Principle and Design of switching power supply inverse series switching power supply energy storage filter capacitor calculation (7)

1-3-3. Calculation of the filtering capacitor of the inverted series Switching Power Supply
The parameter calculation of the energy storage filtering capacitor of the inverted series switching power supply is basically the same as that of the series switching power supply. However, even if the duty cycle D is equal to 0.5, the charging and discharging times of the filter capacitor are not the same, and the charging time of the filter capacitor is less than half a working cycle, the discharge time of the capacitor is more than half a working cycle, but the charge of the capacitor charge and discharge is equal, that is, the current when the capacitor charge is greater than the current when the discharge. This is the general law of the rectification and filtering circuit.

From Figure 1-8, we can see that when the duty cycle D is equal to 0.5, the capacitor charging time is 3 TB/8, and the average capacitor charging current is 3ilm/8 or 3io/2; the discharge time of the capacitor is 5 TB/8, and the average discharge current of the capacitor is 0.9 Io.
Therefore:

In formula, △q is the charge charged by the capacitor, and the average current of Io flowing through the load. T is the working cycle. When the capacitor is charged, the voltage at both ends of the capacitor is charged from the minimum value to the maximum value (absolute value), and the corresponding Voltage Increment is 2 △uc. Thus, the corrugated voltage at both ends of the capacitor is obtained as follows:

(1-33) and (1-34) are the formulas used to calculate the filtering capacitance of the inverted series switching power supply (D = 0.5 hours ). Formula: Io is the average value of the load current, T is the switching cycle, and △up-P is the ripple of the filtered output voltage, or the voltage ripple. Generally, the ripple voltage is the peak-peak value of the Voltage Increment. Therefore, when D = 0.5, the ripple voltage equals the Voltage Increment of the capacitor charging, that is, Delta up-P = 2 △uc.
Similarly, the calculation results of the (1-33) and (1-34) modes only show the median value or average value of the c used for the energy storage filtering capacitor of the inverted series switching power supply, in extreme cases, we can multiply the average value by a factor greater than 1.
When the duty cycle of switch K is less than 0.5, because the current flowing through the energy storage filter inductance L will be discontinuous, the discharge time of the capacitor will be much greater than the charging time of the capacitor, the ripple of the output voltage filtered by the switch power supply will increase significantly. In addition, the load of the switching power supply is generally not fixed. When the load current increases, the ripple of the output voltage filtered by the switching power supply will also increase. Therefore, there must be sufficient balance when designing the switching power supply. In actual application, it is best to calculate the parameters of the energy storage filter capacitor by more than twice the formula (1-33.