The concept of filtering is an engineering concept based on Fourier analysis and transform. An electrical signal is a linear superposition of sine waves of different frequencies; these sine waves of different frequencies that make up the signal are called the frequency components or harmonic components of the signal. A circuit that allows only signal components within a certain frequency range to pass through while blocking other frequency components is called a filter circuit. According to advanced mathematics, any signal that meets certain conditions can be considered as a superposition of an infinite number of sine waves. In other words, an engineering signal is a linear superposition of sine waves of different frequencies; these sine waves of different frequencies that make up the signal are called the frequency components or harmonic components of the signal. A circuit that allows only signal components within a certain frequency range to pass through while blocking other frequency components is called a classical filter or filter circuit. Filter circuits are commonly used to remove ripple from rectified output voltages. They are generally composed of reactive components, such as a capacitor C connected in parallel across the load resistor, or an inductor L connected in series with the load, as well as various complex filter circuits composed of capacitors and inductors.
What is a filter circuit? What is the function of a filter circuit?
Broadly speaking, filtering can be divided into active filtering and passive filtering.
1. Passive filter circuits are simple in structure and easy to design, but their passband amplification and cutoff frequency vary with the load, making them unsuitable for applications with high signal processing requirements. Passive filter circuits are typically used in power circuits, such as filtering after DC power supply rectification, or using LC (inductor and capacitor) circuits for filtering under high current loads.
What is a filter circuit? What is the function of a filter circuit?
2. Active filter circuits do not have their filtering characteristics affected by the load, therefore they are often used in applications with high signal processing requirements. Active filter circuits generally consist of an RC network and an integrated operational amplifier, thus requiring a suitable DC power supply to operate. They can also provide amplification. However, the circuit composition and design are more complex. Active filter circuits are not suitable for high-voltage, high-current applications; they are only suitable for signal processing.
What is a filter circuit? What is the function of a filter circuit?
The basic function of a filter circuit is to allow current of a certain frequency to pass through or to block current of a certain frequency from passing through.
The function of the filter circuit is to minimize the AC component in the pulsating DC voltage while retaining the DC component, thereby reducing the output voltage ripple coefficient and making the waveform smoother.
The output voltage of a rectifier circuit is not pure DC. When observed on an oscilloscope, the output voltage differs significantly from DC, containing substantial ripple components. To obtain a more ideal DC voltage, a filter circuit composed of reactive components with energy storage capabilities (such as capacitors and inductors) is needed to remove the ripple components from the rectifier circuit's output voltage.
What is a filter circuit? What is the function of a filter circuit?
Pulse coefficient (S) = Maximum value of the fundamental frequency of the AC component of the output voltage / DC component of the output voltage
The ripple factor of the output voltage of a half-wave rectifier is S = 1.57, while the ripple factor of the output voltage of a full-wave rectifier and a bridge rectifier is S ≈ 0.67. After using a type C filter circuit for the full-wave and bridge rectifier circuits, the ripple factor S = 1/(4(RLC/T-1). (T is the period of the rectified DC ripple voltage.)
The main function of EMI filter circuit
The main function of an EMI filter is to filter out high-frequency pulses from the external power grid that interfere with the power supply, while also reducing electromagnetic interference from the switching power supply itself. Essentially, it utilizes the characteristics of inductors and capacitors to allow alternating current with a frequency of around 50Hz to pass through the filter smoothly, while high-frequency interference and noise above 50Hz are filtered out. Therefore, it is also known as a low-pass filter (as used on color TVs), meaning that low frequencies can pass through while high frequencies are filtered out.
Secondary EMI filter circuit
High-quality power supplies typically have two EMI filter circuits: one at the power outlet and the other on the power supply's PCB (in some cases, both EMI filter circuits are integrated onto the PCB). These two EMI circuits effectively filter out high-frequency noise and in-phase interference current from the power grid, while minimizing electromagnetic radiation generated within the power supply, ensuring that the amount of electromagnetic radiation leaking outside the power supply does not adversely affect people or other equipment. Inferior power supplies often omit the first-stage EMI filter circuit, and may even omit the second-stage EMI filter circuit altogether.
