The destructive effects of hot spots can be avoided by using a bypass diode. The bypass diode is connected in parallel with the solar cell, but the polarity is opposite. Under normal operation, each solar cell is positively biased, so the bypass diode will be reverse biased, which is actually an open circuit. However, if a solar cell (single crystal, polycrystalline, Sunpower) has a reverse bias due to the mismatch of the short-circuit current between several solar cells in series, the bypass diode will be turned on, so that the current of good solar cells flows in the external circuit, rather than the forward bias of each good solar cell. The maximum reverse bias voltage on poor quality solar cells is reduced to about one diode voltage drop, limiting current and preventing hot spot overheating.
The influence of bypass diode on the IV curve can be determined by first finding the IV curve of a single solar cell with bypass diode, and then combining this curve with the IV curve of other solar cells. The bypass diode only affects the solar cell under the reverse bias voltage. If the reverse bias is greater than the voltage of the solar cell, the diode will turn on and conduct current.
However, in practice, one bypass diode per solar cell is usually too expensive. On the contrary, the bypass diode is usually placed between groups of solar cells. The voltage on shadow or low current solar cells is equal to the forward bias voltage of other series cells sharing the same bypass diode plus the voltage of the bypass diode. The voltage on the unshielded solar cell depends on the degree of shielding of the low current battery. For example, if the solar cell is completely shielded, the uncovered solar cell will be positively biased by its short-circuit current, and the voltage will be about 0.6V.
If only part of the poor solar cell is shielded, then part of the current from the good solar cell can flow through the circuit, and the rest is used to conduct positive bias on each solar cell junction, resulting in a decrease in the positive bias on each cell. The maximum power dissipation of shadow solar cells is approximately equal to the generating capacity of all solar cells in the group. For silicon cells, the maximum number of groups of each diode is about 15 cells/bypass diodes without damage. Therefore, for a normal 36 solar panels, 2 bypass diodes need to be used to ensure that the solar panels are not vulnerable to “hot spot” damage.
If you want to know more about the solar charging panel, please contact us, the solar panel manufacturer – Huanqi Technology (Shenzhen) Co., Ltd.