CM6500UN (1MHz PFC)
http://www.championmicro.com.tw
EPA/90+ ZVS-Like PFC CONTROLLER
Design for High Efficient Power Supply at both Full Load and Light Load
Oscillator (RAMP1, or called RTCT)
Soft Start (ISS)
There is a ~ 10uA to charge ISS pin. The PFC-soft-start function
is implemented with ISS pin.
In CM6500UN, fRTCT = fpfc fRTCT =68Khz, when
VEAO=0V, it provides the best performance in the PC
application.
The oscillator frequency, fRTCT is the similar formula in
CM6800:
After PFC Brown Out condition is removed (Vrms is greater than
1.75V.), ISS potential will be raised by the 10uA charge current.
ISS potential also determines the VFB threshold until ISS is
greater than 2.5V. Therefore, before ISS reaching 2.5V, PFC bulk
output voltage is determined by ISS potential until ISS reaching
2.5V.
1
fRTCT =
t
RAMP + tDEADTIME
The dead time of the oscillator is derived from the
following equation:
V
REF − 0.78
tRAMP = CT x RT x In
V
REF − 3.7
at VREF = 7.52V:
tRAMP = CT x RT x 0.5678
The dead time of the oscillator may be determined using:
2.92V
tDEADTIME
=
x CT = 292 x CT
10.0mA
The dead time is so small (tRAMP >> tDEADTIME ) that the
operating frequency can typically be approximately by:
1
fRTCT =
t
RAMP
Ct should be greater than 470pF.
Let us use 1000PF Solving for RT yields 27K. Selecting
standard components values, CT = 1000pF, and RT = 27kΩ
The dead time of the oscillator determined PFC minimum
off time which is the dead time.
2014/11/11 Rev. 1.0
Champion Microelectronic Corporation
15
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
