Silicon Germanium Rectifiers Provide High Efficiency, Thermal StabilityAugust 24, 2020 by Hailey Stewart
AEC-Q101 approved devices with 120V, 150V, and 200V combine best attributes of Schottky and fast recovery diodes
Silicon Germanium (SiGe) rectifiers combine the efficiency of Schottky rectifiers with the thermal stability of fast recovery diodes, enabling engineers to optimize their 100-200 V power designs for higher efficiency. The devices target applications in automotive, server markets and communications infrastructure. By offering an extended safe-operating area with no thermal runaway up to 175 °C, the AEC-Q101 compliant SiGe rectifiers are particularly suitable for high ambient temperature applications.
Previously when designing rectifier circuits in the 100-200 V range, engineers had to compromise between efficiency and operating temperature. While Schottky rectifiers offer very high efficiency, they experience thermal runaway after a certain temperature threshold. So, they offer limited suitability for power circuits in Engine Control Units (ECU) or fuel-injection systems for example, which routinely see temperatures above 150 °C. The alternative is to use Fast Recovery rectifiers. These are extremely thermally stable, but they do have a very high forward voltage and therefore low efficiency.
SiGe and Ideal Rectifier Performance
SiGe technology has a smaller band-gap, a faster switching frequency, and higher electron mobility than silicon. That brings an advantage in high-frequency switching behavior which is why it has been employed in radio-frequency transistors within ICs for some time. But until now, SiGe diodes have only been the subject of university papers.
Figure 1: Leakage current vs. case temperature for a Schottky and a SiGe rectifier. Thermal runaway occurs when the increase in leakage current becomes super-exponential.
Nexperia has been developing SiGe rectifier technology for a few years and already has several patents on the process that address the apparently conflicting demands for high efficiency and high-temperature operation. To further enhance the performance of these SiGe rectifiers, they are housed in the two-pin Clip-bonded FlatPower (CFP) packages (CFP3 and CFP5), which themselves offer excellent thermal dissipation. It also enables pin-for-pin compatibility and drop-in replacements for Schottky and fast recovery rectifiers.
As Figure 1 shows, the new devices retain a high thermal stability, extending the Safe Operating Area – in this example from 140 °C, the temperature at which Schottky rectifiers start to go into thermal runaway. However, the SiGe rectifiers remain stable at up to and beyond 175 °C, the specified limit of the CFP package.
Figure 2 addresses efficiency. It shows that typically a Fast Recovery rectifier has a forward voltage of about 0.9 V. Nexperia’s first SiGe diode currently has lowest leakage current (1 nA) of the family, which as the curve shows equates to a forward voltage of around 0.75 V, and an advantage of 150 mV. The result is a reduction in conduction losses of around 20%. How this translates into efficiency is dependent on multiple factors, most significantly the duty cycle of the application.
Figure 2: Trade-off in forward voltage vs. leakage current for Schottky, SiGe, and Fast Recovery rectifiers. The SiGe rectifier shows around two orders of magnitude lower leakage current than a Schottky rectifier, and around 20% lower forward voltage drop than a Fast Recovery rectifier.
As a rough estimate, a 5 to 10 % efficiency increase could be expected with the same thermal stability as the best fast recovery diodes.
An extension of the Silicon Germanium portfolio to higher currents, up to 15 A is planned for 2021.
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