Over the past few years, one form of specific lidar, Time-of-Flight (TOF) ranging, has been widely used. When a laser is used as an optical source, it can measure distances to small parts at a great distance, and when used with adjustable optics, it can measure distances to those points and create three-dimensional maps of the object.

EPC has developed the EPC9126 and EPC9126HC laser drivers to bring the performance of eGaN FETs to LiDAR systems and advance the technology level of LiDAR.
The basic principle of this laser driver seems simple, but it is the parasitic components that many engineers ignore in their designs that enable high-speed, high-voltage, and high-current characteristics.

This technical contribution is intended to provide in-depth information on laser driver design and answer commonly asked questions so that users can get the most out of their drivers.
Laser and Pulse Requirements
TOF lidar typically uses near-infrared (NIR) semiconductor laser diodes, side-emitting epitaxial lasers, or vertical cavity surface emitting lasers (VCSELs).

Some common laser diodes are shown in Figure 2. The laser diode acts as an electrical rectifier. When forward-biased above a certain threshold current, it emits laser radiation with an output optical power roughly proportional to the forward current.

Therefore, by driving it with a current pulse, a laser light pulse can be obtained. The laser light pulse has two main parameters, pulse width and energy, and these two factors greatly affect the distance resolution and range, respectively.
The pulse width of the transmitted optical signal has a significant impact on the range resolution of the lidar system. Figure 1 illustrates this case.

In the case above, the lidar emits a narrow pulse of light. Since this light pulse must travel to the target, reflect, and return, the time td between transmission and reception of the pulse at the target at a distance d is: Here c is the speed of light in the atmosphere, which is about 30 cm/ns (about 1 foot/ns). By measuring the time td, we can calculate the distance. Now suppose we transmit a pulse with a longer duration, as shown in the case below.

If the pulse length becomes too long, it begins to overlap with the reflected pulse, making it difficult to distinguish features from the environment. To get an idea of what pulse length is actually desirable, consider a 1 ns current pulse width driving a laser diode.

This corresponds to a 30 cm optical pulse length. When the target characteristic reaches a distance of 15 cm, it begins to overlap with the received pulse and becomes more difficult to distinguish. Although various signal processing techniques can be used to improve the resolution for a given pulse width, it is clear that shorter pulses provide better precision, and pulses of several nanoseconds or less are desirable for resolutions that are compatible with human scale.

Pulse energy determines the range of the lidar. As the pulse width is designed narrower due to the demand for better resolution, the diode current must increase to maintain sufficient pulse energy. Typical pulse currents can range from a few amps to hundreds of amps.

Many laser diodes have nominal pulse currents in the tens of amperes range. For example, under typical data sheet test conditions of Pulse Repetition Frequency (PRF) = 1 kHz, Pulse Width t _w = 100 ns, Peak Current I _DLpk = 30 A, and Operating Temperature TOP = 23 to 25°C, the maximum electrical input power can be as high as 300 W for a triple junction edge emitting laser.

The average test duty cycle is often ≤0.1% to prevent overheating of the laser die. This allows the laser diode to be operated at higher currents with shorter pulse widths, resulting in better peak optical output.

In summary, typical laser diode requirements for commercial laser diodes suitable for LiDAR systems require pulse widths ranging from 1 ns to 10 ns and peak pulse currents from several amperes to hundreds of amperes. The following sections will explore how to achieve these extreme pulses.
Laser Driver
A typical pulsed laser driver for LiDAR uses a semiconductor switch and an electrical energy source connected in series with the laser. Performance is limited by stray inductance and the speed of the semiconductor power switches.

Over the past decade, gallium nitride (GaN) power FETs have become commercially available, offering extremely low inductance and switching figures of merit (FOMs) up to 10 times better than comparable silicon MOSFETs.
Figure 4 shows the EPC2016C FET, a 100 V eGaN FET capable of delivering 75 A pulses. Compared to conventional silicon MOSFET technology, eGaN FETs offer a significant performance improvement, enabling much faster switching for a given peak current capability and, although not yet able to meet both simultaneously, can deliver >100 A current and <2 ns pulse width at laser loads.

There are many different types of laser driver topologies, but the two main topologies suitable for high power are the advanced controlled resonant laser driver and the current-limited dual-edge controlled driver. For high-speed applications, resonant laser drivers are the most commonly used, so we will primarily discuss this type of driver.

EPC9126 and EPC9126HC Laser Diodes
Driver The EPC9126 laser driver is a versatile platform for testing the performance of eGaN FETs and laser diodes. The PCBs for the EPC9126 and EPC9126HC are identical.

The two drivers have several different components, so the commercial EPC9126 has lower peak current and shorter pulses, while the EPC9126HC has higher output current and longer duration pulses.

The major differences are summarized in Table 1. Other than these differences, the boards are identical, and all of the contents of this application apply to both boards unless otherwise specified.
Therefore, we will refer to both drivers collectively as EPC9126xx. When shipped, both drivers are configured as resonant laser drivers. The basic operation of this driver and its design equations are discussed in the following sections.

Design of a resonant capacitive discharge laser driver
Figure 5 shows a simplified circuit diagram of a resonant capacitive discharge laser driver, and Figure 6 shows the main waveforms.
Assuming Q1 to be an ideal switch and DL to be an ideal diode with fixed forward voltage drop V _DLF , the driver operates as follows: Q1 starts in the off state, so i _DL = 0. The capacitor voltage v ₁ = V _IN is charged through R ₁ . At t = t ₀ , the V _command triggers the gate drive, and at t = t ₁ Q ₁ is fully turned on, discharging C ₁ through the laser D _L and inductor L ₁ . Since C ₁ and L ₁ form a resonant network, i _DL and v _C1 form a sinusoidal ring.

The effective initial capacitor voltage is V _C1,0 = V _IN – V _DLF due to the forward drop of the laser diode. At t = t ₂ , i _DL returns to zero, and v _C1 = 2 V _DLF – V _IN . At this point, D _L prevents the current from reversing, and C ₁ is recharged through R ₁ . Switch Q ₁ is turned off before V ₁ crosses zero at t = t ₃ .

The capacitor charging time constant T _chrg and the resonant period t _res are: In general, since T _chrg ≫ t _res , R ₁ has little effect on the L ₁ –C ₁ resonance. The resonant characteristic impedance R ₀ and the full width half maximum (FWHM) pulse width t _w are as follows: This laser driver topology has the following advantages:

• This topology utilizes stray inductance.
• Stable pulse shape
• The pulse energy is set via the V _IN value.
• The switches are ground-referenced for simple drive.
• Precise control is required only at gate turn-on (single edge control)
• The laser current pulse width can be shorter than the minimum pulse width of the gate drive.

Effect of floating inductance
The peak laser diode current I _DLpk can be calculated using the following equation: Inductance has a major impact on design. Vin can be obtained from (3), (4), (5), (6): Equation 7 shows the voltage V _IN calculated from (7) versus L ₁ for a 30 A, 4 ns pulse that produces a 9 V forward diode drop in the laser. It can be clearly seen that the required V _IN for this laser and pulse shape increases linearly with L ₁ .

Driver Switch Properties
The above analysis assumes an ideal switch, but real semiconductor switches have non-zero switching times and saturation current limitations. Additionally, the switch and its package may have significant inductance, which not only increases the voltage required for the pulse shape, but also slows down switch turn-on.

In the past, silicon power MOSFETs were chosen as the switching technology. However, silicon power MOSFETs have become a major limitation in realizing higher performance in lidar systems for two reasons.

First, the gate charge increases because it requires a large die size to meet the current and voltage requirements, which significantly delays the turn-on of the MOSFET.

Second, large MOSFETs are vertical devices connected on both sides of the die. This forces them to use external packages, which adds significant inductance to both the power loop and the gate drive loop. The former requires higher voltage and increases die size, while the latter delays device turn-on.

Over the past few years, new power FETs based on GaN have become commercially available. GaN FETs have several overwhelming advantages over silicon MOSFETs in lidar applications. First, they have up to 10 times lower input capacitance CISS than MOSFETs of the same rated current, and GaN FETs can be turned on much faster.

Second, GaN FETs are lateral devices that can use WLCSP (Wafer Level Chip Scale Package). WLCSP offers the advantages of very low inductance, excellent thermal performance and high stability, and minimal additional costs.

Finally, the die of GaN FETs is much smaller than that of silicon power MOSFETs with comparable voltage and current ratings, which allows for reduced inductance and closer spacing between adjacent lasers for applications such as multi-channel lidar.

In the next session, we will discuss the basic design process and overall experimental results that can be implemented based on this theory, using eGaN FETs.

#About this article E4ds.com is conducting an experimental discussion on GAN technology for LiDAR. If you have any questions about this article or would like to contribute further, please leave a comment below.