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

PV bypass diode failures appear as string underperformance or a damaged junction box, and the evidence chain runs from the shading pattern to the junction-box temperature. This guide covers the failure mechanisms, the hot-spot damage patterns, the thermal evidence, and the module-versus-device attribution.

Last updated: October 16, 2026

Why Bypass Diodes Fail in the Field

The bypass diode conducts the string current around a shaded or damaged cell, limiting the cell’s heating when the string forces current through it in reverse. The role is thermally demanding: in a prolonged partial-shading event, the diode carries the full string current while the junction box holds its heat, and the diode’s own leakage at the elevated temperature adds to the dissipation. The thermal-runaway mechanism—where leakage heating drives temperature and temperature drives leakage—is the failure physics, owned by the dedicated guide; the field article’s point is that the mechanism appears as a junction-box temperature story.

The failure appears in two electrical forms. A shorted bypass diode removes its sub-string’s contribution: the module underperforms, and the string I-V curve shows the missing step. An open bypass diode leaves the shaded cells unprotected, and the hotspot returns to the cell with the damage the bypass was meant to prevent. Both forms have distinct signatures.

Hot-Spot-Induced Damage Patterns

The hot-spot pattern starts at the cell: a shaded cell under reverse bias dissipates the string current as heat, and the localized heating can damage the cell itself. The bypass diode limits that heating when it conducts, and the damage pattern follows the history—if the bypass conducts too late, too weakly, or not at all, the cell shows the burn pattern; if the bypass conducts but overheats, the diode and the box show the damage instead.

The evidence is read on both sides: thermal imaging shows a hot cell or a hot bypass in the same module, and the I-V curve shows which sub-string is missing or weak. The pattern names the suspect, and the next step moves to the junction box for the thermal record.

Junction-Box Thermal Evidence

The junction box is where the bypass diode’s thermal story is written. During a bypass event, the box temperature rises by an amount set by the diode’s dissipation, the potting, the terminal and solder resistance, and the box’s cooling. The thermal test methods in the module standards—the bypass-diode thermal tests in IEC 61215-2 and the junction-box safety and thermal requirements in IEC 62790—define the measurement conditions, and the field reading applies the same logic with a thermocouple or an imaging camera on the box.

The Field Measurement Recipe.

The field reading is a comparison, not a single number: the suspect module’s box temperature is measured during a controlled bypass event and compared with its healthy siblings in the same string, under the same irradiance and current. The thermocouple is fixed to the box body at the diode location, the imaging camera is used to map the temperature distribution, and the measurement is repeated after the module has reached its steady condition rather than taken at the first power-on. The comparison removes the ambient and the irradiance as variables, and the delta between the suspect and the healthy box is the evidence the attribution reads.

The signs of sustained overheating are physical: discolored potting, softened or reflowed solder at the terminals, cracked diode bodies, and a box that runs measurably hotter than its siblings in the same string. The temperature data, the event history, and the physical signs are the junction box’s evidence.

Module-Level vs Device-Level Causes

The attribution separates module design from device quality, and the possible causes are not mutually exclusive. Device-level inputs include excessive leakage at the working temperature, an under-rated current class for the string, and surge damage from a lightning or switching event. Module-level inputs include insufficient junction-box cooling, high ambient, potting that traps heat, and a layout that puts the diode too close to the heat source. Operating conditions—prolonged partial shading, high string currents in large modules—load both sides.

Cause layer	Inputs	Evidence
Device	Leakage, current class, surge survival	Datasheet range, lab measurement
Module	Box cooling, potting, layout	Box temperature, design review
Operation	Shading duration, string current	Event history, I-V records

The analysis reads all three layers together: a returned diode is measured for leakage and surge damage, the box is assessed for cooling and potting, and the string history is checked for the shading and current profile. The cause is the layer where the evidence closes, not the first suspect.

Selection and Design Corrections

The corrections follow the attribution. Where the device class is the limit, the selection moves to a bypass diode with a higher current class, controlled leakage, and a surge margin checked against the actual event—the AMBRP10H100, a 10 A, 100 V Schottky in PDFN56 with AEC-Q101 qualification available, is the class example on the Good-Ark site. Where the module is the limit, the junction-box cooling, the potting, and the terminal design are revised, and the thermal test is re-run with the bypass event.

The design correction is verified the same way the failure was found: the bypass event is reproduced, the box temperature is measured, and the margin to the diode’s limit is confirmed at the module’s rated ambient. The selection and qualification methods own the details; the field article’s closing point is that the fix is proven by the same thermal evidence that named the failure.

Engineering note. The failure attribution follows the possible-cause method: device, module, and operation inputs are read together, and the standards’ thermal tests are the measurement reference, with the current editions confirmed before the assessment.

The Correction’s Verification.

The correction is verified with the same recipe: the bypass event is reproduced with the revised diode or the revised box, the temperature delta is measured against the healthy baseline, and the margin to the diode’s limit is confirmed at the module’s rated ambient and string current. The verification record—the event, the temperature, and the margin—is filed with the module’s qualification documents, and it is what the next field complaint reads first.

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.

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.

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

• Good-Ark — AMBRP10H100 Product Page
• IEC — IEC 61215-2:2021 (MQT 18 bypass-diode thermal test)
• IEC — IEC 62790:2020 (junction-box thermal test)
• Good-Ark — PV Inverter Application
• Good-Ark — Contact

Continue through this engineering topic

For the complete selection, design, verification, and troubleshooting path, explore the Solar and PV Electronics Hub.

• Practical guide: Blocking vs Bypass Diodes in Solar Arrays: Who Blocks What and Where the Difference Sits
• Practical guide: Selecting a PV Bypass Diode: VF, Leakage, Surge, and the Junction-Box Temperature Budget
• Practical guide: PV Bypass Diodes in Plain Language: What the Module Contains and When It Saves the Panel
• Ask Good-Ark technical support about this application