PLECS 2.0 Online Help

Thermal Modeling

Thermal management is an important aspect of power electronic systems and is becoming more critical with increasing demands for compact packaging and higher power density. PLECS enables you to include the thermal design with the electrical design at an early stage in order to provide a cooling solution suitable for each particular application.

Heat Sink Concept

The core component of the thermal library is an idealized heat sink depicted as a semitransparent box in the figure below. A heat sink absorbs the thermal losses dissipated by the components within its boundaries. At the same time, a heat sink defines an isotherm environment and propagates its temperature to the components which it encloses.

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Heat conduction from one heat sink to another or to an ambient temperature is modeled with lumped thermal resistances and capacitances that are connected to the heat sinks. This approach allows you to control the level of detail of the thermal model.

Implementation

Each heat sink has an intrinsic thermal capacitance versus the thermal reference node. All thermal losses absorbed by the heat sink flow into this capacitance and therefore raise the heat sink temperature. Heat exchange with the environment occurs via the external connectors.

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You may set the intrinsic capacitance to zero, but then you must connect the heat sink either to an external thermal capacitance or to a fixed temperature, i.e. the Constant Temperature block or the Controlled Temperature block.

Thermal Loss Dissipation

There are two classes of intrinsic components that dissipate thermal losses: semiconductor switches and ohmic resistors.

Semiconductor Losses

Power semiconductors dissipate losses due to their non-ideal nature. These losses can be classified as conduction losses and switching losses. For completeness the blocking losses due to leakage currents need to be mentioned, but they can usually be neglected.

Semiconductor losses are specified by referencing a thermal data sheet in the component parameter Thermal description. See section Thermal Description Parameter and Thermal Library for more details.

Conduction Losses

The conduction losses can be computed in a straightforward manner as the product of the device current and the device voltage. By default the on-state voltage is calculated from the electrical device parameters as v = Vf + Ron ⋅i  .

However, PLECS also allows you to specify the on-state voltage used for the loss calculation as an arbitrary function of the device current and the device temperature: v = von(i,T)  . This function is defined in the Conduction loss tab of the thermal description as a 2D look-up table (see Thermal Editor).

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A setting of 0 V for a single temperature and current value means no conduction losses. If you do not specify a thermal description in the device parameters, the default will be used, i.e. the losses are calculated from the electrical device parameters.


Note  If you specify the Thermal description parameter, the dissipated thermal power does not correspond to the electrical power that is consumed by the device. This must be taken into account when you use the thermal losses for estimating the efficiency of a circuit.
Switching Losses

Switching losses occur because the transitions from on-state to off-state and vice versa do not occur instantaneously. During the transition interval both the current through and the voltage across the device are substantially larger than zero which leads to large instantaneous power losses. This is illustrated in the figure below. The curves show the simplified current and voltage waveforms and the dissipated power during one switching cycle of an IGBT in an inverter leg.

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In other simulation programs the computation of switching losses is usually challenging because it requires very detailed and accurate semiconductor models. Furthermore, very small simulation time-steps are needed since the duration of an individual switching transition is in the order of a few hundred nanoseconds.

In PLECS this problem is bypassed by using the fact that for a given circuit the current and voltage waveforms during the transition and therefore the total loss energy are principally a function of the pre- and post-switching conditions and the device temperature: E = Eon(vblock,ion,T)  , E  = Eoff(vblock,ion,T)  . These functions are defined in the tabs Turn-on loss and Turn-off loss of the thermal editor as 3D look-up tables (see Thermal Editor) .

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A setting of 0 J for a single voltage, current and temperature value means no switching losses.


Note  Due to the instantaneous nature of the switching transitions, the dissipated thermal energy cannot be consumed electrically by the device. This must be taken into account when you use the thermal losses for estimating the efficiency of a circuit.

Semiconductor components that implement this loss model are

In addition, the Set/Reset Switch is also included in this group to enable you to build your own semiconductor models.

Ohmic Losses

Ohmic losses are calculated as i2 ⋅R   resp. u2∕R  . They are dissipated by the following components:

Heat Sinks and Subcircuits

By default, if you place a subcircuit on a heat sink, the heat sink temperature is propagated recursively into all subschematics of the subcircuit. All thermal losses dissipated in all subschematics flow into the heat sink. In some cases this is not desirable.

The implicit propagation mechanism is disabled if a subschematic contains one or more heat sinks or the Ambient Temperature block. This latter block provides a thermal connection to the heat sink enclosing the parent subcircuit block.

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As an example the figure above shows the subschematic of the Diode with Reverse Recovery. By default, this diode model would only dissipate the ohmic losses from the three resistors and the conduction losses of the internal ideal diode. However, the losses from the reverse recovery current injected by the current source would be neglected because current sources (and also voltage sources) do not dissipate thermal losses.

The Diode with Reverse Recovery therefore uses a Controlled Heat Flow block to inject the electrical power loss into the thermal model via the Ambient Temperature block. The power loss is calculated by multiplying the device voltage and the device current.

Thermal Description Parameter

Most semiconductor components in PLECS have a parameter Thermal description. The parameter can be used in two ways:

Assigning Thermal Data Sheets

Thermal data sheets can be assigned to semiconductors with the menu entry From library.... PLECS only displays data sheets that match the device type; e.g. in the dialog box of a thyristor only those data sheets appear that have their Type field set to Thyristor.

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Selecting a data sheet from a thermal library

If no data sheet is available the menu entry is disabled. In thermal parameters of masked subcircuits all data sheets are accessible, regardless of their type. See section Thermal Library for more information on how to create new data sheets.

Using References

To use a reference in the Thermal description parameter select the menu entry By reference in the parameter menu. Afterwards the referenced variable can be entered in the text field.

The referenced variable must be either a thermal parameter in a subcircuit mask or a MATLAB workspace variable. If a MATLAB workspace variable is referenced it must contain a string value that specifies the URL to a thermal data sheet. It is possible to use an absolute path, e.g.

thLosses = ’file:C:\Thermal\Vendor\mydiode.xml’

Alternatively, the name of a data sheet from the library can be specified. The data sheet must be on the thermal search path. Its name must be provided as a relative path without the .xml extension, e.g.

thLosses = ’file:Vendor/mydiode’