Soft switching technology and circuit classification

乱世煮酒论天下

Soft switching technology and circuit classification [Copy link]

According to the principle of soft switching technology, it can be divided into the following four categories:

1. Full resonant conversion circuit, which can be divided into series resonant conversion circuit and parallel resonant conversion circuit according to the resonance type, and can be divided into series load and parallel load resonant circuit according to the connection in the circuit;

2. Quasi-resonant conversion circuit, which can be divided into zero voltage switching circuit and zero current switching circuit;

3. Zero switching PWM circuit can be divided into zero voltage switching PWM conversion circuit and zero current switching PWM conversion circuit;

4. Zero-transition PWM conversion circuit can be divided into zero-transition switch PWM conversion circuit and zero-current transition conversion PWM circuit.

1. Quasi-resonant circuit is a circuit in which small inductors and small capacitors are connected in series or in parallel in the main switching circuit. The characteristic is that the resonant element only participates in a certain stage of the energy conversion circuit, rather than the whole process. The voltage waveform or current waveform in the resonant circuit is a sine wave, so it is quasi-resonant. The resonant circuit realizes soft switching of the switch tube, but because the resonant period of the resonant circuit changes with the input voltage and load, the circuit can only be controlled using pulse modulation.

2. Zero-switch PWM circuit is formed by adding auxiliary switches on the basis of quasi-resonant circuit. The introduction of auxiliary switches is used to control the start time of resonance, so that resonance only occurs before and after the switching process, so that the circuit can adopt constant frequency control mode, namely PWM control mode. Since the writing time of the resonant element is very short relative to the switching cycle, and the resonant frequency of the resonant element is generally only a few MHz, the switching frequency of the zero-switch PWM circuit can only be several hundred kHz to 1MHz, which is lower than that of the quasi-resonant circuit. However, the switching devices of the two resonant circuits generally have to withstand very high voltages, so they are generally used in low-power and low-voltage occasions.

3. The resonant capacitor and resonant inductor of the first two resonant circuits are always involved in energy transfer, so the energy loss is large, so a zero-conversion PWM circuit is proposed. The advantages are as follows: the auxiliary circuit only works when the switch tube is switched, and does not work at other times. At the same time, the auxiliary circuit is connected in parallel with the main power circuit, thereby reducing losses; because the auxiliary circuit is connected in parallel with the main circuit, the input voltage and load current have little effect on the resonance process of the circuit, and the circuit can work in a soft switching state in a wide input voltage range and from zero load to rated load.

The most common soft switching applications are LLC resonant converters, full-bridge DCDC conversion, main circuit output resonant capacitor C, resonant inductor Lr, excitation inductor Lm, and main power transformer T.

The topological structure of the resonant circuit shows that it has two inductors and correspondingly two resonant frequencies, one is the resonant frequency fr of the resonant inductor Lr and the resonant capacitor Cr, and the other is the resonant frequency fm of the resonant inductor Lr and the excitation inductor Lm in series with the resonant capacitor Cr.

There are three working modes according to the relationship between the switching frequency fs and the resonant frequency:

1. fs>fr, the excitation inductance Lm is always clamped by the output voltage through the transformer at nVo and does not participate in the resonance. At this time, Lm is considered to be a passive load connected in series with the resonant converter, and the secondary diode current continuously reverse recovers.

2. fs=fr, the excitation inductance Lm is always clamped by the output voltage through the transformer at nVo and does not participate in the resonance. At this time, Lm is considered to be a passive load connected in series with the resonant converter, and the secondary diode current is critically continuous, which can achieve zero current shutdown.

3. fm<fs<fr. After the excitation inductance Lm is equal to the resonant inductance current, the excitation inductance is no longer clamped by the output voltage, but resonates with Lr and Cr. At this time, the secondary diode current is intermittent, and zero current shutdown can be achieved.

chejm

The technical knowledge points summarized by the host are very comprehensive and detailed, which is very helpful for technical beginners like me. Thank you for the host

乱世煮酒论天下

chejm posted on 2024-7-22 06:07 The technical knowledge points summarized by the host are very comprehensive and detailed, which is of great help to technical beginners like me. Thank you for the host

I don't really understand the LLC photo at the end. Why is the output used to clamp the input? It's a bit like proof by contradiction. How can we infer it directly from the beginning to the end? I have the following questions:

1. It is LLC resonance, but why not consider the primary inductance of the transformer and the mutual inductance of the secondary side of the transformer to the primary side?

2. The full-bridge output is a square wave, which is a multi-harmonic wave. The resonance is an LC circuit structure. When the switching frequency fs is greater than fr, why is the excitation inductance clamped? What is the process?