EIC Diodes in Solar photovoltaic (PV) Systems Diodes and PV cells are both devices made from P-type and N-type silicon that allow electrical current to flow in one direction only. Low Vf Rectifiers F1200 SMC1200 Silicon PV cells react to sunlight, causing electrons to flow into the cell, whilst current flow in the reverse direction is prevented using the electrostatic field at the PN junction. FT2000 FTB2000 SMC2000 MUR1520 Problems can arise when the PV modules are connected in series, or if part of the cell array is shaded. In these cases there is the need for external diodes that play two important roles. MURB1520 MUR1520S MURB1520S SMC1520 P600J BYPASS DIODES Schottky Diodes MBR1645 MBR2045 MBR2545 MBRB2545 (dual) MBRB4030 SD945 SD1545 SD1645 Here a diode is fitted across each panel (in parallel) in a multi panel set up. When one of the panels in the system is damaged, weak, or (most likely) in shade, then there is the risk that full current from the rest of the panels will pass through the shaded panel and cause overheating damage. In this case the bypass diode conducts, thereby allowing the current from the good solar cells to flow in the external circuit rather than forward biasing each good cell. The maximum reverse bias across the poor cell is reduced to about a single diode drop, thus limiting the current and preventing hot-spot heating. SD2045 BLOCKING DIODES It is often recommended that a diode be fitted to prevent reverse current flow back through a solar panel at night time. This occurs because in battery charging systems, the module potential drops to zero at night, and the battery could discharge all night backwards through the module. This would not be harmful to the module, but would result in loss of precious energy from the battery bank. A blocking diode is not necessary if a charge controller is being used in the system. Selecting the correct diode Critical to any PV system is overall efficiency. Solar panels usually have an efficiency rating somewhere between 40% and 60%. To achieve this, losses in every single device must be minimised. For the diodes, this is particularly important at times of peak sunshine when the device is blocking current flow, and therefore a low leakage part is essential. Additionally when the part is active as a bypass circuit, low forward losses are also required. A fairly obvious requirement is that the part is able to operate at high temperatures. Diodes are tested at an ambient temperature of 75oC, for at least one hour, to be considered suitable for these applications. EIC has a range of diodes that are ideally suited to solar applications because they have: * Overvoltage protection * High forward surge capability * High frequency operation * High efficiency * Lower power losses * Low forward voltage drop * Packaging to assist low junction temperatures (D2Pak and T0-220) * Space saving SMC package option Finally the diode must be able to withstand power surges; such as those caused by lightening or when cables are disconnected. Type No. Max. Average Forward Rectified Current IF(A V) @ T a (A) (°C) Max. Max. Repetitive Repetitive Max. Peak Peak Peak Forward Reverse Forward Surge Voltage Current Current Max. Forward Voltage D rop V RR M (V) IFR M (A) IFSM (A) at Ta=25°C VF @ IF (V) (A) Max. Reverse Current Package Max. Reverse Outline Recovery Time at Ta=25°C IR (µA) T rr (ns) Low Vf Rectifiers P600J 6 60 600 - 400 0.9 6 5 - D6 F1200D 12 50 200 80 390 0.85 5 25 200 D6 SMC1200D 12 50 200 80 390 0.85 5 25 200 SMC FT2000AD/KD 20 100(Tc) 200 80 390 0.84 5 25 200 TO-220AC FTB2000AD/KD 20 100(Tc) 200 80 390 0.84 5 25 200 D PAK SMC2000D 20 100(Tc) 200 80 390 0.84 5 25 200 SMC MUR1520/S 15 150(Tc) 200 - 200 1.05 15 10 35 TO-220AC MURB1520/S 15 150(Tc) 200 - 200 1.05 15 10 35 D PAK SMC1520 15 150(Tc) 200 - 200 1.05 15 10 35 SMC 2 2 Schottky Rectifiers MBR2045C T 20 135(Tc) 45 - 150 0.57 10 0.1mA - TO-220AB MBR2545C T 30 130(Tc) 45 30 150 0.82 30 0.2mA - TO-220AB MBRB2545CT 30 130(Tc) 45 - 150 0.82 30 0.2mA - D PAK MBRB4030 40 115(Tc) 30 - 300 0.55 40 0.35 - D PAK SD1545 15 25(Tc) 45 - MBR1645 16 25 45 SD1645 16 25 45 SD2045 20 25(Tc) 45 - 2 2 300 0.54 15 0.2mA - DO-201 300 0.55 16 0.2mA - TO-220AB 300 0.55 16 0.2mA - DO-201 410 0.55 20 0.26mA - D6