CM6800A_09 Datasheet, PDF (13/18 Pages) Champion Microelectronic Corp. – LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO

CM6800A
LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO
where C_SS is the required soft start capacitance, and the
t_DEALY is the desired start-up delay.
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.
Solving for the minimum value of C_SS:
C_SS = 5ms x ^20μA = 142nF
0.7V
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 V_IN OK comparator at
start-up. The magnitude of V_FB at start-up is related both to
line voltage and nominal PFC output voltage. Typically, a
1.0 μ F soft start capacitor will allow time for V_FB and PFC
out to reach their nominal values prior to activation of the
PWM section at line voltages between 90Vrms and
265Vrms.
In today’s PC power supply, since it has the house-keeping
IC in the secondary, the time sequence maybe different
from the last paragraph. The 5mS delay and the 1uF Soft
Start Capacitor may vary and they may vary base on the
application conditions.
Generating V_CC
After turning on CM6800A at 13V, the operating voltage
can vary from 10V to 17.9V. The threshold voltage of VCC
OVP comparator is 17.9V. The hysteresis of VCC OVP is
1.5V. When VCC see 17.9V, 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 CM6800A system. The bootstrap winding can
be either taped from PFC boost choke or from the
transformer of the DC to DC stage.
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.
R_FILTER x I_VCC ~ 2V, I_VCC = I_OP + (Q_PFCFET + Q_PWMFET ) x fsw
I_OP = 3mA (typ.)
If anything goes wrong, and VCC goes beyond 17.9V, 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
CM6800A itself (5mA, max.) plus the current required by the
two gate driver outputs.
EXAMPLE:
With a wanting voltage called, V_BIAS ,of 18V, a VCC of 15V
and the CM6800A driving a total gate charge of 90nC at
100kHz (e.g. 1 IRF840 MOSFET and 2 IRF820 MOSFET),
the gate driver current required is:
I_GATEDRIVE = 100kHz x 90nC = 9mA
R_BIAS = ^{VBIAS − VCC}
ICC + IG
R_BIAS = ^{18V − 15V}
5mA + 9mA
Choose R_BIAS = 214Ω
The CM6800A should be locally bypassed with a 1.0 μ F
ceramic capacitor. In most applications, an electrolytic
capacitor of between 47 μ F and 220 μ F is also required
across the part, both for filtering and as part of the start-up
bootstrap circuitry.
2009/08/10 Rev. 1.6
Champion Microelectronic Corporation
Page 13