Replacement evaluation of electrolytic capacitors is demonstrated. Extensive improvements in the characteristics and low profiles can be achieved, by replacing electrolytic capacitors with conductive polymer capacitors or monolithic ceramic capacitors.
(07:35)
Multi-functionalization, small size, light weight, and energy savings are required of electronic devices. According to these requirements, the expectations of compact/large capacity, low ESR, high reliability, etc. have been increasing, particularly in ceramic capacitors and polymer capacitors.
The demonstration shown in the following is to display and compare the ripple voltage waveform on a personal computer, when replacing an output capacitor of a DC-DC converter from a common electrolytic capacitor type, with a Murata ceramic capacitor or polymer capacitor.
The output capacitor of a DC-DC converter is used to suppress the ripple voltage. When a ceramic capacitor with an extremely low ESR is used, the output voltage may oscillate abnormally. In that case, the abnormal oscillation can be suppressed by tuning the constant of the phase compensation portion.
Circuit Diagram of Voltage Step Down DC-DC Converter
Applicable Product
|
IC Manufacturer's Name |
: | ROHM |
|
Model No. |
: | BD9778F |
|
DC-DC Specification |
: |
Input Voltage 12 V |
|
Output Capacitor |
: |
(1) Common Electrolysis: Tantalum/100uF |
The ESR of a polymer capacitor is lower than a common electrolytic capacitor, and reduces the ripple voltage.
The ESR of a ceramic capacitor is even lower, and the ripple voltage can be reduced even when the capacity is reduced.
However, since the ESR is extremely low, an abnormal oscillation may occur in some cases. In order to suppress the abnormal oscillation, it is necessary to tune the phase compensation circuit.
The ESR of a ceramic capacitor is extremely low compared to an electrolytic capacitor, and the phase of the LC filter may reverse rapidly. Therefore, if the capacitor is replaced as is, an abnormal oscillation may occur.
The phase compensation circuit suppresses the phase rotation of the LC filter. An abnormal oscillation can be prevented by tuning the constant of this circuit appropriately. It is preferable to keep the phase margin at 45 degrees or more.
Ripple Voltage Estimation Formula
Since the ESR of a ceramic capacitor is low, the ripple voltage can be reduced even when the capacity is reduced.
Note: A ceramic capacitor has a voltage characteristic that changes the capacitance (reduces). It is necessary to select an item where the voltage to be used has been confirmed.
Effectiveness of replacing an electrolytic capacitor with a ceramic capacitor
| Effectiveness | Electrolytic Capacitor |
Ceramic Capacitor |
|---|---|---|
| Cost reduction | △ | ○ |
| Mounting space reduction | △ | ◎ |
| Ripple & spike | △ | ○ |
| Reliability | ○ | ◎ |
| Load response | ○ | ○ |
| Phase tuning difficulty | ◎ | △ |
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