As a starting point, the output voltage is selected and then we proceed in the direction of the transformer (OK so). Originating from 24V output voltage, 3V more must be added in front of the voltage regulator. The voltage regulator requires this so-called dropout voltage to work. Through the bridge rectifier, we also lose the forward voltage from two diodes—that is, around 1V (here Schottky diodes were used). Here, the transformer must deliver 24V+3V+1V=28V to the secondary side. This is the peak-to-peak voltage. The output voltage of the transformers is typically emitted as an effective value. The calculated peak-to-peak voltage of 28V then needs to be divided by a factor of 1.4142 (square root of 2). We thus get an effective secondary voltage of 28V / 1.4142 = 19.8V. A transformer is required that, at a voltage of 230V, delivers an output voltage of around 19.8V. Unfortunately, that is not the whole truth. The supply voltage can amount to 230V+/-23V. Hence, with the minimum supply voltage of 203V, the transformer must already deliver 19.8V at the outset. Therefore, a 24V transformer is selected, which can deliver the minimum voltage of 19.8V at a low supply voltage.
What does the voltage ratio now look like with the selected transformer? It will provide a peak-to-peak voltage of 24V x 1.4142 = 33.94V at the bridge rectifier. After the bridge rectifier, it will still be 32.94V (1V less because of two conductive Schottky diodes). The voltage regulator will convert a power loss of Pv=(32.94V – 24V)x0.25A=2.24W, which will be emitted in the form of heat. In addition, the regulator, which is housed in a SMD SOT-263 package, will conduct the heat to the copper of the circuit board. For measuring how warm the voltage regulator becomes, we conducted measurements (see fig. 2). As a load,we used an adjustable resistor and, for the circuit in fig. 1, we produced three circuit boards that varied in the number of copper layers and copper thickness (see table 1). Subsequently, all three circuits were measured on the different circuit boards. Moreover, 10V were charged at the input of the voltage regulator, and 0.25A at the output. This ensures that all three measurements were carried out with exactly the same power loss of 2.5W. The warming of the voltage regulator and the circuit board were recorded with an infrared (IR) camera.
The results can be seen in figs. 3a–3c: with the single-sided circuit board, the voltage regulator has less copper surface for the discharge of heat and warms up to ca. 88°C. The copper surface of this circuit board is approximately 1000mm2 (0.1dm2, see fig. 3a). With the double-sided circuit board with a copper thickness of 35µm, both layers are connected to one another by 70 vias. This way, portions of the heat are also conducted to the lower layer and the available copper surface is roughly twice as large (ca. 2160mm2, 0.216dm2). It must be noted, however, that because of the vias, the thermal coupling is not optimal. The voltage regulator warms up to around 74.5°C (see fig. 3b).Finally, fig. 3c shows the measurement for the double-sided circuit board with a copper thickness of 70µm. Here, the thicker copper allows for better heat distribution and the voltage regular heats up to ca. 69.8°C.
Cooling the SMD voltage regulator requires a large copper surface. This can also be realized with a two-layer circuit board, by through-connecting two copper layers with several vias, in order to draw the heat onto the lower copper layer. With this step, the warming of the voltage regulator can be reduced from 88°C to 74.5°C. Increasing the copper thickness from 35µm to 70µm, and thus the cross section of the surface,resulted in a further temperature reduction, to 69.8°C.