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About Thermal Simulations of DC-DC Converters

2025.01.29

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In this article, a simulation environment is explained, together with its method of use, that enables the simultaneous execution of circuit operation simulations using the ROHM Solution Simulator of a power supply circuit based on the BD9G500EFJ-LA 80 V DC-DC converter IC with a 5 A output, as well as temperature simulations of the RB088BM100TL Schottky barrier diode, which is an external component of the IC.

About the Thermal Simulation Using the ROHM Solution Simulator

This simulation uses the thermal analysis functions added to the ROHM Solution Simulator. First the thermal analysis functions are summarized.

The thermal analysis functions added to the ROHM Solution Simulator are provided as additions to a Solution Circuit, which is a simulation model used by the ROHM Solution Simulator. At present there are ten Solution Circuits that include the thermal analysis functions; hereafter more such Solution Circuits will be added as they become available. The BD9G500EFJ-LA Solution Circuit used here is one of these ten Solution Circuits.

The thermal analysis functions of the ROHM Solution Simulator have the following features.

  • ・Electrical-thermal coupled analysis of circuits formed from power semiconductors, ICs, and passive components
  • ・In addition to semiconductor chip temperatures (junction temperatures) during circuit operation, terminal temperatures and thermal interference of components on a board can also be analyzed.
  • ・Thermal analysis simulations that in the past required nearly one day can be executed within ten minutes.
  • ・Because thermal analyses can be performed by simple means, thorough thermal design is possible in advance, resulting in less prototype reworking and reduced development man-hours.

The thermal analysis functions use thermal fluid analysis tools to perform 3D modeling of parameters related to the calculated heat dissipation from an actual mounting board, reduce the dimensions to one dimension so as to enable thermal analysis by an electrical circuit simulator, and perform electrical-thermal coupled analysis. In coupled analysis, coupled phenomena involving two or more different domains such as electricity, heat, flows and the like are processed, and steady states and transient states are calculated. A visual representation appears below.

Electrical-Thermal Coupled Analysis, the Basis of Thermal Analysis Functions

In these thermal simulations, in addition to semiconductor junction (chip) temperatures TJ, package top surface temperatures TT, and solder surface temperatures TFIN, all of which change during circuit operation, it is also possible to check temperatures of peripheral components on the board, thermal interference of chips within modules, and other parameters that are inaccessible using SPICE-based thermal models.

Confirming Points for Actual Measurement/Visual Rendering of Complex Thermal Interference between Components

Moreover, simulation times have been shortened dramatically. The time required when using conventional thermal analysis simulations had ranged from over ten hours to nearly a full day, but using the ROHM Solution Simulator, simulations can be executed in under ten minutes.

Thermal Analysis Functions of ROHM Solution Simulator

Using the ROHM Solution Simulator, thermal analyses such as those shown above are possible.

DC-DC Converter Thermal Simulation Circuits

From this point begins a detailed explanation of use of the ROHM Solution Simulator and the solution circuit, complete with thermal analysis functions, for the BD9G500EFJ-LA DC-DC converter. It would be best for the reader to actually start up the ROHM Solution Simulator and operate it while reading this article.

Before beginning, please check out the following.

Simulation Circuit to be Used in Thermal Simulation of the DC-DC Converter

Upon starting up the Solution Circuit for the above-described BD9G500EFJ-LA DC-DC converter, a circuit like that in Fig. 1 is opened. The part surrounded by the green line is a thermal simulation circuit; other areas represent an electrical simulation circuit.

Simulation circuit of the BD9G500EFJ-LA DC-DC converter
Fig. 1. Simulation circuit of the BD9G500EFJ-LA DC-DC converter

This electrical circuit is an application circuit for a 1-channel step-down DC-DC converter using the BD9G500EFJ-LA, with a maximum 5 A output.

The thermal simulation circuit inputs the losses of the BD9G500EFJ-LA (hereafter simply “the IC”) calculated in electrical simulations as well as the losses of the RB088BM100TL Schottky barrier diode (hereafter “the SBD”) into a thermal simulation model, and calculates the temperatures of the IC and the SBD.

Simulation Method

Simulation settings are set and simulations are executed from the “Circuit diagram toolbar” in the upper part of the circuit diagram (for reference: “ROHM Solution Simulator Toolbar Functions and Basic Operations“).

The simulation time, convergence options, and other simulation settings can be set from “Simulation Settings” appearing in Fig. 2. Table 1 shows the initial simulation settings.

When there are issues relating to simulation convergence, these can be resolved by modifying detailed options. Simulation temperatures and various parameters for an electrical circuit are defined in “Manual Options”.

Simulation settings and execution

Fig. 2. Simulation settings and execution（▶）

Parameter Initial value Notes
Simulation Type Time-Domain Do not change the simulation type
End time 7 msecs
Advanced Options More Speed
Manual Options .PARAM For details, see Table 2 of the next section

Table 1. Initial values of simulation settings

Simulation Conditions

Simulation conditions must be set or selected before performing a simulation.

Parameter Definitions

The components shown in blue in Fig. 3 are components for which simulation conditions must be set; for this reason, parameters are defined in “Manual Options”. Upon clicking the “Simulation Settings” button and then clicking on “Advanced Options”, a “Manual Options” text box is displayed (Fig. 4).

Table 2 shows initial values for each parameter. These values are written into the “Manual Options” text box of simulation settings, as shown in Fig. 4.

Components requiring parameter definitions (in blue)

Parameter Variable name Initial value Unit Description
Temperature Ta 25 ℃ Ambient temperature
Voltage V_VIN 48 V Input voltage set in the range 7 to 76 V
Voltage V_VOUT 5 V Set in the range 1 V to (0.97×V_VIN)
Current I_IOUT 1 A 5A(MAX)
Inductance L_PRM 33 μH Smoothing inductor

Table 2. Simulation conditions; initial values appearing in “Manual Options”

'Manual Options' text box

Component Value Settings

For the method used to set the switching frequency, output LC filter component value, output voltage, and other values, refer to “Selection of Components Externally” in the BD9G500EFJ-LA data sheet. Component values can be set by double-clicking on the component to open the “Property Editor” and then changing the value (See ” Customization of Simulations” and “Exporting Circuit Data to PartQuest™ Explorer“).

BD9G500EFJ-LA data sheet

Thermal Circuit

The symbol indicated by the green arrow in Fig. 5 is a thermal simulation model of the BD9G500EFJ-LA. The junction temperatures, package surface temperatures, and FIN surface temperatures can be confirmed at the nodes on the red wiring. Details of nodes appear in Table 3.

BD9G500EFJ-LA thermal simulation model

Node name Description
BD9G500EFJ_Tj Monitors the BD9G500EFJ-LA junction temperature
SBD_Tj Monitors the RB088BM100 junction temperature
BD9G500EFJ_Tt Monitors the package top center temperature of the BD9G500EFJ-LA
SBD_Tt Monitors the package top center temperature of the RB088BM100
SBD_Tfin Monitors the FIN center temperature of the RB088BM100

Table 3. Explanation of the thermal simulation model nodes of Fig. 5

Select a Thermal Simulation Model

The components shown in Table 4 are available for use in thermal simulation models; any of these can be selected. Fig. 6 illustrates the selection method. First, either double-click on a component of the BD9G500EFJ-LA, or right-click and select “Properties” to open the Property Editor. Select the value of the “SpiceLib Part” in the Property Editor from the values shown in Table 4 (options are displayed in the “SpiceLib Part” box) to set the value and change the thermal simulation model.

Method for selecting a thermal simulation model

Component name SpiceLib Part value Description
BD9G500EFJ-LA 2s Thermal selection model for a 2-layer PCB
2s2p Thermal selection model for a 4-layer PCB

Table 4. List of selectable thermal models

Thermal Simulation Model

BD9G500EFJ-LA Thermal Simulation Model

For reference, Fig. 7 is a graphic image of the 3D model used in creation of the thermal simulation model for the BD9G500EFJ-LA. Structural information appears in Table 5.

3D image of the BD9G500EFJ-LA thermal simulation model

Structural part Description
PCB outline dimensions 114.3mm×76.2mm, t=1.6mm
PCB material FR-4
Layout pattern See “Single Buck Switching Regulator BD9G500EFJ-LA EVK User’s Guide”
2-layer PCB, layered configuration Top Layer：70μm（2oz） Bottom Layer：70μm（2oz）
4-layer PCB, layered configuration Top Layer：70μm（2oz）
Middle1 & Middle2 Layer：35μm（1oz）
Bottom Layer：70μm（2oz）

Table 5. Structural information for the BD9G500EFJ-LA thermal simulation model

As indicated in “Select a Thermal Simulation Model” in “Simulation Conditions”, there are 2-layer PCB and 4-layer PCB options when selecting a BD9G500EFJ-LA thermal simulation model. When performing thermal evaluations, simulations of both a 2-layer and a 4-layer PCB can be performed to identify temperature differences, and these can be reflected in the thermal design.

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