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FS8S0765RCBYDTU Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FS8S0765RCBYDTU Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 19 page  11 www.fairchildsemi.com FS8S0765RCB Rev. 1.0.2 Functional Description 1. Start up: To guarantee stable operation of the control IC, FS8S0765RCB has UVLO circuit with 6V hysteresis band. Figure 22 shows the relation between the supply current (Icc) and the supply voltage (Vcc). Before Vcc reaches 15V, the FPS consumes only startup current of 80mA, which is usually provided by the DC link through the 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 normal operation until Vcc goes below the stop voltage of 9V. Figure 22. High Voltage Current Source 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 23 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 comparator (Vfb*). When MOSFET turns on, usually there exists high current spike in the MOSFET current caused by primary-side capacitance and secondary-side rectifier reverse recovery. To prevent premature termination of the switching pulse due to the current spike, the FPS employs leading edge blanking (LEB). The leading edge blanking circuit inhibits the PWM comparator for a short time after the MOSFET is turned on. Figure 23. Pulse-Width Modulation (PWM) Circuit 3. Protection function: FS8S0765RCB has four self- protective functions such as abnormal over current protection (AOCP), overload protection (OLP), over- voltage protection (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 μA. Then, Vcc begin to increase with the current provided through the start-up resistor. When Vcc reaches 15V, the FPS resumes 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 24. Figure 24. 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 25. It turns off the SenseFET within 300ns after the abnormal over-current condition is sensed. Figure 25. AOCP block Vstop=9V Vstart=15V 80uA OVP Vcc Icc 15mA 4 OSC Vcc Vref 2uA 0.9m A V SD R 2.5R FB Gate driver OLP D1 D2 Vfb* Vfb Cfb D Z R Z V psr time time V V 15V 9V Vcc Vds Protection is activated (OCP,OLP,OVP or TSD) Auto restart Vcc GATE DRIVER AOCPCOMP. Vsense : 1V SenseFE T S Q' R OVP OLP TSD UVLO PWMComp |
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