Ask Direct to Engineers
What capacitor and inductor are the best for a switching power supply?
Need to Know not Only Electrical Specs, But Also Their Characteristics Including Materials and Packaging Cases
Capacitor – Part 2 -
－Based upon what you have told us thus far, would it be safe to assume that the MLCC offers significant advantages in ESR, capacitance/size, and reliability/lifetime?
There is no question that the MLCC provides a favorable option in terms of ESR, capacitance/size, reliability/lifetime, as well as in extremely low equivalent series resistance (ESR). The thickness of the dielectric component contributes not only to a high degree of reliability, but alto to high voltage tolerance. However, the MLCC is not without problems. This table, summarizing the advantages and disadvantages of different technologies, identifies several issues.
As you can see, there is no denying that in terms of temperature characteristics on the high-temperature side and DC bias characteristics, the MLCC offers less favorable bias characteristics when compared with other technologies.
－Could you tell us more in terms of specific characteristics?
I’ve brought with me a graph on temperature characteristics of the capacitance and a graph with respect to DC bias. In the temperature characteristic graph, the blue and red lines represent the MLCC, and the green line indicates aluminum electrolyte, and the orange line represents tantalum one. Aside from an extreme drop in capacitance for the aluminum electrolyte on the low-temperature side, on the high-temperature side the MLCC exhibits a significant decline in capacitance when compared with other technologies. The blue temperature characteristics, representing X6S, indicate an applicable temperature range from -55℃ to 105℃, with a rate of change amounting to 22%. The red line, representing X7R, has a temperature range of -55℃～105℃/±15%.These grades, often used in DC/DC converters, offer relatively favorable temperature characteristics. The low-end grades such as Y5V are compatible with a -30℃～85℃ temperature range, with a change of rate of +22%/-82%. These grades may not be applicable to regular DC/DC converters.
Incidentally, the temperature characteristics for this capacitor are coded in EIA. Although In Japan JIS coding is used in most cases, worldwide, EIA seems to be the standard code. I’ve brought some supporting data for your reference.
The DC bias characteristics, as the name implies, indicate a change in capacitance when subjected to a DC voltage. In this case, too, in the MLCCs indicated by red and blue lines, the capacitance declines substantially as the bias voltage rises. Because of a large rate of change, in situations where this type of change is likely to occur, the capacitor must be used with a grain of salt.
－You explained to us that the MLCC tends to have low ESR. Because DC/DC converters are subject to ripples, how do their ESR frequency characteristics behave?
In this case, too, for a quick answer see graphs. The graph on the left shows changes in ESR and impedance as a function of frequency. The magenta and thin blue lines are MLCCs. The other lines show the characteristics of electrolyte and functional polymer capacitors. Whereas for the MLCC the curve declines in a straight line down to the resonance point, for the other capacitors the curve behaves loosely, resulting in a significant difference in ESR as well. Also, the table below shows numerical values in ESL and points.
Another graph shows the relationship between ripple current and temperature increase. Obviously, the MLCC with low ESR values undergoes only a small rise in temperature. The attendant low heat dissipation is a critical point as it has a significant impact on the life of the capacitor.
－Are there any other features of the MLCC that we need to be aware of?
There is one thing I need to mention, that is, the MLCC exhibits different ESR and ESL values, depending on the size of the case in which it is packaged. The actual tendency is that the smaller the case, the smaller are these quantities. Let us look at the graph, showing five types of data for 10µF capacitors, ranging in size from 1005 to 3225. Assuming that the application is to DC/DC converters, available switching frequencies that are worth noting seem to range from 100 kHz to 6 MHz in newer and faster converters, and even boasting 10 MHz, most of them occurring in the range enclosed in vertical blue lines.
In the switching frequency band, the ESR tends to become small in MLCCs that are packaged in small cases. Past the resonance point, when the capacitance component becomes an inductance component, that is, ESL, the lines 3216 and 3225 become reversed in a case aspect ratio relationship. In terms of the overall trend, however, the smaller the case the smaller the ESL.
－Whereas in the case of an IC the electrical characteristics tend to be predominant, I can now see that given the same capacitance and voltage tolerance, capacitors have unique characteristics, depending on the particular structure and materials in which they are made.
In fact, that is a quite important point. Although capacitors are simple components, they can be the main culprit in a problem that arises unless they are selected with an adequate understanding of differences in characteristics due to materials and precautions that must be complied with. In DC/DC converter application examples, for instance, there may be cases where not only capacitance but also the types and brands of capacitors employed are specified. There is a reason for it: We must be aware that if capacitors are changed arbitrarily even though retaining the same capacitance and voltage tolerance, the possibility of the changed capacitor not operating properly could arise.
What capacitor and inductor are the best
for a switching power supply?
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