The half-bridge configuration is used in EV power conversion systems. This article delves into the design of the power conversion using IGBT and its gate driver.
Power is everything in EVs. Large battery packs provide high voltages and currents to various power conversion systems, while the main DC-DC converter provides power to the vehicle’s low-voltage systems. The mechanical power to the wheels is supplied by the traction inverter.
Finally, the battery charging system supplies power to the battery, restarting the entire process. Power is converted from one form to another by each system.
The Half-Bridge Configuration
The half-bridge configuration is at the heart of these systems, and it is one of the key building blocks of today’s power conversion systems. A high-side switch and a low-side switch rapidly toggle the load’s connections between the high-voltage positive and negative rails in this configuration.
Driving the gates of these switches is critical to maximizing efficiency by making them behave as close to ideal switches as possible. Understanding how power flows from smart gate drivers into switching devices allows designs to achieve simplified board layouts, lower costs, and easy reuse in future designs.
The high-voltage positive and negative rails are frequently referred to in EV systems as DC Link+ and DC Link–. Figure 1 depicts a half-bridge circuit made of IGBTs and another made of SiC FETs. The voltage from the gate to the emitter (VGE) must exceed a certain threshold in order for an IGBT to turn on.
In the case of a SiC FET, this voltage also appears from the gate to the source (VGS). The remainder of this article will refer to an IGBT half-bridge design for ease of understanding; however, the principles discussed also apply to SiC FET designs. Isolated gate drivers are also depicted in Figure 1.
Due to the high voltages present in many EV systems, isolation is frequently required to separate a low-voltage system controller from a high-voltage power stage. Isolated smart gate drivers act as a bridge between these two domains, allowing the system controller to control the power stage’s IGBTs or SiC FETs. To keep things simple, the rest of this article will only cover the gate driver’s high-voltage side (output stage).
To turn on an IGBT, the gate drive optocoupler must first raise the gate voltage to at least the VGE threshold, and then provide enough current to charge the gate and turn on the IGBT fully. This is fairly straightforward for the low-side gate driver connected to DC Link.
The gate driver’s output stage, as shown in Figure 1, is connected to DC Link– as its ground and the positive rail of “Power Domain 2” for the output stage’s VDD. The gate is then pulled to VDD to turn on the low-side device. This works because VDD is linked to DC Link–, which is connected to the IGBT’s emitter, resulting in a positive VGE. Things aren’t so simple for the high-side gate driver.
The ground of the high-side gate driver must be connected to the emitter of the high-side IGBT in order to generate a positive VGE. Without this connection, the gate driver is essentially floating with respect to the high-side IGBT’s emitter, and it cannot drive the gate. This also implies that the high-side gate driver must be in its own power domain.
If it is connected to the same power domain as the low-side gate driver, the high-side IGBT’s emitter will be tied to DC Link– breaking the half-bridge configuration. As a result, the architecture of gate driver power domains has a significant impact on system complexity, especially in systems with multiple half-bridge circuits.
Topologies of Converters with Multiple Half-Bridge Configurations
In many complex converter topologies, there are multiple half-bridge configurations. Motors used in electric vehicle drivetrains, for example, are typically three-phase motors with each phase turned on and off to produce motion.
To power each phase of the motor, the traction inverter uses three half-bridge circuits. The gate drive optocoupler power distribution has a significant impact on performance with six power devices and gate drivers. The three-phase inverter also demonstrates the trade-offs for various power distribution configurations, which are applicable to other systems that use only one or two half-bridge circuits.
All low-side devices in a three-phase inverter share a common DC Link– connection to their emitter, allowing the low-side gate drivers to share a common power domain. Unfortunately, because the high-side gate drivers’ emitters are connected to different phases of the system, three separate power domains are required, as shown in Figure 2.
A common solution to this problem is to connect the low-side drivers to a single power domain and then use a single DC-DC converter to generate all four power rails (as shown in Figure 2). This approach, however, frequently results in complex board layouts and long PCB traces, which can cause EMI issues in high-frequency systems.
It is also difficult to achieve tight voltage regulation on all four output rails when using a single DC-DC controller, and it can result in noise from the high side coupling into the low side via the shared transformer. This is particularly troublesome in high-frequency SiC designs. Another approach is to divide the DC-DC converter into multiple, independent DC-DC converters.
Dividing the DC-DC converter into multiple independent DC-DC converters simplifies PCB layout, reduces trace lengths, and provides clean regulation to each output rail.
It also significantly reduces noise between power domains, allowing SiC-based systems to achieve high switching frequencies while remaining efficient. Furthermore, the design of the independent DC-DC converter can be reused in other half-bridge configurations with fewer switches, such as full-bridge systems.
DC-DC Controller Integration in Gate Drivers
To save money, the system is typically divided into four converters rather than six independent DC-DC converters (one for each isolated gate driver).
Some gate drivers, such as the ACPL-302J, integrate the DC-DC controller to further reduce cost and board space, and offer the same gate driver with and without an integrated DC-DC controller, as shown in Figure 3. This configuration strikes the right balance between complexity, cost, and performance in many cases.
Electric vehicles, as well as the power conversion systems upon which they rely, are here to stay. Power systems will be pushed to achieve faster switching speeds, more complex topologies, and higher voltages as demands for higher efficiency and longer range continue to grow.
New power switch devices and advancements in gate drive optocoupler technology will push half-bridge circuit efficiency to new heights. Even as the half-bridge circuit evolves, power domain architecture will continue to be an important design consideration for many years to come.