Application Note
Solar Bypass Diodes
T case = TL!
Tcase = TC!
62.5 ±0.5
Ø 8±0.1
Ø 1.6±0.05
„Low Vf“ Bipolar Diodes for Standard Modules
F1200D, FX2000D, FT2000AD/KD
Reverse Voltage up to 200 V
= more rugged against voltage spikes
Lower leakage current than Schottky diodes
= reduced losses in normal mode of operation
Forward losses smaller than for standard rectifiers
= acceptable losses in bypass mode
Schottky Diodes for High Current Modules
SB1240, 15SQ045, SB1540, 20SQ045
SBX2040, SBX2540, SBX3040
SK2045YD2, SK3040CD2
Low Forward Voltage Drop
= reduced losses in bypass mode
Lower Leakage current than “ultra low Vf“ Schottky’s
= acceptable losses in normal mode of operation
High Voltage Diodes for Thin Film Modules
BY880-1000, P2000M, P2000MTL, P2500Y
Reverse Voltage up to 2000 V
= for thin film modules and for blocking operation
Forward current up to 25 Amp
= acceptable losses in normal mode of operation
© Diotec Semiconductor AG
Version 2014-07-01
Application Note
Function of Bypass Diodes
Normal Mode
Bypass Mode
(module partly shaded)
During construction of solar modules, single cells are switched in series to so called “strings” to achieve higher
system voltages, see left picture above.
If one or more cells are shaded (e. g. by branches of trees, antennas, etc), the affected solar cells are no more
acting like a current source, but as power consumers. Non-shaded cells are delivering further current through
them, generating high power losses. “Hot spots” may occur and even cell breakdowns.
To overcome this problem, bypass diodes are switched parallel to every single or some combined cells,
bypassing current flow across the darkened strings, right picture above.
Like every semiconductor device, also bypass diodes have got a certain leakage current, which in normal
mode of operation reduces the current supplied by the cells and therefore decreases efficiency of the solar
module (see red turn-down arrows in the picture above). Therefore leakage current especially at higher
temperatures (full sun irradiation!) should be as low as possible. Compared to that, partly shading of modules
is only an extreme operation mode which should be completely avoided or at least occurs only during short
time periods. For this mode of operation,l low forward losses are desirable. Finally, the bypass diode has to be
rugged against overvoltage spikes. Such spikes may occur during assembly of the system, if e. g. current
conducting cables are interrupted, or during operation, caused by lightning etc.
© Diotec Semiconductor AG
Version 2014-07-01
Application Note
Bypass Diode Tests
Existing standards (e. g. IEC 61730-2, IEC 61215) describe a bypass diode test, applying the module short
circuit current for one hour, at an ambient temperature of 75°C. At this test, the junction temperature of the
diode has to stay below the maximum admissible value. From 2014, IEC 61215-2 describes two procedures to
estimate this junction temperature.
IEC 61215-2, Procedure 1
Uses this formula:
Tj = TL/C + RthL/C * Vf * ISC
junction temperature of the diode, maximum admissible value see datasheet
temperature of contact leads (L) resp. cooling fin of case (C) 1
thermal resistance junction – contact lead (L) resp. cooling fin of case (C)
forward voltage drop across diode
module short circuit current
This procedure requires the correct measurment of temperature and correct usage of thermal resistance
values given in the data sheet1.
IEC 61215-2, Procedure 2
At this procedure, the junction box equipped with diodes has to be placed in a thermal chamber and Vf to be
tested at ISC and several ambient temperatures (30°C, 50°C, 70°C, 90°C). Care has to be taken that Vf is
tested at very short pulses (about 1ms), to avoid self-heating and thus misreading. Furthermore, four-wiresensing (also called Kelvin contacting) has to be used. The resulting function
Tj = f(Vf)
can be extrapolated linearly towards higher temperatures. In a second step, the junction box is kept at 75°C,
and ISC applied for one hour. By measuring the forward voltage drop Vf (Kelvin contacting!), from above
function the according Tj can be derived. This procedure does not require thermal measuring nor thermal
resistance calculations, so avoids a lot of misunderstandings and wrong results. It requires however dedicated
semiconductor test equipment, providing short pulses and Kelvin contacting. We recommend using this
To comply with the bypass diode test, products by Diotec are offering several advantages:
Reduced power losses inside the diode by reduced forward voltage drop Vf. Possible with „Low Vf“-bipolar
diodes as well as Schottky diodes.
The maximum admissible junction temperature is normally based on a DC load of 80% of the maximum
reverse repetitive voltage VRRM. If the actual occuring reverse voltage is much below this value, as typical for
solar modules (e. g < 30% VRRM), a higher Tj can be specified. See parameters in the Diotec datasheets.
At higher currents, the junction temperature can be reduced by suitable cooling measures. Ideally suited in
this case are package outlines like TO-220 und D2PAK, having low thermal resistance junction to case.
Refer to http://diotec.com/tl_files/diotec/files/pdf/service/applications/thermal-measurements-on-bypass-diodes.pdf
© Diotec Semiconductor AG
Version 2014-07-01
Application Note
Standard/High Voltage Rectifier
+ Reverse voltage up to 2000 V
+ Rugged against over voltage
+ Nominal current up to 25 Amp
+ Low leakage current, therefore reduced losses in normal mode of operation
- Higher forward voltage drop (typ. 1.0 Volt), therefore higher losses in bypass operation
Types e. g.
BY255/BY2000 – 3A, BY550 – 5A, BY880 – 8A, P600 – 6A, P1000 – 10A, P2000 – 20A, P2500 – 25A
„Low Vf“ Rectifier
“Low Vf” rectifier by Diotec are bipolar diodes, having also at higher temperatures a low leakage current. By
optimizing the Si-chip it was possible to reduce forward losses compared to standard rectifiers by more than
10%, having positive impact on forward losses.
+ Reverse voltage up to 400 V
+ Rugged against over voltage
+ Nominal current up to 20 Amp
+ Reduced forward voltage drop (typ. 0.8 to 0.9 V), therefore lower losses/lower temperature in bypass
+ Low leakage current, therefore reduced losses in normal mode of operation
Types e. g.
F1200 – 12A, FX2000 – 20A, FT2000 – 20A (heatsink assembly!)
Schottky Rectifier
+ Reverse voltage up to 100 V
+ Nominal current up to 30 Amp
+ Low forward voltage drop (typ. 0.5 to 0.7 Volt), therefore low losses/low temperature in bypass operation
- High leakage current, therefore higher losses in normal mode of operation;
it is recommended not to load the diodes with DC voltage higher than 30% VRRM 2
Types e. g.
80SQ05 – 8A, SB12.. – 12A, 15SQ – 15A, SB15.. – 15A, 20SQ – 20A (Axial lead)
SBX20 – 20A, SBX25 – 25A, SBX30 – 30A (Low Rth Axial lead)
SBT10.. – 10A, SBT18.. – 18A, SBCT20.. – 20A (TO-220)
PPS1045 – 10A, PPS1545 – 15A (Power-SMD)
SK18...D2 – 18A, SK20...CD2 – 20A, SK30...CD2 – 30A (D²PAK)
SK1545YD2 – 15A, SK2045YD2 – 20A, SK2545YD2 – 25A (D²PAK in “Y” configuration)
Datasheets of all mentioned products can be found at www.diotec.com – “Search”
From some manufacturers offered “ultra low Vf” Schottky diodes have got beside a very high hot leakage current also a lower maximum
admissible junction temperature, and are therefore less suited for bypass operations.
© Diotec Semiconductor AG
Version 2014-07-01
Application Note
Customized solutions
Axial bypass diodes are inserted into the junction box with leads bent. Care has to be taken to do this bending
properly, i. e. with strain relief of lead wires during bending3. As a special service, Diotec offers to supply the
leads already bent for direct insertion into the connectors, saving time and effort during assembly, at only low
additional costs. Also possible is the welding of the leads towards metal leadframes; please inform Diotec if
you want to do so, in order that we can offer to you devices having weldable contacts.
Don’t hesitate to contact us for further questions:
Phone +49 (0)7634-5266-0 or [email protected]
Refer to http://diotec.com/tl_files/diotec/files/pdf/service/applications/correct-bending.pdf
© Diotec Semiconductor AG
Version 2014-07-01