Many offline-powered systems today need a low-voltage bias supply to power its control circuitry and intelligence. In some cases, bias supply outputs are not required to be isolated from the AC mains, but are used to power a system microcontroller, LED display, drive relays or AC switches. Some examples include home automation, e-metering, standby power supplies in TVs, home appliances, and many others.
End equipments have become more power-hungry with time as more and more “smarts” and functionalities are being added. Electricity meters, for instance, have transformed from simple metrology with just energy measurement and mechanical displays, to smart e-meters that incorporate radio frequency (RF) communication such as Wi-Fi, ZigBee® and/or power line communication (PLC), LCD displays, AC disconnect relays, and so on. Consequently, their AC/DC power supply requirements have transformed from a single output rail with a few mA to multiple rails with hundreds of mA.
There are a few different topology options to consider for bias-supply designs. The classic 60-Hz step-down transformer and AC capacitive-drop solutions are both well-known and robust solutions. However, they fall short when it comes to efficiency, size and standby power performance. Similarly, an isolated-flyback switch-mode power supply would be far too complex and expensive to design for this need.
Equipment power consumption regulations  have further driven the need for high efficiency bias-supply designs. Bias supplies with very good light-load efficiency are required to enable more active system functions in standby mode, while keeping the total end equipment consumption to a minimum. In this article we discuss the key requirements and challenges in designing off-line, non-isolated bias supplies and how high-voltage IC technology can help simplify such designs.
Let’s first discuss some basic requirements and key attributes when it comes to designing off-line bias supplies. Some features and system benefits are summarized in Table 1. The priority and importance of each feature listed is highly specific to application and equipment.