Tantalum electrolytic capacitors, usually used in the SMD version, have a higher specific capacitance than the aluminum electrolytic capacitors and are used in devices with limited space or flat design such as laptops. They are also used in military technology, mostly in axial style, hermetically sealed. Niobium electrolytic chip capacitors are a new development in the market and are intended as a replacement for tantalum electrolytic chip capacitors. It had a capacitance of around 2 microfarads.
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With the capacitor ESR at a certain frequency set as "R" and the ripple current set as "I", "RI2" becomes the power heat loss and the capacitor self-heats. While large-capacitance is acquired using an electrolytic capacitor, significant heat develops due to ripple current and a high ESR, which is a weakness of electrolytic capacitors. The upper limit of the ripple current which the capacitor allows is called the "allowable ripple current". The life of the capacitor will decrease when the usage exceeds the allowable ripple current.
Note: ESR and ripple currents Figure 3: ESR equivalent series resistance The ideal capacitor would possess only the properties of capacitance, but in reality it also contains resistor and inductor components due to the electrodes. The resistor component, not shown in the ideal capacitor, is called the "ESR equivalent series resistance " and the inductor component is called the "ESL equivalent series inductance ".
Figure 4: Ripple currents DC Direct Current is when current flows in one direction, but in dc power supplies in addition to DC current there are various superimposed alternating current components which adds ripple to the current. For example, the direct current resulting from the rectification full-wave rectification of commercial alternating current contains pulsating ripple currents at twice the cycle of the commercial alternating current.
In addition, the pulsating current of the switching cycle in a switching DC-DC converter is superimposed on the direct current voltage. This is called the "ripple current". Aluminum Capacitors Have a Lifetime of 10 Years Aluminum electrolytic capacitors are widely used in electronic devices, because they have high capacitance and are inexpensive, but caution is required due to their limited lifetime.
The typical lifetime of an aluminum electrolytic capacitor is said to be ten years. This is because the capacitance decreases as the electrolytic solution dries up capacitance loss. The amount of electrolytic solution lost is related to temperature and closely follows the "Arrhenius equation" of chemical reaction kinetics. For this reason, the lifetime is reduced even more when used under conditions with significant self-heating due to ripple currents.
The drying up of the electrolytic solution also increases the ESR. Attention should be noted that the peak value of the ripple voltage does not exceed the rated voltage withstanding voltage when the ripple voltage is superimposed on the direct current voltage. A capacitor used in a power supply circuit has a rated voltage that is three times of the input voltage. Figure 7 shows the fundamental circuit of a miniature step-down DC-DC converter which is used as a POL converter in many electronic devices.
This type of output capacitor is the primary target for replacement of electrolytic capacitors with MLCCs in DC-DC converters as a solution for the self-heating issue, space reduction and improved reliability. The main converter circuit has been made into an IC, and the capacitor and inductor are attached externally on the PCB internally attached products also exist. The capacitor which comes before the IC is called the "input capacitor Cin " and the one that comes after is the "output capacitor Cout ".
In addition to collecting an electrical charge and smoothing the output voltage, the output capacitor in a DC-DC converter plays the role of grounding and removing the ripple component of the alternating current. The ripple voltage, which causes the self-heating, follows a similar pattern.
The functional polymer aluminum electrolytic capacitor uses a conductive polymer as the electrolyte and is a type designed for a low ESR. Compared to the typical aluminum electrolytic capacitor, the ripple voltage is significantly smaller, but the form factor is slightly large and the price is expensive. Figure The impedance-frequency characteristics and ESR-frequency characteristics for various capacitors As the capacitor ESR becomes lower, the ripple voltage can be kept to a smaller amount.
For this reason, the MLCC displays optimal performance as a replacement for an electrolytic capacitor. Figure The relationship between ESR and the ripple voltage switching frequency of kHz Merits of Replacing an Electrolytic Capacitor in a DC-DC Converter with an MLCC Replacing an electrolytic capacitor with an MLCC provides various advantages such as ripple control as well as space reduction of the circuit board due to the miniature and low-profile form factor, a long lifetime and an improvement in reliability.
Figure Switching from an aluminum electrolytic capacitor to an MLCC Question note: Can the ripple voltage be controlled by increasing the capacitance of the electrolytic capacitor? The ESR of an electrolytic capacitor decreases slightly when the capacitance increases. However, it is fundamentally difficult to control the ripples by increasing the capacitance.
This is because the time constant increases together with the increase in capacitance. The response speed of a transient phenomenon such as the charging and discharging process of a capacitor can be expressed as the time constant index called T. The time required for charging and discharging of the capacitor is short when the time constant is small and becomes longer as the time constant increases.
The time constant becomes extremely large when using an electrolytic capacitor with an excessively large capacitance. In a DC-DC converter with repeated switching of a short duration, the discharging does not complete within the switch OFF time and charge remains in the electrolytic capacitor.
As a result, the voltage does not sufficiently decrease, distortions occur in the voltage waveform, and the output becomes unstable, which does not allow for favorable ripple control Figure Figure Distortions occurring in the waveform of a large capacitance aluminum electrolytic capacitor MLCCs, on the other hand, do not have this kind of problem because of the low ESR over a wide frequency band, which achieves favorable ripple control in the place of an electrolytic capacitor.
The low ESR is a feature of the MLCC, but it is so much lower compared to an aluminum electrolytic capacitor that on the contrary, the output voltage of the DC-DC converter becomes unstable and causes oscillations to occur.
As shown in the figure on the right, the DC-DC converter compares the output voltage with the reference voltage, amplifies the error amount with the error amp error amplifier , and performs negative feedback to achieve a constant and stable direct current voltage. However, signal phase lag occurs due to the inductor L and the capacitor C of the smoothing circuit. Figure Negative feedback circuit in a DC-DC converter Question note: What is the phase compensation that is used to prevent anomalous oscillations?
There is a board diagram used as a chart to determine whether the negative feedback will operate in a stable manner. The horizontal axis of the graph is the frequency and the vertical axis is the gain and phase. Connect a capacitor and resistor near the error amp to reduce the phase lag and adjust to cancel it.
This is called "phase compensation". Previous designs which used an aluminum electrolytic capacitor with a high ESR for the output capacitor did not have this problem. However, the MLCC has insufficient compensation, which causes anomalous oscillations, so caution is required when replacing capacitors. This can be effectively used for phase compensation.
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