Super Barrier Rectifiers Deliver Design-Free Efficiency

Successive generations of power-semiconductor devices based on existing technologies strive to deliver incremental improvements in energy efficiency. Now there is a device, realized through proprietary technology, that enables power supplies to achieve a significant leap forward in performance and efficiency—the Super Barrier Rectifier (SBR).

Maximising power efficiency has become a key concern for designers of almost all types of electrical and electronic systems, such as mobile and smart appliances, automotive electronics, building automation, and data centres. In addition to improving energy ratings, greater power efficiency can also allow simplified thermal management, reduced size and weight, and longer battery runtime.

Focus on Automotive Lighting

The automotive sector is experiencing a wholesale shift towards LED-based external lighting, not least because it can deliver a reduction in electrical power consumption in vehicles. With the increased awareness of the need for energy efficiency, hybrid and electronic vehicle characteristics as well as the subsequent link between electronic power and fuel economy--or driving range--is becoming more widely understood.

To encourage even wider market appeal, the industry is constantly seeking to further improve the efficiency of LED lighting systems and, in particular, Daytime Running Lamps (DRLs). As DRLs remain on continuously while the car is running, predominantly as a safety feature, they have also come to define the signature look of certain models and brands. As an ‘always on’ feature, one way to improve LED DRLs is to tackle the efficiency of power-conversion that takes place in the LED driver/controller circuitry.

A buck-boost topology is typically used in automotive applications to provide DC-DC conversion for various applications, including the drive voltage required for the LED string. Figure 1 shows a simplified circuit featuring the ZXLD1371 buck-boost LED driver/controller from Diodes Incorporated. This is a generic circuit that normally contains a switching MOSFET (Q1) and a freewheeling diode (D1).

Because this is a boost converter, the peak current in the MOSFET and freewheeling diode is much greater than the average LED current, hence the conduction and switching losses of these two components can have a significant impact on the overall converter power consumption.

Historically, Schottky diodes have been selected as the most efficient option due to their lower forward voltage drop (V_F) and faster switching capability compared to conventional rectifier diodes; however, reverse leakage current is relatively high and increases with temperature.

While the Super Barrier Rectifier (SBR) behaves like a Schottky diode, the SBR delivers higher efficiency when used in switching converters, and although its construction means that the forward voltage and reverse recovery time are comparable, leakage current is much lower and more stable with increases in temperature. The avalanche capability is also significantly higher, leading to greater ruggedness. Table 1 compares the key parameters that govern freewheeling performance for an SBR and Schottky diode with similar reverse-voltage and current ratings.

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Table 1: Comparing the Diodes Incorporated SBR with a Typical Schottky Diode

SBR Under the Skin

SBR is a proprietary and patented Diodes Incorporated technology fabricated using a Metal Oxide Semiconductor (MOS) manufacturing process. The presence of the MOS channel forms a low potential barrier for majority carriers, resulting in forward-bias performance similar to that of the Schottky diode at low voltages. However, the leakage current is much lower due to overlapping P-N depletion layers and the absence of potential barrier reduction.

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

Other than displaying lower leakage with superior temperature stability and avalanche capability, an SBR behaves like a diode in any circuit, and as such it is a drop-in replacement for comparable Schottky devices. Without needing to redesign a PCB or add additional components, the SBR delivers immediate improvements in efficiency and a reduction in device case temperature that enables simplified thermal management and greater reliability.

Higher Efficiency, Cooler Running

Table 1 compared the SBR and Schottky diode in identical buck-boost DRL power supplies controlled by the ZXLD1371, as shown in Figure 1. The SBR shows a significant efficiency advantage, increasing at higher ambient temperatures where the Schottky circuit efficiency reduces by as much as 6%, as shown in Figure 2 and Figure 3.

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Figure 2. Efficiency Comparison at 25°C Ambient Temperature

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Figure 3. Efficiency Comparison at 85°C Ambient Temperature

Plotting the efficiency of both circuits against ambient temperature (Figure 4) shows that the efficiency reduces with temperature due to a combination of increasing diode V_F, leakage current, and switching loss, as well as overall system losses. The SBR’s superior temperature stability minimises this loss of efficiency compared to the Schottky-diode circuit.

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Figure 4. The SBR Efficiency Advantage is Greater at Higher Ambient Temperatures

The SBR’s superior efficiency delivers a twin benefit, both saving energy and resulting in lower device operating temperature. Figure 5 shows how the SBR case temperature is consistently about 5°C lower than that of the Schottky diode across the full ambient-temperature range. This lower temperature allows the DRL designer greater freedom to manage heatsink size and cost while also achieving the desired system reliability.

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Figure 5. Lower SBR Case Temperature eases Thermal Management and Design for Reliability

Drop-In Upgrade from 10V to 300V

The Q series of SBRs, including the SBR10M100P5Q, are optimised for automotive applications; however, Diodes offers SBRs covering a wide variety of voltage ratings and package styles to deliver efficiency and reliability advantages for other sectors, such as industrial, consumer electronics, communications, and computer systems, and with environmental technology, such as bypass-diodes in solar panels. Extremely low V_F minimises temperature rise to maintain system reliability, and the devices have a wide operating temperature window that ensures compliance with the solar-industry safety standard IEC 61730-2.

Devices in higher voltage ratings, up to 300V, are suited to applications such as switched-mode power supplies (SMPS) and solar inverters. In addition to superior efficiency and cooler surface temperature, SBRs have high surge-current ratings to withstand hazards, such as unpredictable power flow and lightning strikes.

Conclusion

In today’s energy-conscious and efficiency-focused world, the SBR enables a valuable step-change in power conversion performance. With reduced leakage current, improved switching performance, comparable or lower V_F, and outstanding temperature stability, the SBR offers superior efficiency without any additional design effort to deliver a reduced time to market for numerous applications. With the added advantage of cooler operating temperatures, power converters for systems covering automotive LED lighting, consumer adapters, and renewable energy systems can deliver superior performance and reliability while meeting the latest eco-design objectives and safety standards.

Diodes, Inc. 

www.diodes.com