Traction inverters are the main component in electric vehicles (EVs) that consume battery power and can be available in power levels of 150kW or more. The efficiency and performance of traction inverters directly affect the driving range of electric vehicles after a single charge. Therefore, to build the next generation of traction inverter systems, silicon carbide (SiC) field-effect transistors (FETs) are widely adopted to achieve higher reliability, efficiency, and power density.

The isolated gate driver integrated circuit (IC) shown in Figure 1 provides galvanic isolation from low voltage to high voltage (input to output), drives the high-side and low-side power modules for each phase of the inverter, and monitors and protects the inverter from various faults. According to Automotive Safety Integrity Level (ASIL) functional safety requirements, gate driver ICs must comply with ISO 26262 to ensure fault detection rates of ≥99% for single faults and ≥90% for potential faults.

In this article, we will highlight the technical benefits of real-time variable gate drive strength, a new feature that allows designers to optimize system parameters such as efficiency (affecting EV range) and SiC overshoot (affecting reliability).

Figure 1. Block diagram of traction inverter for electric vehicle

Increase power with real-time variable gate drive strength

Gate driver ICs must turn on the SiC FETs as efficiently as possible while minimizing switching losses. The ability to control and vary the gate drive current intensity reduces switching losses at the cost of increasing transient overshoot at the switching node during switching. Changing the gate drive current controls the switching speed of the SiC, as shown in Figure 2.

Figure 2: Controlling the SiC switching speed by varying the gate driver IC drive strength

Real-time variable gate drive current enables transient overshoot management and design optimization over the entire high-voltage battery energy cycle. Batteries that are fully charged and 100% to 80% state of charge should use a lower gate drive strength to keep SiC voltage overshoot within limits. As the battery level decreases from 80% to 20%, the use of higher gate drive strength reduces switching losses and improves traction inverter efficiency, which is the case for 75% of the charge cycle, so the improvement in system efficiency is significant. Figure 3 shows a typical transient overshoot vs. battery peak voltage and charge state.

Figure 3: Transient overshoot vs. battery peak voltage and charge state

The UCC5880-Q1 is a SiC gate driver up to 20A with multiple protection features for traction inverters in automotive applications. Its gate drive strength ranges from 5A to 20A and can be adjusted via a 4MHz bidirectional serial peripheral interface SPI bus or three digital input pins. Figure 4 shows an implementation of dual split outputs with variable gate drive strength.

Figure 4: The UCC5880-Q1 has a dual output split gate drive structure

Use the DPT to evaluate the power stage switch

The standard method for evaluating the switching performance of a traction inverter power stage is a double pulse test (DPT), which closes and disconnects the SiC power switch at different currents. By varying the switching time, the SiC on and off waveforms under operating conditions can be controlled and measured, helping to evaluate efficiency and SiC overshoot, which affects reliability. Figure 5 shows the connection diagram of a variable-intensity gate driver and a SiC half-bridge for the UCC5880-Q1 low-side DPT setup.

Figure 5: Low-side DPT block diagram

The results in Table 1 show how SiC gate drivers with variable strength can help control overshoot while maximizing efficiency and optimizing thermal performance. EON and EOFF are the energy loss of the switch on and off, respectively. VDS, MAX is the maximum voltage overshoot, TOFF and TON dv/dt are the switching speeds of VDS during turn-on and shutdown, respectively.

Table 1: Summary of DPTs (800V Bus, 540A Load Current,

Highest to lowest gate drive from left to right)

Mitigate overshoot

The waveform in Figure 6 shows the effect of variable gate drive strength on SiC overshoot because the UCC5880-Q1 gate drive resistance and drive strength are controlled in real time. Using a lower gate drive (SiC off) mitigates power stage overshoot.

(a)

(b)

Figure 6: Effect of real-time variable gate drive strength on SiC overshoot:

SiC strong drive shutdown (a); SiC Weak Driver Shutdown (b)

Table 2 lists the actual measurements used for comparison. Depending on the system parasitics and noise control goals, you can make trade-offs between overshoot, DV/dt, and switching losses accordingly.

Table 2: Gate drive strength vs. SiC FET switching speed, overshoot results, and energy loss

Extend your mileage

When using the powerful gate drive control features of the UCC5880-Q1 to reduce SiC switching losses, the efficiency gains can be significant, depending on the power level of the traction inverter. As shown in Figure 7, modeling using the Globally Harmonized Light Vehicle Test Procedure (WLPT) and actual driving meter speed and acceleration shows that SiC power stage efficiency gains of up to 2%, equivalent to 11 kilometers more range per battery. These 11 kilometers may determine whether consumers find charging stations or get stuck on the road.

Figure 7: WLPT and true metered speed and acceleration histogram

The UCC5880-Q1 also includes SiC gate voltage threshold monitoring, which performs a threshold voltage measurement every time the EV pushes a key during the system lifetime and provides power switch data to the microcontroller to predict power switch failures.

epilogue

With the power level of electric vehicle traction inverters approaching 300kW, there is an urgent need for higher reliability and higher efficiency. Selecting a SiC isolated gate driver with real-time variable gate drive strength can help achieve this goal. The UCC5880-Q1 comes with design support tools, including an evaluation board, user guide, and functional safety manual, to assist you with your design.