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FSQ510H Datasheet(PDF) 10 Page - Fairchild Semiconductor |
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FSQ510H Datasheet(HTML) 10 Page - Fairchild Semiconductor |
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10 / 15 page  © 2006 Fairchild Semiconductor Corporation www.fairchildsemi.com FSQ510, FSQ510H Rev. 0.0.3 10 PRELIMINARY DATASHEET 7. Advanced Quasi-Resonant Operation : To minimize switching loss and Electromagnetic Interference (EMI), the MOSFET turns on when the drain voltage reaches its minimum value in QRC converters. Due to Discontinuous Conduction Mode (DCM) operation, the feedback voltage is not changed, despite the DC link voltage ripples, if the load condition is not changed. Since the slope of the drain current is changed depending on the DC link voltage, the turn-on duration of MOSFET is variable with the DC link voltage ripples. The switching period is changed continuously with the DC link voltage ripples. Not only the switching at the instant of the minimum drain voltage, but also the continuous change of the switching period, reduces EMI. So QRC converters can scatter the EMI spectrum inherently. Typical products for QRC turn on the MOSFET when the first valley is detected. In this case, the range of the switching frequency is very wide as a result of the load variations. At a very light load, for example, the switching frequency can be as high as several hundred kHz. Some products for QRC, such as Fairchild’s FSCQ-series, define the turn-on instant of SenseFET change at the first valley into at the second valley when the load condition decreases under its predetermined level. The range of switching frequency narrows somewhat. For details, consult an FSCQ-series datasheet at: http://www.fairchildsemi.com/pf/FS/FSCQ1265RT.html The range of the switching frequency can be limited tightly in FSQ-series. Because a kind of blanking time (tB) is adopted, as shown in Figure 11, the switching frequency has minimum and maximum values. Once the SenseFET is enabled, the next start is prohibited during the blanking time (tB). After the blanking time, the controller finds the first valley within the duration of the quasi-resonant detection window time (tW) (Case A, B, and C). If no valley is found in tW, the internal SenseFET is forced to turn on at the end of t B W (Case D). Therefore, FSQ510 and FSQ510H have minimum switching frequency of 93.5kHz and maximum switching frequency of 130kHz, as shown in Figure 12. T s max=10.7us T s max=10.7us t B=7.7us T s t B=7.7us T s t B=7.7us T s I ds I ds I ds I ds I ds I ds I ds I ds t B=7.7us A B C D t W=3us Figure 11. Advanced QRC Operation 93.5kHz 130kHz 103kHz Burst mode Constant frequency D C B A P o When the resonant period is 2us Figure 12. Switching Frequency Range of the Advanced QRC |
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