Understanding the Frequency Parameter
In PSIM’s thermal element parameters, the frequency determines the interval for calculating losses. For instance, if set to 50 Hz, the losses are averaged over an interval of 20 milliseconds. Alternatively, if the frequency is the same as the switching frequency, losses are obtained for each switching cycle.
It is important to highlight that the frequency parameter does not affect the PWM waveform or the switching process itself. Rather, it determines how losses are calculated by specifying the interval for averaging the energy lost. Whether the frequency is set to the switching frequency or a lower value, it determines the duration over which the losses are evaluated.
The thermal calculation calculates losses, including energy loss during switch’s turn-on process (Eon), turn-off process (Eoff), and conduction state (Econduction). It sums up the losses within the time interval defined by the frequency parameter. This time interval is calculated as 1/frequency. Dividing the total energy losses by this frequency yields the average power loss.
When the frequency parameter matches the switching frequency, losses are obtained per-cycle, offering precise insight into individual switching events. Conversely, a lower frequency evaluates losses over a broader timespan. Typically, in an ac system, the frequency should be set to be the same as the fundamental frequency so as to obtain the average power losses within one fundamental cycle.
3-ph Inverter Example
Let's take a look at the "3-ph inverter - HybridPack FS800R07A2E3 (Infineon)" example. This particular example is one of the thermal examples available in PSIM.
By default, the thermal calculation frequency is set to 60 Hz. This configuration allows the calculation of total losses for the 6-pack IGBT module based on the sinusoidal period of the resulting AC current waveform.
As a result, the losses will initially appear as zero until the first current period's time frame (1/60 Hz) is exceeded. The losses are then updated at each subsequent 1/60 Hz time frame. It is worth noting that the losses will reach a steady state after the thermal equivalent circuit of the device reaches the steady state. The average cycle losses will remain constant at about 840 Watt after the steady state is reached.
Now, let's observe the impact of changing the thermal calculation frequency to the switching frequency (10 kHz):
The losses are now calculated and updated at the rate of the switching frequency (10 kHz):
As the losses are observed in smaller time frames, a more detailed representation is provided. When attempting to average the steady state losses over the entire period of the AC current waveform (1/60 Hz), the resulting value should align with the value shown in Figure 1, which is 840 Watts. Indeed this is validated in Figure 4:
Conclusion
The Frequency parameter affects how energy losses are calculated and waveforms displayed. It determines the interval for loss calculation without affecting the PWM waveform itself. Matching the frequency with the switching frequency provides per-cycle precision, while lower frequencies offer broader loss assessment over time. Choose the appropriate frequency based on the analysis requirements.