PLECS 3.1 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.

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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   (in K/W) and Ci   (in J/K) for each chain element need to be entered.

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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   in (K/W) and τi   (in s) for each chain element need to be entered. Foster networks can be converted to Cauer networks by pressing the button Convert to Cauer.

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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.

Semiconductor Loss Specification

Care must be taken to ensure the polarity of the currents and voltages are correct when specifying conduction and switching loss data for semiconductor switches and diodes. If one or both polarities are in the wrong direction, the losses will be zero or incorrect. The voltage and current polarities of a single semiconductor switch, diode and semiconductor switch with diode are defined in PLECS as shown in the figure below.

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Voltage and current polarity of single semiconductor switch, diode and semiconductor switch with diode

Single Semiconductor Switch Losses

The blocking voltage experienced by a single semiconductor switch is positive; therefore, switching losses are defined in the positive voltage/positive current region. Conduction losses are also defined in the positive voltage/positive current region.

Diode Losses

The voltage and current waveforms during a typical diode switching cycle are shown in the next figure. Turn on losses occur at t = t1   and turn off losses at t = t2  . The switching energy loss in both cases is calculated by PLECS using the negative blocking voltage and positive conducting current at the switching instant. These values are shown in the figure as dots. Therefore, the lookup tables for the turn-on and turn-off switching losses must be specified in the negative voltage/positive current region.

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Diode voltage and current during switching

Conduction losses occur when t1 < t < t2  . During this time period, the current and voltage are both positive. Therefore the conduction loss profile must be specified in the positive voltage/positive current region.

Losses of Semiconductor Switch with Diode

Semiconductor switches with an integrated diode such as the IGBT with Diode model allow losses for both the semiconductor switch and diode to be individually specified using a single set of lookup tables. The conduction and switching loss tables for the semiconductor switch are specified for the same voltage/current regions as for the single semiconductor switch without diode. Due to the polarity reversal of the diode, the diode losses are appended to the loss tables of the semiconductor switch by extending the tables in the negative voltage/negative current direction for the diode conduction losses, and in the positive voltage/negative current direction for the diode switching losses. An example turn-off loss table and conduction loss profile for a semiconductor switch with diode are shown in the next two figures. A summary of the valid voltage and current regions for defining conduction and switching losses for the different types of semiconductors is given below:

 DiodeSwitchSwitch with Diode
SwitchDiode
VIVIVIVI
Conduction Loss  ++++++--
Switching Loss-++++++-


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Turn-off loss lookup table for semiconductor switch with diode

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Conduction loss profile for semiconductor switch with diode