By Craig Hunter, Senior Director, Global Marketing Communications and Dave Richardson, Senior Manager of Field Applications,
OEMs are today suffering from shortages of Multi-Layer Ceramic Capacitors (MLCCs), especially for devices in large case sizes and in high capacitance values. This has led them to evaluate the polymer tantalum capacitor as an alternative for functions such as filtering and in applications such as voltage stabilization and buffering. This Design Note describes some tips to help streamline the evaluation and testing process necessary for a successful substitution of a polymer tantalum capacitor for an MLCC.
These two capacitor types are commonly used surface-mount devices, and are ideal for many applications. To ensure a substitution is successful, it is necessary to look at the main differences in performance that result from the different materials and construction in each type. The designer then needs to consider different values of the main parameters by which a capacitor is specified to see whether the polymer tantalum alternative is compatible with the circuit’s performance requirement.
The MLCC devices most likely to be replaced use a Class II dielectric material. This Class II ceramic, typically X7R or X5R, has a capacitance value which will vary over the operating-temperature range. This characteristic is called the Temperature Coefficient of Capacitance, or TCC, as shown in Figure 1.
For a typical X5R device the TCC is ±15% over a temperature range of -55°C to 85°C. Devices with a Class II dielectric also have a Voltage Coefficient of Capacitance, or VCC, as shown in Figure 2. As the voltage applied to the MLCC approaches the rated voltage, capacitance will drop markedly. These TCC and VCC characteristics are additive. So for a Class II device operating at 85°C and near its rated voltage, the capacitance could be as little as 30% of the specified datasheet value.
By comparison, polymer tantalum capacitors have no material VCC effect, and therefore the capacitance value under applied voltage conditions remains fairly stable. In addition, the capacitance of these devices actually increases slightly as temperature increases, as shown in Figure 3.
Overall then, for surface-mount applications that require high capacitance values, such as bulk energy storage or power filtering, polymer tantalum capacitors provide superior capacitance performance over MLCCs with similar ratings. In fact, if capacitance is the driving factor in the application, it might be possible to replace multiple MLCCs with a single polymer tantalum capacitor.
Rated voltage, de-rating and polarity
It is generally considered safe to run MLCCs at up to their full rated voltage. In practice, many designers de-rate by around 20% to provide for the VCC effect, which yields lower effective capacitance values in the circuit.
However, it is mandatory for designers to de-rate a polymer tantalum capacitor by 20% for 10V ratings and above, and by 10% for products below 10V. By comparison, traditional tantalum (MnO2) parts must be de-rated by 50%. These parts can, however, be better from a cost point of view if they meet the technical requirements of the application.
While polarity is immaterial for MLCCs, it must be maintained for polymer and tantalum devices. This precludes the use of polymer and tantalum devices in switching applications in which reverse voltage spikes can occur.
Equivalent Series Resistance (ESR)
ESR is the real part of the impedance (Z) value. It represents all of the resistive losses in the capacitor. When a signal is passed through a capacitor, energy is lost in the form of waste heat, a function of ESR.
An MLCC has a lower ESR than a polymer tantalum capacitor of the same voltage and capacitance rating. Devices with a lower ESR more efficiently decouple noise to ground, can handle higher average ripple current, and more effectively provide momentary high currents.
It is also worth noting that parts with a low ESR meet pulse-current demand while avoiding voltage drops during discharge, and enable the specification of a lower input voltage. On the other hand, the use of capacitors with very low ESR can sometimes lead to instability in feedback-loop circuits.
Equivalent Series Inductance (ESL)
A capacitor’s ESL is primarily determined by its physical dimensions. The reason that MLCCs are often designed with long side terminations is to decrease inductance in high-speed applications. But overall, for similarly sized devices of normal construction, there is unlikely to be an important difference in performance due to the inductive component.
High-speed circuits are an exception, however: inductive loads may delay the delivery of the required current from the capacitor, and this can have an impact on the performance of the circuit. The effects on impedance vary with case size and can be seen in Figure 4.
DC leakage current
This value is specified differently depending on capacitor type. In short, MLCCs have a lower leakage current and outperform polymer tantalums by a factor of around five.
Scorecard and other relevant information
MLCCs offer superior ESR and DC leakage current characteristics, and are also non-polarized. So if polymer tantalum capacitors are selected, polarity must be maintained on the PCB. Mechanically, MLCCs are more susceptible to cracking when using larger case sizes on boards during the pick-and-place and assembly processes.
|Range of Case Sizes||Capacitance||Voltage||Vishay Polymer Tantalum||Vishay Tantalum MnO2|
|1608 / 0603||0.68μF to 22μF||2.5V to 50V||T55 standard range T58 extended range||TMCJ 298D|
|2012 / 0805||0.1μF to 47μF||2.5V to 50V||T55 standard range T58 extended range||TMCP|
|3216 / 1206||0.1μF to 220μF||4V to 75V||T55 standard range T58 extended range||TMCS 293D|
Fig. 5: Capacitor replacement reference table
High-capacitance MLCCs tend to suffer from interference by high-frequency signals. This is perceived in the form of audible noise, such as whistling, and the piezo effect, making them a poor choice in some DC-DC conversion and audio applications.
Polymer tantalum capacitors provide high and stable capacitance values, which remain almost unaffected by the application of voltage. But they have higher ESR and DC leakage current values than Class II MLCCs. Their materials and construction make them less susceptible to mechanical damage caused by board flexing or high-temperature reflow processes.
When substituting one capacitor type for another, designers should consider:
• The effect of TCC and VCC on capacitance values
• Case size, especially height
• Voltage rating and de-rating
• Polarization, for polymer and tantalum capacitors
• The dynamic parameters of ESR, ESL and DC leakage current
Figure 5 provides a reference table which may be used as a starting point in the evaluation process when replacing MLCCs with polymer and tantalum MnO2 capacitors.