ML4804IS 查看數據表（PDF） - Fairchild Semiconductor

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ML4804IS

Power Factor Correction and PWM Controller Combo

Fairchild Semiconductor 

FUNCTIONAL DESCRIPTION (Continued)

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 2.75V, the PFC output driver is

shut down. The PWM section will continue to operate. The

OVP comparator has 250mV of hysteresis, and the PFC

will not restart until the voltage at VFB drops below 2.50V.

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

passive external power components and the ML4804 are

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

interfere with the boost voltage regulation loop.

VCCOVP

The VCCOVP feature of the ML4804 works along with the

TriFaultTM Detect as a redundant PFC buss voltage limiter,

to prevent a damaged and broken connection or

component from causing an unsafe fault condition.

VCCOVP assumes that VCC is generated from a bootstrap

winding on the PFC boost inductor, or by some other

means whereby VCC is proportional to VBUSS. If the

proportionality is exact, then a nominal VBUSS of 385V at

VCC = 15.0V will cause the VCCOVP comparator to shut

the PFC down when VBUSS = [(16.4/15.0) x 385V] = 444V.

The PFC will then remain in the shutdown state until VCC

declines to 13.0V, at which time the PFC will restart. If

the PFC VCC again encounters an over voltage condition,

the protection cycle will repeat. Note that the PWM stage

of the ML4804 remains operational even when the PFC

goes into VCCOVP shutdown.

For a real-world example, assume that the bootstrap

supply is derived from a conventional boost inductor

winding and rectified using Shottky diodes. Then it follows

VREF

ML4804

that the voltage from the bootstrap winding must equal

15.8V during regular circuit operation, and will increase

to 17.2V at the point of VCCOVP shutdown. Then the

output voltage from the PFC will have increased from a

noninal VBUSS of 385VDC to (17.2/15.8) x 385V =

419VDC. When VBUSS reaches 419V, the PFC will shut

off, thereby protecting the output (BUSS) capacitor and

the semiconductors in both the PFC and PWM stages.

To assure reasonable headroom in which to operate this

device, VCCOVP tracks with UVLO. The VCCOVP

threshold is always at least 2V above that of the UVLO.

To assure reliable operation of the ML4804, VCC must be

operated from a bootstrap winding on the PFC’s inductor,

or from an external power supply whose output is

regulated to 15.0V (nominal). In the case of a regulated

power supply powering the ML4804, the VCCOVP function

will be rendered non-operational.

Error Amplifier Compensation

The PWM loading of the PFC can be modeled as a

negative resistor; an increase in input voltage to the PWM

causes a decrease in the input current. This response

dictates the proper compensation of the PFC's two

transconductance error amplifiers. Figure 2 shows the

types of compensation networks most commonly used for

the voltage and current error amplifiers, along with their

respective return points. The current loop compensation is

returned to VREF to produce a soft-start characteristic on

the PFC: as the reference voltage comes up from zero

volts, it creates a differentiated voltage on IEAO which

prevents the PFC from immediately demanding a full duty

cycle on its boost converter.

There are two major concerns when compensating the

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

REV. 1.0.2 3/9/2001

VBIAS

RBIAS

VCC

ML4804

GND

0.22µF

CERAMIC

15V

ZENER

Figure 3. External Component Connections to VCC

9

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