Introduction to LiDAR
Light detection and ranging (LiDAR) is a sophisticated sensing system that allows autonomous vehicles to “see” surroundings in real-time for navigation, mapping, and object avoidance. LiDAR systems provide a high spatial resolution of objects, detecting distances in real-time by measuring the time that it takes to transmit electromagnetic waves, bounce them off surrounding objects, and receive them (often called “Time-of-Flight”). Autonomous vehicles can continuously view 360 degrees for situational awareness that’s beyond human capability.
LiDAR operates at high frequencies in the near-infrared-to-visible region of wavelengths. Higher frequency LiDAR signals provide higher resolution visibility, but higher performing capabilities also require the system to switch high-current, pulsed lasers very rapidly. Solid-state LiDAR benefits from newer, wide-bandgap (WBG) semiconductor materials like gallium nitride (GaN), which can drive pulsed lasers at higher switching speeds with ultra-short transition times of under one nanosecond.
Figure 2: A schematic of a GaN LLC Converter. (Image: KEMET )
Regulations on Automotive LiDAR
As with any technology, there’s always competition to produce higher performance LiDAR; performance improvements translate to a higher resolution (e.g., visualizing objects just centimeters in size) and an extended range (e.g., visualizing objects hundreds of meters away). A LiDAR system can achieve these improvements by applying faster switching rates and more power. Yet applying more power can cause eye damage, and safety goggles are unlikely to gain pedestrian use. Thus, automotive LiDAR transmissions are restricted to safe levels, adding to the challenge of improving performance.
According to the IEC60825-1 safety specification, lasers are classified concerning human exposure, including the maximum permissible exposure (MPE) of the human eye. Classifying lasers involves calculations based on laser wavelength, the period of exposure, average power, and the energy per pulse of pulsed lasers. MPE equates to a light source’s highest permissible energy density that is deemed safe.
Legal LiDAR can only operate Class 1 lasers under normal, reasonable operating conditions. However, increasing LiDAR resolution requires faster rate sampling, and a longer range means more power. To improve performance while maintaining safety, LiDAR must execute an enormously high-energy pulse in a very short speck of time, at a very low duty cycle.
The Advantages of Wide Bandgap
From a materials science perspective, the GaN bandgap of 3.4 electronvolts (eV) holds serious advantages over a silicon bandgap of 1.1eV, especially for applications in optoelectronics, high-power, and high-frequency devices. Therefore, substituting silicon with WBG materials can result in considerable improvement of switched mode power performance and pulsed lasers. WBG materials demonstrate improved reliability, efﬁciency, and power density (i.e., WBG-based chips can produce more power than a similarly-sized silicon-based chip). WBG-based devices can also operate at higher voltages and perform with more efficiency at higher temperatures. For designers, the benefits of WBG-based devices include lower switching losses, less capacitance that results in lower losses in charging and discharging devices, less power required for a WBG-based circuit driver, a smaller footprint for WBG devices on a PCB, and the ability to operate at higher temperatures and voltages than comparable Si-based devices.
GaN is more suitable for middle power applications, whereas silicon carbide (SiC), another WBG material, is more suitable for very high-power applications (e.g., high current, high voltage). SiC has a voltage rating high enough to accommodate hybrid and electric vehicles (>500 V), while GaN has found its way into automotive electronics, board mounted power supply chips and communication equipment. Both WBG materials are found in switch mode power supplies (SMPS) and general-purpose inverters. WBG technology reduces the size of an automotive system, including EV/HEV, by enabling operation at higher temperatures and higher switching frequencies.
The Challenges of Wide Bandgap
Applications using WBG-based devices face a variety of challenges. Although one can achieve lower capacitances in WBG devices, they can be more sensitive to parasitic components as a natural byproduct of higher switching frequencies. WBG-based devices can operate at high temperatures, but high switching frequencies create more heat. Excessive heat can impact surrounding devices. Thus, thermal management, high voltage insulation, and electromagnetic interference (EMI) need more attention when substituting WBG- for Si-based devices.
How Ceramic Components Maintain the Benefits of WBG
Passive component selection is critical for retaining WBG benefits at the system level, as WBG devices operate at higher voltages, temperatures, and frequencies. As switching frequency increases, both current and voltage ripple decrease, so the required capacitance also decreases. Large aluminum capacitors start to lose their effectiveness beyond a few hundred kilohertz and are ineffective at megahertz frequencies. Compact ceramic capacitors are an effective replacement for lower impedances.
Switching power supply circuits need capacitors on both the input (VIN) and output (VOUT). Fast switching means that a capacitor’s voltage overshoot must be lower, and a multi-layer ceramic capacitor (MLCC) reduces ripple voltage better than a tantalum capacitor. A ceramic capacitor develops a lower equivalent series inductance (ESL) than a polymer capacitor and is generally smaller than comparable capacitors of different materials. Ceramic capacitors also provide a lower effective series resistance (ESR), which minimizes heat since the resistive component generates heat in response to ripple current.
KEMET Electronics’ KC-LINK compact ceramic capacitors meet the challenges of WBG-based devices with a range of high capacitance values, high power ratings, extreme operating temperatures, extremely low ESL and ESR, low thermal resistance, and low-profile form factors. KC-LINK capacitors leverage KEMET’s proprietary C0G/NPO base metal electrode dielectrics for applications where high efficiency is a primary objective.
KC-LINK capacitors operate at a very high ripple current without changing capacitance versus DC voltage and experience only a negligible change in capacitance versus temperature. At an operating temperature of 150°C, they can be mounted near fast-switching, power-dense semiconductors with minimal cooling. KC-LINK ceramic capacitors meet the higher performance requirements of both GaN and SiC with no piezoelectric noise and high thermal stability. The KC-LINK series also offers AEC-Q200 automotive rated capacitors.
Figure 3: KEMET’s MLCC class 1 capacitors both show the ability to carry a high amount of current and a low-temperature rise in response to increasing ripple current up to 10 Amps (RMS). (Image: KEMET)
Stacking with KONNEKT
Designers can create higher capacitance values with extremely high ripple current capability with KONNEKT, a new packaging technology by KEMET, to bond ceramic capacitors together in resourceful configurations. KONNEKT enables more capacitance under a single footprint by stacking leadless components, which is ideal for high efficiency and power-dense applications. “KONNEKT-ing” KC-LINK (or other) multiple layer ceramic capacitors (MLCCs) enables one to increase power density by occupying one footprint while stacking capacitance and thus maximizing current handling capability.
Further information can be found at http://www.kemet.com/kc-link.
Figure 4: KEMET’s KONNEKT is a new packaging technology that notably benefits high power density applications that utilize GaN- or SiC-based devices. The low-loss orientation on the right provides benefits over standard orientation, such as an overall lower inductance, lower ESR, and lower thermal resistance. (Image: KEMET)