easily two ZL9117 modules can be combined to build a two-phase current sharing rail. Digital power robustness is enhanced by ability to monitor and respond, in an optimal way, to environmental changes. The ZL6105 monitors both the internal die temperature and external temperatures allowing the controller to compensate temperature sensitive measurements for accurate control and monitoring. Input, output voltage and output current monitoring allows the device to detect system faults and prevent catastrophic consequences for power supply and load by configurable fault reaction. Stability is critical for power supplies. In
regulated power supplies it is controlled by the characteristics of the feedback path. Power supply engineers need to ensure stable operation over all load conditions, environmental conditions and component characteristic variations. Designing the feedback loop to be stable under all of these conditions is time consuming.
Digital power solutions provide an alternative to analogue compensation. There are no external components and can be tuned just by changing the gain values stored in digital registers. Digital filters are not simply replacements of analogue filters. Digital filters can perform functions that go far beyond the capabilities of analogue filters. For example, in high Q (>0.5) second order circuits, the poles in the plant are complex conjugate poles which may require complex conjugate zeros in the compensation network to effectively compensate. Conventional analogue compensators only provide real zeros for compensation. On the other hand, digital filters can easily provide the complex conjugate zeros to compensate high Q power supplies. Nonetheless, even this advantage, in many cases is not enough to stabilise and optimise a power supply over all conditions. Initial inductor and capacitor values can vary by +/- 10% . This can significantly change the control loop even to the extent that the power supply has
substantially degraded stability. For example, electrolytic capacitor characteristics such as the capacitance and ESR can greatly change with temperature. What is really needed is a method for compensating power supplies that is automatic. Intersil’s Zilker Labs has
recently released several parts with auto compensation. All of those use an advanced digital algorithm to
characterise the plant and to determine appropriate compensation settings for stable operation. All these converters use a dedicated state machine for the digital PWM controller and an embedded microcontroller to monitor the circuit, environmental conditions, and configuration profile to setup and modify the state machine operation in real time. During auto-compensation, the
Figure 3. Two ZL9117 modules using communication via DDC bus for current sharing
microcontroller adjusts the state machine to stabilise the power conversion process by adjusting the compensation coefficients in a systematic way while observing the response of the system. While this does produce a slight perturbation on the output it is almost imperceptible and well within the allowed transient envelop.
In practice, auto compensation is easy to use. Simply enable the power supply and the controller does all of the work. An additional benefit: the plant is characterised by the compensation algorithm. The values of Gain, Q, and Natural Frequency can be monitored over the life of the power supply and significant changes in the plant can be observed, many
times, before failure of the system. This allows the user to incorporate predictive diagnostics of the system health for improved reliability. Auto-compensation saves design time,
producing a more stable power supply and potentially improves the reliability and robustness of the power supply system. Digital power control offers many advantages over traditional analogue controllers in terms of optimal performance, reliability, high number of features, and ease of use. Switching from traditional analogue power control to digital power control is easy and rewarding.
Intersil |
www.intersil.com
Alexei Zernov and Yngve Wernqvist are both Lead Field Application Engineers at Intersil
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May 2013 CIE Power Supplement
15
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