Multi-layer Ceramic Capacitor (MLCC) with large-capacitance can be used as smoothing-capacitor in power supply circuits.Compared to other capacitor types such as an electrolytic capacitor, MLCC differs in frequency characteristics, temperature characteristics, and DC voltage characteristics. Using unmatched MLCC may not obtain required target characteristics for power supply circuit and may cause abnormal operation. This application note explains the important points while using MLCC.
Types of Multi-layer ceramic capacitor (MLCC)
MLCC can be broadly classified into two types; for temperature compensation and for high-dielectric constant. MLCC for temperature compensation uses Titanium oxide (TiO2) and Calcium zirconate (CaZrO3) as material for its paraelectric. Therefore, it cannot suit for capacitor with large-capacitance as relative permittivity is small, about 20 to 300. Moreover, since relative permittivity changes linearly with temperature, the temperature coefficient can be restricted between +100 to -4700ppm/°C by adjusting composition of the dielectric material. The capacitor for temperature compensation in power circuit is used in time-constant circuits such as Snubber-circuit and for SOFT-START.
MLCC for high-dielectric constant uses Barium titanate (BaTiO3) as main material for its paraelectic. With its big relative permittivity of 1,000 to 20,000, it can be capacitor with small in size and high-capacitance. But, this material changes tremendously with temperature, so using it for time-constant circuits needs caution.
In later part, the explanation will focus on high-dielectric constant MLCC used as input/output capacitor in a power circuit.
MLCC has extremely small ESR (Equivalent Series Resistance) compared to other capacitor types such as electrolytic capacitor.
Figure1 shows frequency characteristics of all major types of capacitors. Ripplevoltage of output is expected to become smaller by using MLCC in a power circuit. However in earlier power-IC designs, usage of MLCC with ultra-low ESR was not considered. Therefore, phase of feedback circuit rotates too much at high-frequency area, causing power circuit to operate unstably and may cause oscillation in worst case. When MLCC has to be used at any cost, connecting a low-ohmic resistor more than 10mΩ in series is recommended, and may deteriorate the frequency characteristics.
High-dielectric series MLCC with high-capacitance has products with various temperature characteristics. Table 2 and Figure 2 show typical temperature characteristics. The characteristic curve changes in various ways within the tolerance range of the each product. Since the Temperature characteristics are standardized, it’s easy to judge by the capacitor’s type name
Products with smaller capacitance tolerance（±15%）, such as B, X5R, R, X7R, X8R are recommended for temperature characteristics used in the power circuits. The characteristics of X7U, F, Y5V, Z5U, Z5V are inexpensive, but capacitance tolerance (-82%) is large, making it to operate stable only at room temperature. Do not use capacitors with such characteristics in power circuit as it may cause trouble.
Judging from the operating temperature range of application equipment used, and choosing from B, X5R, R, and X7R characteristics are recommended.