Feature: Industrial electronics
From Equation 1, Tj (~5˚C below Tjmax
). Without adding cost and volume by using heatsinks, this
device would be unsuitable for the application because of its very limited temperature overhead. A better solution would be to select a device with an
extended temperature range. T ere are many commercial DC-DC regulators that off er a Tjmax
extend it to 150˚C, such as the Recom Power solution. Secondly, the input and output voltages could be more
closely matched, which raises the effi ciency of a linear regulator and thus lowers power dissipation. And thirdly, the designer should select a device with the lowest thermal impedance. Considering these criteria, here’s a second example:
Ta
Figure 4: Shown is a graph of effi ciency vs. percent output load for the Recom Power’s R05C05TE05S-CT. The isolated switching converters deliver good effi ciency over a wide load range
to keep component temperatures down and provide satisfactory overhead between Tj
and Tjmax ; see Figure 5. IIoT sensors oſt en operate in confi ned environments with little
ventilation. T is can cause ambient temperature to rise, which in industrial environments can easily reach 70˚C – impacting the components’ temperature overheads. Consider the following example for a typical DC-DC converter:
Ta = 70˚C Tjmax = 125˚C
Power loss = (6.5 – 5.0V) x 370mA = 555mW Rθja
component). = 90˚C/W (a typical value for a conventional surface-mount = 70˚C Tjmax = 150˚C
Power loss = (5.5 – 5.V) x 370mA = 185mW Rθja
= 64˚C/W T en Tj = 70 + 0.185 x 64 = 82˚C (68˚C below Tjmax )
T is option provides considerable temperature overhead, which helps extend product life. In certain circumstances, such as an enclosed space near a
machine powered by large electric motors that radiate a lot of heat, the ambient temperature can rise even higher. In these situations, chip makers recommend derating – limiting the output power of the device to lower power dissipation and, in turn, Tj
. For example, consider the second DC-DC converter
example: a rise in temperature to 110˚C would leave only around 38˚C of temperature overhead, which is less than recommended for extended product life.
of 125˚C, and a few that
= 70 + (0.555 x 90) = 119.95˚C
Figure 5: Component makers specify a maximum junction temperature (Tjmax dissipation, Tj
is largely determined by the component’s total thermal impedance and the ambient temperature (Ta
) for an active device, to ensure proper operation. For a given power )
22 September 2022
www.electronicsworld.co.uk
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