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 DIN1 to DIN4) provides power to the converter after getting filtered on CIN. This is shown in Figure 1.

 AEP3094 Fairchild Intro to Flyback Converter Tools Part 6 Figure 1

Figure 1. Steady-state operation

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

 AEP3094 Fairchild Intro to Flyback Converter Tools Part 6 Figure 2

Figure 2. Waveforms of steady-state operation

The black, bold curve at the top of Figure 2 illustrates the capacitor voltage VIN on CIN. The blue curve is the AC sine waveform from the input side. T1 is the charging time on CIN from the AC input. T2 is the half cycle of the AC input. fL is the AC line frequency.

During T1 , the AC input charges CIN. During T2-T1, the capacitor CIN discharges and provides power to the flyback converter. While CIN is discharging, the voltage on CIN drops to VDC_MIN. As the capacitance in CIN goes up, there is less voltage drop VDIP on CIN.

The relationship between the VDC_MIN and the CIN capacitance can be derived as follows:

Assume PIN is the input power. The energy released from CIN while the capacitor voltage drops from VDC_MAX to VDC_MIN can be addressed as in Equation (1).

AEP3094 Fairchild Intro to Flyback Converter Tools Part 6 Eq 1

(1)

VLINE is the line voltage of the AC input. The VDC_MAX can be expressed as in Equation (2).

AEP3094 Fairchild Intro to Flyback Converter Tools Part 6 Eq 2

(2)

Combining equations (1) and (2), we can calculate the VDC_MIN as in Equation (3).

AEP3094 Fairchild Intro to Flyback Converter Tools Part 6 Eq 3

(3)

Based on the expression in Equation (3), we can see that the value of VDC_MIN depends on several parameters, including the line frequency and line voltage of the AC input, the capacitance of CIN, and the input power level.  Generally speaking, with a given value for VLINE, PIN and fL, the capacitance of CIN determines the value of VDC_min.

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