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What capacitor and inductor are the best for a switching power supply?

Summary

Capacitor – Part 8 -

Keyword
  • Multilayer ceramic
  • MLCC
  • Larger capacitance
  • Functional polymer
  • Ripple
  • ESR
  • ESL
  • High-temperature characteristic
  • DC bias characteristic
  • Compact
  • Lifetime
  • Cracking
  • Audible ringing

-Addressing the theme of "Capacitors and Inductors Best for Switching Power Supplies", we began by learning about various capacitor-related issues. Moreover, rather than focusing on capacitors alone, we learned of their uses in relation to switching power supplies. This information should be highly useful to engineers involved in the design of switching power supplies.

We have addressed capacitors in seven sections, and so here shall provide a simple summary.

Series on Capacitors

To start with, as a discussion of capacitors themselves, we described the structure and basic characteristics of multilayer ceramic capacitors (MLCCs), and focusing on cases of use in DC/DC converters, separately considered their use in input and in output circuits. We also devoted two sections to issues relating to mounting of MLCCs.

-Well then, could you please review the key points of each of those topics?

In “Part 1: Increasing Capacitance of Multilayer Ceramic Chip Capacitors”, we explained that TAIYO YUDEN is able to deliver 470 μF MLCCs. There are probably many people who think that the lineup of MLCCs has relatively small capacitances, but we'd like them to be aware that MLCCs are available with capacitances fully adequate to the needs of onboard devices in DC/DC converter design. Moreover, when device capacitances are this large, they can compete in areas that until now had been reserved for electrolytic capacitors. Here the low ESR and ESL, high capacitance/size ratio, and long lifetimes of MLCCs give these devices important advantages.

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-In "Part 2: Need to Know not Only Electrical Specs, but also their Characteristics including Materials and Packaging Cases" you talked about the issues related to MLCC characteristics.

The high-temperature characteristic and DC bias characteristic must be known in advance when using an MLCC. It should also be remembered that characteristics will differ with the grade and case size, and that an X7R or X6S grade is necessary for use in DC/DC converters.

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In "Part 3: In Selecting an Input Capacitor, Focus on the Ripple Current, ESR, and ESL ", the discussion focused on a case in which an MLCC is used as an input capacitor in a DC/DC converter. Because the input capacitor handles charging/discharging of high currents, ripples and spikes may occur due to the ESR and ESL parasitic components. Heat generation must also be considered. An MLCC with extremely small ESR and ESL values is highly effective in such cases.

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-In the Part 4 and Part 5, things to beware of when using MLCCs as output capacitors were explained.

In “Part 4: In the Evaluation of Output Ripples, ESL for Output Capacitor Requires Particular Attention”, although the relation between output ripple and ESR is well known, we wanted to clarify the effect of ESL in a case in which a ripple voltage close to a square wave appears in the output.

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In "Part 5: ESR of the Output Capacitor Exerts a Significant Impact on Output Fluctuation at Decreased Load", the phenomenon was explained in which, when the load decreases suddenly, the output voltage rises, depending on the ESR. This is something that needs to be thoroughly grasped when evaluating output ripple.

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-In Part 6 and Part 7, mounting related issues were explained.

These are the problems of cracking, and audible ringing. There are MLCCs designed to address both these issues, and it was explained that metal frame type capacitors are effective for both. It was also explained that when using these products, the device height must be studied, and modification of board layout is necessary.

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-Could you please summarize the overall position of MLCCs in the design of DC/DC converters?

First, the capacitances of MLCCs are steadily increasing, and from the standpoint of capacitance, they are able to substitute for capacitors using functional polymer materials. And, although this is something we have repeated many times, the parasitic components ESR and ESL are now very low compared with those for functional polymer capacitors. This does not just means that the ESR and ESL are low; it also means that whereas with functional polymer capacitors, a product with large capacitance must be selected in order to keep the ESR and ESL low, an MLCC with just 1/2 to 2/3 the capacitance can substitute for a functional polymer capacitor from the standpoint of ESR and ESL. In other words, when an MLCC replaces a functional polymer capacitor, the same size capacitance is not needed. In addition, MLCCs have large capacitance per unit size, and so contribute to more compact designs, And, their long lifetimes are an important advantage for use in harsh environments.

Thus MLCCs offer a range of advantages, but they do have a few issues as well. Where high-temperature characteristics, DC bias characteristics, grades and case sizes are concerned, capacitors must be studied carefully before deciding on use. In actual mounting, one should be aware that there are products developed to address such problems as cracking and audible ringing; these can be included in the engineer's studies of candidate devices. And in the event that such problems occur, they can be addressed promptly.

Finally, as the manufacturer, we have a wealth of data that was the basis for these technical pages and boundless related experience, and the company invites customers to feel free to ask for consultations regarding any unexplained phenomena in their circuits or other problems or difficulties.

-Thank you very much. In the next series, we'd like to ask about your inductor products.

Power Supply Design Technical Materials Free Download

Power Supply Design Technical Materials Free Download

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