Thermal Editor

The Thermal Editor is used for creating, viewing and editing thermal data sheets. To create a new data sheet, choose Thermal description… or Thermal package description… from the File > New… menu. In order to access the data sheet in a PLECS model, you must save it in a directory on the thermal search path. See section Thermal Library for details of the structure of the thermal library.

Existing library data sheets can be edited either in the Thermal library browser (accessible from the Window menu) or by assigning a data sheet to a semiconductor in the Thermal description parameter and then selecting the menu entry Edit….

../../_images/thermaleditor.png — Fig. 63 Thermal Editor window

The top row of the Thermal Editor window shows three input elements:

Manufacturer, Part number: These text fields are for documentation purposes only.
Type: This string is used by the Thermal Description parameter of semiconductor devices and thermal mask parameters to filter matching thermal descriptions (see Selecting Thermal Data Sheets).

Thermal Description for a Single Device

When you edit the thermal description of a single device, the editor shows the following tabs:

Turn-on Loss, Turn-off Loss, Conduction Loss

On these tabs you define the switching and conduction losses of the device. See Editing Switching Losses and Editing Conduction Losses.

Thermal Chain

On this tab you define the thermal impedance between the junction and the case of the device. See Editing the Thermal Equivalent Circuit.

Constants

On this tab you define custom constants that can be used in the loss formulae for switching and conduction losses.

Variables

On this tab you define custom variables that can be used in the loss formulae for switching and conduction losses. Custom variables can be edited in the parameter dialog of the component using the thermal description (see Specifying custom variable values). On this tab you can also specify hard limits both for your custom variables and for intrinsic variables, i.e. the blocking voltage, the device current and the junction temperature.

Custom Tables

On this tab you define custom lookup tables that may be used to define device losses.

Comment

This tab provides you with a text field that you may use for documentation purposes.

Editing Switching Losses

Switching losses are defined on the Turn-on Loss and Turn-off Loss tabs. The Computation method popup specifies whether the loss function is defined as a 3D lookup table, a functional expression or a combination of both.

If you select Lookup table, the pane below will show a 3D lookup table with the blocking voltage, the device current and the junction temperature as input variables. For more information regarding lookup tables see Editing Lookup Tables.

If you select Formula, the pane below will show a text field that allows you to enter a functional expression. A formula may consist of numerical constants including pi, arithmetic operators (+ - * / ^), mathematical functions (abs, acos, asin, atan, atan2, cos, cosh, exp, log, log10, max, min, mod, pow, sgn, sin, sinh, sqrt, tan, and tanh), brackets and the function arguments. The default function arguments are the blocking voltage v, the device current i and the junction temperature T. You may define additional function arguments on the Variables tab (see Adding Custom Variables). You may also reference custom lookup tables using the function lookup (see Adding Custom Lookup Tables).

If you select Lookup table and formula, the pane below will show both lookup table and formula field. With this method, an energy E is first computed from the lookup table and may then be used in the formula to calculate the final loss energy value. For instance, in order to quickly increase the switching loss by \(20\,\%\), you could enter 1.2*E into the formula field.

Editing Conduction Losses

Conduction losses are defined by means of the on-state voltage drop on the Conduction Loss tab. The Computation method popup specifies whether the voltage drop is defined as a 2D lookup table, a functional expression or a combination of both.

If you select Lookup table, the pane below will show a 2D lookup table with the device current and the junction temperature as input variables. For more information regarding lookup tables see Editing Lookup Tables. On using the import wizard for constructing lookup table data from vendor plots, see Importing Data from Graphical Datasheets.

If you select Formula, the pane below will show a text field that allows you to enter a functional expression. The default function arguments are the device current i and the junction temperature T. You may define additional function arguments on the Variables tab (see Adding Custom Variables). You may also reference custom lookup tables using the function lookup (see Adding Custom Lookup Tables).

If you select Lookup table and formula, the pane below will show both lookup table and formula field. With this method, a voltage v is first computed from the lookup table and may then be used in the formula to calculate the final voltage drop value. For instance, in order to quickly increase the voltage drop by \(20\,\%\), you could enter 1.2*v into the formula field.

Gate Dependent Conduction Losses

A MOSFET can conduct current in both directions, and so the conduction losses of a MOSFET with integrated anti-parallel diode depend on the gate signal when the current flows in reverse direction. To account for this effect, you can provide two separate conduction loss definitions, one of which is used when the gate signal is non-zero, and the other, when the gate signal is zero.

To create separate tabs that define the conduction losses with respect to the gate signal, select the MOSFET with Diode device type, right-click on the tab bar and select Use gate dependent conduction losses from the context menu.

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. Due to the simple structure of these thermal equivalent circuits, these networks may simply be referred to as thermal (RC) chains. They can be edited on the Thermal Chain tab of the thermal editor. The thermal equivalent circuit is specified in either Cauer or Foster form.

../../_images/thermalchaintab.png — Fig. 64 Thermal Chain Tab of the Thermal Editor

The structure of a Cauer network is shown in Fig. 65. In the thermal editor, the number of chain elements \(n\) and the values for \(R_i\) in \((\mathrm{K}/\mathrm{W})\) and \(C_i\) in \((\mathrm{J}/\mathrm{K})\) for each chain element need to be entered.

../../_images/cauer.svg — Fig. 65 Cauer network

Fig. 66 illustrates the structure of a Foster network. In the thermal editor, the number of chain elements \(n\) and the values for \(R_i\) in \((\mathrm{K}/\mathrm{W})\) and \({\tau}_i\) in \((\mathrm{s})\) for each chain element need to be entered. Foster networks can be converted to Cauer networks by pressing the Convert to Cauer button.

../../_images/foster.svg — Fig. 66 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 meaningful 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.

Detecting and Fixing Parameterization Problems

Sometimes the conversion from the Foster to the Cauer type can lead to unreasonably high thermal capacitance values. This is because some Foster networks contain elements with a large time constant (\(\tau\)) and small thermal resistance or different elements with very similar time constants. This is typically the result of a non-optimal fit of the thermal impedance response curve obtained by measurement or finite element simulation. Because there is a unique transformation between Foster and Cauer network coefficients for circuits with constant ambient temperature, these non-optimal coefficients will also be reflected in the Cauer chain. PLECS is able to fix Foster coefficients (and therefore Cauer coefficients) in the following situations:

A Foster chain features multiple elements with a similar \(\tau\) value. In this case the number of Foster elements can be reduced. For identifying similar \(\tau\) values, an internal tolerance is used.
A Foster chain features elements that are barely visible in the impedance curve but have a large impact on the thermal capacitance coefficients when converted to the Cauer type. These elements can be eliminated by an overall fix which uses a model order reduction algorithm.

If PLECS detects one of these two problems, you can press the Fix Coefficients button. Otherwise it is grayed out. Clicking this button opens a dialog that displays a preview of the fixed Foster coefficients and shows the transient thermal impedance curves before and after fix (see Fig. 67).

../../_images/thermalchainfix.png — Fig. 67 Preview of Fixed Foster Coefficients after Conversion Issues

In this particular example of the Foster coefficients of an IGBT device, a pair reduction (the two elements with close \(\tau=0.0913\,\textrm{s}\) and \(\tau=0.0914\,\textrm{s}\), as shown in Fig. 64 of the Thermal Chain tab, have been combined into a single element) and an overall fix (addressing the last Foster element with \(\tau=28.98\,\textrm{s}\)) have reduced the original five-element thermal chain to a three-element one. The importance of this fix can be understood by comparing the total capacitance of the two Cauer-type networks derived from the Foster coefficients before and after the fix. The original coefficients correspond to a total heat capacity of \(C_\textrm{tot}^\textrm{original}=25.1\,\textrm{MJ/K}\), while the fixed coefficients result in \(C_\textrm{tot}^\textrm{fixed}=4.96\,\textrm{J/K}\). These figures become even more tangible when calculating the equivalent mass of copper required to realize such a heat capacity. Assuming a specific heat capacity of copper of \(385\,\textrm{J/(kg K)}\), the original case would correspond to a copper mass of about 65 tons (!), while the fixed case would correspond to only \(12.9\,\textrm{g}\) of copper. In this way, the fix feature can help to avoid massively delayed heat flows through the final Cauer-type network, while at the same time preserving the shape of the transient thermal impedance curve used to derive the original coefficients.

Adding Custom Variables

Custom variables, such as gate resistance or stray inductance, that you wish to use in the definition of device losses may be defined on the Variables tab (see Fig. 68).

../../_images/thermaleditorvariables.png — Fig. 68 Thermal Variables Tab

Use the Add plus , Remove minus , Up and Down buttons on the left to add or remove custom variables or to reorder them. Note that the first three lines in the list are reserved for the intrinsic variables and may not be removed or reordered.

A custom variable is defined by a Prompt, which should provide a brief description of the purpose of the variable, and a Variable, which must be a unique identifier. This identifier may then be used in the formula expressions that define the device losses. You may specify a Default value that is used if the end user of the thermal description does not provide a value.

In the Min and Max columns you may enter minimum and maximum allowed values for both intrinsic and custom variables. During a simulation, PLECS will monitor the actual values of the variables and raise a diagnostic message if a variable value exceeds a specified limit. The default action is to show an error and stop the simulation but this may be changed in the simulation parameters dialog using the diagnostic parameter Loss variable limit exceeded (see PLECS Standalone Parameters and PLECS Blockset Parameters).

Specifying custom variable values

When you select a thermal description with custom variables on the Thermal tab of a semiconductor parameter dialog, the dialog will show additional parameter fields for the custom variables using the prompts mentioned above. An example dialog is shown in Fig. 69.

../../_images/thermalparametervariables.png — Fig. 69 Example Dialog for Custom Thermal Parameters

Instead of static values that remain constant during a simulation you may also specify the label of a Signal Goto block for a custom variable. The label is a string consisting of a prefix for the scope (g: for global, s: for schematic and m: for masked subsystem) and the tag name of the Goto block. For example, if your model contains a Goto block with global scope and the tag name Rg, you would enter 'g:Rg' (including the quotation marks) in order to reference this signal in a custom variable of a thermal description. This can be used e.g. to simulate the effect of a gate drive that can dynamically change the effective gate resistance.

Example Model

See the example model “Custom Variables in Thermal Descriptions”.
Find it in PLECS under Help > PLECS Documentation > List of Example Models.

Adding Custom Lookup Tables

Custom lookup tables are defined on the Custom Tables tab. To add a new table, click on New… and specify a unique name and the number of dimensions of the new table. Use the Duplicate…, Rename… and Remove buttons to duplicate, rename or remove an existing custom table.

Custom tables can be used in function expressions for device losses using the lookup function, which is called with a string specifying custom table name and one to three numeric arguments depending on the number of dimensions of the table.

For example, consider that you have defined a custom variable Rg for the gate resistance and a custom table Gate Resistance Eon Scaler that describes how the turn-on losses scale in terms of the gate resistance. You could then use the Lookup table and formula method on the Turn-on Loss tab, specify the nominal losses in the turn-on-loss lookup table and enter the following function expression:

E*lookup('Gate Resistance Eon Scaler', Rg)

Editing Lookup Tables

When editing an intrinsic lookup table on one of the three loss tabs or a custom lookup table, you can add and remove new interpolation points for a table dimension with the Edit menu or the context menu in the table. To enter multiple values at once, separate them by semicolons or spaces.

To rotate and tilt a three-dimensional table view, click on an empty space with the left mouse button and drag the mouse while keeping the mouse button pressed.

Lookup method

When calculating function values from a lookup table, PLECS uses linear interpolation if an input value lies within the index range for the corresponding table dimension. If the input value lies outside the index range, PLECS will extrapolate using the first or last pair of index values.

Copy, Paste and Scaling

Thermal data can be copied and pasted within the tables of the thermal editor, and to or from other programs, like e.g. Microsoft Excel. This can be done using the context menu or by pressing Ctrl+C (on macOS: Cmd+C) and Ctrl+V (on macOS: Cmd+V). To specify the target location for the data, you have to select a part of the table that has the same number of rows and columns as the copied data. When copying from another program, only the first correctly formatted number in each table cell will be copied, any additional information (e.g. units) will be discarded.

Values selected in a table can be scaled by a given factor by right-clicking and choosing Scale selected values… from the context menu. To convert a value from \(0.23\,\mathrm{J}\) to \(0.23\,\mathrm{mJ}\), e.g., you can scale it with a factor of \(0.001\). To only change the unit but not the actual value, i.e. to change \(0.23\,\mathrm{J}\) to \(230\,\mathrm{mJ}\), use the Energy scale drop box at the top right.

Thermal Package Description

A thermal package description is used to describe the thermal behavior of a power module that consists of multiple semiconductor chips. It contains the loss descriptions of the individual semiconductors as well as a structural model of the thermal coupling between the semiconductors and the package case.

The concept is illustrated using the example of a T-type inverter module shown in Fig. 70.

../../_images/T-Type-Package.svg — Fig. 70 Example of a T-type inverter power module consisting of multiple semiconductor chips

The module contains two different types of IGBTs and two different types of diodes because the high-side and low-side semiconductors (1 and 4) need to block higher voltages than the mid-point semiconductors (2 and 3). The package description therefore needs to contain four semiconductor device descriptions and a thermal model for the coupling between eight semiconductors and the module case.

Editing a Thermal Package Description

When you edit a thermal package description, the editor shows the following tabs:

Device Types

On this tab you define the different semiconductor types that the package contains. The left hand side shows the list of device types, the right hand side shows tabs to define the switching and conduction losses and the thermal impedances of the individual device types. Each device type has a name filter that is used to associate the thermal description for this device type with the individual components in the module.

For the example module, the thermal package description would contain four device types with name filters IGBT_HILO-*, IGBT_MID-*, D_HILO-* and D_MID-* as shown in Fig. 71.

../../_images/thermalpackagedevices.png — Fig. 71 Thermal Package Description window containing four device types

Thermal Impedance

On this tab you define the thermal impedance between the individual semiconductor devices and the case of the package. This definition considers devices to be heat sources, the case to be a temperature source and the impedance to be an LTI system with or without internal states.

The heating of an IGBT or diode device inside the T-type inverter module not only increases the temperature directly at the heat source device but also causes the temperature of all other devices to rise. A straightforward way to describe such thermal coupling effects is to model the temperature rise of one device by summing the individual contributions to that temperature rise from all the heat sources contained in the package. For the eight devices of the T-type inverter module such a thermal linear superposition model can be formulated in matrix notation as

\[\begin{split}\left[ \begin{array}{c} \Delta T_\mathrm{IGBT\_HILO-1} \\ \Delta T_\mathrm{D\_HILO-1} \\ \vdots \\ \Delta T_\mathrm{D\_HILO-4} \\ \end{array} \right] \; = \; \left[ \begin{array}{cccc} Z_{1,1} & Z_{1,2} & \cdots & Z_{1,8} \\ Z_{2,1} & Z_{2,2} & \cdots & Z_{2,8} \\ \vdots & \vdots & \ddots & \vdots \\ Z_{8,1} & Z_{8,2} & \cdots & Z_{8,8} \\ \end{array} \right] \cdot \left[ \begin{array}{c} Q_\mathrm{IGBT\_HILO-1}\\ Q_\mathrm{D\_HILO-1}\\ \vdots \\ Q_\mathrm{D\_HILO-4}\\ \end{array} \right]\end{split}\]

A single component of the impedance matrix describes a thermal impedance that relates a heating power to a temperature difference. For example, heating up the second device with the heating power \(Q_\mathrm{D\_HILO-1}\) contributes also to increasing the temperature difference \(\Delta T_\mathrm{IGBT\_HILO-1} = T_\mathrm{IGBT\_HILO-1} - T_\mathrm{case}\) of the first device, which is captured in the transient thermal impedance \(Z_{1,2}(t)\). In PLECS, \(Z_{1,2}(t)\) is described by a Foster or Cauer thermal chain that can be defined by clicking on a matrix element and setting type, number of elements and chain parameters as exemplified in the following screenshot.

../../_images/thermalimpedancematrix.png — Fig. 72 Thermal impedance matrix

From the provided impedance matrix, PLECS will then transform all thermal chains to the Cauer type and generate the matrices of a state-space model

\[\dot{\mathbf{x}} = \mathbf{A}\mathbf{x} + \mathbf{B}\mathbf{u}\]

\[\mathbf{y} = \mathbf{C}\mathbf{x} + \mathbf{D}\mathbf{u}\]

where \(\mathbf{u}\) is a vector comprising the device heat sources and case temperature source(s), \(\mathbf{y}\) is a vector of the same length comprising the device temperatures and the heat flow out of the case and \(\mathbf{x}\) is the vector of internal states.

The corresponding numerical representation of the state-space model can be displayed by clicking on the Edit State-Space Matrices… button at the bottom right. In the opened window (shown in Fig. 73), it is then possible not only to view the generated state-space matrices, but also to make changes to them that go beyond the linear superposition model. On the left-hand side you specify the number of internal states, the number of devices in the package, the width of the case connection and the device names which must match the names of the actual devices in the module subsystem. On the right-hand side you enter the numerical values of the state-space matrices. Note that direct changes made to the state-space matrices are not converted back to the thermal impedance matrix when you return to the linear superposition model by clicking the Edit Impedance Matrix… button at the bottom right.

../../_images/thermalpackagezth.png — Fig. 73 Thermal impedance state-space matrix

Constants, Variables, Custom Tables, Comment

Constants, variables and custom tables are shared by all device types in a package description. Please see section Thermal Description for a Single Device for a description of these tabs.

Example Model

See the example model “Thermal Coupling”.
Find it in PLECS under Help > PLECS Documentation > List of Example Models.

Using a Thermal Package Description

A thermal package description is typically used in conjunction with a masked subsystem that defines the electrical behavior of the module. The subsystem mask defines a Thermal parameter with a Device type filter that matches the type of the thermal package description (see also Mask Dialog).

The mask variable that is associated with this thermal parameter (here: thermal) is then referenced as is in the Thermal description parameters of the individual semiconductor devices in the subsystem. The component names (e.g. IGBT_HILO-1 or IGBT_HILO-4) are matched against the name filters (e.g. IGBT_HILO-*) to extract the appropriate loss definitions and thermal impedance for the semiconductor device.

../../_images/thermalpackagemask.png — Fig. 74 Dialog tab of the thermal package description mask

The thermal impedance between the devices and the module case is modeled by the Thermal Package Impedance component that is placed on top of the semiconductor devices similar to a heat sink. This component also has a parameter Thermal description that expects the thermal mask variable thermal to extract the definition of the thermal network from it.

This is illustrated in Fig. 75.

../../_images/thermalpackageigbt.png — Fig. 75 Modeling of thermal impedance between device and module case by means of Thermal Package Impedance

../../_images/thermalpackageimpedance.png — Fig. 75 Modeling of thermal impedance between device and module case by means of Thermal Package Impedance

Importing Data from Graphical Datasheets

PLECS provides an Import Wizard that facilitates the import of data from graphs that are typically used on real datasheets. The wizard is opened by clicking on the magic wand icon () that appears in the top right corner of any page that allows you to enter tabular data, i.e. the loss and custom tables and the thermal impedance.

../../_images/thermalwizard.png — Fig. 76 Importing the turn-on loss characteristic of a device using the Import Wizard feature

When you open the import wizard for the first time on a particular page, you are requested to provide a graph image. To import an image, drag an image file to the empty wizard area or click on the appropriate underlined text to open a file browser that lets you choose an image file. Image files must have a bitmap file format (PNG, GIF, BMP, JPG or XPM). To import graphs from a PDF file, take a snapshot of the desired graph, then select Paste from the Edit menu of the editor window or press Ctrl+V (on macOS: Cmd+V) to paste the snapshot into the wizard.

After the image has been imported, a green coordinate system is drawn on top of it. Your first task should be to align the green axes with the coordinate system in the image. You can move an axis or change its length by dragging the axis itself or its end point with the mouse.

Ensure that the axes have the proper dimensions. For turn-on and turn-off losses, the x-axis is expected to be in amperes \((\mathrm{A})\) and the y-axis, in joules \((\mathrm{J})\); for conduction losses, the x-axis is expected to be in amperes \((\mathrm{A})\) and the y-axis, in volts \((\mathrm{V})\). If the dimensions in the image are swapped, you can flip the image by clicking on the Mirror axes button in the image configuration dialog (see below).

Configuring the Graph Import

After you have aligned the green coordinate system, you need to enter the axis limits into the configuration dialog that has opened automatically when the image was imported. If you have closed the dialog, you can open it again by clicking on the button in the wizard toolbar or on one of the green axis limit labels.

In addition to the minimum and maximum settings, the x-axis has a Snap property that is initialized automatically from the axis limits. You can override the snap value by entering a number here; entering 0 disables snapping. To restore the automatically calculated value, click on the button.

Both the x-axis and y-axis have a Scale property that lets you choose between a linear and logarithmic scale. By default, double-logarithmic scaling is used for thermal impedances only; for all other imports the scaling defaults to linear.

The Opacity slider lets you change the opacity of the graph image from fully transparent (or invisible) to fully opaque. The Mirror axes button will mirror the graph image diagonally so that the two axes are exchanged. This is useful e.g. when importing conduction losses where the graph typically shows Volts on the x-axis and Amperes on the y-axis.

Adding and Moving Points

Points are added by double-clicking anywhere in the coordinate system. If snapping is enabled, the x-value is adjusted to the nearest snap value. To move points, drag them with the left mouse button. If snapping is enabled, you can temporarily disable it by pressing and holding the Shift key when you click on a point.

To add new curves (e.g. for another temperature value) use the corresponding entry from the Edit menu or from the context menu of the table at the bottom of the editor. If there is more than one curve, a double-click will add new points to all curves: The point at the mouse location is added to the current curve, i.e. the curve that you added or interacted with most recently. For all other curves, points are added based on their neighboring points’ coordinates.

Also, if there is more than one curve, the movement of points is restricted to the y-direction. The reason for this is that all curves in the lookup table share the same x-values, so changing the x-value of a point in one curve will affect all other curves as well. You can temporarily override this restriction by pressing and holding the Shift key when you click on a point.

Zooming and Panning

You can zoom into the graph for more precise placement of points and axes. Zooming is controlled via the View menu and the corresponding buttons in the toolbar. You can also zoom in and out by holding the Ctrl (on macOS: Cmd) key while rolling the mouse wheel. When you have zoomed into the graph, you can pan the image using the sliders or by holding the Ctrl (on macOS: Cmd) key while pressing and dragging the left mouse button. Pressing the spacebar will zoom the graph to fit the window.

Adding and Managing Graph Images

Sometimes, the curves for one lookup table come from different graphs. To add a new graph image to the wizard, click the button in the toolbar. To change the visibility of a particular curve on the current graph, use the check box in the corresponding row header in the table at the bottom. Press the button in the toolbar to rename the current graph; graph names are used purely for documentation purposes. To remove a graph (but not the curves), press the button.

Fitting Thermal Impedances

When a vendor datasheet provides the thermal impedance as a heating curve rather than Foster or Cauer network coefficients, you can use the import wizard to fit Foster coefficients to a given heating curve. First, import the graph of the heating curve as described above and place a number of points on the heating curve, then choose the desired number of Foster elements. As a general rule, you must place at least two points per Foster element. As soon as these requirements are met, PLECS will calculate a set of Foster coefficients and display the result as an orange curve on top of the graph.

../../_images/thermalwizardimpedance.png — Fig. 77 Importing and fitting the thermal impedance characteristic of a device using the Import Wizard feature

PLECS uses a non-deterministic optimization algorithm to minimize the error between the calculated curve and the points that you have placed. This algorithm may not converge at all or converge at a local instead of the global minimum. If the current fit is not satisfactory, you can calculate a new one by pressing the Recalculate button. Once the results are acceptable, press the Accept button to close the wizard and transfer the calculated Foster values.

Note

The fitting algorithm can handle only single pulse curves. Vendor datasheets sometimes also show heating curves for repeated pulses with different duty cycles; these curves cannot be used to calculate Foster coefficients with PLECS.

If the fitting algorithm repeatedly does not find a satisfactory solution, you may need to increase the number of Foster elements. Typically, three to five Foster elements should yield good results.

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 Fig. 78.

../../_images/comb_sw_polarity.svg — Fig. 78 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 Fig. 79. Turn-on losses occur at \(t = t_1\) and turn-off losses at \(t = t_2\). 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.

../../_images/diode_conduction.svg — Fig. 79 Diode voltage and current during switching

Conduction losses occur when \(t_1 < t < t_2\). 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 following 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:

	Diode		Switch		Switch with Diode
	Diode		Switch		Switch		Diode
	V	I	V	I	V	I	V	I
Conduction Loss	+	+	+	+	+	+	-	-
Switching Loss	-	+	+	+	+	+	+	-

../../_images/comb_turnoff_lookup.svg — Fig. 80 Turn-off loss lookup table for semiconductor switch with diode

../../_images/comb_conduction_lookup.svg — Fig. 81 Conduction loss profile for semiconductor switch with diode