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The Important Points of Multi-Layer Ceramic Capacitor Used in Buck Converter Circuit

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The Important Points of Multi-Layer Ceramic Capacitor Used in Buck Converter Circuit © 2017 ROHM Co., Ltd. No. 60AP001E Rev.001 2017.4 Application Note Switching Regulator Series The Important Points of Multi-layer Ceramic Capacitor Used in Buck Converter circuit 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. Figure1 shows frequency characteristics of all major types of Types of Multi-layer ceramic capacitor (MLCC) capacitors. Ripple voltage of output is expected to become smaller by using MLCC in a power circuit. However in earlier MLCC can be broadly classified into two types; for temperature power-IC designs, usage of MLCC with ultra-low ESR was not compensation and for high-dielectric constant. MLCC for considered. Therefore, phase of feedback circuit rotates too temperature compensation uses Titanium oxide (TiO2) and much at high-frequency area, causing power circuit to operate Calcium zirconate (CaZrO3) as material for its paraelectric. unstably and may cause oscillation in worst case. When MLCC Therefore, it cannot suit for capacitor with large-capacitance as has to be used at any cost, connecting a low-ohmic resistor more relative permittivity is small, about 20 to 300. Moreover, since than 10mΩ in series is recommended, and may deteriorate the relative permittivity changes linearly with temperature, the frequency characteristics. temperature coefficient can be restricted between +100 to - 4700ppm/°C by adjusting composition of the dielectric material. 100 The capacitor for temperature compensation in power circuit is used in time-constant circuits such as Snubber-circuit and for C=47µF 10 SOFT-START. AL electrolytic MLCC for high-dielectric constant uses Barium titanate (BaTiO3) 1 as main material for its paraelectic. With its big relative permittivity of 1,000 to 20,000, it can be capacitor with small in 0.1 POSCAP size and high-capacitance. But, this material changes tremendously with temperature, so using it for time-constant Impedance (Ω) circuits needs caution. 0.01 OS In later part, the explanation will focus on high-dielectric constant 0.001 Ceramic MLCC used as input/output capacitor in a power circuit. 0.0001 0.001 0.01 0.1 1 10 Frequency (MHz) Frequency Characteristics Figure 1. Frequency Characteristics of Capacitors MLCC has extremely small ESR (Equivalent Series Resistance) compared to other capacitor types such as electrolytic capacitor. Parameter Temperature compensation MLCC High-dielectric constant MLCC Paraelectric material Titanium oxide (TiO2) Barium titanate (BaTiO3) Calcium zirconate (CaZrO3) Relative permittivity 20 to 300 1,000 to 20,000 Temperature characteristics +100 to -4700ppm/°C +30 to -82% Capacity ≤ 0.1µF ≥ 68pF Capacitance-change when Almost no change Changes voltage input Capacitance-change over time Almost no change Changes Application circuits Snubber, Time-constant Smoothing power, Decoupling High-frequency circuit, Audio circuit Table 1. Characteristics of Laminated Ceramic Capacitor © 2013, 2020 ROHM Co., Ltd. No. 62AN089E Rev.002 1/4 JANUARY 2020 The Important Points of Multi-layer Ceramic Capacitor Used in Buck Converter circuit Application Note Temperature Characteristics High-dielectric series MLCC with high-capacitance has products with various temperature characteristics. Table 2 and Figure 2 20 B show typical temperature characteristics. The characteristic 10 X7R curve changes in various ways within the tolerance range of the 0 each product. Since the Temperature characteristics are -10 X7U standardized, it’s easy to judge by the capacitor’s type name. -20 X5R -30 Products with smaller capacitance tolerance（±15%）, such as -40 B, X5R, R, X7R, X8R are recommended for temperature F characteristics used in the power circuits. The characteristics of -50 X7U, F, Y5V, Z5U, Z5V are inexpensive, but capacitance (%) Capacitance change -60 tolerance (-82%) is large, making it to operate stable only at -70 room temperature. Do not use capacitors with such -80 characteristics in power circuit as it may cause trouble. -90 -75 -50 -25 0 25 50 75 100 125 150 Judging from the operating temperature range of application Temperature (℃) equipment used, and choosing from B, X5R, R, and X7R Figure 2. Major Temperature Characteristics of High-dielectric characteristics are recommended. constant type MLCC Basic Temperature Capacitance Standard Characteristics temperature range tolerance JIS B 20°C -25～+85°C ±10% X5R ±15% X5S -55～+85°C ±22% EIA X5T 25°C +22%, -33% X6S ±22% -55～+105°C X6T +22%, -33% JIS R 20°C -55～+125°C ±15% X7R ±15% X7S ±22% -55～+125°C EIA X7T 25°C +22%, -33% X7U +22%, -56% X8R -55～+150°C ±15% JIS F 20°C -25～+85°C +30%, -80% Y5V -30～+85°C +22%, -82% EIA Z5U 25°C +22%, -56% +10～+85°C Z5V +22%, -82% Table 2. Major Temperature Characteristics of High-dielectric constant type MLCC © 2013, 2020 ROHM Co., Ltd. No. 62AN089E Rev.002 2/4 JANUARY 2020 The Important Points of Multi-layer Ceramic Capacitor Used in Buck Converter circuit Application Note DC Voltage Characteristics 10 The capacitance value changes by applying DC voltage to high- 10µF, 10V, B 0 dielectric series type MLCC. This is peculiar only to this capacitor type and does not happen in other types, such as electrolytic -10 3225(1210) capacitor and temperature-compensation type MLCC. -20 T=2.70mm Let’s take a look at MLCC of Murata Manufacturing Co., Ltd. as -30 3216(1206) T=1.25mm an example of DC voltage characteristics. Figure 3 shows -40 characteristics of a capacitor with 10µF/10V (B) that differs in size (L×W). Both of these products are of same 0.95mm -50 thickness. Tendency of bigger capacitance reduction, with (%) Capacitance change 3216(1206) -60 smaller size part can be observed, when DC voltage is applied. T=0.95mm 1608 size capacitor can only be used for 1V even though it is -70 rated for 10V. Caution is required in confirming the changing -80 characteristics, when switching to smaller-size capacitors to 0 1 2 3 4 5 6 7 8 9 10 reduce PCB mounting area. DC voltage (V) Figure 4 shows characteristics of 10µF/10V (B) capacitor with Figure 4. DC voltage characteristics different thickness (T), and same size (L×W). Tendency of Difference in thickness (T) lesser capacitance reduction, with larger thickness part (bigger volume) can be observed, when DC voltage is applied. Caution 10 is required in confirming the changing characteristics, when 0 smaller-thickness of capacitors are used due to PCB height- 10µF, B, 3216(1216), T=1.80mm -10 restriction in slimmer designs. -20 16V Figure 5 shows DC voltage characteristics with capacitors of -30 different rated-voltages. Both capacitors are of 10µF (B), size -40 3216(1216), thickness 1.80mm. Product with 50V reduces -50 capacitance more than product with 16V rated-voltage, on -60 applying DC voltage. 50V -70 Capacitance (%) Capacitance change -80 10 -90 0 -100 10µF, 10V, B -10 0 10 20 30 40 50 DC voltage (V) -20 -30 Figure 5. DC voltage characteristics 2012(0805) Difference in Rated-voltage -40 T=0.95mm -50 As to choosing the capacitor with higher rated-voltage, does not -60 always guarantee higher performance. Selecting a MLCC simply 3216(1206) by checking only the specification of capacitance and rated- Capacitance (%) Capacitance change -70 T=0.95mm 1608(0603) voltage can deteriorate the characteristics of a power circuit. -80 T=0.95mm Always request the manufacturer for detailed characteristics -90 data. 0 1 2 3 4 5 6 7 8 9 10 DC voltage (V) MLCC of Murata Manufacturing Co., Ltd can be easily confirmed by ‘SimSurfing’, a design support system on their website which Figure 3. DC voltage characteristics shares various characteristics. (as of 2020/ January) Difference in size (L×W) © 2013, 2020 ROHM Co., Ltd. No. 62AN089E Rev.002 3/4 JANUARY 2020 The Important Points of Multi-layer Ceramic Capacitor Used in Buck Converter circuit Application Note Chronological change Heat generation characteristics High-dielectric series type MLCC has a characteristic that When ripple current (AC, alternating current) flows through a deteriorates capacitance-value with time. Figure 6 shoes the capacitor, the resistor element generates heat and temperature example of chronological change. “0” represents the point of capacitor itself rises. But since MLCC has extremely small where 24-hours have passed after mounting. Capacitance value ESR (equivalent series resistance), amount of heat is less and decreases linearly when time axis is shown in logarithmic. MLCC ripple resistance capability is excellent. Many MLCC for temperature-compensation type does not have this kind of manufacturers recommend surface temperature to be below chronological changes. 20°C while usage. Capacitor with reduced capacitance by chronological change 100 recovers its capacitance by being heated to more than Curie temperature (about 125°C) by solder, etc. Also that same 10µF, 10V, B, 3216 (1206) capacitor starts chronological change when it cools down to below Curie temperature. ) ℃ Capacitance calculation for the chronological change is necessary while designing for long-term active equipment such 10 as in industrial application. Temperature rise Temperature ( 10 5 B char. 1 0 0 1 2 3 4 5 6 Ripple current (Arms) -5 F char. Figure 7. Example of self-heating of high-dielectric series -10 MLCC -15 -20 Capacitance (%) Capacitance change -25 -30 10 100 1000 10000 100000 Time (h) Figure 6. Example of Chronological change in High-dielectric series MLCC © 2013, 2020 ROHM Co., Ltd. No. 62AN089E Rev.002 4/4 JANUARY 2020 Notice Notes 1) The information contained herein is subject to change without notice.
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