Why Isolation is Critical in the Auxiliary Power Module
When under medium to heavy loading, APM may be in the 2 to 3 kW range with expected efficiency in the 92-95% range. The return/ground for the low-voltage side is normally connected to the vehicle chassis, so galvanic isolation between the LV side and HV side is required for safety, as well as for protection of the LV controller. Isolation between the HV input and the LV output of the converter is accomplished using a transformer. Switching frequencies can be relatively high (e.g. 100 kHz) to keep transformer magnetics sizes minimal.
In some applications like a hybrid electric vehicle (HEV), the dc-dc can be run in reverse to boost the power from a 12 V domain to a 48 V or HV domain, to assist in starting or to provide backup power.
The APM depicted uses a full bridge on the high-voltage primary side of the transformer, and a full-bridge synchronous rectifier on the low-voltage secondary side of the transformer. The HV switches are often implemented as fast IGBTs (as shown) or high-voltage Silicon Carbide (SiC) MOSFETs; the HV switches on the primary side of the transformer require isolated gate drivers. The gate drive for these switches uses isolated gate drivers, for instance Si8239x,
Si828x. The latest dc-dc converters are implemented using SiC MOSFETs to increase power density and reduce size.
The converter may be unidirectional, supplying energy only from the high-voltage domain to the low-voltage domain, or it may be bidirectional. While a unidirectional dc-dc can use diodes instead of switches on the LV side, a bidirectional module requires switches. However, gate drivers for the low-voltage switches generally do not need to be isolated.
The APM controller will also need to sense the input and output voltages of the converter to provide closed-loop control. The Si8931 isolated voltage sensor is well-suited for this purpose.
The entire system can be monitored and controlled via an automotive bus such as CAN (“Comm” in the diagrams). The CAN is isolated with digital isolators like Si86xx and digital isolators with integrated dc-dc power converters like Si88xx.