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2019.08.21 DC/DC

Supplement-Selection of Input Capacitors

Selection of Inductors and Capacitors for DC/DC Converters

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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 CIN explained in the preceding article, in many actual circuits CIN is combined with a capacitor for high-frequency noise reduction known as CBYPASS. Hence in this article, an explanation of CBYPASS is appended.

Roles of Input Capacitor CBYPASS

An input capacitor CIN with a comparatively large static capacitance is required at the input to a DC/DC converter; its role is as a current supply source that suddenly draws current from the input when the output switch is turned on, to reduce the ripple voltage that occurs at this time. By using an appropriate CIN, a stable input voltage can be maintained even when the output is switched. The diagram is a schematic drawing that is often used when explaining this.

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, CIN 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 CIN 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 CIN, it is referred to as CBYPASS. 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 CIN, and C4 corresponds to CBYPASS. As examples, impedance characteristics are shown for CIN = 22 μF and CBYPASS = 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 CIN and CBYPASS. 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, CBYPASS (C4) and CIN (C2) are positioned in that order, as seen from PVIN of the IC. CIN is generally a small multilayer ceramic capacitor, and so should be easy to place very close to the VIN pin of the IC.

Key Points:

・As input capacitors, in addition to a large-value capacitor for ripple reduction, a decoupling capacitor for reduction of high-frequency noise is often used as well.

・Input capacitors are positioned as close as possible to the VIN pin of the IC, with the decoupling capacitor placed on the VIN side.

PCB Layout and Selection of Peripheral Parts

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