Predictive Energy Balancing for Control of Switched-Mode Power
A popular electronics catalog lists 180,000 parts in the Power Management Integrated Circuit category. Add to that the 224,000 listings for DC/DC converters, and the area may seem to be adequately covered by existing products. But, a problem lies hidden in those numbers. There are so many parts because each one is intended for a very specific task. The need for such specialization is driven by the difficulties encountered when stabilizing these controllers over more than a narrow range of operating conditions.
A popular electronics catalog lists 180,000 parts in the Power Management Integrated Circuit category. Add to that the 224,000 listings for DC/DC converters, and the area may seem to be adequately covered by existing products. But, a problem lies hidden in those numbers. There are so many parts because each one is intended for a very specific task. The need for such specialization is driven by the difficulties encountered when stabilizing these controllers over more than a narrow range of operating conditions.
Instead of an overcrowded field, CogniPower sees an opportunity. Predictive Energy Balancing (PEB) is a new, patented control technique that closely follows the underlying physics of switched power. The key calculations for a PEB control loop are made in terms of energy instead of voltage or current. That simple difference allows the gain of a PEB loop to be constant. In other words, there is no compensation needed. That inherent stability offers huge savings in time and trouble when designing and applying Switched-Mode Power Supplies (SMPSs).
The same PEB controller chip can run a buck, boost, flyback, forward or even a SEPIC converter. These SMPSs can be scaled from milliwatts to kilowatts and can operate in discontinuous or continuous mode, or both. The same uncompensated controller can do all that because the same fundamental physics apply to the movement of energy in all the various types of power converters. That flexibility makes PEB easy to apply and digitally control.
The circuitry required for PEB looks a lot like a conventional controller, but the results are dramatically better. After testing a PEB power converter with his sophisticated instrumentation, Steve Sandler of Picotest reported that "this is likely the best performing loop I have seen in my 35+ years as a power guy."
For discontinuous operation, PEB achieves true single-cycle regulation. For continuous operation, one additional cycle is needed beyond the minimum time required for the change in inductive current. To dramatize the difference between conventional loop control and PEB, CogniPower built a power converter so agile that it can serve as an audio amplifier. Imagine substituting an audio input for the DC reference in your power converter. How would it sound if you connected a speaker to the output? The CogniPower PEB amplifier produces a high fidelity output, even though it was intended for only voice-quality application.
The screen shot above shows a PEB amplifier running as a power converter with a 166 kHz clock. The load is modulated at 10 kHz from 20% to 80% of full load. The horizontal time scale is expanded so that each individual control cycle is clearly differentiated. Note the single-cycle response.
Describing the math and basic structure of a PEB controller requires more information than will fit in this format. A short article at cognipower.com/CogniPowerPEB.pdf provides the necessary background, or, see the DPF presentation at cognipower.com/pdf/CogniPowerDarnell2013.pdf.
Substituting PEB for conventional control offers better transient response, more flexibility, and incremental advantages in efficiency, size, and cost. Beyond that, PEB can take better advantage of whichever power-moving switches are in place in a particular topology. Buck, boost, flyback and forward topologies are inherently bidirectional. That capability is usually ignored, or, occasionally is seen as an alternative operating mode. PEB controls can use the available switches to best advantage on a cycle-by-cycle basis.
That means a PEB controller can keep the loop closed under conditions that confound a conventional controller. If you have an SMPS that might run away if presented with the wrong stress, you coddle it. If you have an SMPS that can handle unexpected insults gracefully, you look for more demanding applications.
Over a period of years, CogniPower has explored many implementations of this technology, including multi-output forms, AC/DC forms (with or without Power Factor Correction), bidirectional continuous mode forms, versions with built-in energy storage, bipolar output forms, constant current forms for LED lighting, and quasi-resonant topologies. All of the above can include various levels of isolation. We have built demonstration systems from 50 mW to 1 kW. There are nine issued patents, and more than that many again in process.
Aren’t the old ways good enough? Existing controllers can switch operating modes, for example, to get better efficiency at low loads. Yes, but at a price in complexity, and the discontinuity between multiple modes invites instability.
DSP is now routinely used to improve the dynamic behavior of SMPSs. Doesn’t that solve the problem? The answer is sometimes. DSP does the job for file server power modules, but there is not the space or power available for DSP-enhanced SMPSs in handheld devices. Also, DSP is already challenged to keep up with faster silicon-based switching. GaN and SiC switches can run a hundred times faster than silicon. DSPs will not be able to keep up. PEB, on the other hand, requires a minimum of real-time calculation. Even the simple, low power analog computer intended for cell phone audio can run at 1 megahertz. It could be sped up to 10 or 50 times that fast with minor modifications.
Applications include DC/DC, DC/AC, AC/DC, AC/AC, motor drive, envelope tracking, power amplifiers, piezo drivers, LED lighting, digital power, audio, from hearing aids to theater systems, uninterruptible power supplies, etc.
CogniPower has built a variety of PEB systems, both analog and digital. A hybrid approach would be excellent for digital power applications. Our analog computers are built from inexpensive pairs of ordinary bipolar signal transistors. As an IC, a MOS process is the most likely choice. The point being that there are many ways the PEB controller could be integrated. Once in integrated form, there is an almost endless list of SMPSs that could benefit from performance and efficiency upgrades.
Maybe one more power controller would be a good idea, after all.
For more information, see www.cognipower.com, or contact Tom Lawson, [email protected].
