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Gate-Drive Strength in EVs

Written by George Lakkas

Why Real-Time Variable Gate-Drive Strength Enables SiC Traction Inverter Efficiency Improvements

  • How does real-time variable gate-drive strength enable SiC traction inverter efficiency improvements?
  • What do I need to know about gate-drive strength in EVs
  • What does TI offer for improving SiC traction inverter efficiency?
  • SiC -FETs

With the electrification of light vehicles and trucks replacing fossil fuels as an energy source, the traction inverter is the most important power conversion and delivery subsystem of the electric vehicle (EV) powertrain. A traction inverter’s role is to convert the EV battery’s high-voltage DC to the AC that the electric motor needs. The traction inverter controls the speed and torque of the motor, and the efficiency of the traction inverter has a direct impact on the power and thermal dissipation of the system, as well the EV’s driving range over a single battery charge. “Traction” indicates the act of pulling the EV over a surface in coordination with the motor. As traction inverter power levels approach 300kW, reducing energy losses throughout the drive cycle is the number one design consideration for car original equipment manufacturers worldwide.

To improve efficiency, the industry has widely adopted silicon carbide (SiC) field-effect transistors (FETs); the isolated gate drivers that power these switches, as shown in Figure 1, have become more sophisticated, and now include isolated analog-to-digital converter sensing, multiple modes of overcurrent protection, bias supply monitoring, gate monitoring, programmable safe states, built-in self-test and a new feature called real-time variable gate-drive strength.

FIGURE 1
EV traction inverter block diagram

Additionally, depending on the Automotive Safety Integrity Level (ASIL) functional safety requirements, the gate-driver integrated circuit (IC) may have to be International Organization for Standardization 26262-compliant, ensuring fault detection of ≥99% and ≥90% for single and latent faults, respectively.

In this article, I will focus on the benefit of real-time variable gate-drive strength on improving SiC FET switching losses and efficiency, and therefore extending EV operating ranges.

SIC FET SWITCHING LOSSES

SiC FET switching losses are the total of the turnon (EON) and turnoff (EOFF) losses, and they depend on the drain-to-source voltage (VDS), drain current (ID) and switching frequency (fSW), as shown in Equation 1. The first part in the equation is the turnon energy, the second is the turnoff energy, and multiplying both by the switching frequency determines the switching losses.

Equation 1
HIGHER SIC EFFICIENCY THROUGH REAL-TIME GATE-DRIVE STRENGTH CONTROL

Modern gate-driver ICs have to turn on and turn off the SiC FETs as quickly as possible through a voltage slew-rate control method (dv/dt), minimizing the time (dt component) in Equation 1, reducing turnon and turnoff energy, and thus reducing overall switching losses.

This ability to control and vary the gate-drive current strength provides an impressive reduction in switching losses, at the expense of increasing transient overshoot at the switch node during switching, as shown in Figure 2.

FIGURE 2
SiC slew-rate control by varying gate-driver IC drive strength

The advantage of a real-time variable gate-drive strength gate driver is that it offers designers the flexibility to optimize their traction inverter designs for both efficiency and transient overshoot mitigation.

WHAT IS A GATE DRIVER WITH REAL-TIME VARIABLE GATE-DRIVE STRENGTH?

A gate driver that employs real-time variable gate-drive strength has a dual split-output power stage.

The UCC5880-Q1 from Texas Instruments is a 20A SiC gate driver that has advanced protection features for traction inverters in automotive applications [1]. Its gate-drive strength varies from 5A to 20A, and is variable through both a 4MHz bidirectional Serial Peripheral Interface (SPI) bus or three digital input pins.

Both the OUTH 1 and 2 pullup and OUTL 1 and 2 pulldown are split into two, as shown in Figure 3, allowing for the installation and independent control of separate resistors. The UCC5880-Q1 output is selectable between each output, or the parallel combination of both, in real time using either the GD* digital inputs or the SPI bus. Using the GD* digital input pins makes it easier to set the drive strength at power on without needing to use the SPI bus.

FIGURE 3
The UCC5880-Q1’s dual-output split gate-drive structure
OPERATING CONDITIONS FOR VARYING THE GATE-DRIVE STRENGTH

It is possible to realize SiC efficiency by varying the gate-drive strength (through SiC FET slew-rate control, as I discussed earlier) from the moment the battery starts discharging (from 80% to 20%). Because that is approximately 75% of the charge cycle, the efficiency gains can be quite significant. A fully charged battery with a state of charge from 100% to 80% should use a low gate-drive strength to maintain SiC voltage overshoot within the limits. As the battery charge decreases, using a high gate-drive strength reduces switching losses and increases traction inverter efficiency. Figure 4 illustrates a typical transient overshoot vs. battery peak voltage and state of charge.

FIGURE 4
Efficiency zone during battery peak voltage vs. state of charge
SELECTION CRITERIA FOR WEAK VS. STRONG GATE-DRIVE STRENGTH

Selecting a weak or strong gate-drive strength depends on certain criteria. If the load current is high (the motor load increases with higher speeds, rapid acceleration or deceleration), the battery is >80% full and the ambient temperature is cold, then it’s best to use a weak gate-drive strength. If the load current is low (which is true for the majority of EV driving profiles), the battery is <80% full, and the temperature is room or warm, then employ a strong gate-drive strength. The control logic should include parameters such as phase current, current threshold, and external gate resistor values.

Figure 5 summarizes the criteria, while the results in Figure 6 illustrate how the combined drive with variable strength in the High-Power, High-Performance Automotive SiC Traction Inverter Reference Design enables lower power losses in the power module, maintaining stable thermal performance [2].

FIGURE 5
Weak vs. strong gate drive strength criteria
FIGURE 6
800V, 300kW SiC-based traction inverter reference design data

So, what does this mean for the ultimate goal—longer range? Consumers choose EVs today for their reliability, performance, and looks, but range is an important decision factor.

EXTENDING DRIVING RANGE

In tests performed with the UCC5880-Q1, the strong gate drive strength enabled a large reduction in SiC switching losses on an already 90% efficient traction inverter. Thus, the efficiency gains can be impressive, depending on the traction inverter’s power level.

Modeling with the Worldwide Harmonized Light Vehicles Test Procedure (WLPT) and real drive log-speed and acceleration settings, as shown in Figure 7, resulted in a 2% boost in efficiency of the power stage, corresponding to an additional 7 miles of range per battery or perhaps a smaller battery for the same amount of range at a lower cost.

FIGURE 7
WLPT and real log-speed and acceleration histograms
CONCLUSION

With EV traction inverters approaching 300kW power levels, the need for higher reliability and higher efficiency is imperative. SiC-isolated gate drivers with real-time variable gate-drive strength are useful in achieving these goals. The UCC5880-Q1 comes with design support tools including evaluation boards, user’s guides, and a functional safety manual to assist you with your designs up to ASIL D.

RESOURCES
Texas Instruments | www.ti.com

PUBLISHED IN CIRCUIT CELLAR MAGAZINE • OCTOBER #399 – Get a PDF of the issue

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Product Marketing Engineer, High Power Drivers at | + posts

George Lakkas has been in the power supply and power management IC industry for 33 years, having held positions as a power supply test engineer, assistant design engineer, applications engineer, product marketing engineer, product marketing manager, and product line manager for power management ICs. Since joining TI in 2006 George has worked in various product marketing roles for non-isolated DC-DC conversion solutions, and currently is a product marketing engineer for automotive traction inverter isolated gate drivers. George lives in Raleigh, NC.

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Gate-Drive Strength in EVs

by George Lakkas time to read: 5 min