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FSCQ0765RT Datasheet(PDF) 15 Page - Fairchild Semiconductor |
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FSCQ0765RT Datasheet(HTML) 15 Page - Fairchild Semiconductor |
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15 / 24 page  FSCQ0765RT 15 Figure 12. Auto restart mode protection 4.1 Over Load Protection (OLP) : Overload is defined as the load current exceeding its normal level due to an unexpected abnormal event. In this situation, the protection circuit should trigger in order to protect the SMPS. However, even when the SMPS is in the normal operation, the over load protection circuit can be triggered during the load transition. In order to avoid this undesired operation, the over load protection circuit is designed to trigger after a specified time to determine whether it is a transient situation or an overload situation. Because of the pulse-by-pulse current limit capability, the maximum peak current through the Sense FET is limited, and therefore the maximum input power is restricted with a given input voltage. If the output consumes more than this maximum power, the output voltage (Vo) decreases below the set voltage. This reduces the current through the opto-coupler LED, which also reduces the opto-coupler transistor current, thus increasing the feedback voltage (Vfb). If Vfb exceeds 2.8V, D1 is blocked and the 5uA current source starts to charge CB slowly up to Vcc. In this condition, Vfb continues increasing until it reaches 7.5V, when the switching operation is terminated as shown in Figure 13. The delay time for shutdown is the time required to charge CB from 2.8V to 7.5V with 5uA. In general, a 20 ~ 50 ms delay time is typical for most applications. This protection is implemented in auto restart mode. Figure 13. Over load protection 4.2 Abnormal Over Current Protection (AOCP) : When the secondary rectifier diodes or the transformer pins are shorted, a steep current with extremely high di/dt can flow through the SenseFET during the LEB time. Even though the FSCQ0765RT has OLP (Over Load Protection), it is not enough to protect the FSCQ0765RT in that abnormal case, since sever current stress will be imposed on the SenseFET until OLP triggers. The FSCQ0765RT has an internal AOCP (Abnormal Over Current Protection) circuit as shown in Figure 14. When the gate turn-on signal is applied to the power Sense FET, the AOCP block is enabled and monitors the current through the sensing resistor. The voltage across the resistor is then compared with a preset AOCP level. If the sensing resistor voltage is greater than the AOCP level, the set signal is applied to the latch, resulting in the shutdown of SMPS. This protection is implemented in latch mode. Figure 14. AOCP block 4.3 Over voltage Protection (OVP) : If the secondary side feedback circuit were to malfunction or a solder defect caused an open in the feedback path, the current through the opto-coupler transistor becomes almost zero. Then, Vfb climbs up in a similar manner to the over load situation, Fault situation 9V 15V Vcc Vds I op 4mA 25uA tttt Fault occurs Fault removed Normal operation Normal operation Power on V FB t 2.8V 7.5V Over load protection T 12= CB*(7.5-2.8)/Idelay T 1 T 2 2 S Q Q R OSC R 2.5R GND Gate driver LEB PWM Vaocp AOCP R sense |
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