Selection and selection of power filter capacitors In the rectifier filter circuit, the selection of the filter capacitor is mostly using the formula RC≥ (3 ～ 5) T / 2, and in the actual circuit design, some people also think that the larger the filter capacitor, the better. In fact, this idea is one-sided. This article will discuss this issue in depth. The article first elaborates the necessity of researching the selection of filter capacitors, and secondly analyzes and calculates the circuit theoretically. Then, the program is written according to the theoretical calculation results to simulate the working process of the circuit. Finally, the influence of the filter capacitor on the current, voltage and other component parameters in the circuit is discussed through examples, thus laying a foundation for optimizing the circuit design. Fiber Optic Patch Panel,Fiber Patch Panel,Fiber Distribution Panel,Optical Patch Panel Cixi Dani Plastic Products Co.,Ltd , https://www.danifiberoptic.com
Keywords: rectification; filtering; filter capacitor 1. Introduction In most power circuits, a filter circuit must be added after the rectifier circuit to reduce the pulsation of the rectified voltage and meet the needs of the voltage regulator circuit. In many literatures, the selection of filter capacitor C mostly uses the empirical formula RC ≥ (3 ~ 5) T / 2 [1,2], and think that the larger the filter capacitor C, the better; in the maintenance of some filter circuits, The technician often replaces the broken capacitor with a capacitor larger than the original circuit capacity. Practice has proved that in many cases, this will not work, is the larger the capacitor selection, the better? How does the choice of capacitor affect the performance of the pre-stage device and the overall power supply? Is there an optimal value for the selection of capacitors? This article will discuss these issues in depth.
As shown in the simple rectifier filter circuit shown in Figure 1, in theory, increasing the capacitance of the filter capacitor C in the circuit can indeed make the waveform of the output voltage smoother and less fluctuating, but at the instant the circuit is turned on, the circuit The resulting inrush current cannot be ignored. This is because almost all electronic components have a maximum current value that they can pass. Therefore, when selecting electronic components, the flow caused by the inrush current must be considered. The maximum instantaneous current of related components, the greater the inrush current, the higher the requirements for electronic components, and the cost of the circuit will increase.
In the maintenance of some filter circuits, the replacement of the filter capacitor also has the problem of inrush current. Replacing the original capacitor with a large-capacity filter capacitor will increase the inrush current, and simply improve the filter without replacing other components. The capacity of the capacitor is dangerous, it will greatly shorten the actual service life of the entire circuit, or even burn the entire circuit. Moreover, simply increasing the capacity of the filter capacitor has a limited effect on improving the output voltage, and blindly increasing the capacity of the filter capacitor only increases the cost of the circuit in vain.
2. Calculation of simple filter circuit
figure 2
The simple rectifying and filtering circuit as shown in the figure uses the common 220v50Hz sinusoidal alternating current as the input voltage.
1. Calculation of the charging circuit During the charging process of the capacitor, the equivalent resistance of the diode is R1, then the circuit is equivalent to Figure 2
Figure 2 has to be brought into the U expression and organized:
This is a first-order non-homogeneous differential equation, and its solution is: =
2. Calculation of the discharge circuit During the discharge process of the capacitor, the capacitor only forms a loop with the resistance, and its discharge equation is:
Among them, U is the maximum voltage reached when the capacitor is charged.
3. Computer simulation and discussion of the filter circuit In the foregoing, we have mathematically deduced the working process of the filter circuit. To simulate the working process of the filter circuit, a lot of calculations must be performed before the power output waveform can be plotted point by point. Down. Through the observation of the waveform, the component parameters are selected so that the circuit can meet the requirements of the user in terms of efficiency, power, ripple, cost, etc. under the premise of normal operation. The following is a brief description of the concept and application of the program.
Figure 1 is the circuit simulated by the program. Through the operation of the program, we will obtain a set of voltage and current data on the capacitor C. By substituting a large number of small intervals t (0.00001 seconds), the current and the capacitor C can be obtained. Voltage waveform. The calculation of the program voltage value is obtained by the recursive method, and the current value is calculated by the voltage value.
The simple flow chart of the program operation is as follows:
image 3
1. Example of computer program optimization component selection Load resistance R = 500Ω, the voltage and current waveforms of different capacitances simulated by the computer are as follows:
(1) R = 500Ω, C = 100uF
Figure 4
(2) R = 500Ω, C = 200uF
Figure 5
(3) R = 500Ω, C = 500uF
Image 6
(4) R = 500Ω, C = 1000uF
Picture 7
2. The discussion of the selection of the filter capacitor in the rectifier filter circuit can be observed from the above image: when the load resistance is unchanged, the curve becomes smoother and smoother as the capacitance C capacity increases, but this method has a certain one-sidedness It can be seen from Figure 6 and Figure 7: The effect of simply increasing the filter capacitor on improving the waveform and increasing the DC voltage value is very limited, and as the capacitance C capacity increases, the time for the circuit to enter a steady state after switching on also increases. At the same time, as the capacity of the filter capacitor increases, the inrush current flowing through the circuit also continues to increase. If you do not pay attention to the change of the inrush current, blindly increase the capacity of the filter capacitor to obtain a better The voltage output waveform will inevitably lead to damage to the components through which the current flows due to excessive surge current. Otherwise, to ensure the safety of the entire circuit, we have to increase the indicators of other components, which greatly increases the cost of the circuit. . Therefore, in order to achieve the highest cost performance of the rectifier filter circuit, a capacitor value that can match the load resistance R must be selected, so that the output voltage of the circuit can meet the requirements without improving the performance indicators of other components, and the circuit can reach The best price-performance ratio. Application of this program can accurately and quickly draw the current waveform and output voltage waveform of the circuit shown under any given parameters. We can compare the output voltage waveform and current waveform of different parameter circuits to determine the optimal filter capacitor parameter value.