By Lars Foerster, DC/DC Business Development Manager EMEA, TDK-Lambda
DC-DC converters are relatively easy to use and, with a few precautions, will provide years of reliable operation. However, there are some basic tips to consider before starting a project with them.
For a start, let’s examine the filtering components, (excluding the fuse, output adjustment and remote on/off functions), as shown in Figure 1.
Figure 1: Basic DC-DC converter connections
C1 and C2 can be one, two or a combination of electrolytic and ceramic capacitors. Their purpose is twofold: First, to offset the input wiring or printed wiring board trace inductance so the converter has low-impedance DC source-to-source peak currents from. Second, to reduce electronic noise 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.
Figure 2: Typical printed wiring board traces for a 1” x 1” converter and the CCG15 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 current between the layers.
To minimise input inductance, capacitors C1 and C2 must be located close to the converter and the input printed circuit broad (PCB) traces should be kept close together and made as short as possible. The output traces should also be laid out in a similar manner, to keep the output inductance low.
There should be 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 1.5-10W DC-DC converters have a five-sided metal shielded case, 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 the converter’s input or output. This can be left floating initially until system testing is verified. Larger industry standard packages like the popular 1” x 1” CCG series have six-sided shielded cases, with no need for grounding. The use of a ground plane on multiple layer PCBs can also reduce radiated noise from the converter.
How much input power?
DC-DC converters are not 100% efficient, so planning the power of the DC source is important. If ten 15W-output DC-DC converters are needed for a project, and the efficiency of each converter is about 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!
It’s important to always read the product’s installation manual, application note and other technical information before choosing the power supply or converter for the project. Here’s a good example of technical data for TDK-Lambda’s DC-DC converters and AC-DC power supplies: www.emea.lambda.tdk.com/uk/technical-data/