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FAN6861 데이터시트(PDF) 11 Page - Fairchild Semiconductor |
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FAN6861 데이터시트(HTML) 11 Page - Fairchild Semiconductor |
11 / 15 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6861 • Rev. 1.0.1 11 Protections Self-protective functions include VDD Over-Voltage Protection (OVP), Open-Loop/Overload Protection (OLP), Over-Current Protection (OCP), Over- Temperature Protection (OTP). Among them, OLP, OCP, and SCP are auto-restart mode protections; while OVP and OTP are latch-mode protections. Auto-Restart Mode Protection: Once a fault condition is detected, switching is terminated and the MOSFET remains off. This causes VDD to fall because no more power is delivered from auxiliary winding. When VDD falls to VDD-OFF (9.5V), the protection is reset and the operating current reduces to startup current, which causes VDD to rise. FAN6861 resumes normal operation when VDD reaches VDD-ON (17.5V). In this manner, the auto-restart can alternately enable and disable the switching of the MOSFET until the fault condition is eliminated (see Figure 21). Latch-Mode Protection: Once this protection is triggered, switching is terminated and the MOSFET remains off. The latch is reset only when VDD is discharged below 4V by unplugging AC power line. Fault Situation 9.5V 17.5V VDD Vds Protection Triggers Fault Removed Normal Operation Normal Operation Power On Operating Current 3mA 8uA Figure 21. Auto Restart Operation Two-Level Over-Current Protection (OCP) FAN6861 has two levels of over-current protection thresholds. One is for pulse-by-pulse current limit, which turns off MOSFET for the remainder of the switching cycle when the sensing voltage of MOSFET drain current reaches the threshold. The other threshold is for the over-current protection, which shuts down the MOSFET gate when the sensing voltage of MOSFET drain current is above the threshold longer than the shutdown delay time (780ms). This two-level OCP protection is designed for applications with peak load characteristics, such as printers and scanners. These applications have motor load and inherently impose over-load condition on the power supply during acceleration mode. Therefore, the protection circuit should be triggered after a specified time to determine whether it is a transient situation or an abnormal situation. Figure 22. Two-Level OCP Operation Open-Loop / Over-Load Protection (OLP) When the upper branch of the voltage divider for the shunt regulator (KA431 shown) is broken, as shown in Figure 23, there is no current flowing through the opto- coupler transistor, which pulls up the feedback voltage to 5.2V. When the feedback voltage is above 4.6V longer than 780ms, OLP is triggered. This protection is also triggered when the SMPS output drops below the nominal value longer than 780ms due to the overload condition. 5.2V VFB-OLP OLP Shutdown Delay Time OLP Triggers VFB (4.6V) Figure 23. OLP Operation |
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