**6. Discussion**

#### *6.1. The Analysis of System E*ffi*ciency*

In this paper, the system efficiencies at different switching frequencies (50 kHz, 80 kHz, and 100 kHz) and under different load levels are explored. Figure 25 shows the system block diagram of the efficiency tests. In this test, the AC terminals of the proposed three-phase GaN-based APF are connected to the three-phase power grid having the line to line voltage of 110 V and its DC terminal voltage is regulated at the designed 200 V by the proposed voltage controller of the APF. For testing the APF efficiencies under different load levels, a programmable electronic load is connected to the DC terminal of the APF. By setting different *Pdc* and measuring the corresponding *Pac,* the efficiency at a specific power level and switching frequency can be readily calculated. In this paper, three switching frequencies, i.e., 50, 80, and 100 kHz were tested. The calculated results are graphically shown in Figure 26. As can be seen in Figure 26, the maximum efficiency appears at about 50% of the rated load and it is found that when the switching frequency increases the efficiency decreases. This is mainly due to the increase in switching losses.

**Figure 25.** The system block diagram of the efficiency tests.

**Figure 26.** Efficiencies of GaN-based shunt APF system at different switching frequencies.

In the aspect of the efficiency comparison with different technologies, it is indeed difficult to establish a fair comparative basis due to some system constrains, e.g., the switching technique used, system capacities, quality of components used, and the control functions designed for the circuits. To provide a set comparative result, three recently published technical papers [16–18] using different technologies are summarized in Table 5.


**Table 5.** Performance comparison of different technologies.

#### *6.2. The Thermographic Analysis of the System*

Figure 27 shows a set of thermographic photos (using FLIER-E63900, T198547, E4) of the proposed GaN-based three-phase APF prototype operating under the rated capacity of 2 kW with different switching frequencies. As can be seen in the photos, the temperature of the switching devices increases as the switching frequency increases and the temperature of capacitors and the circuit board remain under 25 ◦C. Table 6 shows the summary of the recorded temperature data gathered from the presented thermographic photos. Based on the results of thermographic analysis, it has been found that the greatest losses are located at the six power-switching devices and the three relays which are designed for ensuring a successful synchronizing control with the power grid to which the proposed APF is connected. It should be noted that in practice, these relays can be removed after the overall control system of the APF has been properly tuned. This means that the maximum efficiency of the proposed GaN-based APF can be further improved.

**Figure 27.** Thermographic photos of the proposed GaN-based three-phase APF prototype: (**a**) TPH3207 device switching at 50 kHz; (**b**) TPH3207 device switching at 80 kHz; (**c**) TPH3207 device switching at 100 kHz; (**d**) DSP; (**e**) communication interface; (**f**) inductors; (**g**) relay; (**h**) signal processing integrated circuits (ICs).


**Table 6.** Summary of operating temperatures of individual devices.

#### *6.3. Related Technical Issues*

As mentioned previously, the proposed GaN-based three-phase APF circuit prototype is demonstrated for the first time. There are some technical issues to be further improved. These include: (1) the driving circuits can be improved to achieve higher switching frequency and to reduce the size of inductors; (2) the three relays can be removed or replaced with new devices having better quality in conduction losses to increase the overall system e fficiency; (3) the layout of the circuit can be improved to reduce the noise level in current- and voltage-sensing mechanisms. It is important to note that the noise level in sensing signals constitutes the operating limits of the switching frequency of the proposed GaN-based APF circuit.
