Load-Shared Power Supplies Boost System Reliability

Sep 1, 2002 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

System integrators can improve system reliability with redundant, paralleled power supplies that share the load. Load sharing distributes load currents equally among paralleled voltage-stabilized supplies and can add “hot-swap” capability. A load-shared power system also allows the addition of power supplies, if system requirements change and load currents increase.

Hot-swap combined with load sharing allows replacement of the failed power supply without shutting down the entire system. This requires hot-swap power supply connectors that disconnect and connect during extraction of one supply and insertion of the new supply.

For shared supplies to operate efficiently, no supply can hog the load current while other supplies are idle. Also, the power system must tolerate the failure of any one supply — as long as there is sufficient current capacity from the remaining supplies. This requires the power supplies to act as one large power supply with equal stress on each unit.

Individual load-shared supplies require an external controller, otherwise the supply with the highest output voltage will contribute most of the output current. Output impedance of typical power supplies is in the milliohm range, so small differences in output voltages can cause large differences in output currents. This might cause the supply providing the majority of load current to enter the current-limit mode, increasing its thermal stress. A load-shared system should have a common, low bandwidth share bus interconnecting all supplies. It should also have good load-sharing transient response and the ability to margin the system output voltage with a single control.

Use of an external load-share controller adds a voltage feedback loop, which allows measurement of each supply's output current and compares it with the load-share bus. The voltage feedback loop adjusts the output voltage of each supply so each one delivers an equal output current. To accomplish this, each supply must have true remote sense capability. Each paralleled supply requires its own load-share controller and minimal compensation components.

In operation, the load-share controller senses the power supply output current in either the high or low side of the load-shared output. When using high-side current sensing, the load-shared power supply output voltage must be within the controller's absolute maximum voltage ratings.

Depending on the amount of noise in an application, you can use a single-ended or differential load-share bus for the shared modules. While true differential mode offers the most noise immunity, the single-ended variety yields excellent results when designed with a high amplitude load-share signal. However, single-ended configurations reduce pin count for dedicated, ground-referenced applications.

Texas Instruments' UCC39002 is a single-ended load-share controller employing automatic master-slave architecture. The figure shows an implementation of the UCC39002 using high-side current sensing for sharing two power supplies. It is compliant with the Intel SSI (server system infrastructure) specification that describes a single-line load-share bus and a scalable load-share voltage independent of the current sense resistor. For power supplies, this specification addresses physical dimensions, voltage range, and electromechanical interface parameters.

With features such as high accuracy (better than 1% current share error at full load) and full-scale adjustability, the UCC39002 can help to improve system reliability.

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