Page 175 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Stability and Tempco Issues

168 Chapter Eight

can be thermally bonded to the photodiode or LED package. Mechanical clamp-
ing, mounting in a common high thermal-conductivity block, and/or bonding
with thermally conductive adhesives can all be effective in delivering a tem-
perature value, as measured by the thermistor, which is representative of that
of the optoelectronic component. This is especially the case with photodiodes
with metal or ceramic packages with good thermal-conductivity.
In the case of lower-cost, plastic-encapsulated LEDs, bonding to the ther-
mistor gives only limited temperature tracking. These LEDs suffer from larger
temperature variations due to their significant (ª50mW) internal dissipation
and higher chip-ambient thermal resistance. The coupling can be improved by
bonding the thermistor to the LED lead which holds the semiconductor chip
(Fig. 8.4), directly outside the polymer encapsulation. The other lead is less
useful, as it is thermally isolated by the fine topside wire-bond. If in addition
the leads can be cropped short and connected to the PCB with much finer lead
wires, isolation from the environment can be improved. Machining away the
underside of the LED polymer to position the thermistor in contact with the
chip support metalwork performs even better, but is tricky to do. Surface-mount
LEDs can also be well coupled due to their very small dimensions.
Of course, thermistors are not the only temperature sensors which can be
used for compensation. In principle, all the other available sensor types could
be applied, compensating either via analog circuitry as in Fig. 8.2, or by tem-
perature measurement, and compensation in software. Table 8.1 shows a few
of the different semiconductor types available, providing either analog voltage
or current outputs, or a digital signal most easily read by a microprocessor.
However, despite the electronic convenience of the semiconductor ICs, and espe-
cially the digital devices, it is difficult to beat the small size and hence mechan-
ical convenience of some of the available bead thermistors.

Molded
lens
LED
chip
Wire bond
to other lead

Bead thermistor
bonded to chip
support lead

PCB

Figure 8.4 Thermal coupling to the chip of a plastic encapsu-
lated LED is most efficient through the substrate lead.

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