PLECS 2.0 Online Help

Thermal Editor

The Thermal Editor is used for creating, viewing and editing thermal data sheets. To open a new editor window select New... + Thermal description... from the File menu. Existing library data sheets can be edited either in the Thermal library browser (accessible from the View menu) or by assigning a data sheet to a semiconductor in the Thermal description parameter and then selecting the menu entry Edit.... The Thermal Editor faciliates editing switching losses, conduction losses and the thermal equivalent circuit of a component.

[Picture]

The text entries Manufacturer, Part number, and Comment are for documentation purposes only. The Type selector serves as a filter for the Thermal description menu entry. It must be set according to the semiconductor type it is intended to be used with.

In order to access the data sheet in a PLECS model it must be saved in a subdirectory on the thermal search path. See section Thermal Library for details of the structure of the thermal library.

Editing Switching Losses

Switching losses are defined as a 3D lookup-table in the Turn-on loss and Turn-off loss tabs. The energy for each switching event depends on the blocking voltage, the device current and the device temperature. PLECS uses a linear interpolation technique to calculate the actual losses from the given values.

New interpolation points for temperature, voltage and current are added and removed with the Edit menu or the context menu in the table. Multiple values can be added separated by semicolons or spaces.

To rotate and tilt the 3D view move the mouse within the view while keeping the left mouse button pressed.

Editing Conduction Losses

Conduction losses are defined by means of the on-state voltage drop as a 2D lookup-table in the Conduction loss tab. The voltage drop depends on the device current and the device temperature. PLECS uses linear interpolation to calculate the actual voltage drop from the given values.

New interpolation points for temperature and current are added and removed with the Edit menu or the context menu in the table. Multiple values can be added separated by semicolons or spaces.

Editing the Thermal Equivalent Circuit

The thermal equivalent circuit of a component describes its physical structure in terms of thermal transitions from the junction to the case. Each transition consists of a thermal resistor and a thermal capacitor. They can be edited in the Therm. impedance tab of the thermal editor. The thermal equivalent circuit is specified either in Cauer or Foster form.

The structure of a Cauer network is shown in the figure below. In the thermal editor the number of chain elements n   and the values for Ri   and Ci   for each chain element need to be entered.

pict
Cauer network

The figure below illustrates the structure of a Foster network. In the thermal editor the number of chain elements n   and the values for Ri   and τi   for each chain element need to be entered. Foster networks can be converted to Cauer networks by pressing the button Convert to Cauer.

pict
Foster network


Note  Internally, PLECS always uses the Cauer network to calculate the thermal transitions. Foster networks are converted to Cauer networks at simulation start. Strictly speaking, this conversion is only accurate if the temperature at the outer end of the network, i.e. the case, is held constant. For practical purposes the conversion should yield accurate results if the external thermal capacitance is much bigger than the capacitances within the network.