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High-voltage IC technology for bias supply design

Posted: 13 Apr 2015 ?? ?Print Version ?Bookmark and Share

Keywords:bias supply? radio frequency? AC/DC? DC/DC switching converter? MOSFET?

Nowadays, many offline-powered systems require a low-voltage bias supply to power their 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 60Hz 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 [1]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 summarised in the table. The priority and importance of each feature listed is highly specific to application and equipment.

Table: Typical bias-supply requirements and system benefits.

The classic linear regulator, also known as an low-dropout (LDO) solution (figure 1) presents an easy-to-use solution. The front-end, implemented with a high-voltage depletion-mode MOSFET, handles the majority of voltage drop and power dissipation, allowing the downstream stage to employ standard low-voltage, low-power LDO regulators. The regulated bias output VOUT can be easily modified for other output voltages by changing the LDO rating.

Power dissipation in Q1

Albeit simple, one of the key limitations with this design is power dissipation in Q1, which can be calculated using equation (1), where VD can be approximated to the RMS value of the input voltage and IZ is the sum of the bias currents for the zener D1 and U1. A typical 230V AC input design with 10V/50 mA output results in approximately 10W power dissipation in Q1 alone. This yields a very poor conversion efficiency and large heat-sinking to dissipate this power.

Figure 1: Linear drop bias supply.


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