Multirate techniques for digital power conversion
Digital power management and control provide real-time intelligence that enables system developers to build power systems that automatically adapt to their environment and provide optimised efficiency for each specific use-case. The use of intelligent digital power ICs allows automatic compensation for changes in load and system temperature and enables energy savings through the use of adaptive dead-time control, dynamic voltage scaling, frequency shifting, phase dropping and discontinuous switching modes.
One obstacle to the rapid adoption of digital power has been its perceived expense, but any differential between analogue and digital control is fast disappearing with the introduction of the latest components, such as Intersil's ZL8800. Digital power efficiency and cost is now equalling or bettering comparable analogue power-conversion solutions, while providing more advanced features.
Most importantly, the pulse width modulation (PWM), loop control and feedback are implemented digitally. Analogue signals are converted to digital using analogue-to-digital converters (ADCs) and once the signals are digital, microcontrollers, digital-signal processors or computational state machines control the digital PWM and the feedback loop. This has important advantages in terms of maintaining stability without the compromises on responsiveness from which analogue control often suffers.
Although digital control offers advantages, many manufacturers are not taking full advantage of what the technology offers and have, in many cases, simply implemented in digital form the core analogue PWM techniques. Digital control makes it possible to build far more flexible control loops and take advantage of multi-rate control in which individual algorithms are tuned to handle events that happen at different speeds.
Traditional digital PWM controllers are uniformly sampled. The controller samples the error in output voltage and from that computes the required duty cycle for the next switching cycle. The downside of uniformly sampled controller is the latency or group delay from sampling the error to when the PWM controller is able to switch the power-supply circuitry appropriately. The group delay translates to phase lag, which increases with frequency and places an upper bound on the achievable closed-loop bandwidth.
Multirate control makes it possible to provide stable power but react almost instantaneously to sudden changes in voltage, providing an appropriate response within a single PWM switching cycle. The only way to achieve with this conventional architectures is to employ variable-frequency switching techniques, using higher frequency sampling and control when the voltage is changing rapidly. But this is not a useful approach for many systems. Modern telecommunications equipment and other applications with stringent electromagnetic compatibility (EMC) demands require fixed-frequency operation so that they can maintain tight control of the noise spectrum.
Another approach is to apply a proportionate gain that is linear to the error voltage deviation. Using only a proportionate gain it is possible to achieve single cycle reaction using fixed-frequency switching but a fast-response loop gain can lead to instability.
The ChargeMode technology developed by Intersil and used in the ZL8800 dual-channel/dual-phase DC/DC controller uses a mixture of uniform and multi-rate sampling techniques, which samples the error and computes the modulation signal multiple times during a switching period. This technique significantly reduces group delay and therefore supports very high bandwidth operation. The phase lag is significantly reduced due to the reduction of group delay. The device also uses a dual-edge modulator, which outperforms competitive and so-called 'leading-edge' modulators in terms of total group delay.
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