DC Converters convert power from one DC voltage source to another DC voltage, though sometimes the output is the same voltage. They are usually regulated devices, taking a possibly varying input voltage, and providing a stable, regulated output voltage, up to a design current (amperage) limit. The switch mode units rely on microprocessors for a high efficiency ratio, and less losses and heat. Converters are typically used to provide electrical noise isolation, or a voltage conversion, or provision of a stable voltage level for voltage sensitive equipment. DC converters are available for step-up and step-down applications, and isolated and non-isolated designs.

The switch mode units that carries offer advantages over linear designs. The switching efficiency can be higher than a linear unit, resulting in less energy loss in the transfer, which means less heat, smaller components, and lower thermal management issues. The linear types may be used in integrated designs (built in), and may be less expensive in this application, but switch mode has almost completely replaced linear power supplies in most situations.

Step-Down DC Converters

Step-down DC to DC converters are called 'buck' converters. A typical example would be a 24 volt to 12 volt converter, having an input DC voltage range from 20 to 30 volts DC, and an output of 13.8 volts DC (VDC) at, say, 12 amps (maximum). The input voltage may simply be some available system voltage in this range, or a 24 volt battery system with fluctuating voltage due to battery state of charge. The output is regulated by the microprocessor at 13.8 VDC in this case, which is a typical float voltage for a 12 volt DC battery system, and usually acceptable input to a '12 volt DC' device.

Some Example Voltage Ratios

9 - 18 VDC12.5 VDC
20 - 35 VDC12.5 VDC
30 - 60 VDC12.5 VDC
60 - 120 VDC12.5 VDC
9 - 18 VDC24 VDC
20 - 35 VDC24 VDC
30 - 60 VDC24 VDC
60 - 120 VDC24 VDC

Obvious uses for step-down DC converters are military, RV, or marine applications with a DC system voltage of 24 volts, and a regulated 12 volt DC source is required for radio communications, sonar, depth finder, computers, and of course audio or video equipment for entertainment.

Battery Imbalance and DC Converters

Why not use a 12 volt tap if the system (24 volts for instance) is comprised of a series connection of lower voltage batteries (two 12 volts for instance)? The batteries may (probably) become imbalanced for voltage/charge status. In a parallel configuration (postive connected to positive, negative to negative), the batteries will equalize over time and settle at a common voltage. In a series connection, equalization of voltage/charge status is not a natural condition. The system and any battery charger involved, sees the combined output voltage, and the charger will try to raise the voltage to its set point that indicates a full charge, by pushing current to accomplish this. The untapped battery which has the higher voltage to begin with, will attain its 'full charge voltage' faster, but current is still being passed as the charger seeks to raise the combined voltage from the two batteries to this same full charge level. There may be gassing, and overcharging occurring in extreme cases.

The DC converter draws equally from the parent voltage, and provides a regulated output voltage. The battery pack remains balanced, providing for a proper charge cycle, and maximum battery life.

Step-Up DC Converters

Step-up DC to DC converters are called 'boost' converters. A typical example would be a 12 volt to 24 volt converter, having an input DC voltage range from 11 to 15 volts DC, and an output of 24 volts DC (VDC) at, say, 5 amps (maximum). An application might be a piece of military equipment designed for a 24 volt system, being used in a 12 volt system.

Isolated versus Non-Isolated Converters

Non isolated converters share a common negative, and are usually very suitable for a typical electronic application (radio, stereo, sonar, etc.). Certain safety requirements or hazardous applications may require input to output isolation. The isolated converters are appropriately more expensive than non-isolated types.

Sizing the Converter

DC converters are rated in watt capacity, and some have a surge rating as well. Most devices used in DC applications indicate their watt or amp consumption. Devices with motors or compressors, or employing capacitor start circuits may need surge wattage consideration. Most electronics (radio, DVD, sonar, GPS, etc.) would not. For converting watts and amps, the following basic electrical formulas can be used:

P = E x I     Power = Volts times Current
Watts = Volts x Amps
Amps = Watts/Volts
Volts = Watts/Amps

So, given any two values above, you can calculate the third. As an example, you have a stereo rated at 60 watts, designed for a 12 volt system. Dividing the 60 watts by 12 volts gives a 5 amp current draw. If you are given the current draw only, and you need to calculate watts for DC converter sizing, you can multiply the amps by the system voltage, giving watts. For the 5 amp draw, 12 volt stereo above, you have 5 amps x 12 volts = 60 watts.

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