed by differential mode current.

2. Three important laws of EMC

Rule 1: EMC cost-effectiveness ratio rule: The earlier EMC issues are considered and resolved, the lower the cost and the better the effect.

Carrying out EMC design during the new product development stage can greatly save costs and improve efficiency, rather than waiting until the product fails the EMC test to make improvements. Otherwise, efficiency will be greatly reduced and costs will be greatly increased.

Experience tells us that it is most time-saving and cost-effective to carry out EMC design at the same time as functional design and to pass EMC testing when samples and prototypes are completed.

On the contrary, if EMC is not taken into consideration during the product development phase, and improvements are only made after EMC is found to be non-compliant after production, this will not only bring great difficulties in terms of technology, but also the rework will inevitably lead to a huge waste of costs and time. There may even be defects in the structural design and PCB design, making it impossible to implement improvement measures, resulting in the product being unable to be launched on the market.

Rule 2: The larger the high-frequency current loop area S, the more serious the EMI radiation.

High-frequency signal current flows through the path with the least inductance. When the frequency is high, the wiring reactance is generally greater than the resistance, and the connection is an inductor for high-frequency signals. The series inductor causes radiation. Electromagnetic radiation is mostly generated by the high-frequency current loop on the EUT device under test. The worst case is the open-circuit antenna form.

The corresponding treatment method is to reduce or shorten the connection, reduce the area of the high-frequency current loop, and try to eliminate any antennas that are not required for normal operation, such as discontinuous wiring or excessively long pins of components with antenna effect.

One of the most important tasks to reduce radiated interference or improve the ability to resist RF radiation interference is to find ways to reduce the high-frequency current loop area S.

Rule 3: The higher the loop current frequency f, the more serious the EMI radiation caused, and the electromagnetic radiation field strength increases in direct proportion to the square of the current frequency f.

The second most important way to reduce radiated interference or improve the anti-interference ability of RF radiation is to find ways to reduce the frequency f of the high-frequency current of the interference source, that is, to reduce the frequency f of the interference electromagnetic wave.

3. Conclusion

The three elements of EMC problems and understanding the three laws of EMC will make EMC problems follow certain rules. Adhering to the laws of EMC will save time and effort in solving EMC problems and achieve twice the result with half the effort.