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Texas Instruments TVS Surge Protection in High-Temperature Environments Application notes
Application Report
SLVA930 – March 2018
TVS Surge Protection in High-Temperature Environments
Benjamin Balazsi, Alec Forbes
ABSTRACT
To ensure reliability during events like power switching, hot plugs, lightning, and various other fault
situations, systems must account for surge protection; however, most common surge-protection TVS
diodes have significant variation over temperatures that is rarely accounted for. This variation can lead to
failures in released products that were not seen in lab testing; to minimize these failures, TVS diode
performance must be understood and accounted for. This report examines the temperature variation in
conventional SMA and SMB type TVS diodes and compares them to the TI Flat-Clamp surge protection,
and then shows laboratory testing in a real-world system that highlights the improved high-temperature
performance of the TI Flat-Clamp devices.
1
2
3
4
Contents
Introduction ...................................................................................................................
Surge Protection Considerations ..........................................................................................
System Surge Protection Example ........................................................................................
Summary ......................................................................................................................
1
2
5
8
List of Figures
1
Stand-off, Break Down and Clamp Voltage .............................................................................. 2
2
Process and Temperature Variation on SMAJ33A ...................................................................... 3
3
Littelfuse® SMAJ33A Series Temperature Derating..................................................................... 4
4
Functional Block Diagram of the Flat-Clamp Technology .............................................................. 4
5
8/20 µs Surge Clamping Waveform of Flat-Clamp Diode vs. Conventional TVS Diode ........................... 5
6
Modified TPS7A4901DGN EVM Schematic.............................................................................. 6
7
TPS7A4901DGN with SMAJ33A on the Input Voltage Line ........................................................... 6
8
TPS7A4901DGN with TVS3300 on the Input Voltage Line ............................................................ 6
9
Test Setup Block Diagram .................................................................................................. 7
10
SMAJ33A Protection at 8.38 A at 85ºC ................................................................................... 7
11
TVS3300 From TI Protection at 9 A at 85ºC ............................................................................. 7
12
TVS3300 Clamping Waveform at 24 A at 85ºC
13
TVS3300 Clamping Waveform at Last Passing Current at 85ºC ...................................................... 8
.........................................................................
8
List of Tables
1
..................................................................................
1
SMAJ33A Temperature Derating Table
2
TI Unidirectional Flat-Clamp Devices ..................................................................................... 8
4
Introduction
It is essential to have robust protection for systems in environments where lightning strikes, switching of
power systems, load changes, or hot pluggable interfaces are present. The most common approach for
surge protection is to use transient voltage suppressor (TVS) diodes to clamp fault voltages to a safe level
at the input to the system.
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Surge Protection Considerations
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However, it is not easy or straightforward to choose the best diode for a system, especially in hightemperature environments. Usually TVS diodes have high clamping voltages, wide temperature variation,
and significant performance degradation over their operating temperature range. To make matters more
difficult, TVS diode data sheets rarely discuss or specify their performance at any temperature but 25°C,
leaving the system designer to either guess or run time consuming tests to verify their system is protected
over their operating temperature range.
TI has designed a new clamp technology to protect against transient surge events that helps to ensure
robust protection against a wide temperature range. TI`s Flat-Clamp technology provides a solution to
dissipate surge transients while simultaneously providing a precise, flat-clamping voltage across –40°C to
125°C which minimizes residual voltage to the protected system.
The surge-protection capabilities of this new technology are tested and evaluated in this application note
and compared to a traditional TVS device. During the tests, a surge pulse is introduced to a system which
is protected by each type of protection device and the results are compared to show that the TI FlatClamp device provides better protection for interfaces in harsh environments.
2
Surge Protection Considerations
2.1
TVS Diodes Specifications
In noisy environments, engineers design their systems to pass the IEC 61000-4-5 standard for systemlevel surge immunity which defines the test setup and procedures for surge immunity testing. Compared to
ESD events, which are covered by the IEC 61000-4-2 standard, surge events (which usually occur during
power system switching transients or lightning discharge scenarios) have 200 times longer pulse durations
and much higher energy.
For details about the IEC electromagnetic compatibility (EMC) standards and related testing, see IEC
61000-4-x Tests for TI’s Protection Devices .
Protection against surge transients is typically done by a TVS diode on the input of the interface.
Designers must work around the limitations and constraints imposed by their TVS protection stage to
ensure reliable operation and protection. The TVS breakdown voltage, VBR, defines the applied voltage
where the TVS begins to conduct current. During a fault event, the TVS diode will shunt significant
amounts of current to ground up to the maximum TVS pulse current of IPP, causing the voltage across the
device to increase above VBR due to the intrinsic dynamic resistance (RDYN) of the diodes. This additional
voltage must be considered when protecting your system. The voltage which is seen by downstream
devices during a surge event is VCLAMP and is calculated using Equation 1.
VCLAMP = VBR + IPP * RDYN
(1)
For the most reliable operation, the maximum VCLAMP of a TVS diode should be below the absolute
maximum rating of the downstream device. Any condition violating the absolute maximum ratings results
in a risk of damaging the component.
These defined voltage levels are illustrated in Figure 1:
Figure 1. Stand-off, Break Down and Clamp Voltage
2
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Surge Protection Considerations
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The RDYN of conventional TVS diodes is generally high, producing a VCLAMP significantly above VBR. To
ensure a robust system, designers must use components tolerant to the high VCLAMP voltages or find TVS
diodes with much lower RDYN. Both solutions have problems; the first results in bigger and more expensive
system components and the latter results in TVS diodes with higher leakage, capacitance, footprints, and
cost.
2.1.1
Process and Temperature Variation
In addition, over process and temperature, VBR and VCLAMP vary significantly. Process variation is usually
specified in device data sheets, while temperature variation can usually be calculated based on
Equation 2. However, when αT, the thermal coefficient, is not determined in the data sheet, temperature
variation cannot be calculated.
VBR@Tj = VBR@25°C x (1 + αT x (Tj - 25))
(2)
8/20 µs Current (A)
Figure 2 illustrates the shift in the I/V curve over process and junction temperature (Tj) for the SMAJ33A
TVS diode. The green region below VRWM is the region used for nominal system input voltages. The red
region illustrates margin that is required by the system for robust protection during a surge event. In this
region, the system cannot nominally operate; however, it must be tolerant to these voltages. The dashed
vertical purple line is the maximum voltage the protected system could detect during a surge event.
Nominal IV Curve
í40:125°C Variation
Voltage (V)
Figure 2. Process and Temperature Variation on SMAJ33A
The SMAJ33A nominal I-V curve is represented by the green curve with a VBR of 39 V and a VCLAMP of 60
V. The red curves illustrate the maximum I/V curve spread over process and temperature. As illustrated in
Figure 2, at 125°C VCLAMP extends to nearly 70 V while at –40°C VBR drops to 34 V. Accommodating these
shifts requires designing in extra margin to the system, increasing cost and complexity even further from
nominal TVS operation.
2.1.2
Temperature Derating
In addition to the deviation in VBR and VCLAMP, TVS diodes often suffer from severe temperature derating in
their surge performance. The IEC 61000-4-5 surge standard requires testing only at 25°C, but in reality,
surge events occur with the same probability at higher temperatures as at room temperature. Figure 3
shows the temperature derating of the Littelfuse® SMAJ33A. The maximum dissipated current at 25°C is
33 A; however, the device current dissipation derates significantly over temperature until at 125°C the
dissipation level drops to 6.6 A.
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Figure 3. Littelfuse® SMAJ33A Series Temperature Derating
Table 1. SMAJ33A Temperature Derating Table
Temperature
MAX IPP Dissipation
25°C
33 A
75°C
19.8 A
105°C
13.2 A
125°C
6.6 A
This is one reason why a system that passes IEC testing at room temperature can still see field failures.
System designers must take into account their protection stages derating over temperature.
2.2
TI's Flat-Clamp Technology
To attempt to simplify protection stage design, TI has developed Flat-Clamp technology that offers much
improved performance in harsh environments. The Flat-Clamp technology offers a solution for designers
who need precise, reliable clamping performance even under rough environmental conditions. Figure 4
shows the functional block diagram of the technology. The primary difference between TI's Flat-Clamp
technology compared to conventional TVS diodes is that in Flat-Clamp devices, the surge current is
clamped by an active FET with a feedback loop (as shown in Figure 4) rather than clamped by a passive
diode.
Figure 4. Functional Block Diagram of the Flat-Clamp Technology
4
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This improved topology allows for significantly lower RDYN, and therefore VCLAMP, than conventional TVS
diodes. Figure 5 shows clamping waveforms at 30 A, compared between a conventional TVS diode
(SMF33A and TI`s TVS3300 Flat-Clamp diode.)
Figure 5. 8/20 µs Surge Clamping Waveform of Flat-Clamp Diode vs. Conventional TVS Diode
In addition, over temperature the RDYN of the Flat-Clamp device has minimal variation compared to
conventional RDYN of the TVS, which helps to improve design reliability and simplify system design.
For more detail on the Flat-Clamp family operation and system advantages, see TI's Flat-Clamp surge
protection technology for efficient system protection .
3
System Surge Protection Example
In this section, an example of a real world situation is highlighted where the TI Flat-Clamp device improves
system robustness.
In this example, input protection is needed for a TPS7A4901 Low Dropout Voltage Regulator in a 4–20
mA loop input design. The equipment operates in an environment where the temperature can reach 85°C
and according to the IEC 61131-2 standard for industrial PLCs, the equipment must survive a ±1-kV surge
pulse through a 42-Ω impedance (the waveform is defined by IEC 61000-4-5), generating close to 24 A of
surge current. The TPS7A4901 is a positive linear regulator with an input voltage range of 3–35 V and an
absolute maximum applied voltage to the IN pin of 36 V.
To ensure reliability, it is best to ensure that the clamping voltage stays as close to the 36 V absolute
maximum input voltage of the low dropout (LDO) as possible. Because the event is a short transient, a
voltage slightly above the absolute maximum is unlikely to harm the system; however, a clamping voltage
significantly above the absolute maximum input voltage will cause failures even with the short transient
length. Satisfying this condition during a fault event can be simple at 25°C, but, as the temperature
increases, the I-V curve of a conventional TVS device will shift higher, increasing VCLAMP and leading to
reliability problems. The diode will also derate, lowering its ability to dissipate fault energy.
3.1
TVS Diode Specifications
TVS manufacturers specify characteristics like IPP or VCLAMP in the data sheet at room temperature;
however, there is rarely information about higher temperature operation. For example, the SMAJ33A data
sheet varies by vendor, but generally only specifies the data sheet at TA = 25°C with rarely any indication
of the performance as the temperature rises. An experienced designer will have an understanding of how
the parameters can be expected to shift, but an inexperienced designer can easily overlook that a shift will
occur. Even an experienced designer must estimate, often leading to over-designed systems.
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In contrast, TI's Flat-Clamp devices specify performance over the operating temperature range of the
device. Designers can confidently predict the clamping performance of the device even at high
temperature, leading to more efficient system design.
In this example, the performance between the TI TVS3300 and a competitor SMAJ33A is compared, an
industry standard device for this type of application. Because the SMAJ33A does not have performance
specified over temperature, testing is required to ensure robustness.
3.2
Test Setup
The test setup is a modified TPS7A4901EVM which is adjusted by adding an external protection diode as
shown in Figure 6, where U2 is the TPS7A49 LDO and the input TVS diode is highlighted. The input TVS
is tested with both a SMAJ33A TVS diode and a TVS3300 to compare the performance during a fault
event.
Figure 6. Modified TPS7A4901DGN EVM Schematic
Figure 7 shows the SMAJ33A with a footprint area of 13 mm2 and Figure 8 shows the TI TVS3300 (on an
adapter board) with a footprint of 1.2 mm2. These adapter boards are designed to enable testing of the
WCSP or SON packages on existing industry standard SMA or SMB footprints, for ease of evaluation
without a new PCB layout. The diode and TVS3300 adapter board are soldered on top of the 1-µF input
capacitors of the TPS7A49. Other than the TVS, the two boards are identical.
Figure 7. TPS7A4901DGN with SMAJ33A on the Input
Voltage Line
6
Figure 8. TPS7A4901DGN with TVS3300 on the Input
Voltage Line
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The overall test setup is shown in Figure 9, where the design under test (DUT) in question is the modified
TPS7A49EVM.
Figure 9. Test Setup Block Diagram
The DUT is heated under a thermal stream to 85ºC to simulate the high-temperature operating region. A
DC offset voltage is applied to the TPS7A49 input to simulate standard operating conditions and a surge
generator is used to generate the IEC 61000-4-5 surge pulse. An oscilloscope is used to measure the
input current of the surge pulse, the clamping voltage, and the TPS7A49 output voltage. For redundancy,
a multimeter is used to monitor the output voltage of the LDO.
3.3
Test Results
The DUT is placed in a 85ºC environment and exposed to surge events of increasing energy until a failure
occurs. Figure 10 shows that, when protected with the SMAJ33A, the TPS7A49 breaks during a test of
8.38-A surge current and a VCLAMP of 42.16 V. This is well below the required 24 A of surge immunity,
showing that at this temperature range, the SMAJ33A cannot protect effectively, despite the data sheet
nominal specifications. The failure occurs because the SMAJ33A high RDYN causes poor regulation of the
voltage during the surge, causing a rise to 42 V which is significantly above the 36 V absolute maximum of
the TPS7A49 device.
However, during the same surge with the TVS3300, the VCLAMP is 37.2 V and no harm is seen to the
TPS7A49. This waveform is shown in Figure 11. Because this voltage is very close to the maximum input
voltage rating of the TPS7A49 and the duration of the transient is short, there is no damage to the
TPS7A49. In the waveforms, the blue curve is the current caused by the surge event, the purple curve is
the voltage measured at the TPS7A49 input, and the green curve is the TPS7A49 output voltage.
Figure 10. SMAJ33A Protection at 8.38 A at 85ºC
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Figure 11. TVS3300 From TI Protection at 9 A at 85ºC
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Summary
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By continuing to increase the output of the surge generator, it is found that the TVS3300 enables
protection of up to four times more surge current than the SMAJ33A. Figure 12 shows the TVS3300
protecting at the required 24 A of surge current, and Figure 13 shows the final passing level of the
TVS3300 at 33.7 A. These results show that the TVS3300 provides significant margin to pass the required
surge test, even in high-temperature environments. In addition, there is no shift in the TPS7A49 output
voltage during the surge event, showing that the TVS3300 can enable protection without disruption to
normal system operation.
Figure 12. TVS3300 Clamping Waveform at 24 A at 85ºC
Figure 13. TVS3300 Clamping Waveform at Last Passing
Current at 85ºC
It is clear that the Flat-Clamp TVS3300 provides more robust protection for the input to this piece of
equipment than conventional TVS diodes. The very low RDYN of the TVS3300 enables using the 36-V
absolute maximum LDO. If a traditional TVS diode is used, the selected TPS7A49 device must be
swapped with a device that is rated for 60 V or higher input tolerance, increasing cost and complexity of
the design.
4
Summary
Unlike conventional TVS diodes, the TI Flat-Clamp devices enable robust protection in high-temperature,
harsh EMC environments. Conventional TVS diodes have poor, difficult-to-predict performance at higher
temperatures and can lead to system failures that are not seen during lab testing. For proper input
protection design with conventional TVS diodes, additional margin and time-consuming lab
characterization is required. However, TI Flat-Clamp diodes guarantee minimal performance shift over
temperature and provide an easy solution for designers to ensure long system lifetimes.
TI offers several voltage options in the Flat-Clamp family as shown in Table 2
Table 2. TI Unidirectional Flat-Clamp Devices
Device
8
Vrwm
Vclamp at Ipp
Ipp (8/20 µs)
Vrwm leakage
(nA)
Package Options
Polarity
TVS0500
5
9.2
43
0.07
SON
Unidirectional
TVS1400
14
18.4
43
2
SON
Unidirectional
TVS1800
18
22.8
40
0.5
SON
Unidirectional
TVS2200
22
27.7
40
3.2
SON
Unidirectional
TVS2700
27
32.5
40
1.7
SON
Unidirectional
TVS3300
33
38
35
19
WCSP, SON
Unidirectional
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