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AN5001 Datasheet, PDF (1/5 Pages) Dynex Semiconductor – On-state Characteristic Model
AN5001 Application Note
Replaces February 2003 version, AN5001-6.0
AN5001
Use Of The VTO, rT On-state Characteristic Model
Application Note
AN5001-7.0 February 2004
The use of the VTO, rT on-state characteristic model and a
more accurate alternative.
The inclusion of the theoretical terms VTO and rT in power
semiconductor data sheets allows a simple means of calculating
power loss, but this can lead to many incorrect assumptions.
The terms in question are the two coefficients of a simple straight
line model of the device on-state characteristic curve. To
calculate the power the following formula is used:
P = VT0 IT(AV) + rT k2 IT(AV)2
[1]
where k is the current waveform form factor, eg 1.57 for half
sine wave.
The use of VTO and rT to approximate to the forward volt drop
curve of a power semiconductor originates from pre-computer
days when engineers used slide rules, calculators and, later
on, simple computers for their calculations. The use of modern
computers means that better approximations to the characteristic
can easily be used. The most popular of these is the model
proposed by General Electric:
VTM = A + B*lnI + C*I + D*sqrt(I)
[2]
where A, B, C and D are constants with values specific to the
device in question.
The use of this model is described below.
VT0, rT DEFINITIONS
Although the straight line model is basically simple, variations in
definition can lead to significant differences in calculated powers.
Different manufacturers of power semiconductors have defined
VT0 and rT in different ways. Here are 4 variations:
1) As fig. 1, where the line is the tangent to the VTM vs IT curve
at the average current.
2) As fig. 2, where a chord is drawn through IT(AV) and 3xIT(AV).
This variation is the one used by Dynex for the calculation of
data sheet power losses and current ratings. The definition
is commonly used for thyristors. For rectifier diodes a chord
through 3xIT(AV) and 5xIT(AV) sometimes gives a better result.
3) A variation of 2 which uses two straight lines instead of one
to approximate to the true curve. In this version the lines
IT
IT
IT(AV)
VTO
Fig.1
www.dynexsemi.com
rT
3x IT(AV)
IT(AV)
VT
rT
VTO
Fig.2
VTM
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