One factor complicating ensuring blade protection is the trend toward concentrating ever-higher levels of computing power on each blade. Once, blades typically ran at maximum power levels of 200–400W; today, however, blade power levels of 600–800W, sometimes even 1000W, are increasingly common. The trend toward increasing power has developed at the same time as a trend has emerged toward decreasing blade operating voltages in order to reduce DC-DC conversion costs and power efficiency losses within the data center power distribution scheme. In some popular designs, a typical low side voltage is 12VDC, which can drive input current levels to greater than 80A per blade. For the blade’s circuit designers, the challenge is to ensure over-current fault protection in a cost-effective, reliable, safe and feasible way to prevent costly hardware damage and service outages.
Because blades are intended to be hot-swappable (i.e., to allow maintenance personnel to remove them from the chassis and plug in new ones while the main power is still on), they are designed with a hot-swap controller IC. This IC provides the primary fault protection mechanism by first sensing over-current and other fault conditions, then signaling a MOSFET to shut down power to the board. The objective is to prevent adjacent blades within a chassis from going down due to undervoltage lock-out (a short circuit can cause the backplane power rail voltage to go down due to droop) and also prevent damage to the faulted blade and backplane. Critical components within this system are the backplane traces, backplane connectors, and power connector on the board. Backplane damage can be particularly problematic because it could lead to the loss of an entire chassis within the data center.
Secondary protection with physical fuses is necessary to back up the primary hot-swap controller IC. These physical fuses provide independent over-current fault protection as a backup to the main IC in case of damage to the MOSFET or IC during fault conditions. To maximize the number of blade servers that will fit into a standard 19-inch chassis, blades must be as thin as possible, so the use of low profile surface mount fuses is essential. However, no manufacturer currently produces a 60A- or 80A- capable surface mount electronic fuse. To support the blade’s full input current, a blade’s circuit protection designer must parallel two or more fuses. However, doing so safely can be complicated for very high current applications.
The highest current SMT fuse now on the market is rated for 40A. To take proper fuse de-rating calculations and the de-rating necessary when paralleling fuses into account, a designer would need about 150A of total rated current from the fuses or four 40A fuses in parallel. In addition, circuit designers must be concerned with how the fuses will behave and carry current between them because slight differences in fuse resistance will generate different carrying currents on each fuse. They also need to understand how the system of fuses they’ve created will react during a short-circuit event. To maintain high system reliability, it’s critical that the physical fuses do not trip before the hot-swap controller IC has the time needed to remove power from the board.
Circuit protection designers can parallel multiple fuses safely to increase a blade’s current carrying capability if they understand these points:
• When fuses are used in parallel, they must have the same current rating and the same voltage rating. The maximum interrupting voltage for the parallel combination is equal to that of one individual fuse only. The maximum interrupting current is at least equal to the individual maximum interrupting current.
• Fuses in parallel will share the load current in inverse proportion to the number of fuses used. However, due to the fact the fuse resistances are not exactly identical, there will be variations in the current each fuse actually carries. Due to the variability inherent in the production of all components, Littelfuse recommends applying at least a 20% derating factor to the individual fuse rating to compensate for this effect. (This derating factor is to be applied in addition to all other recommended de-rating factors.)
• Close thermal tracking is required to keep the fuses at the same temperature, with respect to both ambient temperature and normal operating temperature. Ensure all fuses are exposed to the same airflow and have similar heat conduction mechanisms acting on the wire leads or fuse clips.
• Always remember that the melting integral (I²t) will increase by the square of the number of fuses in the parallel combination. For example, if two fuses with an I²t = 3.5A²s are used in parallel, the effective I²t will be 3.5A²s x 2² = 14A²s. If three are used, I²t = 3.5A²s x 3² = 31.5A²s.
To learn more about choosing and using fuses safely for high current circuit protection applications, download a free copy of Fuseology: Fuse Characteristics, Terms and Consideration Factors from http://www.littelfuse.com/technical-resources/~/media/electronics_technical/application_notes/fuses/littelfuse_fuse_characteristics_terms_and_consideration_factors_application_note.pdf.
Figure 1. A blade server chassis is designed to hold multiple blade servers and provides services such as power cooling, networking, various interconnects, and management.
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