3 Semi-distributed Isolated Bias Power Supply Architecture

In semi-distributed architecture, a two-stage bias power supply architecture is used. At the first stage, a wide input voltage range device is used to generate regulated voltage rails. At the second stage, other devices are used to provide isolated bias power to the gate drivers. In this case, not only a closed loop device but an open loop device can also be used because of available regulated voltage rail as an output of first stage. A common occurrence is that the device used at the first stage also generates other required voltage rails for supplying power to microcontrollers, sensors, isolators (and so forth) of the onboard charger circuit. Depending on the requirements, at the first stage an isolated (flyback or push-pull) or non-isolated (SEPIC or buck-boost) topology can be chosen.

Figure 3-1 Semi-distributed Isolated Bias Power Supply Architecture

For the first stage, flyback and push-pull devices can be used as mentioned in the centralized architecture section for the choice of isolated topology. For non-isolated topologies, SEPIC and buck-boost converters can be selected. For the second stage, a closed loop or open loop isolated bias power device can be chosen. The following topologies and associated devices can be used as the preferred choice for the semi-distributed bias power supply architecture:

• LLC resonant converter: UCC25800-Q1
• Flyback controller: LM5155x-Q1, LM5156x-Q1, LM34xx-Q1
• Flyback converter: LM518x-Q1, LM2518x-Q1
• Push-pull converter: SN6507-Q1, SN6505-Q1
• SEPIC: LM5155x-Q1, LM5156x-Q1, LM5157x-Q1, LM5158x-Q1
• Buck-boost : TPS55287x-Q1, LM51xx-Q1

The UCC25800-Q1 is a transformer driver device based on LLC resonant open loop operation to generate isolated bias power. The device provides several benefits including good efficiency, low EMI, high CMTI and so forth. Due to open-loop operation, a regulated voltage rail is preferred for this device. As the leakage inductance in an LLC is a component of the power train, the topology can enable a higher leakage inductance transformer to be used with an associated reduction in the parasitic primary-secondary capacitance across the isolation barrier of the transformer. These features help to better EMI performance and higher CMTI. Using the advantage of high CMTI capability, the LLC resonant topology can be an excellent choice for an onboard charger design using GaN switches with high slew rate and high-frequency operation.