High-efficiency LED COB device combined diced
V-shaped pattern and remote phosphor
Zongtao Li (李宗涛)
1
, Huiyu Wang (王卉玉)
1
, Binhai Yu (余彬海)
1,
*,
Xinrui Ding (丁鑫锐)
2
, and Yong Tang (汤 勇)
1
1
The Key Laboratory of Surface Functional Structure Manufacturing of Guangdong High Education Institutes,
South China University of Technology, Guangzhou 510641, China
2
The Department of Mechanical Engineering, University of California at Berkeley, Berkeley, California 94720-5800, USA
*Corresponding author: bhaiyu@yeah.net
Received August 11, 2016; accepted December 28, 2016; posted online January 22, 2017
To further improve the luminous efficiency of LED lightings, this Letter proposes a chip-on-board (COB) device
by combining diced staggered V-shaped patterns and remote phosphors. The results show that the V-shaped
patterned COB (V-COB) with vertex angles from 120º to 150º can achieve a ∼17% output power increase (OPI)
compared to the conventional COB. V-COB remote phosphor devices (RPDs) are then manufactured and
tested. The luminous efficiency of the proposed RPD represents an 11.6% increase at the correlated color temper-
ature of ∼3000 K. Such an improvement can be attributed to both the decreases of total internal reflections and
phosphor backscatterings.
OCIS codes: 230.3670, 220.0220, 250.5230.
doi: 10.3788/COL201715.042301.
Phosphor-converted white-light LED devices, insofar as
they are used for general lightings
[1]
, backlights
[2]
, and wire-
less communications
[3]
, can be categorized into those with
discrete components and those with chip-on-board (COB)
devices. The former are typically comprised of a lead
frame, an LED chip, and an optical encapsulant. In prac-
tice, multiple discrete LEDs (using a surface-mounting
assembly) are required for gaining sufficient luminous flux,
which tends to be high cost, forms a multishadow pattern
in the illumination area
[4,5]
, and makes the human eyes feel
tired. By way of contrast, COBs are able to offer a bright
lighting environment, though they utilize only one source.
In effect, this allows engineers to easily design high-
performance illumination systems with a low cost and
without the multishadow pattern.
In a conventional COB, a silicone or phosphor-silicone
mixture is directly dispensed to the LED-chip-array for
blue-light transmission and downconversion, resulting in
a flat encapsulant surface. Light emitted from the chips
or the phosphor particles undergoes total internal reflec-
tions (TIRs) since the critical angle on the silicone-air in-
terface is only 38° to 45° (refractive index of the silicone is
1.4 to 1.6)
[6]
. Techniques such as dome-shaped lenses
[7]
,
textured silicones
[8,9]
, and TiO
2
encapsulations
[10]
were pro-
posed to improve the COB light outcouplin g. For the first
method, the volume of the lens has been improved to
∼18000 mm
3
, which was 17 times higher than that of
the original structure, and made it high cost and difficult
to fabricate. In the textured silicone case, a special and
unique mold was required for one COB size, and it was
against the increasing requirements of product diversity.
Introducing of scattering TiO
2
material was an easy way
to gain high light-extraction efficiency; however, the set-
ting phenomenon of the exotic particles was rather
complex
[11,12]
, which would significantly affect the product
consistency. Therefore, these methods were still lacking a
practical application. In the recently released CREE flip
chip DA1000, the brilliant engineer utilized a V-shaped
pattern to cover the surface of the LED chip, and achieved
a >60% electro-optical efficiency
[13]
. This technique was
quite impressive to the LED researchers because the pat-
tern was manufactured by a dicing saw, and it was flexible
to be adapted to different types of optical microstructures
with a high efficiency and low cost
[14]
. The LED COB and
the CREE chip faced the same problem: how to improve
the light extraction in a high refractive index difference
interface. Both the lecture and the industry are now curi-
ous about whether this technique is effective in the former
case (LED COBs).
Moreover, the contigui ty configuration in the conven-
tional COBs (the chip and the phosphor) would simulta-
neously backscatter about 60% of the luminous flux to the
chips, leading to a large portion of energy absorption
[15]
.
The existing methods only focused on the COB TIR prob-
lems, and never considered the backscattering energy loss.
Therefore, the efforts to increase the luminous flux were
significantly diminished as the phosphor concentration in-
creased
[10,16]
. This made it a great challenge to fabricate
high efficiency COBs with a low correlated color temper-
ature (CCT). Placing the phosphor layer a certain dis-
tance away from the chip therefore constituted a better
option
[17–19]
, which was referred to as remote phosphor
devices (RPDs). This structure reduced the probability
of the phosphorescence directly hitting the chip and
increased the luminous efficiency
[20]
.
In this Letter, both the diced V-shaped patterns and the
remote phosphor are combined to increase the luminous
efficiency of low CCT LED COBs. The ray dynamics of
COL 15(4), 042301(2017) CHINESE OPTICS LETTERS April 10, 2017
1671-7694/2017/042301(4) 042301-1 © 2017 Chinese Optics Letters