[Design View / Design Solution]
Improve Your Card Power System's Reliability
In addition to choosing the proper dc-dc converters, pay careful attention to the power-management design, and make sure you thoroughly qualify your system.
Over the last few years, the voltages in typical equipment cards have dropped dramatically—in many cases down to 1 V or lower—while the total card power continues to soar. An increase in different rail voltages also has added complexity to the power system in the form of sequencing and tracking between rails. Meanwhile, expectations for reliability and availability are rising due to the ongoing drive to reduce equipment downtime.
There are several ways to meet the added power-system design requirements without compromising reliability. High-reliability power converters make up a key part of these solutions, but they need support from a well-chosen overall equipment architecture. Also, attention must be paid to details in the power-system integration.
ON-CARD POWER SYSTEM Current products no longer rely on a simple 5-V power-distribution system. Nowadays, it's not uncommon to have six or more voltages on a single card. Some high-end systems may have up to 20 or more separate power rails, with most below 2 V. These very low voltages must be delivered efficiently at high current, and they must meet increasingly tight regulation, ripple, and transient specifications. Consequently, distributed power systems are now the norm, with multiple dc-dc converters on each card to generate the low voltages very close to the load.
In addition to the need for very low-voltage rails, many ICs impose requirements for sequencing and tracking between power rails during startup and shutdown. The power rails must be controlled so that the difference between them doesn't exceed the specified voltage and/or time limits, even under short-term transient conditions. Combine these requirements with the need to monitor all rails for overvoltage (OV) and undervoltage (UV) protection, and it's plain to see that card power systems have moved out of the "simple-to-build" realm.
POWER-SYSTEM IMPLEMENTATION The figure illustrates an example of a card power system. In this case, a typical product is powered from 48 V dc, such as a communications system or a high-end compute server. The dc-dc converters supply the voltage rails needed for the card and maintain the required isolation between the 48-V input and the logic outputs. In this example, a single isolated dc-dc converter (usually called a brick) generates an intermediate bus voltage of 5 V that feeds a number of non-isolated point-of-load (POL) power converters.
A wide range of manufacturers offer bricks and POL converters as standard products in many output voltage and current combinations. They can conveniently function as building blocks in a card power system. A high degree of commonality exists among manufacturers, both for the electrical performance and the physical details (e.g., dimensions and pinouts). Though the figure shows a single brick, two or more bricks are often used to generate the rails that require the highest power, with POLs for the lower-power rails. Many combinations of power converters will meet the specific needs of any particular card.
To coordinate the operation of the dc-dc converters, the card power system requires an overall management function. Some degree of management also is necessary on both the primary and secondary sides of the isolation, as shown. While details vary, power-management functions typically include some or all of the following:
Startup and shutdown of the power system at a specified input voltage
Controlled startup and shutdown of all outputs in the required sequence
Monitoring of all outputs for OV and UV faults
Controlled shutdown if a fault occurs
Adjustment (trim) of output voltages if required
Margining of rail voltages during system testin
g
Reporting power status to the system controller
Thus, a high-reliability power system requires careful attention to the power-management design, which is equally important as the choice of dc-dc converters.
POWER RELIABILITY Power reliability can be seen in two very different ways:
Component level, using a bottom-up approach based on component failure rate. This aspect of reliability is typically expressed as predicted mean time between failure (MTBF), or failures in time (FITs). Since 1 FIT = 1 failure in 109 device hours, 1000 FITs = 1 million hours MTBF. The two most commonly used prediction methods are MIL-HDBK 217 and Telcordia TR-332. This type of prediction only considers component failures, and it doesn't take into account such aspects as design errors or inadequate specifications.
System level, using a top-down approach based on ability to perform the required functions. This can be addressed by worst-case design, simulation, and testing of the complete system. The testing must be sufficient to ensure that the design meets all required functions under all operating conditions—a process called qualification. As always, good design practices must be followed. Testing alone can't guarantee proper performance under all conditions.
It's important to consider both of these aspects in your design. A good predicted MTBF is necessary, but not sufficient. MTBF itself offers little value to the customer if the power system shuts down for every thunderstorm in the area.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
