FAN4800IMX View Datasheet(PDF) - Fairchild Semiconductor

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FAN4800IMX

Low Start-Up Current PFC/PWM Controller Combos

Fairchild Semiconductor 

Functional Description

The FAN4800 consists of an average current controlled,

continuous boost Power Factor Correction (PFC) front

end and a synchronized Pulse Width Modulator (PWM)

back end. The PWM can be used in either current or

voltage mode. In voltage mode, feed forward from the

PFC output bus can be used to improve the PWM’s line

regulation. In either mode, the PWM stage uses conven-

tional trailing-edge, duty-cycle modulation. This patented

leading/trailing edge modulation results in a higher

usable PFC error amplifier bandwidth and can signifi-

cantly reduce the size of the PFC DC bus capacitor.

The synchronization of the PWM with the PFC simplifies

the PWM compensation due to the controlled ripple on

the PFC output capacitor (the PWM input capacitor). The

PWM section of the FAN4800 runs at the same fre-

quency as the PFC.

In addition to power factor correction, a number of pro-

tection features are built into the FAN4800. These

include soft-start, PFC over-voltage protection, peak cur-

rent limiting, brownout protection, duty cycle limiting, and

under-voltage lockout (UVLO).

Power Factor Correction

Power Factor Correction treats a nonlinear load like a

resistive load to the AC line. For a resistor, the current

drawn from the line is in phase with and proportional to

the line voltage, so the power factor is unity (one). A

common class of nonlinear load is the input of most

power supplies, which use a bridge rectifier and capaci-

tive input filter fed from the line.

The peak charging effect, which occurs on the input filter

capacitor in these supplies, causes brief, high-amplitude

pulses of current to flow from the power line, rather than

a sinusoidal current in phase with the line voltage. Such

supplies present a power factor to the line of less than

one (i.e., they cause significant current harmonics of the

power line frequency to appear at the input). If the input

current drawn by such a supply (or any nonlinear load)

can be made to follow the input voltage in instantaneous

amplitude, it appears resistive to the supply.

To hold the input current draw of a device drawing power

from the AC line in phase with and proportional to the

input voltage, that device must be prevented from load-

ing the line except in proportion to the instantaneous line

voltage. To accomplish this, the PFC section of the

FAN4800 uses a boost mode DC-DC converter. The

input to the converter is the full-wave, rectified, AC line

voltage. No bulk filtering is applied following the bridge

rectifier, so the input voltage to the boost converter

ranges (at twice line the frequency) from zero volts to a

peak value of the AC input and back to zero. By forcing

the boost converter to meet two simultaneous conditions,

it is possible to ensure that the current drawn from the

power line is proportional to the input line voltage.

One of these conditions is that the output voltage of the

boost converter must be set higher than the peak value

of the line voltage. A commonly used value is 385VDC, to

allow for a high line of 270VAC rms. The second condition

is that the current drawn from the line at any given

instant must be proportional to the line voltage. Estab-

lishing a suitable voltage control loop for the converter,

which in turn drives a current error amplifier and switch-

ing output driver, satisfies the first of these requirements.

The second requirement is met by using the rectified AC

line voltage to modulate the output of the voltage control

loop. Such modulation causes the current error amplifier

to command a power stage current that varies directly

with the input voltage. To prevent ripple, which necessar-

ily appears at the output of boost circuit (typically about

10VAC on a 385VDC level), from introducing distortion

back through the voltage error amplifier, the bandwidth of

the voltage loop is deliberately kept low. A final refine-

ment is to adjust the overall gain of the PFC section to be

proportional to 1/VIN2, which linearizes the transfer func-

tion of the system as the AC input voltage.

Since the boost converter in the FAN4800 PFC is current

averaging, no slope compensation is required.

1. PFC Section

1.1 Gain Modulator

Figure 1 shows a block diagram of the PFC section of

the FAN4800. The gain modulator is the heart of the

PFC, as the circuit block controls the response of the

current loop to line voltage waveform and frequency,

RMS line voltage, and PFC output voltages. There are

three inputs to the gain modulator:

1. A current representing the instantaneous input voltage

(amplitude and wave shape) to the PFC. The rectified

AC input sine wave is converted to a proportional cur-

rent via a resistor and is then fed into the gain modula-

tor at IAC. Sampling current in this way minimizes

ground noise, required in high-power, switching-power

conversion environments. The gain modulator

responds linearly to this current.

2. A voltage proportional to the long-term RMS AC line

voltage, derived from the rectified line voltage after

scaling and filtering. This signal is presented to the

gain modulator at VRMS. The output of the gain modu-

lator is inversely proportional to VRMS2 (except at

unusually low values of VRMS, where special gain con-

touring takes over to limit power dissipation of the cir-

cuit components under heavy brownout conditions).

The relationship between VRMS and gain is called K

and is illustrated in Figure 5.

© 2005 Fairchild Semiconductor Corporation

FAN4800 Rev. 1.0.5

9

www.fairchildsemi.com

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