May 2020

28 POWER CONVERTER DESIGN www.ti.com Issue 2 2020 Power Electronics Europe www.power-mag.com The Landscape of Isolated DC/DC Bias Power Supplies Moving signals and power across an isolation barrier is a common challenge for designers. Isolation might be required for safety, noise immunity or large potential differences between system domains. Understanding system-level specifications like the number of outputs, regulation requirements, output power, insulation rating, operating temperature and input voltage range are critical. From there, designers can derive the lowest-cost solution that meets all of the system requirements. Ryan Manack, Business Lead, Texas Instruments, USA For example, a cellphone charger is internally isolated to prevent humans from becoming electrically tied to the mains if the connector short-circuits. In factory robots, sensitive control circuitry sits on a separate ground and is isolated from the motors that draw large DC currents and create noise and ground bounces. Communication and sensing are commonly implemented across an isolation barrier. Traction inverters and motor drives take a PWM signal from the motor controller and pass it through an isolator to tell the gate driver to turn an power transistor on or off. Isolated bias converters enable isolated communication and sensing by providing bias power from one side of the isolation barrier to the other. Current and voltage sensors, digital isolators and gate drivers typically require less than 15 W and as little as tens of milliwatts of power. Figure 1 shows an example of each of these applications. Isolated DC/DC bias supply requirements There are many solutions that can provide isolated bias power, from controllers, which have external power switches, to converters, which integrate a controller with power switches, to finally power modules, which integrate controllers, power switches and transformers in one package. Because of this wide variety of bias power solutions and diverse applications that they go into, it is important to fully understand the application requirements in order to meet the specifications at the lowest cost. At a minimum, the designer should understand the bias supply input voltage range, the output voltage and the output power requirements. Some applications will require more than one bias voltage so it is important to define the acceptable regulation for each output. System requirements such as insulation rating, Figure 1: Isolated bias applications Figure 2: Simplified schematic of the flyback converter