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ML4827CP-2

Fault-Protected PFC and PWM Controller Combo

Micro Linear Corporation 

ML4827

FUNCTIONAL DESCRIPTION (Continued)

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

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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 termed 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.

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:

IGAINMOD

=

IAC ´ VEAO

VRMS 2

´ 1V

(1)

More exactly, the output current of the gain modulator is

given by:

IGAINMOD = K ´ (VEAO - 1.5V) ´ IAC

where K is in units of V-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 average 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 negative voltage being impressed

upon the ISENSE pin (current into ISENSE ≅ VSENSE/3.5kΩ).

The negative voltage on ISENSE represents the sum of all

currents flowing in the 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 transformers are

sometimes used, one to monitor the ID of the boost

MOSFET(s) and one to monitor the IF of the boost diode.

As stated above, the inverting input of the current error

amplifier is a virtual ground. Given this fact, and the

VREF

PFC

OUTPUT

16

VEAO

VFB

15

2.5V

IAC

2

VRMS

4

ISENSE

3

VEA

–

+

GAIN

MODULATOR

1

IEAO

IEA

+

+

–

–

Figure 2. Compensation Network Connections for the

Voltage and Current Error Amplifiers

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

its duty cycle until the voltage on ISENSE is adequately

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

ISENSE pin.

Cycle-By-Cycle Current Limiter

The ISENSE pin, as well as being a part of the current

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

voltage DC output of the PFC is fed to VFB. When the

voltage on VFB exceeds 2.7V, the PFC output driver is shut

down. The PWM section will continue to operate. The

OVP comparator has 125mV of hysteresis, and the PFC

will not restart until the voltage at VFB drops below 2.58V.

The VFB should be set at a level where the active and

passive external power components and the ML4827 are

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

interfere with the boost voltage regulation loop.

8

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