Supplement－Selection of Input Capacitors

This article is a supplement relating to the article “Selection of Input Capacitors” in “Selection of Inductors and Capacitors for DC-DC Converters “.

Selection of Input Capacitors – Supplement

In the previous article, in preparation for discussing selection of input capacitors, the roles of input capacitors and output capacitors were reviewed. Important points when selecting input capacitors were described, and voltage and ripple current ratings, ripple heat generation characteristics, and the temperature characteristics and DC bias characteristics of ceramic capacitors were explained.

In addition to the main capacitor C_IN explained in the preceding article, in many actual circuits C_IN is combined with a capacitor for high-frequency noise reduction known as C_BYPASS. Hence in this article, an explanation of C_BYPASS is appended.

Roles of Input Capacitor C_BYPASS

In actual inputs, however, not only is there ripple due to the on-off switching of the input current, there are also voltage spikes and noise, caused by high-frequency current transitions that accompany switching. As noise, these components can have adverse effects, and must be reduced. The following diagram illustrates the relation between ripple and noise accompanying transitions in the input current.

For these reasons, noise is present in two frequency bands at the input: the ripple at the switching frequency, and high-frequency noise.

As explained above, C_IN is used mainly to reduce the ripple voltage, and so is a capacitor with a relatively large static capacitance. However, in general, capacitors that are suitable for use as C_IN have poor impedance characteristics in high-frequency bands, and although effective for reducing ripple voltages, cannot adequately alleviate high-frequency noise. Hence a capacitor with low impedance at higher frequencies is added to deal with high-frequency noise. To distinguish this capacitor from C_IN, it is referred to as C_BYPASS. In general, a ceramic capacitor of about 0.1 μF is used. This is essentially the usual “decoupling (bypass) capacitor for high-frequency noise reduction”.

The following is a circuit example in which C2 serves as C_IN, and C4 corresponds to C_BYPASS. As examples, impedance characteristics are shown for C_IN = 22 μF and C_BYPASS = 0.1 μF. As the input, the impedance characteristic resulting from the two capacitors is obtained.

If only a relatively small input capacitance is needed, a single ceramic capacitor may be used as both C_IN and C_BYPASS. However, the impedance characteristic of the capacitor, and the resulting ripple and noise frequencies, must be checked.

Points Relating to Placement on the Board

Positioning an input capacitor as close to the VIN pin of an IC as possible is a fundamental principle of board layout. If the capacitor is placed far away, parasitic inductances appear in the board wiring as the distance increases, and it is well known that such inductances together with sudden current switching give rise to unexpectedly high spike voltages.

Another rule is that capacitors with smaller capacitance should be positioned near noise sources. In this case, the VIN (PVIN) pin constitutes a noise source, and so as shown in the circuit diagram, C_BYPASS (C4) and C_IN (C2) are positioned in that order, as seen from PVIN of the IC. C_IN is generally a small multilayer ceramic capacitor, and so should be easy to place very close to the VIN pin of the IC.