How to improve SiC traction inverter efficiency with real-time variable gate drive strength

The traction inverter is the main component that consumes battery power in electric vehicles (EVs), with power levels of up to 150kW or more. The efficiency and performance of the traction inverter directly affect the mileage of an EV on a single charge. Therefore, in order to build the next-generation traction inverter system, the industry has widely adopted silicon carbide (SiC) field effect transistors (FETs) to achieve higher reliability, efficiency, and power density.

The isolated gate driver integrated circuit (IC) shown in Figure 1 provides electrical isolation from low voltage to high voltage (input to output), drives the high-side and low-side power modules of each phase of the inverter, and monitors and protects the inverter from various faults. According to the automotive safety integrity level (ASIL) functional safety requirements, the gate driver IC must comply with the ISO26262 standard to ensure a fault detection rate of ≥99% and ≥90% for single faults and latent faults, respectively.

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: Electric vehicle traction inverter block diagram

Improve efficiency with real-time variable gate drive strength

The gate driver IC must turn on the SiC FET as efficiently as possible while minimizing switching losses. The ability to control and vary the gate drive current strength reduces switching losses, but at the expense of increased transient overshoot at the switch node during switching. Varying the gate drive current controls the switching speed of the SiC, as shown in Figure 2.

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

The real-time variable function of the gate drive current enables transient overshoot management and design optimization throughout the high-voltage battery energy cycle. A fully charged battery with a state of charge of 100% to 80% should use a lower gate drive strength to keep the SiC voltage overshoot within limits. As the battery capacity decreases from 80% to 20%, using a higher gate drive strength can reduce switching losses and improve traction inverter efficiency, which is the case for 75% of the charging cycle, so the improvement in system efficiency is very obvious. Figure 3 shows the relationship between typical transient overshoot and battery peak voltage and state of charge.

Figure 3: Transient overshoot versus battery peak voltage and state of charge

The UCC5880-Q1 is a 20A maximum SiC 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 the implementation of dual split outputs for variable gate drive strength.

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

Evaluating Power Stage Switches Using DPT

A standard method for evaluating the switching performance of traction inverter power stages is the double pulse test (DPT), which closes and opens the SiC power switch at different currents. By varying the switching time, the SiC turn-on and turn-off waveforms under operating conditions can be controlled and measured, which helps evaluate efficiency and SiC overshoot, which affects reliability. Figure 5 shows the connection diagram of the variable strength gate driver and 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 with variable strength helps control overshoot while maximizing efficiency and optimizing thermal performance. EON and EOFF are the turn-on and turn-off switching energy losses, respectively. VDS,MAX is the maximum voltage overshoot, and TOFF and TON dv/dt are the switching speeds of VDS during turn-on and turn-off, respectively.

Table 1: DPT Summary (800V bus, 540A load current, highest to lowest gate drive from left to right)

Mitigating overshoot

The waveforms in Figure 6 show the effect of variable gate drive strength on SiC overshoot, as the UCC5880-Q1 gate drive resistance and drive strength are controlled in real time.

shutdown) can reduce power stage overshoot.

(a)

(b)

Figure 6: Effect of real-time variable gate drive strength on SiC overshoot: SiC strong drive turn-off (a); SiC weak drive turn-off (b)

Actual measured values ​​for comparison are listed in Table 2. Depending on the system parasitics and noise control goals, you can make trade-offs between overshoot, dv/dt, and switching losses accordingly.

Table 2: Relationship between gate drive strength and SiC FET slew rate, overshoot results and energy loss

Extended driving range

When using the powerful gate drive control capabilities 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 Worldwide Harmonized Light Vehicle Test Procedure (WLPT) and real-world driving meter speeds and accelerations shows that SiC power efficiency can improve by up to 2%, which is equivalent to an additional 11 km of driving range per battery. Those 11 km can be the difference between a consumer finding a charging station or being stranded on the road.

Figure 7: WLPT and real taximeter speed and acceleration histograms

The UCC5880-Q1 also includes a SiC threshold monitoring function that performs threshold voltage measurements at each push-button start of the electric vehicle over the system lifetime and provides power switch data to the microcontroller to predict power switch failures.

Conclusion

As the power level of EV traction inverter approaches 300kW, there is an urgent need for higher reliability and higher efficiency.

The UCC5880-Q1 comes with design support tools including an evaluation board, user guide, and functional safety manual to assist you in your design.