Thermal Modeling of AC-DC Power Adapter With Current Boost Regulator

This example shows the capability of modeling the electrothermal aspects of power dissipating circuits. The design is a 5V regulator (non-switching), driven from a 120 Vac/60 Hz input and using a transformer/rectifier circuit to step down to a much lower DC-link voltage.

The load current capability is 5A, which is well above the current limit of the linear regular component itself. This is thanks to the load sharing role of the bypass PNP transistor. The design is based on an example application circuit shown in Figure 11 of the On Semiconductor Datasheet MC7800/D, November 2014 - Rev. 27.

All of the power dissipating electronics' models are from the "Thermal and Electro-thermal" Components Library. They have a thermal port that connects to the thermal network (i.e. red wires). These models output all the power dissipated in the device as a thermal heat-flow into that network. This includes the rectifier diodes, the linear regulator and the BJT, as well as the current sense resistor and the effective winding resistances of the transformer primary and secondary.

The thermal network includes the heat-sink's heat capacitance (0.1 J/degC) and heat transfer resistance to the ambient (1 degC/Watt), as well as the datasheet published values for the junction-to-lead thermal resistances of all the active electronic components. An assumed value for the thermal capacitance of the BJT (0.005 J/degC, not provided by the manufacturer) was added for purposes of illustration. Obtaining the actual value would require deeper analysis or measurement of this important component characteristic. However, in this example, both thermal capacitance values were selected solely to give sufficiently fast thermal time constants, so that steady-state operating temperatures could be reached with minimal simulation time.

This design is based on another shared design:
"https://www.systemvision.com/design/ac-dc-power-adapter-current-boost-re..., That design has been modified not only to add the electrothermal aspects, but it was also adjusted to improve its observed thermal performance. For example, the DC-link capacitance (c3) was increased from 4700uF to 22000uF, to allow a reduced DC-link operating voltage (i.e. to improve efficiency) while avoiding regulation drop-out at AC zero-crossings under heavy load conditions.

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Mike Donnelly

Joined November 8, 2013
Design added Saturday, April 1, 2017 | 1:28 pm PDT