Back to Basics: Paralleling Power Components for Current Sharing

July 4, 2013
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There can be many reasons for paralleling power components such as DC-DC converters for power delivery. One common situation is when the load current exceeds a single module’s rating. Another arises when a degree of power redundancy or fault tolerance is required. Paralleling of modules can provide an effective solution in either case, provided they share the load equally. However, most power components will not do this inherently.

In power converter arrays, one or more of the devices will try to assume a disproportionate or excessive fraction of the load. Typically, the converter with the highest output voltage may deliver current up to its current limit setting, and beyond its rated maximum. Its voltage will then drop back, and other converters will contribute current, but the load will not be shared equally. An overloaded module can become temperature-stressed and prone to failure. A module failure will create a significant step load change for other modules, possibly causing system problems such as resets or a shutdown.

Managed current sharing, then, is important because it improves system performance and increases reliability. It is essential to most systems that use multiple power supplies or converters for higher output power or for fault tolerance. Accordingly, a number of current sharing schemes are available: Output current sensing, droop-share, master-slave, analogue current-share and synchronous current-share.

Output current sense

This forces load sharing by sensing each converter’s output current using a resistor, and comparing it to the average current. The output current for each converter can then be trimmed until its contribution equals the average. Selecting the right resistor value is a tradeoff between power dissipation and sensitivity or noise immunity.

Droop-share

Droop-share forces equal current by artificially increasing converters’ output impedance, based on injecting an error signal into each converter’s control loop. It works with any topology, and is fairly simple and inexpensive to implement. However, it does require a current-sensing device for each power unit. It also incurs a small but often unimportant penalty in load regulation.

Master-slave arrays

These usually contain one intelligent master, and one or more slave modules or boosters. Quasi-resonant converters within these arrays can share a load equally if set up properly, because the per-pulse energy of each converter is the same. The master determines the transient response, and dynamic load sharing is achieved without trimming, adjustments or external components. Sharing is usually within 5%, and the arrays are stable with excellent transient response. However the design is not fault-tolerant, as maintenance of correct output voltage is lost if the master module fails.

Analogue current sharing

Analogue current-share control involves paralleling two or more identical intelligent modules, with the output voltage of each module being actively adjusted to achieve balanced current sharing. The approach supports a level of redundancy but is susceptible to single-point failures. Also, each converter requires its own voltage regulation loop, current sensing device and current control loop.

Synchronous current sharing

Designs using variable frequency zero-current-switching converters can use synchronous current sharing. This allows a truly ‘democratic’ array in which any converter module can take control. All modules are interconnected via a bidirectional PR pin as shown below. The module in command transmits a pulse on PR which synchronises each converter’s high-frequency switching. The PR pulse becomes a current-sharing signal for power expansion and fault-tolerant applications. If the lead module relinquishes control, another module can change from Receive to Transmit mode, transparently assuming control of the system. 

 Synchronous power architecture simplifies current sharing and enhances
fault tolerance

This pulse-based design allows improved fault tolerance: transformers or capacitors can be used for DC-blocked coupling, containing certain failure modes  within their module of origin.  Transformer coupling also provides a high level of common-mode noise immunity while maintaining SELV isolation from the primary source.

Synchronous current sharing in democratic arrays allows power architects to achieve simple, non-dissipative current-share control. It eliminates the need for current-sensing or measuring devices on each module and provides uncompromised load regulation with excellent stability and transient response.

This method applies to quasi-resonant, variable frequency converters with the necessary intelligence, such as the Vicor Maxi, Mini or Micro high-density DC-DC converters with fixed-energy pulses.
Related posts:

Back to Basics: What are Y-Capacitors?

Back to Basics: What does Power Factor Mean and Why Must We Correct it?

Back to Basics: Understanding and Mitigating the Growing Problem of Distribution Losses

Back to Basics: Handling High Input Transients

Back to Basics: Meeting EMI for AC-DC Systems

 

Related Links:

Current Sharing Methods for DC-DC Converters in Parallel

Design Guide: Using Boosters & Parallel Arrays

 

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