Back to Basics: Using Hot Swap to Minimize Downtime

June 25, 2014
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Given the critical role of modern telecommunications and data communications equipment, maximizing its online availability must always be the first priority. Clearly, any equipment’s availability depends critically on that of its power supply. Fortunately, close to zero power system downtime can be effectively achieved by combining two techniques – redundancy and hot-swapping. In this back-to-basics post we will see how these work together, and how systems can be designed for hot swapping.

Consider for example a rack-mounted telecommunications system comprising modules plugged into the rack’s backplane, with a total power requirement of 120 W DC. This could be met in many ways.  If, for example, two 60 W power converter units, also mounted on the backplane were to be used a failure of either unit will cause significant downtime while the faulty unit awaits replacement. A better solution is to share the load between three 60 W converters, so that uninterrupted operation continues even if one unit fails. This is known as N+M redundancy, where N units satisfy the load and M provide redundancy.

However, to benefit fully from the uptime ‘bought’ by the redundant converter, the system must inform an operator about the failed unit, then allow them to replace it without shutting down or otherwise interrupting the power supply. Adding hot swap circuitry to the converter modules facilitates this by protecting the converters and the rest of the system from the problems associated with live insertion and withdrawal.

Removing the faulty unit from the rack is relatively easy, provided the remaining power units can support the step increase in load that will occur if the faulty unit continues to deliver current. Plugging in the replacement unit has more potential for problems, as it will present an uncharged capacitor load and draw a large inrush current. This could cause a momentary, but unacceptable, interruption or sag to the backplane power bus if not limited. Problems can also arise if ordinary power module connectors are used, since the connector pins will engage and disengage in a random and unpredictable sequence during insertion and withdrawal.

One relatively simple – but very manual, and therefore prone to human error – method is known as ‘warm swapping’. This involves plugging a power module into the backplane and then applying the AC or 48 VDC power input, so the power module rather than the supplied load absorbs the step change and inrush current. Problems with random pin insertion sequences are also avoided.

A more automated, true hot swap approach uses an active MOSFET switching device in the power line, and a connector with staggered pins. During insertion, the MOSFET is driven into a resistive state to limit the inrush current; then when the inserted module’s capacitor has charged, the MOSFET becomes fully conductive to avoid voltage drop losses. The staggered pin connector can be configured with the Ground pin longer than all others, to ensure the module remains safely grounded and protected from random pin connect/disconnect sequences during insertion and withdrawal.

Fig. 1 below shows how such a scheme can be implemented using our Picor PI2211 Hot Swap Controller and Circuit Breaker. It demonstrates how the device can be used with an external N-channel MOSFET. The circuit employs the MOSFET’s transient thermal characteristics, to ensure that it operates within its dynamic safe operating area (SOA). Emulation and protection based on the specific MOSFET’s transient thermal performance optimizes the safe operating limits and allows designers to take advantage of the latest power MOSFET technologies. The PI2211 has an operating range of +0.9 to +14 V, and is designed to work with a specific set of Vicor converters (appropriate parts can be selected using the PowerBench Solution Selector tool).

hot-swap-figure-1-pi2211-application

Fig.1: Typical PI2211 application with N-channel MOSFET

An alternative approach, using Vicor’s Input Attenuator Module (IAM48) or now more usually Filtered IAM48 (FIAM48), is available for Advanced TCA (ATCA) and Micro ATCA telecommunications, data communications and other 48 VDC systems. Like the PI2211 example, it uses a staggered pin connector, but configured with a short On/Off pin. The FIAM also provides inrush current limiting and transient overvoltage protection. This part is generally recommended for telecom applications to ensure compliance with EMC (Electromagnetic Compatibility) standards.

Vicor provides other integrated hot swap solutions that comply with Advanced TCA PICMG 3.0 requirements for hot insertion and board level conducted noise limitations. One of these is the QPI-8 QuietPower hot swap controller with active EMI filter, which is designed for applications such as ATCA blades, telecom base stations, IBA and distributed power, network switches and routers, optical line-in cards and TD-SCMA wireless infrastructure installations.  And the QuietPower QPI-10 Advanced TCA hot swap SiP with EMI filter, which is designed for use with telecom and ATCA PICMG 3.0 boards using VI Chip technology.

Overall, Vicor offers different solutions for different hot swap applications, but the objective always remains the same; to give systems designers the opportunity to build hot swap capability into redundant power module arrays. This allows telecoms and other mission-critical applications to continue operation without interruption even through the failure and replacement of one or possibly more of their power modules.

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