This 12V/5W reference design is optimized for very low cost and yet maintains good efficiency. Two key factors for minimizing cost are a BJT is used for the main power switch, and primary-side regulation eliminates the need for an optocoupler. The UCC28720 quasi-resonant flyback controller is able to implement both of these features. With quasi-resonant switching, efficiency at full load is over 80%, even with a BJT switch. Light-load losses are kept below 100mW due to burst mode operation.