Patent Application
20110084656
1. Field of the Invention
The present invention relates to systems and methods for transferring power wirelessly from a sending device to a receiving device.
2. Background
The applications of wireless transfer to charge batteries of cell phones and other portable electronics devices are still emerging technology. The current technologies have several drawbacks. First the maximum efficiency of the existing inductive power transfer technology is significantly lower than those of wired regulators. Second, the wireless power transfer inherently has larger load range requirements since the coupling conditions can vary significantly. Thus progress needs to be made in two areas for its successful commercialization: 1) High efficiency. Energy transfer efficiency has to be reasonably close to wired power transfer and potentially has to meet Energy Star standard, which means the efficiency has to be above 80% over entire range of operational loading conditions, 2) Noise issue. The High energy levels needed for wireless power transfer and the low efficiency together create a strong noise for the electronics inside the phone, which can make the phone's wireless communication non-functional during charging.
These and other problems are solved by a system that includes a transmitter with a feedback loop for efficient switching of a MOSFET or other electronic switch in a switched amplifier/oscillator. In one embodiment, the MOSFET gate and drain voltage is used to tune the switching frequency and duty cycle to ensure Class-E/zero voltage switching operation transmitter.
In one embodiment, a push-pull coupled transmitter planar coil pair and a planar receiving coil that can be embedded into a thin PCB/FPC are used. This wireless power transfer construction allows highly efficient coupling between transmitter and receiver, and relatively insensitive to the environment. It also effectively provides a shielding for the electronics which are adjacent to the receiving coil in the real applications.
One embodiment includes a bi-directional data channel. The data channel includes: 1) sensing instantaneous voltage on the transmitter coil sensing on the transmitter side (this voltage varies with the switching on the receiving side because of the magnetic coupling is bi-directional, and 2) Dual (frequency) series LC resonance sensing on the receiver side.
One embodiment includes an efficient power transmitter having an electronically-controlled switch controlled by a first pulse width modulation control circuit, the switch configured to pull current through a first inductor when the switch is closed, the first pulse width modulation control circuit outputting a PWM output signal to control the switch, a first feedback signal obtained from a control input of the switch, and a second feedback signal obtained from a terminal of the inductor, wherein a control feedback signal is computed, at least in part, as a difference between the first feedback signal and the second feedback signal is provided to the first pulse width modulation control circuit, the control circuit adjusting a frequency of the PWM output signal, at least in part, in response to the control feedback.
In one embodiment, the transmitter of the electronically-controlled switch includes a MOSFET. In one embodiment, the transmitter includes an electronically-controlled delay line controlling a time delay of the control. In one embodiment, an output of the transmitter is provided to a transmitting coil. In one embodiment, the transmitter includes a filter that filters frequency components of voltages across the transmitting coil and provides a filtered signal to the first pulse width modulation control circuit.
One embodiment includes an efficient wireless transmission system having a transmitter that includes an electronically-controlled switch controlled by a first pulse width modulation control circuit, the switch configured to pull current through a first inductor when the switch is closed, the first pulse width modulation control circuit outputting a PWM output signal to control the switch, a first feedback signal obtained from a control input of the switch, a transmitting coil, a second feedback signal obtained from a terminal of the inductor, wherein a control feedback signal is computed, at least in part, as a difference between the first feedback signal and the second feedback signal is provided to the first pulse width modulation control circuit, the control circuit adjusting a frequency of the PWM output signal, at least in part, in response to the control feedback.
In one embodiment, a receiver includes a receiving coil provided to a receiver switch; and a second pulse with modulation controller, the second pulse width modulation controller controlling the receiver switch to deliver a desired power from the receiving coil to a load.
In one embodiment, the electronically-controlled switch includes a MOSFET. In one embodiment, an electronically-controlled delay line controls a time delay of the control. In one embodiment, the receiver includes a filter that filters frequency components corresponding to a switching frequency of the second pulse width modulation controller provides a filtered signal to the first pulse width modulation control circuit.
In one embodiment, the transmitting coil includes a substantially flat conductor, the conductor comprising a series of holes/via spaced along a length of the conductor. In one embodiment, the receiving coil includes a substantially flat conductor, the conductor comprising a series of holes/via spaced along a length of the conductor.
Class-E RF amplifier is a classical amplifier that provides relatively high efficiency when delivering power to a known fixed load. As shown in
Unfortunately, in the wireless power applications, the coupling of through the air is a variable, and thus the amplifier 200 will not operate efficiently in this application.
The resistance 309 includes resistive losses in the transmit coil, radiative losses, and losses due to power delivered to the receiver 103.
The amplifier 300 uses the feedback loop formed from the bandpass filter 320, the bandpass filter 321, the adder 323, the PWM 310 and the delay line 324 to control the switching of the MOSFET 202 to produce efficient operation in the presence of the varying load 307. It is not practical to directly sense the instantaneous load resistance 307, and the drain voltage and current waveform are noisy for non-ideal MOSFET and driver devices. Thus, the amplifier 300 uses feedback to adjust the frequency and/or duty cycle and/or timing of the gate drive to ensure high efficiency operation for large range of air distance between coils.
In the transmitter 300, the timing at the MOSFET 202 gate is controlled by the PWM 310 and the delay line 324. The output from the adder 323 is integrated fed back to control the delay line 324. When the voltage waveforms at MOSFET 202 gate and drain are approximately 180 degree out of phase, the output of adder 323 is maximized (because the gate voltage of the MOSFET 202 is inverted). The bandpass filters 320 and 321 have pass-band at the designed switch frequencies. These two filters remove the spurious signals due to the noise and parasitic capacitances of the MOSFET 202.
The output signal from adder 323 is feedback to control frequency and the timing of PWM signal for the exact turn-on and turn-off of the MOSFET gate. When the air distance between transmit and receive coil is varied, the inductance (206) will also change. This will change the resonance frequency of LC oscillator (205 and 206). However, the feedback loop will adjust PWM frequency and the delay of the turn-on and turn-off timing of the MOSFET gate accordingly, ensure that MOSFET is turned-off when the drain voltage is nonzero. This ability to digitally control the exact cycle-by-cycle timing of MOSFET gate voltage makes it possible to minimize the switching power loss in all loading conditions.
Energy coupled from the coil 441 to the coil 442 is provided to the resistor 407. Thus, the resistor 407 appears as a load to the amplifier. Changes in the resistor, or changes in the separation between the coils 441 and 442 will change the impedance reflected back to the amplifier. These impedance changes will change the relationship between the gate and drain voltages of the MOSFET 202. The feedback loop formed from the bandpass filter 320, the bandpass filter 321, the adder 323, the PWM 310 and the delay line 324 will sense these changes in the gate and drain voltages and adjust the switching of the MOSFET 202 to produce efficient operation in the presence of the varying load.
The zener diode 547 is used to regulate supply voltage to the PWM controller 548 at start-up. The PWM controller 548 adjusts the switching frequency based on load conditions. The optional frequency-sensing filter 550 senses the switching frequency of the PWM 310 and provides the frequency information to the PWM controller 548.
The purpose of PWM controller 548 is to regulate the received energy into a DC current required for the load. The switching frequency is typically much lower frequency than PWM 310. In addition, this frequency is coded to varies with voltage and/or current desired by the load.
For many batteries, including Li-Ion rechargeable batteries, two modes are typically used. In the first stage (CI) when the battery energy is low, a constant current is supplied to fast charge the battery. When the battery energy level reached a voltage threshold, this threshold voltage (CV) is maintained while a decreasing current is supplied to charge the battery to full capacity.
The CICV (constant current constant voltage) operation can be implemented by using the switching frequency as a feedback to transmitter side. When the MOSFET 549 is turned on, the energy stored in the LC tank circuit will be drained. This will be reflected back to the transmitter side as a higher impedance load. The switching frequency on the receiver side can thus be detected on the transmitter side via the bandpass filter 650. The transmitter PWM controller 310 adjusts the delay of the start time of next cycle to reduce the energy to be transferred.
For the strong resonance coupling through the air, the receiver coil design is important. For cell phone and portable electronics applications, planar spiral inductors are typically used to capture magnetic flux.
Planar coils are typically in the form of a spiral. However, planar spirals are simply never used in conventional high efficiency power supplies. Spiral coils have poor efficiency because the inter-winding flux cancellation and the loss increases very rapidly as the number of turn increases. The air gap between windings cannot be too small as the proximity effects and intertwining capacitance work against the resonance. On the other hand, for efficient flux collection, number of turns need to be sufficient as required for the resonance inductance. One additional requirement for strong resonance is that coil must have as low AC impedance as possible.
For strong coupling, it is desirable to reduce inductance as much as possible for a given resonance resonant frequency. However, the flux collected by a given coil is proportional to the surface area, which increases with inductance. This leads to the following design for much more efficient coupling:
The push-pull action of the transmitter functions as a source/drain for magnetic flux. The dipole magnetic flux concentrator minimizes the energy loss to the stray magnetic flux in the typical spiral coils.
This topology allows reduced inductance without reducing the flux receiving area, and maximizes the coupling efficiency by providing a closed circuit path for magnetic flux (as the two transmitter coils have different polarities). The receiving structure acts as a flux concentrator to minimize the coupling loss. The flux concentrators can be combined as a single piece and goes through the receiving coil. This will increase the inductance of the receiving coil. As a result, the maximum energy transferred will be reduced but the coupling efficiency will not be reduced.
The transmitter 101 generates energy at a desired frequency to provide coupling between the transmitter 101 and receiver 103. In order to limit the size of transmit and receive coils which is required for portable electronics, it is advantageous to use a relatively high frequency. As frequency increases, the current flows on the surface of conductor, i.e., the skin effect. The copper skin depth is 210 um at 100 khz, 66 um at 1 Mhz and 21 um at 10 Mhz. The copper thickness of standard 1 oz PCB/FPC is 34 um, and ½ oz is 18 um. This means that standard PCB/FPC process are good for 10 MHz range of operation because it minimizes AC impedance of the conductor, and allows the maximum resonance. Unlike the previous arts which uses Litz wire to minimize the impedance of coil, an improved planar PCB coil is disclosed below:
Although the above-disclosed embodiments have shown, described, and pointed out the fundamental novel features of the invention as applied to the above-disclosed embodiments, it should be understood that various omissions, substitutions, and changes in the form of the detail of the devices, systems, and/or methods shown can be made by those skilled in the art without departing from the scope of the invention. For example, although the above specification describes MOFETS 202 and 549, one of ordinary skill in the art will recognize that the MOSFETS 202 and 549 are merely examples of electronically-controlled switches and other electronically-controlled switches can be used, such as, for example, FETS, transistors, etc. Moreover, various portions of the circuits shown in
