Datasheet
VV
V
F
/ V
F (25°C)
Relative Forward Voltage
1,06
1,04
1,02
1
0,98
0,96
0,94
0,92
10 20 30 40 50 60 70 80
T
B
[°C]
Figure 14
Relative forward voltage
V
F
versus ACULED VHL
board temperature T
B
for
red, green and blue chips.
Influence on flux and intensity
The flux,
Φ
e
and
Φ
V
, and their deducted values, such as luminance, radiance, luminous
intensity or radiant intensity, decreases with increasing temperature. Generally speaking, the
intensity drop of blue and green chips is usually small, whereas the drop with yellow, amber and
red chips is larger. Figure 15 shows typical curves representing the relative luminous drift for the
chips of the RGYB ACULED VHL. These charts can be found in the specific datasheets of the
ACULED VHL products. For the ACULED DYO, an approximation is given by the luminous or
radiant flux temperature coefficient (TC
Φ
V
resp. TC
Φ
e
) in the specific datasheets of the chips.
The change
∆Φ
of the luminous or radiant flux can be calculated analogous to the calculation of the
forward voltage drift over temperature by the following equation:
∆Φ
= TC
Φ
·
∆
T
J
(10)
Φ
1
= TC
Φ
· (T
J1
- T
J0
) +
Φ
0
(10a)
Φ
0
is the known flux at a known temperature T
J0
given by the datasheets. In a steady state, the
junction temperature T
J
can be interchanged with the substrate temperature T
B
in the equation:
Φ
1
= TC
Φ
· (T
B1
- T
B0
) +
Φ
0
(10b)
If a certain flux is necessary in your application, it is important to level out the intensity drop. A
good thermal management will also help you keep the drift as low as possible. The balancing of
the drift over temperature is important, particularly when having chips of different colors on your
ACULED like RGGB or RGYB, to keep the same intensity ratio and, therefore, the same color
appearance. With the RGYB for example, the color mix drifts to a blue-greenish light with
increasing temperature, since yellow and red fade out much more than blue and green, as
shown in figure 15. Due to the excellent suppression of any thermal crosstalk, each chip can be
levelled out individually without regard for how its temperature and heating change influence its
neighbors.
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