Datasheet LT3759 (Analog Devices) - 17

LT3759
APPLICATIONS INFORMATION Flyback Converter: Switch Duty Cycle and Turns Ratio
According to the preceding equations, the user has relative The flyback converter conversion ratio in the continuous freedom in selecting the switch duty cycle or turns ratio to mode operation is: suit a given application. The selections of the duty cycle and the turns ratio are somewhat iterative processes, due VOUT = NS • D to the number of variables involved. The user can choose VIN NP 1−D either a duty cycle or a turns ratio as the start point. The following trade-offs should be considered when select- where NS/NP is the second to primary turns ratio. ing the switch duty cycle or turns ratio, to optimize the Figure 7 shows the waveforms of the flyback converter 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 = NS • D D VIN NP D2 = 1 D+D2 3 (for discontinuous mode operation with a given D3) gives V the power MOSFET the lowest power stress (the product DS 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 diode. The choice, ISW D = 2 ISW(MAX) D+D2 3 (for discontinuous mode operation with a given D3) gives ID the diode the lowest power stress (the product of RMS current and peak voltage). An extreme high or low duty ID(MAX) cycle results in high power stress on the MOSFET or diode, and reduces efficiency. It is recommended to choose a DTS D2TS D3TS t T duty cycle, D, between 20% and 80%. S 3759 F07
Figure 7. Waveforms of the Flyback Converter in Discontinuous Mode Operation
3759fc For more information www.linear.com/3759 17 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Related Parts