Datasheet SCM1212A (Mornsun) - 6

The volt-second product VLp×tON defines the magnetization degree of each switching period.If the volt-second products in the above “push” and “pul ” periods are not identical,a smal direct current component may be generated to cause the deviation of magnetic flow.If balance cannot be restored,the deviation of magnetic flow wil gradual y increase in the each of the fol owing switching period,making magnetic core become saturated gradual y.The phenomenon of the deviation of magnetic flow is usual y caused by the unequal on resistance or switching speed of two power switching components.Although the on resistance or switching speed of the two power components are approximately equal through integrating them into the same wafer with the help of high matching advantage of semiconductor integrated circuit technology,the manufacturing error stil exists,causing the smal deviation for the breakover time. Figure 12 Magnetization Curve Figure 13 Self-adapting Positive Temperature Coefficient Rds(on) Fortunately,the on resistance RDS(on) of MOSFET has positive temperature coefficient,with which SCM1212A has the self-correcting effect to restrain the imbalance of volt-second.Under the condition that there is smal deviation between the breakover time of two MOS transistors,the transistor which has longer breakover time tON generates more quantity of heat,and the temperature of the transistor rises to improve RDS(on),then in the breakover period when the load remains unchanged,the drain-source voltage of this transistor VDS is relatively high,as shown in Figure 13,the voltage of primary winding VLP conforms to the formula VLP=VIN-VDS,thus the VLP of the transistor which has larger tON wil gradual y decrease to make volt-second recover balance. Product Operation Mode SCM1212A has three operation modes,which respectively are start-up mode,operation mode and short mode. In start-up mode,SCM1212A provides sufficient charging time for output capacitor,to avoid the abnormal start caused by output short circuit which is incorrectly identified due to the excessive low voltage of output capacitor when it is just started,at the same time,the MOS transistor in start-up mode is always operating in current-limiting drive status which means that the drive voltage of MOS transistor in start is limited,thereby making the current flowing through MOS transistor restrained within the safe range of components,that is to restrain the output switching current of primary winding to ISOFT to achieve the soft start of the system,thereby avoiding the over-current impact and the generation of excessive heat. In operation mode,MOS transistor is always in ful drive status which means that the MOS transistor is operating in switching status and the breakover voltage is very low,which guarantee the efficiency of converter. In short mode,it wil stop driving the converter in sleep mode and the heat generated in start-up mode wil be dissipated,then the product changes to start-up mode. The three operation modes can be freely switched.Only when there is abnormality of output short circuit,the product wil repeatedly switch between the start-up mode and short mode,when the abnormality disappears,the product wil automatical y change to operation mode,al of which can ful y guarantee the reliability of converter and have no influence on the performance of converter in normal operation. Start-up Mode The voltage of output capacitor is zero when the converter is just started,and the converter is firstly in start-up mode.The flow diagram is shown in Figure 14,that is,start → Drive the selected MOS transistor in current-limiting drive method → check the switch-on voltages of MOS transistors(VVD1,VVD2) → judge whether the voltages (VVD1,VVD2) are more than the set value. If (VVD1,VVD2) are more than the set value,then calculate the duration of over-voltage→ judge whether the duration is more than TD_OSP (100ms,typ,).If yes,then the system turns into short circuit mode,if no,then the above process is repeated. If (VVD1,VVD2) are not more than the set value,then the system turns into operation mode. 2019.12.13-A/3 Page 6 of 11 MORNSUN Guangzhou Science & Technology Co.,Ltd.reserves the copyright and right of final interpretation Document Outline Mornsun Guangzhou Science & Technology Co.,Ltd.