Capacitor Characterisation in Switching Power Supply EMC Design
Many electronic designers are aware of the role of filter capacitors in the power supply, but in the switching power supply output with the filter capacitor, and industrial frequency circuits are not the same as the filter capacitor selected in the industrial frequency circuit used as a filter of ordinary electrolytic capacitors, the frequency of its pulsating voltage is only 100 hertz, charging and discharging time is milliseconds order of magnitude, in order to obtain a smaller coefficient of pulsation, the need for the capacitance of up to hundreds of thousands of microfarads! In order to obtain a small pulsation coefficient, the required capacitance is as high as hundreds of thousands of microfarads, and thus the general low-frequency capacitors are manufactured with ordinary aluminium electrolytic capacitors, and the goal is to improve the capacitance mainly.
In the switching regulator as an output filter with electrolytic capacitors, the frequency of the sawtooth wave voltage up to tens of kilohertz, or even tens of megahertz, its requirements and low-frequency applications are different, the capacity is not the main indicator to measure its good or bad is its impedance and frequency characteristics, the requirements of the switching regulator to have a low impedance within the frequency band, at the same time, for the power supply, due to the semiconductor devices to start At the same time, for the internal power supply, due to the semiconductor devices began to work as high as hundreds of kilohertz generated by the spike noise, but also have a good filtering effect, generally low-frequency with ordinary electrolytic capacitors in the 10 kilohertz or so, the impedance will begin to show inductive, can not meet the requirements of switching power supply use.
Switching regulated power supply special high frequency aluminium electrolytic capacitors, it has a four-terminal son, positive aluminium piece of the two ends of the positive electrode as a capacitor, the negative aluminium piece of the two ends of the negative electrode as a negative electrode. Regulated power supply current from the four-terminal capacitor of a positive end of the inflow, through the capacitor inside, and then from another positive end to the load; from the load back to the current from the capacitor of a negative end of the inflow, and then from another negative end to the negative end of the power supply.
Because the four-terminal capacitor has good high-frequency characteristics, it is to reduce the pulsating component of the output voltage as well as the suppression of switching spike noise provides an extremely favourable means.
High-frequency aluminium electrolytic capacitors also have the form of multi-core, it will be divided into a number of short sections of aluminium foil, with more than the lead sheet connected in parallel in order to reduce the resistive component in the capacitance impedance, at the same time, the use of low-resistivity materials and screws as the lead terminals, in order to enhance the capacitor to withstand the ability of the high current.
Stacked capacitors are also known as non-inductive capacitors, general electrolytic capacitors are rolled into a cylindrical core, the equivalent series inductance is larger; stacked capacitors and the structure of the book is similar to the opposite direction of the flux generated by the flow of current is cancelled out, thus reducing the inductance of the value of a more excellent high-frequency characteristics of the capacitor is generally made into a square to facilitate the fixing, but also can be appropriately reduced to account for the size of the machine.
In addition, there is a combination of four-terminal and laminated four-terminal laminated high-frequency electrolytic capacitors, which combines the advantages of both, high-frequency characteristics better.
