SGP400 Datasheet, PDF (9/15 Pages) Fairchild Semiconductor – Low-Power Green-Mode PWM Flyback Power Controller without Secondary Feedback

Product Specification
Low-Power Green-Mode PWM Flyback Power Controller without Secondary Feedback
SGP400
OPERATION DESCRIPTION
SGP400 devices integrate many useful functions for
low-power switch-mode power supplies. The following
descriptions highlight key features of the SGP400.
Start-up Current
The required start-up current is only 8µA, which allows a
high-resistance, low-wattage start-up resistor to be used
to supply the controller’s start-up power. A
1.5MΩ/0.25W start-up 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_DD hold-up capacitance. A 10µF/25V V_DD
hold-up capacitor can be used over a wide input range
(100V-240V AC) with very little power loss.
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 SGP400 can easily meet
international restrictions regarding standby power
consumption.
Constant Voltage (CV), Constant
Current (CC) without Feedback
The SGP400 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.
Over-Temperature Protection (OTP)
The SGP400 has a built-in temperature sensing circuit to
shut down PWM output if the junction temperature
exceeds 140°C. While PWM output is shut down, the
V_DD voltage gradually drops to the UVLO voltage. Some
of the internal circuits are shut down and V_DD gradually
starts increasing again. When V_DD reaches 17V, all the
internal circuits, including the temperature sensing
circuit, start operating normally. If the junction
temperature is still higher than 140°C, the PWM
controller shuts down immediately. This situation
continues until the temperature drops below 110°C. The
PWM output is then turned back on. The temperature
hysteresis window for the OTP circuit is 30°C.
V_DD Over-Voltage Clamping
V_DD over-voltage clamping is built-in to prevent damage
from over-voltage conditions. When V_DD exceeds 22.7V,
PWM output is shut down. Over-voltage conditions may
be caused by an open photo-coupler loop or a
short-circuit in the output.
Oscillator Operation
The oscillation frequency is fixed at 65KHz.
Leading-Edge Blanking
Each time the power MOSFET switches on, a spike
occurs 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 the blanking period, the current-limit
comparator is disabled and cannot switch off the
gate drive.
© System General Corp.
Version 1.0.1 (IAO33.0084.B0)
-9-
www.sg.com.tw • www.fairchildsemi.com
September 24, 2007