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LT8331 데이터시트(PDF) 16 Page - Analog Devices |
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LT8331 데이터시트(HTML) 16 Page - Analog Devices |
16 / 30 page LT8331 16 Rev. C For more information www.analog.com APPLICATIONS INFORMATION Figure 7. Waveforms of the Flyback Converter in Discontinuous Mode Operation 8331 F07 ISW VSW ID t DTS D2TS D3TS ID(MAX) TS smaller transformer inductances and easy loop compen- sation, and the disadvantage of higher peak-to-average current and lower efficiency. Flyback Converter: Switch Duty Cycle and Turns Ratio The flyback converter conversion ratio in the continuous mode operation is: VOUT VIN = NS NP • D 1 − D where NS/NP is the second to primary turns ratio. D is duty cycle. Figure 7 shows the waveforms of the flyback converter in discontinuous mode operation. During each switching period TS, three subintervals occur: DTS, D2TS, D3TS. During DTS, M is on, and D is reverse-biased. During D2TS, M is off, and LS is conducting current. Both LP and LS currents are zero during D3TS. The flyback converter conversion ratio in the discontinu- ous mode operation is: VOUT VIN = NS NP • D D2 According to Figure 6, the peak SW voltage is: VSW(PEAK) = VIN(MAX) + VSN where VSN is the snubber capacitor voltage. A smaller VSN results in a larger snubber loss. A reasonable VSN is 1.5 to 2 times of the reflected output voltage: VSN = k • VOUT • NP NS k = 1.5 ~ 2 According to the Absolute Maximum Ratings table, the SW voltage Absolute Maximum value is 140V. Therefore, the maximum primary to secondary turns ratio (for both the continuous and the discontinuous operation) should be: NP NS ≤ 140V − VIN(MAX) k • VOUT According to the preceding equations, the user has rela- tive freedom in selecting the switch duty cycle or turns ratio to suit a given application. The selections of the duty cycle and the turns ratio are somewhat iterative pro- cesses, due to the number of variables involved. The user can choose either a duty cycle or a turns ratio as the start point. The following trade-offs should be considered when selecting the switch duty cycle or turns ratio, to optimize Figure 6. A Simplified Flyback Converter NP:NS VIN CIN CSN VSN LP D SUGGESTED RCD SNUBBER ID ISW 8331 F06 GND SW LT8331 LS + – RSN DSN + – + VOUT COUT + |
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