TISP7xxxF3 (MV, HV) Overvoltage Protector Series
APPLICATIONS INFORMATION
Lightning Surge (continued)
ITU-T 10/700 Generator (continued)
VC
2.8 kV
R2
15 Ω
R3
25 Ω
70 A
5/310
SW
R
T
T
R
R
T
G
R1
50 Ω
C1
20 µF
C2
200 nF
70 A
5/310
G
G
T AND G
TEST
R AND G
TEST
R AND T
TEST
10/700 GENERATOR - SINGLE TERMINAL PAIR TEST
95 A
4/250
R4
25 Ω
VC
5.2 kV
95 A
4/250
R2
15 Ω
R3
25 Ω
SW
T
R
C1
20 µF
R1
50 Ω
C2
200 nF
190 A
4/250
G
DUAL
10/700 GENERATOR - DUAL TERMINAL PAIR TEST
T AND G,
R AND G
TEST
Figure 33.
With the generator output open circuit, when SW closes, C discharges through R . The decay time constant will be C R , or 20 x 50 =
1
1
1 1
1000 µs. For the 50 % voltage decay time, the time constant needs to be multiplied by 0.697, giving 0.697 x 1000 = 697 µs which is rounded to
700 µs.
The output rise time is controlled by the time constant of R and C , which is 15 x 200 = 3000 ns or 3 µs. Virtual voltage rise times are given
2
2
by straight line extrapolation through the 30 % and 90 % points of the voltage waveform to zero and 100 %. Mathematically, this is equivalent to
3.24 times the time constant, which gives 3.24 x 3 = 9.73 which is rounded to 10 µs. Thus, the open circuit voltage rises in 10 µs and decays in
700 µs, giving the 10/700 generator its name.
When the overvoltage protector switches, it effectively shorts the generator output via the series 25 Ω resistor. Two short circuit conditions
need to be considered: single output using R only (top circuit of Figure 33) and dual output using R and R (bottom circuit of Figure 33).
3
3
4
For the single test, the series combination of R and R (15 + 25 = 40 Ω) is in shunt with R . This lowers the discharge resistance from 50 Ω to
2
3
1
22.2 Ω, giving a discharge time constant of 444 µs and a 50% current decay time of 309.7 µs, which is rounded to 310 µs.
For the rise time, R and R are in parallel, reducing the effective source resistance from 15 Ω to 9.38 Ω, giving a time constant of 1.88 µs.
2
3
Virtual current rise times are given by straight line extrapolation through the 10 % and 90 % points of the current waveform to zero and 100 %.
Mathematically, this is equivalent to 2.75 times the time constant, which gives 2.75 x 1.88 = 5.15, which is rounded to 5 µs. Thus, the short
circuit current rises in 5 µs and decays in 310 µs, giving the 5/310 wave shape.
The series resistance from C to the output is 40 Ω, giving an output conductance of 25 A/kV. For each 1 kV of capacitor charge voltage, 25 A
1
of output current will result.
For the dual test, the series combination of R plus R and R in parallel (15 + 12.5 = 27.5 Ω) is in shunt with R . This lowers the discharge
2
3
4
1
resistance from 50 Ω to 17.7 Ω, giving a discharge time constant of 355 µs and a 50% current decay time of 247 µs, which is rounded to
250 µs.
For the rise time, R , R and R are in parallel, reducing the effective source resistance from 15 Ω to 6.82 Ω, giving a time constant of 1.36 µs,
2
3
4
which gives a current rise time of 2.75 x 1.36 = 3.75, which is rounded to 4 µs. Thus, the short circuit current rises in 4 µs and decays in 250
µs, giving the 4/250 wave shape.
MARCH 1994 - REVISED MARCH 2006
Specifications are subject to change without notice.
Customers should verify actual device performance in their specific applications.
BOURNS [ BOURNS ELECTRONIC SOLUTIONS ]
