Unregulated and Yet Continuous Short Circuit Protected - Is That Possible?

The following article illuminates the topic of converter reliability from new point of view and offers the developer a solution that will not only increase customer satisfaction but also relieve the sleepless nights of the poor DC/DC manufacturer.

Specifically, we would like to speak to all developers working on networks, interfaces and bus-systems that need to design universal solutions operating in Worst-Case conditions, where all of the variables of mounting, installation and interconnectivity are unknowns during the development phase. The selection of the components for galvanic separation is always a difficult weighing up process between the design specifications and operational safety on the one hand and costs on the other hand, but a responsible solution is to use a prefabricated DC/DC converter module for the isolation. Very often the outputs of the converter appear externally in the application via terminals, connectors and inter-connection cables and consequently inadvertent short circuits can sometimes occur.

For most DC/DC converters this means immediate failure, because typical low power 1W or 2W unregulated interface converters are is not protected against continuous short circuits across their outputs. The datasheet often specifies that a maximum short circuit duration of 1second is allowable, but de facto the converter is not designed to operate into a short circuit and this figure of 1 second is attributed purely to the robustness of the internal components when overloaded. Why is this actually? Although a continuous short circuit protection is very desirable, such well-defined overload behaviour of DC/DC converters is normally only offered by more expensive regulated converter designs, and not in the simple unregulated types.

However, the clear price differential between unregulated and regulated converters leads many developers to select the cheaper unregulated part, reasoning that the extra costs involved with a regulated and short-circuit protected converter is not worth it for the ‘rare’ occurrence of a short circuit, which should only occur during fault conditions.We at RECOM have the opinion that this forced choice between price and protection leads to increased field failures and therefore we have supplemented our entire spectrum of unregulated low power converters, additionally to the standard unprotected implementation , with so named P-Options; where P stands for protected. The P-versions are still simple, low-cost unregulated converters which are noticeably cheaper than their equivalent regulated converters, but with the additional feature of the ability to resist a continuous short circuit.

Through clever balancing of selected materials for the transformer core and a careful specification of the transistors used in the primary side oscillator, we have succeeded in a construction that can guarantee the survival of the converter with a continuous output short circuit, without needing to resort to the costly method of using feedback loops to cope with overload shorts. A converter can thus be built that is fully safeguarded against the condition that causes certain failure for the wide mass of interface converters available on the market from other manufacturers.The excessive short circuit currents that normally destroy the switching transistors in standard designs are simply avoided in the P-option converters.

As simple as the solution of the problem is, so simple can the P-version also be used, as all converter specifications applying to the standard variation are the same for the P-option. We have already convinced many of our customers who manufacture interface products to upgrade to this improved solution as no PCB changes or specification changes are required in their existing designs. A few designers were so enthusiastic, that they immediately instructed their buyers only to purchase P-Option converters in the future. Within a very short time, they could convince their colleagues from quality control or their colleagues from the customer complaint department of the considerable advantages of the P-Option for only a very modest price increase over the cost of industry standard parts. Convince yourself today and test the P version converters from RECOM!

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