Selection and Selection of Power Filter Capacitors
Time:2023-08-17
Views:668
In rectifier filtering circuits, the selection of filtering capacitors is often based on the formula RC ≥ (3-5) T/2. In actual circuit design, some people also believe that the larger the filtering capacitor, the better. However, this idea is one-sided. This article will delve into this issue in depth. The article first elaborates on the necessity of studying the selection of filtering capacitors, followed by theoretical analysis and calculation of the circuit. Then, based on the theoretical calculation results, a program is written to simulate the working process of the circuit. Finally, by discussing the effects of filter capacitors on the current, voltage, and other component parameters in the circuit through examples, this lays the foundation for optimizing circuit design.
Keywords: rectification; Filtering; Filter capacitor
1、 Introduction
In most power supply circuits, a filtering circuit is added after the rectifier circuit to reduce the fluctuation of the rectifier voltage and meet the needs of the voltage stabilizing circuit. In many literature, the empirical formula RC ≥ (3-5) T/2 is often used for the selection of filter capacitor C, and it is believed that the larger the filter capacitor C, the better; In the maintenance of some filtering circuits, technicians often use capacitors with larger capacity than the original circuit to replace damaged capacitors. Practice has proven that in many cases, this is not feasible. Is the selection of a larger capacitor better? How does the selection of capacitors affect the performance of the front-end devices and the overall power supply? Is there an optimal value for selecting capacitors? This article will delve into these issues in depth.
A simple rectification and filtering circuit. In theory, increasing the capacity of the filtering capacitor C in the circuit can indeed make the waveform of the output voltage smoother and less fluctuating. However, at the moment of circuit connection, the impact current factor generated in the circuit cannot be ignored. This is because almost all electronic components have their maximum current value that can be passed through. Therefore, when selecting electronic components, It is necessary to consider the maximum instantaneous current flowing through relevant components caused by the impact current. The larger the impact current, the higher the requirements for electronic components, and the higher the cost of the circuit.
In the maintenance of some filtering circuits, there is also a problem of impulse current when replacing the filter capacitor. Replacing the original capacitor with a large capacity filter capacitor will increase the impulse current. Without replacing other components, simply increasing the capacity of the filter capacitor is dangerous, as it will greatly shorten the actual service life of the entire circuit and even burn the entire circuit. Moreover, simply increasing the capacity of the filter capacitor has a limited effect on improving the output voltage. blindly increasing the capacity of the filter capacitor is only a futile increase in the cost of the circuit.
2、 Calculation of Simple Filter Circuit
A simple rectification and filtering circuit with a common 220v50Hz sinusoidal alternating current as the input voltage.
2.1. Calculation of charging circuit
During the charging process of the capacitor, if the equivalent resistance of the diode is R1, the circuit is equivalent to Figure 2
According to Figure 2
Bring it into the U expression and organize it into:
This is a first-order non-homogeneous differential equation, whose solution is:=
2.2. Calculation of discharge circuit
During the discharge process of the capacitor, the capacitor only forms a circuit with the resistor, and its discharge equation is:
Among them, U is the maximum voltage reached during capacitor charging.
3、 Computer Simulation and Discussion of Filter Circuits
We have already conducted mathematical derivation on the working process of the filtering circuit earlier, and to simulate the working process of the filtering circuit, a large amount of calculations must be carried out in order to depict the output waveform of the power supply point by point. By observing the waveform and selecting component parameters, the circuit can meet the user‘s requirements for efficiency, power, ripple, cost, and other aspects of the circuit while working normally. Below is a brief explanation of the concept and application of the program.
Figure 1 shows the circuit simulated by the program. By running the program, we will obtain a set of voltage and current data on capacitor C. By substituting a large amount of small interval time t (0.00001 seconds), we can obtain the current and voltage waveform of capacitor C. The calculation of the program voltage value is obtained by the recursive method, and the current value is calculated from the voltage value.
The simple flowchart of program operation is as follows:
1. Example of selecting computer program optimization components
Select a load resistor R=500 Ω, and simulate the voltage and current waveforms under different capacitance conditions using a computer to obtain the following figures:
(1) R=500 Ω, C=100uF
(2) R=500 Ω, C=200uF
(3) R=500 Ω, C=500uF
(4) R=500 Ω, C=1000uF
2. Discussion on the selection of filtering capacitors in rectifier filtering circuits
From the above image, it can be observed that when the load resistance remains constant, the curve becomes smoother as the capacitance C capacity increases. However, this method has a certain degree of one-sidedness. As shown in Figures 6 and 7, simply increasing the filtering capacitance has a very limited effect on improving the waveform and increasing the DC voltage value. Moreover, as the capacitance C capacity increases, the time for the circuit to enter steady state after connection also increases; At the same time, as the capacity of the filter capacitor increases, the impact current flowing through the circuit also continues to increase. If we do not pay attention to the changes in the impact current and blindly increase the capacity of the filter capacitor to obtain better voltage output waveform, it will inevitably lead to damage to the current flowing components due to excessive impact current. Otherwise, to ensure the safety of the entire circuit, we have to improve the indicators of other components, This significantly increases the cost of the circuit. Therefore, in order to achieve the highest cost-effectiveness of the rectification and filtering circuit, it is necessary to select a capacitor value that can match the load resistance R. This way, the output voltage of the circuit can meet the requirements without improving the performance indicators of other components, and the circuit can achieve the best cost-effectiveness. Applying this program can accurately and quickly draw the current waveform and output voltage waveform of the shown circuit under any given parameter. We can compare the output voltage waveform and current waveform of different parameter circuits to determine the optimal filter capacitor parameter value.
|
Disclaimer: This article is transferred from other platforms and does not represent the views and positions of this site. If there is any infringement or objection, please contact us to delete it. thank you! |
Business consulting
13823761625
Mail me