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FSGM0465RBWDTU Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FSGM0465RBWDTU Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 18 page  © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FSGM0465RB • Rev. 1.0.2 11 4.3. Output-Short Protection (OSP): If the output is shorted, steep current with extremely high di/dt can flow through the SenseFET during the minimum turn- on time. Such a steep current brings high-voltage stress on the drain of the SenseFET when turned off. To protect the device from this abnormal condition, OSP is included. It is comprised of detecting VFB and SenseFET turn-on time. When the VFB is higher than 2V and the SenseFET turn-on time is lower than 1.2μs, the FSGM0465RB recognizes this condition as an abnormal error and shuts down PWM switching until VCC reaches VSTART again. An abnormal condition output short is shown in Figure 22. Figure 22. Output-Short Protection 4.4 Over-Voltage Protection (OVP): If the secondary-side feedback circuit malfunctions or a solder defect causes an opening in the feedback path, the current through the opto-coupler transistor becomes almost zero. Then VFB climbs up in a similar manner to the overload situation, forcing the preset maximum current to be supplied to the SMPS until the overload protection is triggered. Because more energy than required is provided to the output, the output voltage may exceed the rated voltage before the overload protection is triggered, resulting in the breakdown of the devices in the secondary side. To prevent this situation, an OVP circuit is employed. In general, the VCC is proportional to the output voltage and the FSGM0465RB uses VCC instead of directly monitoring the output voltage. If VCC exceeds 24.5V, an OVP circuit is triggered, resulting in the termination of the switching operation. To avoid undesired activation of OVP during normal operation, VCC should be designed to be below 24.5V. 4.5 Thermal Shutdown (TSD): The SenseFET and the control IC on a die in one package makes it easier for the control IC to detect the over temperature of the SenseFET. If the temperature exceeds ~140 °C, the thermal shutdown is triggered and the FSGM0465RB stops operation. The FSGM0465RB operates in auto-restart mode until the temperature decreases to around 110 °C, when normal operation resumes. 5. Soft Burst-Mode Operation: To minimize power dissipation in standby mode, the FSGM0465RB enters burst-mode operation. As the load decreases, the feedback voltage decreases. As shown in Figure 23, the device automatically enters burst mode when the feedback voltage drops below VBURL (500mV). At this point, switching stops and the output voltages start to drop at a rate dependent on standby current load. This causes the feedback voltage to rise. Once it passes VBURH (700mV), switching resumes. At this point, the drain current peak increases gradually. This soft burst- mode can reduce audible noise during burst-mode operation. The feedback voltage then falls and the process repeats. Burst-mode operation alternately enables and disables switching of the SenseFET, thereby reducing switching loss in standby mode. Figure 23. Burst-Mode Operation |
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