CM6802
NO BLEED RESISTOR GREEN MODE PFC/PWM CONTROLLER COMBO
Soft Start
Generating VCC
Start-up of the PWM is controlled by the selection of the
external capacitor at SS. A current source of 20µA supplies
the charging current for the capacitor, and start-up of the
PWM begins at 1.25V. Start-up delay can be programmed
by the following equation:
After turning on CM6802 at 15V, the operating voltage can
vary from 10V to 19.4V. The threshold voltage of VCC OVP
comparator is 19.4V. The hysteresis of VCC OVP is 1.5V.
When VCC see 19.4V, PFCOUT will be low, and PWM
section will not be disturbed. That’s the two ways to generate
VCC. One way is to use auxiliary power supply around 15V,
and the other way is to use bootstrap winding to self-bias
CM6802 system. The bootstrap winding can be either taped
from PFC boost choke or from the transformer of the DC to
DC stage.
20µA
x
CSS = tDELAY
1.25V
where CSS is the required soft start capacitance and the
DEALY is the desired start-up delay.
t
The ratio of winding transformer for the bootstrap should be
set between 18V and 15V. A filter network is recommended
between VCC (pin 13) and bootstrap winding. The resistor of
the filter can be set as following.
It is important that the time constant of the PWM soft-start
allow the PFC time to generate sufficient output power for
the PWM section. The PWM start-up delay should be at
least 5ms.
RFILTER x IVCC ~ 2V, IVCC = IOP + (QPFCFET + QPWMFET ) x fsw
IOP = 3mA (typ.)
Solving for the minimum value of CSS
:
If anything goes wrong, and VCC goes beyond 19.4V, the
PFC gate (pin 12) drive goes low and the PWM gate drive
(pin 11) remains function. The resistor’s value must be
chosen to meet the operating current requirement of the
CM6802 itself (3mA, max.) plus the current required by the
two gate driver outputs.
20µA
CSS = 5ms x
= 80nF
1.25V
Caution should be exercised when using this minimum soft
start capacitance value because premature charging of the
SS capacitor and activation of the PWM section can result if
VFB is in the hysteresis band of the VIN OK comparator at
start-up. The magnitude of VFB at start-up is related both to
line voltage and nominal PFC output voltage. Typically, a
1.0µF soft start capacitor will allow time for VFB and PFC
out to reach their nominal values prior to activation of the
PWM section at line voltages between 90Vrms and
265Vrms.
2003/06/25 Preliminary
Champion Microelectronic Corporation
Page 13
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
