A transient voltage suppressor is selected by three voltages, in order: the stand-off voltage (VRWM) that the circuit must tolerate continuously, the breakdown voltage (VBR) where conduction starts, and the clamping voltage (VC) that the protected node will see during the surge. This guide explains the three, the waveform behind the power class, and the placement rules that make the numbers real.
What a TVS Does, and the Failure It Prevents
A TVS is a diode optimized to clamp transients: it blocks like a normal diode below its stand-off voltage, then conducts hard when a surge pushes the node past its breakdown point, shunting the energy and holding the voltage near the clamping level. The response is fast and the action is repeatable—within its power rating, the part absorbs surge after surge without dying.
The repeatability matters: within its pulse rating a TVS can clamp event after event, while a crowbar-style clamp destroys itself in the act—the TVS is the part that survives to protect again.
The failure it prevents is the overvoltage breakdown of the protected circuit: a switching spike, a load-dump, or a lightning-coupled transient can punch through an IC’s oxide or a rectifier’s junction in microseconds. The TVS’s job is to be the first thing that breaks the voltage rise, at a level the protected parts can survive.
The Three Voltages: VRWM, VBR, VC
Three datasheet numbers define the whole selection, and each answers one question:
- VRWM (stand-off / working voltage): the maximum voltage the line carries in normal operation without the TVS conducting. The TVS must sit above the nominal rail with margin, or it will conduct during normal operation and fail.
- VBR (breakdown voltage): where the TVS starts to conduct, stated as a range at a test current. It sits above VRWM, and it is where clamping begins.
- VC (clamping voltage): the maximum voltage the TVS holds the node at when it carries the rated surge current. This is the number that must stay below the protected circuit’s absolute maximum—the lower VC, the better the protection, at the cost of more power dissipation.
The selection order follows the questions: pick VRWM for the operating rail, confirm VBR clears the worst normal transient without false conduction, and choose the part whose VC protects the downstream components during the actual surge waveform.
A worked pair makes the order concrete: a 12 V rail with tolerance to 14.4 V needs a stand-off above 14.4 V, a breakdown that clears normal transients, and a clamping voltage below the 30–40 V the downstream driver can survive. The three numbers are tied—raising the stand-off pushes the breakdown and clamping up with it—so the rail tolerance is where the whole chain starts.
Unidirectional vs Bidirectional TVS
The direction choice follows the line. A unidirectional TVS conducts in one direction and clamps one polarity of transient; it suits DC rails where the normal bias is one polarity and the threat is an overvoltage spike in the same direction. A bidirectional TVS clamps both polarities and suits AC lines, data lines, and any node that swings both ways.
The rule is practical: if the node is always positive with respect to ground, a unidirectional part gives a lower clamping voltage for the same power class; if the node swings negative or carries AC signals, use bidirectional. The automotive lighting example later in this article is a DC line, which is why the unidirectional choice fits.
Power Class: What 600 W Actually Means
The power class—600 W, 1500 W, 3000 W—is a pulse rating, and the waveform is part of the number. In the TVS industry convention, a 600 W class part is quoted for a standard 10/1000 µs pulse: the current rises in 10 µs and decays to half in 1000 µs. The peak pulse power the part can handle at other pulse widths changes—shorter pulses allow more power, longer pulses less—so the rating is only meaningful with the waveform stated.
The ASMBJ28CA is a 28 V, 600 W class part in SMB; its exact clamping values belong to the datasheet, which should be confirmed with the supplier before the design is locked. What the class tells you now: it is a single-event capability for surge protection, not a continuous power rating, and the actual surge waveform must fit inside the pulse-power curve.
The pulse-power curve matters because real surges are rarely the exact 10/1000 µs shape: a shorter, higher-current event may be inside the peak-power capability while a longer low-current event is not. Match the event’s current and duration to the curve, and keep the protected circuit’s clamp budget above the worst clamping voltage at that point.
Placement Principles in Three Rules
The best TVS fails if it is in the wrong place. Three rules cover most layouts:
- Place it at the entry point. The TVS goes as close to the connector or the surge entry as possible, so the transient is clamped before it travels through the board.
- Keep the ground path short. A long TVS-to-ground trace adds inductance that turns the clamp into a spike; short, wide connections preserve the clamping voltage.
- Minimize the loop. The TVS and its ground return form a loop with the protected line; smaller loops radiate and couple less.
The detailed layout treatment belongs to the TVS layout guide; the selection takeaway is that placement is part of the protection design, not an afterthought.
The three rules interact: a TVS placed near the entry but with a long ground return still shows the inductance spike, and a short ground path at the wrong location leaves the line unprotected before the clamp. The layout review treats the TVS, its ground, and the protected line as one loop.
Automotive Example: Protecting a 12 V Lighting Line
A 12 V automotive lighting line runs at 12–14.4 V in normal operation and faces load-dump and switching transients that can reach tens of volts. The selection starts with a stand-off voltage above the normal rail—28 V on the ASMBJ28CA clears the 14.4 V operating maximum with margin—and the 600 W class covers the surge events the line is exposed to. The clamping voltage then decides whether the downstream driver sees a survivable level.
The example works because the three questions stay in order: the rail sets VRWM, the transients set the power class, and the protected components set the clamping budget. The ASMBJ28CA product page on the official site lists the part as 28 V / 600 W in SMB (DO-214AA) for automotive lighting-driver applications; the detailed electrical values are confirmed with the datasheet.
Good-Ark TVS Portfolio
Good-Ark’s TVS portfolio—including the automotive parts on the New Release page—and the ASMBJ28CA product page cover the automotive and power-line protection roles in SMB and related packages. The selection framework in this article—three voltages, waveform-matched power class, and placement—applies across the catalog; filter by stand-off voltage and package before opening individual datasheets.
Measurement note. The three-voltage framework above follows the standard TVS datasheet structure—VRWM, VBR, and VC—and the 600 W class convention of a 10/1000 µs pulse. Confirm the exact clamping voltage and pulse-power curve of the selected part from its datasheet, and verify the stand-off margin against the normal rail plus tolerance before ordering.
Frequently Asked Questions
What is the difference between VRWM, VBR, and VC?
VRWM is the normal operating voltage the TVS tolerates without conducting; VBR is where conduction starts; VC is the maximum voltage the node sees while the TVS carries the rated surge. Selection runs in that order.
Should I use a unidirectional or bidirectional TVS?
Unidirectional for DC rails with one polarity and one-directional threats; bidirectional for AC lines, data lines, or nodes that swing both ways. The unidirectional part usually clamps lower for the same power class.
What does 600 W mean on a TVS?
It is a pulse-power capability quoted for a standard surge waveform (10/1000 µs in the TVS convention), not a continuous rating. Match the actual surge waveform to the part’s pulse-power curve, not to the class name alone.
Can a TVS replace a Zener regulator?
No. A TVS is built to clamp fast, high-energy transients repeatedly within its pulse rating; a Zener is built to regulate continuously. The two solve different problems, and using a TVS as a regulator or a Zener as a surge clamp both end badly.
Where should the TVS be placed?
As close to the surge entry as possible, with a short, wide ground connection and a small loop to the protected line. Placement determines whether the clamping voltage on the datasheet is the voltage the circuit actually sees.
Conclusion
TVS selection is a three-voltage procedure with a waveform attached: set the stand-off for the rail, confirm the breakdown clears normal transients, choose the clamping voltage that protects the downstream parts, and match the power class to the real surge pulse. Then place it where it can do the job.
Review the ASMBJ28CA product page on the Good-Ark site for the 28 V / 600 W SMB part, and contact Good-Ark with your rail voltage, surge level, and protected component limits for a TVS recommendation.