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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.

Frequency Characteristics

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.

Temperature 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.

……未完待续……

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