Introduction to capacitor characteristics in EMC design of switching power supply
Many electronic designers are aware of the role of filtering capacitors in power supplies, but the filtering capacitors used at the output of switch mode power supplies are different from those 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 on the order of milliseconds. To obtain a smaller pulsation coefficient, a capacitance of up to hundreds of thousands of microfarads is required. Therefore, ordinary aluminum electrolytic capacitors are generally used for low-frequency manufacturing, with the goal of increasing capacitance as the main factor. The capacitance, loss tangent value, and leakage current of capacitors are the main parameters used to distinguish their advantages and disadvantages.
As an electrolytic capacitor used for output filtering in switch mode power supplies, the frequency of the sawtooth voltage on it can reach tens of kilohertz, or even tens of megahertz. Its requirements are different from those for low-frequency applications, and capacitance is not the main indicator. The measure of its quality is its impedance frequency characteristics, which require it to have low impedance within the operating frequency range of the switch mode power supply. At the same time, for the internal of the power supply, due to the sharp noise generated by semiconductor devices starting to work, it can also have good filtering effect. Generally, for ordinary electrolytic capacitors used at low frequencies around 10 kilohertz, their impedance begins to show inductance and cannot meet the requirements of switch mode power supply.
A high-frequency aluminum electrolytic capacitor specifically designed for switch mode stabilized power supply, which has four terminals. The two ends of the positive aluminum plate are respectively led out as the positive electrode of the capacitor, and the two ends of the negative aluminum plate are also respectively led out as the negative electrode. The current of the regulated power supply flows from one positive terminal of the four terminal capacitor, passes through the inside of the capacitor, and then flows from the other positive terminal to the load; The current returning from the load also flows from one negative terminal of the capacitor, and then from the other negative terminal to the negative terminal 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 have a multi-core form, which divides aluminum foil into several shorter sections and connects them in parallel with multiple lead out pieces to reduce the resistance component in the capacitance impedance. At the same time, low resistivity materials are used and screws are used as lead out terminals to enhance the capacitor's ability to withstand high currents.
Stacked capacitors, also known as non inductive capacitors, typically have cylindrical cores for electrolytic capacitors, resulting in a higher equivalent series inductance; The structure of a stacked capacitor is similar to that of a book, as the magnetic flux generated by the current flowing in the opposite direction is cancelled out, thereby reducing the inductance value and having better high-frequency characteristics. This type of capacitor is generally made in a square shape for easy fixation and can also reduce the size of the device appropriately.
