Patent Application
20140185327
Power electronics are widely used in a variety of applications. Power electronic devices are commonly used in circuits to modify the form of electrical energy, for example, from alternating current (AC) to direct current (DC), from one voltage level to another, or in some other way. Such devices can operate over a wide range of power levels, from milliwatts in mobile devices to hundreds of megawatts in a high voltage power transmission system. Despite the progress made in power electronics, there is a need in the art for improved electronics systems and methods of operating the same.
The present invention relates generally to methods and systems for providing power to electronic devices. More specifically, the present invention relates to an AC/DC power adapter with a power density capability of >50 W/in3. Embodiments of the present invention are applicable to a wide variety of power electronics including power supplies for various electronic devices and applications, power converters, and the like.
According to an embodiment of the present invention, a power supply is provided. The power supply includes a filter stage configured to receive an AC input voltage, a bridge circuit configured to rectify the filtered AC input voltage, an AC/DC converter, and a DC/DC converter. The AC/DC converter includes a primary transistor and an auxiliary circuit including an auxiliary transistor and configured to convert the rectified AC input voltage to a first DC output voltage, wherein the primary transistor and the auxiliary transistor are at least one of gallium nitride (GaN) transistors or silicon carbide (SiC) transistors. The DC/DC converter is configured to convert the first DC output voltage to a second DC output voltage.
According to another embodiment of the present invention, a method of providing a DC voltage is provided. The method includes receiving an AC input voltage, filtering the AC input voltage using an electromagnetic interference (EMI) filter, and rectifying the filtered AC input voltage. The rectified AC input voltage is then converted to a first DC output voltage using an AC/DC converter having a primary transistor and an auxiliary circuit, wherein the auxiliary circuit includes an auxiliary transistor and wherein the primary transistor and the auxiliary transistor are at least one of gallium nitride (GaN) or silicon carbide (SiC) transistors. The method further includes converting the first DC output voltage to a second DC output voltage using a DC/DC converter.
Embodiments of the invention provide a number of benefits and advantages over conventional techniques and power adapters. For example, embodiments of the present invention decrease the physical size of the power adapter in order to make it more convenient to carry. The weight of the power adapter is also reduced, increasing its portability, and applicability in mobile device applications. Another advantage provided by embodiments of the invention is a reduction in the heat dissipated by the power adapter, thereby improving the efficiency, performance, and resulting in cooler operation of the power adapter. Some embodiments of the present invention achieve an increase in power density through using a combination of advanced circuit topologies and state-of-the-art power devices made from GaN materials. These and other embodiments of the invention, along with many of its advantages and features, are described in more detail in conjunction with the text below and attached figures.
The present invention relates generally to methods and systems for providing power to electronic devices. More specifically, the present invention relates to an AC/DC power adapter with a power density capability of >50 W/in3. Embodiments of the present invention are applicable to a wide variety of power electronics including power supplies for various electronic devices and applications, power converters, and the like.
The growing popularity of laptop computers and other portable electronic devices for use at home, office, and in-transit for both work and leisure in the recent years has created a trend of size and weight reduction of electronic devices and power adapters that are used to charge them. Conventional power adapters available today that are provided with typical laptop computers are bulky, heavy, and cumbersome to carry around, and therefore are not as portable as the consumer devices they are intended to charge. Typical full-size power adapters typically are about 85-90 W in output and the best in class power density available is about 17 W/in3. Despite progress made in miniaturizing portable consumer electronics, there is a need in the art for improved power adapter devices and methods of charging the portable consumer devices with optimal performance and minimized size, weight, and portability.
Conventional power adapters and other electronics generally are silicon-based. However, GaN-based electronic devices are undergoing rapid development, and generally are expected to outperform competitors in silicon (Si) and silicon carbide (SiC). Desirable properties associated with GaN and related alloys and heterostructures include high bandgap energy for visible and ultraviolet light emission, favorable transport properties (e.g., high electron mobility and saturation velocity), a high breakdown field, and high thermal conductivity. In particular, electron mobility, μ, is higher than competing materials for a given background doping level, N. This provides low resistivity, ρ, because resistivity is inversely proportional to electron mobility, as provided by equation (1):
where q is the elementary charge.
Another superior property provided by GaN materials, including homoepitaxial GaN layers on bulk GaN substrates, is high critical electric field for avalanche breakdown. A high critical electric field allows a larger voltage to be supported over smaller length, L, than a material with a lower critical electric field. A smaller length for current to flow together with low resistivity give rise to a lower resistance, R, than other materials, since resistance can be determined by equation (2):
where A is the cross-sectional area of the channel or current path.
These superior properties of GaN can give rise to improved semiconductor devices, including vertical semiconductor devices. Traditional semiconductor devices are typically lateral devices that utilize only the top side of a semiconductor wafer, locating electrical contacts such that electricity travels laterally along the semiconductor surface. This tends to consume a large footprint on the semiconductor. Vertical semiconductor devices, on the other hand, utilize a smaller footprint to achieve the same performance as lateral devices. Vertical semiconductor devices have electrical contacts on both the top surface of the semiconductor and on the bottom surface, or backside, such that electricity flows vertically between the electrical contacts. Thus, GaN and vertical GaN semiconductor devices can be utilized in high power and/or high voltage applications, such as power electronics, to improve performance, and reduce physical size of conventional power electronics.
The AC voltage from AC power supply 100 is then fed to an electromagnetic interference (EMI) filter stage 101. Electromagnetic interference (also called radio frequency interference or RFI when in high frequency or radio frequency) is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. EMI may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit. Effects of EMI can range from a simple degradation of data to a total loss of data; therefore embodiments of the invention provide an EMI filter to reduce the effects of EMI in the power adapter. The EMI filtered AC input may then be rectified by a full-wave rectifier circuit, converting the filtered AC voltage into a rectified AC voltage. In an embodiment of the present invention, a bridge circuit 102 may be used as the full-wave rectifier circuit.
Referring to
For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (cathode-to-cathode or anode-to-anode, depending upon output polarity utilized) may form a full-wave rectifier. Twice as many turns are utilized on the transformer secondary to obtain the same output voltage than for a bridge rectifier, but the power rating is unchanged.
The rectified AC voltage (
In typical electric power systems, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. High currents increase the energy lost in the system, and require larger wires and other equipment. PFC may involve using a passive network of capacitors and/or inductors to correct linear loads with low power factors. For non-linear loads, such as rectifiers, PFC involves distorting the current drawn from the system. In such cases, active or passive PFC may be used to counteract the distortion and raise the power factor. PFC devices or circuits to correct the power factor may be at a central substation, spread out over the power supply system, or built into power-consuming equipment, depending on the application.
Similar to typical boost PFC pre- regulation, the soft-switching boost converter circuit 300 may comprise a control circuit 301 that regulates an input current (i.e., current through inductor Lmain 308) waveform to follow the rectified AC input voltage, while maintaining the DC voltage between DC-link nodes. An auxiliary circuit may be added to the circuit for soft-switching transitions of a main circuit comprising a main transistor 310 and diode 309. In an embodiment of the invention, the auxiliary circuit may include an auxiliary transistor 314, two diodes 312 and 313, and an inductor Lr 311. The auxiliary circuit is active only briefly to facilitate a turn-on transition of the main transistor 310, as illustrated in
Referring to
Between time t1 and t2 after the diode 309 is turned off, the inductor, Lr, resonates with the capacitance 310(), Coss, between a drain and a source terminals of the main transistor 310. The voltage across capacitance 310(a) Coss which is monitored by a comparator 306, then falls to a threshold level, Vth at time t2. The main transistor 310 is then turned on at time t2, as shown in
At time between t2 and t3, a time delay is introduced before turning off the auxiliary transistor, 314, at t3 after the main transistor 310 is turned on at t2, as shown in
In an embodiment of the present invention, a frequency of operation of the soft-switching boost circuit of AC/DC converter 300, shown in
The main transistor 310 and auxiliary transistor 314 may be a silicon (Si) transistor, a GaN transistor or a silicon-carbide (SiC) transistor, and either in normally-off or normally-on operation. However, it is commonly known to one of ordinary skill in the art that switching a Si transistor at such high frequencies leads to high switching losses, owing to a large input and output capacitance of Si transistors. SiC or GaN transistors are capable of having much smaller switching and transition losses compared to Si transistors, which is advantageous for high frequency operation. Gate drivers 302, 303 are driven by signals from the control circuit 300 and may be customized per the kind of transistor being driven, either normally-off or normally-on.
Similarly, diodes 309, 312 and 313 may also be Si, SiC or GaN diodes. However, for the high switching frequency operations of the soft-switching boost circuit 103 to achieve the target power density of greater than 50 W/in3, Si diodes are susceptible to reverse recovery losses. Therefore, according to an embodiment of the present invention, GaN and/or SiC diodes are used in the soft-switching boost circuit of the AC/DC converter, because GaN and SiC diodes do not suffer from reverse recovery losses that are common to Si diodes. Furthermore, simulations indicate the operational efficiency of the soft-switching boost circuit shown in
Other AC/DC converter circuit topologies other than the soft-switching boost converter circuit 300 of
A half-bridge network according to an embodiment of the present invention generates a square wave voltage from an input voltage from the output voltage of the AC/DC converter 103 stored across capacitor CAC/DC 104. The half-bridge network may include the same type (N-Channel) transistors. The first transistor 506 and the second transistor 507 may be driven at 50% duty cycle. Diodes 508 and 509 may be antiparallel to transistors 506 and 507, respectively, such that diode 508 is parallel to transistor 506, but in reverse polarity, and diode 509 is parallel to transistor 507, but in reverse polarity. Antiparallel diodes aid in ensuring safe operating conditions for the transistors by handling currents forced by inductive loads when the transistors are turned off.
A resonant network 510, may consist of a leakage inductor LR 510(b), a magnetizing inductor LM 510(c), and a capacitor CR 510(a). In an embodiment of the invention, the resonant network 510 is an LLC (inductor-inductor-capacitor) circuit, as shown by leakage inductor LR 510(b), a magnetizing inductor LM 510(c), and a capacitor CR 510(a). However, other types of resonant networks are available, for example LLCC, LC, or RLC (resistor-inductor-capacitor). Only AC current flows through the resonant network 510, and is operated such that an input current into the resonant network 510 lags the input voltage to the half-bridge network, which is the output voltage of the AC/DC converter 103 stored across capacitor CAC/DC 104. As a result, the transistors 506 with zero-voltage transitions.
A rectifier network according to embodiment of the present invention may comprise transistors 511 and 513, and antiparallel diodes 512 and 514. The rectifier network converts the AC current from the resonant network 510 into a DC output voltage.
The transistors 506, 507, 511, and 513, and the diodes 508, 509, 512, and 514 may be either GaN or SiC, and normally-on or normally-off according to an embodiment of the present invention in order to achieve a power density range of the power adapter of 35 W/in3 to 80 W/in3. Further, using GaN or SiC transistors and GaN or SiC diodes in the DC/DC converter enables the DC/DC converter 105 to switch at high frequencies of greater than 1.5 MHz so that the resonant network 510 may be physically smaller in size. Typically inductors and capacitors are relatively large, causing increases in the physical size of resonant networks that comprise multiple inductors. The control circuit 501, which drives the transistor drivers 502, 503, 504, and 505, operates at high frequencies. Furthermore, the transistor drivers 502, 503, 504, and 505 may be modified to drive either normally-on or normally-off transistors, depending on their corresponding transistors.
The method further includes converting the DC voltage from the SiC or GaN AC/DC converter to a target DC voltage using a GaN or SiC DC/DC converter circuit (610). The DC voltage from the SiC or GaN AC/DC converter may be higher than the target DC voltage, for example, the DC target voltage from the SiC or GaN AC/DC converter may be 400V, which may not be appropriate for a particular application. Therefore, the target DC voltage may be delivered at a target DC voltage level (e.g., 19V) and a target power density range (e.g., 35 W/in3 to 80 W/in3), and in an embodiment of the present invention, a target power density may be greater than 50 W/in3.
It should be appreciated that the specific steps illustrated in
It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.
