|Reverse current (IR) flows through the diode junction when reverse voltage is applied.
Factors influencing the amplitude of the reverse current include:
a) Thickness of the silicon
b) Amplitude of the reverse voltage
c) Junction temperature
d) Cross-sectional area of the junction
e) Exposure of the junction to light
f) Dopants used
g) Amount of radiation impinging on the junction
A common approximation is to assume that IR doubles every 10°C, however, actual view of the reverse current is the sum of three distinct current flows:
1) Current due to diffusion
2) Current due to surface leakage
3) Current due to charge generation
High voltage junctions differ from their low-voltage counterparts in that, at room temperature, the dominant leakage path is usually along the surface while current due to junction capacitance is much lower in amplitude. Careful attention to the leakage source is important. It is quite common, for example, to carefully match the reverse current measurements of diodes that are then connected in a string at 25°C, then subject them to elevated temperatures. It is probable that the reverse current flowing through the diodes at even slightly elevated temperatures would change significantly and produce a severe mismatch. This, in turn, would result in an over-voltage condition on some diodes.
To appreciate how this can happen, consider a situation in which two diode exhibit exactly the same reverse current at 25°C:
Thus a pair of diodes that were perfectly matched at 25°C would be terribly mismatched at higher temperatures.
Vf varies with: resistivity, thickness of silicon, level of dopant concentration, type of dopant, temperature, and current density.
Forward drop in a high-voltage rectifier is usually not as important an issue as it is in low- voltage applications. Consider, for example, a 1000-watt, 5-volt power supply. If its rectifiers exhibit a 0.7-volt forward drop, the power loss with 200 amps flowing would be 140 watts, or 14 percent of the power being handled. In contrast, the rectifiers forward drop in a typical 1000-watt, 1000-volt supply ranges between 1 and 1.5 volts.
At full output, the current is 1 amp, which with a 1.5-volt forward drop, translates to a 1.5 watt power loss. This represents less than two-tenths of one percent of the power being passed through the rectifier.
REVERSE RECOVERY TIME
Trr varies with resistivity, dopant concentration, type of dopant, junction width, forward current, temperature, and change in reverse current with time (dI/dT).
In applications, these factors translate into high voltage junctions having slower recovery times than similarly doped low voltage junctions. Because of this, high-voltage junctions typically contain considerably higher concentrations of dopants- like platinum or gold- than do similar- speed low-voltage junctions. These higher dopant concentrations result in higher forward voltage drops and reverse leakage currents.
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|Last revised: 30 May 2014|