How to select capacitive and inductive components for portable systems (Part 1)

When designers think about passive components, they think about the production tolerances for inductors and capacitors, which are typically ±20% or ±10%. This is true in theory, but not in practice. At a certain frequency, adding a DC bias voltage to a ceramic capacitor or loading a current to an inductor will change the characteristics of these components, so they are called "active passives". For example, a 10µF, 0603, 6.3V capacitor measures 4µF at -30°C with a DC bias of 1.8V. A 3.3 μH inductor measures 0.8 μH when used in a practical application at 85°C.

In addition, component manufacturers are becoming more aggressive and are likely to continue to introduce some very good parts to remain sufficiently competitive in the size-to-value war. This is similar to various practical situations. For example, a car with an EPA (U.S. Environmental Protection Agency) test rating of 30 Mpg (miles per gallon) may only have 20 mpg in actual driving. This means car owners have to go to the gas station more often than expected.

This example can be extended to portable power systems. Every component used by each module in the system has a direct impact on system performance. Key performance indicators for portable power systems include battery life, solution size, ease of use of system resources, etc. For example, in portable power systems, charging devices too frequently will make the so-called "portable" meaningless.

System designers have taken the first step in achieving these key performance indicators by selecting switching regulators to power the different system modules. The next step is to ensure that the selected switching regulator is operating at maximum efficiency. The key performance indicators of switching regulators are efficiency, accuracy and output voltage tolerance (including transient response, voltage ripple, solution size, etc.). To meet these performance specifications, the switching IC must work in harmony with external components.

The external components of a switching regulator typically include an inductor, an input capacitor, and an output capacitor. Just as the success of any game relies on a team working together, external components and switches must work together to meet the expected performance specifications of the DC-DC converter solution.

When designing the switching regulator, a series of compensations for the inductor value and input and output capacitance values ​​are optimized. The part's output current capability also depends on many factors, one of which is the inductor value.

This article introduces some of the parameters that capacitors and inductors are susceptible to, discusses what system designers must know, and explains when to select external components for the smallest yet most efficient solution for portable power systems.

Select capacitor

Let's look at ceramic capacitors first. This capacitor is ideal for portable applications due to its size, cost and performance advantages, and is also well suited for high-frequency applications due to its low equivalent series resistance (ESR) and equivalent impedance at the switching frequency. Low ESR minimizes output voltage ripple, and low impedance produces excellent filtering characteristics. The temperature coefficient of Y5V dielectric capacitors is very poor, and may drop by 80% at 85°C. It is generally not recommended for portable applications, so this section focuses on X5R/X7R capacitors.

Figure 1 shows the history of case size changes for 10μF, 6.3V, X5R ceramic capacitors. The main benefit of the smaller housing size is to save the switch's footprint and reduce the height of the overall solution. Currently, the maximum height limit for components used in phones by mainstream mobile phone manufacturers is 1.2mm. As phone models become slimmer, this limit will decrease further. Today's ceramic capacitors are well able to meet these requirements.

So, is there anything else a system designer needs to know beyond ceramic capacitors? Absolutely needed! For example, when selecting the capacitance value and case size of a ceramic capacitor, its DC bias effect must be taken into consideration. Incorrect capacitor selection can wreak havoc on the stability of the system. DC bias effects typically occur in ferroelectric dielectric (Class 2) capacitors, such as X5R, X7R, and Y5V class capacitors.

The basic calculation formula for ceramic capacitors is as follows:

C=K×[(S×n)/t]

Here, C=capacitance, K=dielectric constant, n=number of dielectric layers, S=electrode area, t=thickness of dielectric layer