Why Must Pulse Waveforms Be Considered When Selecting TVS Diodes for Automotive Electronics? Key Points for ISO 7637 Applications

When selecting TVS devices for automotive electronics, it is essential to consider the pulse waveform, as pulse polarity, duration, source impedance, and repetition rate collectively determine the shunt current and junction temperature. Two transient events with the same peak voltage—one lasting 50 microseconds and the other several hundred milliseconds—impose vastly different thermal stresses on the device; likewise, TVS components with the same nominal 6,600 W rating cannot be compared without reference to their rated waveforms.

ISO 7637-2:2011 specifies test methods for electrical transients conducted along supply lines in 12 V or 24 V road vehicle equipment and remains the current standard. During project implementation, the specific pulse levels, number of pulses, and functional status requirements stipulated by the OEM or customer take precedence. While the ASIM SM6S24V and SM6S28V models can be used to demonstrate trade-offs between voltage ratings, the datasheet parameters of a single TVS component cannot substitute for system-level testing.

The pulses defined in ISO 7637-2 are not the same type of surge.

Pulse 1 is typically used to simulate negative supply transients associated with the disconnection of inductive loads; Pulse 2a represents a positive transient; Pulses 3a and 3b are repetitive negative and positive switching disturbances characterized by rapid rise times. They differ in amplitude, internal resistance, pulse width, and repetition patterns, thereby imposing varying levels of stress on capacitors, TVS devices, reverse-polarity protection components, and downstream power management ICs.

Fast pulses may first expose issues related to PCB parasitic inductance and local decoupling. Pulses of longer duration, however, are more likely to drive up the TVS junction temperature, potentially necessitating active shutdown, series current limiting, or centralized clamping. Passing a single ESD test does not guarantee that the power port can withstand all automotive transients.

The test voltage is not equivalent to the voltage that appears across the TVS. Once clamping occurs, the internal resistance of the pulse source, the impedance of the wiring harness, and any series components at the front end collectively limit the current. The actual TVS current should be calculated based on the test generator's parameters and the circuit topology, and then verified using a current probe or a shunt resistor.

First, confirm which standard and version the project is using.

The statement "designed according to ISO 7637" lacks sufficient information. At a minimum, it must specify the standard part, the year of publication, whether it applies to a 12 V or 24 V system, the pulse type, test level, number of pulses, functional status, and test temperature. ISO 7637-2 covers conducted transients along power supply lines; other signal lines or coupling methods may fall under different parts of the standard.

Load dump conditions must also be verified individually. Current projects are typically tested in accordance with ISO 16750-2 or the OEM's own electrical specifications; the designations "Pulse 5a" and "Pulse 5b"—common in older documentation—cannot be directly applied to all new projects. If the customer documentation specifies both clamped and unclamped conditions, these must be evaluated separately.

Standards provide the testing framework, whereas OEM specifications often adjust parameters such as amplitude, duration, source impedance, functional class, and sample size. Obtaining the official test matrix from the customer before selecting a TVS device is more reliable than relying on a "typical waveform table" found online.

VRWM must cover the vehicle's normal high-voltage operating state.

Automotive 12 V systems do not remain constantly at 12 V; factors such as engine operation, charging, temperature fluctuations, and power management states alter the bus voltage, and some specifications even require operation under short-term overvoltage conditions. The TVS device's reverse standoff voltage (VRWM) should exceed the maximum normal continuous voltage defined by the project specifications, while also accounting for increases in leakage current associated with rising temperatures.

If the VRWM is set too low, the TVS may exhibit significant leakage or even enter avalanche breakdown prematurely under high-voltage charging conditions; conversely, if it is set too high, VBR and VC typically rise, thereby reducing the available voltage headroom for downstream components such as MOSFETs, input capacitors, and DC-DC chips.

Unidirectional TVS diodes are commonly used to clamp positive supply lines to ground; however, during negative-polarity events, they may conduct in the forward direction (exhibiting a forward voltage drop). Whether this behavior is permissible depends on factors such as reverse-polarity protection, fuse specifications, wiring harness impedance, and negative pulse testing requirements. Bidirectional TVS diodes do not conduct in the forward direction in the same manner under standard reverse-polarity voltages and should not be treated as direct replacements for unidirectional devices.

Selecting Voltage Ratings for SM6S24V and SM6S28V

Both the SM6S24V and SM6S28V are unidirectional devices in the SM8S/DO-218AB package. According to the specifications, the SM6S24V has a VRWM of 24 V and a VBR range of 26.7 V to 29.5 V; under specified pulse conditions, it features an IPP of 170 A, a maximum VC of 38.9 V, and a peak pulse power of 6600 W. The SM6S28V has a VRWM of 28 V and a VBR range of 31.1 V to 34.4 V; it features an IPP of 145 A and a maximum VC of 45.4 V, with the same peak pulse power rating of 6600 W.

This set of parameters illustrates a classic trade-off: the 24 V option offers a lower clamping voltage—which is more favorable for the voltage tolerance of downstream components—but provides less operating margin against normal high-voltage conditions; conversely, the 28 V option is less likely to trigger during normal high-voltage operation but results in a higher maximum clamping voltage during surges. If downstream components can withstand a maximum of only 40 V, the 45.4 V rating of the SM6S28V indicates a mismatch; however, if the bus voltage might consistently approach 24 V, the SM6S24V may lack sufficient operating margin.

The IPP and 6600 W ratings apply only under the specific pulse and temperature conditions defined in the specifications. If the target pulse is longer, the initial junction temperature is higher, or the repetition rate is more frequent, the 170 A or 145 A figures cannot be used directly as the operational limits. You must also consult the pulse derating curves, temperature derating specifications, and the official datasheet for the specific device version.

Calculate TVS current based on generator conditions.

As a preliminary estimate, the test generator can be modeled as an open-circuit pulse voltage source in series with a source impedance, with the TVS clamping voltage and upstream voltage drops subsequently subtracted. This yields a peak-value estimate suitable for screening purposes; however, the actual waveform will also be influenced by input capacitance, wiring harnesses, reverse-polarity protection MOSFETs, and PCB inductance.

Avoid circular logic during component selection: do not calculate an ideal clamping voltage ($V_C$) based on the maximum peak pulse current ($I_{PP}$) from the datasheet, only to then use that $V_C$ to verify that the current remains within the maximum $I_{PP}$ limit. A more robust approach involves iterating using the TVS device's voltage-current (V-I) curve, or directly observing the current and residual voltage through simulation and prototype testing.

After the peak current has passed, it is also necessary to evaluate the single-pulse energy and cumulative thermal effects from repetition. When the pulse interval is very short, the junction temperature does not fully recover, causing the subsequent stress to be superimposed. Additionally, a high initial junction temperature within a high-temperature chamber significantly reduces the operating margin.

The voltage-withstand chain of the output stage determines the upper limit of VC.

Downstream of the TVS, there are typically components such as a reverse-polarity protection MOSFET, input capacitors, a filter, an electronic fuse, and a DC-DC converter. Each node has different absolute maximum ratings, and the weakest link determines the permissible residual voltage. Since the absolute maximum rating of a chip represents the damage threshold rather than a suitable long-term operating point, engineering design requires incorporating margins for tolerances, temperature effects, and voltage ringing.

The clamping voltage (VC) specified in a TVS datasheet applies only to the device terminals; the actual voltage seen by the downstream circuit may include an additional voltage spike resulting from the $L \cdot di/dt$ effect. While a larger package enhances pulse-handling capability, it cannot compensate for the limitations of a long, thin grounding path. During verification, the oscilloscope probe should be connected close to the protected device, using the smallest possible measurement loop.

If no operating window can be found that simultaneously satisfies both VRWM and VC requirements, the front-end architecture should be modified. For example, one could employ active surge suppression, series current limiting, or a front-end stage with a higher voltage rating, rather than continuing to search for an ideal TVS that is simultaneously non-conducting and capable of extremely low clamping.

Negative pulses and sustained overvoltage must not be overlooked.

Only positive surges are addressed; negative pulses may propagate to downstream stages via input capacitors, the body diodes of protection MOSFETs, or the signal ground. It is necessary to analyze the TVS polarity, reverse-polarity protection topology, and the negative voltage tolerance of the components to determine the current return path.

TVS devices are designed for short-duration transients and are not suitable for independently withstanding conditions such as incorrect power supply connection, regulator runaway, or sustained short circuits. If a TVS is designed to fail as a short circuit to trigger a fuse, one must verify the fuse's operating time, the wiring harness current, and the TVS's short-duration energy handling capability; one cannot simply assume the device will fail in a safe manner.

Prototype testing must also verify functional performance levels. The fact that the device did not burn out does not mean it has passed the test. Communication interruptions, system resets, unintended output actions, and data storage anomalies can all constitute violations of the functional status requirements specified by the customer.

Prototype verification must yield data that can be reviewed.

For each pulse, record the generator settings, polarity, level, number of pulses, ambient temperature, power supply status, and load status. Measure the voltage across the TVS, the residual voltage at critical downstream nodes, the shunt current, and the device temperature rise; also record whether the equipment resets or experiences performance degradation.

Retain the raw oscilloscope files for critical waveforms rather than just screenshots. This allows for a comparison of peak values, pulse widths, and ringing—rather than relying on a simple "it passed this time"—after replacing TVS components, modifying the layout, or changing input capacitors.

Replacement components intended for mass production must also be re-evaluated. Even if TVS components share the same VRWM and package, parameters such as dynamic resistance, VC test current, leakage current, and derating curves may differ; therefore, system-level test results from the original component cannot be directly applied to the replacement.

Automotive TVS and ISO 7637: Frequently Asked Questions

Is selecting a high-power TVS sufficient to pass ISO 7637 testing?

No. The performance of the complete system also depends on reverse-polarity protection, input capacitance, wiring harnesses, PCB circuit paths, the voltage withstand capability of downstream components, and functional status. Matching at the component level is achieved only when the TVS provides sufficient margin regarding the target waveform, temperature, and current.

Can automotive TVS diodes with the same peak power rating be directly interchanged?

No. To compare VRWM, VBR, VC, IPP, leakage current, and derating curves under identical waveform conditions, you must also verify polarity, package thermal characteristics, and certification status.

Is the test voltage specified in ISO 7637 the voltage that the downstream stage must withstand?

No. The test voltage is a parameter of the generator setup; once the TVS conducts, it interacts with the source impedance to determine the resulting current and residual voltage. The actual voltage experienced by the downstream stage must be measured at the corresponding node.

Is load dump still covered by ISO 7637-2?

While legacy documentation often describes load dump events using "Pulse 5a" and "Pulse 5b," current projects typically reference ISO 16750-2 or specific OEM standards. Testing must be conducted in accordance with the standard version and test matrix specified by the customer, rather than simply relying on outdated terminology.

The final step in selecting an automotive TVS is to create an auditable cross-reference table linking each pulse to specific generator settings, estimated TVS current, maximum residual voltage, device energy, downstream voltage withstand capability, and functional requirements. Only by reaching this stage does component selection evolve from a simple matter of "checking power ratings" into a verifiable engineering decision.