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Exploring the ideal diode

Posted: 26 Aug 2014 ?? ?Print Version ?Bookmark and Share

Keywords:DC-DC converters? EMI? rectifier diode? Schottky? PIN?

The era of analogue power systems is long gone, giving way to modern power systems that invariably apply switching topologies in their operation. Today's systems, such as power factor correction, motor drives, DC-AC inverters, bridge converters and DC-DC converters are all called upon to operate at high levels of efficiency, generate low EMI, be physically small, light in weight and low cost to manufacture. These requirements all point to high-frequency switching, the higher frequency the better, provided that EMI regulations can be met. Successfully meeting requirements often relies on the characteristics of what may be regarded as a relatively simple component, the rectifier diode.

Whenever an inductor is switched in a power circuit, a diode is generally present to carry the freewheel current through the inductor during each cycle. When a power electronic system is scanned for temperature rise while operating, the components found to be dissipating most heat will often be the rectifier diodes, so attention given to careful selection of diodes at the design stage will be well rewarded. A range of technologies and architectures are now applied in the manufacture of diodes, attempting to produce the ideal diode for each application. A number of diode technologies are identified in the table.

Table: Comparison of diode technologies.

The first four technologies employ bulk silicon and the fifth uses a compound semiconductor material to achieve breakdown voltages higher than achievable with silicon. The simplest is the silicon standard diode which consists of P-type and N-type silicon forming a single junction at the interface. If reverse bias is applied while the diode is conducting a high forward current, a finite amount of time (tRR) is required for minority carriers to be removed from the junction and a depletion region established. Because the tRR of standard diodes is in the range 1-2 ms they are limited to only low-frequency applications.

A Schottky diode is produced by replacing the P-type material with a metal contact. Current flow is only carried by electrons so there are no minority carriers. When reverse bias is applied, electrons are attracted to the opposite pole, creating a depletion region to block current. Because there are no minority carriers and the depletion region is very thin, the Schottky diode reverse recovery is much faster than with a standard silicon diode. The penalty is that the breakdown voltage for silicon Schottky is limited to around 200V and leakage can be significant. Higher breakdown voltages can be achieved by using compounds such as silicon carbide instead of silicon, but at the high material cost, limits the use of silicon carbide only to the most demanding applications.

The P intrinsic N (PIN), often known as ultra-fast, diode uses a region of lightly doped N-type silicon between the normally doped P and N regions. The lightly doped N region is often doped with platinum to create recombination centres that reduce minority carrier lifetime. When the diode is reverse biased, holes and electrons are attracted to the recombination centres in the drift region where they recombine. This creates a much faster reverse recovery than with a standard diode.

The larger junction thickness results in a much higher forward voltage to carry the same amount of current when compared with a Schottky diode.

As can be seen from the table, the Schottky diode has lower VF and faster tRR than ultra-fast diodes but is limited to 200V VBR. The ultra-fast diode can be used above 200V but efficiency is compromised.

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