Download Parallel Stacking of TVSs For Higher Surge Currents

MicroNote™ 113
Parallel Stacking of TVSs For
Higher Surge Currents
By Mel Clark and Kent Walters
In addition to series stacking of transient voltage suppressors (TVSs) to attain higher surge current ratings
described in MicroNote™ 112, they can also be placed in parallel for low voltage applications when series
stacking is not feasible. One major advantage of parallel stacking over series stacking is the reduction of
inductance in the surge current path, minimizing L(di/dt) effects.
Control of the ambient temperature during matching is important since TVSs have a positive temperature
coefficient of voltage, ranging from 0.04% / OC to 0.10% / OC. This parameter is found on the data sheets
and is the same for both TVSs and zener diodes of the same breakdown voltage. Poor ambient temperature
control during matching will result in greater voltage mismatch and can reduce the IPP rating of the paralleled
TVSs.
The most severe requirements for parallel stacking have been associated with meeting the very high current
surges required for lightning protection across 28 V aircraft bus lines per RTCA/DO-160 Waveform 5A and
for military vehicles per MIL-STD-1275. Since the present interest in lightning protection of aircraft is quite
significant, examples that can be extrapolated to other applications will be used as a guide to arriving at
solutions for parallel stacking low voltage TVS devices. For equally rated TVSs, Peak Pulse Current (IPP) ratings are additive when perfectly matched. It is important
that “A” suffix devices be used for this application. The non suffix, +/- 10% tolerance devices, cover too large
of a voltage spread to conveniently match. The practical limit of devices in parallel is 10-12 devices. The
objective is to have each device equally share the current conducted during the unexpected surge. To be assured that surge current ratings are additive when matched in parallel, they must be matched in
groups to a tight tolerance of +/- 0.5% or less for optimum performance. Typical pulse test matching currents
range from 3A to 10A to assure that the devices are well into the avalanche mode and will match closely at
maximum IPP.
Although two TVSs can be matched under surge conditions to provide 2X the power of one device, keeping
matched pairs identified requires serialization and separate packaging for each pair, that adds extra labor
and additional cost to this process. This is one of the downsides to parallel device requirements. For some
applications this is a necessity. Rev. A
7/2008
Microsemi - Scottsdale Division
8700 East Thomas Road, Scottsdale, AZ 85252
Ph: (480) 941-6300
© 2008 Microsemi Corporation
Fax: (480) 947-1503
Micronote 113
However, for some applications, very close matching may not be necessary. For those requirements having
only a 40% - 50% increase above the rated surge current level of a single off-the-shelf A suffix device with
a specified +/-5% VBR tolerance, these components are matched sufficiently for these requirements. The
additive surge current rating of two off-the-shelf devices with worst case mismatch of +/- 5% tolerance
device might suffice for many applications. Other options may require closer matching of +/- 3 % or +/-1 %.
The graph shown in figure 1 is used to illustrate the effectiveness of two RT100KP48As in parallel at the
maximum tolerance extremes in voltage (+/-5%). This unmatched pair has a minimum of 78% of the surge
capability of the total of both devices in parallel as illustrated in subsequent paragraphs or where the highest
voltage device provides 56% additional surge capability, as shown in Figure 1, beyond the contribution of the
lowest voltage device. The graph denotes the minimum and maximum breakdown voltage (V(BR)) points at the test current (IBR) of 5
mA on the lower horizontal coordinate at 53.3 V and 58.9 V respectively with the values taken from the data
sheet. The Clamping Factor (CF), the amount of voltage rise resulting from the combination of temperature
and impedance of the chip during surge, is 1.33 times the V(BR), producing a Clamping Voltage (VC) level
with a peak surge current rating of 1285A for this device. VC values are so marked on the graph at the IPP
level (vertical coordinate) of 1285 A (6.4/70 µs), the top line on the graph, for devices having -5% and +5%
tolerance along with the median value.
The two heavy outside lines in the graph are the +/-5% voltage limits of an RT100KP48A TVS device on
each side of the median voltage, also indicated with a heavy line. The top line of the graph is the IPP for the
RT100KP48A, 1285 Amps, for a 6.4/69 µs waveform (DO-160E waveform 4.) Note the VC values have a
slightly greater spread than the VBR values. This information is illustrated in figure 1 shown below.
1300
1285A IPP
77.7 Vc Max
69 Vc Min
nV
alu
e
1000
56%
%
Lim
it
dia
Lim
725A IP
for +5% Device
Me
it
800
-5%
+5
600
400
53
58
.3V
.9V
V
V
BR
200
BR
Mi
n
Ma
x
Peak Pulse Current - A 6.4/69 µs
1200
5mA
50
55
60
65
70
75
80
Breakdown (VBR) and Clamping (VC) Voltages It and Vc in Volts
Figure 1. Peak Pulse Current vs Breakdown and Clamping Voltages for RT100KP48A
On the graph, a perpendicular line is dropped from the max VC of the -5% device to the +5% high end of the
VC curve presenting the worst case mis-match resulting in only 56% performance of the highest voltage in
the TVS pair at extreme limits of the voltage spectrum. This minimum performance factor is valid for two TVS
devices of any voltage or power level. The net result is the maximum IPP for two randomly selected parallel
TVSs of the same type having an A suffix has 156% of the rating of one device or 78% of the rating of the
sum of the IPP rating of both devices. The 78% value is derived as illustrated below:
Micronote 113
% IP1 = [(IP2 + IPP) / 2X IPP ] x100% [Eq. 1.]
= [(725A + 1285A) / 2570A] x100%
= 78% of the IPP of both devices in parallel Where IP1 = the net lowest IP rating of both devices in parallel
IP2 = the 56% effective rating of the highest V(BR) device IPP = The IPP rating of a single device.
Two randomly selected RT100KP48As in parallel at extreme ends of the high and low voltage tolerances
would have a minimum surge current rating of 1285A (6.4/69 µs waveform) x 1.56 = 2004A. This 1.56 factor
Microsemi - Scottsdale Division
8700 East Thomas Road, Scottsdale, AZ 85252
Ph: (480) 941-6300
© 2008 Microsemi Corporation
Fax: (480) 947-1503
is valid for any set of two devices from a TVS with an A suffix type and a specified VBR min and maximum
equating to a +/-5% tolerance, regardless of its voltage or power rating. However, most devices from
individual processing lots typically range over a distribution of +/- 3.5% to +/- 4% with high probability of
yielding a greater Ipp rating for two or more randomly selected parts in parallel. Some TVS families do not
specify a maximum VBR and hence may have a broader range between min and max values up to +/- 8%.
When matched by the user, precautions must be taken such as measuring the device VBR to assure +/- 5%
match. Greatly expanding this graph for two devices and dividing down into finer increments of +/-1%, +/-2%, +/-3%
and +/-4%, results in the following performance:
Table I: Effect of Performance vs % of VBR Matching
Effective % Ipp of % Effectiveness of Additive
Percent Match Two Parallel Devices Ipp Rating of 2 devices
+/-1% 195.0 97.5 +/-2% 185.2 92.6
+/-3% 175.5 87.8
+/-4% 165.8 82.8
+/-5% 156.0 78.0
We observe from the above data, that the relationship between the percent of matching is virtually linear with
percent of Ipp of the matched devices. This is valuable information when the IPP demand is significantly more
than one device but less than two very closely matched parts. For higher surge current requirements, 4 or more parallel parts from the same manufacturing lot will normally
yield an average distribution of +/-4% or less with individual voltages distributed over the spectrum limits.
Under these conditions the effective Ipp could approach 85% to 88% for each added TVS device for a larger
group of standard A suffix devices in parallel. For specific needs of matched parts for higher IPP performance, Microsemi can supply TVSs in matched sets
of up to 10 parts down to a level of +/- 0.25%. High power modules can also be supplied with components
encapsulated in a single package with terminals, mounting through-holes or threaded inserts as required to
suit specific needs.
IMPORTANT NOTICE
The equations derived and information discussed above applies only to +/- 5% A suffix TVS devices having
a clamping factor of 1.33 with the high power devices largely used for parallel device applications. The vast
majority of our devices fit this description; however there are caveats that emerge unexpectedly and these
will be addressed at this time. For additional technical information, please contact one of our technical personnel listed below:
Kent Walters 480-941-6524
Joe Fulton 480-941-6405
Mel Clark 480-941-6433
[email protected]
[email protected]
[email protected]
Microsemi - Scottsdale Division
8700 East Thomas Road, Scottsdale, AZ 85252
Ph: (480) 941-6300
© 2008 Microsemi Corporation
Fax: (480) 947-1503
Micronote 113
A small percentage of our TVS types have clamping factors approximating 1.20 which renders the
information herein of little value when stacking in parallel. These include 500 W and 1.5 kW glass
encapsulated TVS devices which are least likely to be used for parallel stacking. Should questions arise on
stacking these types, please contact the factory through one of the technologists listed below.