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Q0365R Datasheet(PDF) 12 Page - Fairchild Semiconductor |
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Q0365R Datasheet(HTML) 12 Page - Fairchild Semiconductor |
12 / 19 page © 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com FSQ0365RN, FSQ0265RN, FSQ0165RN, FSQ311 Rev. 1.0.0 12 4. Protection Circuits: The FSQ-series has several self-protective functions, such as Overload Protection (OLP), Abnormal Over-Current protection (AOCP), Over- Voltage Protection (OVP), and Thermal Shutdown (TSD). All the protections are implemented as auto- restart mode. Once the fault condition is detected, switching is terminated and the SenseFET remains off. This causes VCC to fall. When VCC falls down to the Under-Voltage Lockout (UVLO) stop voltage of 8V, the protection is reset and start-up circuit charges VCC capacitor. When the VCC reaches the start voltage of 12V, the FSQ-series 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 23. 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 the 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 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 5µA current source starts to charge CB slowly up to VCC. In this condition, VFB continues increasing until it reaches 6V, when the switching operation is terminated, as shown in Figure 24. The delay time for shutdown is the time required to charge CB from 2.8V to 6V with 5µA. A 20 ~ 50ms delay time is typical for most applications. Figure 24. 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 LEB time. Even though the FSQ-series has OLP (Overload Protection), it is not enough to protect the FSQ-series in that abnormal case, since severe current stress is imposed on the SenseFET until OLP triggers. The FSQ-series has an internal AOCP (Abnormal Over-Current Protection) circuit as shown in Figure 25. 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 latch, resulting in the shutdown of the SMPS. Figure 25. Abnormal Over-Current Protection Fault situation 8V 12V VCC V DS t Fault occurs Fault removed Normal operation Normal operation Power on FSQ0365RN Rev. 00 V FB t 2.8V 6.0V Overload protection t 12= CFB*(6.0-2.8)/Idelay t1 t2 FSQ0365RN Rev.00 1 S Q Q R OSC R 3R GND Gate driver LEB 200ns PWM V OCP AOCP R sense FSQ0365RN Rev.00 |
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