AN-6902 Datasheet, PDF (1/12 Pages) Fairchild Semiconductor – Control a CCM PFC and Flyback/PWM Power Supply

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AN-6902
Applying SG6902 to Control a CCM PFC and
Flyback/PWM Power Supply
Summary
This application note shows a step-by-step design to a
120W/24V power adapter. The equations also can be
applied to different output voltages and wattages.
Features
ƒ Interleaved PFC/PWM Switching
ƒ Green-Mode PFC/PWM Switching
ƒ No PFC Switching at Light Loads for Power Saving
ƒ Innovative Switching Charge Multiplier-divider
ƒ Low Startup and Operating Current
ƒ Innovative Switching Charge Multiplier-divider
ƒ Multi-vector Control for Improved PFC Output
Transient Response
ƒ Average-Current-Mode Control for PFC
ƒ Programmable Two-Level PFC Output Voltage to
Achieve the Best Efficiency
ƒ PFC Over-voltage and Under-voltage Protections
ƒ PFC and PWM Feedback Open-loop Protection
ƒ Cycle-by-cycle Current Limiting for PFC/PWM
ƒ Slope Compensation for PWM
ƒ Maximum Power Limit for PWM
ƒ Brownout Protection
ƒ Over Temperature Protection
ƒ Power-on Sequence Control and Soft-start
ƒ 20-Pin SOP and SSOP Packages
Description
SG6902 is designed for power supplies that consist of boost
PFC and flyback PWM. It requires few external
components to achieve green-mode operation and versatile
protections and compensations.
The proprietary interleave switching synchronizes the PFC
and PWM stages and reduces switching noise. At light
loads, PFC stage is turned off to save power and the PWM
switching frequency is decreased in response to the load.
For PFC stage, the proprietary multi-vector control scheme
provides a fast transient response in a low-bandwidth PFC
loop. The overshoot and undershoot of the PFC voltage are
clamped. If the feedback loop is broken, SG6902 shuts off
the switching to protect the power supply and its load.
For the flyback PWM stage, the synchronized slope
compensation ensures the stability of the current loop.
“Hiccup” operation limits a maximum output power during
the overload situations.
The difference between members of this family are shown
in the table below.
Parameter
Start Threshold Voltage
Minimum Operating Voltage
The Interval of OPFC Lags
Behind OPWM at Startup
PFC On/Off
OTP
Soft-Start
SG6902
16V
10V
11.5ms
O
O
O
SG6901A
12V
10V
11.5ms
X
O
O
© 2007 Fairchild Semiconductor Corporation
Rev. 1.2.1 • 5/1/08
www.fairchildsemi.com