Switching power supplies are highly valuable due to their compact size, low cost, and high efficiency. However, a major drawback is the high output noise caused by switching transients, which makes them unsuitable for high-performance analog circuits that typically use linear regulators. Fortunately, with proper filtering, a switching converter can be used to produce a low-noise power supply, making it a viable alternative.
Designing an optimized and damped multi-stage filter is essential to eliminate the output noise of a switching power supply. This article focuses on a boost converter example, but the design principles can be applied to any DC-DC converter. Figure 1 shows the basic voltage and current waveforms of a boost converter in constant current mode (CCM). In practice, the output waveform often appears more like Figure 2, even with good layout and ceramic capacitors, due to parasitic inductance in the switch, layout, and output capacitors.
The switching ripple, although small compared to the undamped ringing, is still a significant issue. This noise typically ranges from 10 MHz to over 100 MHz, beyond the self-resonant frequency of most ceramic capacitors, making additional capacitance ineffective. Various types of filters can be used to address this noise, and each will be explained with detailed design steps.
While the formulas in this paper are simplified, some assumptions have been made to make them more practical. Iterations may still be required as components affect each other. The ADIsimPower design tool helps simplify the process by optimizing component values before selection, but for initial designs, using a simulator like ADIsimPE or hands-on testing can yield satisfactory results with minimal effort.
Before designing a filter, it's important to consider what can be achieved with a single-stage RC or LC filter. A secondary filter is typically used to reduce ripple to a few hundred μV pp and suppress switching noise below 1 mV pp. However, once ripple is reduced to the μV level, noise coupling between component parasitics and the filter becomes a limiting factor. For very low noise power supplies, combining a secondary filter with an LDO at the output may be more effective.
Several key parameters are defined in the design process, such as ΔIPP (peak-to-peak current into the filter), ΔVRIPOUT (output voltage ripple), RESR (ESR of the output capacitor), FSW (switching frequency), CRIP (assumed current through the output capacitor), ΔVTRANOUT (voltage change during load step), ISTEP (load step), TSTEP (response time), and Fu (crossover frequency).
The simplest filter is an RC filter, suitable for low-current applications. It has the advantage of being low-cost and not requiring damping, but it is limited by power consumption. For higher current sources, replacing resistors with inductors in a pi filter offers better performance without excessive power loss. However, this introduces a potential resonance issue, requiring careful damping techniques.
Three damping methods are discussed: adding a resistor, using a high-ESR capacitor, or adding a damping capacitor. Each has its own trade-offs in terms of cost, size, and performance. The first technique is often preferred due to its lower cost and ease of implementation.
Compensation is also crucial. Placing the filter inside the feedback loop helps suppress noise and improves transient response. The Bode plot of a boost converter with an LC filter illustrates how the filter affects the control loop, necessitating proper compensation.
The design process for LC filters involves selecting appropriate capacitors and inductors, calculating damping resistance, and iterating to meet ripple and transient requirements. Care must be taken when choosing actual components, especially considering the DC bias effect on ceramic capacitors.
In conclusion, this article provides various techniques for filtering switching power supplies, offering a step-by-step approach to reduce guesswork and streamline the design process. With these methods, you can achieve a low-noise power supply efficiently.
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