The Smarter the Car, the More It Needs GaN

Author:
Alex Lidow, CEO, Efficient Power Conversion

Date
07/02/2020

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Gallium nitride is helping the transition from internal combustion engines to electric and smart cars

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Figure 1: The MOSFET solution (top) for a 3 kW 48 V – 12 V DC-DC converter requires five phases compared with the eGaN FET solution (bottom) with only four phases. The eGaN FET solution is 35% smaller and 20% less expensive

As the new decade emerges, it is becoming clearer that the trend in automotive is electric. Internal combustion engines (ICE) are succumbing to hybrid and full-electric vehicles.  Along with that major shift in propulsion comes a variety of accessories that need to convert to electric.  In addition, new features such as advanced driver assistance systems (ADAS) and centralized infotainment and feature control systems are adding to the increased electrical loads that must be supported while constantly driving down the overall energy consumption of the vehicle. This is where GaN technology thrives as it enables systems to be smaller, lighter, more efficient, and less costly to build.

In this article, four applications where GaN technology is driving changes in automotive systems will be discussed. These applications include 48V power distribution bus for mild hybrid cars, high-frequency DC-DC conversion for infotainment systems, brushless DC (BLDC) motors, and light detection and ranging (lidar) used for autonomous navigation.

48 V Power Bus for Mild Hybrid Cars

By 2025, one of every 10 vehicles sold worldwide is projected to be a 48 V mild hybrid. 48 V electrical power distribution systems boost fuel efficiency, deliver four times the power without increasing engine size, and reduce carbon-dioxide emissions without increasing system costs.  These systems require a 48V – 12 V bidirectional converter, with power ranging from 1 kW to 3. 5 kW. The design priorities for these power systems are size and cost, both of which are very dependent upon improvements in conversion efficiency.

For 48 V bus systems, GaN technology increases efficiency, shrinks size, and reduces overall system cost. For example, in a 3kW bidirectional converter with a multiphase buck/boost topology, a GaN-based solution can efficiently operate at 250 kHz per phase as opposed to 125 kHz per phase for traditional MOSFET solutions. The higher frequency allows a smaller inductor value, 2.2 µH versus 4.7 µH, to be used; and, consequently, the inductor direct current resistance (DCR) is less, 0.7 mΩ versus 1.7 mΩ.  All these reductions result in less losses and a smaller size for the GaN-based solution.

The added efficiency from the eGaN devices additionally enables a reduction in the number of required point of load (POL) phases. As an example, in the 3 kW converter, the higher frequency, and higher efficiency results in the reduction from a five-phase MOSFET system to a four-phase GaN-based system, using automotive qualified EPC2206 eGaN® FETs, thus reducing both size and cost. As seen in figure 1, the GaN-based solution is 35% smaller and 20% less expensive than its similar MOSFET-based system.

With one less phase and double the switching frequency, the EPC GaN FET solution is also more efficient than the five-phase MOSFET solution.  The graph in figure 2 shows efficiency on the vertical axis versus the load power on the horizontal axis.  The plot show that the eGaN FET solution is 0.7% more efficient at full load (15% less power loss) than the MOSFET solution and 5% more efficient at 10% load (30% less power loss).  That represents a reduction of 21 W of power loss at full load.

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Figure 2: Efficiency vs. load power for the five-phase MOSFET-based 3 kW converter (red) compared with the four-phase eGaN FET based solution (blue).  At full load the eGaN FET solution saves 21 W which is 15% lower power loss

Infotainment

Worldwide shipments of in-vehicle infotainment systems are expected to exceed 183 million units by the year 2022. Modern infotainment systems contain many advanced features such a touch screen capability, variable console and interior lighting, Bluetooth communications, digital and high-definition TV, satellite radio, GPS navigation, advanced driver assistance systems, powerful 16-speaker class D audio, and even gaming.

These power-hungry systems put additional demand on the vehicles overall power system.  At the same time, there is limited space inside the dashboard monitor for additional power systems and there is little tolerance for additional heat generation despite significantly higher required power levels.

One approach to meeting the size constraints is to go to higher frequencies.  GaN devices are very suitable for infotainment DC-DC converters because they can efficiently be switched at 2 MHz, the frequency required to avoid interference with the AM band.  In hard switching applications, such as the case with the low-cost and high power-density buck converter topology, the very low gate-drain charge (QGD) and low switching losses of eGaN FETs result in considerable improvement in efficiency and heat generation compared with silicon MOSFET solutions.

Figure 3 shows a comparison between a MOSFET-based (red), and GaN-based (blue) 12 V – 3.3 V DC-DC buck converter switching at 2 MHz. Efficiency is reflected on the left side vertical axis of the graph with power loss on the right-side vertical axis and output current on the horizontal axis.  At 12 V to 3.3 V using eGaN FETs instead of Si MOSFETs. there is an almost 5% efficiency improvement and a savings of 2 W, or 50% lower power losses. Furthermore, the eGaN FET solution runs 10 oC cooler.

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Figure 3: A comparison between a MOSFET-based (red), and GaN-based (blue) 12 V – 3.3 V DC-DC buck converter switching at 2 MHz.  The GaN-based solution dissipates 2 W less than the MOSFET solution at 10 A, a 50% reduction in losses

Brushless DC Motors

There are, on average, ten electric motors per car for operations such as door locks, trunk latches, retracting roof, gas pumps, ventilation, battery management, heating control, electric steering, just to name a few (see figure 4).

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Figure 4: Typical applications for BLDC motor drives in modern automobiles requiring a power range from 30 W to 1 kW

In internal combustion cars, automakers use three different types of electric motors: the BLDC motor, brushed DC motor, and AC induction motor.

A brushless DC motor has a permanent-magnet rotor surrounded by a wound stator. The winding in the stator gets commutated electronically, without the use of brushes. This makes the BLDC motor simpler to maintain, more durable, smaller, much more energy efficient, able to respond faster and at higher operating speeds, and they are considerably lighter. BLDC motors are also less prone to the types of failures experienced by brushed motors, leading to lower warranty costs. As cars migrate to a 48 V-bus architecture, BLDC motors become even more attractive for power levels between 30 W to 1 kW compared with brushed or AC induction motors.

The value for eGaN devices in powering 48 V automotive motors is that they can reduce the size and weight of the motors, work efficiently at frequencies above the audible spectrum, have better torque, and higher efficiency.

In March 2020, Efficient Power Conversion introduced the EPC2152 ePower® Stage that has a monolithically integrated 80 VIN half bridge including drivers, level shift, synchronous bootstrap circuit and input logic ideally suited for BLDC motors below about 500 W.  By integrating several functions onto a single chip, an entire BLDC drive can be made with just three ePower Stage ICs, a digital controller, plus sensing and filtering elements (see figure 5).

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Figure 5: A 500 W motor drive solution 98.5% efficient when driving BLDC motors up to 500 W

Light Detection and Ranging (Lidar)

Lidar remote object sensing systems have been in use for decades in military, aerospace, robotics, and meteorological applications.  More recently, lidar has quickly become a key technology to enable autonomous driving. 

A contributing factor to this rapid expansion of lidar in self-driving vehicles has been the advent of commercially available eGaN FETs and integrated circuits that enable a laser to be driven with high-current pulses that have extremely short pulse widths.  For a direct time-of-flight measurement (DTOF), a short pulse width leads to higher resolution, and the higher pulse current forces more photons from the laser, thus allowing the lidar system to see farther with better resolution.

These two characteristics, short pulse width and high pulse current forces, along with their extremely small size, make eGaN FETs ideal for lidar.  Figure 6 presents the state-of-the art for long-distance DTOF lidar.  This figure shows a 135 A laser current pulse with a pulse width of only 2.5 ns.  This high current and short pulse width in combination enables 300-meter visibility with centimeter resolution.

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Figure 6: A 135 A laser current pulse with a pulse width of only 2.5 ns delivered in an OSRAM SPL S4L90A_3 A01 laser driven by an EPC2001C eGaN FET

For close-up high precision lidar applications, indirect time-of-flight (ITOF) lidar can be attractive. Often referred to as flash lidar, it can capture an entire megapixel frame at a time. These lidar systems do not need the extreme power of long-range DTOF lidars, but they benefit from extremely fast edges and very high pulse repetition frequencies.

An example of a short-range lidar pulse generated by an eGaN FET is shown in figure 7. The blue oscillogram trace is the VDS across the FET as it turns on and off.  Note that the 10 A pulse has a rise time is 556 pico seconds and the fall time is 203 pico seconds. For ITOF lidar systems, the pulse repetition rates can be as high as 100 MHz, and therefore nanosecond pulse widths are essential.

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Figure 7: ITOF circuit delivering a 10 A pulse with a rise time of 556 pico seconds and fall time of 203 pico seconds that can be driven a pulse rates greater than 100 MHz

Summary

In many ways the automotive industry is going through a Renaissance, driven by technological advances electronics. The over-arching goals for the automotive industry are safer cars, a better passenger experience, higher power efficiency, and lower cost of ownership. In this article, four ways GaN technology is helping in a significant way in all these categories were outlined.

GaN technology is relatively new compared with silicon and is still seeing rapid technological development with a way to go before its performance limitations are reached. GaN devices are becoming more efficient and integration is just starting the journey toward delivering complete power systems on a chip.

Enjoy the ride!

Efficient Power Conversion Corporation

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