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.... 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.
The Thermal Editor window has the following input elements:
Manufacturer, Part number These text fields are for documentation purposes only.
Type This selector serves as a filter for the Thermal description menu entry on the Thermal tab of a semiconductor parameter dialog. It must be set according to the semiconductor type it is intended to be used with.
Turn-on loss, Turn-off loss, Conduction loss On these tabs you define the switching a conduction losses of the device. See Editing Switching Losses and Editing Conduction Losses.
Therm. impedance On this tab you define the thermal impedance between the junction and the case of the device. See Editing the Thermal Equivalent Circuit.
Variables On this tab you define custom parameters (such as gate resistance or stray inductance) that may be used to define device losses. Here you can also specify hard limits both for your custom parameters 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.
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.
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 on 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 and the values for
(in K/W) and
(in J/K) for each chain element need to be entered.
Cauer network
The figure below illustrates the structure of a Foster network. In the thermal
editor the number of chain elements and the values for
in (K/W)
and
(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.
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.
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.
Use the Add , Remove
, 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.
When you add a custom variable, you may specify a Prompt, which should provide a brief description of the purpose 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.
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 Blockset Parameters and PLECS Standalone 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 below.
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.
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/Ctrl-V (or cmd-C/cmd-V on Mac OS X). 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 J to 0.23 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 J to 230 mJ, use the Energy scale drop box at the top right.
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.
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 and turn off losses at
. 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.
Diode voltage and current during switching
Conduction losses occur when . 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:
Diode | Switch | Switch with Diode | ||||||
Switch | Diode | |||||||
V | I | V | I | V | I | V | I | |
Conduction Loss | + | + | + | + | + | + | - | - |
Switching Loss | - | + | + | + | + | + | + | - |
Turn-off loss lookup table for semiconductor switch with diode
Conduction loss profile for semiconductor switch with diode