ML4826

8

Figure 2. Compensation Network Connections for the

Voltage and Current Error Amplifiers

3) The output of the voltage error amplifier, VEAO. The

gain modulator responds linearly to variations in this

voltage.

The output of the gain modulator is a current signal, in the

form of a full wave rectified sinusoid at twice the line

frequency. This current is applied to the virtual-ground

(negative) input of the current error amplifier. In this way

the gain modulator forms the reference for the current

error loop, and ultimately controls the instantaneous

current draw of the PFC from the power line. The general

form for the output of the gain modulator is:

I

I

VEAO

V

V

GAINMOD

AC

RMS

≅

×

×

2

1

More exactly, the output current of the gain modulator is

given by:

I

K

VEAO

V

I

GAINMOD

AC

≅×

–.

15

(1)

Note that the output current of the gain modulator is

limited to

≅ 200µA.

Current Error Amplifier

The current error amplifier’s output controls the PFC duty

cycle to keep the current through the boost inductor a

linear function of the line voltage. At the inverting input to

the current error amplifier, the output current of the gain

modulator is summed with a current which results from a

PFC circuit, and is typically derived from a current sense

resistor in series with the negative terminal of the input

bridge rectifier. In higher power applications, two current

boost diode. As stated above, the inverting input of the

current error amplifier is a virtual ground. Given this fact,

and the arrangement of the duty cycle modulator polarities

internal to the PFC, an increase in positive current from

the gain modulator will cause the output stage to increase

negative to cancel this increased current. Similarly, if the

gain modulator’s output decreases, the output duty cycle

will decrease, to achieve a less negative voltage on the

There is a modest degree of gain contouring applied to the

transfer characteristic of the current error amplifier, to

increase its speed of response to current-loop

perturbations. However, the boost inductor will usually be

the dominant factor in overall current loop response.

Therefore, this contouring is significantly less marked than

that of the voltage error amplifier. This is illustrated in the

Typical Performance Characteristics.

Cycle-By-Cycle Current Limiter

feedback loop, is a direct input to the cycle-by-cycle

current limiter for the PFC section. Should the input

voltage at this pin ever be more negative than -1V, the

output of the PFC will be disabled until the protection

flip-flop is reset by the clock pulse at the start of the next

PFC power cycle.

Overvoltage Protection

The OVP comparator serves to protect the power circuit

from being subjected to excessive voltages if the load

should suddenly change. A resistor divider from the high

down. The PWM section will continue to operate. The

OVP comparator has 125mV of hysteresis, and the PFC

passive external power components and the ML4826 are

within their safe operating voltages, but not so low as to

interfere with the boost voltage regulation loop.

19

VEAO

IEAO

VFB

IAC

VRMS

ISENSE

2.5V

-

+

20

2

4

3

VEA

-

+

IEA

+

-

VREF

1

AGND

11

PFC

OUTPUT

GAIN

MODULATOR