Through the application of transformer technology, the L Cancel Transformer (LCT) reduces the ESL in the decoupling capacitor connected to it and the ESL generated in the board (through holes, etc.) from the terminal on the GND side to the GND layer of the multilayer ceramic capacitor (MLCC). With this, the component maximizes the noise elimination performance of the MLCC and greatly suppresses noise in high-frequency bands.
Fig. 1 shows a comparison of the insertion loss characteristics in the case of a standalone 10 uF MLCC (black line) and in the case when the MLCC is connected to an LCT (red line).
Insertion loss increases in high-frequency bands due to the self-resonant frequency of the MLCC with a connection to the LCT. This is outstandingly effective in suppressing harmonic noise of several MHz to 1 GHz in power lines caused by DC-DC converters and other components.
Two coils are built inside the component in an L Cancel Transformer (LCT). Employing a transformer structure in which these two coils are placed close to each other and linked leads to negative mutual inductance (−M) occurring in the middle of the two coils. This negative mutual inductance realizes a highly accurate and stable M value with Murata Manufacturing's ceramic multilayer technology.
In addition, non-magnetic materials (dielectric materials) are used in the component housing. Therefore, there are no DC superimposition characteristics and stable noise suppression against changes in the current is possible.
A decoupling capacitor reduces noise by circulating power supply noise to the GND. However, there is ESL (L3) inside the capacitor. In addition, ESL (L4) occurs due in through holes and other areas between the capacitor and the GND layer. This ESL degrades the ability to reduce noise in the GND.
An LCT can cancel this ESL with negative inductance (−M). When the value of L3 + L4 becomes equal to the M value, the ESL is completely canceled and the ideal capacitor performance can be obtained. Noise can then be efficiently reduced in the GND layer.
An L Cancel Capacitor (LCT) can improve the noise elimination performance of decoupling capacitors. Accordingly, noise suppression becomes possible with a smaller number of MLCCs. It also becomes possible to cut the number of small-capacity MLCCs and other components to reduce harmonic noise.
Figure 5 shows an example of a circuit with a reduced number of MLCCs by adding an LCT. This circuit case is an example in which it is possible to reduce the noise level compared to the initial circuit even after reducing the number of MLCCs by 10. In the case of a buck converter, it is possible to efficiently eliminate the switching noise leaking into the input power line with an LCT.
Fig. 6 shows the results of a comparison of the conducted noise in the circuit example in Fig. 5.
We see it is possible to reduce noise even after cutting the number of MLCCs by 10 with the addition of an LCT. The effect is mainly seen in the frequencies between 20 to 108 MHz, which includes the FM radio band, in particular.