GaN HEMT Circuit Topologies for High-resolution LiDAR

LiDAR is based on the principle of estimating distance by shining a beam of light on an object and accurately measuring the reflected time of flight. By sweeping the emitted light across a real-world scene, 3D representations can be gathered for further processing by a computer system. Applications for LiDAR are numerous and growing and include aerial surveying, object scanning for 3D modeling, and autonomous vehicle control.

With that in mind, this article will first introduce some of the key components of a high-performance LiDAR system before demonstrating the primary circuits and providing simulation examples of the two different design topologies of GaN HEMTs: dual-edge square wave and single-edge resonant circuits. 

 

High-power Pulse Laser Diodes for LiDAR

The most common source of light in LiDAR systems is a high-powered pulsed laser diode, often centered on a wavelength outside the absorption range of the human eye. The quality of beams produced by these lasers is critically important to the resolution of LiDAR systems. As shown in Figure 1, resolution can be significantly improved by using high-power, sharp emission area devices.

 

Figure 1. High-power laser diodes improve resolution for 3D LiDAR sensing. 

 

For example, ROHM Semiconductor offers laser diodes with a peak output power of up to 120 W at 905 nm for use with LiDAR and simultaneous localization and mapping (SLAM). 

 

Semiconductor Switches for LiDAR Pulse Generation

To create pulses of light for LiDAR, these lasers must be driven by a semiconductor switch. Again, the quality of the switch directly affects the resolution of the LiDAR image and is typically characterized by turn-on time, peak current, and switching loss. 

One example of technology for LiDAR switches is the high electron mobility transistor (HEMT), built using gallium nitride (GaN). Compared to traditional silicon devices, these switches can exhibit extremely fast speeds and up to 65% lower power loss, as shown in Figure 2.

 

Figure 2. HEMT switching loss compared to traditional silicon transistors.

 

For high-power applications that include LiDAR, GaN HEMT devices are available to support various system operating voltages and power levels. For example, ROHM Semiconductor’s portfolio of GaN HEMTs includes 150 V and 650 V models. 

 

HEMT Gate Drivers

To take advantage of the speed of HEMT devices, their gates must be properly driven with a high-speed signal. At the very front of the LiDAR output stage resides the HEMT gate driver, as shown in Figure 3. 

 

Figure 3. Using a specialized high-speed gate driver IC for HEMT circuits.

 

These drivers are specially designed to provide the appropriate bias voltage and speed to interface with GaN HEMT transistors while also offering convenient features like dual outputs and output-enable control.

Building a LiDAR system with all of these optimized parts for controlling, driving, and generating the output light pulses guarantees higher resolution in the final output image. The exact circuit topology for a particular application, however, is often chosen from one of two types: dual-edge square wave or single-edge resonant. 

 

Dual-edge Square Wave GaN HEMT Circuit

The simplest method for pulsing a laser diode is to control the current using a series switch, as shown in the simulation schematic of Fig. 4. 

 

Figure 4. Simulation schematic using a square wave HEMT driver configuration.

 

In the above figure, U1 is a GaN HEMT that directly sinks current from the power supply Vin through an RLD90QZWD 35 W laser diode. When the gate of U1 is driven high, the laser turns on, and when driven low, the laser turns off. These two edges control the pulse width of the optical output, hence the term “two edge” or square wave configuration.

The simulated waveform for this control scheme is shown in Figure 5, where the gate voltage on the HEMT is depicted in red and the current through the HEMT in green. 

 

Figure 5. Gate voltage (red) and drain current (green) using a square wave HEMT gate driver.

 

In Figure 6, the optical output power of the laser diode is indicated in blue.

 

Figure 6. Optical output power using a square wave HEMT gate driver.

 

While this type of driver can be easy to implement and offer flexibility in output pulse width, there are several drawbacks that make it a rather unpopular choice for high-performance LiDAR. 

1. The speed of turning on the laser is directly limited by the turn-on speed of the HEMT and the series inductance in the circuit. 
2. The pulse shape is asymmetrical, with both the turn-on and turn-off edges requiring careful timing consideration. 

Despite these drawbacks,both of these characteristics can be improved upon by using the more popular resonant configuration.

 

Single-edge Resonant GaN HEMT Circuit

As shown in the simulation schematic of Figure 7, the resonant topology places the HEMT in a completely different role. 

 

Figure 7. Simulation schematic of a resonant configuration HEMT gate driver.

 

Rather than controlling the current directly through the laser, the HEMT (U1) is used to kick off a resonant discharge through the inductance L1 and the capacitor C2. In this way, only the leading edge of the control signal is important, as the pulse width is entirely determined by the LC circuit in series with the laser.

The gate voltage and drain current of the resonant configuration are shown in Figure 8, with the optical output power illustrated in Figure 9. 

 

Figure 8. Gate voltage (red) and drain current (green) using a resonant configuration. 

 

Figure 9. Optical output power using a resonant configuration

 

As these figures show, the rising edge of the gate drive initiates the discharge of stored energy in C2 through the laser. It is important to note that the pulse width of the laser signal is not related to the falling edge of the gate signal. 

Compared to its square wave counterpart, the resonant design offers several unique benefits:

1. The LC resonance allows for much narrower pulses with well-defined symmetry—an important factor for LiDAR in particular. 
2. The parasitic series inductance within the components and circuit wiring can be used as part of the LC resonance. Instead of hindering speed, as in the square wave design, the total inductance can be tuned for optimal performance. 
3. The energy of the laser pulse is only a factor of the input voltage. This enables precise control without regard to the timing of the gate drive in applications where the total energy is critical. 

The trade-off for all of these advantages is the complexity of designing the resonant conditions. Stray inductance must be modeled, with the physical positioning and layout of components and traces of important factors that affect overall performance. One useful tool that can help with designing these circuits, is ROHM Semiconductor’s online circuit simulator, which includes pre-populated driver topologies. 

As an example of the impact of stray inductance, the simulations of Figures 7 through 9 were repeated with an increased L2 term to see how the optical output was affected. Additionally, as shown in Figure 10, when L2 is increased from 3 to 6 nH, the peak output power was reduced by 26%, while the pulse width increased by nearly 50%.

 

Figure 10. The optical output power of the resonant configuration with increased series inductance (L2)

 

Sensitivity to these parameters often requires that the circuit be modeled ahead of time, and it is likely that multiple iterations of design and testing be undertaken. In addition, in applications where a short pulse isn’t required or variable pulse width is preferred, the resonant advantages cannot be employed. 

 

Select The Right Topology to Meet Your LiDAR Requirements

As LiDAR and similar distance-measuring technologies become even more common in today’s world, the underlying devices must also evolve to meet the ever-increasing performance requirements. At the same time, engineers must understand the different design methodologies and tools available to achieve success in any particular application. 

As we have discussed, single-edge resonant circuits often provide improved performance for these high-speed GaN HEMT switching applications, but at the cost of increased design complexity. If your application does not require this higher level of sophistication, the dual-edge square wave circuit topology presents an easier solution.

 

All images used courtesy of ROHM Semiconductor

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