Until recently, engineers were faced with two conventional options for the diode-based rectifiers at the heart of their fast switching AC/DC power supplies: Schottky rectifiers or fast recovery rectifiers. Schottky rectifiers offer low-loss switching and good efficiency but are subject to thermal runaway in designs subject to elevated temperatures such as automotive LED headlamps or electronics control units (ECUs). Fast recovery diodes are more stable at higher temperatures but are less efficient.
Silicon germanium (SiGe) rectifiers provide a new third option and eliminate many of the trade-offs of the other types by combining the best characteristics of Schottky rectifiers with fast recovery devices. In particular, SiGe rectifiers feature high thermal stability, making them a good option for elevated temperature applications.
This article will briefly discuss rectifier basics and associated challenges, including a comparison of conventional Schottky and fast recovery rectifiers. It will then show how a SiGe rectifier architecture combines the benefits of both. Using example devices from Nexperia, the article will then outline key SiGe rectifier characteristics and how SiGe devices can be applied to solve the problems associated with high temperature, fast switching, AC/DC applications.
The basics of rectifiers
Rectifiers are essential circuits for power supplies which are used to convert an AC input voltage into a DC voltage supply that can then be used to power Electronic components. Although there are many topologies (for example, half-wave and full-wave rectifiers), the key components of rectifiers are one or more diodes.
The simplest form of diode is a doped silicon (Si) p-n junction. When the diode is forward biased (with the positive terminal of the power source connected to the p-type side of the component and the negative to the n-type side) with a sufficient voltage to overcome the diode’s inherent “barrier potential” or forward voltage drop (which is around 0.7 volts for a Si diode), a large forward current (IF) flows. IF then climbs in proportion to increased voltage (VF) from the supply. Above the barrier potential, the gradient of the VF vs IF curve is largely determined by the bulk resistance of the diode but is typically very steep, as shown for Nexperia’s BAS21H (Figure 1.) For this reason, the diode is often connected in series with a resistor for device overcurrent protection.
Figure 1: VF vs. IF characteristic for Nexperia’s BAS21H switching diode. Note how conduction starts at approximately 0.7 volts for this p/n-type Si diode. (Image source: Nexperia)
When the voltage is reversed (VR), a corresponding low reverse leakage current (IR) occurs. At low operating temperatures, IR is insignificant, but since it is temperature dependent, at high operating temperatures it can become more of a problem. When VR is large, the diode enters an avalanche mode, and a large current flows, often sufficient to permanently damage the component. This reverse voltage threshold is known as the breakdown voltage (Vbr). In their datasheets, manufacturers typically advise a working peak reverse voltage (Vrmax) which is less than Vbr to allow for a safety margin (Figure 2).
Figure 2: Key parameters are shown for a p/n-type diode V-I curve, including the forward voltage (VF), the reverse current (IR), and breakdown voltage (Vbr). (Image source: Wikipedia)
In a switching application, once the reverse bias is flipped, there is still sufficient charge on the diode to allow significant current flow in the reverse direction. This so-called reverse recovery time (trr) is an important design parameter, especially for high-frequency applications. The use of additional dopants such as gold or platinum in the p and n-type semiconductors forming the diode junction dramatically shorten trr. So-called fast recovery diodes using these materials feature a trr of a few tens of nanoseconds (ns). The trade-off for this fast switching performance is an increased VF; this can typically rise from 0.7 to 0.9 volts with a subsequent decrease in efficiency. However, the IR of a fast recovery diode remains similar to a conventional p/n-type Si diode.
In a practical application, the characteristics of the diode allow a large current to flow in one direction only, blocking the negative half of the sinusoidal AC wave, effectively rectifying the voltage source to a DC supply.
Thermal design challenges
In AC/DC conversion applications, engineers generally look for the most efficient components to reduce power dissipation and limit thermal problems.
VF is the most significant factor in determining the efficiency of a diode. Schottky diodes represent an improvement on standard diodes through the replacement of the p and n-type Si junction with a metal/n-type Si alternative. As a result, the forward voltage drop is reduced to between 0.15 and 0.45 volts (depending on the choice of barrier metal). An additional advantage of the Schottky diode is very fast trr (on the order of 100 picoseconds (ps)). These characteristics make the Schottky a popular rectifier choice in applications such as high-frequency switch-mode power supplies.
But there are significant downsides to the Schottky rectifier. For example, it features a relatively low Vrmax compared to p/n-type Si diodes. Second, and perhaps more critically, Schottky rectifiers feature a relatively high IR, which can be as high as hundreds of microamperes (µA) compared with hundreds of nanoamperes (nA) for p/n-type Si diodes in comparable applications. Worse yet, IR climbs exponentially with junction temperature (Tj) (Figure 3).
Figure 3: VR vs. IR characteristic for the Nexperia 1PS7xSB70 general purpose Schottky diode. IR is typically much higher than for an equivalent p/n-type Si diode and increases exponentially with temperature. (Image source: Nexperia)
The thermal stability of a diode-based rectifier is determined by the delicate balance of the self-heating generated by IR and the capability of the rectifier to dissipate heat through the thermal resistance of the system (Figure 4). If the rectifier is in thermal equilibrium, Tj (with a fixed ambient temperature (Tamb) as the thermal “ground”) can be described as:
Rth(j-a) = The thermal resistance between the diode junction and ambient
Pdissipated = The power dissipated in the device
Figure 4: Shown are the thermal resistances presented to an operational diode. (Image source: Nexperia)
In operation, provided the generated power through self-heating is less than the dissipated power, the Tj of the device will converge toward a stable condition (Figure 5). However, if more self-heating is generated than can be dissipated, Tj increases until the device eventually becomes thermally unstable. The situation quickly turns to thermal runaway because IR increases exponentially with temperature, effectively triggering a positive feedback loop.
Figure 5: The stable operating condition of an example diode is determined by the balance of: the capability of the thermal system to dissipate heat through the thermal resistance (blue line (1)), and the self-heating of the rectifier caused by its own reverse leakage current (IR) (and switching losses) (red line (2)). Note how self-heating increases exponentially as the system temperature rises, resulting in thermal runaway. (Image source: Nexperia)
The designer runs a high risk of thermal runaway if a Schottky diode used in an application is subject to high ambient temperatures unless its operation is significantly derated for temperatures above 145°C. For this reason, engineers tend to shy away from the Schottky diode in applications such as fast switching LED drivers or under-the-hood automotive electronic control units. Until now, that left the engineer with only the fast recovery diode—which features a low IR and hence is much less prone to thermal runaway—with its subsequent lower efficiency trade-off.
The SiGe rectifier alternative
The narrow choice of fast recovery diodes for high temperature and/or high Vrmax designs has been extended by the emergence of SiGe diode technology that combines the advantages of Schottky and fast recovery diodes in a single device. These rectifiers replace the Schottky’s barrier metal/n-type Si junction with one based on SiGe/n-type Si (Figure 6).
Figure 6: The SiGe rectifier replaces the Schottky metal barrier with SiGe. The result is a smaller bandgap, greater electron mobility, and higher intrinsic charge carrier density. (Image source: Nexperia)
SiGe, as the name suggests, is an alloy of silicon and germanium; the key benefits of the semiconductor are a smaller bandgap (where the bandgap is the energy difference in electron volts (eV) between the semiconductor’s valence band and conduction band), the ability to switch at higher frequencies, greater electron mobility, and higher intrinsic charge carrier density than silicon. The lower bandgap of SiGe lowers the VF of the Si/n-type SiGe junction to around 0.75 volts, around 150 millivolts (mV) lower than a fast recovery diode.
In practice, the lower VF reduces the diode’s conduction losses by around 20 percent compared to a fast recovery diode. While component efficiency is dependent on multiple factors, including the duty cycle of the application, an engineer might reasonably expect an improvement of 5 to 10 percent in like-for-like applications. In addition, the SiGe diode features a lower IR than a Schottky diode (Figure 7).
Figure 7: SiGe rectifiers feature a lower IR than Schottky devices (for superior high-temperature operation) and a lower VF than fast recovery rectifiers (for higher efficiency). (Image source: Nexperia)
Due to the SiGe diode’s high intrinsic charge density and electron/hole mobility, it features low trr, so it is capable of fast switching. This fast switching is also enabled by relatively low parasitic capacitance and inductance. Moreover, because the SiGe diode has a lower reverse-recovery charge (QRR) and lower reverse-recovery current (IRR) than a comparable Schottky rectifier, it features lower switching losses. This is critical because, in high-frequency applications, these switching losses are a major contributor to overall losses. The combination of low IR and low switching losses nearly eliminates thermal runaway challenges.
Selecting and applying SiGe diodes
While SiGe transistors have been on the market for several years, SiGe diodes are a more recent arrival. For example, Nexperia’s PMEG120G10ELRX, PMEG120G20ELRX, and PMEG120G30ELPJ SiGe rectifiers are part of a family that come in the size and thermally efficient Clip-bonded FlatPower (CFP3) and CFP5 packages (Figure 8). This package has become the industry standard for power diodes.
Figure 8: The PMEG120G10ELRX SiGe rectifier comes in a CFP5 package that saves space while boosting heat transfer. (Image source: Nexperia)
The package’s solid copper clip minimizes thermal resistance to boost heat transfer, thereby allowing designers to use more compact pc board designs. The CFP3 reduces rectifier space requirements by 38 percent, while the CFP5 saves up to 56 percent, compared to SMA and SMB packages.
Often when a new technology is introduced, designers need to be concerned about implementation variables. In the case of the Nexperia SiGe diodes, the same packaging is also used for the company’s Schottky and fast recovery diodes, enabling drop-in replacement for high-temperature applications including LED lighting, automotive ECUs, server power supplies, and communications infrastructure.
The SiGe rectifiers offer a Vrmax up to 120 volts (150 and 200-volt versions are available for sampling), well beyond the 100-volt limit imposed by most Schottky diodes. Moreover, the devices have been tested up to 200°C without any thermal runaway or derating (Figure 9). Note that the components’ operating temperature limit (safe operating area (SOA)) of 175°C is determined not so much by the diode but by the component package. Figure 10 shows how the SiGe diodes’ thermal runaway immunity allows for a greater extended safe operating area compared with Schottky diodes.
Figure 9: Nexperia SiGe rectifiers don’t suffer the thermal runaway of Schottky rectifiers at high temperatures. (Image source: Nexperia)
Figure 10: Thermal runaway immunity allows for an extended safe operating area for SiGe rectifiers compared with Schottky rectifiers. (Image source: Nexperia)
The Nexperia SiGe rectifiers offer IF capabilities of 1, 2 and 3 amperes (A) with a low IR of 0.2 nA (VR = 120 volts (pulsed), Tj = 25°C), rising to 10 µA at elevated temperatures (VR = 120 volts (pulsed), Tj = 150°C). Like Schottky diodes, the rectifiers are a good choice for fast switching options with low switching losses and a trr of 6 ns. The products are qualified to AEC-Q101.
Schottky rectifiers are a proven option for efficient, high frequency, AC/DC converters, but their relatively high IR can lead to damaging thermal runaway in high-temperature applications. As a result, designers had to resort to lower-efficiency but thermally stable fast recovery diodes for their high-temperature switching converters.
However, as shown, proven SiGe technology from transistors has been made commercially available in diodes. This new class of devices combine the efficiency and fast switching characteristics of Schottky’s with the thermal stability of fast recovery diodes. As such, they provide a good solution for designs going into high-temperature environments such as LED lighting, automotive ECUs, server power supplies, and communications infrastructure.