To conclude this issues 12-page power section, Electronics Sourcing asked Dengrove Marketing to walk readers through a typical DC/DC converter datasheet, explaining key issues along the way Input range
Input ranges for DC/DC converters depend on the input source, likelihood of the source to vary and by what amount. There are different input types including single inputs, 2:1 inputs and 4:1 inputs. Single inputs provide an input range that generally varies between five and 10 per cent of the nominal input. They are suited to a source with a steady output such as a power supply or front end converter. 2:1 and 4:1 inputs are called wide and ultra wide as they offer more flexibility in terms of input range. For example, a 2:1 input will have an input range of nine to 18VDC for a nominal 12V input. The term 2:1 means the highest input voltage can be divided by the lowest input voltage twice. The same applies to 4:1 where a nominal 24V input will have and input range of nine to 36V DC. These input types suit variable sources such as batteries.
DC/DC converters are available with single and multiple outputs. Normally, lower wattage converters provide single and dual outputs, while higher wattage converters provide up to four outputs. Standard voltages are provided for easy application in digital and analogue circuits. Dual isolated converters provide two outputs isolated from each other. Thus, one converter can power two circuits while maintaining isolation.
Regulation level is a combination of line and load regulation performance. Selection of regulated or unregulated converters depends on the application and requirements of the devices being powered. An unregulated converter is generally used where the input and output requirements are stable or when the output voltage can vary with load. A regulated converter is generally used where there is more variation in input and output conditions.
Ripple and noise
Switching of DC/DC converters means they produce ripple and noise. The ripple observed on the input and output is twice the switching frequency as a result of full wave rectification. Switching noise is seen on top of the ripple and arises from the interaction between the switching transistors and the inductance of the transformer core. If the transistors are driven hard, they will switch more efficiently, but generate more noise. If they are driven soft, they generate less noise but more heat, so the design of the converter switching circuit is a compromise between efficiency and noise.
Input to output isolation
Input to output isolation is used to prevent current flowing between two circuits. The isolation level depends on the application. In general, an isolation voltage of 1kVDC is selected. However, in certain applications, such as medical equipment, a minimum isolation voltage of 5.2kVDC is required. It is also important to know how long the barrier must withstand the isolation voltage: the difference between the isolation and rated working voltage. Isolation voltage provides a specification for the short-term ability to withstand the specified voltage, normally a 1s flash test. Rated working voltage is the voltage a converter will withstand indefinitely.
To meet European EMC legislation, OEMs must ensure they meet the requirements of appropriate European directives. DC/DC converters are generally exempt as they are specified as a component part of a system which itself will need to comply. Converters are available with metal cases to reduce radiated emissions. Some are provided with internal filters to meet conducted emissions levels. It is reasonable to assess this when selecting a converter but designers must appreciate the potential need for external filtration to help meet EMC requirements.
This can have a major effect on a converters ability to operate. Generally there are three different levels of operating temperature range: commercial (0degC to 70degC); industrial (-25degC to 85degC) and military (-40degC to 125degC). Cooling methods, heat sinks and maximum case temperature must also be understood. Operating temperature range can be extended at the high temperature end by de-rating or running the converter under full power.