Design of filtering and voltage stabilizing circuitx
1. Filter circuit and device selection
The rectifier filter circuit usually has filter circuits such as capacitors, inductors, and RC. Inductive filtering is realized by using the inductance to generate counter electromotive force to the pulsating current and hinder the current change. The larger the inductance, the better the filtering effect. It is generally used in the field where the load current is large and the filtering requirements are not high. The RC filter circuit is a filter circuit used by connecting resistors and capacitors. Since the resistor will reduce a part of the DC voltage, the DC output voltage will decrease, so it is only suitable for small current circuits. Capacitor filtering is to use the charging and discharging effect of the capacitor to make the rectified output voltage stable, and the voltage amplitude increases, the filtering effect is good, and it is suitable for various rectifying circuits. The selection of the filter capacitor is mainly the determination of the type, capacity and withstand voltage value. Commonly used rectifier filter capacitors include aluminum electrolytic, tantalum electrolytic, polyester, and monolithic capacitors. Aluminum electrolytic capacitors have large leakage current, low withstand voltage and operating temperature (up to +70°C), but large capacity; tantalum electrolytic capacitors have small leakage current, higher withstand voltage and operating temperature than aluminum electrolytic capacitors, and are generally used for higher requirements places; polyester capacitors have large insulation resistance, low loss, low operating temperature (up to +55°C), small capacity, but high withstand voltage; monolithic capacitors can be made small in size and high in withstand voltage. The performance and thermal performance are relatively stable, but the capacity is small. Generally, when the rectified output current is large, electrolytic capacitors must be used to filter and stabilize the voltage; if the output current is small, ordinary capacitors or electrolytic capacitors can be used for filtering. If the DC output voltage has ripple coefficient requirements or in order to prevent high-frequency noise, use electrolytic capacitors It is better to be used in parallel with small-capacity non-polar capacitors: small-capacity capacitors can filter out high-order harmonics in pulsating DC, and electrolytic capacitors can filter out large-value low-frequency components, and the voltage stabilization range is wide and the effect is good. The rectification and filtering circuit does not require too much capacity and withstand voltage of the capacitor. Generally, the capacity of the capacitor is estimated according to the output current. If the output current is large, the capacity will be large; if the current is small, the capacity will be small. However, if the capacity is too large, the output voltage value will be reduced, and if it is too small, the voltage ripple will be large and unstable. Refer to Table 1 to determine the capacity. The withstand voltage value is generally 1.5 to 2 times the working voltage of the connected circuit.
