LTC Design Notes: Micropower Isolated Flyback Converter with Input VoltageRange from 6V to 100V
Flyback converters are widely used in isolated DC/DC applications because of their relative simplicity and low cost compared to alternative isolated topologies. Even so, designing a traditional flyback is not easy—the transformer requires careful design, and loop compensation is complicated by the well known right-half plane (RHP) zero and the propagation delay of the opto-coupler.
Linear Technology’s no-opto flyback converters, such as the LT3573, LT3574, LT3575, LT3511, and LT3512, simplify the design of flyback converters by incorporating a primary-side sensing scheme and running the converter in boundary mode. The LT8300 high voltage monolithic isolated flyback converter further simplifies flyback design by integrating a 260mA, 150V DMOS power switch, an internal compensation network and a soft-start capacitor. The LT8300 operates with input supply voltages from 6V to 100V and delivers output power of up to 2W with as few as five external components.
The LT8300 operates in boundary mode and offers low ripple Burst Mode operation, enabling the design of converters that feature high efficiency, low component count and minimal power loss in standby.
Simple and Accurate Primary-Side Voltage Sensing
The LT8300 eliminates the need for an opto-coupler by sensing the output voltage on the primary side when the output diode current drops to zero during the primary switch-off period. This greatly improves the load regulation since the voltage drop is zero across the transformer secondary winding and any PCB traces. This allows an LT8300-based flyback converter to produce ±1% typical load regulation at room temperature. Figure 1 shows the schematic and Figure 2 the load regulation curves of a flyback converter with a 5V output.