GaN HEMT in class E power amplifiers
Charging the wireless way
Fortsetzung des Artikels von Teil 1
Decisive GaN characteristics for wireless systems
The advantages of GaN devices in wireless power transfer can be summarized as follows:
Lower gate charge loss
GaN devices are typically driven with 5 V gate drive voltage as opposed to standard silicon MOSFETs typically driven around 10 V. The gate charge (QG) for the GaN device is about one fifth of that for a MOSFET or similar RDS(on) and VBRR, which results in dramatically lower gate drive current and far lower losses in the gate driver IC. To minimize gate charge loss it is preferable not only to choose low QG, but also to use device technology with low gate threshold voltage, allowing the designer to use lower driving voltage, thus decreasing overall losses related to the driving circuitry.
Gate charge losses can be calculated as:
PGATE=QG_SYNC ∙ fSW ∙ Vdr
Where QG_SYNC is the gate charge at voltage Vdr without the QGD (since a ZVS transition is assumed), fSW is the switching frequency and Vdr is the driving voltage.
Body diode losses
Although GaN HEMT devices do not have an actual body diode like a MOSFET, they do exhibit a diode-like behavior. Another important source of system losses is the body diode forward voltage, which is in fact higher in GaN devices. In class E topology diode conduction is avoided by correct tuning of the circuit.
PDT=USD ∙ IOUT ∙ fSW ∙ TDT
In evaluation of body diode losses it is important to compute the correct VSD value, which will vary with current and temperature.
Designing wireless charging systems with GaN
Above it was explained why GaN technology provides many opportunities to increase overall efficiency of the system. But this does not come for free, the GaN technology has some attributes that need to be considered during system design. Design criteria to be taken into consideration:
Driving voltage accuracy
As specified in the data sheet, the absolute maximum rating for the VGS of a MOSFET is typically ±20 V. This provides the designer some freedom to keep the voltage regulator of the driving stage relatively simple and cheap. With GaN this is not the case. The absolute maximum rating is limited to roughly 5 to 6 V. That is mainly due to the diode nature of the gate structure. If in operation the gate-source voltage exceeds this limit in worst cases, it could create severe damage to the device and at best reduction of lifetime. For this reason design of the voltage regulator used to create the driving voltage must be very careful, because a solution that works for silicon may not be suitable for GaN.
Gate current
Behavior is different to silicon-based products, in which the gate is controlled through a gate oxide isolator. The gate connection for GaN devices takes the form of a Schottky barrier where the leakage current is consequently not in the range of nanoamperes but milliamperes. Care should be taken when selecting the gate drive voltage and drive network components.
Device area
As seen above, GaN technology provides greater power density resulting from the low RDS(on) x area figure. This results from the high conductivity of the electron gas (2DEG), which provides a very attractive feature to designers who want to increase the power density of their applications. But it also creates some challenges. The fact that the area is smaller implies that there will be less contact area to extract the power dissipated inside the device. During the layout phase, design of the power connections between the device(s) and PCB will be more demanding and the thermal resistance of the device could suffer. Since the most important thermal resistance is junction to ambient, which is mainly dictated by PCB characteristics, the smaller dimensions of the GaN device package should not create too much additional thermal resistance. In any case particular care should be taken during design of the PCB to minimize this thermal resistance, since the smaller area of the GaN might partially counteract the advantage of the technology.
| Key factors of CoolGaN HEMTs from Infineon |
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Continuous growth of the world’s population and acceleration of social development have led to an increasing demand for electricity, and the increasingly urgent environmental pressure forces us to do more with less energy. For years engineers have sought ways to improve circuit design using existing silicon-based semiconductors, constantly wringing more efficiency from designs. While this has clearly delivered some benefits, there are very few advances still left in this area and engineers are seeking other opportunities for efficiency. Since the beginning of solid-state electronics, silicon (Si) has been the material of choice for power devices. But a new generation of wide bandgap materials including silicon carbide (SiC) and gallium nitride (GaN) are entering the market, providing significant opportunities for power designers. These technologies are key for the next essential step towards an energy-efficient world by allowing for greater power efficiency, smaller size, lighter weight, lower cost – or all of these together. Compared to silicon, the breakdown field of Infineon’s CoolGaN enhancement (e-mode) HEMTs is ten times higher and the electron mobility is double. Both output charge and gate charge are ten times lower than with Si and the reverse recovery charge is almost zero, which is key for high-frequency operations. GaN is the suitable technology of choice in hard switching as well as resonant topologies, and is enabling new approaches in current modulation. Infineon’s GaN solution is based on the most robust and performing concept in the market – the e-mode concept offering fast turn-on and turn-off speed. CoolGaN products focus on high performance and robustness, and add significant value to a broad variety of systems across many applications such as server, telecom, wireless charging, adapter and charger, and audio. GaN devices are by nature normally-on devices, since the 2DEG channel is immediately present in a GaN/AlGaN heterojunction. The power electronics industry, however, strong¬ly wishes normally-off devices. There are two ways to achieve that: the socalled cascode approach, or to realize a real monolithic enhancement mode device. Infineon is focusing on the e-mode GaN concept for its CoolGaN 400 V and 600 V devices, suitable for all consumer and industrial applications. The reduced switching losses associated with GaN deliver smaller and lighter designs. On the one hand, the SMD packaged device allows compact and modular designs, while secondly, smaller heatsinks and less components can be used. And thirdly, moving to higher switching frequency in certain applications (when required) reduces the size of the passives. At system level, higher power density achieved by GaN-based power supplies allows more computing power to be installed within the same volume. To support accurate lifetime prediction Infineon has developed a highly structured and accurate qualification plan, built upon four key areas including expected profile, quality requirements of an application, reliability data col¬lected during product development, and degradation models. During the quality management process of CoolGaN, not only the device is tested but also its behavior in the application. The performance of CoolGaN goes beyond other GaN products in the market. It offers a predicted lifetime of more than 15 years, with failure rate less than 1FIT rate. |
The Authors
Milko Paolucci
graduated in 2000 from Poli¬tecnico di Milano in system and signal engineering. He joined Infineon Technologies AG in 2006, where he has worked as an applications engineer for MOSFET technology definition in different application fields and voltage classes. Prior to joining Infineon, he was an applications engineers for drivers and controllers at STMicroelectronics.
Peter Green
is a graduate of Queen Mary College, University of London and has an MSEE from the University of Wisconsin Madison. He has over 30 years’ experience in the electronics industry with 17 years combined at International Rectifier and Infineon as an applications engineer. He specializes in SMPS and lighting and currently heads the renewable energy applications group covering UPS, solar and wireless charging at Infineon based in El Segundo, California.
- Charging the wireless way
- Decisive GaN characteristics for wireless systems
