The auxiliary power system is typically controlled by a dedicated module, called the Auxiliary Power Module (APM). This is effectively a dc-dc converter that takes the high voltage (HV) from the traction battery and converters to a low voltage (LV). This LV bus supplies the auxiliary systems and charges the 12 V battery. Initially, this may seem like a relatively simple function, however the need for galvanic isolation introduces additional complexity.
Many DC-DC converter topologies use a transformer to provide both voltage step-down and galvanic isolation in a single step. While this is an effective way of isolating the HV and LV circuits, it does require additional conversion steps in order to make use of the transformer. Specifically, the HV voltage needs to be converted from DC to AC and then the LV needs to be converted from AC back to DC. The circuit diagram in Figure 2 shows a common full bridge implementation.
Figure 2: Circuit diagram of an APM (Source: Silicon Labs)
The full bridge converts the DC voltage into an AC voltage, so it can excite the primary side of the insulating transformer and induce a current in the secondary side. The secondary side AC voltage then needs to be converted back to DC. In order to use smaller magnetic components, and reduce the size and weight of the end solution, many systems use a switching frequency of 100 kHz or higher.