WE NEED some advice about installing redundant, DIN-rail, 24 V, switch-mode power supplies in parallel. Over the years, we’ve mix-and-matched competing brands. Lately, we’re experiencing more failures than we should. What primary device performance characteristics should we focus on? What installation tips can you offer up? This doesn’t seem like it should be a problem.

—From November 2006 Control Design

Understand the Differences
Some power supplies are marked parallel-capable, which allows the unit to be connected in parallel, but it doesn’t automatically mean the unit can do load sharing. Load-sharing balances the total load between the two supplies connected in parallel, and actually can prolong the device life as it might not operate at full capacity. Check the manufacturer’s data sheet to ensure the power supply is intended for paralleling, otherwise irreparable damage could occur.

Here’s what occurs when two power supplies are connected in parallel but don’t have load-sharing capabilities. Because of several factors, it’s impossible to set both supplies to the exact same output voltage. The unit with the higher output voltage will become the lead supply and carry the entire load current. If the switching power supply used follows a typical fold-forward curve, the output voltage will start to drop once the unit has exceeded its rated current. Once the output voltage has dropped enough to match the second supply, the second unit will start to carry current, but typically the first supply is running in an overload condition. This can lead to premature failure due to excess heat.

	1+1 REDUNDANCY

When configuring a 1+1 redundant system (See Figure 1), it’s permissible to use non-load-sharing supplies because only one unit is needed to run the load, and therefore it’s never driven into over-current. N+1 redundancy circuits should use only load-sharing devices to prevent over driving one or more supplies.

On the other hand, using two supplies that have load-sharing capabilities delivers a different result because the supplies are designed with an additional internal feedback circuit. This method is called passive load sharing. The same situation will occur in the first example with one unit having a slightly higher output voltage. However, because of the feedback circuit, the supply can sense that too much current is being delivered too quickly and internally tapers back the output voltage. The lag supply initially carries no current because the output voltage is lower than the lead supply, so the feedback circuit mechanism increases the output voltage. This back-and-forth adjustment between the two power supplies occurs very rapidly but doesn’t affect the output voltage supplying the load. This rapid adjustment is what helps balance the current between the two supplies, but the current supplied by each unit is not 100% the same. The difference in output voltages of each supply also determines how much balance occurs, and therefore the voltages should be set as close as possible to each other.

N+1 REDUNDANCY

Because each manufacturer’s load-sharing circuit is designed differently, using the same power supply brand is recommended when connecting in parallel. The same design can react appropriately to the rapid adjustments to the internal voltage, and unforeseen problems can occur when two different brand supplies are used together, which can lead to premature failure.

When connecting in parallel, power supplies should be mounted side-by-side. Placing one unit above the other can cause excess heating of the top unit as heat rises from the lower. Consider using a distribution block for the load wires to prevent possible over-heating of the supply terminals and to keep the wire lengths the same, avoiding voltage drops that affect the load sharing.

Two power supplies connected in parallel offer a certain degree of redundancy, but a true redundant power supply has a de-coupling diode incorporated into the design. The diode is used to prevent a de-energized unit from becoming a burden to the remaining energized unit and also blocks a short circuit from bringing the entire DC buss down to zero volts.

Ed Merkle, engineering manager and technical support, Puls Power Supplies, St. Charles, Ill.

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