Introduction: Transient Voltage Suppressors (TVS) for Automotive Electronic Protection. SM8/5Z Series APPLICATION NOTE

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1 Introduction: Because of the benefits from the booming development of automotive electronics, cars are integrating more and more sophisticated electronics into their systems. For example, entertainment systems, high-end lighting systems and precise sensor systems. However, these systems are very complicated and are made from sensitive discrete semiconductors designed into each car system. There are a lot of issues regarding reliability due to these complicated systems. These issues are being studied and examined by engineers and industry officials which has resulted in current standards and regulations. In terms of the functional safety of vehicles, ISO and Chapter 8 in this regulation have set up the framework. In terms of electronic component reliability, the test conditions are defined by AEC-Q101. Regarding electromagnetic compatibility issues, there are ISO 7637, ISO 10605, etc. Regarding production, TS-16949, is implemented in the quality management system. It mandates that the automotive supplier must be a certified automotive manufacturer. Overall, the goal is to provide the best product with high reliability and stable quality through different considerations and demands. Currently there are three types of components used: Gas tube, MOVs, and Avalanche TVS (Transient Voltage Suppressors). These are the main stream of solutions to eliminate EMC threats. The table below shows the basic strengths and weaknesses of those devices. Among the three types, TVS (Transient Voltage Suppressors) has the advantage with high reliability, fast response time, low leakage current and etc. Therefore, TVS (Transient Voltage Suppressors) is the optimum solution for automotive electronics. Revision February 3, / 2013

2 Device Advantages disadvantages Gas tube MOVs Withstand high current surge Low capacitance Withstand high power surge Low cost Small size Slow Turn-On time Poor voltage accuracy High ratio value of VC/VB Large size Wear-out problem after surge Poor voltage accuracy High VC and Leakage current Aging problem Avalanche TVS Diodes Fast <1ns Response time Small size Very precise voltage accuracy High reliability performance Low ratio value of VC/VB Low IR value High cost High capacitance Table 1. The basic characteristics of Gas tube, MOVs, Avalanche TVS Diodes. Revision February 3, / 2013

3 The basic characteristics of TVS Figure 1 shows the basic IV characteristics of TVS, the following is the introduction of these basic parameters: Breakdown Voltage (V B ): The point when device goes into avalanche breakdown and this breakdown voltage can be measured at a specific current on the datasheet. Maximum Breakdown Voltage (V C : Clamping Voltage): When the surge event appears, TVS will clamp the transient voltage to an acceptable voltage level. The clamping voltage can be measured at specific current, IPP. Revision February 3, / 2013

4 Stand-Off Voltage (V RWM : Working Stand-Off Reverse Voltage): The maximum voltage can be applied on TVS when it s not in breakdown phase, is called Working Stand-Off Reverse Voltage. In general, this voltage is the same or higher than the maximum working voltage of the circuits. Power rating: The power rating is a way of evaluating the surge-absorbing capability of TVS. In the industrial standard, the test condition is defined at 10/1000us pulse as shown in figure 2. The test waveform is completely different compared to ESD test or 8/20us lightning surge. However, this standard test is also a way to evaluate the voltage level after clamp Revision February 3, / 2013

5 The introduction of load dump phenomenon THE MAJOR THREAT OF THE AUTOMOTIVE POWER LINE (LOAD DUMP EVENT) Figure 3 illustrates the concept of vehicle power bus. The power sources are the battery and alternator, both have unstable output voltages. All the automotive electronics are supplied by this power line, such as, electronic control unit (ECU), sensor, and entertainment system. Currently, there are the two main international regulations for American, Japanese, and international markets. First, ISO :2004 comes from ISO organization. Second, JASO comes from Japan. These two well-known standards provide the simulation of the Electro-Magnetic Interference (EMI) in automotive environments, and provide the definitions of the pulses and the noise immunity levels for the industry. Revision February 3, / 2013

6 As mentioned, the load supplied by the same power line and the power source is an unstable DC supply system which is a combined, generator and battery. It leads to generate severe noise sources. According to the definitions of ISO :2004, the main hazards can be separated into five different surge waveforms. Among those pulses, the pulse 5a, load dump transient, represents the highest energy and destructive pulse. Therefore, it can cause severe damage. Load dump may occur on account of a battery being disconnected as a result of cable corrosion, poor connection or of intentional disconnection with the engine running. The possible reasons for the disconnection of the battery is described as below: - cable corrosion - poor or loose connection - intentional disconnection with the engine running Revision February 3, / 2013

7 The pulse shape and parameters defined by ISO are shown in figure 4 and table 2, respectively. The setting of 12V system and 24V system is different. Where U A : the working voltage of the 12V or 24V system U S : the peak voltage R i : The internal resistance of an alternator, in the case of load dump, is mainly a function of alternator rotational speed and excitation current. T d : the pulse duration time The internal resistance Ri of the generator shall be obtained from the following relationship: Where U nom : the specified voltage of the alternator; I rated : the specified current at an alternator speed of min 1 N act : the actual alternator speed, in reciprocal minutes. Revision February 3, / 2013

8 By reason of TVS s features, it is the ideal solution for automotive electronic protection. Figure 5 is the basic circuit of the protective framework. The pulse 5a will be generated by the surge tester. When the surge wave is produced, TVS will respond to the clamping voltage and the peak current rating. The clamping voltage and the peak current rating can be estimated from the following relationship: Where I PP : the peak current rating (Unit: A). V IN : the peak voltage of surge wave (Unit: V). V C : the clamping voltage (Unit: V). R i : the line impedance (Unit: Ω). Revision February 3, / 2013

9 The TVS s capability of UN technologies for automotive electronic protection SM8Z series It is clear from the introduction that a load dump event is a significant hazard for automotive electronics. The ISO not only has the simulation of pulse waves, it also has defined the classification of test pulse severity. The highest grade is level 4 as shown in the table 3 with the maximum output voltage for each system. The duration time and internal resistance is relative to generators. However, due to the highly reliable demand of automotive application, in this chapter of the product capability s introduction, the test setting for samples will be defined by the strict conditions as shown in the table 4. Regarding the 12V system, the maximum duration time, 400ms, and the lowest resistance, 0.5Ω, is applied. Regarding the 24V system, the maximum duration time, 350ms, and the low resistance, 2Ω, is applied. Therefore, TVS has to resist the severe impact. SYSTEM Test Level Us td (ms) Ri(Ω) Number of pulse 12V IV pulse 24V IV pulse Table 4. The setting of test conditions for 12V and 24V system Revision February 3, / 2013

10 The following figure 6 and figure 7 are the output of surge tester for 12V and 24V systems, respectively. The waveforms meet the ISO s definition and demand. For the shape of pulse waveform, those pulses have a high output voltage and long duration time. Consequently, the impact energy is significant. Since the power supply which is supplied by the generator and battery is not a stable DC source, the voltage level fluctuates. For the sake of safety, 12V system can choose TVS SM8Z22A for protection and 24V system can choose TVS SM8Z33A for protection. Figure 8 is the appearance of those devices and the outward dimensions of two components are the same. The following table shows the electrical characteristics of SM8Z33A and SM8Z22A. Take SM8Z22A as an example, when the reverse bias is applied to it, the leakage current is lower than 10uA at 22V. SM8Z22A will start to go into avalanche breakdown mode at 24.4V and the maximum clamping voltage is 35.5V at specific test current. Revision February 3, / 2013

11 Table 5. The electrical characteristics of SM8Z33A and SM8Z22A Part number Breakdown Voltage V T Maximum Reverse Maximum I RWM Working Peak Maximum Reverse Surge Maximum Clamping Min( Max( V) V) I T (ma) Leakage T J =175 Reverse Current Voltage I RWM (ua) Voltage V RWM I PP (A) (1) V PP (V) (V) (ua) SM8Z22A SM8Z33A NOTE 1: Surge current waveform is defined at 10/1000uS waveform The examinations are applying 20pcs for SM8Z22A to 12V test conditions and applying 20pcs of SM8Z33A for 24V test conditions. As shown in the table 6 data, all the samples pass the first impact. Furthermore, all the samples pass the second test after the devices cool down to ambient temperature. The performance shows the reliable capability of the products. As described in the previous section, TVS has a great ability to clamp the voltage when it faces a surge event, and the clamping voltage of TVS can protect the loads from the impact of the high voltage surge. The following figures are the clamping effects of SM833A and SM8Z22A. Also, the devices absorb the most surge energy. The clamping voltages of SM833A and Revision February 3, / 2013

12 SM8Z22A which is measured by oscilloscope are 48.4V and 31.2V, respectively. The performance immunizes the damage of the load dump hazard. SM5Z series Because of the different specifications of generators and the variety of application designs, the levels of threats are also different. The small power rating of SM5Z series can provide the designer other options. Here, the capability of SM5Z series is examined and the classification of test pulse severity is defined at level 4 as well. Also, the maximum duration time, 350ms, and the low resistance, 3.5Ω, is applied for 24V system as in below table. Revision February 3, / 2013

13 24V system can select TVS SM5Z33A for protection. The following table shows the electrical characteristics of SM5Z33A. When the reverse bias is applied, the leakage current is lower than 10uA at 33V. SM5Z33A will start to go into avalanche breakdown mode at 36.7V and the maximum clamping voltage is 53.3V at specific test current. The examination is applying 20pcs of SM5Z33A for 24V test condition. The table 6 is the test result, as shown in the table, all the samples pass the first impact, and pass the second test after devices cool down to ambient temperature. The performance shows the reliable capability of products. Revision February 3, / 2013

14 The clamping voltages of SM533A measured by oscilloscope is 48.8V. The performance immunizes the damage of the load dump hazard. Conclusion: Due to the complicated design of automotive electronics, the automotive system should be protected by protection components to avoid damage from EMI hazards. According to the phenomenon described by ISO , the pulse 5a load dump transient, is the most powerful impact. Consequently, the damage is severe. Based on the test results of SM8Z and SM5Z series, the products have high reliability to overcome the impact from pulse 5a. Therefore, SM8Z and SM5Z series are the ideal solutions for automotive electronic protection Revision February 3, / 2013