The growth of distributed power architecture (DPA) has increased the demand for dc-dc power conversion. As a result, a number of "off-the-shelf" packaged dc-dc converters have become available. Careful design with these converters can help engineers to save space, reduce component count, and simplify the layout of power conversion stages by eliminating the need for discrete solutions.
Not surprisingly, a growing number of engineers are choosing the packaged approach. Designers, though, should be aware of certain issues that arise from using these components. Because isolation is often necessary for either safety or performance reasons, it's important that engineers understand the impact of using isolated dc-dc converters in their designs.
The first question that any engineer must address is whether to use an isolated or a nonisolated dc-dc converter. For instance, when designing safety-critical applications such as products for the telecommunications or medical sector, isolated converters will almost always be required. Very often these devices will have to be certified to the rigorous UL1950 safety standard. In such cases, only UL1950-approved converters that have been independently assessed by a third party for both isolation strength and physical separation of the windings within the component, should be used.
Safety, however, isn't the sole reason for isolation. In most modern noise-sensitive circuitry, it will be necessary to isolate the load and noise presented to the local power-supply rails from the main supply rails of the entire system. To achieve this goal, dc-dc converters with a high galvanic isolation should be the automatic choice. The basic input-to-output isolation can then be used to provide a simple, isolated-output power source. Or, it could be used to generate different voltage rails, dual-polarity rails, and/or nonstandard voltages.
When using isolated converters, it's vital that the designer fully understands the difference between the converter's rated isolation voltage and its rated working voltage. Isolation is defined as the voltage that the device can withstand between input and output, for a fixed time period. An example is one second. This parameter is a measure of the electrical strength of the insulating materials used.
Rated working voltage, on the other hand, is defined as the maximum continuous voltage that can be sustained across the component's isolation barrier. Typically, it's lower than the rated isolation voltage. Determining a converter's working voltage from its specified isolation voltage is a controversial matter. Their relationship depends on the construction of the individual device and the materials used. That aside, the IEC-950 specification for the safety of information technology equipment provides guidelines for calculating working voltage from isolation voltage (Fig. 1).
Despite the many benefits offered by packaged dc-dc converters, many engineers decide to design and implement their own discrete power-conversion stages. This is often because they have a particular custom requirement that cannot, at first glance, be solved by using a standard part. Fortunately, many custom requirements can be addressed through the creative use of packaged, isolated converters.
Different Configurations At the most basic level, the input-to-output isolation of a converter can be used to generate different voltage rails and/or dual-polarity rails (Fig. 2). Furthermore, a negative supply can be generated by connecting the isolated positive output to the input ground rail. Alternatively, it's possible to produce a voltage above that of the main supply by referencing the output to a voltage other than ground (Fig. 3).
The output is isolated from the input, so the choice of reference for the output side can be relatively arbitrary. Where a floating output isn't required, the 0-V output can be connected to the input ground. Doing this converts a single-ended voltage to a dual rail, although the converter's inherent isolation is then lost.
It should be emphasized that, when mixing reference levels, regulated converters will need more careful consideration than their unregulated counterparts. The former usually only have a series regulator in the positive output rail. Therefore, referencing the isolated ground will work only if all the current returns via the converter rather than through external components, such as diodes and transistors.
The galvanic isolation of the output from a dc-dc converter also allows nonstandard voltage rails to be generated by connecting multiple converters in series. This is accomplished by simply attaching the positive output of one converter to the negative output of another. This configuration allows a wide variety of voltage variations to be produced provided that one rule is obeyed: the output current for the whole cannot exceed the output current rating of the dc-dc converter with the highest output voltage.
When connecting converters in series, additional filtering is strongly recommended because the converters' switching circuits won't be synchronized. An unfiltered output produces a ripple voltage that's the sum of the ripple voltages from the individual converters. It can also generate relatively large beat frequencies. (Filtering will be covered in detail later.)
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