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Document Number: 88490 For technical ques tions within your region, please contact one of the following: www.vishay.com Revision: 09-Aug-10 DiodesAmericas@vishay.com, DiodesAsia@vishay.com, DiodesEurope@vishay.com 1 VISHAY GENERAL SEMICONDUCTOR Transient Voltage Suppressors Application Note Transient Voltage Suppressors (TVS) for Automotive Electronic Protection APPLICATION NOTE By Soo Man (Sweetman) Kim, Senior Application Manager A major challenge in automo tive design is protecting electronics - such as cont rol units, sensors, and entertainment systems - against damaging surges, voltage transients, ESD, and noise that are present on the power line. Transient voltage suppressors (TVS) are ideal solutions for automotive electronic pr otection and have several important parameters for th ese applications, including power rating, stand-off voltage, breakdown voltage, and maximum breakdown voltage. Fo llowing are definitions for these parameters. Power rating The power rating of a TVS is its surge-absorbing capability under specific test or application conditions. The industrial- standard test condition of 10 μs/1000 μs pulse form (Bellcore 1089 spec.), as show n in Figure 1. This test condition differs from the TVS ESD test condition of 8 μs/20 μs pulse form, as shown in Figure 2. Fig. 1 - Test Waveform of TVS Fig. 2 - Test Waveform of TVS ESD Breakdown Voltage (VBR) The breakdown voltage is the voltage at which the device goes into avalanche breakdown, and is measured at a specified current on the datasheet. Maximum Breakdown Voltage (V C: Clamping Voltage) The clamping voltage appears across the TVS at the specified peak pulse current rating. The breakdown voltage of a TVS is measured at a ve ry low current, such as 1 mA or 10 mA, which is different from the actual avalanche voltage in application conditions. Thus, semiconductor manufactures specify the ty pical or maximum breakdown voltage in large current. Stand-Off Voltage (V WM): Working Stand-Off Reverse Voltage The stand-off voltage indicates the maximum voltage of the TVS when not in breakdown, and is an important parameter of protection devices in circuits that do not operate under normal conditions. In auto motive applications, some regulation of the automotive electronics is provided by “jump-start protection”. This condition supplies 24 V DC in 10 min to 12 V type electronics, and 36 V DC in 10 min to
24 V type electronics without damage or malfunction of the
circuit. Thus, the stand-off voltage is one of the key parameters in TVS for automotive electronics. 0 0.5 1.0 2.0 2.5 0 % 100 % 50 % (ms) 10 µs IPPM 1.5 IPPM 10 μs/1000 μs 0 5 10 20 25 0 % 100 % 50 % (μs) 8 μs IPPM 8 μs/20 μs IPPM
Transient Voltage Suppressors (TVS) for Automotive Electronic Protection www.vishay.com For technical qu estions within your region, please contact one of the following: Document Number: 88490 2 DiodesAmericas@vishay.com, DiodesAsia@vishay.com, DiodesEurope@vishay.com Revision: 09-Aug-10 Application Note Vishay General SemiconductorAPPLICATION NOTE Fig. 3 - Parameters of Voltage and Current PRIMARY PROTECTION OF THE AUTOMOTIVE POWER LINE (LOAD DUMP) Automotive electronics, such as electronic control units, sensors, and entertainment systems, are connected to one power line. The power sources for these electronics are the battery and alternator, both of which have unstable output voltages that are subject to temperature, operating status, and other conditions. Additionally, ESD, spike noise, and several kinds of transient and surge voltages are introduced into the power and signal line from automotive systems that use solenoid loads, such as fuel injection, valve, motor, electrical, and hydrolytic controllers. Fig. 4 - Typical Vehicle Power Bus What is Load Dump? The worst instances of surge voltage are generated when the battery is disconnected when the engine is in operation, and the alternator is supplying current to the power line of the vehicle. This condition is known as “load dump”, and most vehicle manufacturers and industry associations specify a maximum voltage, line impedance, and time duration for this load dump status, as shown in Figure 5. Fig. 5 - Output Voltage of Alternator in Load Dump Condition Two well-known tests simulate this condition: the U.S.’s ISO-7637-2 pulse 5 and Japan’s JASO A-1 for 14 V powertrains and JASO D-1 for 27 V powertrains. In this section we review the applicat ion of TVS for load dump in 14 V powertrains. Specification and Results of Load Dump Tests The U.S.’s ISO-7637-2 pulse 5 and Japan’s JASO A-1 tests for 14 V powertrains are simulated in Table 1. Reverse Voltage IF VF IR IPP VWM VBRVC Reverse Current Forward Voltage Forward Current A TCU ECU Air-Bag ABS Car-Audio Battery Regulator ESD or Lightning High Voltage induced by Alternator Reaction Transient from Motor and Spark Plugs A Load X Output Voltage of Alternator Battery Connected Battery Disconnected VP VN TABLE 1 - MAJOR LOAD DUMP TEST CONDITIONS FOR 14 V POWERTRAINS V TOTAL (VP) (V) VS (V) VA (V) Ri (Ω) TIME (ms) CYCLE TIME JASO A-1 70 - 12.0 0.8 200 1 88 - 12.0 1.0 200 1
Transient Voltage Suppressors (TVS) for Automotive Electronic Protection www.vishay.com For technical qu estions within your region, please contact one of the following: Document Number: 88490 4 DiodesAmericas@vishay.com, DiodesAsia@vishay.com, DiodesEurope@vishay.com Revision: 09-Aug-10 Application Note Vishay General SemiconductorAPPLICATION NOTE Two Groups of Load Dump TVS There are two kinds of load dump TVS for the primary protection of automotive electronics: EPI PAR TVS and Non-EPI PAR TVS. Both product groups have similar operating breakdown characteristics in reverse bias mode. The difference is that EPI-PAR TVSs have low forward voltage drop (V F) characteristics in forward mode, and non-EPI PAR TVSs have relatively high V F under the same conditions. This characteristic is important to the reverse voltage supplied to the power line. Most CMOS ICs and LSIs have very poor reverse voltage capabilities. The gates of MOSFETs are also weak in reverse voltage, at - 1 V or lower. In the reversed power input mode, the voltage of the power line is the same as the voltage of the TVS V This reverse bias mode causes electronic circuit failure. The low forward voltage drop of EPI PAR TVSs is a good solution to this problem. Another meth od to protect circuits from reversed power input is utilizing a polarity protection rectifier into the power line, as show n in Figure 7. A polarity protection rectifier should ha ve sufficient forward current ratings, and forward surge and reverse voltage capabilities. Fig. 7 - Reverse Bias Status SECONDARY PROTECTION OF THE AUTOMOTIVE POWER LINE The primary target of protect ion circuits in automotive systems is high surge voltages, but the clamped voltage is still high. Secondary protection is especially important in 2 4 V p o w e r t r a i n s , s u c h a s f o u n d i n t r u c k s a n d v a n s . T h e main reason for this is the maximum input voltages for most regulators and DC/DC converter ICs for automotive applications are 45 V to 60 V. For this kind of application, using secondary protection, as shown in Figure 8, is recommended. Fig. 8 - Secondary Protection Circuit Adding resistor R onto the power line reduces the transient current, allowing smaller power-rating TVSs as the secondary protection. Current requirements for microprocessor and logic circuits in electronic units are 150 mA to 300 mA, and the minimum output voltage of a 12 V battery is 7.2 V at - 18 °C, or 14.4 V for a 24 V battery under the same conditions. In a 24 V battery under the above conditions, the supply voltage at a 300 mA load is 8.4 V at R = 20 Ω, and 11.4 V at R = 10 Ω at a minimum voltage of battery of 14.4 V (24 V battery voltage in - 18 °C). Power rating of R = I This supply voltage is high er than the minimum input voltages for most voltage re gulators and DC/DC converter ICs. Protected Load TVS VFCurrentReverse Power Input Load Current Limiter Primary Protection + R Secondary Protection Power Line Transient Voltage Clamped Voltage Clamped Voltage VL Vmin. Vmin. IL⁄()⁄() Vmin. IL⁄() R–()×= VL: Voltage to load Vmin.: Minimum input voltage IL: Load current R: Resistor value