APPLICATION NOTE: AN027
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eGaN FETs for Lidar – Getting the Most Out of the EPC9126 Laser Driver
eGaN FETs for Lidar –
Getting the Most Out of the
EPC9126 Laser Driver
EFFICIENT POWER CONVERSION
John S. Glaser, Director of Applications Engineering; Efficient Power Conversion Corporation
Figure 1. Basic lidar system
Figure 2. Some typical laser diodes used for TOF lidar.
Lidar is a form of radar where the electro-
magnetic radiation happens to be in the
optical band [1, 2]. In the last few years,
one particular form of lidar, time-of-ight
(TOF) distance measurement, has become
popular. If a laser is used as the optical source,
one can measure the distance of a small spot
even at a long range. When combined with
steerable optics, one can sweep the spot
distance measurement and map objects in
3-D space.
EPC has developed the EPC9126 and
EPC9126HC laser drivers to demonstrate the
performance of eGaN FETs in lidar systems,
and to advance the lidar state of the art
[3, 4]. The EPC9126 and EPC9126HC are both
provided with Quick Start Guides (QSG) that
provide basic information on getting up and
running along with schematics and bills of
materials. Layout les for the drivers are also
freely available.
However, while the basic principle of the
laser drivers seems simple, the high speeds,
voltages, and currents result in the dominance
of parasitic components that many engineers
are used to ignoring or designing away.
This document was written to answer com-
mon questions and to provide more depth on
laser driver design, so that the user can get the
most out of their driver.
Please note that this application note is
intended as a complement to the QSG, and
the user should have both handy when
learning about and applying the EPC9126 and
EPC9126HC.
Lasers and pulse requirements
TOF lidar usually uses near-infrared (NIR) semiconductor laser diodes, either
side-emitting epitaxial lasers or vertical cavity surface emitting lasers (VCSELs).
Some typical laser diodes are shown in
Figure
2 [5, 6]. Electrically, the laser diode
behaves as a rectifier. When forward biased above a certain threshold current, it
emits laser radiation with the output optical power approximately proportional to
the forward current. Thus, if we drive it with a pulse of current, we get a pulse of laser
light [7]. The laser optical pulse has two main parameters: pulse width and energy.
These two factors have a large effect on the distance resolution and the range,
respectively.
3-D point cloud
Target
Laser
transmitter
Receiver
Signal processing
Scan optics
Transmitted beam
Reected beam
SPL PL90_3 TPGAD1S09H
APPLICATION NOTE: AN027
EPC – THE LEADER IN GaN TECHNOLOGY | WWW.EPC-CO.COM | COPYRIGHT 2019 | | 2
eGaN FETs for Lidar – Getting the Most Out of the EPC9126 Laser Driver
The pulse width of the transmitted optical
signal has a great influence on the distance
resolution of a lidar system [8, 9]. Figure 3 helps
show why this is the case. If we look at the top
case, we send narrow pulses of light out from
the lidar. Since the light pulse must travel to
the target, be reflected, and travel back, for a
target at distance d, the time t
d
between pulse
transmission and reception is:
t
d
= 2d/c
Where c is the speed of light in air,
approximately 30 cm/ns (about 1 foot/ns for
the imperialists among us). By measuring
the time t
d
, we can compute the distance.
Now suppose that we send longer duration
pulses, as shown in the bottom case.
We see that if the pulse length becomes
long enough, the reected pulses begin to
overlap, and it becomes harder to distinguish
features in the environment.
For an idea of what pulse lengths are
desirable in practice, consider an electrical
current pulse width of 1 ns driving the laser
diode, which corresponds to an optical pulse
length of 30 cm. As features of the target
approach 15 cm, the received pulses begin
to overlap and become harder to distinguish.
While various signal processing techniques
can improve the resolution for a given pulse
width, it is clear that a shorter pulse gives
better inherent precision, and that pulses on
the order of a few nanoseconds or less are
desirable for human-scale resolution.
Pulse energy determines the range of the
lidar. As demand for better resolution drives
designs towards narrower pulses, the diode
current must increase in order to maintain
sucient pulse energy. Typical pulse current
can range from a few amps to hundreds of
amps. A number of laser diodes are specied
with nominal pulse currents in the range of
several tens of amps. Under typical data sheet
test conditions, e.g. Pulse repetition frequency
(PRF) = 1 kHz, pulse width t
w
= 100 ns,
peak
current I
DLpk
= 30 A, operating temp T
OP
= 23-
25°C,
the peak electrical input power can
approach 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. It is possible to operate these laser
diodes at higher currents with shorter pulse
widths and obtain greater peak optical power.
In summary, typical laser diode requirements
for commercial o-the-shelf laser diodes in
lidar systems suitable result in desired peak
pulse current ranges from a few amps to a few
hundred amps, with pulse widths from 1 ns to
10 ns. In the next section, we will see how to
obtain these extreme pulses.
t
p
t
p
Lidar
Transmission
Reection
Transmission
Reection
Lidar
Figure 3. Eect of lidar pulse width on resolution. Top: narrow pulses allows reections to be easily distinguished.
Bottom: wider pulses can overlap, making them harder to distinguish and reducing distance resolution.
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