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LT8331 bảng dữ liệu(PDF) 19 Page - Analog Devices |
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LT8331 bảng dữ liệu(HTML) 19 Page - Analog Devices |
19 / 30 page LT8331 19 Rev. C For more information www.analog.com APPLICATIONS INFORMATION In a SEPIC converter, no DC path exists between the input and output. This is an advantage over the boost converter for applications requiring the output to be disconnected from the input source when the circuit is in shutdown. SEPIC Converter: Switch Duty Cycle and Frequency For a SEPIC converter operating in CCM, the duty cycle of the main switch can be calculated based on the output voltage (VOUT), the input voltage (VIN) and the diode for- ward voltage (VD). The maximum duty cycle (DMAX) occurs when the con- verter operates at the minimum input voltage: DMAX = VOUT + VD VIN(MIN) + VOUT + VD Conversely, the minimum duty cycle (DMIN) occurs when the converter operates at the maximum input voltage: DMIN = VOUT + VD VIN(MAX) + VOUT + VD Be sure to check that DMAX and DMIN obey: DMAX < 1 – Minimum Off-Time(MAX) • fOSC(MAX) and DMIN > Minimum On-Time(MAX) • fOSC(MAX) where Minimum Off-Time, Minimum On-Time and fOSC are specified in the Electrical Characteristics table. SEPIC Converter: The Maximum Output Current Capability and Inductor Selection As shown in Figure 8, the SEPIC converter contains two inductors: L1 and L2. L1 and L2 can be independent, but can also be wound on the same core, since iden- tical voltages are applied to L1 and L2 throughout the switching cycle. For the SEPIC topology, the current through L1 is the converter input current. Based on the fact that, ideally, the output power is equal to the input power, the maximum average inductor currents of L1 and L2 are: IL1(MAX)(AVE) =IIN(MAX)(AVE)=IO(MAX) • DMAX 1 − DMAX IL2(MAX)(AVE)= IO(MAX) In a SEPIC converter, the switch current is equal to IL1 + IL2 when the power switch is on, therefore, the maximum average switch current is defined as: ISW(MAX)(AVE) = IL1(MAX)(AVE)+ IL2(MAX)(AVE) =IO(MAX) • 1 1 − DMAX and the peak switch current is: ISW(PEAK) = 1 + c 2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ • IO(MAX) • 1 1 − DMAX The constant c in the preceding equations represents the percentage peak-to-peak ripple current in the switch, relative to ISW(MAX)(AVE), as shown in Figure 9. Then, the switch ripple current ∆ISW can be calculated by: ∆ISW = c • ISW(MAX)(AVE) The inductor ripple currents ∆IL1 and ∆IL2 are identical: ∆IL1 = ∆IL2 = 0.5 • ∆ISW Figure 9. The Switch Current Waveform of the SEPIC Converter 8331 F09 ∆ISW = χ•ISW(MAX)(AVE) ISW t DTS ISW(MAX)(AVE) TS |
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