3 Options for Implementing Bias Supply in AC-DC Applications

“The bias supply is important in the power supply. Inside the power supply, a bias supply is required to power up the IC. External to the power supply, a bias supply may be required to power up the MCU and/or other local power supplies in the system.

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Author: Sheng-yang Yu1

The bias supply is important in the power supply. Inside the power supply, a bias supply is required to power up the IC. External to the power supply, a bias supply may be required to power up the MCU and/or other local power supplies in the system.

Many low-voltage ICs (rated less than 100V DC) have a bias supply integrated into the IC. For AC-DC power supplies, though, things get more complicated, as much higher input voltages now need to be handled.

There are several ways to design a bias supply. Today, I will introduce 3 options for implementing bias supply in AC-DC applications: linear, buck converter or flyback converter.

Linear Bias Supply

The BJT linear circuit provides a simple bias power solution with minimal component count. However, the main disadvantage of using this scheme is inefficiency.

Figure 1. BJT Linear Bias Supply

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

Flyback Bias Supply

A flyback converter bias supply, such as the PMP8764 in Figure 2, is a common bias supply solution that not only provides safe isolation, but also provides multiple bias outputs.

Figure 2. Flyback Bias Supply

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

Buck Bias 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, a buck converter, such as the PMP9087 in Figure 3, is a better option for a low-cost bias supply solution because standard inductors are used. In addition, fast load transient response can be achieved using the PSR controller (UCC28700 series) because of the cycle-by-cycle voltage sensing feature of the PSR controller.

Figure 3. Buck Bias Supply

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

More bias supply solutions can be found in the PowerLab reference design library, here are some design examples:

Low Power Flyback Bias Supply Solution:

PMP8968 ― 176VAC-263VAC input, 5.5V/250mA bias power
PMP9059 ― 90VAC-265VAC input, 15V/250mA bias power supply
PMP9066 ― 96VAC-768VAC input, 20V/550mA bias power supply
PMP9235 ― 90VAC-132VAC input, 5V/250mA bias power

Low Power Buck Bias Supply Solutions:

PMP5412 ― 120VAC-250VAC Input, 5V/0.75A Transformerless Bias Power Supply
PMP9087 ― 85VAC-265VAC input, 12V/200mA transformerless bias power supply
PMP9176 ― 50VAC-275VAC Input, 9V, 1.1W Transformerless Bias Power Supply