Bypass Diodes in Solar Panels: Function and Failure

A solar panel is a series string of cells, and a series string carries the same current through every element. That is the whole reason a bypass diode exists. When one cell or cell group cannot pass the current the rest of the string is producing, it does not simply stop contributing. It becomes a reverse-biased load, dissipating power as heat, and the heat is concentrated in a small area of a sealed laminate. The bypass diode gives the current an alternate path around that group so the rest of the panel can keep working and the affected cells are not driven into thermal damage.

What the diode does electrically

A crystalline silicon cell produces roughly half a volt under load. A 60-cell panel therefore has an open-circuit voltage near 37 to 40 volts at standard test conditions, and a 72-cell panel near 45 to 46 volts. Cells are wired in series, so the current through the panel is set by the weakest cell in the string unless something routes around it.

A bypass diode is connected antiparallel across a group of cells, typically 20 or 24 cells in a 60- or 72-cell panel. Under normal operation the diode is reverse-biased: the cell group’s forward voltage holds it off, and it conducts essentially nothing. When the group is shaded or damaged and its forward voltage collapses, the diode becomes forward-biased and carries the string current around the group. The panel loses the voltage of that group, roughly a third of its total, but keeps producing from the remaining groups.

This is why a partly shaded panel often loses about a third of its output rather than all of it, and why the pattern in production data is stepped rather than smooth. The diode is not a optimizer and does not recover the shaded group’s energy. It limits the damage.

What it looks like on an IV curve

A bypass diode changes the shape of the current-voltage curve. With one group bypassed, the curve shows a step: the current holds at the string value up to the reduced voltage, then the bypassed group’s contribution is missing. A curve tracer or inverter sweep may show one or more plateaus, and the maximum power point sits on the lower plateau.

In production data, a failed or conducting bypass diode often appears as a persistent fractional loss that does not track irradiance the way shading does. Shading moves with the sun and the season; a diode that has failed shorted removes a group from the circuit all the time. A diode that has failed open leaves the group unprotected, so the symptom may be heat damage or a hot spot rather than an obvious electrical loss.

Failure modes and how they present

Bypass diodes fail in two directions. A shorted diode keeps the cell group bypassed permanently, so the panel produces roughly two-thirds of its normal voltage and the mismatch may show up as a string that is consistently low. An open diode removes the protection, and the affected group can be driven into reverse breakdown under mismatch, producing a hot spot that may crack the laminate or burn the junction box.

Junction box diodes are potted or clamped and are not field-serviceable in the way a connector is. Diagnosis means measuring string voltage and current under known conditions and comparing against the panel’s datasheet, then confirming with a curve trace or thermal imaging. That work is for a qualified technician. A PV array produces dangerous DC voltage whenever there is light on it and cannot be switched off at the panel, so the DC side is not a place for probing or for opening a combiner.

The design decision: how many groups, and what margin

The number of bypass diodes is a design choice fixed at manufacture, usually three in a 60- or 72-cell panel, one per cell group. More diodes mean finer granularity under partial shade but more components and more junctions in the junction box. The decision has to satisfy two limits: the diode’s reverse voltage rating must exceed the group’s open-circuit voltage at the coldest expected temperature, and its forward current rating must exceed the string current at the highest expected irradiance and temperature.

That cold-morning margin is where designs get caught. A panel rated at 40 volts open circuit at 25 °C rises to roughly 44 volts at −10 °C with a voltage coefficient near −0.3 %/°C (40 × (1 + 0.003 × 35) ≈ 44.2 V). Each diode spans only one cell group, so the reverse voltage it has to block is that group’s share - about a third of the panel figure in a three-diode panel - and the module manufacturer sizes the diode against the cold-weather value, not the 25 °C one. The margin a system designer controls is the string: the inverter’s maximum input voltage must clear the cold-temperature open-circuit voltage of the whole string.

What people get wrong

A common misdiagnosis is to read a one-third loss as shading when the shade has moved on, or to read a persistent loss as a bad panel when the cause is a shorted bypass diode inside the junction box. The mistake is natural because the two produce similar numbers: a shaded group and a bypassed group both remove about a third of the voltage. What separates them is time and pattern. Shading follows the sun and the season; a failed diode does not.

A second mistake is to assume the bypass diode protects against all shade. It protects the cell group it spans, and only when the group is reverse-biased enough to forward-bias the diode. Partial shade across several groups, or shade that covers part of every group, can leave the panel with no clean bypass path and can still produce hot spots. The diode is a coarse protective device, not a shade-tolerant architecture.

A third is to treat the diode as a service item. It is a sealed component, and the work of testing or replacing it belongs to a qualified technician on an isolated, covered panel. Describing the failure is useful; opening a junction box on a live array is not.

Checking the specification

When a design is reviewed, the bypass diode rating is one line in a datasheet that has to be checked against the cold-temperature open-circuit voltage of the cell group, not the panel’s nominal voltage. The same check applies to the string maximum voltage at the inverter. The arithmetic is short:

V_oc_cold = V_oc_stc × (1 + temp_coeff_V × (T_cold − 25))

The output is the open-circuit voltage at the coldest expected cell temperature, which, multiplied by the number of panels in series, is the number the inverter’s maximum input voltage must exceed with margin. A design that uses the 25 °C figure has no margin at all.

V_mp_string = N_panels × V_mp_panel

The output is the string voltage at maximum power, which sets the operating window and must sit inside the inverter’s MPPT range across the expected temperature range, not just at 25 °C.

I_string_max = I_sc_stc × irradiance_factor

The output is the current the bypass diode may have to carry when conducting, which must sit below its forward current rating with margin for the highest expected irradiance.

Local rules and what to check

Interconnection, permitting, and equipment approval are set by state and by utility and they change. The bypass diode rating itself is a manufacturer specification, but the string voltage and current limits it interacts with are enforced at the inverter and at the point of interconnection. What the reader has to check is the local electrical code edition in force, the utility’s interconnection requirements, and the inverter’s datasheet limits for the specific site’s record low temperature. None of those can be assumed from another jurisdiction.