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FSD210DH Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FSD210DH Datasheet(HTML) 11 Page - Fairchild Semiconductor |
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11 / 15 page  © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com FSQ510, FSQ510H, and FSQ510M • Rev. 1.2.0 11 V FB V ds 0.75V 0.85V I ds Vo Voset time Switching disabled Switching disabled t 4 t 3 t 2 t 1 Figure 18. Burst-Mode Operation 7. Advanced Valley Switching Operation : To minimize switching loss and Electromagnetic Interference (EMI), the MOSFET turns on when the drain voltage reaches its minimum value in VS converters. Due to the 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. VS converters inherently scatter the EMI spectrum. Typical products for VSC turn the MOSFET on 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 VSC, 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, such as: 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 19, 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 valley 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, FSQ510H, and FSQ510M have minimum switching frequency of 94.3kHz and maximum switching frequency of 132kHz, typically, as shown in Figure 20. Tsmax=10.6µs Ts max=10.6µs tB=7.6µs Ts_A tB=7.6µs Ts_B tB=7.6µs Ts_C I DS I DS I DS I DS IDS I DS I DS Ids tB=7.6µs A B C D tW=3µs Figure 19. Advanced VS Operation 94.3kHz 132kHz 104kHz Bur st mode Con stant frequ ency D C B A P o When the resonant period is 2µs Figure 20. Switching Frequency Range of the Advanced Valley Switching |
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