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LT8331 Datasheet(HTML) 17 Page  Analog Devices 

LT8331 Datasheet(HTML) 17 Page  Analog Devices 
LT8331 17 Rev. C For more information www.analog.com APPLICATIONS INFORMATION the converter performance. A higher duty cycle affects the flyback converter in the following aspects: n Lower switch RMS current ISW(RMS), but higher switch VSW peak voltage n Lower diode peak reverse voltage, but higher diode RMS current ID(RMS) n Higher transformer turns ratio (NP/NS) It is recommended to choose a duty cycle between 20% and 80%. Flyback Converter: Maximum Output Current Capability and Transformer Design The maximum output current capability and transformer design for continuous conduction mode (CCM) is chosen as presented here. The maximum duty cycle (DMAX) occurs when the con verter has the minimum VIN: DMAX = VOUT â€¢ NP NS ï£« ï£ ï£¬ ï£¬ï£¬ ï£¶ ï£¸ ï£· ï£·ï£· VOUT â€¢ NP NS ï£« ï£ ï£¬ ï£¬ï£¬ ï£¶ ï£¸ ï£· ï£·ï£· + VIN(MIN) Due to the current limit of its internal power switch, the LT8331 should be used in a flyback converter whose maximum output current (IO(MAX)) is: IO(MAX) â‰¤ VIN(MIN) VOUT â€¢ DMAX â€¢ 0.5A âˆ’ 0.5 â€¢ Î”ISW ( ) â€¢ Î· where Î· (< 1.0) is the converter efficiency. Minimum possible inductor value and switching frequency should also be considered since they will increase inductor ripple current âˆ†ISW. The transformer ripple current âˆ†ISW has a direct effect on the design/choice of the transformer and the converterâ€™s output current capability. Choosing smaller values of âˆ†ISW increases the output current capability, but requires large primary and secondary inductances and reduces the cur rent loop gain (the converter will approach voltage mode). Accepting larger values of âˆ†ISW allows the use of low primary and secondary inductances, but results in higher input current ripple, greater core losses, and reduces the output current capability. It is recommended to choose a âˆ†ISW of approximately 0.2A to 0.3A. Given an operating input voltage range, and having cho sen the operating frequency and ripple current in the pri mary winding, the primary winding inductance can be calculated using the following equation: L = VIN(MIN) Î”ISW â€¢ fOSC â€¢ DMAX The primary winding peak current is the switch current limit (maximum 0.7A). The primary and secondary maxi mum RMS currents are: ILP(RMS) â‰ˆ POUT(MAX) DMAX â€¢ VIN(MIN) â€¢ Î· ILS(RMS) â‰ˆ IOUT(MAX) 1 âˆ’ DMAX Based on the preceding equations, the user should design/ choose the transformer having sufficient saturation and RMS current ratings. Flyback Converter: Snubber Design Transformer leakage inductance (on either the primary or secondary) causes a voltage spike to occur after the MOSFET turnoff. This is increasingly prominent at higher load currents, where more stored energy must be dis sipated. In some cases a snubber circuit will be required to avoid overvoltage breakdown at the MOSFETâ€™s drain node. There are different snubber circuits (such as RC snubber, RCD snubber, etc.) and Application Note 19 is a good reference on snubber design. An RCD snubber is shown in FigureÂ 6. The snubber resistor value (RSN) can be calculated by the following equation: RSN = 2 â€¢ V2SN âˆ’ VSN â€¢ VOUT â€¢ NP NS I2SW(PEAK) â€¢ LLK â€¢ fOSC 
