DC/DC Controller Design without Input Filter
DC/DC converters are often designed assuming an ideal input voltage source, capable of absorbing the switching ripple generated by the converter. In the example below, a buck converter supplied by an ideal source delivers power to a resistive load. This ideal assumption implies that the input source can tolerate the converter’s 100 kHz current ripple without adverse effects.
In practice, however, all commercial converters must comply with conduction EMI requirements to prevent interference with other equipment connected to the same power bus. To attenuate high-frequency switching noise, a second-order input filter, typically consisting of a differential-mode choke in series with a capacitor, is added at the input. It is essential that the introduction of such a filter does not severely degrade converter performance or, in the worst case, cause instability. The figure below shows the initial controller design without an input filter, which exhibits sufficient gain (greater than 11 dB) and phase margin (~50°).
Impact of an Undamped Filter on Controller Performance
An undamped LC input filter is added to the system to illustrate the potential adverse effects such a filter can introduce. The figure below compares the controller performance without an input filter (green curves) to the degraded performance when an undamped LC filter is present (red curves).
The transient response highlights how a step change in load induces prolonged oscillations, indicating a significant loss in stability margin. Although the system eventually settles, the resulting performance would generally be considered unacceptable. Reviewing the Bode plot, the system now exhibits multiple crossover frequencies, making it difficult to define a single, meaningful phase margin [1].
Impact of Different Damping Strategies on Converter Performance
The model below demonstrates three different damping strategies designed to mitigate the adverse effects of the input filter on converter performance. All strategies employ a single damping resistor, differing only in the blocking element (or placement of blocking element) used to prevent excessive power loss. For detailed filter design methodology, see [2].
The strategies are:
- R-Cd Parallel damping: A resistor and capacitor in parallel across the filter capacitor.
- R-Ld Parallel damping: A resistor and inductor in parallel across the choke.
- R-Ld Series damping: A resistor in series with the choke but with a low impedance path for the DC current via the parallel damping inductor Ld.
The different filter configurations are shown in the figure below:
The converter performance of these strategies is illustrated in the figure below.
All three damping strategies provide significant attenuation of the input current ripple without degrading converter performance.
Finally, the figure below compares the input ripple current for five cases: no input filter, undamped filter, and the three damping strategies. Each strategy presents its own design challenges—for example, the parallel R-Cd approach uses a damping capacitor (C_d) that is 2.5× the filter capacitor, which may introduce space and layout constraints in the converter design. Alternate filter designs, such as multi-stage filters, may offer a more optimal solution to meet the converter’s multi-objective design requirements. During design iterations, it is critical to ensure that controller stability is maintained across the entire operating range to avoid last-minute surprises.
References:
- R. W. Erickson and D. Maksimović, Fundamentals of Power Electronics, 2nd ed. Boston, MA: Springer, 2001.
- R. W. Erickson, “Optimal single resistors damping of input filters,” APEC '99. Fourteenth Annual Applied Power Electronics Conference and Exposition. 1999 Conference Proceedings (Cat. No.99CH36285), Dallas, TX, USA, 1999, pp. 1073-1079 vol.2, doi: 10.1109/APEC.1999.750502.
Model:
buck_converter_input_filter_stability_impacts.plecs (69.7 KB)