Saturday, December 18, 2010
【 Weak current College 】 Aluminum Electrolytic Capacitor selection parameters 】
Circuit system performance stable reliable, and selection of components parameters, grade, quality, etc. Designers should address product application environment as well as the electrical properties of the request, are accurate to the specific requirements of component parameters, including the nominal value, precision, and stability of error requirements, requirements, temperature requirements, installation dimensions and circuit performance closely related to other requirements. For all passive components, Aluminum Electrolytic Capacitor failure rate of most high, so the selection is particularly important.
Aluminum Electrolytic Capacitor selection:
Capacity, pressure and temperature range, component packaging forms and dimensions
Ripple current and ripple voltage
Leakage current, and ESR, the escape factor, the frequency characteristic impedance/
Actual needs, performance and cost, and other considerations
Electronic component technology network survey engineers in aluminum electrolytic type selection and problems they encountered in the application, pay attention to the pressure, volume, temperature and the size of several parameters, but also pay attention to the aluminum electrolytic capacitor on the stability of the entire circuit.
Aluminum Electrolytic Capacitor is etched foils as anode of high purity, to soak a cloth of tissue paper or electrolyte do cathode consisting of polar capacitors.
Advantage: large capacity, high pressure resistant, low price
Disadvantages: leakage current, error, poor stability, life with the temperature dropping soon
Digital circuit in the use of aluminum electrolytic capacitor is generally used to power smoothing filter, in addition to capacity, pressure resistance, capacity error, operating temperature, package size, and other familiar arguments, there are infants on the important parameters of the quality of the capacitor, including loss angle tangent, leakage current, equivalent series resistance ESR, allowed ripple current, service life, etc. These parameters are marked in a production package rind, only in the product specification is embodied in, but these parameters it is possible that the relationship between critical circuit performance.
Capacity and rated operating voltage
Aluminum Electrolytic Capacitor body marked capacity and pressure, these two parameters is important, is the selection of the capacitance of the most basic content.
In the actual Capacitance type selection, the current change tempo to use larger capacitors, but is not the larger the better, first of all, capacity increase, the cost and volume may rise, in addition, the greater the charging current, capacitance, the longer the charging time. These are actual application to be considered in the selection.
Rated working voltage: the operating temperature range, the capacitor long-term reliable work, it can withstand the maximum DC voltage. In AC circuits, it is necessary to pay attention to the AC voltage maximum value cannot exceed the capacity of the DC voltage values. Commonly used with fixed capacitor voltage 10V, 6.3V, 16V, 25V, 50V, 63V, 100V, 2500V, 400V, 500V, 630V. Capacitor in circuit voltage actually bear cannot exceed its pressure value.
In the filter, the capacitor voltage value not less than AC RMS 1.42 times. Also note that a problem is the operating voltage margin, as a general rule to 15 percent. For example, a capacitor of rated voltage is 50V, although surge voltage may be high to 63V, but generally the most high will impose 42V voltage.
Let the rated voltage of the capacitor has a lot of margin, can reduce resistance, low leakage current, reduce losses, increased life expectancy. Although the use of voltage 48V 50V aluminum electrolytic capacitors for a short period of time will not be a problem, but using long, life is likely to decrease.
Capacitor inthe electric field consumption of energy, usually with a loss of power and a capacitor of reactive power ratio, i.e. the tangent of loss angle (the equivalent circuit in the capacitor, in tandem with the equivalent resistance ESR capacitance 1/C ratio called ω T a n δ, ESR is here at 120Hz calculated values obtained. Obviously, T a n δ with measuring frequency increases, the larger, with the measurement temperature drop of growth). Loss angle increases, the greater the loss of the capacitor, loss angle large capacitance is not suitable for high frequency work. Escape factor dissipationfactor (DF) exist in all capacitors, sometimes DF values to loss angle t a n δ said. This parameter, the lower the better. But this argument of aluminum electrolytic capacitor is relatively high.
DF values is high or low, in respect of the same brand, same series capacitor, and temperature, voltage, frequency, capacity, ... Has relationship; when the same capacity, pressure resistance, the higher the value of DF is lower. In addition, the higher the temperature, the higher the DF values, frequency, the higher the DF values will be higher.
Overall dimensions and weight and PIN pattern. Singleended is radial lead type, screw lock screw-type, SMD aluminum electrolytic capacitors, etc. With regard to the weight, with the capacity to withstand, but brand different two capacitors do comparison, weight must be different; the overall dimensions and shell programming. In General, the diameter is the same, the same capacity low capacitance, high can substitute capacitors highly significant, but the length of the high alternative low capacitance will consider institutional interference problems.
A capacitor is due to its structure and produce various impedance, inductive reactance. ESR equivalent series resistance and ESL equivalent series inductance is a pair of important parameters-this is the capacitance of the Foundation. An equivalent series resistance (ESR) very small capacitance relatively large-capacity external capacitor can absorb fast conversion time of peak (ripple) current. Using ESR capacitor parallel more cost-effective. However, this need in the number of PCB area, devices and costs between seek compromise.
Ripple current and ripple voltage
Where the information is called the ripple current and ripple voltage, is actually ripplecurrent, ripplevoltage。 Meaning is the capacitor can withstand ripple current/voltage values. Ripple voltage equal to ripple currents and the product of the ESR.
When the ripple current increases, even in ESR remain unchanged, the ripple voltage will increase exponentially. In other words, when a ripple voltage increases, ripple current is increased, this is also called capacitor with lower ESR values. Fold to join the ripple current, because of the internal equivalent capacitance series resistance (ESR) cause the heat, thus affecting the life of the capacitor. General, ripple current proportional to the frequency, low frequency is also relatively low ripple current.
Rated ripple current in the highest working temperature conditions defined values. But the practical application of the capacitance of ripple and endurance by its use of ambient temperature and the temperature grade capacitors itself. Specifications of the bibliography will usually provide a specific temperature conditions the temperature grade capacitors can withstand the maximum ripple current. Even provides a detailed chart to help users quickly find to a certain environment temperatures to achieve a desired service life of the allowed amount of capacitance ripple.
Capacitor's media on DC has great impediment. However, because the aluminum oxide film media soaked electrolyte, voltage, and fix again formation of oxide films will have a very small called leakage current of electricity. Generally, residual flows with temperature and voltagebut increased. Its formula is: I = K×CV. Leakage current I is A μ, K is a constant. In General, the higher the capacitor leakage current is greater. From the formula to the higher rated voltage, leakage current is larger, therefore reducing voltage can also reduce leakage current.
First of all be made clear that the Aluminum Electrolytic Capacitor must be bad, it's just a matter of time. Influence the life of the capacitor for many reasons, overvoltage and reverse voltage, high temperature, rapid charge and discharge, and so on, normal usage, the greatest impact is temperature, because the higher the temperature of the electrolyte, the faster the volatile loss. Note that here the temperature is not a surface temperature of the environment or refers to foil the operating temperature. Manufacturers normally capacitance life and test temperature callout in the capacitor ontology.
Because of the capacitance of the working temperature 10 ℃ life each increase by half, so don't think 2000 hour life of aluminum electrolytic capacitor is better than 1000 hours, pay attention to confirm that the life of the test temperature. Each manufacturer has a temperature and life calculation formula, when in the design of the capacitor to be calculated by reference to the actual data. Need to know is to improve the life of Aluminum Electrolytic Capacitor, first to lower the operating temperature, away from sources of heat in the PCB, secondly consider using the maximum working temperature high capacitance, of course, the price will be high.
In the specific frequencies, impeding the AC current through a resistance which is the so-called impedance. It and capacitance equivalent circuit of capacitance, inductance values are closely related, and are also related with ESR. Capacitor capacitance in the low frequency range as the frequency increases gradually to reduce the frequency, continue to increase reach medium frequency range reactor to ESR values. When the frequency reaches high frequency range inductive reactance becomes dominant, so the impedance is as the frequency increases.
Switching power supply's output filtering Electrolytic Capacitor, its sawtooth wave voltage frequency up to 10 kHz, or even tens of MHz when capacitance is not its main indicators to measure the pros and cons of high frequency Electrolytic Capacitor is the "impedance-frequency characteristics, called" switching power supply operating frequency to lower equivalent impedance, for semiconductor devices work when the high-frequency peak signal has a good filter.
From the face of it DF, leakage current, the lower the ESR, higher ripple current, Aluminum Electrolytic Capacitor performance better, but the price is a performance increase of mast and price increases. Therefore, the choice of Aluminum Electrolytic Capacitor must carefully, it is necessary to take into account the performance requirements, but also to consider the package size, be sure to at design time for system requirements, carefully check the related product manual carefully to determine the appropriate model, and the actual test.