ML4827 Datasheet, PDF (1/16 Pages) Fairchild Semiconductor – Fault-Protected PFC and PWM Controller Combo

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ML4827
Fault-Protected PFC and PWM Controller Combo
Features
• Pin-compatible with industry-standard ML4824-1
• TriFault Detect™ to conform to UL1950™ requirements
• Available in 50% or 74% max duty cycle versions
• Low total harmonic distortion
• Reduces ripple current in the storage capacitor between
the PFC and PWM sections
• Average current, continuous boost leading edge PFC
• High efficiency trailing-edge PWM can be configured for
current mode or voltage mode operation
• Average line voltage compensation with brown-out
control
• PFC overvoltage comparator eliminates output
“runaway” due to load removal
• Current fed gain modulator for improved noise immunity
• Overvoltage protection, UVLO, and soft start
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 ML4827-1 (Duty CycleMAX = 50%) and
the ML4827-2 (Duty CycleMAX = 74%). The higher
maximum duty cycle 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.
Block Diagram
16
VEAO
1
IEAO ^{POWER FACTOR CORRECTOR}
VFB
15
2.5V
IAC
2
VRMS
4
ISENSE
3
VEA
–
+
3.5kΩ ^IEA
+
–
GAIN
MODULATOR
3.5kΩ
0.5V
+
–
BROKEN WIRE
+ ^COMPARATOR
2.7V
–
OVP
+
–
–1V +
–
PFC ILIMIT
RAMP 1
7
OSCILLATOR
2µA
VREF ^VCCZ
13.5V
13
VCC
7.5V ^VREF
REFERENCE
14
SQ
RQ
SQ
RQ
PFC OUT
12
RAMP 2
8
8V
VDC
6
VCC
1.25V
SS
50µA
5
8V
DC ILIMIT
9
–
+
–
+
DUTY CYCLE
LIMIT
VIN OK
VFB –
1V –
2.5V +
+
DC ILIMIT
PULSE WIDTH MODULATOR
VCCZ
SQ
RQ
UVLO
PWM OUT
11
GND
10
REV. 1.0.1 6/27/01