7

ML4821

Micro Linear

6

7

9

–

+

S.S

INV

+8V

+8V

6.2k

Ω

EA OUT

The output of the gain modulator is a current which

appears on IA+ to form the reference for the current error

amplifier and is given as:

IK I

V

GM

SINE

EA

=×

×

−

(. )

08

where:

is the output of the error amplifier and K is a

The output current of the gain modulator is limited to:

I

R

GM MAX

T

()

.

= 25

This sets the system current limit.The multiplier output

current is converted into the reference voltage for the

current (IA) amplifier through a resistor to ground on IA+.

Figure 6 shows the gain adjustor (K) with respect to the

parts. The right hand part is for operation under normal

conditions in the voltage range from minimum line

voltage to maximum line voltage (90VAC to 260VAC).

85VAC on the curve has been chosen to account for

tolerances. Under normal operating conditions as input

voltage decreases the gain increases compensating for the

drop in the loop gain.

Under brownout conditions (below 85VAC) the gain

decreases to limit the amount of current that is drawn

from the line thus preventing an overload condition. This

is a very useful feature since in many cases the load for a

PFC is a constant power load. The input current has to go

high to compensate for a drop in the input voltage.

Figure 5. Output Saturation Voltage vs. Output Current.

Figure 6. K-factor. Gain Modulator gain with

0.5

0.4

0.3

0.2

0.1

0

135

7

46

0

85VAC

0.23

DESIGN

FOR

NORMAL

OPERATIONS

THIS IS THE MINIMUM

OPERATING

VOLTAGE POINT

THIS GAIN CURVE TAKES

DEPENDENCY OF THE

VOLTAGE CONTROL LOOP

120VAC

OPERATING BOUNDRY

220VAC

K

2

DESIGN

FOR

BROWNOUT

Figure 3. Error and Current Amplifier Configuration

Figure 4. Error Amplifier Open-loop Gain and Phase vs.

Frequency.

GAIN

PHASE

100

80

60

40

20

0

–20

10

100

1.0k

10k

100k

1.0M

10M

0

–30

60

90

120

150

180

f, FREQUENCY (Hz)

SOURCE SATURATION

(LOAD TO GROUND)

80

µs PULSED LOAD

120 Hz RATE

SINK SATURATION

GND

0

–1.0

–2.0

3.0

2.0

1.0

0

0

200

400

600

800