Explore the Lesser-Known Benefits of Digital Power

Nov 1, 2006 12:00 PM
By Marty Pandola, Product Manager, Zilker Labs Inc., Austin, Texas

System Monitoring

In a typical power-distribution system today, the voltage rails are monitored for voltage faults only. The added components required to monitor the currents effectively, and thus the power, for each rail adds much cost to the system. Having the power management integrated with the power conversion allows for a more reliable system where all of the rails are monitored for voltage and current. Monitoring both the voltage and current of each rail is easily realized and inexpensive in a digital power system. Temperature monitoring provides additional benefits. Each digital power IC has an internal temperature diode and the ability to monitor external temperature diodes. With the known temperature data, current measurements at each power rail are easily compensated for temperature changes. The external temperature data can provide a thermal map of the entire system, which can be used to reduce hot spots in the system.

Having all this monitored data can be useful. The efficiency of each power rail can be calculated from the VIN, VOUT, IOUT and duty cycle. The efficiency can be tracked over time to predict pending failures in the system. Early warnings of a system failure are easier to deal with than catastrophic failures. All voltage rails can be monitored and fault detection can be acted upon accordingly to improve the operation and reliability of the system.

A digital power solution provides multiple methods of reacting to a fault. Overcurrent and undercurrent, overvoltage and undervoltage, and overtemperature faults are common faults to monitor in a power system. Fault and warning thresholds can be configured and adjusted throughout the life cycle of the product. When a fault does occur, the supply can shut down, wait for a period of time to see if the fault clears, and then either shut down or act as a warning and do nothing. Equally important, the fault is communicated to the other voltage supplies in the system. This allows for the other supplies to react to the fault as well.

Faults occur because a voltage, current or temperature went beyond certain limits. It is important for the safety of the system that the power supplies react according to the fault when it occurs. It is also beneficial to the design engineer to understand what the fault was that caused the system to shut down. This information is stored in the digital power IC to allow the engineer to debug and improve the reliability of the system.

Digital solutions allow for all the power rails to be monitored for voltage, current and temperature faults to provide a more reliable system.

Lower Cost of Ownership

Many functional elements in a digital power system contribute to the lower cost of ownership. The lower component count makes a more reliable and longer-lived system. The highly configurable digital power IC allows the designer to use the same device for each voltage rail and to make changes to the operation of the device without having to make hardware changes. The power engineer does not need to learn how to use multiple devices to support high power rails versus low power rails. Design tools allow the engineer to build and model the entire power system prior to building hardware. This reduces the development cost of the system by having a faster development time and fewer board spins.

The monitoring and fault management with reporting allow for a system design that will have a longer life, lower service costs and lower warranty costs. The fault reporting can help the engineer troubleshoot system issues in order to improve the design of the system and future systems.

Improved Efficiency

Boards are getting denser and are consuming more power. Inefficient power supplies cause boards to run extremely hot. Improving the power-supply efficiency can help reduce the generated heat in the system. Efficiency is measured as the output power divided by the input power. Power lost in the power conversion leads to a lower efficiency. Contributors to lost power in a synchronous buck converter are the on-resistance of the switching FETs, the energy required to turn on and turn off the FETs, the power required to bias the digital controller circuit and the resistance of the output inductor.

A digital solution can help improve the efficiency and thus reduce the heat. In the synchronous buck converter case, power is lost when both switching FETs are on (cross conduction) and when both FETs are off (body diode conduction). This is known as dead time since no power is being delivered to the load. Ideally, one FET should be on while the other FET is off and there would be zero dead time. Digital controllers can optimize the switching of the FETs and minimize the dead time to a point that the dead time nears zero. This optimization can improve the efficiency by 1% to 2%, depending on the application.

The figure below shows scope plots before and after optimization. The trace on the left is prior to optimization. Note there is a period where both the high-side and low-side switching FETs ore off. This is the dead time. The trace on the right shows the waveforms after optimization. Here both switching FETs switch at just about the same time, but with opposite polarity, eliminating just about all of the dead time.

More on Buck Converters

• Buck-Converter Design Demystified
• Optimizing Voltage Selection in Buck Converters
• Power Conversion Synthesis Part 1: Buck Converter Design
• Improving Efficiency in Synchronous Buck Converters

> Buck Converter Archive 

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.