CM6807
10-PIN Green-Mode PFC/PWM Combo CONTROLLER for High Density AC Adapte
r
UVLO
The UVLO threshold is 13V providing 3V hysteresis.
PFCOUT and PWMOUT
Both PFCOUT and PWMOUT are CMOS drivers. They both
have adaptive anti-shoot through to reduce the switching
loss. Its pull-up is a 30ohm PMOS driver and its pull-down
is a 15ohm NMOS driver. It can source 0.5A and sink 1A if
the VCC is above 15V.
PWM Section
Green Mode
CM6807 has the green mode function to improve the light
load efficiency. PWM Green Mode will happen when the
PWMCMP (PWM Comparator) Duty Cycle is less than ~
6%, in the next cycle, the PWMOUT pulse will be removed
until PWMCMP Duty Cycle is greater than 6%, then the
next cycle, PWMOUT pulse appears.
In other words, during the green mode, PWM switching
frequency will reduce to improve the efficiency. With the
proper external components, CM6807 can easily meet
energy star and blue angel specification.
After 10mS digital soft start, CM6807’s PWM is operating
as a typical current mode. It requires a secondary
feedback, typically, it is configured with CM431, and photo
couple.
Since PWM Section is different from CM6800 family, it
needs the emitter of the photo couple to connected with V +
I instead of the collector. The PWM current information also
goes into V + I. Usually, the PWM current information
requires a RC filter before goes into the V + I.
Therefore, V + I actually is a summing node from voltage
information which is from photo couple and CM431 and
current information which is from one end of PWM sensing
resistor and the signal goes through a single pole, RC filter
then enter the V + I pin.
This RC filter at V+I also serves several functions:
1.) It protects IC.
2.) It provides level shift for voltage information.
3.) It filters the switching noise from current
information.
At normal operation, the threshold voltage of the V + I pin is
1.5V. When the V + I is greater than 1.5V, PWM output driver
will turn off the PWM Power MOSfet.
When the Soft Start is triggered, the V+I threshold is around
150mV.
Soft Start Can be triggered by the following conditions:
1.) During the startup (VCC is less than 10V)
2.) DC to DC short (PWMtrifault is greater than VCC-0.7V)
During above 2 conditions, the V + I threshold is around
150mV until the conditions have been removed.
After above 2 conditions have been removed, the internal
Soft Start D to A will ramp up the voltage from ~150mV to
2V. Each Soft Start Ramp can last around 10mS.
Short (PWMtrifault)
When PWMtrifault is greater VCC-0.7V, PWMOUT will be
turned off. It can be used to detected the following 2 things:
1.) Short Protection
2.) Thermal Shut Down
To achieve above item, it requires a negative temperature
coefficient Resistor.
The following figure shows the typical circuit for PWMtrifault
and V + I pins.
Turn off PFC(PWMtrifault)
When PWMtrifault is less than (VCC-1.4V)/2, CM6807 will
turn off PFC. Usually; it means load has been reduced to a
level, which is the level of the Green Mode threshold.
Usually, we set the Green Mode threshold around 20% of the
full load.
After turning off PFC, the efficiency will be increased due to
the input voltage is higher and less switching events.
Component Reduction
Components associated with the VRMS and IEAO pins of a
typical PFC controller such as the CM6800 have been
eliminated. The PFC power limit and bandwidth does vary
with line voltage.
2010/04/20 Rev1.0
Champion Microelectronic Corporation
Page 12
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
