Capacitors Blog

Suntan Technology Company Limited
---All Kinds of Capacitors

Effect of Frequency: Frequency affects both capacitance and dissipation factor..

The variation of impedance with frequency is an important consideration in the application of multilayer ceramic capacitors. Total impedance of the capacitor is the vector of the capacitive reactance, the inductive reactance, and the ESR. As frequency increases, the capacitive reactance decreases. However, the series inductance (L)  produces inductive reactance, which increases with frequency. At some frequency, the impedance ceases to be capacitive and becomes inductive. This point, at the bottom of the V-shaped impedance versus frequency curves, is the self-resonant frequency. At the self-resonant frequency, the reactance is zero, and the impedance consists of the ESR only.

Suntan Technology Company Limited
---All Kinds of Capacitors

Of much less concern, yet still important, especially in audio applications, is the micro phonic or piezoelectric effect. Barium titinate which is the base ceramic material for most dielectric systems will exhibit microphonic effects. It is not very difficult to take a Y5V capacitor and put a DC bias with a small signal 1kHz sine wave and get the capacitor to “sing”. Tantalum capacitors exhibit no microphonic effects. The experiment done by AVX involved the opposite phenomena whereby the part was shaken while under bias and the resulting generated voltage was measured. This was done for a series of 1µF devices and the results are shown in Fig below.

Power supply designers are often concerned about the ripple current capabilities of capacitors on both the input and output sides of converters. The biggest concern is the internal temperature rise caused by the I2R power consumption of the capacitor. Since tantalums are a polar device, this ripple should always be accompanied by a DC bias. Since ESR is so dependent on frequency and temperature, the power ratings listed should be viewed as rules of thumb and not laws written in stone. The tantalum capacitors have a published data set for a 10°C rise above the ambient. The experimental set-up used to arrive at this number was emulated and done for a series of ceramic capacitor chips. It should be noted that different mounting techniques can alter the thermal conductivity greatly.