2 Selecting Devices

In general for automotive high voltage input flyback power supply designs, all component ratings should be optimized based on minimizing cost, without violating creep age and clearance rules. For example, a single larger bulk input capacitor might work, but it may be cheaper or smaller to use multiple smaller capacitors. Large components also can have larger parasitics, which can make complying with EMI regulations more difficult.

Many factors affect the design and selection of magnetic components like transformer and EMI filter inductor. Magnetics are especially critical for isolated topologies, since they play a significant role in determining the overall system performance. First, the components must be designed to accommodate both peak and root mean squared currents with sufficient margin and be qualified for isolation sufficient for automotive products. Then, shape and core types must be considered to optimize flux density. Finally, winding and termination structure must be chosen. Magnetics design is a complex and iterative process. It’s common and recommended to work alongside an external vendor that can provide custom solutions.

The maximum input voltage must be considered when selecting the power device. For 400-V battery systems, a Silicon or Gallium Nitride FET can be used. For 800-V battery systems, Silicon Carbide (SiC) MOSFETS are becoming more popular due to their fast switching and high voltage rating (typically 900 V or greater). In either case having appropriate drive strength ensures reliability over lifetime.

The Silicon MOSFET driver integrated in the flyback controller may not be able to drive a SiC MOSFET directly. In these cases, external gate driver circuitry, or a low-side gate driver like the UCC27531A-Q1, are recommended.

Careful consideration must be given to the UVLO turn-off threshold of the flyback controller. For SiC applications, a UVLO turn-off of greater than 8 V is recommended to provide increased lifetime reliability of the SiC MOSFET. Drive voltages less than 8 V can cause the power device to operate in the saturation region, which results in high conduction losses and heating. However, silicon MOSFETs can typically support drive voltages below 8 V.