Low-Power Green-Mode PWM Flyback Power Controller without Secondary Feedback SG6849
FEATURES
Linearly Decreasing PWM Frequency
Green-Mode under Light-Load and Zero-Load
Conditions
Constant Voltage (CV) and Constant Current (CC)
No Secondary Feedback
Low Startup Current (5uA)
Low Operating Current (3.6mA)
Leading-Edge Blanking
Constant Power Limit
Universal AC Input Range
Synchronized Slope Compensation
150°C OTP Sensor with Hysteresis
V
Over Voltage Clamping
DD
Cycle-by-Cycle Current Limiting
Under Voltage Lockout (UVLO)
Fixed PWM Frequency (65kHz/100kHz)
Gate Output Maximum Voltage Clamped at 16.7V
Small SOT-26 Package
APPLICATIONS
Low-power flyback power converters, such as
Battery chargers for cellular phones, cordless phones,
PDAs, digital cameras, and power tools.
Power adapters for ink jet printers, video game
consoles, and portable audio players.
Open-frame SMPS for TV/DVD standby and other
auxiliary supplies, home appliances, consumer
electronics, and PC 5V standby-power.
Replacements for linear transformers and RCC
SMPS.
DESCRIPTION
This highly integrated PWM controller provides several
features to enhance the performance of low power flyback
converters. To minimize standby power consumption, the
proprietary green-mode function provides off-time
modulation to linearly decrease the switching frequency
under light-load and zero-load conditions. This
green-mode function enables the power supply to easily
meet international power conservation requirements. The
supply voltage V
compensation, to regulate the output voltage without
requiring a conventional TL431 and a photo-coupler.
Another advantage of the SG6849 is that the typical
startup current is only 5uA, while the typical operating
current can be as low as 3.6mA. A large startup resistance
could be used to achieve even higher power conversion
efficiency. Built-in synchronized slope compensation
maintains the stability of peak current-mode control.
Proprietary internal compensation ensures constant output
power limiting over a universal range of AC input
voltages, from 90VAC to 264VAC.The SG6849 provides
many protection functions. Pulse-by-pulse current
limiting ensures a constant output current, even if a short
circuit occurs. Also, the internal protection circuit will
disable PWM output, if V
output is clamped at 16.7V to protect the power MOS
from over-voltage damage. The built-in over temperature
protection (OTP) function will shutdown the controller at
150°C with a 20°C hysteresis. The SG6849 is designed to
provide a low-cost total solution for flyback converters. It
is available in a small footprint, 6-pin, SOT-26 package.
Low-Power Green-Mode PWM Flyback Power Controller without Secondary Feedback SG6849
OPERATION DESCRIPTION
SG6849 devices integrate many useful functions for low
power switch mode power supplies. The following
descriptions highlight the key features of the SG6849.
Startup Current
The required startup current is only 5uA. This allows a
high-resistance, low-wattage startup resistor to be used to
supply the controller’s startup power. A 1.5 MΩ/0.25W
startup resistor can be used over a wide input range
(100V-240V AC) with very little power loss.
Operating Current
The operating current is normally 3.6mA. The low
operating current results in higher efficiency and reduces
the required V
hold-up capacitor can be used over a wide input range
(100V-240V AC) with very little power loss.
hold-up capacitance. A 10uF/25V VDD
DD
Green Mode Operation
The proprietary green-mode function provides off-time
modulation to linearly decrease the switching frequency
under light-load and zero-load conditions. The on-time is
limited to provide better protection against brownouts and
other abnormal conditions.
This green-mode function dramatically reduces power
consumption under light-load and zero-load conditions.
Power supplies using the SG6849 can easily meet
international restrictions regarding standby
power-consumption.
Constant Voltage (CV) and Constant
Current (CC) without Feedback
Over Temperature Protection (OTP)
The SG6849 has a built-in temperature sensing circuit to
shut down PWM output once the junction temperature
exceeds 150°C. While PWM output is shut down, the V
voltage will gradually drop to the UVLO voltage. Some of
the SG6849’s internal circuits will be shut down, and V
will gradually start increasing again. When V
17V, all the internal circuits, including the temperature
sensing circuit, will start operating normally. If the
junction temperature is still higher than 150°C, the PWM
controller will be shut down immediately. This situation
will continue until the temperature drops below 130°C.
The PWM output will then be turned back on. The
temperature hysteresis window for the OTP circuit is
20°C.
DD
DD
DD
reaches
VDD Over Voltage Clamping
VDD over voltage clamping is built in to prevent damage
from over-voltage conditions. When V
PWM output is shut down. Over-voltage conditions may
be caused by an open photo-coupler loop or a short circuit
in the output.
exceeds 22.7V,
DD
Oscillator Operation
The oscillation frequency is fixed at 65kHz for the
SG6849-65, and at 100kHz for the SG6849-100.
Leading Edge Blanking
Each time the power MOSFET is switched on, a turn-on
spike will inevitably occur at the sense-resistor. To avoid
premature termination of the switching pulse, a 310ns
leading-edge blanking time is built in. Conventional RC
filtering is not necessary. During this blanking period, the
current-limit comparator is disabled, and it cannot switch
off the gate drive.
The SG6849 can tightly regulate the output voltage and
provide over current protection without requiring
secondary-side feedback signals. For improved CV and
CC accuracy, the transformer leakage inductance should
be reduced as much as possible.
When the SENSE voltage across the sense resistor RS
reaches the threshold voltage (around 1.0V), the output
GATE drive will be turned off following a small
www.sg.com.tw
Product Specification
×
Low-Power Green-Mode PWM Flyback Power Controller without Secondary Feedback SG6849
propagation delay TPD. This propagation delay will
introduce an additional current proportional to
T
PD*VIN/LP
regardless of the input line voltage V
. The propagation delay is nearly constant
. Higher input line
IN
voltages will result in larger additional currents. Thus,
under high input-line voltages the output power limit will
be higher than under low input-line voltages.
Over a wide range of AC input voltages, the variation can
be significant. To compensate for this, the threshold
voltage is adjusted by adding a positive ramp. This ramp
signal can vary from 0.77V to 1.05V for SG6849-65 and
0.70 to 1.03V for SG6849-100, and it flattens out at 1.05V
and 1.03V respectively. A smaller threshold voltage
forces the output GATE drive to terminate earlier, thus
reducing total PWM turn-on time and making the output
power equal to that of the low line input. This proprietary
internal compensation feature ensures a constant output
power limit over a wide range of AC input voltages
(90VAC to 264VAC).
Under Voltage Lockout (UVLO)
The turn-on/turn-off thresholds of the SG6849 are fixed
internally at 17V/8V. To enable the SG6849 during
startup, the hold-up capacitor must first be charged to 17V
through the startup resistor.
The hold-up capacitor will continue to supply V
energy can be delivered from the auxiliary winding of the
main transformer. V
must not drop below 8V during
DD
this startup process. This UVLO hysteresis window
ensures that the hold-up capacitor can adequately supply
V
during startup.
DD
before
DD
Gate Output
The SG6849 BiCMOS output stage is a fast totem pole
gate driver. Cross-conduction has been avoided to
minimize heat dissipation, increase efficiency, and
enhance reliability. The output driver is clamped by an
internal 16.7V Zener diode in order to protect the power
MOSFET transistors against any harmful over-voltage
gate signals.
Slope Compensation
The sensed voltage across the current sense resistor is
used for current mode control and pulse-by-pulse current
limiting. The built-in slope compensation function
improves power supply stability. Furthermore, it prevents
sub-harmonic oscillations that normally would occur
because of peak current mode control. A positively sloped,
synchronized ramp is activated by the SG6849 with every
switching cycle. The slope of the ramp is
33.0
Duty
.
(max)
Duty
Noise Immunity
Noise from the current sense or the control signal may
cause significant pulse width jitter, particularly in
continuous-conduction mode. Slope compensation helps
alleviate this problem. Good placement and layout
practices should be followed. The designer should avoid
long PCB traces and component leads. Compensation and
filter components should be located near the SG6849.
Finally, increasing the power-MOS gate resistance is
advised.
Low-Power Green-Mode PWM Flyback Power Controller without Secondary Feedback SG6849
PACKAGE INFORMATION
The SOT-26 Package
D
b
64
Detail A
Dimensions:
Symbol
b 0.30 0.500.0110.020
c 0.08 0.220.0030.009
D 2.90 0.114
E 2.80 0.110
E1 1.60 0.063
e 0.95 0.037
e1 1.90 0.075
L 0.300.45 0.600.0200.0180.24
L1 0.60 0.024
L2 0.25 0.010
R 0.100.004
R1 0.10 0.250.0040.25
Low-Power Green-Mode PWM Flyback Power Controller without Secondary Feedback SG6849
The DIP-8 Package
D
°
85
Θ
E1
41
A2
A1
L
b1
e
b
E
A
eB
Dimensions:
Symbol
5.3340.210
1 0.3810.015
2 3.1753.302 3.4290.1250.1300.135
b 1.524 0.060
b1 0.457 0.018
D 9.0179.271 10.1600.3550.3650.400
E 7.620 0.300
E1 6.2236.350 6.4770.2450.2500.255
e 2.540 0.100
L 2.9213.302 3.8100.1150.1300.150
e