Switching Power Supply Principle and Design (Series 44)

1-8-3-2. Rectifier output full-bridge transformer switching power supply

Figure 1-48 is the working principle diagram of the full-wave rectifier output full-bridge transformer switching power supply; Figure 1-49 is the working principle diagram of the full-bridge transformer switching power supply with adjustable output voltage.

The working principle of the rectifier output full-bridge transformer switching power supply is very similar to that of the rectifier output push-pull transformer switching power supply and the rectifier output half-bridge transformer switching power supply, except that the transformer excitation method and the working power supply access method are slightly different. For the detailed working principles of the switching power supplies such as Figures 1-48 and 1-49, as well as the calculation of the parameters of the transformer and energy storage filter original components, please refer to the analysis of the previous relevant chapters, and I will not repeat them here.

1-8-3-3． Calculation of inductor and capacitor parameters for energy storage filter in full-bridge switching power supply

The calculation of the energy storage filter inductor and capacitor parameters of the full-bridge switching power supply is mainly for the selection of energy storage filter inductor and capacitor parameters in the full-bridge transformer switching power supply with adjustable output voltage as shown in Figure 1-49. In fact, the selection method of energy storage filter inductor and capacitor parameters in the full-bridge transformer switching power supply with adjustable output voltage in Figure 1-49 is basically the same as the selection method of energy storage filter inductor and capacitor parameters in the push-pull transformer switching power supply with adjustable output voltage in Figure 1-33. Therefore, only the formulas for calculating energy storage filter inductor and capacitor parameters are listed here. For detailed analysis, please refer to the content of the section "1-8-1-3. Calculation of energy storage filter inductor and capacitor parameters of push-pull transformer switching power supply".

A) Calculation of the energy storage filter inductor parameters of the full-bridge switching power supply

According to the previous analysis and Figure 1-35, the duty cycle of the two control switches K1 and K2 of the push-pull transformer switching power supply with adjustable output voltage must be less than 0.5, so that the switching power supply can work normally; when the output voltage adjustable range is required to be the maximum, the duty cycle is best taken as 0.25. This analysis result is also valid for the full-bridge switching power supply.

When the duty cycles of the two control switches K1 and K2 are both 0.25, the calculation of the energy storage filter inductor L and the output voltage Uo in the push-pull transformer switching power supply with adjustable output voltage is determined by equations (1-144) and (1-145), that is:

L ≥nUiT/12Io =nUi/12FIo —— When D is 0.25 (1-144)

Uo =2nUi/3 —— When D is 0.25 (1-145)

The above formulas (1-144) and (1-145) are both expressions for calculating the energy storage filter inductance and filter output voltage of the push-pull transformer switching power supply with adjustable output voltage (when D is 0.25), and expressions for calculating the energy storage filter inductance and filter output voltage of the full-bridge transformer switching power supply with adjustable output voltage (when D is 0.25). Where: Ui is the input voltage of the full-bridge transformer switching power supply, Uo is the output voltage of the full-bridge transformer switching power supply, T is the working cycle of the control switch, F is the working frequency of the control switch, and n is the turns ratio of the secondary coil N2 winding of the switching power supply to the primary coil N1 winding.

The calculation results of formulas (1-144) and (1-145) above only give the middle value or average value of the energy storage filter inductance L of the full-bridge transformer switching power supply with adjustable output voltage. For extreme cases, the calculated average value can be multiplied by a coefficient greater than 1.

B) Calculation of the parameters of the energy storage filter capacitor of the full-bridge switching power supply

According to the previous analysis and Figure 1-35, when the duty cycles of the two control switches K1 and K2 are both 0.25, the calculation of the parameters of the energy storage filter capacitor C in the push-pull transformer switching power supply with adjustable output voltage is determined by formula (1-149), that is:

C >Io*T/ 8ΔUP-P —— When D is 0.25 (1-149)

(1-149) In the formula: Io is the current flowing through the load, T is the working cycle of the control switches K1 and K2, and ΔUP-P is the ripple voltage of the output voltage. The ripple voltage ΔUP-P is generally taken as the peak-to-peak value, so the ripple voltage is exactly equal to the voltage increment when the capacitor is charged or discharged, that is: ΔUP-P = 2ΔUc.

Although formula (1-149) is a formula for calculating the energy storage filter capacitor of a push-pull transformer switching power supply with adjustable output voltage (when D = 0.25), it is also valid for calculating the energy storage filter capacitor in a full-bridge transformer switching power supply with adjustable output voltage.

Similarly, the calculation result of formula (1-149) only gives the middle value or average value of the energy storage filter capacitor C of the full-bridge transformer switching power supply, that is, the case when the control switch operates at a duty cycle D of 0.25. For extreme cases, the calculated average value can be multiplied by a coefficient greater than 1.