Interleaved buck-boost converters and cascaded non-inverting buck-boost converters in power electronics can reduce voltage and current stress, improve efficiency, and increase power density. Learn more about it in this article.

DC-DC power electronics converters find extensive usage in various applications, including microgrids, electric cars, renewable energy sources, and uninterruptible power supplies.

How can we improve the efficacy of the buck-boost converter?

The buck-boost converter uses basic circuitry and control loops to adjust the input voltage's magnitude. On the other hand, its inverted output voltage results in a restricted voltage gain. It was suggested that the buck-boost converter be run in discontinuous conduction mode (DCM) to boost the voltage gain.

Operating under DCM results in smaller converter passive parts and better dynamic performance. However, power electronics devices experience increased current stress.

Researchers have proposed two distinct topologies to lessen current stress.

  • First topology: During the discharging process, the two buck-boost converter inductors are linked in parallel and charged in series.
  • Second topology: During the charging and discharging process, the inductors are connected in parallel. 

It was determined that the second topology successfully decreased the current load on power electronics components.

What are interleaved converters?

By connecting two or more converters in parallel, interleaving converters can produce efficient performance. An interphase transformer and a common inductor are employed to double the ripple frequency and boost the power density.

Advantages

  • Raises both the output voltage and the converter's rated output power.
  • Moreover, because the inductors' currents from several interleaved converter phases charge a common capacitor, the current ripple is reduced.
  • Power electronics devices of various phases experience a distribution of current stress, which tends to reduce interleaved converter power losses.
  • The reduction in passive component size and weight resulting from a rise in ripple frequency is another benefit of interleaved converters.
  • The ripple frequency of a two-phase interleaved converter is twice that of the switching frequency.

Continuous conduction mode

The continuous conduction mode (CCM) is an operating mode for the coupled inductor-based interleaving converter. Nevertheless, the converter's weight increases during CCM operation. 

Zero-voltage switching

An interleaved converter that can perform zero voltage switching (ZVS) has been proposed. This topology's primary drawback is that it necessitates the use of an extra inductor to accomplish ZVS, which runs at CCM, increasing the converter's weight and footprint size.

Discontinuous conduction mode

Both the converter's size and weight are taken into account for PV applications and electric vehicles. Operating in discontinuous conduction mode (DCM) is therefore recommended. Compared to CCM operation, the interleaved converter has more operating modes in a single switching cycle when it runs in DCM. The primary drawback of this topology is the increased number of magnetics.

What is a cascaded non-inverting buck-boost converter (CNIBBC)?

Despite having a simple circuit design, the traditional interleaved buck-boost converter places a lot of voltage stress on its power electronics components. 

The benefits of the cascaded non-inverting buck-boost converter (CNIBBC) are as follows: 

  • Non-inverted output voltage polarity
  • Decreased voltage stress

To increase the PV input voltage for the grid-connected inverter, two interleaved CNIBBCs are connected in parallel.

Proposed Non-Inverting Interleaved Buck-Boost Converter Topology

The non-inverting interleaved buck-boost converter with reduced switch count described in this article is depicted in Fig. 1.

Fig. 1: The proposed interleaved converter: (a) circuit, (b) switching signals. Source: IEEE Access

A single-buck converter is cascaded with n parallel-interleaved boost converters to form the suggested topology. The suggested converter has fewer switches. The suggested interleaved converter is easy to modify, has an odd or even number of phases, and has a straightforward architecture.

The suggested three-phase interleaved buck-boost converter is made up of 

  • Three interleaved parallel-connected boost converters (L1 to L3, S1 to S3, and D1 to D3)
  • One-buck converter (S0 and D0)

S0 is turned on at the same time as any of the switches from S1 to S3. As a result, the gate signal for the buck converter switch S0 is obtained using an OR logic gate with input signals S1 to S3. As a result, the buck converter's switching frequency is three times that of the boost converters. 

As seen in Fig. 1(b), the three boost converter switches (S1 to S3) all function with the same duty cycle, but each gate signal is shifted by 120° from the one before. 

The proposed interleaved converter will function as a traditional interleaved boost converter if the modulation index is greater than 1/n without altering the control loop or switching pattern. This is because the suggested buck switch is always on. 

Because of this feature, the converter can handle continuous input current and has a high voltage gain. The single buck switch also gives the suggested interleaved topology soft start and shutdown capabilities.

For non-overlapping gate signal functioning, the duty cycle k must be less than 1/3, as shown in Fig. 1(b). If the duty cycle exceeds one-third, S0 becomes constantly conducting, and the proposed converter functions as an interleaved boost converter. This operation is appropriate for situations that demand a large DC voltage gain. 

In Fig. 2(a), a rectifier with an LC filter is added to the proposed interleaved DC/DC converter to make it an AC/DC converter. Figure 2(b) depicts a cascade control system that may operate the proposed converter in either DC/DC or AC/DC modes. 

Fig. 2: The proposed AC/DC converter system. (a) topology, (b) controller. Source: IEEE Access

Examination of the Proposed Topology

The features of the proposed buck-boost interleaved converter when operating with non-overlapping gate signals are examined.

It was discovered that the proposed converter operates in 

  • Boost mode during five zones 
  • Buck mode during two zones 
  • A buck-boost mode in one zone

The voltage gain and peak current for each zone are calculated, and relation to the duty cycle, converter parameters, and load resistance are studied. An experimental prototype is created to test the suggested interleaved buck-boost converter's analysis and capabilities. 

Conclusion

To conclude, the suggested converter was determined to have an efficiency of 98.5%. It may perform a soft shutdown and a soft startup for both DC-DC and AC-DC operations. Consequently, the suggested interleaved buck-boost converter performs better than the traditional interleaved boost converter.

Summarizing the Key Points

  • Interleaved buck-boost converters offer reduced voltage stress and increased power density, making them advantageous for various applications.
  • The proposed non-inverting interleaved buck-boost converter topology provides a straightforward architecture and improved efficiency.
  • Operating buck-boost converters in discontinuous conduction mode can lead to smaller passive components and better dynamic performance.
  • Cascaded non-inverting buck-boost converters help decrease current stress on power electronics components, enhancing overall system reliability and performance.

Reference

Alajmi, B. N., Marei, M. I., Abdelsalam, I., & Ahmed, N. A. (2022). Multiphase Interleaved Converter Based on Cascaded Non-Inverting Buck-Boost Converter. IEEE Access, 10, 42497–42506. https://doi.org/10.1109/access.2022.3168389