Bias power is very important in the power supply. Inside the power supply, a bias power supply is required to power up the IC. Outside the power supply, a bias power supply may be needed to power up the MCU and/or other local power supplies in the system. Many low-voltage ICs (with a nominal voltage less than 100V DC) integrate a bias supply into the IC. However, things will get more complicated with AC-DC power supplies because now much higher input voltages need to be handled. There are several ways to design a bias supply. Today, I will introduce three options for implementing bias power in AC-DC applications: linear, buck converters, or flyback converters.

Linear bias power supply

        The BJT linear circuit offers a simple bias power solution with the lowest component count. However, the main drawback of using this solution is its low efficiency.

Figure 1. BJT Linear Bias Supply

        In the example of Figure 1, to use a universal AC input (85VAC to 264VAC) for a 12V/50mA bias supply, a 2kΩ maximum resistor is required for R2. Even with a series resistor, the losses on the BJT still need to be compensated, which can be as high as 5W at an input of 264VAC.

Flyback bias power supply

        The flyback converter bias supply (PMP8764 in Figure 2) is a common bias supply solution that not only provides safety isolation but also provides multiple bias outputs.

Figure 2. Flyback bias power supply

        Due to primary side regulation (PSR) technology, the optocoupler and compensation circuitry can be removed from the flyback bias supply while maintaining 5% output regulation.

Buck bias power supply

        In certain applications such as electronic metering, isolation is not required, and custom transformers in flyback converters are too expensive for cost-sensitive markets. In this case, the buck converter (PMP9087 in Figure 3) is a better option to implement a low-cost biased AE power solution because of the use of a standard inductor. In addition, using a PSR controller (UCC28700 Series) also enables fast load transient response because the PSR controller has cycle-by-cycle voltage sensing.

Figure 3. Buck bias power supply

       For applications requiring a low output voltage (9V or lower) bias supply, the p-channel MOSFET buck converter (PMP5412 in Figure 4) is the perfect alternative to a flyback bias supply.