Stability and transient response are critical to LDO performance.
An electrolytic capacitor used for smoothing, energy storage, or filtering a rectified ac voltage has a ripple current that causes power loss and self-heating. Because its life is determined by its internal temperature, designers must minimize this temperature.
Electrolytic capacitors are a critical element in power electronics designs. Different applications in power electronics have different requirements when it comes to selecting electrolytic capacitors. However, one common requirement in most of these applications is the need for high ripple current capacity. This need is often combined with elevated ambient temperatures.
An electrolytic capacitor is one of the most expensive components in a power electronics circuit. For this reason special attention is often paid to the end of life of this component - especially when dealing with capacitor banks that include a number of these capacitors. In most cases, the electrolytic capacitor is the life-limiting device. Therefore, it's important to understand the factors that can contribute to the capacitor's end of life so the anticipated lifetime meets the overall system reliability requirements.
Several factors can cause electrolytic capacitors to fail, such as severe cold temperatures, heat (soldering, ambient, ac ripple), high voltages, transients, extreme frequencies or reverse bias. However, heat is the most influential factor on the operational life (L subscript op) of electrolytic capacitors. Apart from abnormal failures, the life of electrolytic capacitors has an exponential temperature dependency. With non-solid electrolytic solutions, the life of the capacitor is determined by how fast the electrolyte solution evaporates, causing degradation in the electrical parameters. Those parameters are the capacitance, leakage current and the equivalent series resistance, ESR.
The temperature rise in the capacitor depends on the ESR and the rms value of the current flowing through it, in combination with the thermal properties of the device. At some spot inside the capacitor, the highest temperature will be found. This is known as the Hot Spot temperature (T subscript h). The value of the hot spot temperature is the major factor influencing the operational life of the capacitor. The hot spot temperature is dependent on several factors. Those factors are the outside temperature in the application-also called the ambient temperature (T subscript a)-the thermal resistance (R subscript th) from hot spot to ambient, and the power loss (P subscript LOSS) caused by the ac current. The temperature rise inside the capacitor is linear with the power loss.
When the capacitor is charged and discharged, the current flowing through it causes losses in the ohmic resistance. The change in voltage across the dielectric causes losses as well. Add to that the losses caused by the leakage current. These losses result in a temperature rise inside the capacitor. With that in mind, the operational life can be calculated:
P subscript LOSS4(I subscript rms) subscript 22ESR T subscript h4T subscript a + P subscript LOSS2R subscript th L subscript OP 4A x 2 superscript (B-Th /C) Hr
B4Reference temperature (typically 85C).
A4Life at reference temperature (varies by capacitor diameter).
C4Number of degrees temperature rise needed to reduce the life by half (typically 12 C for screw terminal type capacitors).
In the non-solid electrolytic capacitor (Fig. 1, on page 64), the dielectric is the oxide layer of the anodic foil. The electrolyte acts as the electric contact between the cathode foil and the oxide layer of the anode foil. Paper layers absorbing the electrolyte acts as a spacer between the cathode and anode foils. The foils are connected to the capacitor terminals by aluminum tabs.
Fig. 2 shows what happens inside an electrolytic capacitor. Conduction capability depends on the electrolyte dissociation and viscosity. When the temperature is lowered, the viscosity is increased, causing lower ion mobility and lower conductance. When the electrolyte freezes, ion mobility becomes very low, leading to very high resistance. On the other hand, excessive heating will accelerate electrolyte evaporation. When the amount of electrolyte is reduced to a critical amount, the end of life of the capacitor is reached.
Factors of Life What are the main factors that directly and indirectly contribute to electrolytic capacitor life? Screw-terminal, non-solid electrolytic capacitors are designed specifically to provide long life in power electronics applications where they are often exposed to very high ripple currents at elevated ambient temperatures.
Electrolytic capacitors must be designed for operation under severe climatic conditions and for heavy ripple current load. This requires low internal losses and an efficient heat transfer between the capacitor "Hot Spot" and the ambient. Long life of electrolytic capacitors is achieved in three ways:
- Internal heating due to the flow of ripple current in the capacitor is reduced by lowering the ESR. This is accomplished by the use of multiple, laser-welded electrode tabs. The temperature rise in the capacitor depends on the ESR and the ripple current. ESR is frequency dependent, adding complexity to the power loss calculations. Typically, the ESR is lower at higher frequencies; though some designs offer a flatter ESR characteristic, resulting in better performance at lower AC frequencies.
One of the main contributors to excessive ESR is the connection between the outer electrodes and the winding, usually made with one or more metal tabs. The more tabs added to the winding, the lower the ESR. However, the number of tabs that can be added without reducing reliability depends on the process used to connect the tabs to the terminals. Special laser welding allows you to add more tabs, providing lower ESR. This means more ripple current capability and less internal heating, which means longer life.
This also contributes to a higher shock and vibration resistance, which reduces internal short circuits, high leakage currents, capacitance losses, increased ESR and open circuits.
- Internal heat is dissipated out the bottom of the can to the equipment chassis, via a good mechanical connection between the capacitor winding and the can. This heat is also dissipated by an internal heat sink running through the middle of the winding.
The internal thermal design is important for the reliability and operational life of the capacitor. In one design, the negative foil is extended to make direct contact with the thick base of the capacitor aluminum can. The base then becomes a heat sink to the winding through which heat is dissipated away from the hot spot. With the "Stud" mounting option, securely mounting the capacitor into a plate (usually aluminum) provides a better thermal solution with a lower total R subscript th.
- Electrolyte loss is greatly reduced through the use of a solid phenolic lid with insert-molded terminals, joined to the can with a double-seal employing a special rubber gasket.
Operational life of long life electrolytic capacitors is also dependent on evaporation of the electrolyte through the seal. As the electrolyte evaporates out of the capacitor over time, the capacitor will eventually fail. (This is accelerated by heat.) One design provides a double sealing technique that slows the evaporation rate, keeping the electrolyte inside the capacitor for the longest possible life.
The combination of these features allows very long life in demanding applications. It is also so that through a good understanding of the application, the design of the capacitors can be optimized to provide the desired combination of ESR, life, size, and cost.
End of Life As discussed earlier, heat is of paramount concern. The hotter the capacitor, the faster the end of life is reached. With an increase in temperature the change in capacitance, electrolyte conductivity, aluminum resistivity, leakage current, chemical instability and corrosion processes increase.
As the capacitor ages, capacitance decreases and ESR increases, as shown in Fig. 3. Lifetime is defined by the application. In some circuits, only small changes in capacitance and ESR are tolerable, which means the capacitor will cause a failure in a shorter time than if the application were more tolerant.
ESR consists of three components. Those are the resistance in the aluminum tabs and foil, which increases with temperature, resistance of the electrolyte (oxide layer) that strongly decreases with temperature and the dielectric resistance that decreases with frequency. The last is negligible above 1.5 KHz.
Electrolytic capacitor end of life is defined as when one or more of its parameters changes by a given amount (Fig. 4). Those parameters are capacitance (C), ESR, dissipation factor (DF), leakage current (I subscript L), and rated voltage (V subscript r). Different manufacturers define those limits differently, depending on their capacitor capabilities. One definition for electrolytic capacitor end of life is when:
DC415% for V subscript r<160Vdc 10% for V subscript r.160 Vdc. ESR>2 times the initial value. DF (tand)>1.3 times the rated value. I subscript L>the rated value.
Reducing Cost Although specific requirements vary, it's usually taken for granted that to handle higher ripple current, higher capacitance is needed. This is true to an extent, but varying capacitor technologies among suppliers often affects the capacitance actually needed to achieve a given ripple current and operational life. A higher capacitance may be required from one supplier compared to another depending on the capacitor design.
Design, materials, and fabrication processes determine the life time and the reliability of the capacitor. A good capacitor design makes it possible to meet the application ripple current requirements with less capacitance than expected - especially in circuits that require less capacitance if it were not for the ripple current load.
In other words, the circuit may perform well with a certain C-value except that the high ripple current would cause the operational life to be too low. In this case, the designer must choose an "over-designed" capacitor just to survive the current. With a well-designed electrolytic capacitor, the need for over-designing is reduced, resulting in potentially considerable cost savings.