WESTCODE An IXYS Company

Medium Voltage Thyristor Types K0885N#440 to K0885N#520

11.2 Calculating VT using ABCD Coefficients

The on-state characteristic IT vs. VT, on page 6 is represented in two ways;

(i) the well established V0 and rs tangent used for rating purposes and

(ii) a set of constants A, B, C, D, forming the coefficients of the representative equation for VT in

terms of IT given below:

VT = A + B ⋅ ln(IT ) + C ⋅ IT + D ⋅ IT

The constants, derived by curve fitting software, are given below for both hot and cold characteristics. The

resulting values for VT agree with the true device characteristic over a current range, which is limited to

that plotted.

25°C Coefficients

A 1.817265

B -0.137011

C 4.436071e-4

D 0.0205661

125°C Coefficients

A 0.1339075

B 0.1991792

C 8.79778e-4

D -5.972062e-3

11.3 D.C. Thermal Impedance Calculation

∑rt

=

p=n

rp

p =1

⋅

1

−

−t

eτ p

Where p = 1 to n, n is the number of terms in the series and:

t = Duration of heating pulse in seconds.

r

t

=

Thermal resistance at time t.

rp = Amplitude of pth term.

τp = Time Constant of rth term.

The coefficients for this device are shown in the tables below:

Term

rp

τp

1

0.01249139

0.8840810

D.C. Double Side Cooled

23

6.316833×10-3

1.850855×10-3

0.1215195

0.03400152

4

1.922045×10-3

6.742908×10-3

5

6.135330×10-4

1.326292×10-3

Term

rp

τp

1

0.02919832

6.298105

2

4.863568×10-3

3.286174

D.C. Single Side Cooled

34

3.744798×10-3 6.818034×10-3

0.5359179

0.1186897

5

2.183558×10-3

0.02404574

6

1.848294×10-3

3.379476×10-3

Provisional Rating Report. Types K0885N#440 to K0885N#520 Issue 1

Page 5 of 12

March, 2009