Datasheet LT3757, LT3757A (Analog Devices) - 18

link to page 18 LT3757/LT3757A
APPLICATIONS INFORMATION Flyback Converter: Switch Duty Cycle and Turns Ratio
According to the preceding equations, the user has rela- The flyback converter conversion ratio in the continuous tive freedom in selecting the switch duty cycle or turns mode operation is: ratio to suit a given application. The selections of the duty cycle and the turns ratio are somewhat iterative pro- VOUT N D cesses, due to the number of variables involved. The user = S • can choose either a duty cycle or a turns ratio as the start VIN NP 1−D point. The following trade-offs should be considered when where NS/NP is the second to primary turns ratio. selecting the switch duty cycle or turns ratio, to optimize Figure 8 shows the waveforms of the flyback converter the converter performance. A higher duty cycle affects the in discontinuous mode operation. During each switching flyback converter in the following aspects: period TS, three subintervals occur: DTS, D2TS, D3TS. • Lower MOSFET RMS current ISW(RMS), but higher During DTS, M is on, and D is reverse-biased. During MOSFET VDS peak voltage D2TS, M is off, and LS is conducting current. Both LP and • Lower diode peak reverse voltage, but higher diode LS currents are zero during D3TS. RMS current ID(RMS) The flyback converter conversion ratio in the discontinu- • Higher transformer turns ratio (N ous mode operation is: P/NS) The choice, VOUT N D = S • D 1 VIN NP D2 = D + D2 3 (for discontinuous mode operation with a given D3) gives VDS the power MOSFET the lowest power stress (the prod- uct of RMS current and peak voltage). However, in the high output voltage applications, a higher duty cycle may be adopted to limit the large peak reverse voltage of the ISW diode. The choice, D 2 = ISW(MAX) D + D2 3 I (for discontinuous mode operation with a given D3) D gives the diode the lowest power stress (the product of RMS current and peak voltage). An extreme high or low ID(MAX) duty cycle results in high power stress on the MOSFET DTS D2TS D3TS t or diode, and reduces efficiency. It is recommended to TS 3757 F08 choose a duty cycle, D, between 20% and 80%.
Figure 8. Waveforms of the Flyback Converter in Discontinuous Mode Operation
Rev. F 18 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts