Polymer Tantalum Capacitors Continue to Toughen Up to Meet Automotive Demands
ew materials and manufacturing processes enable polymer tantalum capacitors to pass automotive AEC-Q200 stress test requirements, as well specific electrical transient tests. Although the market supply chain challenges with high CV MLCC‘s availability and new automotive requirements with electrification, digitalization and autonomous driving are bringing new opportunities.
Tantalum Polymer SMD construction is shown in Figure 1. The anode body is a sintered tantalum powder, the dielectric is a thin film of Ta2O5 generated by electrochemical oxidation, and the cathode is a highly conductive polymer layer deposited in the oxide layer. The contact layers consisting of carbon and silver require prior assembly and molding protection. Tantalum Polymer SMD parts are polar capacitors; therefore, is important to pay attention to the polarity marking in the component. Reverse polarity is permitted only up to values indicated in this paper. Although, several application guidelines should be consider by the designers.
Figure 1. Basic Construction of Ta Polymer SMD
KEMET Automotive grade products are designed by Ta polymer SMD T598 (125ºC) and T599 (150ºC) series
|Series||Operating Temperature Range|
|T598 Series||-55ºC to 125ºC
TLL(lower limit) = -55ºC
TUL (upper limit) = 125ºC
|T599 Series||-55ºC to 150ºC
TLL(lower limit) = -55ºC
TUL (upper limit) = 150ºC
Table 1 – Ta Polymer SMD Operating Temperature
The range for voltage is 2,5V to 35V Rated Voltage (Ur), and is defined as maximum peak DC operating voltage from -55ºC to 105ºC for continuous duty. Above 105ºC the voltage is de-rated linearly to 0,67Ur up to maximum operational temperature, Table 2a and 2b.
Ta polymer SMD are solid state capacitors that demonstrate no wearout mechanism when operated within their recommended guidelines. While they can be operated at full rated voltage, most circuit designers seek a minimum level of assurance in long term reliability, which should be demonstrated with data. A voltage derating can provide the desired level of demonstrated reliability based on industry accepted acceleration models. Since most applications do require long term reliability.
KEMET recommends that designers consider a voltage derating, according the Table 2a and 2b, for the maximum steady state voltage.
Table 2a – Derating T598 Series
Table 2b – Derating T599 Series
In Automotive applications, as described schematically in figure 2, the out-put position component (capacitor 2) selection follow Table 2a and 2c depending in the mission profile/temperature requirements. For the 12V line KEMET recommendation is at minimum a 35V rated voltage capacitor taking in consideration the existing ISO Pulse requirements defined by ISO7637 Specification.
Figure 2. Schematic diagram of an Automotive Applications
In the Table.2c we represent the recommended capacitor per application voltage organized by maximum mission profile application temperature.
Table 2c – Application Voltage – Recommended Capacitors
Ta Polymer capacitors are polar devices and may be permanently damaged or destroyed if connected in wrong polarity. The positive terminal is identified by a laser mark stripe. The capacitors will withstand a small degree of transient voltage reversal for short periods as shown in Table 3. Please note that these parts may not be operated continuously in reverse even within these limits.
Table 3 . Maximum Permissible Transient Reverse Voltage
In the 12V line special protection is required to protect the component against transient negative pulses.
Ta Polymer SMD automotive series are fully qualified according the AEC-Q200, revision D from June 2010 and follow the table 1 qualification sample size requirements and table 2 table of methods referenced – Tantalum & Ceramic Capacitors. T598 Series offers extended Endurance Life test up to 2,000 hours demonstrating robust design capabilities
In the effort to support new advance driver assistant systems (ADAS) and comply with the new ultra extended mission profiles a new 7343-31 capacitor with 470uF2,5V with 9mOhm (https://api.kemet.com/component-edge/download/specsheet/T598D477M2R5ATE009.pdf) was release to market . The qualification process was performed with maximum operational temperature of 125ºC up to 2000h. The critical ESR stability at endurance and storage evaluation is shown in Figure 3a and 3b. In both tests the post measurements shown that all capacitors keep an ESR level within 9mOhm.
Figure 3a – Endurance 125ºC / 0,67Ur up to 2,000 hours – 470uF 2,5V – 9mOhm
Figure 3b – Storage 125ºC / 0Vr up to 2,000 hours – 470uF 2,5V – 9mOhm
Surge voltage is the maximum voltage (peak value) which may be applied to the capacitor. The surge voltage must not be applied for periodic charging and discharging in course of normal operation and can not be part of the application voltage. Surge voltage capability is demonstrated by application of 1,000 cycles at operating temperature. The parts are charged through a 33O hm resistor for 30 seconds and then discharged though a 33 Ohm resistor for each cycle. Table 4 shows the applied maximum surge overvoltage for T598 and T598 Series.
Table 4 . Maximum Surge Overvoltage
The design of T598/T599 Series in input side requires to explain technological differences with the old Ta-MnO2 products. During the last years KEMET has done efforts to clarify that Ta Polymer capacitors offers typically a benign failures mode, mitigating ignition due to the absence of oxygen.
In addition, extensively tests were performed based in the:
- ISO7637-2 Road vehicles — Electrical disturbances from conduction and coupling — Part 2: Electrical transient conduction along supply lines,
- Special Load Dump testing E05 according the VW80000, issue 06-2013 – Electric and Electronics Components in Motor Vehicles up to 3,5t
Results are summarized in Table 5:
Table 5 Summary of ISO7637-2 and VW80000-E05 Load Dump
We concluded that for the 35V rated voltage part numbers no failures created on Pulses 2a, 2b, 3a,3b, 4, 5b suppressed and in E05, and that application attention is required with:
- Pulse 1 – The 1 Reverse Pulse requires reverse voltage protection with maximum of -1V
- Pulse 5a – The 5a pulse limitation requires up front definition of the pulse shape and parameters for an alternator with centralized load dump suppression or not is use. In most new alternators, the load dump amplitude is suppressed (clamped) by the addition of the limiting diodes. All evaluations with maximum peak of (13,5+40)V pass the acceptance criteria.
An important application requirement is the validation of the mission profile. Ta Polymer SMD capacitors have an average failure rate of 0.5 %/1,000 hours at category voltage, UC, and category temperature, TC. These capacitors are qualified using industry test standards at UC and TC. The minimum test time (1,000 hours or 2,000 hours) is dependent on the product series.The actual life expectancy of polymer capacitors increases when application voltage, UA, and application temperature, TA , are lower than UC and TC.
As a general guideline, when UA < 0.9 * UC and TA < 85°C, the life expectancy will typically exceed the useful lifetime of most hardware (> 10 yrs).
The lifetime of a polymer capacitor at a specific application voltage and temperature can be modeled using the equations below.
A failure is defined as passing enough current to blow a 1-Amp fuse. The calculation below is an estimation based on empirical results and is not a guarantee.
The existing automotive Ta Polymer SMD series offer excellent solution for bulk capacitance requirements and high reliability capability against typical mission profiles, figure 4.
Figure 4 . Histogram Reference Example – Mission Profile (Old – Combustion 8000h and New – Electrification/Autonomous 131,000 hours)
During the mission profile validation we determine the estimated FIT based in the given conditions, from the examples describe above we can estimate for Old Mission Profile and New Mission Profile that Automotive Ta Polymer SMD products are a viable option, Figure 5a and 5b.
Figure 5a. Mission Profile Validation – Old Mission Profile Example
Figure 5b. Mission Profile Validation – Old Mission Profile Example
KEMET continues to expand the Ta Polymer SMD automotive capabilities with robust design validation and extended endurance and storage specification against AEC-Q200. Design application guidelines and demonstration of specific customer requirements have been issued and validation of new extended mission profiles completed. Expectation is that adoption will continue with emergence new and more space efficient automotive modules take advantage of volumetric efficiency and performance of these product series.