The TVS’s placement decides whether the datasheet’s clamp is the circuit’s clamp: close to the connector, short to ground, and a small loop are the rules, and the parasitic inductance is the enemy. This guide covers the rules, the overshoot, the PCB checklist, and the pulse verification.
Why Placement Matters More Than the Part Number
A TVS with the perfect rating in the wrong place clamps late: the transient travels through the board before the clamp, and the ground return’s inductance turns the clamp into a spike. The placement is the protection’s second half, and the layout review reads it with the part number.
The Voltage the Load Really Sees.
The mechanism is inductance: the clamp current rises in nanoseconds, and the loop inductance between the TVS and the protected node adds a voltage proportional to the rate of change. A rough illustration makes the scale visible: at a current edge of 10 A/ns through a 10 nH loop, the induced term reaches about 100 V—more than the clamp voltage itself on many automotive lines. The number is illustrative, and the measurement on the actual board is the real value, but the illustration explains why a few millimeters of trace and a long ground return decide whether the protected part sees the clamp or the spike. The placement review is an inductance review.
Placement Rules: Close to the Connector, Short to Ground
Three rules cover the placement: the TVS sits as close to the connector or the surge entry as possible, its ground return is short and wide, and the loop through the protected line is small. Each rule reduces the inductance the clamp must overcome, and the three together decide the voltage the protected circuit actually sees.
The Connector Entry and the Harness.
The surge enters at the connector, and the harness adds its own inductance and capacitance before the board. The TVS sits as close to the entry as the layout allows, so the clamp engages before the transient has traveled through the board; the connector’s filtering and the bulk capacitance behind it shape the residual. A shielded harness changes the return path, and the review checks the shield’s termination rather than assuming it. The automotive lighting application page on the Good-Ark site lists the module families this placement serves, and the layout rules are read with the harness in the picture, because the clamp’s job is measured at the protected node, not at the connector pin.
Parasitic Inductance and Clamping Overshoot
| Inductance | Effect on the clamp |
|---|---|
| Small | Clamp near the datasheet’s voltage |
| Large | Overshoot above the clamp |
| Very large | The protected part sees the spike before the clamp |
The table is the overshoot story: the ground return’s inductance adds a voltage spike to the clamp, and the overshoot is the reason the placement rules exist. The measurement on the prototype—the clamp waveform at the actual surge—is the placement’s judge.
The Overshoot Budget in the Design Review.
The review reads the clamp waveform against a budget: the clamp level, the overshoot, and the protected component’s absolute maximum are the three numbers, and the margin between the peak the node sees and the limit is the review’s verdict. The budget is set before the layout work so that the measurement has a pass criterion, and the revision cycle—move the part, shorten the return, re-measure—continues until the margin closes. The overshoot budget is the layout’s specification, and the placement rules are the means to meet it.
PCB Checklist for TVS in Lighting ECUs
- Entry: the TVS close to the connector.
- Ground: short, wide return to the reference plane.
- Loop: the protected line’s loop small.
- Via: direct connection, no long stubs.
- Measurement: the clamp waveform at the worst surge.
Verification With Pulse Testing
The pulse verification applies the surge waveform and measures the clamp at the protected node: the overshoot, the clamp level, and the protected part’s margin are the results, and the before-and-after layout change is the evidence.
Measuring the Clamp Without Lying to It.
The measurement is easy to corrupt: a long scope lead adds the same inductance the layout removed, and the reading shows a spike that the circuit never sees. The probe tip is soldered directly at the protected node with a short ground spring, the loop is kept to the same scale as the layout’s, and the waveform is captured at the highest surge level and the most loaded state. The comparison between the original and the revised layout is taken with the same probe setup, so the change in the overshoot is real. The record—waveform, setup, and revision—is the placement proof. The surge-testing article owns the waveform details; the layout article’s point is that the measurement is the placement’s proof.
The Layout’s Place in the Protection Chain.
The TVS placement is one stage of the protection chain: the series element limits the current, the clamp sets the voltage, and the filter shapes the residual. A well-placed TVS makes the downstream stages’ job easier, while a poor placement makes the same parts look inadequate. The layout review reads the whole chain—entry, clamp, filter, and load—and the pulse test verifies the chain at the protected node. The placement is the part of the protection that costs no BOM dollars and decides the real clamp on the protected line.
The Ground Return as the First Priority.
The ground return is the first item in the layout review, because the clamp’s current must return to the source without crossing the protected circuit. A short, wide return to the reference plane keeps the loop small; a return that wanders through the load or the control ground turns the clamp into a noise source. The review walks the surge path and the return path together, and the measurement verifies the walk by comparing the clamp waveform before and after the return is shortened. The return is the layout’s cheapest fix and its most common miss.
The Protected Component’s Margin.
The layout work ends with the protected component’s margin: the peak the node sees during the pulse is compared with the component’s absolute maximum, and the difference is the protection’s verdict. The margin is read at the highest ambient and the highest surge level, because both move the peak. The record states the waveform, the clamp, the peak, and the margin, and the revision cycle continues until the margin closes. The layout is the means; the margin is the proof.
The PCB Checklist in Production.
The checklist is also a production item: the placement is verified on the first articles, the ground return is confirmed against the drawing, and the clamp waveform is re-measured when the board or the harness changes. The production check is quicker than the design review but reads the same three rules—entry, return, loop—and it catches the variance that a design file cannot. The record ties the production result to the design review’s baseline.
Engineering note. The placement rules and the overshoot model follow the TVS layout practice and the surge-testing method; the ASMBJ28CA is the automotive cell, and its clamping is confirmed with the supplier.
Frequently Asked Questions
Why does placement matter more than the part?
Because a TVS in the wrong place clamps late—the transient travels before the clamp, and the ground inductance turns the clamp into a spike.
What are the placement rules?
Close to the connector, short and wide ground, small loop—each reduces the inductance the clamp must overcome.
What does the inductance do?
It adds an overshoot to the clamp; a very large inductance lets the protected part see the spike before the clamp.
What is the PCB checklist?
Entry, ground, loop, via, and measurement—the five items that close the placement.
How is it verified?
With the pulse applied and the clamp measured at the protected node—overshoot, clamp, and margin are the results.
Conclusion
The TVS’s placement is the protection’s second half: close to the connector, short to ground, small loop, and the pulse measurement is the judge. Design the layout with the part, and the clamp on the datasheet becomes the clamp in the circuit.
Review the ASMBJ28CA product page on the Good-Ark site, and contact Good-Ark with your lighting line’s layout and surge data for a TVS placement review.