CUI Inc. – Compensation-free voltage regulators: continuing the evolution of digital compensation

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Traditional power supplies and voltage regulators are able to produce stable output voltages due to the incorporation of control loops with negative feedback. One difficulty associated with the proper implementation of negative feedback, however, is providing the correct frequency compensation for the feedback network. Early voltage regulator designs used analogue circuitry for the control and feedback circuits. Later improvements in technology have allowed digital circuits to replace almost all of the analogue functions in voltage regulators and power supplies.

The incorporation of digital circuits has allowed the development of automatic compensation algorithms, easing the work of the power design engineer. While this is a great improvement over traditional topologies, it still has some limitations due to the requirement to determine the compensation parameters.

Now recent developments in digital regulator technology have led to the introduction of compensation-free topologies. These compensation free designs provide superior voltage regulation while eliminating the problems associated with determining compensation parameters. This Design Note describes the evolution of voltage regulation which has led to the compensation-free regulator.

The first analogue voltage regulators
Analogue voltage regulators require the design engineer to calculate the values for compensation resistors and capacitors, and then to solder these components onto the PCB. The selection, placement and modification of the discrete compensation components add delays and risks to the process of designing power supplies. Some vendors simplify the component selection process by allowing the user to select a single resistor and a single capacitor to compensate the regulator. While this option simplifies the task, it also makes it less likely that the power supply will respond adequately to sharp changes in the load current.

In essence, then, the development and implementation of analogue voltage regulators is an intensive process which exposes the design team to undesirable risks and costs.

Fig. 1: typical analogue switching voltage regulator

Fig. 1: typical analogue switching voltage regulator

Applying a digital wrapper to the analogue voltage regulator
IC vendors next sought to improve the configuration, control and monitoring of their products by adding a digital wrapper to the analogue voltage regulator. This somewhat reduced the difficulties and delays involved in designing with a traditional analogue voltage regulator, but the risks and costs associated with the compensation components still existed.

Fig. 2: typical analogue switching voltage regulator with a digital wrapper

Fig. 2: typical analogue switching voltage regulator with a digital wrapper

The fully digital voltage regulator
A digital voltage regulator topology can give the user complete configurability, controllability and oversight of the power supply via a software interface. Many digital voltage regulators allow the user to select Proportional, Integral and Derivative (PID) coefficients rather than physical components to provide compensation for the voltage regulator feedback loop. With these topologies, the risks and delays of soldering (and unsoldering and then re-soldering) discrete compensation resistors and capacitors are eliminated, since the PID coefficients are entered and altered as software functions. These software compensation techniques reduce many of the delays and risks associated with soldering components, but the design engineer still needs to have extensive knowledge of compensation theory in order to produce an optimised design.

Fig. 3: typical digital switching voltage regulator

Fig. 3: typical digital switching voltage regulator

Enhancing digital regulators with automatic compensation
Newer digital voltage regulators include an automatic compensation topology which eliminates the need for the user to have knowledge of compensation techniques. These regulators are able to determine the optimum compensation values for the circuit when power is applied to the regulator, or at any other time when a software command is sent to the unit to re-calculate the compensation. Automatic compensation eliminates the costs, risks and delays associated with topologies that require a design engineer to determine compensation values.

The latest evolution: compensation-free regulation
There is, of course, one thing better than automatic compensation, and that is to require no compensation at all. This is the promise of CUI’s NDM3Z-90 digital Point-of-Load (PoL) module, which is based on compensation-free technology. It determines the load-current transient response by monitoring and adjusting the charge delivered to the load on a cycle-by-cycle basis. This technique allows the voltage regulator to optimise the load transient response each switching cycle of the regulator without the use of feedback loop compensation.

Fig: 4: a digital PID compensator

Fig: 4: a digital PID compensator

The compensation-free topology is a superior technology because of the low latency involved in the load transient response. Low latency is achieved by the implementation of a faster signal path in the compensator in addition to the traditional slower signal path.

Fig. 5: compensation-free digital compensator

Fig. 5: compensation-free digital compensator

The cycle-by-cycle charge-delivery architecture also incorporates nonlinear transient response characteristics to provide superior output-voltage regulation of the PoL module compared to that offered by conventional feedback loop compensation.

One benefit of low latency and non-linear transient response techniques is a reduced requirement for output decoupling capacitance. Decoupling capacitors provide transient control at frequencies above those to which the voltage regulator can respond. The low latency and non-linear transient response of the no-compensation architecture extend the effective frequency range of the voltage regulator and thus minimise the number, area and cost of the decoupling capacitors required to achieve the desired transient response.

Eliminating need for power-supply expertise
Compensation techniques have come a long way since the days of the manual, trial-and-error methods employed in purely analogue designs. The complexities of the power circuits for today’s advanced semiconductors, coupled with the compression of OEMs’ design cycles, have led to an evolution in compensation methods. The latest compensation technology employed in the CUI NDM3Z-90 PoL module, backed by an easy-to-use graphical user interface also supplied by CUI, now allows for rapid design implementation without the need for deep knowledge of power-supply design.
 

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