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Paralleling power supplies: Benefits and drawbacks

Posted: 23 Mar 2016 ?? ?Print Version ?Bookmark and Share

Keywords:paralleling? power supplies? resistors? amplifier?

Another approach adds small ballast resistors in series with each supply's output, to equalise the distribution of the load current among the supplies in the array even when their control loops are seeking dissimilar output voltages, as shown in figure 1. The ballast resistors create some loss of load regulation, depending on the spread of setpoint errors that the ballasting intends to overcome. However, these ballast resistors also dissipate heat, which degrades system efficiency.

This "OR" that?
A seemingly "simple" solution to this direct-connect dilemma is to just use a diode between each supply and the common tie point of all supplies, a technique commonly referred to as diode-ORing (figure 2). ORing diodes are very effective at preventing a supply from sinking current away from the shared output, but are generally insufficient to address sharing errors among supplies with independent error amplifiers, because the conduction knee is abrupt enough that parametric differences in the supplies' setpoints will still lead to significant sharing imbalances.

Figure 1: One sharing approach is to use relatively low-value ballast resistors on each supply's output, but this has issues due to resistor-related dissipation and overall efficiency.

Figure 2: In principle, multiple DC-supply outputs can be combined by using diodes to isolate one supply from the other, but this configuration has many performance issues related to balance and current sharing. (reference 2)

Diode ORing is generally required for supplies acting independently, whose outputs can both sink and source current (two quadrant-operation). The effect of directly paralleling such supplies without ORing diodes is far worse than it is for single-quadrant supplies. While single-quadrant supplies will only suffer from load-sharing errors, two-quadrant supplies will actively contend for control of the common output voltage. This will cause current in excess of the load current to circulate among the supplies in the array, and likely lead to immediate overload of one or more of the supplies.

Also, if the diodes have a negative temperature coefficient for their conduction threshold, they will actually promote current hogging in the array. One way to minimise the problem is to use a method of rectification with a positive tempco C Schottky diodes, or via a diode-like function but built using FETs and a rectifier in an active-ORing implementation C but diodes can reduce efficiency due to forward voltage drop, and active-ORing can add cost and complexity.

Under some circumstances, diode ORing can still offer reliability improvements at the system level. The chief case of interest is where one of the supplies suffers a shorted output FET or capacitor which could jeopardise the common output voltage rail. ORing diodes will quickly decouple that short from the output bus, and improve reliability and system robustness.

Who's in charge here?
Supplies generally must be designed specifically for parallel operation in order to operate reliably and predictably in an array. Start-up synchronisation, fault-protection coordination, and control-loop stability must all be considered.

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