CM6800
LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO
where CSS is the required soft start capacitance, and the
tDEALY is the desired start-up delay.
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
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
CM6800 itself (5mA, max.) plus the current required by the
two gate driver outputs.
Solving for the minimum value of CSS
:
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.
EXAMPLE:
With a wanting voltage called, VBIAS ,of 18V, a VCC of 15V
and the CM6800 driving a total gate charge of 90nC at
100kHz (e.g. 1 IRF840 MOSFET and 2 IRF820 MOSFET),
the gate driver current required is:
IGATEDRIVE = 100kHz x 90nC = 9mA
V
BIAS − VCC
I
RBIAS
=
=
CC + I
G
Generating VCC
18V −15V
RBIAS
After turning on CM6800 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 CM6800 system. The bootstrap winding can be
either taped from PFC boost choke or from the transformer
of the DC to DC stage.
5mA + 9mA
Choose RBIAS = 214Ω
The CM6800 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.
2008/10/23 Rev. 2.1
Champion Microelectronic Corporation
Page 13
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
