Project type: Failure analysis / power electronics insight

Tags: #MOSFET #PowerElectronics #VoltageSpike #ThermalDesign #HardwareFail

🧩 The Day a “600V” MOSFET Failed Below 500V

You know that feeling when your prototype is finally running smoothly — until something pops?

That’s exactly what happened to our high-voltage motor control board.
It had passed every bench test, survived thermal cycling, and handled all loads… until the internal temperature hit 85 °C.

Then came the “click.”
Followed by silence and a faint smell of burned epoxy.

Post-mortem?
A 600 V MOSFET, split clean in half — and the voltage log showed only 480 V.

🔍 Myth Busted: High Temperature ≠ High Spike Margin

It’s easy to assume that higher temperature gives you more tolerance — after all, the datasheet says breakdown voltage increases slightly with temperature.

But that’s a static condition. Real circuits are never static.

In fast switching environments, parasitic inductances, gate ringing, and layout geometry dominate.
Those 10 ns overshoots don’t care what your nominal V_DS is.

And once the MOSFET’s avalanche region absorbs too much localized energy, it’s game over — even if you never reach the official breakdown voltage.

⚙️ The Deep Dive: Where Spike Margin Actually Comes From

Spike margin isn’t a single parameter. It’s a cocktail of design factors:

• Device structure: trench vs planar vs super-junction

• Package inductance: wire bonds, lead frames, internal loops

• Board layout: every millimeter of copper adds nH

• Gate drive speed: faster isn’t always better

• External snubbers or clamps: RC, RCD, or TVS control real-world overshoot

On paper, two MOSFETs may look identical. On the scope, they behave like completely different species.

🔥 The Test That Changed Our Design Philosophy

We instrumented the next prototype with a high-bandwidth probe across drain-source.

Result:
Every turn-off event showed a transient +80 V spike, lasting 50 ns — enough to push local avalanche current through the die corners.

The solution wasn’t just a “better MOSFET.”
We redesigned the gate driver, shortened return loops, added an RC snubber, and verified avalanche energy under load.

That board hasn’t failed once since.

💡 Lessons Learned (So You Don’t Have to Burn Your Own Board)

1. Design for margin, not miracles. Keep switching peaks < 80% of rated V_DS.

2. Layout is your first line of defense. Stray inductance kills faster than voltage.

3. Use clamps or snubbers to tame energy — don’t let silicon absorb it all.

4. Validate under stress. If you haven’t scoped it at full current and full temperature, you haven’t really tested it.

5. Choose MOSFETs with robust avalanche energy specs, not just pretty voltage numbers.

⚔️ Key Takeaway

High voltage doesn’t guarantee safety.
High temperature doesn’t buy you margin.
Only engineering discipline keeps your MOSFET alive.

If your design depends on luck, it’s already a failure — it just hasn’t burned yet.

📊 Resources / Further Reading

• Application notes on avalanche energy testing (look for TI, Infineon, ON Semi docs)

• Kelvin-source layout tips for low inductance paths

• Snubber design calculator (LTspice model templates available)

💬 Discussion Prompt

What’s the worst transient-related failure you’ve ever debugged?
Did layout, switching speed, or bad luck take the blame?
Drop your scope horror stories below — we’ve all been there.

🔖 Tags for discoverability

#MOSFET #VoltageSpike #PowerElectronics #ThermalDesign #CircuitDesign #HardwareFail #PCBLayout #Reliability #Semiconductors #SwitchingLoss #ElectronicsEngineering #AvanlancheEnergy #DIYPowerDesign