15W Multi Output Offline Flyback Transformer

Next generation battery chargers used in industrial appliance applications such as power tools often require isolated AC/DC power supplies with high efficiency, low profile, low standby power and small form factor along with multiple outputs. Texas Instrument’s reference design PMP21927 takes a universal VAC input and creates three isolated 15 V outputs. The main output can be loaded up to 1 A and achieves over 86% efficiency. The other two isolated 15 V outputs can be loaded to 50 mA each. Using primary side regulation usually gives +/-5% regulation accuracy related to the output voltage and normally it is avoided when multiple outputs are involved. In this design, the low current outputs have LDO linear regulators to keep the voltages within 1% regardless of cross-loading conditions.

The flyback transformer is a principal component in such designs. Building a transformer is often an iterative process with many incremental changes that can affect the overall efficiency. In this application note, we will focus on the flyback transformer design with multi-isolated outputs used in PMP21927 and discuss the many design considerations taken to achieve high efficiency, adhere to UL60950 safety standard with reinforced insulation while maintaining a low profile part for power tool applications.

Download: ANP094