A hybrid integrated circuit in a wafer level package for an implantable medical device includes one or more passive component windings formed, at least in part, along one or more routing layers of the package. The windings may be primary and secondary windings of a transformer, wherein all or part of a magnetic core thereof is embedded in a component layer of the wafer level package. If the core includes a part bonded to a surface of the package, that part of the core may be E-shaped with legs extending into the routing layers, and, in some cases, through the routing layers. Routing layers may be formed on both sides of the component layer to accommodate the transformer windings, in some instances.