Useful information! Murata EMI noise suppression basics class (Part 2)

Shielding refers to excluding the surrounding electromagnetic fields by enclosing the target object with a metal plate or other protective device as shown in Figure 1-9.

Although the effectiveness of shielding generally depends on the conductivity, permeability, and thickness of the material used, noise suppression for conventional electronic devices is more effective with extremely thin metal plates such as aluminum foil. You must be aware that the noise suppression effect of electronic devices will vary depending on the connection method (gap, contact impedance, etc.) used to form the enclosure, regardless of the material specifications.

When making openings in a shield for heat dissipation, it is more important to limit the oversize of each opening than to limit the total area of ​​the openings. As shown in Figure 1-10, if there is a long and narrow opening or slit, this part can act as a slit antenna (especially in the high-frequency range when the length l in the figure exceeds 1/2 of the wavelength), and radio waves can enter and exit the shield. To avoid this, each opening should be kept small. In this regard, sheets with many small holes (such as punched metal and expanded metal) are good materials for both ventilation and shielding.

A filter (Figure 1-11) refers to a component or function that allows necessary components to pass through and eliminates unwanted components in the current flowing in a conductor. Although the noise is shunted to the ground as shown in Figure 1-12, the noise energy is absorbed inside these components or returned to the noise source (increasing impedance).

Because noise tends to be distributed in a relatively high frequency range as shown in Figure 1-13, noise suppression in electronic devices usually uses a low-pass filter to eliminate high-frequency components. General-purpose components such as inductors (coils), resistors, and capacitors can be used as low-pass filters. However, in order to completely isolate noise, dedicated components such as EMI suppression filters can be used. EMI suppression filters will be explained in detail in Chapter 6 of this class .

In addition to these filters that utilize the uneven frequency distribution of noise, there are filters that utilize voltage differences (varistor, etc.) or utilize conduction mode differences (common mode choke coil, etc.). In addition to these filters, transformers, optical cables, or optical isolators can be used as a filter. Although these components can achieve excellent noise reduction effects in some cases, the applicable cases are very limited.

Filters are used for noise conducted through conductors, while shields are used for noise conducted through space. However, since the conductor that conducts noise also acts as an antenna, these two types of conduction also convert into each other by acting as an antenna conductor. Therefore, in order to completely isolate noise, it is necessary to use both filters and shields in one place.

For example, when a shield is used to isolate spatial conduction, if there is a conductor passing through the shield as shown in Figure 1-14, this conductor will pick up the noise inside the shield and absorb it outside the shield, causing noise emission. Therefore, using a shield cannot completely isolate spatial conduction.

Similarly, when a filter is used to isolate conductor conduction, the wires before and after the filter are coupled to each other through spatial conduction as shown in Figure 1-15. Therefore, it is impossible to completely isolate conductor conduction using only a filter.

When a shielding case and a filter are used at the same location as shown in Figure 1-16, isolating both spatial conduction and conductor conduction will completely eliminate noise.

If the conductor length between the noise source and the filter is significantly shorter as shown in Figure 1-17, the effect of the conductor as an antenna can be ignored, and the noise can be eliminated to a certain extent using only the filter. Therefore, if the filter can be used close to the noise source, noise suppression can be achieved using only the filter.

Good grounding is usually required for effective use of filters and shielding.

If there is a built-in bypass capacitor in the filter, the ground becomes a path for the noise current to return to the noise source, as shown in Figure 1-18. You need to consider keeping this component with very low impedance. If the impedance to ground is large as shown in Figure 1-19(a), a voltage will be generated in the ground due to the noise current, so the noise cannot be completely eliminated. If this ground is shared with another wire connected to another filter, the voltage generated at the ground will be transferred back to the other line through the filter capacitor.

This type of noise that is coupled through the ground impedance is called common impedance coupling. This state where there is noise at the ground is also called common mode noise generation. Common mode noise will be explained in the following chapters. Common impedance coupling is one of the mechanisms that cause common mode noise.

Because the filter effect with built-in capacitors is easily affected by the condition of the connected ground, a stable ground with low impedance is required.

The shield also needs to be grounded.

The static shield must be connected to ground, in principle external ground (zero voltage). The conductor connected to ground must have low impedance because currents flow in the conductor due to changes in the electric field of the shield.

In many cases, when using shielded cables, the shield also becomes a return path for current that has flowed through the inner conductor (such as the outer conductor of a coaxial cable). Therefore, it needs to be connected to a ground where this current can return (circuit ground in the case of a signal shield).

Similar to the case of Figure 1-19, when the noise has been guided to the ground, if the shield is connected to the ground, the shield is extended and then the noise is emitted from the ground like an antenna, which may increase the noise. When connecting the shield, it is necessary to select a ground with stable voltage and low impedance.

The shell shield is actually a relatively good ground. If there is a shield covering the entire electronic device, the shield itself may be a good ground for noise suppression, even if it is not connected to the earth (if the discharge current is discharged to the earth due to the need to suppress static electricity or other currents, grounding is required). Therefore, we call this grounding the shell shield grounding.

This case shield ground can also be used as the ground for the shielded cable. However, in order to use this shield as a return path for the above signal, it is necessary to connect it with the circuit ground. Therefore, if the case shield ground and the circuit ground have been separated, the connection will become complicated.

Figure 1-20 shows an example of grounding connections for shielded cables.

As mentioned above, it is necessary to connect to a stable ground to enhance the effect of the filter and shielding case. In addition, when a shielding case covering the entire device is used, the shielding case itself can be used as a stable ground. Therefore, the shielding usually has the function of a stable ground.

If a wire passes through this shield, a hole is provided as shown in Figure 1-14 to allow noise to enter and exit the shield, which will make the shield grounding unstable. In this case, a filter can be used in this wire to prevent noise from entering and exiting, thereby stabilizing the shield grounding.

As mentioned above, a suitable shielding case and filter can serve as a stable grounding, so there is a mutually supportive relationship between the shielding case, the filter and the grounding.

In addition to the above-mentioned shield ground, circuit ground is also a type of ground, and often induces more noise voltage than shield ground. This type of ground is called "stable ground". In contrast, a ground that does not induce noise is called "unstable ground".

It is ideal to connect to the module ground of the shield or filter. However, a circuit ground connection is required to return the signal return current or return the noise current to the noise source. If the circuit ground is unstable, the noise voltage should be minimized by reducing the impedance of the circuit ground, providing a ground plane along the circuit board, or connecting the housing shield ground.

Stabilizing ground noise by reducing the voltage in this way is called "ground reinforcement." Covering a portion of the circuit board with a shield can help to reinforce the ground. Figure 1-21 shows some methods of ground reinforcement .

When connecting a cable to a shield case, a filter is connected to prevent noise from entering or exiting through the cable. The ground of this filter is formed on the circuit board. However, in order to stabilize the ground, it is often connected to the shield case ground instead of the circuit ground. Therefore, a filter ground connected to the shield case ground is often formed at the section where the cable is connected. Here, we call this ground "filter ground".

Normally, the filter ground is connected not only to the shield case ground but also to the circuit ground in order to return the noise generated within the circuit to the noise source. In this case, it also serves as a ground reinforcement for the circuit ground. When using a shielded cable, the shield can be connected to the filter ground. In this case, it must be connected to the shield case ground with extremely low impedance because the effect of the shielded cable varies depending on the quality of the filter ground.

Figure 1-22 shows an example of filter grounding. Regarding the shield grounding, the most important thing is to keep the filter grounded at a very low impedance.

Although Section 1-4-2 has stated that the filter ground should be connected to the noise source with low impedance (regarding circuit ground), Figure 1-22 shows that it is preferentially connected to the shield case ground. This is because it is actually difficult to return to the noise source with low impedance because the connection point of the cable is usually far away from the noise source. In addition, there are many cases where the noise from other circuits is unstable due to the circuit ground, and even if a low-impedance filter ground is connected, it is difficult to improve the effect.

Therefore, when using a filter for a single circuit at a location close to the noise source, connect the filter to the circuit ground as described in Section 1-4-2. However, when the noise source is far away (such as at a junction box) and more than two noise sources need to be considered, it is difficult to achieve this connection. A practical trick is to use a filter at the junction box, find a stable ground such as a shield case ground, and connect the filter ground as shown in Figure 1-22 .