Passive lossless buffer circuit and its new topology

Abstract: Based on the analysis of the topological classification of passive lossless snubber circuits and the switching losses in the hard switching conversion process, the structural principles and general implementation methods of passive lossless snubber circuits are summarized. Two novel topological structures in DC/DC converters are introduced in particular, and their working principles, advantages and disadvantages are briefly analyzed.

Keywords: passive lossless snubber circuit; DC/DC converter; power factor correction

1 Overview

In hard switching circuits, active switching devices are connected to rigid voltage or current sources, resulting in large switching losses, severe electromagnetic interference, and low reliability. This phenomenon becomes more serious as the switching frequency increases. In order to overcome these defects, soft switching technology is widely used.

Active snubber circuit, RCD snubber circuit, resonant converter, and passive lossless snubber circuit are commonly used soft switching technologies. Among them, the active snubber circuit reduces switching losses by adding auxiliary switches, but this also increases the complexity of the main circuit and the control circuit, thereby increasing the cost-effectiveness and reducing reliability; although the RCD snubber circuit has the simplest structure and the cheapest price, it has the worst performance among various soft switching technologies because the resistor consumes energy; and although the resonant converter achieves ZVS or ZCS and reduces switching losses, the resonant energy must be large enough to create ZVS or ZCS conditions, and the circulating current in the resonant circuit is large, and it must also work under the control signal of a specific soft switching controller, which increases the conduction loss, increases the cost, and reduces the reliability. Unlike these three methods, the passive lossless snubber circuit does not use either active devices or energy-consuming components, and therefore has the advantages of the above three methods. Its structure is as simple as the RCD snubber circuit, and its efficiency is as high as the active snubber circuit and resonant converter. It has low electromagnetic interference, low cost, good performance, and high reliability, and has therefore been widely used.

At present, although the passive lossless buffer technology is relatively mature, new topologies and research results are still published from time to time at home and abroad. Based on the research results of passive lossless buffer circuits in the past 20 years, this paper summarizes the structural principles and general implementation methods of passive lossless buffer circuits. In addition, it focuses on introducing its two latest topological structures in PWM DC/DC converters, analyzing their working principles, and comparing their advantages and disadvantages.

2 Topological classification

In the past few decades, many different passive lossless snubber circuit topologies have emerged, which can be described by a set of properties [1]. For this purpose, they can be divided into two categories: one is the minimum voltage stress unit (MVS), as shown in Figure 1 (a) and Figure 1 (b); the other is the non-minimum voltage stress unit (Non-MVS), as shown in Figure 1 (c), Figure 1 (d), Figure 1 (e), and Figure 1 (f). The minimum voltage stress unit ^[2] uses only an inductor and a capacitor with a small capacitance value to minimize the voltage stress of the main switch tube, but the range of soft switching is not large; the non-minimum voltage stress unit ^[3] adds an inductor and also increases the voltage stress of the main switch tube, but compared with the minimum voltage stress unit, under the same inductance and capacitance, its soft switching range is larger. Moreover, it has higher efficiency under low power conditions.

(a) MVS (b) MVS

(c) Non-MVS (d) Non-MVS

(e) Non-MVS (f) Non-MVS

Figure 1 Passive lossless buffer circuit topology

3 Structural principles and implementation methods

There are three types of losses when hard switching circuits are switched:

1) When turned on, the surge current caused by the reverse recovery current of the freewheeling diode will lead to large conduction losses;

2) When turned on, the discharge of the parasitic junction capacitance of the MOSFET will cause losses;

3) When turned off, the rapid increase in the junction capacitance voltage of the MOSFET will result in large turn-off losses.

In view of the above loss structure of hard switching circuit, a basic passive lossless snubber circuit generally includes three functional circuits:

1) Open the buffer circuit;

2) Turn off the buffer circuit;

3) Feeding circuit.

The commonly used method is to use an inductor ( L ) in series with the power tube. When turned on, the current can only increase from zero, so "zero current" softens the turn-on; use a capacitor ( C ) in parallel with the power tube. When turned off, the voltage across the power tube can only increase from zero, so "zero voltage" softens the turn-off; use a diode (D) through a certain topological network, and during the power tube switching process, the stored energy in L and C is fed back to the power supply or fed to the load. Depending on the converter circuit, the capacitor can be directly connected in parallel with the power tube, or it can be connected across the power tube output and the load, or across the power tube input and the positive end of the power supply. The latter two cross-connection methods require that the capacitor C has been charged to the power supply voltage before the power tube is turned off. Commonly used methods of energy feeding circuits are: when the final charging voltage of a capacitor is required to be greater than the power supply voltage, the power supply can charge the capacitor through an inductor. If the loss is ignored, the final charging voltage will reach twice the power supply voltage; if a charged capacitor needs to change the voltage polarity during work, it can be completed by connecting an inductor in series to achieve oscillating discharge; inductors can also be used to transfer the energy stored in one capacitor to another capacitor. Of course, a circuit composed of diodes must be used here to cooperate; energy storage or transfer can also use mutual inductance methods, etc.

The three-function circuit structure of the passive lossless snubber circuit cannot absorb or supply energy under the control timing of the leading or lagging main switch to create a true ZVS or ZCS condition like the active soft switching scheme, but it can significantly reduce the switching losses in the above items 1) and 3) by staggering the voltage and current waveforms during the switching period to minimize the overlapping area between the two. Although the discharge loss of the parasitic junction capacitance in the switching device in item 2) cannot be eliminated by the passive lossless snubber circuit, this loss is much lower than other switching losses and has little effect on improving the overall efficiency. Considering that the passive lossless snubber circuit does not introduce auxiliary active devices, compared with other soft switching schemes, it does not increase the loss of additional auxiliary active devices. Therefore, under the same switching loss power reduction, the passive lossless snubber circuit can achieve higher efficiency improvement [4]. Therefore, the passive lossless snubber circuit is widely used in PWM converters.

4 Application of Passive Lossless Snubber Circuit in DC/DC Converter

With the development of power electronics, computer technology, and communication technology, passive lossless snubber circuits are not only widely used in PWM DC/DC converters, PWMAC/DC rectifiers[5], and PWM DC/AC inverters ^[6] , but are also closely related to multilevel converters and PFC ^[7] . The following introduces two latest topological structures of passive lossless snubber circuits in PWM DC/DC converters.

4.1 Regenerative Passive Lossless Snubber Circuit

Figure 2 shows a passive regenerative soft-switching Boost converter proposed in the literature [8], which is an improvement on the traditional L + RCD composite snubber circuit. Its improvements include:

Figure 2 Regenerative passive lossless snubber circuit

1) Remove the discharge resistor R ;

2) Remove the dedicated power inductor L and cleverly replace it with a _small power winding La coupled to the input _inductor Lp .

The working process of the circuit in Figure 2 is analyzed below. Assumptions in the analysis:

1) The input voltage V _i is constant, the main inductance L _p is much larger than the buffer inductance L _s , so that the input current I _s is constant;

2) The output capacitor C _o is large enough so that the output voltage V _o is constant;

3) Only the reverse recovery current of the freewheeling diode D and the switching transition time of the main switch S are considered, and other components are ideal;

4) The initial state is S off, D on, i _D = I _s .

For the CCM operation of the inductive load, each cycle can be divided into the following six modes in steady state. The corresponding equivalent circuit diagrams and main waveform diagrams are shown in Figures 3 and 4.

(a) Mode 1 ( t ₁ to t ₂ ) equivalent circuit