The TVS’s power class is a pulse-shape statement: 400 W and 600 W are quoted at a defined waveform, and the event’s actual energy decides the right class. This guide covers the waveform conventions, the energy estimate, the decision table, and the derating.
TVS Power Ratings Depend on the Pulse Shape
The TVS’s power rating is not a continuous number—it is quoted at a defined pulse width, and the same device carries more power at a shorter pulse and less at a longer one.
Where the Catalog Number Comes From.
The 400 W and 600 W figures are pulse-power numbers read at a stated waveform and starting temperature, usually the 10/1000 µs convention, and the datasheet’s pulse-power curve shows how the capability decays as the pulse lengthens. The curve is the real specification: at a shorter event the same device carries far more than its catalog class, and at a longer event it carries less. The class number is a convenience for sorting, and the curve is the contract for the actual event. The selection reads the curve at the application’s width and temperature, with the derating applied, rather than trusting the class alone. The 400 W and 600 W classes are the catalog’s shorthand, and the event’s waveform is the real input.
8/20 µs vs 10/1000 µs vs 10/700 µs
The common waveforms are the conventions: the 8/20 µs current wave for the surge-immunity tests, the 10/1000 µs wave for the TVS pulse-power rating, and the 10/700 µs wave for the telecom and data-line tests. Each shape has a different energy for the same peak, and the rating is read at the application’s width.
Matching the Waveform to the Test That Produces It.
The waveform choice follows the test and the line: the 8/20 µs current wave belongs to the surge-immunity tests on power and signal lines, the 10/1000 µs wave is the pulse-power rating convention for TVS components, and the 10/700 µs wave covers the longer telecom events. The automotive power line adds its own family, where the load-dump event is longer still and drives the selection toward the higher class or a series stage. Each waveform is a different energy for the same peak, and the mismatch—reading the 8/20 µs capability for a 10/1000 µs event—is the classic selection error.
Calculating Your Event Energy
| Step | What to do |
|---|---|
| 1 | Define the event’s peak, width, and shape |
| 2 | Estimate the energy or the peak power at the width |
| 3 | Compare with the TVS’s pulse-power curve at the same width |
| 4 | Add margin for temperature and repetition |
The estimate is the selection’s arithmetic: the event’s energy at the actual width is compared with the TVS’s capability at that width, not with the headline class.
400 W vs 600 W Decision Table
| Line type | Typical event | Class leaning |
|---|---|---|
| Small-signal / data | Modest energy | 400 W class |
| Automotive power line | Load-dump and surge energy | 600 W class |
| Industrial power line | High-energy events | 600 W+ |
The table is the starting point: the automotive and industrial power lines land on the 600 W class, and the small lines on the 400 W. The ASMBJ28CA is the 28 V, 600 W cell for the automotive lines.
The Automotive Line’s Fit.
The 28 V stand-off class clears the 12–14.4 V rail with margin, and the 600 W class covers the surge energy the lighting and control lines see, with the pulse width and repetition read against the datasheet curve. The part is listed in SMB (DO-214AA) under the automotive application parts on the Good-Ark site, and the full clamping and pulse-power parameters are confirmed with the supplier before the class is locked. The example shows the method: the class names the candidate, and the waveform, the clamp, and the derating close the selection.
Derating for Temperature and Repetition
The derating reads the environment: the TVS’s pulse-power capability derates with the ambient temperature, and the repetition rate decides whether the die cools between events. The derating curve and the duty are read together, and the class is chosen with the margin the derating demands.
Checking the Clamp, Not Only the Power.
The power class is only half the selection: the clamping voltage at the rated current must stay below the protected component’s limit, and the leakage at the working temperature must stay inside the application’s budget. The two checks are read at the same event—the clamp at the surge, the leakage at the hot steady state—and the class that passes the power check can still fail the clamp check. The margin table records the clamp, the protected limit, and the difference, and the datasheet’s clamping column is confirmed for the exact part and revision.
The Review Order for a New Event.
When a new surge event appears in the specification, the review runs in the same order: name the waveform and its width, estimate the energy at that width, read the curve at the temperature and the repetition, check the clamp against the protected limit, and confirm the derating. The order prevents the class-number shortcut, and the record makes the selection auditable. The same order applies when the environment changes—a hotter ambient, a longer cable, a new line—because the rating that was right for the old event is only a starting point for the new one.
The Temperature Derating’s Shape.
The derating curve is read with the ambient in mind: the pulse-power capability falls as the ambient rises, because the die’s allowed temperature rise shrinks. The review reads the curve at the application’s ambient and at the working temperature, and the margin is the difference between the derated capability and the event’s energy. The repetition rate is read the same way—a single event lets the die cool, while a repeated event keeps the die warm and demands a lower per-event allowance. The two readings are the derating’s practical form.
The Selection’s Documentation.
The selection record states the event’s waveform and width, the energy estimate, the derated capability, the clamp, and the margin, each with the datasheet reference. The record is the protection design’s evidence, and it is what a later revision or a field event reads first. The same table serves the next selection, because the method does not change when the line changes—only the numbers do.
The 600 W Class in the Automotive Line.
The automotive power and lighting lines carry the higher-energy events, and the 600 W class is the catalog’s answer to them: the class absorbs the load-dump and the surge energy at the working width, with the derating applied. The class is a starting point, not a conclusion—the waveform, the clamp, and the protected limit are read before the part is locked, and the ASMBJ28CA’s SMB package fits the board area the lighting modules allow. The fit is confirmed with the actual surge test on the line.
Engineering note. The waveform conventions and the derating follow the TVS selection and surge-rating methods; the ASMBJ28CA’s clamping and pulse-power curves are confirmed with the supplier.
Frequently Asked Questions
Why is the power class a pulse statement?
Because the TVS’s rating is quoted at a defined width, and the same device carries more at a shorter pulse—the event’s waveform is the real input.
What are the common waveforms?
The 8/20 µs current wave, the 10/1000 µs pulse-power wave, and the 10/700 µs data-line wave—each with a different energy for the same peak.
How do I size the class?
Define the event, estimate its energy at the width, compare with the pulse-power curve, and add margin for temperature and repetition.
When do I need the 600 W class?
On the automotive and industrial power lines with the load-dump and surge energy; the small lines can use the 400 W class.
How is the class derated?
By the ambient temperature and the repetition rate—the derating curve and the duty are read together.
Conclusion
The TVS power class is a waveform-and-energy decision: define the event, read the rating at the width, and derate for the environment. The 600 W class serves the automotive lines, and the ASMBJ28CA is its cell.
Review the ASMBJ28CA product page on the Good-Ark site, and contact Good-Ark with your event’s waveform and energy for a TVS class recommendation.