With the increasing speed and wave frequency in electronics, further reduction of ESL (equivalent series inductance) is required for capacitors, or 3-terminal feed-through filters must be used for noise filtering or decoupling. Furthermore, the need for low-ESL type products is increasing in electrical systems of automobiles in order to improve safety and comfort, and to support infotainment systems. Through a series of articles, we will introduce various solutions for automobile electronics using TDK's advanced low-ESL components.
3-terminal feed-through filters can be used as an effective noise countermeasure and for decoupling depending upon the connection method used.
The feed-through connection is a general mounting method in which a component is connected in series with the power line, and is optimal as a countermeasure against high-frequency noise. The shunt-through connection is a mounting method in which a component is connected in parallel with the power line, and produces an excellent effect for decoupling of the power line. This article will explain each mounting method and their advantages in detail.
A 3-terminal feed-though filter is an electronic component in which low ESL is achieved by the special multilayer structure of the internal electrodes to enhance the attenuation characteristic. However, depending on the method by which it is mounted on the PCB, its attenuation characteristic changes due to the effect of the ESL component of the PCB. In particular, when it is mounted on a multilayer PCB, the through-hole (via hole) design greatly affects the ESL value.
Important points for mounting a 3-terminal feed-though filter are introduced below, using an example multilayer PCB with a ground plane (a layer whose entire surface is a ground) on the first layer and the power line on the second layer.
Fig. 1: Example of mounting a 3-terminal feed-through filter on a multilayer PCB
When a ground plane is provided between the PCB surface on which the signal wiring pattern or electronic components are mounted and the power line, the effect of noise is suppressed due to the electrostatic shield effect of the ground plane. Therefore, as a basic rule, a ground plane is more desirable than a ground line.
When mounting a 3-terminal feed-through filter on a multilayer PCB such as the case illustrated in Fig. 1, a part of the power line pattern is cut and the filter is connected to the wiring pattern on the PCB surface via through-holes.
The two ground electrodes on the sides of the 3-terminal feed-through filter are connected to the ground plane on the internal layer of the PCB via through-holes. The connection to the ground pattern should be made as thick and short as possible, in order to achieve low ESL.
The number and length (depth) of the through-holes connecting the ground pattern and ground plane greatly affect the ESL value. Increasing the number of through-holes and making the connections as short as possible are advantageous.
How the number and length (depth) of through-holes connecting the ground pattern and ground plane actually affect the attenuation-frequency characteristic of a 3-terminal feed-through filter is shown in Fig. 2 by comparing 3 mounting conditions.
Fig. 2: Mounting conditions on a multilayer PCB (through-hole, ground plane)
When comparing the case in which through-hole is provided on each side of the ground pattern (condition (1)) and the case in which only one through-hole is provided on one side (condition (2)), the attenuation is greater in the case with two through-holes. In addition, when comparing the two cases that both have two through holes, the attenuation amount is greater under condition (1), with shorter (shallower) through-holes, than under condition (3), with longer (deeper) through-holes (Fig. 3).
Fig. 3: Relationship between the number/length (depth) of through-holes and the attenuation characteristic
Fig. 4 Through-hole arrangement recommended by TDK,
which also takes into consideration cost-effectiveness
As described above, ESL decreases as the number of through-holes increases and the length (depth) of the through-holes becomes shorter (shallower), providing a superior attenuation characteristic.
Fig. 4 illustrates the through-hole arrangement recommended by TDK, which also takes into consideration cost-effectiveness. The through-holes in the center produce an excellent ESL reduction effect.
Voltage fluctuation in the power line of an IC makes the circuit operation unstable and becomes a source of noise. To suppress this voltage fluctuation, a capacitor is inserted between the power line and ground to temporarily supply current when there is a sudden change in the load. This is called decoupling, and the capacitor used for this is called a decoupling capacitor.
The YFF-AC/AH series of 3-terminal feed-through filters offer excellent performance not only as noise countermeasure components, but also for decoupling in power lines of automotive electronics. When using a 3-terminal feed-through filter for decoupling, a special connection method called the “shunt-through” connection is effective. In the shunt-through connection, the filter is mounted on a PCB without cutting the power line pattern, as illustrated in Fig. 5. It is connected in parallel with the power line.
Fig. 5: Mounting method using the shunt-through connection
Connecting a 3-terminal feed-through filter for decoupling using the shunt-through method offers the following advantages.
In the shunt-through connection, a larger amount of current flows in the wiring pattern of the power line as the main flow. Therefore, there is the advantage that a 3-terminal feed-through filter can be used in a circuit with a current larger than its specified rated current.
When a 3-terminal feed-through filter is connected for decoupling using the normal feed-through method, a voltage drop occurs largely due to the internal resistance component (Rdc) of the 3-terminal feed-through filter, which has a large impact, especially on low-voltage circuits. The shunt-through connection offers the advantage of mitigating this impact.
A comparison of the voltage drop in a feed-through connection and in a shunt-through connection is shown in Fig. 6.
Fig. 6 : Comparison of voltage drops in a feed-through connection and shunt-through connection
When a 3-terminal feed-through filter (4.3μF) was inserted into a circuit with a power supply voltage of 3.0 V and a load of 1Ω using the feed-through connection, a voltage drop of 18 mV occurred. However, when the shunt-through connection was used, the voltage drop was suppressed to 8 mV. The shunt-through connection can suppress voltage drop in power lines, and can also be used in low-voltage circuits.
As described above, it is important to choose the appropriate mounting method when using a 3-terminal feed-through filter. The features of the feed-through connection and shunt-through connection are summarized below (Table 1).
Table 1. Comparison of the features of the feed-through connection and the shunt-through connection
|Feed-through connection||Shunt-through connection|
|Mounting method||Mounted by cutting a portion of the power line pattern where noise removal is desired and inserting the filter there.
||Mounted using the parallel (shunt-through) connection without cutting the wiring pattern of the line.
|Major applications||Noise removal in high-frequency regions
|Suppression of the IC voltage
(For decoupling use)
|Advantages||Demonstrates a high noise removal effect over a broad frequency range, including high frequencies.||There is no limit for the rated current because almost no current flows through the chip.
The number of components can be easily changed after the PCB design because the wiring pattern is not cut.
|Disadvantages||There is a limit for the rated current because a current flows through the chip. It is difficult to make changes to the design because the power line or signal line is cut.||ESL becomes slightly higher compared to when using the feed-through connection.|
|Noise removal effect||◎||◯|
|Noise can be deflected to the GND effectively because noise in the line must pass through the chip.||The noise removal effect becomes weaker than that of the feed-through connection because some noise passes through the power line.|
|Voltage fluctuation suppression effect||△
(Voltage drop occurs.)
(Voltage drop is small.)
* Please select the type/product code suited to your use, and use it to improve the reliability of your product.