The LM5155 is a versatile non-synchronous low-side, N-FET controller for switching regulators. the common configurations for the LM5155 include boost regulators, flyback regulators and SEPIC regulators. This design guide focuses on how to configure and design the LM5155 as an isolated flyback regulator. The procedure is generic and focuses on selecting the correct components for stable flyback operation. The design example follows the application specifications of the LM5155EVM-FLY evaluation module, the results are presented in the LM5155EVM-FLY User's Guide. For typical applications the LM5155 Flyback Controller Quick Start Calculator can be used to efficiently complete the calculations described in this report.
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This design guide follows typical design procedures and calculations to implement an isolated nonsynchronous flyback controller operating in continuous conduction mode. The design example uses an unregulated 24V rail (18V-36V) to produce a regulated 5 V of up to 4 A load current. A non-isolated auxiliary winding of 10V is selected to power the LM5155, lowering power dissipation in the LM5155. The switching frequency of 250kHz is selected to help minimize switching losses and conduction losses of the switching MOSFET. Section 3 details the component selection based on the general design parameters shown in Table 2-1.