Resonant converter, what is a resonant converter

The resonant converter consists of a switching network Ns, a resonant tank circuit NT, a rectifier circuit NR, a low-pass filter NF, and other parts. Its structural block diagram is shown in Figure 1. In Figure 1, Vg is a DC voltage source that provides input power. The switching network Ns converts DC energy into AC energy, and its output voltage vs(t) is a square wave power signal. vs(t) contains fundamental waves and higher harmonics, and its spectrum characteristics are shown in Figure 2-a. vs(t)) is the input signal of the resonant tank circuit NT. The resonant network NT is a linear network with a bandpass characteristic. Its transmission ratio is defined as the ratio of the output signal to the input signal. The voltage transfer characteristic describes the frequency response of NT, and its spectrum characteristics are shown in Figure 2-b. As shown in Figure 2-c, if NT is a high-Q resonant network and is relatively close to, the output signal of NT only contains the fundamental wave in vs(t), and the higher harmonic components can be ignored. Therefore, the input signal of the rectifier network NR is a sinusoidal quantity. Assuming that the rectifier network is a full-wave rectifier, the output of the rectifier is a full-wave rectifier waveform. After the full-wave rectifier waveform is expanded, it contains DC components and high-order harmonic components, and its spectrum is shown in Figure 2-d. From the perspective of spectrum analysis, the role of NR is equivalent to "spectrum shifting". Assuming that the turning frequency of the low-pass filter network NF is much smaller than the switching frequency, its spectrum is shown in Figure 2-e.

Figure 1 Resonant converter block diagram
The following is a brief introduction to the regulation principle of the resonant converter. When the switching frequency is equal to the resonant frequency of the resonant network, the DC output voltage reaches the maximum value; when the switching frequency deviates, the DC output power decreases. The farther the deviation is, the lower the DC output voltage is. Therefore, the resonant converter achieves the purpose of regulating the output voltage by changing the degree of deviation from.

Figure 2 Spectrum of resonant converter
According to the type of resonant tank circuit, resonant converters mainly include three basic types: series resonant converter (SRC), parallel resonant converter (PRC) and series-parallel resonant converter (SPRC).