ml4802 Microsemi Corporation, ml4802 Datasheet - Page 12

• Manufacturer Part Number: ml4802
• Description: Pfc/pwm Controller Combo With Green Mode
• Manufacturer: Microsemi Corporation

ML4802
In the case of leading edge modulation, the switch is
turned OFF right at the leading edge of the system clock.
When the modulating ramp reaches the level of the error
amplifier output voltage, the switch will be turned ON.
The effective duty-cycle of the leading edge modulation
is determined during the OFF time of the switch. Figure 6
shows a leading edge control scheme.
One of the advantages of this control technique is that it
requires only one system clock. Switch 1 (SW1) turns off
and switch 2 (SW2) turns on at the same instant to
minimize the momentary “no-load” period, thus lowering
ripple voltage generated by the switching action. With
such synchronized switching, the ripple voltage of the
first stage is reduced. Calculation and evaluation have
shown that the 120Hz component of the PFC’s output
ripple voltage can be reduced by as much as 30% using
this method.
12
FUNCTIONAL DESCRIPTION (Continued)
+
DC
VIN
Figure 6. Leading/Trailing Edge Control Scheme
REF
OSC
I1
L1
U4
+
–
EA
RAMP
CLK
U3
VEAO
SW2
SW1
+
–
CMP
U1
I2
C1
I4
I3
D
R
DFF
CLK
U2
RL
Q
Q
RAMP
VSW1
VEAO
Datasheet
TIME
TIME
August 2000
TYPICAL APPLICATIONS
Figure 7 is the application circuit for a complete 100W
power factor corrected power supply, designed using the
methods and general topology detailed in Application
Note 33.