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AN1606 APPLICATION NOTE

The circuit shown from a functional point of view is similar to the common boost converter. In the traditional to-

pology current flows through two of the bridge diodes in series. In the bridgeless PFC configuration, current

flows through only one diode with the PowerMOS providing the return path.

To analyze the circuit operation, it is necessary to separate it into two sections. The first section operates as the

boost stage and the second section operates as the return path for the AC input signal.

Referring to Figure 4, the left side (Figure 4a) shows current flow during the positive half cycle and the right side

(Figure 4b) shows current flow during the negative half cycle

Figure 4.

Positive "HALF Cycle."

When the AC input voltage goes positive, the gate of M1 is driven high and current flows from the input through

the inductor, storing energy. When M1 turns off, energy in the inductor is released as current flows through D1,

through the load and returns through the body diode of M2 back to the input mains. See Figure 4A

During the-off time, the current throw the inductor L (that during this time discharges its energy), flows in to the

boost diode D1 and close the circuit through the load.

Negative "HALF Cycle".

During the negative half cycle circuit operation is mirrored as shown in Figure 4B. M2 turns on, current flows

through the inductor, storing energy. When M2 turns off, energy is released as current flows through D2, through

the load and back to the mains through the body diode of M1.

Note that the two PowerMOSFETs are driven synchronously. It doesn't matter whether the sections are per-

forming as an active boost or as a path for the current to return. In either case there is benefit of lower power

dissipation when current flows through the PowerMOSFETs during the return phase.

Current Sensing.

The PFC function requires controlling the current drawn from the mains and shaping it like the input voltage

waveform. To accomplish this it is necessary to sense the current and feed its signal to the control circuit.

In average current conventional boost topology, we sense the rectified current rather than the AC input current.

This can be achieved by a simple sensing resistor in the return of the current to the bridge, as shown in Figure5a.

in

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L

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D

controller

L

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M

2

M

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D

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D

Positive half cycle

Fig4a

C

H

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P

P

E

R

return

L

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in

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controller

L

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M

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M

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Negative half cycle

Fig4b

C

H

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R

return