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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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FS8S0765RCBTU 데이터시트(PDF) 11 Page - Fairchild Semiconductor |
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FS8S0765RCBTU 데이터시트(HTML) 11 Page - Fairchild Semiconductor |
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11 / 20 page  FS8S0765RCB 11 Functional Description 1. 1. 1. 1. Start up : To guarantee stable operation of the control IC, FS8S0765RCB has UVLO circuit with 6V hysteresis band. Figure 1 shows the relation between the sup- ply current (Icc) and the supply voltage (Vcc). Before Vcc reaches 15V, the FPS consumes only startup current of 80 µA, which is usually provided by the DC link through start-up resistor. When Vcc reaches 15V, the FPS begins operation and the operating current increases to 15mA as shown. Once the control IC starts operation, it continues its normal operation until Vcc goes below the stop voltage of 9V Figure 1. Strat up with hysteresis 2. Feedback Control : FS8S0765RCB employs primary side regulation, which permits elimination of feedback circuit components in the secondary side such as opto coupler and TL431. Figure 2 shows the primary side control circuit. The primary side regulation voltage (Vpsr) is controlled to the breakdown voltage of zener diode (Dz). Because current mode control is employed, the drain current of the power MOSFET is limited by the inverting input of PWM compara- tor (Vfb*). When MOSFET turns on, usually there exists high current spike in the MOSFET current caused by pri- mary-side capacitance and secondary-side rectifier reverse recovery. In order to prevent premature termination of the switching pulse due to the current spike, the FPS employs leading edge blanking (LEB). The leading edge blanking cir- cuit inhibits the PWM comparator for a short time after the MOSFET is turned on. Figure 2. Primary side control circuit 3. Protection function : FS8S0765RCB has 4 self protective functions such as abnormal over current protec- tion (AOCP), over load protection (OLP), over voltage pro- tection (OVP) and thermal shutdown (TSD). Because these protection circuits are fully integrated into the IC without external components, the reliability can be improved without cost increase. In the event of these fault conditions, the FPS enters into auto-restart operation. Once the fault condition occurs, switching operation is terminated and MOSFET remains off, which forces Vcc to be reduced. When Vcc reaches 9V, the protection is reset and the supply current reduces to 80 uA. Then, Vcc begin to increase with the cur- rent provided through the start-up resistor. When Vcc reaches 15V, the FPS resumes its normal operation if the fault condition is removed. In this manner, the auto-restart alternately enables and disables the switching of the power MOSFET until the fault condition is eliminated as illustrated in figure 3. Figure 3. Auto restart operation after protection 3.1 Abnormal Over Current Protection (AOCP) : When the secondary rectifying diodes or the transformer pins are shorted, a steep current with extremely high di/dt can flow during the LEB time. Therefore, the abnormal over current protection (AOCP) block is added to ensure the reliability as shown in figure 4. It turns off the SenseFET within 300ns after the abnormal over current condition is sensed. Figure 4. AOCP block Vstop=9V Vstart=15V 80uA OVP Vcc Icc 15mA 4 OSC Vcc V ref 2uA 0.9m A V SD R 2.5R FB Gate driv er OL P D1 D 2 Vfb* Vfb Cfb D Z R Z V psr tim e tim e V V 15V 9V Vcc Vds Prote c tion is a c tiva te d (O CP,O LP,O VP or T SD) Au to re sta rt Vc c GATE DRIVER AOCP COMP. Vsense : 1V SenseFET S Q' R OVP OLP TSD UVLO PWM Comp |
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