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10 / 192018-08-16 | Document No.: AN097
Figure 10: Optical bandpass filter for SFH 4780S with operating conditions of 1 A/
10 ms. The design ensures that 75 % of total optical power is transmitted at the
ambient temperature extremes of -20 °C and 65 °C. Note that good connection to a
heat sink (low R
th
) is mandatory as T
j
has direct influence on the passband (upper cut-
off wavelength). No guardband/ angle of incidence effect considered
The following example provides a basic approach to calculate the required
optical filter passband (by using the following operating condition):
•I
F
= 1 A, 10 ms on-time with low duty-cycle
T
j
~ 55 °C (at T
a
= 25°C)
FWHM~30 nm (at T
a
=25°C)
•T
a
= - 20 °C...+ 65 °C
• good heat sink (low R
th
)
•TC
λ
=0.25nm/K
•TC
Ie
= - 0.3 %/K (temperature coefficient of I
e
)
•λ
centroid
distribution ±13 nm
• transmission (optical bandpass filter):
>75 % at the ambient temperature extremes (- 20 °C and 65 °C)
At - 20 °C the actual output power is increased by ~ 13 % versus T
a
=25°C.
Thus the required transmission at the lower cut-off wavelength needs to match
> 62 % to transmit 75 % of the optical power at 25 °C. Assuming a
FWHM ~ 25 nm, the actual lower cut-off wavelength can be estimated to be:
•λ
cut_off_lower
=810nm
- 13 nm (centroid distribution)
- 0.25 nm/K · 45 K (thermal shift)
- 6 nm (cut-off to transmit > 62 %)
= ~ 780 nm
Wavelength range [nm]
Norm. filter transmission
0.6
0.4
0
400 500 800 900 1000
1.0
0.8
0.2
600 700