Using a standard off-the-shelf DC-DC converter reduces the time, risk and cost associated with developing a discrete version. There are usually, however, some additional external components required to enable a standard converter to operate and perform correctly. The manufacturer’s installation manual should provide simple, yet thorough, instructions on what parts to use and where to position them for the best performance. Before starting on the printed circuit board layout, it is suggested that a schematic is drawn, and a prototype built and tested first.
Some of these external components, and their functions, are described as follows:
Input capacitor, input fuse and reverse polarity protection
To negate the impedance of input wiring from the DC source to the converter, a low ESR electrolytic capacitor should be fitted as close to the converter as possible. Ensure that the voltage rating of the capacitor is sufficient. If the ambient temperature is expected to drop below minus 10°C, then the capacitance value may have to be increased to compensate for a drop in the capacitor’s ESR.
The manufacturer’s installation manual or specification will advise if an external fuse is required and give the rating and type of fuse required. Failing to adhere to these recommendations may cause a hazard if the converter were to fail short circuit.
If there is a possibility that the input polarity to the converter may be reversed, then a diode should be added across the input as shown in Figure 1. The diode should be suitably rated for the maximum input voltage and have a higher surge current rating than the fuse.
An electrolytic and/or ceramic capacitor may be fitted across the positive and negative output terminals. This will improve the converter’s response to sudden load changes and reduce output ripple and noise. It can also be used to compensate for the impedance in wiring or PCB traces between the converter and the load. Adhere to any maximum values of capacitance stated by the manufacturer to avoid start-up problems.
Adjusting the output voltage
If fitted, the ‘trim’ connection can be used to adjust the output voltage, see Figure 2.
To increase the output voltage, a resistor is connected from the trim connection to –Vout. To decrease the output, the resistor is connected to +Vout. Fit the trim resistor close to the converter to avoid noise pick-up. The manufacturer should provide the relevant formulae for calculating the resistance. When increasing the output voltage, ensure that the maximum output power is not exceeded.
The DC-DC converter may have a function to remotely turn the output on or off. This can be achieved using a relay, switch or transistor. (Figure 3).
Note there are two types of remote on/off polarity. ‘Negative logic’ requires the RC terminal to be pulled low (or shorted) to –Vin to activate the output voltage, or pulled high (or disconnected) to inhibit the output. ‘Positive logic’ requires the RC terminal to be open to enable the output, or shorted to –Vin to inhibit it.
Some low-cost manufacturers may not provide detailed installation documentation. If you have questions, try contacting their sales or technical support office. Installation manuals and other technical information for TDK-Lambda’s DC-DC converters, power supplies and other products can be found at this link: www.emea.lambda.tdk.com/uk/technical-data/.
For more information about the CC-E series of DC-DC converters, please visit: www.emea.lambda.tdk.com/cc-e