The input voltage and the input capacitance can influence performance in a flyback converter. Let’s look at how to analyze the input voltage and calculate the input capacitance.

When a flyback controller is operating in steady state, the rectified AC input voltage (through D_{IN1} to D_{IN4}) provides power to the converter after getting filtered on C_{IN}. This is shown in Figure 1.

Figure 1. Steady-state operation

Figure 2 gives the waveforms generated by steady-state operation.

Figure 2. Waveforms of steady-state operation

The black, bold curve at the top of Figure 2 illustrates the capacitor voltage V_{IN} on C_{IN}. The blue curve is the AC sine waveform from the input side. T_{1} is the charging time on C_{IN} from the AC input. T_{2} is the half cycle of the AC input. f_{L} is the AC line frequency.

During T_{1 }, the AC input charges C_{IN}. During T_{2}-T_{1}, the capacitor C_{IN} discharges and provides power to the flyback converter. While C_{IN} is discharging, the voltage on C_{IN} drops to V_{DC_MIN}. As the capacitance in C_{IN} goes up, there is less voltage drop V_{DIP} on C_{IN}.

The relationship between the V_{DC_MIN} and the C_{IN} capacitance can be derived as follows:

Assume P_{IN} is the input power. The energy released from C_{IN} while the capacitor voltage drops from V_{DC_MAX} to V_{DC_MIN} can be addressed as in Equation (1).

（1）

V_{LINE} is the line voltage of the AC input. The V_{DC_MAX} can be expressed as in Equation (2).

（2）

Combining equations (1) and (2), we can calculate the V_{DC_MIN} as in Equation (3).

（3）

Based on the expression in Equation (3), we can see that the value of V_{DC_MIN} depends on several parameters, including the line frequency and line voltage of the AC input, the capacitance of C_{IN}, and the input power level. Generally speaking, with a given value for V_{LINE}, P_{IN }and f_{L}, the capacitance of C_{IN} determines the value of V_{DC_min.}

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