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DC-DC converters are used in applications where voltage levels need to shift or where an isolation barrier is necessary. Voltage conversion can be achieved locally or at the point of load (POL).   All converters fall into two types, isolated and non-isolated. Let's look at the differences.   

Isolated Converters
   
As the name implies, isolated converters have an isolation barrier. The isolation barrier is typically provided by a transformer which separates the input and output terminals.
It can withstand hundreds to thousands of volts. Most isolation voltages are in 1500 to 4000V range, depending on the application.

Isolation falls into three categories:
  • Operational:  An isolated output that offers fault protection
  • Basic: A transformer isolation with single fault protection
  • Reinforced: Two isolation barriers that may offer physical separation
There are several reasons to use an isolated DC-DC converter.

  • Safety: Isolation barriers are required to protect humans from dangerous voltages. Whether it be operator safety or as applied in the medical world, the critical safety of the patient.
  • Galvanic Isolation: Impeding an electrical charge through isolation reduces galvanic corrosion.
  • Noise/transients: Isolation results in better noise filtering and the prevention of unwanted ground loops.
  • Floating Ground Requirements: Floating grounds are useful for providing negative or positive grounds. Examples: negative grounds for datacom, positive grounds for telecom.  (See Figure 1)
  • Floating Outputs: Positive  or negative voltages can be obtained  by changing the placement of the output reference ground, regardless of the input polarity. (See Figure 1)
Figure 1.
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Non-Isolated Converters
  
Non-Isolated Converters are used in applications not requiring an isolation barrier. There are several reasons to use a non-isolated converter.

  • Cost: Non-isolated converters are less expensive than their isolated counterparts.
  • Size: Without a transformer and associated circuitry, compact sizes are available including DIP(dual in- line),SIP(single in- line pin) and SMT(surface mount module)
  • POL: Smaller package sizes enable Point of Load placement which reduces I²R board losses, while increasing transient response and load regulation.
  • Output Voltages: Additional board level voltages obtainable using existing converters as their source of input.
  • Efficiency: Efficiencies are quite good given the lack of the copper and switching losses of a transformer.


Figure 2.

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