SG6849 Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Features Description Linearly Decreasing PWM Frequency Green-Mode Under Light-Load and Zero-Load 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 meet international power conservation requirements. Conditions Constant Voltage (CV) and Constant Current (CC) No Secondary Feedback Low Startup Current: 5μA Low Operating Current: 3.6mA Leading-Edge Blanking (LEB) Constant Power Limit Universal AC Input Range Synchronized Slope Compensation 150°C OTP Sensor with Hysteresis VDD Over-Voltage Clamping Cycle-by-Cycle Current Limiting Under-Voltage Lockout (UVLO) Fixed PWM Frequency: 65kHz Gate Output Maximum Voltage Clamped at 16.7V Small SSOT-6 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 The supply voltage VDD is also used for feedback compensation to regulate the output voltage without requiring a conventional TL431 and a photo-coupler. An advantage of the SG6849 is a typical startup current of only 5µA, while the typical operating current can be as low as 3.6mA. A large startup resistance can 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. Pulseby-pulse current limiting ensures a constant output current, even if a short circuit occurs. The internal protection circuit disables PWM output if VDD exceeds 22.7V. The gate output is clamped at 16.7V to protect the power MOS from over-voltage damage. The built-in over-temperature protection (OTP) function shuts down 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, SSOT-6 package. 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 © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 Related Resources AN-6849 - Low Cost, Green-Mode PWM Controller for Flyback Converters www.fairchildsemi.com SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback December 2008 Part Number Operating Temperature Range SG684965TZ -40°C to +105°C Small SOT-26 Package RoHS Tape & reel SG684965DZ -40°C to +105°C 8-pin Dual in-line Package (DIP) RoHS Tube SG684965DY -40°C to +105°C 8-pin Dual in-line Package (DIP) Green Tube Package Eco Status Packing Method For Fairchild’s definition of “green” Eco Status, please visit: http://www.fairchildsemi.com/company/green/rohs_green.html. Application Diagram V IN DO RIN D1 VDD GATE CIN CO VO W AUX Q1 SENSE GND RS Figure 1. Typical Application Internal Block Diagram GND 8(1) V DD OTP SENSE Internal Supply OSC Driver S R VDD 2(5) Q PWM Comp V DD Power Limi ter Slope Compensation UVLO OCP Comp 17V/8V 1(6) GATE Green Mode Controller Blanking Circuit 4(4) SENSE SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Ordering Information 4.5V Σ 7(2) FB VDD VDDZ Figure 2. Functional Block Diagram © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 www.fairchildsemi.com 2 XX: BB=SG6849-65T TT : Die Run Code . . . : Year Code - - - : Week Code *Marking for SG684965TZ (Pb-free) T: D=DIP P: Z= Lead free + RoHS Compatible Null=Regular Package XXXXXXXX: Wafer Lot Y: Year WW: Week V: Assembly location *Marking for SG684965DZ (Pb-free) F- Fairchild Logo Z- Plant Code X- 1 Digit Year Code Y- 1 Digit Week Code TT: 2 Digits Die Run Code T: Package Type (D=DIP) P: Z: Pb-free, Y: Green package M: Manufacture Flow Code ZXYTT DXYTT 684965 TPM *Marking for SG684965DY (Green Compound) Figure 3. © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Marking Information Top Mark www.fairchildsemi.com 3 Figure 4. SSOT-6 Pin Configuration Figure 5. DIP-8 Pin Configuration Pin Definitions DIP Pin # SSOT Pin # Name Description 1 6 GATE The totem-pole output driver for driving the power MOSFET. 2 5 VDD Power supply. NC No connection. 3 Current sense. It senses the voltage across a sensed resistor. To provide overcurrent protection, PWM output is disabled if the voltage exceeds an internal threshold. This pin provides current information for current-mode control. 4 4 SENSE 5 3 NC No connection. NC No connection. Feedback. The FB pin provides feedback information to the internal PWM comparator. This feedback is used to control the duty cycle. When no feedback is provided, this pin is left open. 6 7 2 FB 8 1 GND © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 Ground. SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Pin Configurations www.fairchildsemi.com 4 Stresses exceeding the absolute maximum ratings may damage the device. The device may not function or be operable above the recommended operating conditions and stressing the parts to these levels is not recommended. In addition, extended exposure to stresses above the recommended operating conditions may affect device reliability. The absolute maximum ratings are stress ratings only. Symbol Parameter Min. (1, 2) Max. Unit 25 V VVDD DC Supply Voltage VFB Input Voltage to FB Pin -0.3 7.0 V Input Voltage to Sense Pin -0.3 7.0 V TJ Operating Junction Temperature -40 +125 °C ΘJA Thermal Resistance (Junction-to-Air) SSOT 263.3 °C/W DIP 135.7 °C/W TSTG Storage Temperature Range +150 °C VSENSE TL ESD -55 +260 °C Human Body Model, JEDEC:JESD22-A114 3.0 kV Machine Model, JEDEC:JESD22-C101 300 V Lead Temperature (Wave Soldering or IR, 10 Seconds) Notes: 1. All voltage values, except differential voltages, are given with respect to GND pin. 2. Stresses beyond those listed under "absolute maximum ratings" may cause permanent damage to the device. © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Absolute Maximum Ratings www.fairchildsemi.com 5 Unless otherwise noted, VDD=15V and TA=25°C. Symbol Parameter Conditions Min. Typ. Max. Units VDD Section VDD-ON Turn-on Threshold Voltage 16 17 18 V VDD-OFF Turn-off Threshold Voltage 7.5 8.0 8.5 V 5 20 μA IDD-ST Startup Current VDD=15V IDD-OP Operating Supply Current VDD=15V, CL=1nF 3.6 4.6 mA IDD=10mA 28 30 V VDD-CLAMP Zener Diode Breakdown Voltage VDD-TH-G VDD-Low Threshold Voltage for Exiting Green-Mode OFF VDD- OFF+1.0 VDD- VDD- OFF+1.5 OFF+2.0 1.15 1.40 V Feedback Input Section IFB-ZDC VFB-OPEN ZFB VDD-FB Zero Duty Cycle FB Input Current Open-Loop Voltage 4.5 Input Impedance VDD Feedback Threshold Voltage* V 2.4 IFB=0.1mA to 0.2mA mA kΩ FB is Open 20.0 22.7 25.0 V IFB=0.4mA 18 20 22 V Current-Sense Section tPD VSTHVA tLEB Delay to Output 100 Valley Threshold Voltage for Current Limit VDD=20V 0.84 0.89 0.94 V VDD=15V 0.72 0.77 0.82 V VDD=10V 0.58 0.63 0.68 V Leading-Edge Blanking Time 310 Figure 6. © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 ns ns SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Electrical Characteristics Saw Limit www.fairchildsemi.com 6 Unless otherwise noted, VDD=15V and TA=25°C. Symbol Parameter Conditions Min. Typ. Max. Units Oscillator Section Frequency 60 65 70 kHz fOSC-G fOSC Green-Mode Frequency 20 25 30 kHz IFB-G Green-Mode Ending FB Current 0.90 1.15 1.40 mA 2.00 % 80 % 1.5 V fDV Frequency Variation vs. VDD Deviation VDD=10 to 20V Output Section DCYMAX Maximum Duty Cycle VGATE-L Output Voltage Low VDD=15V, Io=10mA VGATE-H Output Voltage High VDD=15V, Io=10mA tr Rising Time VDD=15V, CL=1nF 200 ns tf Falling Time VDD=15V, CL=1nF 80 ns Output Clamp Voltage VDD=20V VGATECLAMP 70 75 8 V 16.7 18.0 V Over-Temperature Protection (OTP) TOTP TOTPRESTART Protection Junction Temperature +150 °C Restart Junction Temperature +130 °C © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Electrical Characteristics (Continued) www.fairchildsemi.com 7 9.00 18.0 17.5 8.50 V DD OFF (V) 16.5 VDD ON (V) 17.0 16.0 8.00 7.50 15.5 15.0 7.00 -40 -25 -10 5 20 35 50 65 80 95 110 125 -40 -25 -10 5 Figure 7. Turn-on Threshold Voltage (VDD-ON) vs. Temperature 35 50 65 80 95 110 125 Figure 8. Turn-off Threshold Voltage (VDD-OFF) vs. Temperature 4.5 8.0 4.3 7.0 4.0 (mA) 9.0 3.8 OP 6.0 IDD I DD ST (µA) 20 Temperature (°C) Temperature (°C) 5.0 3.5 3.3 4.0 3.0 3.0 -40 -25 -10 5 20 35 50 65 80 95 -40 110 125 -25 -10 5 20 35 50 65 80 95 110 125 Temperature (°C) Temperature (°C) Figure 9. Startup Current (IDD-ST) vs. Temperature Figure 10. Center Frequency (fOSC) vs. Temperature 80.0 70.0 78.0 DC Y MAX (%) fOSC (kHz) 65.0 60.0 55.0 76.0 74.0 SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Typical Performance Characteristics 72.0 50.0 70.0 -40 -25 -10 5 20 35 50 65 80 95 -40 110 125 Figure 11. Center Frequency (fOSC) vs. Temperature © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 -25 -10 5 20 35 50 65 80 95 110 125 Temperature (°C) Temperature (°C) Figure 12. Maximum Duty Cycle (DCYMAX) vs. Temperature www.fairchildsemi.com 8 0.80 4.8 0.78 4.6 0.76 V TH,VA(V) V FB (V) 5.0 4.4 4.2 0.74 0.72 4.0 -40 -30 -15 0 25 50 75 100 0.70 125 -40 -25 -10 5 20 Temperature (°C) 35 50 65 80 95 110 125 Temperature (°C) Figure 13. FB Open-Loop Voltage (VFB) vs. Temperature Figure 14. Valley Threshold Voltage (VSTHVA) vs. Temperature 30.0 0.40 f OSC-G-BK 25.0 fOSC (kHz) V GATE-L (V) 0.30 0.20 20.0 15.0 10.0 0.10 5.0 f OSC-G-OFF 0.00 0.0 -40 -25 -10 5 20 35 50 65 Temperature (°C) 80 95 110 125 8 9 9.5 10 V DD (V) Figure 15. Output Voltage Low (VGATE-L) vs. Temperature Figure 16. Green-Mode Burst Back and Green-Mode Off Frequency vs. VDD 1.10 1.00 0.90 1.05 0.80 V LIMIT (V) V TH,VA (V) 8.5 0.70 1.00 0.95 0.60 0.50 9 10 11 12 13 14 15 16 17 18 19 0.90 20 10 VDD (V) 15 20 25 30 35 Duty Cycle (%) Figure 17. Valley Threshold Voltage (VSTHVA) vs. VDD Figure 18. VLIMIT Ramp vs. Duty Cycle 120 40 SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Typical Performance Characteristics (Continued) VDD Zener Current (mA) 100 80 60 40 20 0 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0 VDD Zener Clamp Voltage (V) Figure 19. VDD Zener Current vs. Zener Clamp Voltage © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 www.fairchildsemi.com 9 SG6849 devices integrate many useful functions for low-power switch-mode power supplies. The following descriptions highlight the key features of the SG6849. VDD Over-Voltage Clamping VDD over-voltage clamping is built in to prevent damage from over-voltage conditions. When VDD 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. Startup Current The required startup current is only 5μA. This allows a high-resistance, low-wattage startup resistor to be used to supply the controller’s startup power. A 1.5MΩ/0.25W startup resistor can be used over a wide input range (90VAC-264VAC) with very little power loss. Oscillator Operation The oscillation frequency is fixed at 65kHz. Leading-Edge Blanking (LEB) Operating Current Each time the power MOSFET is switched on, a turn-on spike occurs at the sense resistor. To avoid premature termination of the switching pulse, a 310ns leadingedge 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 operating current is normally 3.6mA. The low operating current results in higher efficiency and reduces the required VDD hold-up capacitance. A 10μF/25V VDD hold-up capacitor can be used over a wide input range (90VAC -264VAC) with little power loss. Green-Mode Operation Constant Output Power Limit 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. When the SENSE voltage across the sense resistor RS reaches the threshold voltage (around 1.0V), the output GATE drive is turned off following a small propagation delay tPD. This propagation delay introduces an additional current proportional to tPD•VIN/LP. The propagation delay is nearly constant regardless of the input line voltage VIN. Higher input line voltages result in larger additional currents. Under high input-line voltages, the output power limit is higher than under low input-line voltages. This green-mode function dramatically reduces power consumption under light-load and zero-load conditions. Power supplies using SG6849 can meet international restrictions regarding standby power-consumption. Constant Voltage (CV) and Constant Current (CC) without Feedback 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 (VLIMITramp). This ramp signal varies from 0.77V to 1.05V for SG6849 and it flattens out at 1.05V. A smaller threshold voltage forces the output GATE drive to terminate earlier, 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-264VAC). The SG6849 tightly regulates 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 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 VDD voltage gradually drops to the UVLO voltage. Some of the SG6849’s internal circuits are shut down and VDD gradually starts increasing again. When VDD reaches 17V, all the internal circuits, including the temperature sensing circuit, starts operating normally. If the junction temperature is still higher than 150°C, the PWM controller is shut down immediately. This situation continues until the temperature drops below 130°C. The PWM output then turns back on. The temperature hysteresis window for the OTP circuit is 20°C. © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 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. SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Operation Description The hold-up capacitor continues to supply VDD before energy can be delivered from the auxiliary winding of the main transformer. VDD must not drop below 8V during this startup process. This UVLO hysteresis window ensures that the hold-up capacitor can adequately supply VDD during startup. www.fairchildsemi.com 10 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. Avoid long PCB traces and component leads. Compensation and filter components should be located near the SG6849. Increasing the power-MOS gate resistance is advised. The SG6849 BiCMOS output stage is a fast totem-pole gate driver. Cross-conduction is avoided to minimize heat dissipation, increase efficiency, and enhance reliability. The output driver is clamped by an internal 16.7V Zener diode to protect the power MOSFET transistors against 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: 0.36 × Duty Duty(max.) © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 (1) SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Noise Immunity Gate Output www.fairchildsemi.com 11 F1 L1 1 1 L CX1 2 BD1 2 - + 4 + C1 R1 C2 C3 R3 R5 R4 D4 T1 10 2 1 1 L2 VO 2 R2 9 D1 R10 22 8 5 2 3 GATE FB VDD NC SENSE 6 C8 + D5 5 7 4 GND 3,4 R6 R8 D2 1 2 CY1 + C4 Figure 20. Reference Circuit (without Secondary-Side Feedback) BOM BD1 CX1 (Option) CY1 (Option) C1 C2 C3 C4 C7 (Option) C8 C9 D1 D2 D4 D5 (Option) Component BD DI106 1A/600V YC 472pF/400V (Y1) YC 102pF/400V (Y1) CC 0.01μF/500V EC 10μF/400V 105ºC CC 1000pF/500V EC 10μF/50V CC 102pF/100V 1206 EC 470μF/10V 105ºC EC 220μF/10V 105ºC Diode FRI07 Diode FR102 Diode SB360 ZD 6.8V/0.5W © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 C9 Q1 1 SG6849 Reference + 2 GND R7 3 1 U4 C7 1 3 1 N 1,2 Reference F1 L1 L2 Q1 R1,R2 R3,R4 R5 R6 R7 R8 R10(Option) T1 U4 Component R 1Ω/0.5W Inductor 20mH 6*8mm Inductor 10μH 6mm MOSFET 1A/600V R 750kΩ/1206 R 47kΩ/1206 R 47Ω/1206 R 4.7Ω/0.5W R 100Ω/0805 R 10Ω/1206 R 10Ω/1206 Transformer EE-16 IC SG6849 (Green PWM IC) SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Applications Information www.fairchildsemi.com 12 L1 2 BD1 + 2 - + 4 R3 R5 R4 C2 R1 3 N C3 + C1 T1 D4 10 1,2 2 1 2 Vo 9 D1 R10 C7 8 5 22 + C8 D5 + C9 Q1 1 3 2 R7 L2 1 1 1 1 1 F1 L 7 GND 3,4 R6 1 2 3 C6 U1 SG6849 GND GATE FB VDD NC SENSE R2 6 R8 5 1 D2 2 4 + C4 R11 U2 1 3 2 R13 R14 3 4 C10 U3 CY1 1 2 R12 Figure 21. Reference Circuit (with Secondary-Side Feedback) BOM Reference BD1 CY1 (Option) C1 C2 C3 C4 C6 C7 C8 C9 C10 D1 D2 D4 D5 (Option) F1 L1 Component BD DI106 1A/600V YC 102p/400V CC 0.01μF/500V EC 10μF/400V 105ºC CC 1000p/500V EC 10μF50V CC 472p/0805 CC 102p/100V 1206 EC 470μF10V 105ºC EC 470μF10V 105ºC CC 222p/0805 Diode FR107 Diode FR102 Diode SB360 ZD 6.8V/0.5W R 1Ω/0.5W Resistor Inductor 20mH 6*8mm © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 Reference L2 Q1 R1,R2 R4,R3 R5 R6 R7 R10 R8 R11 R12 R13 R14 T1 U1 U2 U3 Component Inductor 10μH 6mm MOSFET 1A/600V R 750kΩ/1206 R 47kΩ/1206 R 47Ω/1206 R 4.7Ω/0.5W R 100Ω/0805 R 10Ω/1206 R 10Ω/1206 R 100Ω/1/8W R 33kΩ/0805 R 33kΩ/1/8W R 4.7kΩ/0805 Transformer EE-16 IC SG6849 (Green PWM IC) IC PC817 IC TL431 SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Applications Information (Continued) www.fairchildsemi.com 13 SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Physical Dimensions 9.83 9.00 6.67 6.096 8.255 7.61 3.683 3.20 5.08 MAX 7.62 0.33 MIN 3.60 3.00 (0.56) 2.54 0.56 0.355 0.356 0.20 1.65 1.27 9.957 7.87 7.62 NOTES: UNLESS OTHERWISE SPECIFIED A) THIS PACKAGE CONFORMS TO JEDEC MS-001 VARIATION BA B) ALL DIMENSIONS ARE IN MILLIMETERS. C) DIMENSIONS ARE EXCLUSIVE OF BURRS, MOLD FLASH, AND TIE BAR EXTRUSIONS. D) DIMENSIONS AND TOLERANCES PER ASME Y14.5M-1994 E) DRAWING FILENAME AND REVSION: MKT-N08FREV2. Figure 22. 8-Pin Dual Inline (DIP) Package Package drawings are provided as a service to customers considering Fairchild components. Drawings may change in any manner without notice. Please note the revision and/or date on the drawing and contact a Fairchild Semiconductor representative to verify or obtain the most recent revision. Package specifications do not expand the terms of Fairchild’s worldwide terms and conditions, specifically the warranty therein, which covers Fairchild products. Always visit Fairchild Semiconductor’s online packaging area for the most recent package drawings: http://www.fairchildsemi.com/packaging/. © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 www.fairchildsemi.com 14 SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback Physical Dimensions (Continued) Figure 23. 6-Pin SSOT-6 Package Package drawings are provided as a service to customers considering Fairchild components. Drawings may change in any manner without notice. Please note the revision and/or date on the drawing and contact a Fairchild Semiconductor representative to verify or obtain the most recent revision. Package specifications do not expand the terms of Fairchild’s worldwide terms and conditions, specifically the warranty therein, which covers Fairchild products. Always visit Fairchild Semiconductor’s online packaging area for the most recent package drawings: http://www.fairchildsemi.com/packaging/. © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1 www.fairchildsemi.com 15 SG6849 —Low-Power, Green-Mode PWM Flyback Power Controller without Secondary Feedback 16 www.fairchildsemi.com © 2008 Fairchild Semiconductor Corporation SG6849 Rev. 1.6.1