Choose the line voltage range of input capacitor ripple current by compromise
Time:2022-07-05
Views:2026
An interesting trade-off occurs when you choose an input filter capacitor for a low-power, offline power supply. You should choose the ripple current rating of the capacitor in a compromise way to suit the voltage range required by the power supply. By increasing the input capacitance, you can get more ripple current and reduce the voltage drop of the input capacitance to narrow the working input voltage range of the power supply. Doing so will affect the transformer turn ratio of the power supply and various voltage and current stresses. The higher the ripple current rating of the capacitor, the smaller the stress, and the higher the power efficiency.
Figures 1 and 2 show two rectifier configurations used in offline power supplies. Figure 1 shows a full wave bridge in which the AC input voltage is sent to the capacitor after simple rectification. This circuit is commonly used in wide range AC and 230 volt AC applications. The capacitor is charged to the peak of sine wave and then discharged in most half cycles. The ripple current of capacitance includes two parts: the first is the charging cycle, and its current is determined by the capacitance value and the applied dv/dt; The second is capacitor discharge. The power supply acts as a constant power load, so the capacitor discharges at a nonlinear rate. The calculation method is: W= ½ * C *V^2 = P * dt。
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Figures 1 and 2 show two rectifier configurations used in offline power supplies. Figure 1 shows a full wave bridge in which the AC input voltage is sent to the capacitor after simple rectification. This circuit is commonly used in wide range AC and 230 volt AC applications. The capacitor is charged to the peak of sine wave and then discharged in most half cycles. The ripple current of capacitance includes two parts: the first is the charging cycle, and its current is determined by the capacitance value and the applied dv/dt; The second is capacitor discharge. The power supply acts as a constant power load, so the capacitor discharges at a nonlinear rate. The calculation method is: W= ½ * C *V^2 = P * dt。
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Figure 1 full wave bridging used in many offline designs
Figure 2 depicts a voltage multiplier rectifier configuration that is used in many 115/230 VAC applications. If you have a 230 VAC application, your input stage needs to deal with a voltage that is as large as the maximum input voltage (265 VAC) multiplied by the peak factor, which is close to 400 volts. When used with a 115 VAC input, the voltage multiplier will boost the rectified voltage to close to the 230 VAC input level. We can design a power supply for 230 VAC line voltage to reduce the rectified voltage range of the power supply. We usually use a jumper or switch to switch between different rectifier configurations. The only disadvantage of this method is the occasional artificial multiplication of 230 VAC input, which causes serious damage to the power supply. Figure 2 shows some waveforms of the voltage multiplier circuit. There is no electricity between capacitors. Two rectifiers alternately apply input voltage to each capacitor. In a cycle, each capacitor is charged to the peak line voltage, so that each of them has a line frequency ripple part. Since the capacitor is charged in different phases, the ripple frequency of its sum is twice the line frequency.
Figure 3 shows the uf/w normalized voltage drop for four rectifier / input voltage methods. There are three full wave bridging methods, which are applicable to low line voltage America (108 vac/60 Hz), low line voltage Japan (85 vac/50 Hz) and low line voltage Europe (216 vac/50 Hz). In addition, there is a Japanese voltage multiplier with low line voltage. For the full wave bridge, the standardization process only needs to divide the capacitance by the power. In a voltage multiplier, the standardization method is to divide the capacitance of one of the two series capacitors by the power. To use this graph, first determine your rectifier configuration and choose an acceptable voltage drop. After that, you only need to read the uf/w of the input capacitance. Finally, by multiplying your power, you can de standardize.
Then, you can use figure 4 to calculate the ripple current rating of the capacitor. Figure 4 shows the comparison between the standardized ripple current and the standardized input capacitance. Interestingly, ripple current is not closely related to capacitance. This is because during discharge, the current is determined by a nearly constant current from the load. Only during the charging cycle will the current be very different. This occurs when the capacitance (uf/w) decreases and the progressive ripple current increases. When the capacitance is larger and the conduction angle is smaller, the peak current is higher. Please note that the curve only includes the ripple current of line frequency, and does not include the ripple current effect of high-frequency power supply.
In short, it is very important for designers to make some compromises when choosing the configuration of input capacitance and rectifier. If full wave bridging is selected for a wide range of applications, the power supply may need to operate in the 4:1 input range. If the designer chooses to use a voltage multiplier in the design to reduce this range, there is a hidden danger of overvoltage caused by user misoperation. Selecting the correct input capacitance according to the curve provided in this paper can limit the working voltage range to a certain extent.
Figure 2 voltage multiplier reduces power line voltage range
Figure 3 shows the uf/w normalized voltage drop for four rectifier / input voltage methods. There are three full wave bridging methods, which are applicable to low line voltage America (108 vac/60 Hz), low line voltage Japan (85 vac/50 Hz) and low line voltage Europe (216 vac/50 Hz). In addition, there is a Japanese voltage multiplier with low line voltage. For the full wave bridge, the standardization process only needs to divide the capacitance by the power. In a voltage multiplier, the standardization method is to divide the capacitance of one of the two series capacitors by the power. To use this graph, first determine your rectifier configuration and choose an acceptable voltage drop. After that, you only need to read the uf/w of the input capacitance. Finally, by multiplying your power, you can de standardize.
Figure 3 large capacitance can reduce the input line voltage range and improve efficiency
Then, you can use figure 4 to calculate the ripple current rating of the capacitor. Figure 4 shows the comparison between the standardized ripple current and the standardized input capacitance. Interestingly, ripple current is not closely related to capacitance. This is because during discharge, the current is determined by a nearly constant current from the load. Only during the charging cycle will the current be very different. This occurs when the capacitance (uf/w) decreases and the progressive ripple current increases. When the capacitance is larger and the conduction angle is smaller, the peak current is higher. Please note that the curve only includes the ripple current of line frequency, and does not include the ripple current effect of high-frequency power supply.
Figure 4 increasing uf/w will not significantly increase the ripple current of input capacitance
In short, it is very important for designers to make some compromises when choosing the configuration of input capacitance and rectifier. If full wave bridging is selected for a wide range of applications, the power supply may need to operate in the 4:1 input range. If the designer chooses to use a voltage multiplier in the design to reduce this range, there is a hidden danger of overvoltage caused by user misoperation. Selecting the correct input capacitance according to the curve provided in this paper can limit the working voltage range to a certain extent.
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