Substituting solid tantalum and tantalum polymer capacitors for surface-mount MLCCs

Article By : David Richardson

Examining key performance characteristics will help streamline the evaluation process of alternative capacitor technologies for the replacement of multilayer ceramic chip capacitors.

Due to current bottlenecks in the procurement of surface-mount ceramic capacitors, designers are looking for substitutes to keep their production lines running smoothly, and to find long-term replacements for hard to find ratings. This article will examine key performance characteristics that will help streamline the evaluation process of alternative capacitor technologies for the replacement of multilayer ceramic chip capacitors (MLCCs). The most likely MLCC candidates for successful replacement by tantalum technology are the higher capacitance values in case sizes from 0402 to 1210. Applications that require high capacitance for filtering or voltage stabilization are good candidates for replacement.

There are two primary capacitor technologies that are most often considered for surface-mount applications: electrostatic and electrolytic. The most common electrostatic types are MLCCs and film capacitors. The most common electrolytic types are aluminum and tantalum (including both solid and polymer tantalum technologies). When looking to substitute for high capacitance MLCCs, it makes sense to pick tantalum electrolytic devices to get the broadest overlap in size, surface mount ability, capacitance values, voltage ratings, and reliability.

Although top-level comparisons of capacitance, voltage, tolerance, and size are useful as a starting point, MLCCs and surface-mount tantalums utilize different designs and materials in their construction. As a result, they have different electrical and mechanical properties. Instead of throwing massive amounts of performance data at the design team, this paper will attempt to look at the key parametric differences that relate to the performance of the capacitors. In addition, some helpful hints and suggestions for testing will be offered with the goal of successfully substituting solid tantalum or tantalum polymer capacitors for MLCCs with maximum efficiency.

The equivalent circuit of a capacitor
To simplify and organize our investigation, we will utilize the capacitor equivalent circuit as a model and discuss how the different elements of the circuit vary between MLCCs and tantalums. Figure 1 shows the universal equivalent circuit of a capacitor:

Figure 1 Universal equivalent circuit of a capacitor

• RESR = equivalent series resistance in ohms. This is the real part of the impedance that produces losses via heat generation
• C = capacitance value in Farads. The reactance of this component is XC = 1 / 2πfC
• L = inductance in Henrys. The reactance of this component is XL = 2πfL
• RIR = insulation resistance (in an ideal capacitor this would equal infinity, but in an actual capacitor it’s a finite resistance value that can be used to calculate the DC leakage current value IDCL)

Figure 2 Capacitor impedance as a function of frequency

Let’s take the components of the equivalent circuit one at a time and consider their effect on the overall circuit performance.

Capacitance (C)
Most MLCCs used in today’s applications are “Class II” types. This means that the capacitance value varies over temperature (temperature coefficient of capacitance (TCC)) in the following ways:

• ± 15% from −55 °C to +125 °C for X7R dielectrics
• ± 15% from −55 °C to +85 °C for X5R dielectrics

 
Figure 3 Temperature coefficient of capacitance for Class II capacitors

MLCCs have another property that affects the capacitance value: the voltage coefficient of capacitance (VCC). As you apply voltage to a Class II MLCC, the closer you get to the rated voltage of the capacitor, the more the capacitance value is reduced.

 
Figure 4 Voltage coefficient of capacitance for Class II capacitors

Furthermore, these TCC and VCC properties are cumulative.

For a Class II MLCC capacitor with rated voltage applied at 85 ºC, the effective capacitance value could be reduced to as little as 30% of the stated value.

For solid tantalum and tantalum polymer devices, though, this VCC effect is insignificant.  Regardless of the applied voltage, the capacitance value remains essentially unchanged.

For both solid tantalum and tantalum polymer capacitors, the capacitance value increases with temperature.

 
Figure 5 Tantalum capacitors increase in value with temperature

In summary, tantalum and tantalum polymer capacitors offer higher and more stable capacitance values than MLCCs, especially at high temperatures. So for applications that require high capacitance values, such as power filtering and bulk energy storage, the tantalum and tantalum polymer electrolytic devices will offer better capacitance retention than MLCCs with the same capacitor rating. In some cases, it may be possible to replace several MLCCs configured in a parallel combination with fewer, or a single tantalum electrolytic.

ESR (R_ESR)
The ESR of a capacitor represents the real part of the impedance (Z) and includes all the resistive elements of the capacitor, including the terminations, lead frame (if used), electrodes, dielectric, and solid or polymer electrolyte system.

In general, MLCCs have lower ESR than either solid tantalum or tantalum polymer capacitors of the same voltage rating and capacitance values.

For a typical 47 µF, 6.3V 1206 capacitor, the maximum 100 kHz ESR levels would be something like this:

• Solid tantalum ~ 0.9 Ω     
• Tantalum polymer ~ 0.1 Ω 
• MLCC ~ 0.003 Ω

These differences in ESR may or may not significantly affect performance in the circuit, depending on how much design tolerance for ESR is built in.

Here are some general comments on ESR:

• Lower ESR typically results in a capacitor that can handle higher RMS ripple current
• Lower ESR capacitors are more efficient for decoupling noise to ground
• Lower ESR is more effective for delivering high momentary current from bulk energy capacitors
• Very, very low ESR can sometimes lead to instability in feedback loop circuits
• ESR vs temperature can vary with different capacitor technologies, so be sure to check ripple current and power dissipation at both the low and high ends of the operating temperature range
• Changes in ESR could cause variations in RC time constants in timing circuits 

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Editor’s note: This article is part of a Special Project by AspenCore Media (EDN’s parent company) on the electronic industry’s IP&E (interconnect, passive, and electromechanical) component shortage. For more on the shortage, including identifying high-demand markets; sourcing solutions and tips; production forecasts; and how suppliers are working with designers and buyers to manage through this crisis, see these other articles from across the AspenCore Network:

Engineering a Solution to the IP&E Shortage – Supply challenges are not something that designers typically concern themselves with, but this is one time when they should.

How the Supply Chain Developed an IP&E Shortage – What distinguishes this scarcity from others is the broad base of markets that are desperate for IP&E.

Passive component shortages drive new supply strategies at Kemet – Currently, many MLCCs and resistors in the industry are on allocation or are quoting lead times well into 2019. But the supply shortages are spread across capacitor technologies.

AVX: Component Shortages Require Better Communications, More Transparency – A common thread running throughout the component market is rising demand, which crosses a variety of end market sectors, contributing to passive component shortages.

Murata: Ceramic Capacitor Lead Times Continue to Stretch; Parts Go EOL – Ceramic capacitor manufacturers including AVX, KEMET, Taiyo Yuden and TDK are all quoting extended lead times and managing product allocation.

Vishay Intertechnology Diversifies as End-Markets Expand – Here’s the first of several supplier profiles on key players in the IP&E market.

IP&E Users Get Creative as Component Shortages Linger –  OEMs are facing idle production lines while they wait for devices that cost less than a nickel. We’ve interviewed IP&E market leaders to find out how they’re meeting customers’ needs.

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