Analysis of Switching Frequency Effects on THD and Switching Losses in a Filtered Single-Phase SPWM Inverter

References

[8] . In this study, values of THD were recorded across all switching frequencies and load condition, allowing comparison of inverter performance under different operating scenarios. 2.5 Switching Loss Modeling Inverter switching losses were estimated based on the overlap of device voltage and current during each transition. For each switching event, a finite rise and fall time occurs, during which the device simultaneously sustains voltage and conducts current, resulting in energy dissipation. The average switching power loss is approximated as: where:  is the input DC bus voltage,  is the load current,  and are the device turn-on and turn-off transition times, and  is the switching frequency. From this relation, it is evident that switching losses increase linearly with frequency. In this study, typical IGBT switching times from manufacturer datasheets were assumed . By applying this model across the tested load conditions, the relative increase in switching losses at different frequencies was quantified. III. RESULTS AND DISCUSSION 3.1 For this study, the simulation was carried out using MATLAB/Simulink. The inverter model consists of four IGBT/diode switches, a DC source, an LC low-pass filter, current and voltage measurement blocks, an SPWM controller, and the PowerGUI analysis tool. The study investigates how different switching frequencies affect the performance of a single-phase SPWM inverter, focusing specifically on total harmonic distortion (THD) and switching losses. Although switching frequency is commonly varied to improve harmonic performance, the presence of a strong LC output filter may change this behaviour. FFT analysis is therefore used to evaluate THD at each switching frequency and determine whether increasing the switching frequency provides any additional benefit in the filtered inverter design. 3.2 Effect of Switching Frequency Effect on Output Quality The simulation results for THD, output voltage, and power at different switching frequencies are presented in Figure 5 and Table 1. Figure 3: simulation results for THD and output voltage Table 1: Effect of Switching Frequency on Output Quality Switching frequency (kHz) THD (%) Power(W) 5 0.02 232.6 511.2 10 0.06 232.4 509.8 15 0.06 232.6 511.2 20 0.04 232.6 511.2 25 0.07 232.8 512.9 The results show that THD remains extremely low (below 0.1%) across all tested switching frequencies, with the lowest value (0.02%) observed at 5 kHz. Interestingly, the THD does not show a consistent decreasing trend with increasing switching frequency as might be expected from theory. This could be attributed to the effective filtering provided by the LC filter design and the high modulation index used. The output voltage remains stable at approximately 232.6 V across all switching frequencies, indicating good voltage regulation. Similarly, the power delivered to the load remains consistent at approximately 512 W, showing that the switching frequency does not significantly affect the power transfer capability of the inverter. Table 2: Effect of Switching Frequency on switching losses Switching frequency (KHz) THD (%) Switching Losses, (W) 5 0.02 232.6 1.19 10 0.06 232.4 2.38 15 0.06 232.6 3.57 20 0.04 232.6 4.77 25 0.07 232.8 5.96 The results in Table 2 show that the switching losses increase almost linearly with switching frequency. The loss rises from 1.19 W at 5 kHz to 5.96 W at 25 kHz, which is roughly a five-fold increase for a five- fold increase in switching frequency. This behavior is consistent with the switching-loss model , where higher switching frequencies produce more voltage–current overlap transitions per second. In contrast, the output-voltage THD remains extremely low (0.02–0.07%) for all frequencies due to the LC filter. Since THD does not significantly improve with higher switching frequency, the increasing switching losses become the dominant factor in selecting the operating point. Therefore, for this inverter configuration, 5 kHz is the most efficient switching frequency, providing the lowest switching loss while maintaining excellent waveform quality Figure 4: Graph showing THD vs. switching frequency These findings suggest that for this inverter design with the specified filter parameters, the switching frequency can be selected based on other considerations (such as switching losses) without significantly compromising the output voltage quality. 3.3 Switching Loss Analysis The switching losses were calculated using the model presented in Section 2.5. As expected, the switching losses increase linearly with switching frequency, as shown in Figure 6. Figure 5: Graph of switching loss VS Switching Frequency At 5 kHz, the switching losses are minimal, while at 25 kHz, they are approximately five times higher. This increase in switching losses directly impacts the overall efficiency of the inverter, particularly at higher frequencies 3.4 Discussion The results of this study clearly show that switching frequency has two distinct effects on the simulated SPWM inverter. First, the switching losses increase steadily with switching frequency, rising from 1.19 W at 5 kHz to 5.96 W at 25 kHz. This behavior is consistent with switching-loss theory, where higher switching frequencies introduce more voltage– current overlap transitions, resulting in greater energy dissipation per second. In contrast, the output-voltage THD remains extremely low across all tested switching frequencies, varying only between 0.02% and 0.07%. This very small variation indicates that the LC output filter plays a dominant role in harmonic suppression. Since the filter already attenuates high-frequency components effectively, increasing the switching frequency does not significantly influence the harmonic content of the output voltage. This result differs from typical unfiltered or lightly filtered SPWM inverters, where increasing switching frequency directly improves THD by shifting harmonics to higher frequencies. The comparison between switching losses and THD demonstrates that raising the switching frequency does not yield any harmonic-quality improvement for this inverter configuration, yet it increases switching losses substantially. As a result, efficiency becomes the determining factor in selecting an appropriate switching frequency. The lowest switching frequency tested, 5 kHz, provides the minimum switching losses while maintaining the same excellent harmonic performance as higher switching frequencies. IV. CONCLUSION This study presented an analysis of the relationship between switching frequency, total harmonic distortion, and switching losses in a single-phase SPWM inverter equipped with an LC output filter. The key findings are: THD remains extremely low (0.02%–0.07%) for all tested switching frequencies, demonstrating that the LC output filter effectively dominates harmonic suppression in this inverter configuration. Switching losses increase nearly linearly with switching frequency, highlighting the efficiency penalty associated with operating at higher switching frequencies. Increasing the switching frequency does not provide any meaningful improvement in THD, since the LC filter already removes most of the switching harmonics. The most optimal switching frequency for this design is 5 kHz, which yields the lowest switching losses while maintaining excellent waveform quality. Overall, the results show that in filtered SPWM inverters, increasing the switching frequency offers no harmonic benefit and only increases switching losses. Therefore, for the modeled inverter parameters, a switching frequency of 5 kHz provides the best balance between power quality and efficiency. These conclusions apply specifically to the inverter configuration, load conditions, and filtering design used in this study. REFERENCES

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[8] IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems, IEEE Standard 519-2014, 2014.

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