Demystifying the digital power compensator design process

Power engineers have worked long and hard to perfect the art of power supply design over the past several decades. In today’s world, they are tackling a new challenge: designing digital compensators for digital power supply designs. Much of the age-old control theory and analog design processes still apply in the digital world with some added idiosyncrasies. For instance, there is an inherent sampling error introduced when the analog signal is discretized by an analog-to-digital converter (ADC). Additionally, there is a phase shift caused by the delay in processing of the control law outputs. And finally, there are obvious bandwidth limitations of the digital power supply control loop as it approaches the Nyquist frequency, which is half the sampling frequency. These small changes in the system prevent the analog theory from mapping uniformly to the digital domain, causing a point of contention for die-hard analog power supply designers trying to convert to the realm of digital power supply design.

Designing a digitally controlled power supply typically involves the following steps, which are similar to the analog control design process:

1)      Design a digital compensator based on a theoretical plant transfer function.

2)      Measure the frequency response of the loop, which in this case is the digital compensator, the power stage (also known as the plant) and the feedback.

3)      Analyze the system frequency response.

4)      Based on the measured response, modify the digital compensator to optimize the gain margin, phase margin and bandwidth of the digital control loop.

5)      Repeat steps 2-4 until the power supply system is properly tuned.

The process of designing and tuning a digital power supply control loop, as Texas Instruments has streamlined with its latest powerSUITE digital power supply software design tools, is described in the following diagram:

The Solution Adapter tool allows you to adapt existing code examples from TI digital power supply kits and configure them to run on your custom digital power supply board that uses the same topology as in the TI kit. The GUI steps you through the process of selecting the solution to adapt, selecting the relevant options for that solution, and customizing those options to adapt the software solution to your custom hardware design.

 The Software Frequency Response Analyzer (SFRA) tool enables measurement of the open loop gain of a closed loop digitally controller power converter using software. This makes measurement of the bandwidth, gain margin and phase margin of your power supply design quick, easy and unobtrusive (see this blog post for more details).

 The Compensation Designer tool allows the design of different styles of compensators to achieve the desired closed loop performance. This can be done using the measured power stage or plant data from the SFRA tool or the modeled power stage as part of the Solution Adapter Tool. The coefficients that need to be programmed on the device are generated by the Solution Adapter and can be copied into the code directly.

 Learning these digital power supply design concepts can be easily applied using the new Digital Power BoosterPack, which implements a single DC-DC buck converter power stage and an on-board adjustable load (controlled by software).

 This development platform simplifies the task of digital compensator and digital control loop design and allows the power designers to focus more on the power stage design. What will you do with this new suite of software tools and development kit? Leave a comment below!

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