2 Centralized Isolated Bias Power Supply Architecture

In this architecture, a single-stage bias power supply architecture is used in which a bias power supply device is directly connected to the low-voltage battery. This connection supports a wide input voltage range and works in closed-loop operation. This kind of architecture can be realized using a single or multiple device depending on the power rating. A multi-winding transformer is used to give isolated output to the different gate drivers. The low-side gate drivers share the same ground can be supplied isolated bias power using the same transformer output winding.

Figure 2-1 shows how three isolated devices with multi-winding transformers are used for PFC, DC-DC primary, and DC-DC secondary stage isolated bias supply. For each stage, low-side gate drivers share the power supply from same output winding of the transformer; whereas, each high-side gate driver has a separate output winding of the transformer.

Figure 2-1 Centralized Architecture With Three Isolated Bias Power Devices

Figure 2-2 shows the use of two isolated devices with multi-winding transformers. From the first isolated bias power device, all five low-side gate drivers of the PFC and DC-DC Pri stage shares the power supply from the same output winding of the transformer since all of these are sharing the same ground. Each high-side gate driver has a separate output winding of the transformer. From the second isolated bias power supply device, the operation is similar to the previous case. There are six output windings in the transformer used for the PFC and DC-DC Pri stage. A higher number of transformer windings results in complex transformer design and increases the challenges in output regulation. Additionally, the load on one output winding, which is used to supply power to all five gate drivers of the low side is higher compared to other five windings used for each of the high-side switches separately.

Figure 2-2 Centralized Architecture Using Two Isolated Bias Power Devices

Figure 2-3 uses a single isolated device with multi-winding transformers. Nine output windings of the transformer is needed to supply the power from a single isolated bias device. Seven output windings of the transformer is used for seven high-side gate drivers, one winding for five low-side gate drivers of PFC and DC-DC Pri stage and one winding for two low-side gate drivers of the DC-DC Sec stage is used. The high number of transformer output winding results in a complex transformer design and increased challenges in output regulation. Also, the load on output windings of the transformer is not same; therefore, this factor must be accounted for during the transformer design. The PCB layout for routing the traces is complicated in this kind of bias power supply architecture since long traces need to be drawn from the transformer output to the gate drivers of the PFC and DC-DC stages. A controller device with external FET can be a better choice compared to a converter with an internal FET in the situation of delivering sufficient power required for gate drivers of the onboard charger.

Figure 2-3 Centralized Architecture Using Single Isolated Bias Power Devices

The following topologies and associated devices can be used as the preferred choice for the centralized bias power supply architecture:

• Flyback controller: LM5155x-Q1, LM5156x-Q1, LM34xx-Q1
• Flyback converter: LM518x-Q1, LM2518x-Q1
• Push-pull converter: SN6507-Q1

Different topologies for the isolated bias power supply come with certain advantages and trade-offs. A flyback device can help to achieve advantages like high efficiency, high load regulation, and high line regulation accuracy for a wide voltage input range. The tightly coupled flyback transformer design has low leakage inductance but this design comes with the trade-off of having comparatively higher parasitic capacitance across the isolation barrier of the transformer. Appropriate measures in the EMI filter design are sometimes needed to suppress the EMI and CMTI due to the parasitic capacitance of the transformer. The push-pull device provides good efficiency, high CMTI, lower EMI, and so forth. An extra inductor is needed in the output side to do the duty cycle control for wide input voltage range operation.