Isolated bias supply provides power to the different gate-driver circuits in HEVs and EVs. There are different topologies to design an isolated bias power supply. The most commonly used topologies are flyback, push-pull, LLC-resonant, and integrated transformer modules. Each topology provides specific advantages but at the same time has trade-offs and challenges. The choice of the topology depends largely on the overall architecture of the isolated bias power supply. Different Si, SiC, GaN, IGBT switches (and possibly others) need different input voltage range for gate-source voltage. Therefore, isolated bias power supply architecture and selection of devices also depend on the switches used in the design.

Isolated bias power supplies take power either from the low-voltage battery or from the high-voltage battery of the HEV or EV. Based on the power source, the isolated bias power supplies can be divided in two groups: low-voltage isolated bias power supplies and high-voltage isolated bias power supplies. The isolated bias supply circuit can be directly connected to the battery or connected to the battery using the pre-regulators. The pre-regulators are needed depending on the wide input voltage range capability of the device. Although low-voltage batteries are common as a power source for isolated bias power supplies, sometimes both low-voltage and high-voltage batteries are used to provide redundancy in the system. A redundant power supply can lead to achieve higher functional safety of an overall system.

Figure 1-1 shows a generic onboard charger circuit with PFC, DC-DC primary, and DC-DC secondary stages. The switches are named as PFC_HS_1, Pri_HS_1, Sec_HS_1, and so forth. The first part of this nomenclature indicates that whether the switch belongs to PFC or DC-DC primary or DC-DC secondary stage of the onboard charger. The second part shows whether the circuit uses a high-side or low-side switch. The third part shows the switch number of the high or low side. In the same fashion, consider the same nomenclature for the gate driver of each of these switches and the isolated bias power supply to the gate drivers. In the figures in this document, this nomenclature is used to describe the different isolated bias power supply architecture.

Figure 1-1 Onboard Charger Generic Circuit

Low-voltage isolated bias power supply circuits usually have a 12V battery as a power source in HEVs and EVs. Although there are some systems with 48V as a low-voltage battery, this paper focuses on the 12V battery system. However, these architectures can be still relevant for 48V low-voltage battery designs. In that case, one option is to have a converter to lower the voltage to use the same devices or another option is to have devices supporting an input voltage range designed for a 48V battery.

Considering the state of charge (SOC) of the 12V low-voltage battery, the wider input voltage range needs to be supported by the isolated bias power supply (as an example: 8V–16V). In case of cold crank and load dump scenarios, the input voltage range requirement goes further down and up, respectively. There can be differences in this wide input voltage range of a 12V low-voltage battery depending on the OEM. Not all types of topologies and the associated devices can support this wide input voltage range. Therefore, in several designs a pre-regulator is needed between low-voltage battery and isolated bias power supply to regulate the input voltage for the isolated bias power supply device.