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High-Speed Load-Disconnects in Redundant Power System Architectures

Tue, 09/14/2010 - 7:44am
Chester Firek, Product Marketing Manager, Picor Corporation
Chester FirekDesigners of high-availability systems understand that the costs of system down-time can have a devastating impact on their end-customer’s business. For critical business and information systems such as data centers and networking and data storage systems, reliable redundant power is essential, and requires a careful analysis of each power stage and each potential mode of failure. In order to achieve up to 100% on-time, a design must be able to tolerate the failure of any single device, and in some cases that could include a failure of an output bus.

In basic terms, a redundant power system may consist of two or more power sources that terminate at common or redundant load points driven from the output buses. Higher power systems may consist of multiple conversion stages and/or large numbers of converters connected in parallel. Each conversion stage creates additional “output buses”, which could also become shorted. In order to protect against having a shorted output bus potentially damage the other devices in the array, an electronic circuit breaker device can be used. The two most common types of electronic or “smart” circuit breakers include hot-swap and load-disconnect switch.

Comparison of a Load-Disconnect to a Hot-Swap under a Fault Condition
Hot-swap solutions are designed primarily to limit potentially harmful transient currents of a board during insertion or removal from a live backplane, eliminating the need to shut down the entire system power during the installation. Hot-swap devices sense and limit a current surge for short intervals, but can also be configured to latch off during a fault. A hot-swap circuit’s response time to a short-circuit will depend on the period of time that it takes for the device to change from “normal mode” to “circuit breaker” mode, which is based on the timer circuit of the device. Under a high-current fault condition, this response time could be far too slow, and could result in a build-up of very high currents (ex: >100 A for a 48V system), which may ultimately disrupt the output load voltage, over-heat the MOSFET and cause damage to the system.

Unlike a hot-swap, a load-switch solution is designed specifically to turn off during a fault. A load-disconnect is similar to a hot-swap circuit in that it will sense a fault but rather than enter a current limit mode, it will quickly latch-off once the current limit set point is reached. Since the load-disconnect does not need to enter a current limiting state, it is able to respond to a short-circuit in a fraction of the time, limiting both the peak current and the amount of time that it takes to remove a failure from the system. The faster response to a fault condition also reduces the stress on the MOSFET and therefore further improves reliability. The key advantage of the load-disconnect over a hot-swap device is that its switch (MOSFET) is always in RDS(on) and its MOSFET is not required to dissipate as much power as does the hot-swap.

Use of Load-disconnects Can Provide Greater Design Flexibility
The faster and more direct response of a load-disconnect to a fault condition can allow a design to be simplified with respect to protection against a failure of the output bus. To illustrate, consider the two different approaches to the same redundant power system design; Figures 1A and 1B show two examples of a power systems with a shorted output bus. Both examples show a redundant power system consisting of two ORed 400 W power stages designed to support a 400 W load. In the first example, without a load disconnect, PS1 and PS2 each require two 200 W DC-DC converters to support a functioning bus if the other bus becomes shorted. In the second example, with a load-disconnect added, the example shows that the two 200 W supplies in PS1 and PS2 can now be replaced by single 400 W converters. This is possible because the response time of the load-disconnect is much faster than the response time of the DC-DC converter preventing the DC-DC converter from entering into current-limit mode. In this example, the addition of the load-disconnects allows the designer to reduce both the size and cost of the power system by reducing the number of converters needed. 

Figure 1A: Redundant power system using two converters per stage, with shorted bus 1.

Figure 1B: Redundant power system using two converters per stage, with shorted bus 1.

Load-Disconnect Combines Fast Response, High Power Density, and Low Power Loss
A load-disconnect takes advantage of two different technologies combining low RDS(ON) MOSFETs with high-density control circuitry to provide a fast circuit breaker solution. The low on-state resistance of the MOSFET minimizes the voltage drop at MOSFET’s max current rating significantly reducing the power dissipation and eliminating the need for heat sinking. An important aspect of the load switch is its ability to quickly and accurately sense the bus current. Figure 2 shows the typical application schematic for a load disconnect controller IC. Sense resistor Rs, is used to sense the load current and is user selectable, allowing the designer to set the trip point, based on the over-current threshold voltage divided by the minimum trip current. Figure 3 illustrates the response time of the device to an output bus short circuit. 

Figure 2: High-side load disconnect application

Figure 3: Load-disconnect response time to short fault condition

Figure 4: Full function load-disconnect solution, in a 7 x 8 mm thermally enhanced packageIntegrated Solutions for Improved Density and Ease of Design 
 For further ease of design and improved density, a full function load-disconnect solution is available combining a very low on-state MOSFET in a high-density thermally enhanced land-grid-array (LGA) package. The device provides a typical 8.5 m Ohm on-state resistance while enabling up to 12 A of continuous load current over a wide range of operating temperature. In addition, the sense resistor RS conducts only a fraction of the current in relation to the internal MOSFET which significantly reduces the total power dissipation and the size of the sense resistor. The LGA package is extremely small (7 mm x 8 mm) thermally enhanced package, providing up to 50% space savings over conventional load-disconnect solutions while enabling very low power loss, it also provides fast dynamic response, 120 ns typical. The fully integrated package eliminates layout and thermal related issues while minimizing the total parts count (see Figure 4). 

Conclusions
When designing redundant power systems a designer must consider the potential impact of a shorted output bus on the system. And when selecting a circuit breaker solution to protect against a shorted output, it is important for the designer to consider whether to select a device, like a hot-swap solution, that is intended to try to “ride-through” a fault condition or to select a device that will be able to quickly disconnect the fault from the system. As previously mentioned, a short-circuited power bus can reach dangerously high levels of current that can damage significant portions of the power bus, potentially resulting in the failure of the entire power bus.

A load-disconnect combines a low-loss MOSFET and a fast and accurate controller IC, providing a high-density, high-performance solution. When selecting a load-disconnect solution, the size, layout and thermal design must also be considered. Fully integrated solutions allow for greater ease of design, providing up to 50% space savings over conventional solutions, requiring a much smaller, lower-loss sense resistor, and can eliminate most layout related issues.
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