DC-DC converter printed circuit board and input power considerations
DC-DC converters are relatively easy to use and, with a few precautions, will provide years of reliable operation. Most manufacturers will have instruction or application notes on their website to assist you. This article provides some basic information that should be considered before starting a project.
To start, let us examine the filtering components (Figure 1). This excludes the fuse, the output adjustment and remote on/off functions which are covered in this article.
C1 and C2 can be one, two or a combination of electrolytic and ceramic capacitors. Their purpose is twofold. Firstly, to offset the input wiring or printed wiring board trace inductance so the converter has a low impedance DC source to source peak currents from. Secondly, to reduce electronic noise from interfering with the DC source and other board mounted components.
PCB layout and noise reduction
Figure 2 shows the printed wiring traces for a 1” x 1” converter and a photo of the top side of the TDK-Lambda CCG15 1” x 1” converter.
Note that the input and output traces are larger in comparison to the RC (remote control – on/off) pin and TRM (output adjustment) as they carry more current. A minimum of a two-sided printed wiring board is strongly recommended for reliability, along with plated through holes and vias for conducting the current between the layers.
To minimize input inductance, capacitors C1 and C2 must be located close to the converter and the input printed circuit broad traces should be kept close together and made as short as possible. The output traces should also be laid out in a similar manner in order to keep the output inductance low.
Also make sure that there is sufficient creepage and clearance spacing between the primary (input) and secondary (output) traces and external components to avoid compromising the safety isolation barrier.
TDK-Lambda’s CC-E 1.5 to 10W DC-DC converters have a five-sided metal shielded case. This is to reduce the effect of radiated noise where the space between plug-in cards is very small. Terminals are available to connect the case to either input or output of the converter. This can be left floating initially until system testing is verified. Larger industry standard packages like the popular 1” x 1” CCG series have 6 sided shielded cases and it is not necessary to ground the case. The use of a ground plane on multiple layer printed circuit boards can also reduce radiated noise from the converter.
How much input power do I need?
DC-DC converters are not 100% efficient, so planning the wattage of the DC source is important. If ten 15W output, DC-DC converters are needed for your project and the efficiency of each converter is say 89%, then the calculation would be as follows:
Required Power from the DC source = (Number of converters) x output power / Efficiency
The required power = 10 x 15 / 0.89 = 168.54W and not 150W!
Always read a product’s installation manual, application note and other technical information before making the choice of power supply or converter to use in your project. This link, for example, will direct you to technical data for TDK-Lambda’s DC-DC converters and AC-DC power supplies: www.emea.lambda.tdk.com/uk/technical-data/ Low cost manufacturers often do not provide this level of data and support, which can lead to product testing issues and field reliability problems.