L4984D

Application information

Equation 12

ΔVout

Δfsw

fsw

=

Vout

1+ ΔVout

Vout

Figure 28. Line-modulated fixed-off-time-controlled boost PFC: current waveforms

Vin = 88 Vac

Line current

Vin = 190 Vac

Vin = 264 Vac

Vin = 230 Vac

Vin = 88 Vac

Boost inductor

current envelope

Vin = 264 Vac

Line voltage phase angle (rad)

Line voltage phase angle (rad)

AM13244v1

As a result of the operation of the circuit in Figure 26, the current that the boost PFC pre-

regulator draws from the power line is not exactly sinusoidal but is affected by distortion that

is lower as the current ripple in the boost inductor is smaller as compared to its peak value.

Figure 28 shows some theoretical waveforms, relevant to full load condition, in a line cycle

at different input voltages.

In the diagram on the left-hand side the line (input) current waveform is shown for different

line voltages, while on the right-hand side the envelope of the inductor current at minimum

and maximum line voltage is shown.

The input current waveform relevant to Vin = 88 Vac shows no visible sign of distortion; the

operation of the boost inductor is CCM throughout the entire line cycle as testified by the

inductor current envelope. The brown waveform is relevant to Vin = 190 Vac, which is the

condition where CCM operation no longer occurs at zero-crossings (this voltage value, for a

given power level, depends on the inductance value of the boost inductor); a certain degree

of distortion is already visible.

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