Analysis of Capacitor Characteristics in EMC Design of Switching Power Supplies
Many electronic designers are aware of the role of filter capacitors in power supplies, but the filter capacitors used at the output end of switching power supplies are different from the filter capacitors used in power frequency circuits. Ordinary electrolytic capacitors used for filtering in power frequency circuits have a pulsating voltage frequency of only 100 Hz and a charging and discharging time of millisecond order of magnitude. To obtain a smaller pulsation coefficient, a capacitance of up to hundreds of thousands of microfacies is required, Therefore, ordinary aluminum electrolytic capacitors are generally used for low-frequency manufacturing, with the goal of mainly improving capacitance. The capacitance, loss tangent value, and leakage current of capacitors are the main parameters for distinguishing their advantages and disadvantages.
As an electrolytic capacitor used for output filtering in a switch regulated power supply, the frequency of the sawtooth wave voltage on it can reach tens of kilohertz, or even tens of megahertz. Its requirements are different from those in low-frequency applications, and capacitance is not the main indicator. Its quality is measured by its impedance frequency characteristics, which require it to have low impedance within the operating frequency range of the switch regulated power supply. At the same time, for internal power supply, Due to the peak noise generated by semiconductor devices starting to work, which can reach hundreds of kilohertz and also have good filtering effect, ordinary electrolytic capacitors are generally used at around 10 kilohertz for low frequencies, and their impedance begins to appear inductive, unable to meet the requirements of switching power supply use.
A high-frequency aluminum electrolytic capacitor specifically designed for switch regulated power supply, which has four terminals. The two ends of the positive aluminum sheet are respectively led out as the positive electrode of the capacitor, and the two ends of the negative aluminum sheet are also led out as the negative electrode. The current of the regulated power supply flows from one positive end of the four terminal capacitor, passes through the capacitor, and then flows from the other positive end to the load; The current returned from the load also flows from one negative end of the capacitor and then from the other negative end to the negative end of the power supply.
Because the four terminal capacitor has good high-frequency characteristics, it provides an extremely advantageous means to reduce the ripple component of the output voltage and suppress switch spike noise.
High frequency aluminum electrolytic capacitors also come in the form of multiple cores, which divide the aluminum foil into shorter segments and connect multiple leads in parallel to reduce the resistance component in the capacitance. At the same time, low resistivity materials are used and screws are used as lead terminals to enhance the capacitor's ability to withstand large currents.
Stacked capacitors, also known as non-inductive capacitors, typically have a cylindrical core, resulting in a larger equivalent series inductance; The structure of a stacked capacitor is similar to that of a book, but it is cancelled out due to the opposite direction of the magnetic flux generated by the current flowing through it, thereby reducing the value of the inductance and having better high-frequency characteristics. This type of capacitor is generally made into a square shape for easy fixation, and can also reduce the machine volume appropriately.
In addition, there is a four terminal stacked high-frequency electrolytic capacitor that combines the advantages of the two, with better high-frequency characteristics.
