November 1998

PRELIMINARY

ML4827

*

Fault-Protected PFC and PWM Controller Combo

1

GENERAL DESCRIPTION

The ML4827 is a controller for power factor corrected,

switched mode power supplies, that includes circuitry

necessary for conformance to the safety requirements of

UL1950. A direct descendent of the industry-standard

ML4824-1, the ML4827 adds a TriFault Detect™ function

to guarantee that no unsafe conditions may result from

single component failure in the PFC. Power Factor

Correction (PFC) allows the use of smaller, lower cost

bulk capacitors, reduces power line loading and stress on

the switching FETs, and results in a power supply that

fully complies with IEC1000-3-2 specification. The

ML4827 includes circuits for the implementation of a

leading edge, average current, “boost” type power factor

correction and a trailing edge, pulse width modulator

(PWM). The device is available in two versions; the

of the -2 allows enhanced utilization of a given

transformer core’s power handling capacity. An over-

voltage comparator shuts down the PFC section in the

event of a sudden decrease in load. The PFC section also

includes peak current limiting and input voltage brown-

out protection. The PWM section can be operated in

current or voltage mode, and includes a duty cycle limit

to prevent transformer saturation.

FEATURES

s

Pin-compatible with industry-standard ML4824-1

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TriFault Detect™ to conform to UL1950™ requirements

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Available in 50% or 74% max duty cycle versions

s

Low total harmonic distortion

s

Reduces ripple current in the storage capacitor

between the PFC and PWM sections

s

Average current, continuous boost leading edge PFC

s

High efficiency trailing-edge PWM can be configured

for current mode or voltage mode operation

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Average line voltage compensation with brown-out

control

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PFC overvoltage comparator eliminates output

“runaway” due to load removal

s

Current fed gain modulator for improved noise immunity

s

Overvoltage protection, UVLO, and soft start

BLOCK DIAGRAM

* Some Packages Are End Of Life

VEAO

IEAO

VFB

IAC

VRMS

ISENSE

RAMP 1

OSCILLATOR

OVP

PFC ILIMIT

UVLO

VREF

PULSE WIDTH MODULATOR

POWER FACTOR CORRECTOR

BROKEN WIRE

COMPARATOR

2.5V

+

–

–

+

7.5V

REFERENCE

VCC

VCCZ

VREF

VEA

+

–

IEA

+

–

+

–

+

–

PFC OUT

S

R

Q

Q

S

R

Q

Q

2.7V

–1V

0.5V

2µA

RAMP 2

PWM OUT

S

R

Q

Q

VDC

SS

DC ILIMIT

VCC

DUTY CYCLE

LIMIT

–

+

1V

–

+

2.5V

VFB

–

+

8V

8V

VIN OK

GAIN

MODULATOR

VCCZ

3.5k

Ω

3.5k

Ω

1.25V

50µA

–

+

13.5V

DC ILIMIT

14

12

11

GND

10

15

2

4

3

7

8

6

5

9

16

1

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