A common control method for off-grid inverters is multiple-loop control with a PI compensator. The output of the voltage loop is the reference value for the current loop. In this model, the common control method is utilized except that the voltage reference and sampling signal is the RMS value of output voltage. Besides, an additional notch filter is used to attenuate an amplitude of 100Hz ripple of output voltage. Conventional voltage loop directly uses sampling AC voltage to produce the error signal. In this application note, using the RMS value can quickly switch the voltage reference to the DC bus voltage when the inverter needs to connect to the grid and can also reduce the issue caused by sampling noise. Figure 2-2 shows a block diagram of the control system.

Figure 2-2 Control Method Block Diagram

Several modulation methods have been proposed over the past few decades. Bipolar, unipolar, and unipolar modulation with multiple frequencies are widely used in current applications. Different modulation methods have different influences on current total harmonic distortion (THD) and the common voltage which is the voltage change between the mid-point of the bridge and neutral point. However, in this application note, totem-pole modulation – also known as 'modified' unipolar modulation – is used to decrease switching loss to optimize the efficiency. The totem-pole bridgeless PFC structure is now widely used in PFC circuits and inverter circuits, such as the PMP23338. In unipolar or bipolar modulation, all four switches operate at high frequency and the switching loss in Si FETs is much higher than GaN FETs. Therefore, if totem-pole modulation is used to let Si FETS operate with low frequency and GaN FET operate with high frequency, totem-pole modulation can fully utilize the characteristic of GaN thus reducing the switching loss.

Figure 2-3 Totem-Pole Modulation

For the low-frequency bridge, the switching point is at the zero of modulation signal and for the high-frequency bridge, the switching point is the intersection point with the sawtooth wave. Table 2-2 shows the switching states using the totem-pole modulation.

Figure 2-4 shows the basic operation situation of each mode.

Figure 2-4 Operation Mode in Each State