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Design Features
Advanced Digital Control Enables Smart Power Solution
Anthony Kelly
Chief System Architect, ZMDI
As solutions benefitting from digital power management proliferate, the system level benefits have become well known. From the availability and use of accurate telemetry data to the sophisticated management of a multitude of power rails, digital power control ICs have found their place in the DC-DC POL field.
The implementation of the control loop comes in a variety of offerings from several vendors, ranging from traditional style analogue control loops to digital control loops that in and of themselves come in all sorts of guises rooted in the fields of linear and nonlinear control.
As digital control matures the analogue versus digital debate has turned firmly in the direction of the speed and resolution afforded by analogue solutions, but with the integration, flexibility, modularity, and enhanced control strategies of digital solutions.
Sadly, the vendors of established digital power controllers have been found wanting. The inventiveness and variety of approaches taken by vendors is tremendous but the Achilles heel of all digital power control solutions is the control delay involved in sampling and processing the output voltage or current. Control loop phase margin or bandwidth must be reduced in such linear systems.
The response of the industry has been to invent novel nonlinear techniques that extend the controller bandwidth to a higher “equivalent bandwidth”, but their application results in issues with ease of use and control loop and electrical robustness. Alternatively, an analogue loop is employed, but the integration with the required digital power management, scaling to multiple rails and flexibility in design and application make their use problematic in systems where digital power management prevails.
By combining techniques, such as ZMDI’s TRU-SAMPLE™ technology, with STATE-LAW™ control techniques, it is possible to deliver transient and steady state behaviour. The TRU-SAMPLE™ technology ensures the lowest possible latency in the control loop whilst the STATE-LAW™ control ensures that the controller always responds appropriately to varying load conditions, on a cycle by cycle basis. In this way the transient response is optimised for both transient and steady state operation.
This article will discuss the operation of various digital control techniques and demonstrate advantages and drawbacks of various approaches, giving designers a useful insight into the detailed operation and selection of digital power controllers for DC-DC conversion.
Superior Transient Response of Digital Control: Why?
Whenever a power application demands the best transient response and narrow output voltage total-operating-band in a switch mode power converter, power system architects have tended towards analogue control loop PWM converters. Digital control loop PWM converter vendors have responded with products that perform “just like” or “as good as” analogue control designs. Consider Figure 1 which illustrates the transient response of the output voltage to a load step for both an analogue PWM based control IC and the ZSPM1000 digital PWM based control IC. It is clear that the digital PWM based product can outperform an analogue PWM based product by a factor of 2.
Figure 1. Load Step Transient Response
In order to understand why, we need to look at the impact of delay on the small –signal model of the Buck converter and how that affects the control performance.
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