A ceramic capacitor that loses capacitance under voltage

Tags: Capacitors

Symptom

A converter’s output ripple is a few times the value the design predicts, or a filter’s corner sits well away from its calculation, although an LCR meter reads the ceramic capacitor close to its rated value on the bench.

Why

A class-2 code such as X7R or X5R states how much the capacitance may change over a temperature range. It implies nothing about the change with the dc voltage across the capacitor, so the data for the specific part must be checked [Analog Devices (Maxim Integrated), Temperature and Voltage Variation of Ceramic Capacitors, or Why Your 4.7 µF Capacitor Becomes a 0.33 µF Capacitor]. The dc voltage alone can leave a small fraction of the rated capacitance: in the source’s example, a 4.7 µF Y5V capacitor rated 6.3 V keeps 0.33 µF at 5 V at room temperature [Analog Devices (Maxim Integrated), Temperature and Voltage Variation of Ceramic Capacitors, or Why Your 4.7 µF Capacitor Becomes a 0.33 µF Capacitor]. How much an X7R or X5R part loses depends on the part: in the article’s data a larger case lost less, and within one case size and dielectric the voltage rating often made no difference [Analog Devices (Maxim Integrated), Temperature and Voltage Variation of Ceramic Capacitors, or Why Your 4.7 µF Capacitor Becomes a 0.33 µF Capacitor]. Temperature changes the value again. An LCR meter’s small test signal with no dc voltage does not show the loss.

The ripple follows the capacitance that is actually there (output voltage ripple). An example buck’s output capacitor of 10 µF would ripple by 12.5 mV (half peak-to-peak); if it keeps 4 µF at its working voltage, the ripple is 31.3 mV.

How to confirm

Read the capacitance with the working dc voltage applied: an impedance meter that can add a dc bias does it, or the manufacturer’s curve of capacitance against dc voltage for that exact part number. In the circuit, compute the capacitance the measured ripple implies and compare it with the rating.

Fix

Size the capacitor from its capacitance at the working dc voltage and at the extreme temperature, taken from the data of the part itself, and compare candidate parts by those data, not by their codes [Analog Devices (Maxim Integrated), Temperature and Voltage Variation of Ceramic Capacitors, or Why Your 4.7 µF Capacitor Becomes a 0.33 µF Capacitor]. Where that capacitance falls short, add capacitors in parallel until the remaining capacitance meets the ripple.

References

• [Analog Devices (Maxim Integrated), Temperature and Voltage Variation of Ceramic Capacitors, or Why Your 4.7 µF Capacitor Becomes a 0.33 µF Capacitor]: class-2 codes say nothing about the change with dc voltage; the dc voltage alone can leave a small fraction of the rated value, and in its data a larger case lost less.
• Real passive components: series resistance, self-resonance and output ripple.
• Low temperature: what the cold changes on the bench.