Nexperia SiGe Rectifiers
Silicon Germanium (SiGe) rectifiers offer superior efficiency and thermal stability
Silicon Germanium (SiGe) rectifiers combine the efficiency of silicon Schottky rectifiers with the thermal stability of Fast Recovery rectifiers, enabling engineers to optimize 100V-200V power designs for high efficiency.
SiGe rectifiers now available from Nexperia are intended for use in applications in vehicles, servers and communications infrastructure. By offering an extended safe-operating area with no thermal runaway at up to 175°C, these AEC-Q101 qualified SiGe rectifiers are particularly suitable for use in applications exposed to high ambient temperatures.
When designing rectifier circuits in the 100V-200V range previously, engineers had to compromise between efficiency and operating temperature. While Schottky rectifiers offer very high efficiency, they suffer from thermal runaway above a certain temperature threshold. This means that their use is limited in power circuits in automotive Electronic Control Units (ECUs) or fuel-injection systems for example, which routinely operate in temperatures above 150°C.
The alternative is to use a Fast Recovery rectifier. These are very thermally stable, but they have a very high forward voltage, and this compromises their efficiency.
SiGe and the ideal rectifier performance
The characteristics of SiGe technology include a smaller bandgap, a faster switching frequency and higher intrinsic charge-carrier density than silicon. These features confer an advantage in high-frequency switching behaviour: this is why SiGe devices are employed in radio-frequency transistors. Before now, SiGe diodes have only been discussed theoretically in academic literature, and not available for practical implementation.
Nexperia has been developing SiGe rectifier technology in recent years, and already has several patents for the process which address the apparently conflicting demands for high efficiency and high-temperature operation.
Figure 1 shows a simplified diagram of the internal structure of Nexperia’s new SiGe rectifiers. To enhance their performance, they are housed in two-pin Clip-bonded FlatPower (CFP) packages (CFP3 and CFP5), which offer excellent thermal dissipation. This package design is pin-compatible with that of Schottky and Fast Recovery rectifiers.
Fig. 1: Structural comparison of a Schottky and SiGe rectifier
As Figure 2 shows, the new devices maintain high thermal stability, extending the Safe Operating Area – in this example from 140°C, the temperature at which Schottky rectifiers begin thermal runaway. The SiGe rectifiers remain stable up to and beyond 175°C, the specified limit of the CFP package. Thermal runaway occurs when the reverse power generated within the chip exceeds the power which can be dissipated by the package. At this point the increase in leakage current becomes super-exponential.
Fig. 2: Leakage current vs case temperature for a Schottky and a SiGe rectifier
As Figure 3 shows, a Fast Recovery rectifier typically has a forward voltage of about 0.9V. Nexperia’s first SiGe diode, by contrast, has low leakage current of 1nA, which, as the curve shows, equates to a forward voltage of around 0.75V, some 150mV better than the Fast Recovery rectifier.
The result is a reduction in conduction losses of around 20%. How this translates into efficiency is dependent on multiple factors, most importantly the duty cycle of the application. As a rough estimate, an improvement in efficiency of 5-10% could be expected with the same thermal stability as the best Fast Recovery diodes.
Fig. 3: 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.
SiGe advantages in high-temperature switching applications
In addition to these benefits, SiGe rectifiers show improved switching performance in comparison to Schottky rectifiers, for example in a 48V/12V DC-DC converter. The SiGe rectifier has a lower reverse-recovery charge and lower reverse-recovery current than a comparable Schottky rectifier, resulting in lower switching losses, in combination with a lower snappiness.
These benefits directly improve the efficiency of the DC-DC converter (see Figure 4). At high frequencies, the switching losses become a major contributor to overall losses: here, the SiGe rectifier is more efficient than the Schottky rectifier.
Fig. 4: Efficiency of a 48V/12V DC-DC converter at switching frequencies between 130kHz and 500kHz. A 3A SiGe rectifier is compared to a 3A Schottky rectifier. The increased efficiency of the SiGe rectifier at high frequencies is because of lower switching losses.
In summary, SiGe rectifiers are a suitable choice for switch-mode power supplies even when operating in high-temperature environments. They combine the high efficiency of a Schottky rectifier with the thermal stability and safe operation of a Fast Recovery rectifier.
1A/2A/3A SiGe rectifiers in space-saving CFP packages
Nexperia has released a new range of Silicon Germanium (SiGe) rectifiers featuring 120V, 150V and 200V reverse-voltage ratings.
The new 1A-3A SiGe rectifiers are particularly suitable for automotive applications that operate at high temperature, such as LED lighting, engine control units or fuel-injection systems. Design engineers using these low-leakage devices can now rely on an extended safe-operating area with no thermal runaway up to 175°C. At the same time, they can optimize their design for power efficiency, which is not possible with the Fast Recovery rectifiers commonly used in high-temperature designs.
Compared to Fast Recovery rectifiers, Nexperia’s SiGe rectifiers offer conduction losses which are lower by some 10-20% thanks to their low forward voltage. The devices’ low reverse-recovery charge results in low switching losses.
The PMEG120Gx, PMEG150Gx and PMEG200Gx SiGe devices are housed in compact, thermally-efficient CFP3 and CFP5 packages.
An extension of the portfolio to higher currents up to 15A is planned for 2021.