What is the difference between bypass diode and anti reverse diode?

1, Working principle: Differentiated application of unidirectional conductivity
Bypass diode: an active defender of the hot spot effect
The bypass diode is connected in reverse parallel to both ends of the photovoltaic module, utilizing the unidirectional conductivity of the diode to achieve thermal spot protection. When the output voltage of a component decreases due to local obstruction, malfunction, or aging, the diode conducts in the forward direction, short circuiting the faulty component and allowing current to bypass the problem area. For example, a certain photovoltaic module is composed of 36 solar cells connected in series. If one of them cannot generate electricity due to shading, its equivalent resistance will suddenly increase, and the total voltage of the series circuit will be concentrated on that cell, resulting in high-temperature heat spots. At this point, the parallel bypass diode conducts, bypassing the faulty battery cell to prevent it from becoming a load and consuming energy from other normal battery cells. At the same time, it prevents the packaging material from deforming or the battery cell from cracking due to excessive component temperature.

Anti reverse diode: a passive blocker of current backflow
Anti reverse diodes are connected in series with photovoltaic strings or DC combiner boxes, utilizing unidirectional conductivity to prevent reverse current flow. Its core functions include:

Anti battery backflow: In an independent photovoltaic system, when the components do not generate electricity at night, the anti reverse diode can block the reverse flow of battery current into the components, avoiding component heating and damage;
Anti string mutual injection: In parallel strings, if a branch experiences a decrease in output voltage due to shadows or faults, the anti reverse diode can block the current from the high voltage branch from flowing back to the low voltage branch, preventing the overall output voltage from dropping. For example, a photovoltaic power station contains 10 strings. If one string experiences a decrease in output voltage due to snow cover and no anti reverse diode is installed, the currents of the other strings will form a circulating current through the faulty string, resulting in a loss of system efficiency; After installing anti reverse diodes, the faulty string is isolated and the system output voltage remains stable.
2, Core role: Differentiated division of labor for functional positioning
Bypass diode: dual guarantee of efficiency and safety
The core value of bypass diodes lies in maintaining system power generation efficiency and component safety. Experimental data shows that components without bypass diodes can reduce output power by 30% -50% when partially obstructed, and the hot spot temperature can reach over 150 ℃, seriously threatening the lifespan of the components; After configuring the bypass diode, the power loss can be controlled within 5%, and the hot spot temperature can be reduced to below 80 ℃. In addition, bypass diodes can reduce system downtime caused by component failures and improve operational efficiency.

Anti reverse diode: the cornerstone of system stability
The core function of anti reverse diodes is to maintain system voltage stability and energy utilization efficiency. In large photovoltaic power plants, voltage differences between strings may cause current backflow, leading to the following issues:

Energy loss: Reverse current consumes effective power generation, reducing overall system efficiency;
Equipment damage: Long term reverse current may cause component heating, junction box burnout, and even fire;
Monitoring failure: Current backflow can interfere with the monitoring system's accurate judgment of component status, increasing the difficulty of operation and maintenance.
By installing anti reverse diodes, reverse current can be effectively blocked, ensuring that the system voltage remains stable within the design range and improving energy transfer efficiency and equipment reliability.
3, Installation location: Differentiated layout of circuit topology
Bypass diode: component level protection
The bypass diode is usually built into the junction box of the photovoltaic module and connected in reverse parallel with the battery pack. Depending on the component design, each component can be configured with 1-3 bypass diodes. For example, a component of 60 battery cells may use 2 bypass diodes, each diode protecting 30 battery cells; If 3 diodes are used, each protecting 20 battery cells can isolate the fault area more finely and reduce the power loss of normal battery cells.

Anti reverse diode: system level protection
Anti reverse diodes are usually installed at the input of DC combiner boxes or inverters, connected in series in the output circuit of the string. In large power plants, each string output terminal may be equipped with one anti reverse diode; In the combiner box, multiple strings may share one anti reverse diode module after converging to reduce costs and space occupation. For example, a 1MW photovoltaic power station contains 20 50kW strings, and 4 anti reverse diode modules can be configured in the combiner box, each module protecting 5 strings.

4, Selection criteria: Differentiated requirements for parameter matching
Bypass diode: Voltage resistance, current and thermal performance are key factors
The selection of bypass diodes must meet the following parameter requirements:

Reverse withstand voltage: It needs to be greater than 1.5 times the open circuit voltage of the component. For example, if the open circuit voltage of a component is 45V, the reverse withstand voltage of the bypass diode needs to be ≥ 67.5V;
Forward current: It should be greater than 1.2 times the short-circuit current of the component. For example, if the short-circuit current of a component is 9A, the forward current of the bypass diode needs to be ≥ 10.8A;
Thermal resistance and junction temperature: It is necessary to consider the high temperature environment inside the component (usually 20-30 ℃ higher than the ambient temperature), and choose diodes with low thermal resistance and high junction temperature. For example, if the internal temperature of a component can reach 85 ℃, the diode junction temperature needs to be ≥ 125 ℃;
Pressure drop: The lower the pressure drop, the smaller the power loss. Schottky diodes are commonly used in low-power components due to voltage drop (0.2-0.3V), while silicon rectifier diodes (voltage drop 0.7-1V) are suitable for high-power components.
Anti reverse diode: Voltage resistance, current and heat dissipation are the core
The selection of anti reverse diodes must meet the following parameter requirements:

Reverse withstand voltage: It needs to be greater than twice the maximum operating voltage of the system. For example, if the maximum operating voltage of a system is 1000V, the reverse withstand voltage of the anti reverse diode needs to be ≥ 2000V;
Forward current: It needs to be greater than 1.5 times the maximum output current of the string. For example, if the maximum output current of a certain string is 12A, the forward current of the anti reverse diode needs to be ≥ 18A;
Heat dissipation design: It is necessary to consider the high temperature environment inside the combiner box (usually up to 60-80 ℃), and choose modules with low thermal resistance and heat sinks. For example, the thermal resistance of a certain anti reverse diode module is 0.5 ℃/W, which can effectively reduce the junction temperature;
Voltage drop and power consumption: The lower the voltage drop, the higher the system efficiency. The voltage drop of photovoltaic dedicated anti reverse diode modules can be as low as 1.0-1.5V, reducing power consumption by 20% -30% compared to ordinary modules (1.5-2V).
5, Practical application case: typical manifestation of collaborative value
Case 1: Optimization of bypass diodes in a 50MW photovoltaic power station
The power station was originally designed with one bypass diode per component, but it was later discovered that the power loss still reached 15% when partially obstructed. By adopting a scheme of using 3 bypass diodes per component, the power loss is reduced to below 5%, and the annual power generation is increased by about 2%. At the same time, the hot spot temperature decreased from 120 ℃ to 70 ℃, and the component failure rate decreased by 40%.

Case 2: Anti reverse diode retrofit of a 10MW photovoltaic power station
The power station was not originally equipped with anti reverse diodes, and the current backflow between the series resulted in an 8% loss of system efficiency, and 3-5 junction box burnout accidents occur every year. By installing photovoltaic specific anti reverse diode modules in the combiner box, the system efficiency is improved by 5%, the junction box failure rate is reduced to zero, and the operation and maintenance costs are reduced by 30%.