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Applications most suited for isolated DC-DC converters


By Lars Foerster, DC/DC Business Development Manager EMEA, TDK-Lambda

Low-power DC-DC converters offer a simple way of deriving multiple system voltages from a higher-power AC-DC supply, a technique commonly known as a “distributed power architecture”, or DPA. Often, that larger wattage supply is providing 24V to drive motors, pumps or other electrical equipment, whereas controllers, logic circuits and other electronic devices require 3.3V, 5V, 12V or 15V for their operation.

In a 19” rack-mount system using the DPA approach, bulk AC-DC power supplies, which can be rated to allow for future system expansion, will usually be located at the bottom to keep them cooler. A 12V, 24V or 48V system bus is then provided to power multiple rack-mounted assemblies, which also minimises the exposure to potentially harmful high-voltage wiring.

From the DC bus, DC-DC converters provide the local voltages of 3.3V, 5V, 12, 15V, ±12V and ±15V. Dual output models can be connected to generate 24V or 30V if required, even from a 5V or 12V input.

Inside each assembly, DC-DC converters can be locally placed on the circuit cards, situated close to the devices needing power. This minimises any voltage drop between the converter and the device, and reduces the possibility of any noise pickup in the wiring, which could cause signal degradation or malfunction. If a part of the circuit is very sensitive to noise, it should be isolated from the system ground with an isolated DC-DC converter.

If an electronic device or circuit is very sensitive, or its performance can be optimised with a particular supply voltage, DC-DC converters often have a trim function. This allows the output voltage to be adjusted from, say, 5V to 4.75-6.0V using a resistor or external voltage.

If the system can be expanded in the field with additional plug-in cards, having DC-DC converter(s) located on that card (powered from the existing DC bus) simplifies the field upgrade. The original (base) system power supply will not need to be upgraded to include additional voltages, reducing the upgrade cost.

Portable equipment will use an internal battery with a voltage most suited to power the device that requires the most energy, but, again, may need other auxiliary voltages to function. Here, DC-DC converters are powered directly from the battery and can provide a stable output voltage whilst the battery is being charged or discharged.

TDK Lambda’s CC-E series of converters are available with 5V, 12V, 24V or 48V nominal inputs. The 24V input model, for instance, can remain in regulation with a maximum deviation of only 80mV over an 18-36V input range.

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