Page 12 - PEN eBook May 2022

Cover Story – Design Cover Story – Design

TOPOLOGIES WITH BIDIRECTIONAL POWER FLOW ENABLED BY

WBG DEVICES
Let’s start with one of the most widely used configurations in a single AC phase for bidirectionality,

allowing the vehicle-to-grid (V2G)/vehicle-to-load (V2L)-AC load functions. For a 3.6-kW power-
class solution, a single phase for the PFC topology is enough. However, for the 7.2-kW power
class, interleaving phases with the totem pole are recommended to keep the proper efficiency and
thermal management. The HV/HV DC/DC converter can be realized with either CLLC or dual-active–
bridge topologies. The selection depends on the desired peak/overall efficiency throughout the load
as well as the preferred controllability.

In terms of power semiconductors, the recommendation for each topology is depicted in Figure 7. The
proper selection of the WBG technology depends on each customer’s value drivers, like efficiency,

power density, cost, system requirements, and the selected topology. It is worth mentioning that if
the OBC in discharging mode (i.e., vehicle-to-everything, or V2X) works as a voltage source with a
power factor equal to 1, then the slow leg (Q5 and Q6) can be populated with silicon superjunction (SJ)
transistors. But if the OBC needs to handle reactive power (power factor does not equal 1), then the
slow leg must be populated with WBG transistors, as hard-commutation events are bound to happen.

Based on the topology shown in Figure 7, keeping Q5 fully off and Q6 fully on makes it possible to

enable the vehicle-to-vehicle (V2V), V2L-DC load, and vehicle-to-DC (V2DC)-microgrid options. In
Figure 5: A gap filler is the preferred thermal bonding approach, with the addition of an insulator for HV applications.
this case, the front-end converter works as an interleaved buck converter.

By sharing the same power circuit, both bidirectional AC/DC and bidirectional DC/DC power
transmission can be realized efficiently and conveniently without additional devices and costs.

The configuration shown in Figure 7 can be the building block for a three-phase AC system. In
other words, each building block is connected to each phase of the AC grid and has the secondary
sides of the HV/HV DC/DC converters tied together. This approach makes it possible to get 11-kW

(3× 3.6 kW) and 22-kW (3× 7.2 kW) OBC designs.

Another attractive and simpler approach could be made by combining WBG devices for three-phase
AC systems, as shown in Figure 8. In this configuration, there are two possible scenarios:

1. Depending on the AC grid supply configuration and consequently the DC bus voltage at the
output of PFC (in this base, the B6/voltage-source converter), the HV/HV DC/DC converters

can be connected in series (for a three-phase input) or in parallel (for single-phase input). The
purpose of this is to keep the same input-to-output voltage-conversion ratio of the transformers.

2. Depending on the power density, thermal management, and efficiency requirements of the OBC
system, the HV/HV DC/DC converter can be either in series or parallel on the primary side.
Figure 6: The TSC-capable QDPAK offers several manufacturability/assembly benefits.

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