PRODUCT SPECIFICATION

ML4800

REV. 1.0.5 9/25/01

7

Functional Description

The ML4800 consists of an average current controlled,

continuous boost Power Factor Corrector (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, feedforward from the PFC output

buss can be used to improve the PWM’s line regulation.

In either mode, the PWM stage uses conventional trailing-

edge duty cycle modulation, while the PFC uses leading-

edge modulation. This patented leading/trailing edge modu-

lation technique results in a higher usable PFC error ampli-

fier bandwidth, and can significantly reduce the size of the

PFC DC buss 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 sec-

tion of the ML4800 runs at the same frequency as the PFC.

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

tion features have been built into the ML4800. These include

soft-start, PFC overvoltage protection, peak current limiting,

brownout protection, duty cycle limiting, and under-voltage

lockout.

Power Factor Correction

Power factor correction makes a nonlinear load look 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 capacitive input filter fed from the

line. The peak-charging effect, which occurs on the input fil-

ter capacitor in these supplies, causes brief high-amplitude

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

sinusoidal current inphase with the line voltage. Such sup-

plies 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 their input). If the input current drawn

by such a supply (or any other nonlinear load) can be made

to follow the input voltage in instantaneous amplitude, it will

appear resistive to the AC line and a unity power factor will

be achieved.

To hold the input current draw of a device drawing power

from the AC line in phase with and proportional to the input

voltage, a way must be found to prevent that device from

loading the line except in proportion to the instantaneous line

voltage. The PFC section of the ML4800 uses a boost-mode

DC-DC converter to accomplish this. The input to the con-

verter is the full wave rectified AC line voltage. No bulk fil-

tering is applied following the bridge rectifier, so the input

voltage to the boost converter ranges (at twice line fre-

quency) from zero volts to the 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 cur-

rent drawn from the power line is proportional to the input

line voltage. One of these conditions is that the output volt-

age 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 270VACrms. The other

condition is that the current drawn from the line at any given

instant must be proportional to the line voltage. Establishing

a suitable voltage control loop for the converter, which in

turn drives a current error amplifier and switching 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. In order to prevent ripple, which will necessarily

appear at the output of the boost circuit (typically about

10VAC on a 385V DC level), from introducing distortion

back through the voltage error amplifier, the bandwidth of

the voltage loop is deliberately kept low. A final refinement

is to adjust the overall gain of the PFC such to be propor-

tional to 1/VIN2, which linearizes the transfer function of the

system as the AC input voltage varies.

Since the boost converter topology in the ML4800 PFC is of

the current-averaging type, no slope compensation is

required.

PFC Section

Gain Modulator

Figure 1 shows a block diagram of the PFC section of the

ML4800. The gain modulator is the heart of the PFC, as it is

this circuit block which controls the response of the current

loop to line voltage waveform and frequency, rms line volt-

age, and PFC output voltage. There are three inputs to the

gain modulator. These are:

1.

A current representing the instantaneous input voltage

(amplitude and waveshape) to the PFC. The rectified

AC input sine wave is converted to a proportional

current via a resistor and is then fed into the gain

modulator at IAC. Sampling current in this way

minimizes ground noise, as is 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 gain modulator s output is

inversely proportional to VRMS

2 (except at unusually

low values of VRMS where special gain contouring takes

over, to limit power dissipation of the circuit

components under heavy brownout conditions). The

relationship between VRMS and gain is called K, and is

illustrated in the Typical Performance Characteristics.

3.

The output of the voltage error amplifier, VEAO. The

gain modulator responds linearly to variations in this

voltage.