Compensation-Free Voltage Regulators: The Continuing Evolution of Digital Compensation

Feedback and compensation of voltage regulators

Traditional power supplies and voltage regulators are able to produce stable output voltages due to the incorporation of control loops with negative feedback. A major challenge to properly implement negative feedback is to provide the correct frequency compensation associated with the feedback network. Initial implementations of voltage regulators employed analog circuitry for the control and feedback circuits. Later improvements in technology have allowed digital circuits to replace almost all of the analog functions in voltage regulators and power supplies. The incorporation of digital circuits has allowed the development of automatic compensation algorithms to ease the burden on the power design engineer. Auto-compensation is a great improvement over traditional topologies but still has some limitations due to the requirements of the circuits to determine the compensation parameters. Recent developments in digital voltage regulator controllers have created "compensation-free" topologies. These compensation-free designs provide superior voltage regulation while eliminating the issues associated with determining compensation parameters.

Analog voltage regulators

Analog voltage regulators require the design engineer to determine 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 power delivery designs. Some vendors simplify the compensation 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 user tasks, it reduces the probability that the resulting load current transient behavior of the power supply is acceptable. The design and implementation of analog voltage regulators is a manually-intensive process and thus carries undesirable risks and costs.

Figure 1: Analog Switching Voltage Regulator

Analog voltage regulators with digital wrappers

When an IC vendor adds a digital wrapper to an analog voltage regulator there are benefits in the ability to configure, control and monitor some of the characteristics of the power supply. Selecting analog voltage regulators with digital wrappers improves upon the challenges and delays in designing with a traditional analog voltage regulator, but the risks and costs associated with the compensation components still exist.

Figure 2: Analog Switching Voltage Regulator with 'Digital Wrapper'

Digital voltage regulators

A digital voltage regulator topology can allow the user complete configurability, controllability and monitoring capability of the power supply via a software interface. Many digital voltage regulators are designed in a manner that allows the user to select proportional, integral and derivative (PID) tap coefficients rather than physical compensation 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. The 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 optimized design.

Figure 3: Digital Switching Voltage Regulator

Digital voltage regulators with automatic compensation

Recent advances in digital voltage regulators include the incorporation of an automatic compensation topology that eliminates the need for the user to have knowledge and experience in compensation techniques. These regulators are able to determine the optimum compensation (values for Kp, Ki and Kd) for the circuit when power is applied to the regulator or at any other time that 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 the compensation values.

Figure 4: Digital PID Compensator

Compensation evolves again with compensation-free designs

A superior digital voltage regulator topology to those that provide automatic compensation is one that requires no compensation at all. CUI offers families of digital point-of-load (POL) modules which are based upon the compensation-free technology; the NDM3Z-90 POL modules are the latest example. These modules determine 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 optimize the load transient response each switching cycle of the regulator without the use of feedback loop compensation. The compensation-free topology is a superior technology due to 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. The cycle-by-cycle charge delivery architecture also incorporates non-linear transient response characteristics to provide superior output voltage regulation of the POL modules as compared to what could be achieved with more conventional feedback loop compensation. One benefit of low latency and non-linear transient response techniques is the reduction in output decoupling capacitors required. 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 minimize the number, area and cost of the decoupling capacitors required to achieve the desired transient response of the digital POL module.

Figure 5: Compensation-Free Digital Compensator

Superior power delivery solutions without power supply expertise

Compensation techniques have come a long way since the days of the manual "trial and error" methods employed in purely analog designs. The complexities of powering today’s advanced semiconductors coupled with increasingly short design cycles have driven an evolution in compensation methods. The latest compensation technology employed in many CUI digital power modules, coupled with an easy-to-use Graphical User Interface also supplied by CUI, now allows for rapid design cycles without the need for advanced power supply knowledge.