ML4802 Просмотр технического описания (PDF) - Fairchild Semiconductor

Номер в каталоге

Компоненты Описание

производитель

ML4802

PFC/PWM Controller Combo with Green Mode

Fairchild Semiconductor 

ML4802

FUNCTIONAL DESCRIPTION

The ML4802 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 unique Green Mode operation and

dramatically reduced start-up and operating currents. The

PWM section has been optimized for use in current mode

topologies. 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 bus 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 ML4802 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 ML4802. These

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

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

and under-voltage lockout.

GREEN MODE OPERATION

Green Mode automatically improves efficiency by up to

20% or more during low power operation. This feature is

particularly helpful in meeting the demands of Energy

Star™ programs. When the PWM’s output falls to

nominally 17% of its design maximum power, Green

Mode operation is initiated. The upper Green Mode

threshold corresponds roughly to 1/3 of rated full power

level. In Green Mode, the PWM operates in a pulse-

START

NO

VFB ≥

2.50V?

YES

NORMAL MODE

OPERATION

VDC <

1.30V?

NO

YES

TURN PFC OFF

VFB <

NO

2.25V?

YES

TURN PWM ON

TURN PFC ON

VDC ≥

NO

2.25V?

YES

ISSUE PWM

POWER PULSE

Figure 1. ML4802 Operational Flow Chart

skipping mode. This significantly reduces the frequency

of operation, and therefore the dissipation in the PWM

output driver and switch. Since the pulse-skipping is

synchronous to the PWM’s master clock, the noise

spectrum of the PWM retains a strong relationship to its

spectrum during continuous-mode operation, which eases

input and output filter design. PWM pulse frequency

reductions in excess of 10:1 are common, with no

increase in peak-to-peak output ripple. During Green

Mode, the PFC also cycles on and off, running only as

often as necessary to maintain its feedback voltage (VFB)

between 2.25V and 2.5V (corresponding typical values of

VBUSS are 382V and 425V). The PFC uses a built-in soft-

start to minimize line current peaks and component stress

when turning on. See Figure 1 for a flow chart detailing

Green Mode and Normal Mode operation.

2.25V

–

+

VFB

+

2.5V

–

SQ

RQ

PFC ON

VDC

–

VTH1

+

VTH2

PWM OFF

Figure 2. Green Mode Section Block Diagram

Entering Green Mode

The Green Mode Controller is detailed in Figure 2. Key to

the ML4802’s operation in Green-Mode is the fact that the

PWM’s output power is related to the voltage on the VDC

(PWM Duty Cycle Control Voltage) pin by a known

transfer characteristic. Therefore, the output power POUT

drawn from an ML4802 supply can be inferred by

monitoring the value of VDC fed back to the ML4802

from the (external) reference/error amplifier combination.

When the output power taken from the PWM is reduced,

the voltage on VDC will decrease. When VDC falls below

VTH1 (1.30V typical), the part enters Green Mode

operation. Once this happens, the threshold to which VDC

is compared for further PWM operation is set to a higher

value VTH2 (1.58V typical). This causes the PWM to enter

a pulse-skipping mode while maintaining the desired

output voltage. Pulse-skipping occurs because VTH2 is a

higher voltage than VTH1, and because the PWM drive

(PWMOUT) is disabled until VDC ⊕ VTH2. Since the

primary current of the PWM output transformer is

determined by VTH2 in Green Mode, and VTH2 > VTH1,

each PWM output pulse will carry slightly more energy

during Green Mode operation than during all but the

highest duty cycle regimes of continuous-mode operation.

In Green Mode, the power in each PWM output pulse is:

PPULSE ∝ IPRIMARY(PWM) x VBUSS

PPULSE ∝ (VGMT/RSENSE[PWM]) x 380V

6

REV. 1.0.1 12/12/2000

Share Link: