I recently did a “tear-down” of a Baofeng BL-8 battery eliminator (for use with a UV82 dual-band tranceiver; see www.hamguyparts.com, articles). The tear-down revealed that the eight volt battery eliminator actually used 2X L7808CV TO-220 voltage regulators connected pin for pin in parallel, which is somewhat unconventional.
Both regularors were mounted to a common small (20 x 34 x 1.8 mm) aluminum heatsink and all enclosed within the plastic housing which was sized to resemble the battery it was replacing.
In the ideal case, each regulator would carry 1/2 the total maximum current taken by the transceiver, which is approx. 0.775 amps. Each regulator produces a nominal 8.0 volt DC output and has to dissipate approx 4.6 watts of heat. The thermal resistance of the TO220 package is 5°C per watt, junction to case (or mounting base).
They are mounted dry, which adds another 1°C per watt for mounting the base to the heatsink.
Q1: What is the temperature rise in the aluminum plate?
Q2: Since there is no direct path to air, what further temperature rise may take place inside the plastic housing?
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I searched my hard disc and found a chart for thermal resistance vs. heat sink area for a 1.5mm thick heat sink. 1.8mm is not much different from 1.5mm so I will use that. This chart ends at 500 sq. cm. so I will extend the line and estimate that the thermal resistance is 2 deg C per watt. I am also assuming the thermal resistance is in deg C/watt since it is not otherwise labeled.
Since the dissipation is 4.6 watts per TO220 and there are two of them, the power to the heat sink is 9.2 watts. The heat sink area is 34X20=680 sq. mm. The temperature rise in the heat sink is: 2 deg C/watt X 9.2 watts = 18.4 deg C. The temperature rise to the junction of the TO220 is: 6 deg/watt X 4.6 watts = 27.6 deg C. Adding these together, the junction of the TO220 will be 46 deg C above the heat sink temperature which will be above the ambient of nominal 25 deg C.
It is not possible to estimate the further rise of the heat sink due to lack of air circulation without more information such as size of the enclosure, thickness of the plastic, thermal resistance of the plastic, etcetera.