AC filter capacitor The core function of AC filtering capacitors is to filter out harmonics and noise in AC signals, making the output DC power smoother. Its characteristics are large capacity and frequency characteristics divided into low frequency and high frequency categories. It is usually connected in parallel at the output end of the rectifier circuit, utilizing the characteristic of "isolated direct current" to bypass unwanted AC ripples and maintain stable DC components. Key Features: 1. Large capacity: Most use electrolytic capacitors, commonly ranging from hundreds to thousands of micrometers. The larger the capacity, the slower the discharge and smoother the filtering. 2. Frequency division work: Low frequency filtering for 100Hz pulsation after 50Hz power frequency rectification; High frequency filtering targets the switching noise of several thousand to tens of thousands of Hz in switching power supplies, and places more emphasis on the "impedance frequency" characteristics rather than just capacity. 3. Soft external characteristics: When the load current increases, the output voltage will significantly decrease, making it more suitable for situations where the load current is small and the change is not significant. 4. Polarity: The positive and negative poles of an electrolytic capacitor cannot be reversed, as reversing them can cause damage. Typical application scenarios: Power rectification and filtering: AC power is rectified into unidirectional pulsating DC power. After parallel filtering capacitors are connected, the capacitors charge when the voltage rises and discharge when it falls, filling the valley and reducing the ripple coefficient. Switching power supply output filtering: The operating frequency ranges from several thousand Hz to tens of thousands of Hz, and capacitance is not the main indicator at this time. The key is to look at the "impedance frequency" characteristics, requiring low equivalent impedance within the operating frequency band. Chip power decoupling: Place small capacity ceramic capacitors next to the power pins of the digital IC to filter out switch noise and provide transient energy. In high-frequency scenarios, capacitors with smaller capacitance values need to be selected based on the noise frequency. AC filter for DC transmission system: In high-voltage converter stations, it is composed of capacitors, reactances, and resistors, used to filter out harmonics generated during the converter process, compensate for reactive power, and regulate bus voltage.
