Bypass Diode Failures in the Field: Hot Spots, Thermal Stress, and How to Diagnose On-Site

Q: Which ratings matter most for PV bypass diodes?

VF at operating current, leakage at temperature, surge, and the thermal path in the junction box.

Updated: October 28, 2026 This page is a focused engineering note in the solar pv cluster—use the checklists and links below to…

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Bypass Diode Failures in the Field: Hot Spots, Thermal Stress, and How to Diagnose On-Site

Updated: October 28, 2026

This page is a focused engineering note in the solar pv cluster—use the checklists and links below to finish selection, not as a substitute for the datasheet.

Topic hub: How to select a PV bypass diode. Related deep-dives sit in the continue list at the end.

Before release	Pass criteria
Ratings vs mission profile	Voltage/current/thermal margins recorded
Package & PCB	R_th path agreed with layout
Docs	Datasheet rev + PCN policy filed
Alternates	At least one workable second source or risk accepted

Three Failure Modes in One Part: Short, Open, and Leaky

A bypass diode fails in three distinct ways, and the failure signature table is the fastest way to read which one you have. A shorted bypass diode conducts in both directions, carrying current when it should block — its substring is bypassed permanently, so the module loses the output of that third even in full sun. An open bypass diode blocks in both directions, carrying no current when it should — its substring is unprotected, and the hot spot returns under shade. A leaky bypass diode conducts a small reverse current when it should block, adding heat in the junction box and a small output loss.

Failure	Forward	Reverse	Effect
Short	Conducts	Conducts	Loses that substring’s output
Open	Blocks	Blocks	Substring unprotected, hot spot returns
Leaky	Conducts	Small reverse	Heat in the box, small loss

The table is the diagnosis in one view: shorted and open are the two clean states, and leaky is the gray one that shows up as heat rather than a clear output loss. The PV bypass failure article documents the three in field data, and this article adds the on-site workflow that identifies them.

Good-Ark Schottky bypass diode module whose failure modes are diagnosed on-site in this guide, from the PV bypass diode module category

On-Site Diagnosis: Meter and IR Camera Workflow

The on-site diagnosis runs as a two-instrument workflow. First, the meter: with the module isolated and the junction box opened safely, test each bypass diode in both directions. A diode-mode reading that conducts both ways is shorted; one that blocks both ways is open; one that conducts forward and leaks reverse is leaky. Second, the IR camera: under controlled shade, image the module and the junction box. A hot cell in the shaded substring points at an open bypass diode; a hot junction box points at a leaky or shorted one.

The two instruments cross-check. The meter names the diode’s electrical state; the IR camera confirms the thermal consequence. A module that reads an open bypass diode on the meter and shows the hot spot on IR is a clear repair case; a module that reads healthy but runs warm may be a leaky diode or a poor thermal joint. The workflow, run in that order, separates the three failure modes without unsoldering anything.

Axial diode package in the same family as discrete bypass diodes whose solder stress and thermal cycling age in the junction box, from the bypass diode module category context

Replacement and Reliability Notes for O&M Teams

The replacement guidance closes the article. A failed bypass diode is replaced with a part matched to the module’s current and the junction-box temperature — the same selection discipline the PV bypass selection article applies, read at the shaded, hot duty rather than at room temperature. An open diode is the priority repair, because it is the hot-spot risk; a shorted one is a production loss that can wait; a leaky one should be checked against the thermal budget.

The reliability notes for a maintenance team are three. First, schedule bypass checks on hot, high-swing installations, where the cycling stress is worst. Second, use the IR camera under shade as the fastest array-wide screen. Third, treat a repeat failure at the same diode position as a signal — a recurring failure in one junction box points at a thermal or assembly problem, not random part luck. The bypass module category lists the replacement parts built for the junction-box duty, and the PV module qualification article gives the reliability context the replacement is chosen against. A bypass diode, diagnosed and replaced by the workflow, keeps a module producing and a hot spot from forming. And in the message to any O&M team, the three failure modes and their priorities are the whole syllabus. Diagnosis first, risk first, replacement second, and the array keeps its harvest. That is the practical shape of this whole guide. It is also the shape of its takeaway. One that sticks.

The IR camera workflow deserves its own detail because it is the fastest array-wide screen. Under a controlled shade, point the IR camera at a row of modules and look for two signatures: a hot cell inside a shaded substring, which points at an open or failed bypass diode, and a warm junction box, which points at a leaky or shorted diode. A single thermal image can flag a suspect module across a whole string, and the meter then confirms which failure mode it is. The IR screen catches the failures that a quarterly electrical check misses, because it sees the thermal consequence before the output loss becomes obvious.

The replacement guidance also carries a cost note for a maintenance team. An open bypass diode is the hot-spot risk and should be repaired promptly; a shorted one costs a third of the module output and earns repair on the next visit; a leaky one that stays within the thermal budget can be logged and scheduled. Prioritizing by risk rather than by convenience is what keeps a team from spending its repair budget on the least dangerous failure first. The bypass selection article gives the part-matching method, and the bypass module category lists the replacements ready for the box.

Frequently Asked Questions

Why do bypass diodes fail?

They carry the full string current during shading while the junction box holds the heat, and leakage at temperature adds to the dissipation until the thermal limit is crossed.

What are the failure forms?

A shorted diode removes a sub-string’s output; an open diode leaves the shaded cells unprotected, and the two have distinct I-V and thermal signatures.

What is the junction-box evidence?

Temperature rise during the bypass event, discolored potting, reflowed solder, and a box measurably hotter than its siblings in the same string.

How do I attribute module versus device?

Measure the returned diode’s leakage and surge survival, assess the box cooling and potting, and read the shading and current history—the cause is the layer where the evidence closes.

What are the corrections?

A higher current class with controlled leakage for the device side, or improved box cooling, potting, and terminal design for the module side, verified by reproducing the bypass event.

Conclusion

The bypass failure is a thermal evidence chain: the shading history, the box temperature, and the diode’s measured behavior are read together, and the correction is verified by the same thermal test that named the failure. Attribute the layers, fix the layer that closes, and the module’s field story is the proof.

Review the PV Inverter Application on the Good-Ark site for the module context, and send Good-Ark your string current, junction-box temperature, and failure evidence for a bypass reliability review.

Sources

Release checklist

Re-read absolute maximum ratings against worst-case spikes.
Confirm pinout/package drawing matches the footprint library.
Capture why this MPN beat the next alternative (loss, cost, lead time, or qualification).

FAQ

What does a bypass diode failure look like in the field?

Hot spots, abnormal string I-V, and modules that heat under partial shade—confirm thermally and electrically on site.

Which ratings matter most for PV bypass diodes?

V_F at operating current, leakage at temperature, surge, and the thermal path in the junction box.

Can I swap bypass diodes by voltage alone?

No—thermal and surge behavior differ; follow the module maker’s qualified parts list when available.

Selection guide?

How to select a PV bypass diode.

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Topic hub: How to select a PV bypass diode — related selection guides and application notes in this cluster.