Surface Mount Power TVS Diodes Deliver
Optimal Protection for Power Supplies
APPLICATION NOTE
PTVS3-xxxC-TH
PTVS3-xxxC-SH
PTVS6-xxxC-TH
PTVS10-xxxC-TH
AC and DC power supplies used in telecom and other exposed applications often require
protection against hostile events such as power line surges and indirect lightning strikes. Avoiding
damage to the power supply by limiting the peak surge voltage to an acceptable level without shortcircuiting the line for an extended period of time is crucial to minimizing the downtime of critical
systems and avoiding costly maintenance calls. Power TVS diodes uniquely meet the demands of
these applications.
Other voltage limiting technologies, such as Metal Oxide Varistors (MOVs), exhibit a response
where the voltage across the device continues to rise as the current through it increases. The peak
voltage at its maximum rated current could be as much as three times its breakdown voltage at
low current. In contrast, after an initial short duration peak, the voltage across a Power TVS diode
folds back to a lower level and then remains relatively constant as the current through the device
continues to increase. This difference in response is shown in figure 1, which compares a Model
PTVS3-076C-TH Power TVS diode with a typical breakdown voltage (VBR) of 90 V to a Model
MOV-10D101K 10 mm MOV with a typical VBR of 100 V. Note the voltage foldback characteristic
of the diode compared to the rising voltage of the MOV. At the peak current of 2500 A, the
diode voltage is almost 200 V lower than that of the MOV. In addition to a lower clamp voltage,
the Power TVS diode is a semiconductor device which provides better reliability and increased
durability against repetitive surges when compared to a typical Metal Oxide Varistor.
Surge Current (A)
PTVS10-xxxC-SH
PTVS15-xxxC-SH
300
2500
250
2000
200
1500
150
1000
100
500
50
0
0
-50
-500
0
5
10
PTVS15-xxxC-TH
20
25
30
35
40
45
50
Time (µs)
Surge Current
Figure 1.
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15
PTVS3-076C-TH Voltage
MOV-10D101K Voltage
Comparison of a PTVS Diode and a Metal Oxide Varistor
Voltage (V)
PTVS Diode vs. MOV
3000
Surface Mount Power TVS Diodes Deliver Optimal Protection for Power Supplies
PTVS3-xxxC-TH
PTVS3-xxxC-SH
PTVS6-xxxC-TH
One factor that can significantly compromise the performance of lower voltage Power TVS
diodes at high currents is the lead inductance of the packaged device. The contribution of the lead
inductance to the voltage response of the device is given by the equation V = L di/dt. Knowing that
the voltage contribution is directly proportional to the rate of change of the surge current (di/dt),
we realize that the voltage drop across the inductance will increase for waveforms with faster rise
and decay times as well as at higher peak currents for the same waveform. It also tells us that this
voltage drop will be less significant at lower peak surge currents for the same waveform and for slower
waveforms since the rate of change of the surge current (di/dt) will be reduced.
Figure 2 shows the simulated effect of the lead inductance on a PTVS diode when it is subjected to a
15kA, 8/20 μs surge current waveform. The simulation shows the response of a diode with 0, 8 and
20 nH of lead inductance. Just 20 nH of lead inductance increases the peak voltage by over 60 V. The
lead inductance of a low voltage (58 or 76 V), 15 kA PTVS diode with a lead pitch of 24 mm is about
20 nH, which means that this is a very realistic scenario. We know we can’t reduce the lead inductance
to zero, but Figure 2 shows that we can significantly improve the performance of the diode if we
reduce the lead inductance to 8 nH. Fortunately, we can achieve this lower lead inductance by using a
surface mount package.
PTVS10-xxxC-TH
180
18000
20 nH
140
PTVS10-xxxC-SH
16000
Current
14000
Voltage (V)
120
PTVS15-xxxC-SH
12000
100
0 nH
80
8000
60
6000
8 nH
40
4000
20
2000
0
0
PTVS15-xxxC-TH
5
10
15
20
25
0
30
Time (µs)
Figure 2.
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10000
PTVS Current (A)
160
Simulated Effect of Lead Inductance on the Performance of a PTVS Diode
2
Surface Mount Power TVS Diodes Deliver Optimal Protection for Power Supplies
Bourns has developed three families of Power TVS diodes in surface mount (SM) packaging - Model
PTVS3-xxxC-SH, PTVS10-xxxC-SH and PTVS15-xxxC-SH. The PTVS15-xxxC-SH high temperature
models are available with standoff voltages of 58 V and 76 V. In addition to improved surge response
which is due to their low lead inductance, they also eliminate an extra assembly step in cases where
they would have been the only through-hole device(s) used on the PCB assembly. The graph in figure
3 compares the voltage waveforms of a surface mount (Model PTVS15-076C-SH) and an axial lead
(Model PTVS15-076C-TH) 15 kA Power TVS diode with identical voltage and peak current ratings.
The lower inductance of the SM package reduces the peak voltage by 35 V when tested using the
1.2/50 µs, 8/20 µs combination wave with a peak current of 15 kA. This is a significant improvement
in protection performance given that limiting the peak surge voltage to an acceptable level is critical
in most power line applications.
PTVS3-xxxC-SH
180
PTVS6-xxxC-TH
18000
161 V peak
160
16000
140
14000
126 V peak
Voltage (V)
120
PTVS10-xxxC-TH
12000
100
10000
80
8000
60
6000
40
20
0
PTVS10-xxxC-SH
4000
PTVS15-076C-SH Voltage
PTVS15-076C-TH Voltage
Surge Current
0
5
10
15
20
Surge Current (A)
PTVS3-xxxC-TH
2000
25
30
35
0
Time (µs)
Figure 3.
Comparison of 15 kA Surface Mount and Axial Lead Devices
SUMMARY
PTVS15-xxxC-SH
In conclusion, Power TVS diodes offer superior protection for AC and DC power line applications
with the best overall protection being achieved by a Power TVS diode in a surface mount package.
Power TVS diodes offer superior protection compared to other devices, such as MOVs. Bourns
offers a wide voltage range of devices with surge current ratings of 3, 6, 10 and 15 kA.
PTVS15-xxxC-TH
ADDITIONAL RESOURCES
For more information and further technical support, visit Bourns online at:
www.bourns.com
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