New Solid Polymer Aluminum Capacitors Improve Reliability

The new, 125°C, solid polymer aluminum (SPA) capacitor has performance advantages over other types of low ESR capacitors when used as dc-dc converter output filters. The principal advantage of an SPA capacitor is that its ultra low ESR permits filtering with fewer capacitors, which reduces the converter's cost and size. Other advantages are an ignition-free, open circuit failure mode, better filtering from lower ESR, higher peak current capability, and more rapid energy transfer.

Although typical SPA capacitors are inherently reliable, they have a wear out mechanism. Entrapped moisture causes a steady increase in ESR. However, the new 125°C, ESRH version eliminates most of the moisture entrapped during manufacturing and can provide an expected life of more than 10 years in typical, hot running dc-dc converters.

The new Type ESRH SPA device, rated for 125°C operation, is a breakthrough for hot running dc-dc converters. Dc-dc converters routinely incorporate an integrated metal substrate (IMS) to handle higher currents, reduce the thermal resistance, and lower the operating temperature of magnetic components and switching devices. However, the IMS permits operation up to 125°C, therefore this requires the output capacitors also be capable of 125°C operation.

The ESR of the output capacitors in a dc-dc converter sets the level of unwanted output ripple voltage, because at the 200 kHz to 900 kHz switching frequencies the capacitive reactance is negligible compared to the ESR. For comparable CV values, an SPA capacitor has an ESR at 100 kHz, typically one-seventh to one-tenth that of low ESR tantalum chips, and one-third of tantalum polymer types. This ultra-low ESR accomplishes output filtering in a dc-dc converter with fewer capacitors. Fewer capacitors are possible because of the combined effect of the SPA's lower ESR, higher ripple current handling capability of 2A or 3A (continuous and up to 60A peak), and the ability to deliver more energy in less time.

While the modeling of solid tantalum capacitors have no wear out mechanism and thus a failure rate that decreases with time, SPA capacitors have a wear out mechanism, and the ESR increases. Entrapped moisture determines the rate by which ESR increases.

You can define expected life as the time until the 400 kHz impedance reaches two times its initial value. Expected life increases with the reduction of applied voltage, ambient temperature, and relative humidity. Relative humidity has the most effect. As a function of these three environmental variables, expected life models demonstrate more than 10 years life for all but the most extreme applications.

Lower ESR

As said, the big advantage of SPA capacitors is their low ESR at high frequency. Compared to solid tantalum and other SMT aluminum capacitors capable of the capacitance values needed for dc-dc converter output filters, the SPA have much lower ESR.

Referring to Fig. 1, on page 50, ESR descends more than two decades as you move from standard SMT aluminum Type AVS, to low ESR SMT aluminum Type AFK, to low ESR tantalum polymer type and finally to the SPA Type ESRE. In the frequency range from 1000 Hz to 100,000 Hz the ESR of the SPA chip is one-third or less than the ESR of the polymer tantalum chip. This demonstrates the need for three tantalum chips to provide the same filtering and output ripple voltage for a dc-dc using one SPA chip. In this case, the SPA capacitor has an ESR one-third that of the new tantalum polymer chip, which has the lowest ESR of any available tantalum chip.

The slow transient response of power supplies embedded in equipment can cause CPUs to malfunction, because the supply current ramps up too slowly, as shown in Fig. 2, on page 50. An appropriate capacitor across the interface between the supply and the CPU can function as a buffer and provide drive current until the supply output current has ramped up, but not just any capacitor will do.

You can test capacitor transient response using the circuit of Fig. 3. Looking at Fig. 4, on page 54, you can see the test circuit that provided the curves comparing the transient response of an 180 μF, 2V SPA vs. a 220 μF, 4V polymer tantalum. Polymer tantalum capacitors perform better on this test than standard, low ESR tantalum capacitors because they have lower ESR resulting from the use of a solid-polymer impregnation of the sintered-tantalum pellet capacitor. This polymer is similar to the one used in SPA capacitors.

Both capacitors were charged to 2V and discharged into a short circuit. The curves show the SPA capacitor delivers a much higher peak discharge current and fully discharges in less time. The peak current for the SPA capacitor is 70A and 30A for the tantalum capacitor. The SPA fully discharges in 15 μs, whereas the tantalum capacitor takes 30 μs. While the two capacitors exhibit the same initial di/dt of 40A/μs, the SPA delivers half its energy in half the time required for the tantalum capacitor (12.3 μs vs. 24.5 μs).

Time Constant

When a capacitor is used as a source stiffening capacitor and is buffering the supply by delivering initial current, a lower RC time constant allows delivering more current. The Table, on page 54, compares a 33 μF, 8V SPA and a 33 μF, 10V tantalum (T491D336(1)010AS). SPA capacitors run 20% to 50% lower RC time constants than same rating, low ESR tantalum chips with best performance typically at 100 kHz.

With microprocessor speeds exceeding 1 GHz and CPU peak current demands of 80A and more, output filter capacitors are being hit hard. The days of conservatively holding the source impedance to one ohm per volt to protect the output capacitor are over. This recent demand for higher peak currents and at faster repetition rates has surpassed the ability of the capacitor industry to respond, and no industry or manufacturer standards exist for specifying peak current capability. However, the transient response test circuit of Fig. 3, on page 52, is also suitable to test a capacitor's peak current capability. While there are no absolute requirements, it's possible to compare performance of different types of capacitors.

You can vary the repetition rate by replacing the mechanical switch shown in Fig. 3 with two power MOSFET switches and appropriate driving circuitry. Thus configured, the test circuit revealed orders of magnitude differences in peak current capability between SPA capacitors and low ESR tantalum capacitors. With a 33 μF, 8V SPA capacitor in the test circuit, the supply voltage was set to 125% of rated voltage, 10V, and the repetition rate was set to 1 kHz. The current pulses occurred twice per cycle with a positive pulse when applying the power source and a negative pulse when shorting the capacitor to ground. Initially the current peaks were 60A as shown in Fig. 5. After 6.5 hr of continuous operation the peaks began to approach 40A. The following day the current peaks were initially 45A; so, the permanent change in the capacitor apparently caused the drop in current.

With a 33 μF, 10V solid tantalum capacitor in the test set, it was a different story. We set the supply voltage again to 10V, but in this case 10V was the rated voltage. The repetition rate was again set to 1 kHz. Initially the current peaks were 45A; however, in 1 min and 55 sec the capacitor erupted in flames, as shown in Fig. 6. A second capacitor lasted 2 min and 5 sec before it too burst into flames.

The tantalum capacitor was able to withstand continuous operation at 1 kHz with the supply voltage reduced from 10V to 6.3V. This is a 60% reduction in power. Note that at 1 kHz and 10V the SPA capacitor lasted nearly 12,000 times as long as the tantalum capacitor before its performance reduced to the initial performance of the tantalum capacitor.

Ignition-Free Failures

While other capacitor types routinely fail short circuit when exposed to overvoltage or overtemperature stresses, SPA capacitors are surprisingly free of short-circuit failures. The physical mechanism that prevents short-circuit failures is in the polymer electrolyte. When a short-circuit failure tries to occur, local heating of the polymer converts it to a high resistance, stable compound, and thus effectively self-heals the capacitor. A similar self-healing occurs in solid tantalum capacitors in which the manganese-dioxide (MnO₂) contact electrode is converted to a more resistive manganese oxide (Mn₂O₃). However, the conversion of the polymer in SPA capacitors produces better disconnects of damaged material. This is because while MnO₂ requires heating above 600°C to increase resistivity, the SPA polymer becomes resistive above 300°C and is open circuit at less than 500°C.

Demonstrating the reliability, 5080 SPA capacitors tested at 105°C with rated voltage applied with less than 0.1Ω source impedance for 20 million units hours exhibited no failures. At 125°C, 1000 SPA capacitors tested for 3.5 million unit hours also exhibited no failures.

The advantage of SPA capacitors is that they don't support combustion. None of the constituent materials is readily flammable and failures are ignition free. In tantalum capacitors the MnO₂ releases oxygen that supports combustion.

The SPA capacitor appears to be an offspring of a delightful marriage between the solid tantalum capacitor with the aluminum electrolytic capacitor. The SPA and its tantalum and aluminum parents create the capacitor's dielectric as an oxide grown on a metal that connects to the positive terminal of the capacitor. However, the SPA's positive side looks like an aluminum electrolytic and its negative side looks like a solid tantalum.

The aluminum electrolytic capacitor starts with an etched and oxidized anode foil, an etched cathode foil and paper separators. Aluminum tabs connect to the foils and the foils and paper wind into a capacitor element. The capacitor element is wet with conductive electrolyte, and the tabs connect to terminals in the sealed capacitor. The aluminum oxide connects to the negative terminal through the electrolyte and cathode foil.

The SPA capacitor replaces the liquid electrolyte of the aluminum electrolytic capacitor with a highly conductive polymer that becomes solid after it penetrates the pours of the etched aluminum anode foil. Then, like the solid tantalum capacitor, it connects to the negative terminal through coatings of carbon and silver paint.

The high conductance of the solid polymer sets the SPA above its parents making it 1000 times as conductive as MnO₂ and 10,000 times as conductive as liquid electrolyte.

Until now, the SPA capacitor has been available only as a 105°C rated component. The problem was that if operated continuously at 125°C, the ESR would increase rapidly and cause the impedance to double in a few hundred hours. Moisture trapped during manufacture at the interface between the conductive polymer and the aluminum-oxide dielectric would react at high temperatures and form aluminum hydroxide, which is conductive and appears as a resistive layer in series with the solid polymer.

Introduction of the new Type ESRH capable of continuous operation at 125°C required changes to the polymer resin and manufacturing environment to avoid entrapping moisture at the polymer aluminum-oxide interface. As shown in Fig. 7, humidity and operating temperature greatly affect expected life.

The expected life equation is

This is expressed as a function of ambient temperature T_a in °C and ambient relative humidity H_a in percent.

References

J. Marshall, J. Prymak, E. Reed, “Lowest ESR Tantalum Chip Capacitor,” CARTS '98 Program, The Components Technology Institute, Inc., Huntington Beach, Calif. March 1998.
Ted von Kampen, “Ensure AC Film Capacitor Reliability with Thermal Analysis,” PCIM March 2001, pp 56-67.
I. Clelland & R. Price, “Multilayer Polymer (MLP) Capacitors Provide Low ESR and are Stable Over Wide Temperature and Voltage Range,” Proceedings of the 8^th Annual European Capacitor and Resistor Technology Symposium, October 1994.
J Prymak, “Performance Issues for Polymer Cathodes in Aluminum and Tantalum Capacitors,” CARTS 2001 Program, The Components Technology Institute, Inc., Huntington Beach, Calif., February 2001.