Following our previous discussion of chip ferrite beads
, in this lesson we will talk about chip 3 terminal capacitors.
<Lead-type ceramic capacitors>
Before discussing chip 3 terminal capacitors, an explanation of lead-type 3 terminal capacitors will make the concepts easier to understand.
Figure 1 shows the structure of a general lead-type ceramic capacitor (2 terminal).
Figure 1. Structure of a lead-type 2 terminal capacitor
In lead-type ceramic capacitors, electrodes on both sides of a single panel dielectric are coated and lead terminals are attached. In this structure, the lead terminal parts have minimal inductance (residual inductance), so when this capacitor is used as a bypass capacitor, there is inductance between it and the ground terminal.
Figure 2. Example of capacitor insertion loss-frequency characteristics
Figure 2 shows an example of insertion loss characteristics when the capacitor is used as a bypass capacitor. As this graph shows insertion loss, noise level decreases towards the bottom of the graph. Impedance normally decreases proportionately to increasing frequency in capacitors, so, even in high-frequency regions, insertion loss should continue to increase along the dashed line in the figure. However, in reality, capacitors have the aforementioned residual inductance, and this minimal inductance interferes, causing a decrease in performance at high frequencies represented by the V-shaped insertion loss curve of the solid line.
<3 terminal capacitors are made by projecting two ends of the lead on one side>
3 terminal capacitors are ceramic capacitors in which the shape of the lead terminals is altered to improve the high-frequency characteristics of 2 terminal capacitors. As shown in Figure 3, one lead in a 3 terminal capacitor has two projections. With this configuration, the projections of the 2 terminal lead are connected to an input and an output of power sources or signal lines, respectively, and the other lead is connected to the ground terminal to create the connections shown in the equivalent circuit schematic on the right. By connecting it this way, the 2 terminal lead inductance does not enter the ground side, thereby making the ground impedance extremely small. Also, as the inductance of the 2 terminal lead works similar to a T-type filter inductor, it works in the direction of reducing noise.
Figure 3. Structure of lead-type 3 terminal capacitors
<Multilayer ceramic chip-type capacitors and chip 3 terminal capacitors>
Currently the most commonly used capacitors are chip-type multilayer ceramic capacitors. Figure 4 shows the structural concept of 2 terminal chip-type multilayer capacitors. A dielectric sheet is placed between the plates and the internal electrodes are connected to alternate projecting ends of the electrodes in a multilayer or layered pattern. Because it is in the shape of a chip, it has no leads, and there is no longer any residual inductance. However, a minimal amount of inductance remains inside, so that performance drops at higher frequencies.
Figure 4. Structure of a 2 terminal multilayer ceramic capacitor
Similar to a lead-type 3 terminal capacitor, the electrode structure is altered in chip 3 terminal capacitors to improve performance at high frequencies. Figure 5 shows the structural concept of a 3 terminal chip-type capacitor. A ground terminal is attached to each side of the chip, the dielectric is placed between the plates, and feed through electrodes and ground electrodes are alternately stacked up to create a feed through capacitor-like structure. As you can see in the equivalent circuit schematic, the inductance of the feed through electrodes works like a T-type filter inductor, similar to the conditions in the 3 terminal lead capacitor, so that residual inductance has less influence. The distance to the ground side is shorter, resulting in minimal inductance in this part. Moreover, as the ground side is connected to both ends, they become connected in series, and inductance becomes apparently cut in half.
Figure 5. Structure of a 3 terminal multilayer ceramic capacitor
Figure 6 compares the insertion loss characteristics of chip 3 terminal capacitors and 2 terminal chip-type multilayer capacitors. The capacitance is the same in each type, so similar characteristics are seen in low-frequency regions. However, the performance of the 2 terminal capacitor begins to drop as it exceeds 10 MHz, while the 3 terminal capacitor maintains its performance until the vicinity of 100 MHz. As the performance in the 3 terminal chip-type capacitor does not decrease until it reaches the high-frequency region, it is useful for applications that require noise suppression until it hits a high frequency.
Figure 6. Improvement in high-frequency characteristics with the 3 terminal chip-type capacitor
<Chip 3 terminal capacitors actually have 4 terminals>
As shown in Figure 5, even though we say that chip 3 terminal capacitors have 3 terminals, they actually have four. 4 terminal types can even further reduce the impedance on the ground side, but even when made into chips, they are still called '3 terminal' capacitors because electrically all terminals have the same potential and because the original lead-type 3 terminal capacitors had 3 terminals.
<3 terminal chip-type capacitor mounting method>
As chip 3 terminal capacitors have feed through terminals and ground terminals, the mounting method differs from that of a regular 2 terminal capacitor. Figure 7 shows the mounting method.
Figure 7. 3 terminal chip-type capacitor mounting method
When mounting a 3 terminal chip-type capacitor as a bypass capacitor, we cut the signal or power pattern and connect a feed through electrode in between, and prepare and connect a ground pattern at the ground terminal. The ground pattern must be connected with the shortest possible connection to a stable ground plane to maintain minimal impedance. When using a double-sided board or multilayer board, it is preferable to connect it to the ground plane via a through hole.
Person in charge: Murata Manufacturing Co., Ltd. Yasuhiro Mitsuya