Safe, Reliable Designs for Non-Isolated Offline Switchers with 400VDC Rated Power Inductors


Although the majority of AC offline converters and switching converters that run from hazardous voltage (> 60 VDC, as defined by safety standards such as IEC-60950) include galvanic isolation for safety, there are many applications where non-isolated circuits can be used. Where mechanical isolation protects the user and service personnel from contact with hazardous voltages or for where both input and output voltage are on the primary side of isolated converters the buck regulator and buckboost regulators are often more efficient, use less PCB area and cost less to implement than flyback regulators. Flyback regulators are without a doubt the least expensive option for isolated circuits, and can of course be non-isolated as well. However, even at power levels of 5W and below the flyback still requires a coupled inductor (often called “flyback transformer”) with varying turns ratios depending upon input voltage, output voltage, the range of output current and the switching frequency. Würth Elektronik eiSos offers several families of off-the-shelf flyback transformers, but when the circuit does not require isolation the buck and buck-boost become attractive options because they can use the much wider selection of off-the-shelf power inductors which are often more affordable.