How to use SBR to improve power conversion efficiency

    Maximizing power efficiency is important for the design of nearly any electronic or electrical system. In mobile devices, better power efficiency provides longer battery life, which is a key selling point. For essentially any application, improved energy efficiency means that size and weight can be reduced, and thermal management can be simplified—thus reducing costs. Of course, using less power also helps the product meet energy class requirements, which may be a legal requirement as well as an attractive feature for customers.

    In many products, the biggest gains in energy efficiency can be achieved in the power supplies or voltage converters used. Therefore, over the years, suppliers of power semiconductor devices have been working to gradually improve energy efficiency.

    Now there is a device, based on a proprietary deep trench process, that enables power supplies to significantly improve their performance and efficiency: the Super Barrier Rectifier (SBR®). It can be used in exactly the same way as a standard Schottky diode in a wide range of applications – from automotive lighting to renewable energy and consumer products.

    In this article, we will introduce SBR, explain its characteristics, and discuss how it can be used in an example application of automotive daytime running lights.

    SBR Technology Explained

    SBR is a proprietary and patented technology developed by Diodes Incorporated that is manufactured using a Metal Oxide Semiconductor (MOS) manufacturing process as opposed to the bipolar process used for traditional Schottky diodes.

    The presence of the MOS channel forms a low barrier for majority carriers, resulting in forward bias performance similar to that of a Schottky diode at low voltages. However, the leakage current is much lower due to the overlapping PN depletion layers and the absence of barrier lowering.

    The SBR is represented by the same electronic schematic symbol as a Schottky diode. In reality, the internal structure is like a MOSFET with the gate and source terminals connected together to form the SBR anode terminal. The MOSFET drain acts as the SBR cathode.

    In addition to excellent temperature stability and avalanche capability, the SBR exhibits lower leakage and behaves like a diode in any circuit, so it can be used as a drop-in replacement for similar Schottky devices.

    This means that SBRs can immediately improve efficiency and reduce device case temperature, simplifying thermal management and improving reliability without redesigning the PCB or adding additional components. In addition, SBRs have high surge current ratings to withstand hazards such as unpredictable power flow and lightning strikes, and have the thermal stability and high reliability characteristics of PN epitaxial diodes.

    Improving efficiency with SBR

    In buck-boost converters, Schottky diodes are often chosen as the most efficient option because they have a lower forward voltage drop (VF) and faster switching capability than conventional rectifier diodes. On the other hand, their reverse leakage current is relatively high and increases with temperature.

    While the SBR behaves similarly to a Schottky diode, the SBR offers higher efficiency when used in switching converters. Its construction means that the forward voltage and reverse recovery time are comparable, but the leakage current is much lower and less temperature dependent. Avalanche capability is also significantly improved, resulting in improved ruggedness.

    Table 1 compares the key parameters that control the freewheeling performance of the SBR and a typical Schottky diode with similar reverse voltage and current ratings. You can see that the leakage current is much lower at 1.7 µA compared to 18 µA at 85°C, and the Schottky diode also shows a greater increase in leakage current at the higher temperature of 125°C.

    Example Application: Automotive Lighting

    LED-based exterior lighting is quickly becoming the standard for automotive applications, due in large part to its lower power consumption.

    The industry is constantly looking to further improve the efficiency of LED lighting systems, particularly daytime running lights (DRLs), which stay on continuously when the car is in use, primarily as a safety feature.

    As an “always on” feature, one way to improve LED DRL is to increase the efficiency of the power conversion that occurs in the LED driver/controller circuit. Buck-boost topologies are commonly used in many automotive applications to provide the DC-DC conversion, including the required drive voltage for the LEDs.

    Figure 1 shows a simplified circuit using the ZXLD1371 buck-boost LED driver/controller from Diodes Incorporated. This is a generic circuit that typically contains a switching MOSFET (Q1) and a freewheeling diode (D1).

    Figure 1. Simplified schematic of a buck-boost LED driver for DRL applications

    Since this is a boost converter, the peak current of the MOSFET and freewheeling diode is much larger than the average current of the LED. This means that the conduction and switching losses of these two components have a significant impact on the overall power consumption of the converter.

    Higher efficiency, cooler operation

    Table 1 compares SBRs and Schottky diodes in the same buck-boost DRL supply controlled by the ZXLD1371 LED driver/controller, as shown in Figure 1. In particular, it shows how the leakage current of the Schottky diode is higher and increases with temperature. This results in a significant efficiency advantage for the SBR. This increases at higher ambient temperatures, where the Schottky circuit efficiency can drop by up to 6%, as shown in Figures 2 and 3.

    Figure 2. Efficiency comparison at 25 ° C ambient temperature

    Figure 3. Efficiency comparison at 85 ° C ambient temperature

    Plotting the efficiency of both circuits versus ambient temperature (Figure 4) shows that efficiency decreases with temperature. This is due to a combination of increased diode VF, leakage current and switching losses, and overall system losses. The superior temperature stability of the SBR minimizes this efficiency reduction compared to the circuit using Schottky diodes.

    Figure 4. SBR efficiency advantage is greater at higher ambient temperatures

    The superior efficiency of SBRs saves energy and reduces device operating temperature. Figure 5 shows how the SBR case temperature is consistently approximately 5°C cooler than a Schottky diode over the entire ambient temperature range. In our DRL LED lighting application example, this lower temperature provides designers with greater flexibility to manage heat sink size and cost while also achieving the desired system reliability.

    Figure 5. Lower SBR case temperature simplifies thermal management and reliability design

    Wide range of applications

    Diodes offers SBRs in a wide range of voltage ratings and package styles, making them ideal for many applications in industrial, consumer electronics, communications, and computer systems, including DC-DC conversion, battery chargers, and reverse polarity protection.

    Devices with higher voltage ratings (up to 300V) are available for applications such as switch mode power supplies (SMPS) and solar inverters. For automotive, the Q-series SBRs, including the SBR10M100P5Q, are optimized for specific application requirements, meet the AECQ101 high reliability automotive standard, and are supported by PPAP Level 3 documentation.

    Another application that can benefit from the efficiency of SBRs is solar panels, where they can replace bypass diodes. Their extremely low VF minimizes temperature rise, which helps improve system reliability, and SBRs have a wide operating temperature window, ensuring compliance with the solar industry safety standard IEC 61730-2.

    in conclusion

    The SBR is a true innovation in power semiconductor design, enabling a step change in power conversion performance and higher efficiency. Compared to Schottky diodes, it offers lower leakage current, higher switching performance, comparable or lower VF, and excellent temperature stability with the added benefit of lower operating temperature.

    It can be used as a drop-in replacement without redesign, thus reducing time to market. SBR provides superior performance and reliability, enabling power converters to meet the latest eco-design goals and safety standards in a wide range of applications.