AN4242 datasheet(5/26 Pages) STMICROELECTRONICS

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AN4242

Features of the SiC diodes

After simplification and introduction of the junction capacitance variation versus the reverse

voltage Cj(V), Qc is defined by the following formula:

Equation 3

This relation demonstrates that Qc is defined by the integral of the junction capacitance Cj

between 0 and VR, the voltage reapplied on the diode. This theoretical approach allows the

direct and accurate evaluation of Qc, avoiding the inaccuracy introduced by potential

measurement problems.

The strict expression of the energy stocked in the junction capacitor for a given reverse

voltage can be determined by:

Equation 4

Due to the non-linearity of the junction capacitance versus the reverse voltage, this relation

is different from the traditional energy formula ½ · C · V² (or ½ · Q · V), which is valid only

when considering a constant capacitance.

1.2

Forward characteristics

Another main feature of SiC diodes is the variation of the forward voltage drop (VF) with the

junction temperature.

Figure 3 shows the forward current versus forward voltage drop characteristics for 3

different junction temperature levels. A crossing-point can be observed at a certain level of

current IC. When the current is lower than this level, the temperature coefficient of the

forward voltage drop (

V

F) is negative. When the current is higher, it becomes positive. The

same crossing point exists for traditional silicon diodes, but it appears at a much higher

current level (>10 times the nominal current). This is linked to the higher forward current

density of SiC diodes.

Qc(V ) =

Cj(V) dV

Qc(V ) =

Cj(V) · V dV

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