Electrothermal Flow For Power Electronics

When simulating the PCBs of power electronics converters, most cases deal with Pulse Width Modulated (PWM) voltage and current sources. From these cases, it is impossible to configure a DC IR setup that both accurately represents power losses and satisfies the current balance in accordance with Kirchhoff's first law. Therefore, an advanced workflow is required to accurately estimate losses on PCB traces for power electronics.

Frequency Domain Workflow Using SIwave and Circuit

Complete the following steps for at least one period of power converter output frequency.

1. Set up a PCB design in SIwave.

   

2. Set up and run an SYZ simulation to compute and extract S-parameters.

   

3. Save and Close the project.
4. Open Electronics Desktop and insert a Circuit Design.

   

5. Create an SIwave dynamic link in Circuit/Nexxim.

6. Create an appropriate Circuit design (Refer to the Nexxim Design Examples).

   

7. Run a Nexxim Transient analysis with appropriate Step and Stop values.

   

   Note:

   SIwave uses Fourier Transform to convert a transient source into a frequency domain source. If users set a high Step variable during the transient analysis, simulation time may increase when run as a DC IR Drop simulation (i.e., step 10).

   

8. Push the analysis excitations from Circuit/Nexxim to SIwave.

   

9. Return to SIwave and reopen the project.

10. Create and run a DC IR Drop simulation. The excitation output from Circuit/Nexxim will be automatically used as the source file (e.g., 4685_NexximTransient_sources.tmp).

    

    Note:

    Passive components (e.g., capacitors and inductors) should be removed from the layout and modeled in the Circuit design.

11. Once the DC IR Drop simulation is complete, navigate to the Results folder. Then right-click the completed simulation (e.g., DC IR) and select one or more of the following:

    

    • Plot Currents/Voltages

      

    • Plot Icepak Power Map

      

    • Plot Solver Convergence

      

Note:

Power loss is the total loss generated by the transient source. Current and voltage correspond with the value at time t= 0s.


Refer to Viewing DC IR Results in SIwave for more information.

Frequency Domain Workflow Using Frequency Dependent Current in SIwave DC IR Solver

Complete the following steps if users intend to use a frequency dependent source file. Generate the data for the frequency dependent current course by performing a Fourier transformation on a transient current obtained from a circuit simulation for at least one period of power converter output frequency.

1. Create a frequency dependent source file (i.e., a *.txt file), adhering to the following guidelines:

   • The file must contain the frequency domain excitations (i.e., Freq, Re, Im).

     SOURCE_RESISTANCE <resistance value of the source (e.g., 5e7)>
     0	-1	0
     1e9	0	-1
     5e9	-1	0

   • If there is more than one frequency dependent current source used for a DC IR setup in a project, all source files must contain the same frequency points.

2. Add the frequency dependent source to the design.

3. Return to step 10 in the Frequency Domain Workflow to run a DC IR simulation. At the Compute DC Current and Voltage Distribution window, ensure Use sources defined in project is selected.

   

Technical Notes: How Frequency-Dependent Resistance is Calculated for the DC IR Solver

If a project includes a DC IR setup with frequency-dependent current sources or excitations pushed from a Dynamic SIwave component in a Circuit Design, the frequencies from the current sources are used in an SYZ analysis, which is run as part of the DC IR simulation. Since the DC IR simulation does not compute frequency-dependent resistance (i.e., RAC), the SYZ analysis computes the RAC. The resistance from the DC IR analysis (i.e., RDC) and the RAC from the SYZ analysis are used to calculate the power loss generated by the AC current source.

Utilizing DC correction to compute the exact DC point (i.e., the ZDC matrix generated by the DC IR solver), the simulation will then compute the power loss density for each frequency point with constant resistance obtained from the SYZ analysis. The final frequency-dependent power loss is the sum of the points generated by each current source using Re and Im current values at different frequencies, incorporating the ratio of the power loss calculated by RAC and RDC.

In summation, for DC point, power loss is computed by the DC IR solver as

where I₀ is DC current and R_dc is resistance at DC point, calculated by the DC IR solver. The AC solver yields an equivalent resistance matrix that is used to compute power loss as

where R_fn is equivalent to the resistance matrix of the model at the frequency f_n, l_{n_re} and l_{n_im} are vectors of real and imaginary parts of the current sources at the frequency f_n. If a coefficient (e.g., k_n) is defined as , the AC power loss at the frequency f_n can be rewritten as

where P_{n_Rdc} is the AC power loss at the frequency f_n computed with constant resistance R_dc, leading to . Since both P_{n_Rf} and P_{n_Rdc} can be computed for the frequency f_n, utilizing the AC solver (i.e., SYZ analysis) and the DC IR solver, k_n can be computed for all frequencies. Finally, the coefficient k_n is incorporated into the current sources as . The power loss result of modified current sources accounts for frequency dependent resistance behavior.

Version 2025 R1 and Beyond

Prior to version 2025 R1, when computing power loss on the design, the DC IR solver considers capacitors open elements and inductors zero impedance elements. Starting with version 2025 R1, if the design contains frequency-dependent or time domain sources, the DC IR solver treats capacitors and inductors as active elements when solving the problem and computing power loss in the design.

When addressing frequency-dependent sources in DC IR, a parasitic resistor of 10 MΩ is introduced in parallel with each capacitor, and a parasitic resistor of 9 µΩ is added in series with each inductor.

The impedance of capacitors and inductors is calculated as in AC mode:

where Z_C and Z_L represent the impedance of the capacitor and inductor, respectively; C and L are the capacitance and inductance, and f is the frequency under study. The impedances of capacitors and inductors are ignored at DC (i.e., f=0.0).

For designs that include RLC components, the DC IR solver uses the constant resistance of the metal at DC to compute power loss. To optimize computation time, the Schur complement method is used to solve the matrix system when frequency-dependent or time domain sources are involved.