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FSGM0765RLDTU Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FSGM0765RLDTU Datasheet(HTML) 10 Page - Fairchild Semiconductor |
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10 / 18 page  © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FSGM0765R • Rev. 1.0.2 10 4. Protection Circuits : The FSGM0765R has several self-protective functions, such as Overload Protection (OLP), Abnormal Over-Current Protection (AOCP), Output-Short Protection (OSP), Over-Voltage Protection (OVP), and Thermal Shutdown (TSD). All the protections are implemented as auto-restart. Once the fault condition is detected, switching is terminated and the SenseFET remains off. This causes VCC to fall. When V B CC B falls to the Under-Voltage Lockout (UVLO) stop voltage of 7.5V, the protection is reset and the startup circuit charges the VCC capacitor. When VCC reaches the start voltage of 12.0V, the FSGM0765R resumes normal operation. If the fault condition is not removed, the SenseFET remains off and VCC drops to stop voltage again. In this manner, the auto-restart can alternately enable and disable the switching of the power SenseFET until the fault condition is eliminated. Because these protection circuits are fully integrated into the IC without external components, the reliability is improved without increasing cost. Figure 19. Auto-Restart Protection Waveforms 4.1 Overload 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 to protect the SMPS. However, even when the SMPS is in normal operation, the overload protection circuit can be triggered during the load transition. To avoid this undesired operation, the overload protection circuit is designed to trigger only after a specified time to determine whether it is a transient situation or a true overload situation. Because of the pulse-by-pulse current limit capability, the maximum peak current through the SenseFET 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 (VOUT) 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 2.4V, D1 is blocked and the 3.3µA current source starts to charge CFB slowly up. In this condition, VFB continues increasing until it reaches 6.0V, when the switching operation is terminated, as shown in Figure 20. The delay time for shutdown is the time required to charge CFB from 2.4V to 6.0V with 3.3µA. A 25 ~ 50ms delay is typical for most applications. This protection is implemented in auto-restart mode. Figure 20. Overload 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 minimum turn-on time. Even though the FSGM0765R has overload protection, it is not enough to protect the FSGM0765R in that abnormal case; since severe current stress is imposed on the SenseFET until OLP is triggered. The FSGM0765R internal AOCP circuit is shown in Figure 21. When the gate turn-on signal is applied to the power SenseFET, the AOCP block is enabled and monitors the current through the sensing resistor. The voltage across the resistor is compared with a preset AOCP level. If the sensing resistor voltage is greater than the AOCP level, the set signal is applied to the S-R latch, resulting in the shutdown of the SMPS. Figure 21. Abnormal Over-Current Protection |
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