Predictive energy balance control for PDN applications
What is PEB?
The PEB control algorithm controls the voltage converter performance from a supply vs. demand perspective. As in most switching converters, energy is stored in an inductor and transferred to the output where the voltage is averaged in a capacitor. The energy stored in the inductor is:
The PEB controller establishes the required supply based on the demand, setting these equations equal during each switching cycle. The result is a “memory-less” control, with every cycle being a “clean slate” resulting in complete dynamic response recovery within a single switch cycle. The output neither overshoots nor undershoots. The controller is inherently stable, since there are no compensating poles added in the control function. The PEB control computational block diagram is shown in Figure 1.
PEB control is adaptable to many switching topologies, including buck and flyback, operating in both discontinuous and continuous operating modes.
Why it might be ideal for PDN
The converter output impedance function under PEB control is a near frequency independent resistance, coincident with the goals of PDN applications. The fixed resistance is a function of the inductor and capacitor ratio. The single cycle response results in the fastest possible recovery time, also ideal for PDN applications. The flat impedance profile is illustrated in Figures 2 and 4, using a low power, discontinuous mode PEB demonstration board. (This measurement was provided by Cognipower and was not designed for any particular application.)
The demonstration board inductor and capacitor values were modified to set a target impedance of 300mO and set in a probing fixture, shown in Figure 3.
The measured converter impedance computed from the 18mV voltage excursion and 58mA current step is approximately 320mO. The output ripple voltage is removed by averaging in this measurement for clarity. Note the total absence of overshoot or undershoot in the voltage response as shown in Figure 4.
While this example provides a 300mO output impedance, representative of a low-power application, the PEB control is scalable to any power level. The computational requirements are minimal allowing either digital or analog implementations. Presumably, the controller could be integrated into a single monolithic chip.
The PEB controller is hardware independent, allowing control of a silicon MOSFET power stage or for much higher frequency operation and optimum efficiency, a GaN power stage. The PEB controller can also be used as an amplifier allowing dynamic voltage programming also with the absence of overshoot or undershoot. When considering the many new switching topologies for PDN applications, the PEB control offers some useful benefits. I hope to explore this topology further in the future.
Tom Lawson, Predictive Energy Balancing for Agile Control of Switched-Mode Power Converters, BODOS Power May 2013 http://www.cognipower.com/pdf/Bodos_May_2013_PEB_Article.pdf
Steve Sandler and Tom Lawson, Quantifying the Difference: Predictive Energy Balancing Controls for Switched-Mode Power Converters PowerViews Nov. 25 2014 http://m.powerpulse.net/powerViews.php?pv_id=85