Ripple Reduction Techniques for Stable Power Conversion
Ripple appears as noise on the output of a power supply.
Providing stable power requires reducing ripple on the output voltage.
Ripple reduction techniques include filtering and precise regulation with feedback.
All power supplies have some level of ripple that appears as noise on the device output. In particular, ripple is an annoying noise problem that is seen on the output from a switching DC-DC converter. Some applications, particularly analog components that receive DC supply voltages, need to have very clean power, otherwise the noise may propagate to the output of the component. In contrast, saturation logic (TTL and CMOS, for example) is more immune to this type of noise.
DC-DC converters that implement switching regulation need to have at least one ripple reduction technique implemented, but the technique needs to be matched with the appropriate switching frequency, power output, and load operating frequency. The load impedance also plays a role in determining the behavior of the switching converter and the effectiveness of the implemented ripple reduction technique. In this article, we’ll give a brief overview of several ripple reduction techniques and where they might best be used in a new power supply design.
Simple and Advanced Ripple Reduction Techniques
Ripple reduction techniques can be implemented with passive circuits or active circuits as well as with linear or nonlinear components. Some algorithms for implementing active control strategies that attempt to compensate for ripple on the output from a DC-DC converter can be a bit complex and are best implemented with a specialty ASIC.
Regulation With an LDO
This strategy is an entry-level ripple reduction technique for components running on DC power. Many digital components will include an on-die LDO that provides ripple reduction. The ripple reduction effectiveness provided by these components can be in the neighborhood of 80 dB, meaning ripple can be reduced by a factor of 100 million! These designs operate by using a comparator and a stable silicon bandgap reference voltage source in the component, which saturates the output voltage. However, because they are reducing noise through saturation, they will generate a lot of heat when the input voltage differential and output power get too high.
Linear regulators can provide high ripple reduction
Filtering with a low-pass filter or a notch filter is a strategy for addressing the ripple frequency. This is preferable in DC regulators, especially when filtering is implemented. Simple higher-order passive filtering implemented in a pi-topology (LC circuits) can provide high rolloff in the transfer function, but requires very large inductors and capacitors to filter out typical PWM frequencies. A more sophisticated strategy uses active filtering with reactance added to the feedback loop, which can provide very high gain at DC and suppress the AC output.
Control With Feedback
Feedback loops are implicitly implemented in some highly integrated switching regulator components, specifically those that implement the PWM oscillator, gate driver, and FET stages into the same package. More advanced designs that run at very high power output need a set of discrete components to implement a control strategy in a feedback loop. Once ripple on the output can be sensed alongside the DC output, a control strategy that involves adjusting the PWM driving signal and the driving frequency allows the output level and ripple to be stabilized simultaneously. This is one area of active research among power systems designers.
Larger Inductor and Higher PWM frequency
These two methods should be mentioned together, as they are the standard tools used to set the initial ripple value in a switching regulator (in standard buck, boost, and buck-boost designs). The ripple level is inversely proportional to the output inductance and the PWM frequency, so increasing both will reduce the ripple measured on the output.
Large inductors are needed at lower switching frequencies to hit a target ripple value
A multiphased converter implements multiple switching stages in parallel, but separated in phase. While these circuits can be much more complex, they act like a single-stage regulator with a much larger switching frequency and inductor. The result is greatly reduced ripple measured on the output. These regulators can be used with DC loads or high frequency AC loads for FM signals as long as the output filtration stage has a cutoff frequency above the baseband frequency. These converters can also provide high current without overstressing components because the switching load is spread across multiple switching stages.
A Summary of Ripple Reduction Techniques
The table below shows when various ripple reduction techniques are appropriate for use in switching DC-DC converters. Designers should be mindful which methods they use to implement ripple reduction, as these mechanisms can fail if used in the wrong situation.
One final point to consider is that these methods are often implemented in tandem in a new design. For example, buck converters for RF power systems and high-power RF transmitters can implement higher-order low-pass filtering, multiphase regulation, and high PWM frequencies (several MHz or more) with a passive or active control method in a feedback loop. Choosing the right combination of techniques requires considering everything from the behavior of the load to the operation of the converter stage. SPICE simulations with highly accurate component models are the starting point for evaluating these designs.
Cadence’s Allegro PSpice Simulator application is an ideal tool for evaluating ripple reduction techniques and designing circuits to implement control strategies. When you’re ready to create your PCB layout, you can use Cadence’s PCB design and analysis software to finish your design. Cadence’s software suite can also be used to perform a range of power and signal integrity simulations and evaluate your system’s functionality before you send your design out for manufacturing.
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