Simplifying EV Power Design by Overcoming the Gate Driver Power Challenge
Power conversion systems in EVs follow the half-bridge configuration. This article explores the IGBT half-bridge design of a high-voltage side (output stage) of the gate driver.
EVs are all about power. Large battery packs deliver power to various power conversion systems through high voltages and currents, and the main DC-DC converter delivers power to the low-voltage systems in a vehicle. The traction inverter delivers mechanical power to the wheels. Finally, the battery charging system delivers power to the battery to start the whole process over again. Each system converts power from one form to another.
The Half-Bridge Configuration
At the heart of these systems lies one of the key building blocks of today’s power conversion systems: the half-bridge configuration. In this configuration, a high-side switch and a low-side switch rapidly toggle the load’s connections between the high-voltage positive and negative rails. Driving the gates of these switches is essential to maximizing efficiency by making them behave, as much as possible, like ideal switches. By understanding how power flows from gate drivers into the switching devices, the gate driver power can be architected to realize simplified board layouts, reduced costs, and easy reuse in future designs.
EV systems often refer to the high-voltage positive and negative rails as DC Link+ and DC Link–. Figure 1 shows a half-bridge circuit built from IGBT devices and one built using silicon carbide (SiC) FETs. To turn on an IGBT, the voltage from the gate to the emitter (VGE) must rise above a certain threshold.
Figure 1. Half bridges with isolated gate drivers and IGBT switching devices and SiC FET switching devices
Likewise, in the case of a SiC FET, this voltage appears from the gate to the source (VGS). For simplicity, the remainder of this article will refer to an IGBT half-bridge design; however, the principles discussed also apply to SiC FET designs. Figure 1 also shows isolated gate drivers. Due to the high voltages involved in many EV systems, isolation is often necessary to separate a low-voltage system controller from the high-voltage power stage. Isolated gate drivers bridge these two domains, allowing the system controller to control the IGBTs or SiC FETs of the power stage. Once again, for simplicity, the remainder of this article will only refer to the high-voltage side (output stage) of the gate driver.
To turn on an IGBT, the gate driver must raise the gate voltage to at least the VGE threshold and then provide enough current to charge the gate and fully turn on the IGBT. For the low-side gate driver connected to DC Link–, this is fairly simple. As shown in Figure 1, the gate driver’s output stage is tied to DC Link– as its ground and the positive rail of “Power Domain 2” for the output stage’s VDD. It then pulls the gate to VDD to turn on the low-side device. This works because VDD is referenced to DC Link–, which is tied to the emitter of the IGBT; so, a positive VGE is created. For the high-side gate driver, things are not so simple.
To create a positive VGE, the high-side gate driver’s ground must be connected to the emitter of the high-side IGBT. Without this connection, the gate driver is essentially floating with respect to the emitter of the high-side IGBT, and it cannot drive the gate. This also means that the high-side gate driver must be on a separate power domain. If it is connected to the same power domain as the low-side gate driver, the emitter of the high-side IGBT will be tied to DC Link– and break the half-bridge setup. Thus, the architecture of gate driver power domains, especially in systems with multiple half-bridge circuits, has a tremendous impact on system complexity.
Converter Topologies with Multiple Half-Bridge Configurations
Many complex converter topologies contain more than one half-bridge configuration. For example, motors used in the drivetrains of electric vehicles are typically three-phase motors where each phase is turned on and off to create motion. The traction inverter uses three half-bridge circuits to power each phase of the motor. With six power devices and gate drivers, carefully planning the gate driver power distribution has a major impact on performance. The three-phase inverter also illustrates the trade-offs for different power distribution configurations, which are also relevant to other systems using only one or two half-bridge circuits.
In a three-phase inverter, all the low-side devices share a common DC Link– connection to their emitter; so, the low-side gate drivers can all share a common power domain. Unfortunately, the high-side gate drivers have their emitters connected to the different phases of the system, so three separate power domains are required, as shown in Figure 2.
Figure 2. Three-phase system with a single DC-DC converter
Connecting the low-side drivers to a single power domain and then using a single DC-DC converter to generate all four power rails (also shown in Figure 2) is a common solution to this problem. However, this approach often leads to complex board layouts and long PCB traces, which can cause EMI issues in high-frequency systems. Achieving tight voltage regulation on all four output rails is also difficult when using a single DC-DC controller, and, finally, it can lead to noise from the high side coupling into the low side through the shared transformer. This is especially problematic in high-frequency SiC designs. A different approach involves breaking the DC-DC converter into multiple, independent DC-DC converters.
Breaking up the DC-DC converter into multiple independent DC-DC converters generally simplifies PCB layout, reduces trace lengths, and gives clean regulation to each output rail. It also greatly reduces noise between the power domains and allows SiC-based systems to achieve high switching frequencies and maximum efficiency. In addition, the independent DC-DC converter design can be reused in other half-bridge configurations with fewer switches, such as full-bridge systems.
Integrating DC-DC Controllers into Gate Drivers
Rather than using six independent DC-DC converters (one for each isolated gate driver), the system is typically broken up into four converters to reduce cost. As shown in Figure 3, some gate drivers, such as the Silicon Labs Si828x, 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. In many cases, this configuration strikes the right balance between complexity, cost, and performance.
Figure 3. Three-phase system using gate drivers with integrated DC-DC controllers and four independent power domains
Electric vehicles, and the power conversion systems they rely on, are here to stay. As demands for higher efficiency and longer range continue to grow, power systems will be pushed to achieve faster switching speeds, more complex topologies, and higher voltages. New power switch devices and advances in gate driver technology will push the efficiency of half-bridge circuits to new heights. However, even as the half-bridge circuit evolves, power domain architecture will remain a critical design consideration for years to come.
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