Quick guide to GaN FETs for LiDAR in autonomous vehicles
Pulse width, peak power, repetition frequency, and duty cycle are primary LiDAR specifications. For example, a typical laser diode used in a LiDAR system may have a pulse width of 100 ns or less, a peak power of >100 watts, a 1 kilohertz (kHz) or higher repetition frequency, and a duty cycle of 0.2%. The higher the peak power, the longer the detection range of the LiDAR, but thermal dissipation is a tradeoff. For a pulse width of 100 ns, the average duty cycle is usually limited to 0.1% to 0.2% to prevent laser overheating. Shorter pulse widths also contribute to LiDAR safety. IEC 60825-1 defines laser safety in terms of the maximum permissible exposure (MPE), which is the highest energy density or power of a light source with negligible potential to cause eye damage. To be negligible, the MPE power level is limited to roughly 10% of the energy density, which has a 50% possibility of causing eye damage. With a constant power level, shorter pulse widths have a lower average energy density and are safer. While a single LiDAR ToF measurement can determine the distance to an object, thousands or millions of LiDAR ToF measurements can be used to create a three-dimensional (3-D) point cloud (Figure 2).
A point cloud is a collection of data points storing large amounts of information called components. Each component contains a value describing an attribute. The components may include x, y, and z coordinates and information about the intensity, color, and time (to measure object movement). LiDAR point clouds create a real-time 3-D model of the target area.
Designers can use EPC’s EPC9179 development board for a fast start by employing the EPC2252 in LiDAR systems with total pulse widths of 2 to 3 ns (Figure 4). The EPC9179 includes an LMG1020 gate driver from Texas Instruments that can be controlled by an external signal or an onboard narrow-pulse generator (with sub- nanosecond precision).
The IC is delivered as a die-size ball grid array (DSBGA). This means the passivated die is directly attached to solder balls without any other packaging. As a result, the DSBGA chips are the same size as the silicon die, minimizing their form factor. In this case, the EPC2252 uses a 9-DSBGA implementation that measures 1.5 x 1.5 millimeters (mm). It has a thermal resistance of 8.3°C per watt (˚C/W) from junction to board, making it suitable for high-density systems.
Use GaN FETs to power LiDAR lasers
Figure 1: LiDAR uses ToF measurements to detect objects and determine their distance. (Image source: ams OSRAM)
GaN FETs switch much faster than their silicon counterparts, making them suitable for LiDAR applications requiring very narrow pulse widths. For example, the EPC2252 from EPC is an AEC-Q101 automotive- qualified 80 volt GaN FET capable of current pulses up to 75 amperes (A) (Figure 3). The EPC2252 has a maximum on resistance (RDS(on)) of 11 milliohms (mΩ), a maximum total gate charge (Qg) of 4.3 nanocoulombs (nC), and zero source-drain recovery charge (QRR).
Figure 4: Shown is the EPC9179 demo board for the EPC2252 GaN FET and other key components. (Image source: EPC)
Figure 2: LiDAR systems combine large numbers of ToF measurements to create 3-D point clouds and images of a target area. (Image source: EPC)
Figure 3: The EPC2252 GaN FET is AEC-Q101 qualified and is suitable for driving laser diodes in automotive LiDAR systems. (Image source: EPC)
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