Achieving Optimized, Minimum-component Input Filters for Transformer-coupled ZVS Buck-Boost DC-DC Converters

Transformer-coupled ZVS buck-boost DC-DC converters provide highly efficient solutions for telecommunication power systems. However they may need input filters for protection from electromagnetic interference (EMI) and high frequency input noise. These filters must be stable over the operating frequency range.

In this Post we describe how it’s possible to design such a filter with just a couple of external components, by using the parasitic parameters of both converter and filter elements. This offers power system designers simplicity, high flexibility and minimized board real estate without sacrificing performance or stability.

Transformer-coupled buck-boost DC-DC converters used in telecommunications power systems can obtain significantly higher efficiency by using double-clamped ZVS buck-boost controllers. They can operate at a very high switching frequency of approximately 1 MHz. This, with the input voltage levels of 36 V to 75 V, 48 V nominal, typical in telecommunications, allows a simplified-design single-stage filter with reduced components and good performance. Component choice is flexible, with only a capacitor and an inductor required. Damping is achieved by using parasitic elements.

Fig 1 shows the topography of an input filter stage based on these principles. ESR is the equivalent series resistance of the capacitor C, DCR is the direct current resistance of the inductor L, while Cint is the internal capacitance of the converter.

Topography of an input filter stage
By considering the input filter circuit as a voltage divider network, its transfer function can be found:

Transfer Function Formula

Starting from this basis, we can make different assumptions about the filter components employed, use these to develop simplified recommendations for choosing their values, and see their impact on the filter’s performance. For example if DCR is sufficiently low to be negligible, this allows a simplified version of the transfer function. From this, we can extract the cutoff frequency and damping factor expressions for a filter providing 40 db/decade of attenuation after cutoff frequency. Both are related to L and Cint, while the damping factor can also be tuned by varying ESR. General recommendations for choosing R, L and ESR using formulae are also possible. Similar recommendations can be made for when Cint is too small to achieve an acceptable cutoff frequency, or if ESR is too large to generate sufficient damping.

The interaction between the input filter and the converter’s negative input impedance may cause stability issues. By making approximations for the converter’s input impedance, the stability of the complete system – filter and converter – can be assessed, using a Routh-Hurwitz table. A list of stability criteria can be concluded – and these expressions in turn include the filter component values. Sometimes, as for the component considerations above, a parameter of the filter is small enough to ignore and simplified stability checks are sufficient.

Practical considerations are also essential to the filter design. Real capacitors and inductors present non-trivial statistical parametric spreads, especially when the frequency is high. The ESRs of electrolytic capacitors also vary with age and temperature.

A full Paper on this topic has been presented 4th IEEE Energy Conversion Congress and Expo (ECCE), Raleigh, North Carolina:

[PAID] An optimal minimum-component input filter design and its stability analysis for a transformer-coupled zero voltage switching buck-boost DC-DC converter

Related filter tool (If downloaded as a ZIP file, rename the extension from .zip to .xlsm and then save; Microsoft Excel 2010 is required to open .xlsm file).

Application Note: Filter Network Design for VI Chip® DC to DC Converter Modules

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