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Diode Characteristics

Although they are a simple device, diodes do have a number of traits to consider. These traits are often application dependent. Some applications include: rectifiers, signal ORing, transistor turn off, freewheeling (inductors and motors) and of course specialized functions such as LEDs and voltage references.

The diode, like its triangular symbol, passes current in one direction while blocking current as well as voltage in the opposite direction. The theory is based on a semiconductor junction and the orientation of the doped regions. The most basic of these is the PN junction diode.

The Basic Diode Symbol, Composition, Package View and Graph (Courtesy of Electronic & Electrical Engineer's Guide)

The Basic Diode Symbol, Composition, Package View and Graph (Courtesy of Electronic & Electrical Engineer’s Guide)

The diode biases on, “automatically” passing current with a low forward voltage drop typical values of which are 0.7 V for PN junctions diodes and 0.3 V for Schottky diodes. Operation occurs in the first quadrant where both voltage and current are positive. The fourth quadrant is the blocking voltage and resulting leakage current. Most of the power loss is calculated based on these two operating points. However, rapid switching of the diode introduces power loss in the form of voltage and currents that are present during switching.

Turn ON waveforms (Courtesy of Vishay)

Turn ON waveforms (Courtesy of Vishay)

Turn OFF waveforms (Courtesy of Vishay)

Turn OFF waveforms (Courtesy of Vishay)

Reverse recovery also plays a role in power loss. This is an area where technology is advancing with new semiconductor materials such as SiC. Many different types of semiconductors are used for diode applications such as GaN and GaAs. The advantages span the needs, based on faster recovery times, higher blocking voltages, and enhanced current capacity.

Reverse Recovery Time Comparisons (Courtesy of Research Gate image)

Reverse Recovery Time Comparisons (Courtesy of Research Gate image)

Because they are semiconductors that operate on an energy gap principle, the introduction of heat in a diode changes its characteristics. Power loss generates heat. Therefore, the power loss would include Ptotal = Pconduction + Pblocking + Pturnoff + Pturnon. Fortunately, turn off losses are averaged over the turn off time. Otherwise they could become quite significant due to the amount of voltage and current that’s present.

One final thought about diodes is, they are not invincible. Diodes have an energy pulse rating for single even as well as repetitive energy absorption in periodic applications. Understanding these limits and the effect on the device’s performance can save headaches. For insurance, it might be better to parallel or series diodes just to spread the stress a little. Unlike siblings, they tend to share well.

References

  1. Vishay application note Rectifiers
  2. The Signal Diode, Electronic & Electrical Engineer's Guide September 25, 2014
  3. Research Gate image

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