Cross reference search for rectifiers

When hardware developers have to make changes, whether for cost reasons or because of adjustments to the supply chain, and have to search for new rectifiers, they will find themselves confronted by problems that can't be solved by simply comparing datasheets.

Hardware developers naturally like to use older circuits with the rectifiers and manufacturers they trust – especially because, in this area, there has not been much technological progress. If changes have to be made, the developer usually searches on Google and compares datasheets. However, in many cases, it is necessary to test the component.

Through a better understanding of the production process, datasheets and technologies, testing can be carried out in a more targeted manner and mass production losses can be avoided. Test programs and statistics should also be taken into account.

The reason for this is straightforward: rectifiers are important components for manufacturers of power-management systems and are among the hottest components on a PCB. They also determine the efficiency and electromagnetic compatibility (EMC), and are subjected to many temperature cycles and high voltages.

Standard rectifiers include, for example, the S1 in an SMA package and the IN4007 in a DO41 package. They are usually employed as 50/60Hz AC/DC rectifiers or for polarity protection.

A rectifier datasheet contains only two absolute maximum values that can be compared: the surge current I_fsm and the breakdown voltage V_rrm. Exceeding these values can lead to serious failures.

► Because the breakdown voltage is comprehensively tested in production, hardware developer can take this parameter for granted. Problems in the cross-reference search, however, can occur when the distribution of V_rrm is not taken into consideration.

Most standard rectifiers have many different component names and cover a range from 100 to 1000V, but may have only one or two silicon wafer sources. In some cases, these sources can show tremendous variation in the distribution of the breakdown voltage. Because different manufacturers can set different testing guidelines, protection scope test conventions, guard bands and distributions on breakdown voltages, this can lead to surprises.

► When producing a prototype or when sample testing in the lab, only a few components from a very large number are observed. If the design deviates even slightly from the specifications on the datasheet, problems will only show up when the manufacturer changes their supplier.

If only one wafer source is available, then the electrical characteristics in the forward direction are the same for all voltages. This information can be useful in dealing with problems in the supply chain.

Also crucial to the search is the comparison of such parameters as the Ifsmsurge current, which is dictated by the construction of the component, the maximum junction temperature Tj, which can be interpreted in three different ways, and a number of others.

With conservative design and a good derating, it should be easy to compare standard rectifiers and, if necessary, to change suppliers. If the design is marginal, however, comprehensive testing needs to be carried out.

An example of conservative design of the circuit would be limiting the PCB temperature to 90°C/95°C, a derating of at least 20% of the breakdown voltage, and a peak surge current below the 10/8.3ms value in the datasheet.

On the subject of second sourcing rectifiers, specialist Taiwan Semiconductor has produced a comprehensive white paper that highlights the general fundamentals that are important for the comparison and selection of alternative rectifiers. The white paper also looks at the specific approach needed for bridge rectifiers, fast recovery and fast efficient rectifiers, Schottky diodes, TVS diodes, small-signal products and zener diodes.

We offer a wide selection of different rectifiers from manufacturer Taiwan Semiconductor direct from stock, among others Fast Rectifier Diodes and Super/Ultra Fast Rectifier Diodes.

Our sales team will advise you if you have any questions.