Power Electronics Europe Issue 4 - November 2022

6 GATE DRIVERS https://www.allegromicro.com/en/ Issue 4 2022 Power Electronics Europe www.power-mag.com Why gate drivers are key to successful electric vehicle designs By Allegro MicroSystems SPONSORED FEATURE Power-conversion technology plays a key role in every electric vehicle. In a typical vehicle, the on-board charger (OBC) alone has as many as three power- conversion stages in addition to further high-power-conversion stages to drive the traction motors. When embarking on the design of a new electric vehicle, gate drivers probably are not the first components that come to mind. Yet, choosing the right gate driver technology can cut costs and help designers create more-reliable, more- efficient vehicles. Gate drivers are key to the design of power conversion systems, whether in electric vehicles or other applications. They enable the flow of large currents through power transistors, which are increasingly moving from silicon insulated gate bipolar transistors (IGBTs) to different transistor structures in materials such as silicon carbide (SiC) MOSFET and gallium nitride (GaN) enhanced mode transistors. SiC transistors can handle higher voltages—up to 1700 V—and higher currents at higher temperatures than silicon IGBTs, while GaN devices support higher switching speeds—up to 2 MHz—enabling smaller system designs. All these power devices, and there can be many of them at each level of conversion in a design, need to be very robust and reliable to meet strict safety standards in an electric vehicle. All these switches need different control voltages to be provided from the gate drivers as efficiently and accurately as possible. The gate drivers need to match the requirements of the power switches, and this is key to the performance of the system. The quickness of the switching of the devices and the quality of the output impact the performance of the whole power conversion. The gate driver also needs to cope with a wide variety of conditions. Gate drivers can be situated on the low-voltage side of a bridge, or on the high-voltage side of the bridge, in the hostile environment of an inverter or in a charger. As a result, a gate driver’s performance is evaluated along multiple criteria: the ability to minimize the cost, size, and weight of the power-conversion modules while maximizing reliability and efficiency. Component count and design complexity influence all of these characteristics in a power-conversion module. Choices for gate drivers are extensive, and choosing the driver is just where the work begins. Designers also need to provide a source of power to drive the gates of the switches. This can be a challenge. Supplies must frequently be isolated from the controlled ground, and multiple supplies are often required, which can create a new set of design challenges and trade-offs. Therefore, choosing the right gate driver technology can have a big impact on the success of a vehicle in the marketplace. However, it is not easy to find the right solution for a gate driver and its power sub-system. Delivering reliable gate-drive signal and energy to the gate of any device is the first job of any gate driver, and this needs to be achieved with a low propagation time from the system controller to the FET gate. Low propagation time gives more flexibility for the management of the dead time between the ON and OFF cycles of the power devices, improving system efficiency. Switch types, such as SiC MOSFETs and GaN transistors, have begun to exhibit improved switching transition speeds in recent years. This means the common mode transient immunity (CMTI) of the gate driver needs to be up to the challenge. Failure to meet this requirement will mean unexpected transitions on the transistor gate and potential destructive events in a system. Conventional designs drive the gates of Figure 1: Gate drivers are an increasingly important part of the design of on-board chargers and traction inverters for lithium ion battery systems in electric vehicles.