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.
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.
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.