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AN3027

TM PFC operation (boost topology)

Doc ID 16134 Rev 4

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TM PFC operation (boost topology)

The operation of the PFC transition mode controlled boost converter can be summarized in

the following description.

The AC mains voltage is rectified by a bridge and the rectified voltage is delivered to the

boost converter. This, using a switching technique, boosts the rectified input voltage to a

regulated DC output voltage (Vo).

The boost converter consists of a boost inductor (L), a controlled power switch (Q), a catch

diode (D), an output capacitor (Co) and, obviously, a control circuit (see figure below). The

goal is to shape the input current in a sinusoidal fashion, in phase with the input sinusoidal

voltage. To do this, the L6563S uses the transition mode technique.

Figure 2.

Boost converter circuit

The error amplifier compares a partition of the output voltage of the boost converter with an

internal reference, generating an error signal proportional to the difference between them. If

the bandwidth of the error amplifier is narrow enough (below 20 Hz), the error signal is a DC

value over a given half-cycle.

The error signal is fed into the multiplier block and multiplied by a partition of the rectified

mains voltage. The result is a rectified sinusoid whose peak amplitude depends on the

mains peak voltage and the value of the error signal.

The output of the multiplier is in turn fed into the (+) input of the current comparator, thus it

represents a sinusoidal reference for PWM. In fact, when the voltage on the current sense

pin (instantaneous inductor current multiplied by the sense resistor) equals the value on the

(+) of the current comparator, the conduction of the MOSFET is terminated. As a

consequence, the peak inductor current is enveloped by a rectified sinusoid. As

demonstrated in

Section 3.3.4, TM control causes a constant on-time operation over each

line half-cycle.

After the MOSFET has been turned off, the boost inductor discharges its energy into the

load until its current goes to zero. The boost inductor has now run out of energy, the drain

node is floating and the inductor resonates with the total capacitance of the drain. The drain

voltage drops rapidly below the instantaneous line voltage and the signal on ZCD drives the

MOSFET on again and another conversion cycle starts.

This low voltage across the MOSFET at turn-on reduces both the switching losses and the

total drain capacitance energy that is dissipated inside the MOSFET.

The resulting inductor current and the timing intervals of the MOSFET are shown in

Figure 3, where it is also shown that, by geometric relationships, the average input current

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