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Posted on 02 July 2019

Smart Bypass Diode Paves the Way for Electronics in Solar Panels

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Reducing power dissipation by more than 80% of a Schottky diode enables smaller junction boxes and the use of electronics inside the new generation of highly efficient solar panels.

By Chris Goeltner, Texas Instruments ti_cgoeltner@list.ti.com

Abstract

Increasing efficiency of photovoltaic (PV) panels is the most effective effort to achieve cost reduction when designing solar panels. Bypass diodes protect each of the three substrings in a PV panel to prevent hot-spots in the event of partial shading. Traditionally, Schottky diodes have been used for this purpose. They operate at a high voltage drop of typically 0.5 – 0.7 V, dissipating up to 7W per diode under sunny conditions. Large heat-sinks inside the plastic junction box cool temperatures to acceptable limits for the guaranteed service life of 25 years. The new generation of PV panels is increasing conversion efficiency and operates at even higher currents, pushing the limits of the thermal balance in the panel. “Active diodes” respond to the industry requirements to reduce losses in the panel and lower operating temperatures. This article describes the functionality of a smart bypass diode, presents field data about the thermal conditions inside a junction box, and quantifies the potential for increasing energy yield in PV systems.

Bypass diodes in solar panels

A typical crystalline Silicon (c-Si) photovoltaic (PV) module consists of 60 to 72 individual PV cells connected in series. A bypass diode is required for every 20-24 cells to avoid hot-spot conditions and damage to the cell. When a panel in a string is completely or partially shaded, the shaded solar cells operate in reverse mode, effectively becoming resistors. At the same time the string current is being pushed through the reverse cells, which eventually will damage the cell and possibly cause fire.

Bypass diodes in a PV panel junction box

Figure 1 shows the arrangement of three bypass diodes in a solar panel, providing an alternative path for each of the three substrings in the PV panel. The bypass diodes are located in a junction box together with DC cabling that connects the PV module to the array.

Bypass diodes are used in all c-Si panels, comprising more than 85% of the PV market. In 2013 close to 500 Million diodes are being sold to the PV industry. Due to the implication of safe operation, their use is required by all certification agencies such as the International Electrotechnical Commission (IEC), Underwriters Laboratories (UL), Technischer Überwachungsverein (TUV) and others.

Power dissipation comparison at 85?C ambient temperature

Market drivers

With the increase in PV panel efficiency, and the industry’s move to use the larger 6-inch instead of 5-inch solar cells, modern PV panels provide substantially higher currents than first generation products. Today panels generate currents under standard test conditions (1000W/m2, 25°C) of close to 10 A. Real world conditions can result in substantially higher currents. UL and IEC standards require panels to be tested at 25% higher currents than standard test conditions.

The first generation of junction boxes used axial lead diodes. Conducting excess heat generated in the diode through thin metal leads was not sufficient at higher currents. As a next step, the industry adopted a surface-mounted device (SMD) type of diode, exposing a much larger contact area to the heat-sink.

A new generation of active or “smart” diodes is needed to enable further efficiency increases in PV panels. An example of a “smart bypass diode” is the SM74611, which offers a drop-in replacement for surface-mounted Schottky diodes with one order of magnitude improved power dissipation. The SM74611 uses a MOSFET plus control logic to substitute the electrical switching behavior of a standard diode.

For junction boxes using Schottky diodes, the main benefits are:

  • Lower power dissipation of 80% or more in forward mode (25 mV vs. 0.5V) (Figure 2)
  • Less leakage current in reverse mode (<0.001 mA vs. 0.5 – 50 mA)
  • Plug-in replacement for existing Schottky diodes (D2Pak SMD package)

Functional elements and basic operation

Figure 3 shows the main functional components of the smart bypass diode. The control logic and MOSFET are two separate integrated circuits (ICs) co-packaged in a multi-chip module. The main functional elements of the device are: charge pump, voltage detector, MOSFET driver, MOSFET and capacitor.

Block diagram of active bypass device and functional equivalent

Smart bypass diodes encounter two operating conditions: forward and reverse mode. Under typical conditions there is no shade on the panel. The voltage on the cathode is higher than the anode, the body diode of the MOSFET is in reverse mode, and the SM74611 is not activated. All three panel substrings contribute to power generation.

When the entire panel or one substring is shaded, the affected cells operate in the negative voltage region causing the respective bypass diode to switch into forward mode. The anode voltage is higher than the cathode, causing the MOSFET’s body diode to conduct. Its voltage drop powers up the control circuitry, and the charge pump starts energizing the capacitor. Once the charge pump has fully energized the capacitor, the MOSFET is turned on. Operation of the MOSFET causes the capacitor to gradually discharge its energy. The cycle repeats when the energy in the capacitor goes below the pre-configured level, then the charge pump is re-activated.

When the bypass diode operates in forward mode, the affected cells and panel substring(s) are not contributing useable energy.

Main applications

Figure 4 shows the temperature comparison between two identical PV panels, one with a standard Schottky diode, the other with a smart bypass diode. Under full-shading conditions, for example, if all three diodes are in forward mode, during moderate summer conditions in Northern California the temperature advantage of the smart bypass diode inside the junction box averages at 50°C. In hotter climates the thermal advantage will be even more pronounced.

Reduced power dissipation gives junction box manufacturers the opportunity to design smaller sized junction boxes that are cheaper and easier to seal against the backside of the panel. Manufacturers with highly automated production lines can replace Schottky diodes and increase the usable current ratings without any changes in the production process with a smart bypass diode, such as the SM74611, in the same package.

Distributed electronic devices in the junction box are experiencing fast growth, adding functionality and new services to the PV system. Typical examples include DC/AC microinverters, DC/DC power optimizer, monitoring and safety shutdown solutions. Due to the high temperatures created by Schottky diodes, integrating the electronics inside a junction box is not practical. Smart bypass diodes with improved thermal conditions allow for advanced compact and integrated electronics solutions to be designed inside the junction box (Figure 4).

Temperature inside junction box with three bypass diodes activated

Energy advantages

During most of its operational life a PV panel is exposed to the sun, and the bypass diode is operating in reverse-mode. A MOSFET based device experiences leakage currents of 0.3 μA, versus up to 100 μA in the Schottky diodes. However, the practical energy gain for active diodes is very small – at only 0.01% of the total energy produced.

Nevertheless, even the best PV array is exposed to some degree of shading, causing some of the bypass diodes to operate in forwardmode. It is this case where smart bypass diodes achieve some noticeable energy gains due to their 80% power advantage. In residential and commercial systems average shading conditions of up to 10% are quite common and acceptable. Examples include shade caused by neighboring buildings, trees, chimneys, and a variety of other things. A PV array with 10% shading will experience up to 0.5% increased energy yield when active bypass devices are used.

Conclusions and outlook

Smart bypass diodes approach “ideal diode” behavior with an order of magnitude performance increase. High-performance panels and integrated electronics will be the first adopters of this new technology, before it will be used in the mass market. The smart bypass diode is the first type of electronic circuitry used in PV panels, opening the path for additional electronic solutions to improve energy yield, new features and services. The energy advantages of the smart bypass diode are noticeable and offer potential for a growing array of opportunities in other markets.

References
“Influence of the shadows in photovoltaic systems with different configurations of bypass diodes,” E. Díaz-Dorado, Department of Electrical Engineering, University of Vigo, ETSEI, Spain, SPEEDAM 2010, International Symposium on Power Electronics
“Partially shaded operation of a grid-tied PV system,” C. Deline, National Renewable Energy Laboratory, presented at the 34th IEEE Photovoltaic Specialists Conference, Philadelphia, Pennsylvania, June 7–12, 2009
Download a datasheet for the SM74611: www.ti.com/sm74611-ca.
For more information on solar solutions, visit: www.ti.com/solar-ca.

About the Author
Christoph Goeltner is the Solar IC Marketing Manager at Texas Instruments where he is responsible for solar ICs and voltage references. He has over seven years of experience working in the PV industry. Chris received his Ph.D. from the Massachusetts Institute of Technology. He can be reached at ti_cgoeltner@list.ti.com.

 

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