AN4213 datasheet(7/42 Pages) STMICROELECTRONICS

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AN4213

PFC section

Figure 3. Boost converter circuit

The boost converter can operate in two modes: discontinuous conduction mode (DCM) and

continuous conduction mode (CCM).

Discontinuous conduction mode is when the Power MOSFET of the boost converter is

turned on when the inductor current reaches zero after a dead time and turned off when the

inductor current meets the input reference voltage. In this way, the input current waveform

follows the input voltage one, therefore obtaining a power factor close to 1. DCM is suitable

for power levels of 300 W or less. DCM uses larger cores and has higher I²R and skin-effect

losses due to the larger inductor current swing. With the increased swing a larger input filter

is also required. On the positive side, since in the discontinuous mode the Power MOSFET

switches on when the inductor current is at zero, there is no reverse-recovery current (IRR)

specification required on the boost diode. This means that less expensive diodes can be

used.

Continuous conduction mode (CCM) is when the current in the energy transfer inductor

never reaches zero during the switching cycle. The Power MOSFET starts conducting when

the current through itself is not zero. Continuous conduction mode (CCM) is suitable for high

power ratings (>300 W). The voltage swing is less than in DCM resulting in lower I²R losses

and the lower ripple current results in lower inductor core losses. Less voltage swing also

reduces EMI and allows for a smaller input filter to be used. Unfortunately, since the Power

MOSFET is not being turned on when the current of the inductor is at zero, a very fast

reverse-recovery diode is required to keep losses to a minimum.

Transition conduction mode which is typically used in lighting applications represents a good

cost-benefit compromise. In the transition mode approach, the switch-on time is held

constant during the line cycle and the switch is turned on when the inductor current falls to

zero, so that the converter operates at the boundary between continuous and discontinuous

conduction mode. In this way, the freewheeling diode is turned off softly (no recovery losses)

and the switch is turned on at zero current, so the commutation losses are reduced. Besides

the simplicity and the few external parts required, this system minimizes the inductor size

due to the low inductance value needed. On the other hand, the high current ripple on the

inductor involves high RMS current and high noise on the rectified main bus, which needs a

heavier EMI filter to be rejected. These drawbacks limit the use of the TM PFC in a lower

power range (typically below 200 W).

AM17317v1

Controller

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