PV modules — hotspot effect and bypass diodes under partial shading

The guide on PV string Voc temperature correction covers how the voltage of a series-connected string is sized. This article covers a different mechanism that occurs within that same series chain as soon as one cell receives less light than the rest: the hotspot effect, and the bypass diode that, per IEC 61215 (PV module qualification), is intended to limit that effect.

Why a shaded cell becomes a problem

The cells in a PV module are connected in series: the same current flows through every cell. When one cell is fully or partially shaded (fallen leaves, bird droppings, chimney shadow, a patch of snow), that cell can no longer supply the same current as the unshaded cells next to it. Because the current through the whole series chain is still forced through that one cell, the shaded cell is driven into reverse bias: instead of delivering power, the cell starts to dissipate power generated by the other cells in the string. This can produce a sharply localised temperature rise at that one cell — the hotspot effect — with a risk of discoloration, delamination, melted solder joints, or, in a severe case, ignition of the backsheet.

Function of the bypass diode

To prevent this, a module is internally divided into a small number of substrings (typically two to three substrings of roughly 18 to 24 cells each), with a bypass diode connected across each substring in opposite polarity. Under normal operation each cell is forward biased and the associated bypass diode is reverse biased — the diode does not conduct and has no effect. As soon as one cell in a substring becomes shaded and goes into reverse bias, the voltage across that substring rises until the diode itself becomes forward biased. The diode then starts conducting and offers the current from the rest of the string an alternative path around the shaded substring, limiting the dissipation in the shaded cell to roughly the forward voltage of the diode (typically 0.5–0.7 V) instead of the full substring voltage.

Note: a bypass diode protects per substring, not per individual cell. Within a shaded substring of, say, 20 cells, the one shaded cell can therefore still dissipate a significant share of the power from the other 19 cells in that substring until the diode starts conducting — the bypass diode limits the effect but does not eliminate it entirely.

When the protection itself fails

A bypass diode can fail in two ways, with opposite consequences:

Diode shorted: the substring is permanently bypassed, even without shading. The yield loss is then immediately noticeable and relatively easy to trace — the affected substring structurally stops delivering power.
Diode open: the protection is lost without this being visible under normal, unshaded operation. Only at the next instance of partial shading on that substring does an uncontrolled hotspot develop, precisely at the moment the diode should have intervened. This failure mode is the most dangerous, because the module looks and performs completely normally until that point.

Detection: thermography during operation

In practice, a hotspot is most reliably located with a thermographic camera while the installation is under load and, preferably, while part of the array experiences some shading (early morning, late afternoon, or a period of patchy cloud cover). An I-V curve measurement with a PV I-V curve tracer can confirm a reduced string yield, but does not by itself pinpoint the exact cell or the exact diode failure — that requires the thermographic image.

Practical relevance

During the periodic inspection of a PV installation, it is important to check not only for physical damage to the modules, but also to flag structural, recurring partial shading on the array (a growing tree, a new rooftop obstruction) and, when a string underperforms, to take a thermographic image during partial shading — precisely because a failed-open bypass diode remains invisible in full sun.

Common mistakes

Attributing an I-V curve deviation to "soiling" without verifying thermographically whether a bypass diode has failed open, leaving the underlying defect in place.
Assuming a bypass diode protects per cell, when in reality it operates per substring of typically 18 to 24 cells.
Carrying out thermographic inspection only in full, unshaded sun, which misses a failed-open bypass diode that only reveals itself under partial shading.
Not flagging or removing structural, recurring shading on the array (growing vegetation, new obstructions), which repeatedly drives cells into hotspot conditions and increases the chance of diode failure over time.