Bypass diodes are a critical safety and performance component in solar modules. When cells are shaded or damaged, they can become reverse-biased and generate heat, a phenomenon known as hot-spotting that can destroy cells and even cause fires. Bypass diodes protect against this by providing an alternative current path. The IEC 61215 MQT 18 bypass diode testing programme verifies both the thermal design adequacy and the functional integrity of these diodes. Here is a complete breakdown.
What is MQT 18 Bypass Diode Testing?
MQT 18, defined in IEC 61215-2:2021, consists of two separate but related sub-tests:
• MQT 18.1 — Bypass diode thermal test
• MQT 18.2 — Bypass diode functionality test
Note: For PV modules without bypass diodes, MQT 18.2 can be omitted.
MQT 18.1 — Bypass Diode Thermal Test
Purpose
To assess the adequacy of the thermal design and relative long-term reliability of the bypass diodes used to limit the detrimental effects of module hot-spot susceptibility.
The test determines the diode's temperature characteristic and its maximum diode junction temperature T_J under continuous operation.
Test Sample Selection
If the module contains three or fewer bypass diodes, all diodes shall be tested. If the module contains more than three diodes, three bypass diodes are selected for testing by the test laboratory, those representative of the bypass diodes subject to the most stress in the design. The test lab must document which three were selected and why.
Selection criteria:
For more than three diodes embedded in a laminate:
1. Closest to the centre of the junction box (may be underneath the junction box)
2. Closest to the module frame (or module edge, for frameless modules)
3. Closest to the module centre
For more than three diodes in a junction box:
1. Closest to the centre of the junction box
2. Next closest to the centre of the junction box
3. Closest to the edge of the junction box
If the bypass diodes are not accessible in the module type under test, a special sample can be prepared. This sample must provide the same thermal environment for the diode as a standard production module but does not need to be an active PV module. The special test sample shall be used only for measuring the bypass diode temperature and shall allow electrical access to the three bypass diodes in locations complying with the above selection criteria.
Apparatus
• Means for heating the module up to a temperature of (90 ± 5) °C
• Temperature monitoring to an accuracy of ±2.0 °C and repeatability of ±0.5 °C
• Means for measuring the junction voltage V_D of the bypass diodes to an accuracy of 2%
• Means for applying a pulsed current equal to 1.25 times the STC short-circuit current of the module, with a pulse width not exceeding 1 ms, and means for monitoring the flow of current through the module throughout the test
Procedure
Step 1 — Preparation
Electrically short any blocking diodes incorporated in the module. Determine the rated STC short-circuit current of the module from its label or instruction sheet.
Step 2 — Connect lead wires
Connect the lead wire for V_D and I_D on both diode terminals. If the diodes are potted, connections shall be made by the module manufacturer before delivery. Lead wires must not cause heat dissipation from the terminal box, current connections should be made as far as possible from the terminal box, and voltage probes made as small and thin as possible.
Step 3 — Establish V_D versus T_J characteristic
Heat the module and junction box up to (30 ± 2) °C. Apply the pulsed current (pulse width 1 ms) equal to the STC short-circuit current of the module, and measure the forward voltage V_D1.
Repeat the measurement at:
• (50 ± 2) °C → measure V_D2
• (70 ± 2) °C → measure V_D3
• (90 ± 2) °C → measure V_D4
From these four measurements, obtain the V_D versus T_J characteristic by a least-squares-fit curve. T_J is assumed to be the ambient temperature of the junction box for these steps.
Step 4 — Measure at continuous operating temperature
Heat the module to (75 ± 5) °C. Apply a current equal to the short-circuit current I_sc ± 2% as determined earlier. After 1 hour, measure the forward voltage of each of the selected diodes.
Note: If the module contains a heat sink specifically designed to reduce the operating temperature of the diode, this test may be performed at the temperature the heat sink reaches under conditions of 1 000 W/m², (43 ± 3) °C ambient with no wind, rather than at 75 °C.
Step 5 — Obtain T_J at operating conditions
Using the V_D versus T_J characteristic obtained above, obtain T_J from V_D at T_amb = 75 °C, with I_D = I_sc of the diode during the test.
Step 6 — Apply 1.25× current stress
Increase the applied current to 1.25 times the STC short-circuit current while maintaining the module temperature at (75 ± 5) °C. Maintain the current flow for 1 hour.
Final Measurements
After the thermal test, repeat:
• MQT 01 — Visual inspection
• MQT 15 — Wet leakage current test
• MQT 18.2 — Bypass diode functionality test
Pass/Fail Criteria
1. The diode junction temperature T_J shall not exceed the diode manufacturer's maximum junction temperature rating for continuous operation.
2. No evidence of major visual defects, as defined in IEC 61215-1:2021.
3. Wet leakage current must meet the same requirements as for the initial measurements.
4. The diode shall still function as a diode after the conclusion of the test, as verified by MQT 18.2.
MQT 18.2 — Bypass Diode Functionality Test
Purpose
To verify that the bypass diode(s) of the test samples remain(s) functional following MQT 09 and MQT 18.1.
This test can be omitted for PV modules without bypass diodes.
Apparatus
Means for measuring the current-voltage (I-V) curve within 1 second, for example, an I-V curve tracer, with an accuracy of ±1% of the open-circuit voltage and short-circuit current measurements respectively.
Procedure — Method A
Conduct within an ambient temperature of (25 ± 10) °C. The module shall not be subjected to illumination during the test.
1. Electrically short any blocking diodes. Note that some modules have overlapping bypass diode circuits — in this case it may be necessary to install a jumper cable to ensure all current flows through one bypass diode at a time.
2. Determine the rated STC short-circuit current from the module's nameplate. For bifacial modules, use the I_sc value measured at STC.
3. Connect the DC power source / I-V curve tracer's positive output to the test sample's negative terminal, and its negative output to the test sample's positive terminal. With this configuration, current passes through the solar cells in the reverse direction and through the bypass diode(s) in the forward direction.
4. Run a current sweep from 0 A to 1.25 × I_sc and record voltage.
Procedure — Method B
Successive I-V measurements of the PV module can be performed in conjunction with maximum power determination (MQT 02), with portions of a string in the interconnection circuit completely shadowed in order to turn on the diode. This procedure should be repeated for each substring.
Pass/Fail Criteria
Method A:
In the current sweep, identify the largest current at which the forward voltage is specified on the data sheet. The diode(s) forward voltage measured at the identified current is defined as VFM and must meet:
VFM = (N × V_FMrated) ± 10%
where N is the number of bypass diodes and V_FMrated is the diode forward voltage as defined in the diode data sheet for 25 °C.
Method B:
The bypass diode is working properly if the characteristic bend in the I-V curve is observed. Example: A crystalline silicon PV module with 60 cells and three strings protected each by one diode will have a power drop to roughly 2/3 if cells in one string are shadowed.
Why MQT 18 Matters for PV Module Certification
Bypass diode failures are one of the more insidious quality issues in solar modules — they often go undetected until a shading event triggers hot-spot heating, by which point cell damage may already be irreversible. A diode that operates too hot under continuous current stress will degrade faster and fail sooner, dramatically shortening effective module lifetime.
MQT 18.1 ensures the thermal design of the junction box and diode mounting is adequate for continuous operation. MQT 18.2 confirms the diodes are still electrically functional after the stress tests. Together they give buyers, insurers, and project developers confidence that the module's protection circuitry will work reliably when it matters most — under partial shading conditions in the field.
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Source: IEC 61215-2:2021 — Terrestrial photovoltaic (PV) modules – Design qualification and type approval – Part 2: Test procedures, Section 4.18