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  Date: 15/12/2016

Basic DC-DC converter ICs selection guide

Introduction:

In many applications, it is required to convert a fixed-voltage DC source into a variable voltage DC source. Such a circuit which converts DC current from one voltage level to another is called a DC to DC Converter. To match the loads to the power supply, to isolate primary and secondary circuits and to simplify power supply systems, the DC to DC converters are very much needed.

The DC to DC converter can step up or step down DC voltage level such that the varying operating voltage levels of different components within the same device can be met. For an example, the switch mode converters used in cellphones and laptops are powered with rechargeable batteries. The individual components such as display, memories, USB drives, etc., within these devices require different operating voltage and current levels. The individual operating needs are met by using the DC to DC converters that converts unregulated DC input voltage into controlled DC output at desired voltage level.

Stable regulated output voltage can be achieved through different types of conversion methods such as electronic, linear, switched mode, magnetic, capacitive, etc. The DC to DC Converter can be termed as a DC equivalent to an AC transformer with a continuously variable turn ratio. These converters range from tiny packaged parts with no external components to large parts with multiple external components for proper operation, making it difficult to choose the best option.

Types (Topologies):

When designing an isolated dc-dc power converter, the first and most critical choice is selection of the topology. Historically, topology selection was based upon the desired output power level.

Based on the existence of an electrical barrier between the input and output, the DC to DC Converters are classified into:

DC to DC Converter without Isolation

These converters do not have isolation transformer between input and output stages.

Although low cost and simple, these converters have the disadvantage in the form of electrical connection presence between the input and output. Many safety agency bodies and/or customers require a separation from the applied input voltage and the output voltage which is often user accessible.

a. Buck converter (a step down converter: input voltage > output voltage)
b. Boost converter (a step up converter: input voltage < output voltage)
c. Buck-Boost converter (Operate in step up or step down mode based on duty cycle of switch)

DC to DC Converter with Isolation

An isolated DC-DC converter will have a high frequency transformer providing the barrier. This barrier can withstand anything from a few hundred volts to several thousand volts, as is required for medical application. A second advantage of an isolated converter is that the output can be configured to be either positive or negative.

These converters often are used to provide galvanic isolation, improve safety, and enhance noise immunity. Moreover, they can be used to generate multiple output voltage rails including dual-polarity rails.

In terms of output voltage regulation accuracy, isolated DC/DC converters usually fall into three categories: regulated, unregulated, and semi-regulated.

a. CUK converter (can also be used in non isolated mode)
b. Fly Back converter
c. Forward converter
d. Full Bridge converter
e. Half Bridge converter
f. Push-pull converter

Topology Comparison Chart

DC to DC Converter Topology


For the basic topologies, the order from lower power to higher power was usually Flyback, Forward, Push-Pull, Half-Bridge and Full-Bridge.

While this basic power order still remains true, for designers to push to new heights in power density, topologies that were once used in much higher power applications are now finding their way into relatively lower power, small form factor, high density power converters. Power management IC manufacturers are enabling this trend by adding not only more features but also by integrating high voltage gate drivers within the controllers.

Applications:

DC/DC converters are widely used in most portable devices as well as distributed power architecture systems, electric motor drives, industrial programmable logic controllers(PLC), DC power distributed systems, networking equipments, industrial fieldbus, etc.,

Key Specifications:

The performance of isolated DC-DC power converter is generally provided by Efficiency and density (Watts/Volume).

However, there are quite a few specifications as mentioned below which are to be considered while selecting the right DC-DC converter. The “Range” given in the below table denotes approximate values and the user is advised to verify the actual data from the suppliers.


DC to DC Converter Specification Range


Top Manufacturers:

Maxim Integrated Products
On Semiconductor
Microchip
Infineon
CUI Inc
Cosel
TRACO Power
TDK-Lambda
RS Components

Sample Parts from leading Suppliers

DC to DC Converter Selection Table1

DC to DC Converter Selection Table2

DC to DC Converter Selection Table3

Note: Please refer the orginal spec data from the manufacturer. We are not responsible for the data presented in the above tables.


 
          
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