The stability and reliability of circuit system performance are closely related to the parameters, grades, and quality of selected components. Designers should accurately propose specific requirements for component parameters based on the product application environment and electrical performance requirements, including nominal values, accuracy and error requirements, stability requirements, temperature range requirements, installation dimensions, and other closely related circuit performance requirements. Requirements. Among all passive components, aluminum Electrolytic capacitor has the highest failure rate, so selection is particularly important.
Key points for selection of aluminum Electrolytic capacitor: capacity, withstand voltage, temperature range, component packaging form and size, ripple current, ripple voltage, leakage current, ESR, dissipation factor, impedance/frequency characteristics, capacitor life, actual needs, performance and cost consideration. The Electronic Components Technology Network raised the problems encountered by investigation engineers in aluminum electrolytic selection and application, paying attention to withstand voltage, capacity, temperature Size and other parameters, and also pay attention to the influence of aluminum Electrolytic capacitor on the whole circuit. Stability issues.
Aluminum Electrolytic capacitor is a polar capacitor composed of etched high-purity aluminum foil as anode and thin paper or cloth soaked with electrolyte as cathode. Advantages: large capacity, high voltage resistance, low price Disadvantages: large leakage current, large error, poor stability, and rapid decline in service life with increasing temperature. Aluminum Electrolytic capacitor used in digital circuit is generally used for smoothing and filtering of power supply. In addition to well-known parameters such as capacity, withstand voltage, capacity error, operating temperature, and packaging size, there are also some important parameters related to capacitor quality, including loss angle. Tangent, leakage current, Equivalent series resistance ESR, allowable ripple current, service life, etc. These parameters are not marked on the outer skin of the finished product packaging, but are only reflected in the product specifications, but these parameters may be the performance of key circuits.
Capacity and rated working voltage
The capacity and withstand voltage marked on the aluminum Electrolytic capacitor body are very important. It is the most basic aspect of selecting capacitors. In the actual selection of capacitors, capacitors with larger capacities should be used in rapidly changing areas, but the larger the capacity, the better. Firstly, the larger the capacity, the higher the cost and volume. In addition, the larger the capacitor, the more charging current it has. The larger it is, the longer the charging time. These are all considered in practical application selection.
Rated working voltage: The maximum DC voltage that a capacitor can reliably operate for a long time within the specified working temperature range and can withstand. In AC circuits, it should be noted that the maximum value of the applied AC voltage cannot exceed the DC operating voltage value of the capacitor. The commonly used fixed capacitor working voltages are 6.3V, 10V, 16V, 25V, 50V, 63V, 100V, 2500V, 400V, 500V, 630V. The voltage that a capacitor must actually withstand in the circuit cannot exceed its withstand voltage value. In the filter circuit, the withstand voltage value of the capacitor should not be less than 1.42 times the RMS value of the AC.
Another issue to note is the issue of operating voltage margin, which usually exceeds 15%. For example, the rated voltage of a capacitor is 50V, but the surge voltage can reach up to 63V, but usually only the maximum voltage of 42V is applied. Allowing more margin to the rated voltage of capacitors can reduce internal resistance, reduce leakage current, reduce loss angle, and increase lifespan. Although it is said that the use of 50V aluminum Electrolytic capacitor with 48V working voltage will not cause problems in a short time, it can reduce the service life after long-term use.
The energy consumed by a capacitor under the action of an electric field is usually represented by the ratio of the reactive power of the capacitor, which is the tangent of the loss angle (in the equivalent circuit of the capacitor, the series equivalent resistance ESR is equal to the capacitance reactance 1/Ω c. It is called Tan Delta, where ESR is calculated at 120 Hz. Obviously, Tan Delta increases with the increase of measurement temperature and with the decrease of measurement temperature, depending on the measurement frequency). The larger the loss angle, the greater the loss of the capacitor. Capacitors with a larger loss angle are not suitable for high-frequency operation.
The dissipation factor (DF) exists in all capacitors, and sometimes the DF value is expressed as the loss angle tan δ。 The lower this parameter, the better. However, this parameter of aluminum Electrolytic capacitor is relatively high. The DF value is related to the temperature, capacity, voltage, frequency, etc. of capacitors of the same brand and series; When the capacity is the same, the higher the withstand voltage, the lower the DF value. In addition, the higher the temperature, the higher the DF value, and the higher the frequency, the higher the DF value.
The size is related to weight and pin type. The single end is of radial lead type, the screw is of locking screw type, and there is an aluminum Electrolytic capacitor chip. As for weight, the same capacity is voltage resistance, but comparing two different brands of capacitors, the weight is definitely different; The overall dimensions are more related to the shell design.
Generally speaking, capacitors with the same diameter and capacity can be replaced by capacitors with smaller heights, but mechanical interference must be considered when replacing capacitors with longer lengths.
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Due to its structure, capacitors will have various impedances and inductances. ESR Equivalent series resistance and ESL Equivalent series inductance are a pair of important parameters, which are the basis of capacitive reactance. Compared with larger external capacitors, capacitors with smaller Equivalent series resistance (ESR) can better absorb the peak (ripple) current during fast switching. Parallel use of capacitors with high ESR is more cost-effective. However, this requires a compromise between PCB area, device quantity, and cost.
Ripple current and ripple voltage
In some data, it is referred to as ripple current and ripple voltage, but it is actually ripple current NT, ripple voltage. This meaning refers to the ripple current/voltage value that a capacitor can withstand. The ripple voltage is equal to the product of the ripple current and ESR.
When the ripple current increases, even if the ESR remains constant, the ripple voltage will exponentially increase. In other words, as the ripple voltage increases, the ripple current also increases, which is why capacitors need to have a lower ESR value. After the ripple current is superimposed, heat is generated due to the Equivalent series resistance (ESR) in the capacitor, which will affect the service life of the capacitor. Usually, the ripple current and frequency are positive