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ML4801

Variable Feedforward PFC/PWM Controller Combo

Micro Linear Corporation 

ML4801

FUNCTIONAL DESCRIPTION

The ML4801 consists of a combined average-current-

controlled, continuous boost Power Factor Corrector (PFC)

front end and a synchronized Pulse Width Modulator

(PWM) back end. It is distinguished from earlier combo

controllers by its dramatically reduced start-up and

operating currents. The PWM section is intended to be

used in current mode. The PWM stage uses conventional

trailing-edge duty cycle modulation, while the PFC uses

leading-edge modulation. This patented leading/trailing

edge modulation technique results in a higher useable

PFC error amplifier 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 reduced ripple on the

PFC output capacitor (the PWM input capacitor). The

PWM section of the ML4801 runs at twice the frequency

of the PFC, which allows the use of smaller PWM output

magnetics and filter capacitors while holding down the

losses in the PFC stage power components.

In addition to power factor correction, a number of

protection features have been built into the ML4801.

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

current limiting, brown-out protection, duty cycle limit,

and under-voltage lockout.

POWER FACTOR CORRECTION

Power factor correction makes a non-linear 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 non-linear 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 filter capacitor

in such a supply 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 a

supply presents a power factor to the line of less than one

(another way to state this is that it causes significant

current harmonics to appear at its input). If the input

current drawn by such a supply (or any other non-linear

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 maintain the input current of a device drawing power

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

input voltage, a way must be found to cause that device

to load the line in proportion to the instantaneous line

voltage. The PFC section of the ML4801 uses a boost-

mode DC-DC converter to accomplish this. The input to

the converter is the full wave rectified AC line voltage.

No filtering is applied following the bridge rectifier, so the

input voltage to the boost converter ranges, at twice line

frequency, 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 current which the converter draws from the power

line matches the instantaneous 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 270VACrms. The other condition is that the

current which the converter is allowed to draw from the

line at any given instant must be proportional to the line

voltage. The first of these requirements is satisfied by

establishing a suitable voltage control loop for the

converter, which sets an average operating level for a

current error amplifier and switching output driver. The

second requirement is met by using the rectified AC line

voltage to modulate the instantaneous input of the current

control loop. Such modulation causes the current error

amplifier to command a power stage current which 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 proportional to 1/VIN2,

which linearizes the transfer function of the system as the

AC input voltage varies.

Since the boost converter topology in the ML4801 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

ML4801. 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 voltage, 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 an (external) 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

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