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FSDL321L Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FSDL321L Datasheet(HTML) 10 Page - Fairchild Semiconductor |
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10 / 20 page  FSDH321, FSDL321 10 Figure 6. Over Load Protection (OLP) 4.2 Thermal Shutdown (TSD) : The Sense FET and the control IC are integrated, making it easier for the control IC to detect the temperature of the Sense FET. When the tem- perature exceeds approximately 145 °C, thermal shutdown is activated. 4.3 Abnormal Over Current Protection (AOCP) : Even though the FPS has OLP (Over Load Protection) and current mode PWM feedback, these are not enough to protect the FPS when a secondary side diode short or a transformer pin short occurs. In addition to start-up, soft-start is also activated at each restart attempt during auto-restart and when restarting after latch mode is activated. The FPS has an internal AOCP (Abnormal Over Current Protection) circuit, as shown in Figure 7. 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, pulse-by-pulse AOCP is triggered regardless of uncontrollable LEB time. Here, pulse-by-pulse AOCP stops the Sense FET within 350ns after it is activated. Figure 7. Abnormal Over Current Protection (AOCP) 4.4 Over Voltage Protection (OVP) : In the event of a mal- function in the secondary side feedback circuit, or an open feedback loop caused by a soldering defect, the current through the opto-coupler transistor becomes almost zero (refer to Figure 5). Then, VFB climbs up in a similar manner to the over load situation, forcing the preset maximum cur- rent to be supplied to the SMPS until the over load protection is activated. Because excess energy is provided to the output, the output voltage may exceed the rated voltage before the over load protection is activated, resulting in the breakdown of the devices in the secondary side. In order to prevent this situation, an over voltage protection (OVP) circuit is employed. In general, Vcc is proportional to the output volt- age and the FPS uses Vcc instead of directly monitoring the output voltage. If VCC exceeds 19V, OVP circuit is activated resulting in termination of the switching operation. In order to avoid undesired activation of OVP during normal opera- tion, Vcc should be properly designed to be below 19V. V FB t 3V 6V Over Load Protection t 12= CFB×(V(t2)-V(t1)) / IDELAY t 1 t 2 V t V V t V A I I t V t V C t DELAY DELAY FB 6 ) ( , 3 ) ( , 5 ; ) ( ) ( 2 1 1 2 12 = = = − = µ Vsense V FB,in Gate Driver Rsense CLK Drain V AOCP PWM COMPARATOR AOCP COMPARATOR LEB R SQ |
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