Patent Publication Number: US-2021175728-A1

Title: Method for determining a quality factor and wireless charger

Description:
BACKGROUND 
     The present disclosure generally relates to a method for detecting a quality factor (Q-factor) and a wireless charger. More particularly, the present disclosure relates to a wireless charger having a complex resonant tank circuit and a method for detecting the Q-factor of the wireless charger with the complex resonant tank circuit. 
     Quality factor (Q-factor) can be used in wireless chargers to determine if an unfriendly foreign object (i.e., a metal object) is present in its charging area, so to avoid the charger from heating up the foreign object and causing damages. Typically, Q-factor is defined as a pole with respect to a resonant frequency band. A signal with a sweeping frequency is applied to the resonant tank circuit of the charger to determine the Q-factor as the largest ratio of V resonant /V drive  over the sweeping frequency range, where V resonant  is the signal voltage on a transmitter coil, and V drive  is the signal voltage applied to the resonant tank circuit. While this method is readily performed on low power charging systems, which use a series LC resonant circuit, it cannot be easily performed in charging systems that use a more complex resonant circuit. For example, in automotive applications, complex resonant tank circuits are used to meet Electro-Magnetic Compatibility (EMC) requirements. The complex resonant circuits cause the system to have multiple poles, making it difficult to determine the Q-factor. 
     It would be advantageous to have a method and an apparatus for determining a Q-factor in a wireless charger having a complex resonant tank circuit. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
     In one embodiment, the present disclosure provides a method for determining a quality factor of a wireless charger. The wireless charger includes an inverter, a filter, and a resonant tank circuit. The inverter receives a supply voltage and generates a PWM signal by switching first, second, third, and fourth switches of the inverter. The filter connects to the inverter and receives the PWM signal and generates a filtered signal. The resonant tank circuit connects to the filter and receives the filtered signal, and provides wireless power to a receiver. The method includes: closing one of the first and the second switches, and one of the third and fourth switches, to issue a current pulse to the resonant tank circuit; and opening the first to fourth switches in a Q-factor determination phase of the wireless charger. The Q-factor determination phase may extend only whilst all four switches are open (single-pole resonance state), or may encompass the single pole resonance state when all four switches are open, together with the prior state when one of the first and the second switches is closed, along with one of the third and fourth switches (current pulse, also known as tank-priming, state). 
     In another embodiment, the present disclosure provides a method for determining a quality factor of a wireless charger. The wireless charger includes an inverter, a filter, and a resonant tank circuit. The inverter receives a supply voltage and generates a PWM signal at a first node and a second node. The filter connects to the first and second nodes of the inverter to receive the PWM signal, and generates a filtered signal at a first terminal and a second terminal of a capacitor. The resonant tank circuit connects to the first and second terminals of the capacitor of the filter to receive the filtered signal, and provides wireless power at an inductor coil to a receiver. The method includes: issuing a current pulse to the resonant tank circuit; and in a Q-factor determination phase of the wireless charger, connecting the resonant tank circuit and only the capacitor of the filter in a resonance network. 
     In another embodiment, the present disclosure provides a wireless charger. The wireless charger includes an inverter, a filter, a resonant tank circuit, and a controller. The inverter receives a supply voltage from a voltage supply and generates a PWM signal. The inverter includes a first branch and a second branch that are connected in parallel between the voltage supply and ground. The first branch includes first and second series connected switches. The second branch includes third and fourth series connected switches. The filter is connected to the inverter for receiving the PWM signal, and generates a filtered signal. The filter includes a capacitor and an inductor connected to the capacitor. The resonant tank circuit is connected across the capacitor of the filter for receiving the filtered signal. The resonant tank circuit provides wireless power to a receiver located within a charging area of the wireless charger. The controller is connected to the first to fourth switches of the inverter. The controller closes one of the first and the second switches, and one of the third and fourth switches, to issue a current pulse to the resonant tank circuit. The controller opens the first to fourth switches during a Q-factor determination phase of the wireless charger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more detailed description is given below, with reference to embodiments, some of which are illustrated in the appended drawings. The appended drawings illustrate only typical embodiments of the disclosure and should not be interpreted as limiting the scope of the disclosure, as the disclosure may have other embodiments, which may be equally effective. The drawings are for facilitating an understanding of the disclosure and thus are not necessarily drawn to scale. Advantages of the subject matter claimed will become apparent to those skilled in the art upon reading this description in conjunction with the accompanying drawings, in which like reference numerals have been used to designate like elements, and in which: 
         FIG. 1  is a circuit diagram of a wireless charger in a normal operation phase; 
         FIG. 2  is a circuit diagram of the wireless charger of  FIG. 1  in a Q-factor determination phase; 
         FIG. 3  is a signal diagram of a resonant voltage of the inductor coil of  FIG. 1 ; 
         FIG. 4  is a circuit diagram of a wireless charger according to an embodiment; 
         FIG. 5  is a current wave form of the inductor coil of  FIG. 4  in the Q-factor determination phase, wherein the inductors of the filter are coupled into the resonance network; 
         FIG. 6  is the control signals from the controller  410  of  FIG. 4  according to an embodiment; 
         FIG. 7  is a current wave form of the inductor coil of  FIG. 4  in the Q-factor determination phase, wherein the switches are controlled by the control signals of  FIG. 6 ; and 
         FIG. 8  is a circuit diagram of a wireless charger according to another embodiment, wherein the wireless charger includes a current sensor. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a circuit diagram of a wireless charger  100 . The wireless charger  100  includes a voltage supply  102 , an inverter  104 , and a resonance tank  106 . The voltage supply  102  provides a supply voltage at an output terminal  112 . The supply voltage from the voltage supply  102  is a DC voltage. The inverter  104 , also named a power inverter, converts the DC voltage into an AC signal, and provides the AC signal to the resonant tank  106 . 
     The inverter  104  is a full-bridge type inverter which includes a first branch  116  and a second branch  118 . The first and second branches  116  and  118  are connected in parallel between the output terminal  112  of the voltage supply  102  and ground  114 . More particularly, the first branch  116  includes first and second series connected switches S 1  and S 2 , where a first terminal of the first switch S 1  receives the DC supply voltage from the output terminal  112  and a terminal of the second switch S 2  is connected to the ground  114 . The second branch  118  includes third and fourth series connected switches S 3  and S 4 , where a first terminal of the third switch S 3  receives the DC supply voltage from the output terminal  112  and a terminal of the fourth switch S 4  is connected to the ground  114 . The inverter  104  has a first output terminal  120  and a second output terminal  122  that provide the converted AC signal to the resonance tank  106 . The first output terminal  120  is connected to a node between the first and second switches S 1  and S 2 , and the second output terminal  122  is connected to a node between the third and fourth switches S 3  and S 4 . 
     As shown by the signal diagrams next to each of the switches S 1  to S 4  in  FIG. 1 , in a normal operation phase, for example in a charging activity, in the first branch  116 , the first and second switches S 1  and S 2  are controlled by a controller (not shown) to be alternately closed. Similarly, in the second branch  118 , the third and fourth switches S 3  and S 4  are controlled by the controller to be alternately closed. The switches S 1  and S 2 , or the switches S 3  and S 4 , are not closed simultaneously to avoid shorting the voltage supply  102 . The switches S 1  through S 4  are shown as MOSFETs each having a gate terminal to receive the control signal from the controller, however the switches can be implemented to be other types of device which are controllable to be conductive. 
     The resonant tank circuit  106  is connected to the inverter  104  to receive the AC signal. To be more specific, the resonant tank circuit  106  is connected to the first output terminal  120  and the second output terminal  122  to receive the AC signal. The resonant tank circuit  106  includes an inductor coil  108  which radiates power, and a capacitor  110  series connected with the inductor coil  108 . The inductor coil  108  of the resonant tank circuit  106  has a first terminal  132  coupled to the first output terminal  120  of the inverter  104  by way of a series connected capacitor  110 , and a second terminal  134  coupled to the second output terminal  122  of the inverter  104 . 
     The radiated power may be used to charge a receiver device that is placed proximate the wireless charger  100 , i.e. within a charging area of the wireless charger  100 . The AC signal drives the inductor coil  108  to generate a magnetic field. The magnetic field causes a receiver coil in a receiver device to generate an induced current, thereby transfers power from the charger to the receiver. As understood, the magnetic field attenuates with an increasing distance, the charging area of the wireless charger  100  is an area where the generated magnetic field is strong enough to cause a required current to be induced in the receiver device, for example within a distance from the inductor coil  108 , which distance being dependent on the AC signal for driving the inductor coil  108 , and the inductance of the inductor coil  108 , etc. 
     As understood by those of skill in the art, Q-factor is used in one method of detecting the presence of a foreign object (FO) within the charging area of the wireless charger  100 . Referring to  FIG. 2 , the signal diagrams next to the switches S 1  to S 4  are control signals applied to the corresponding switches in a Q-factor determination phase. The first switch S 1  is closed for a short time to issue an exciting current into the resonant tank circuit  106 . After that, the switches S 2  of the first branch  116  and S 4  of the second branch  118  are closed, while the switches S 1  and S 3  are open, the resonant tank circuit  106  is connected as a free resonant tank as indicated by the dotted lines in  FIG. 2 , and accordingly enters a free resonant status. 
     Typically, Q-factor is defined as describing how fast an energy stored in the resonant tank circuit  106  is damped due to the internal energy loss of the resonant tank circuit  106 . If an FO is present, the stored energy by the resonant tank circuit  106  fluctuates, and accordingly the Q-factor becomes different as compared to the absence of the FO. For simplicity, a resonant voltage V r  of the inductor coil  108  at its first terminal  132  indicative the stored energy of the resonant tank circuit  106  is measured to determine the Q-factor. As the switches S 2  and S 4  are closed, it is now able to determine a resonant voltage V r  of the inductor coil  108  at its first terminal  132 .  FIG. 3  shows the resonant voltage V r  of the inductor coil  108  of  FIG. 2  over time. 
       FIG. 4  is a circuit diagram of a wireless charger  400  operable according to an embodiment of the present disclosure. The wireless charger  400  includes a voltage supply  402 , an inverter  404 , a filter  406 , a resonant tank circuit  408  and a controller  410 . The voltage supply  402 , the inverter  404 , and the resonant tank circuit  408  of the wireless charger  400  in  FIG. 4  are similar to the voltage supply  102 , the inverter  104 , and the resonant tank circuit  106  of the wireless charger  100  of  FIG. 1 , and will be described in general herein. 
     The voltage supply  402  provides a DC supply voltage at an output terminal  412 . The inverter  404  converts the DC voltage into an AC signal, and provides the AC signal to the filter  406 . 
     Similar to the inverter  104  of  FIG. 1 , the inverter  404  of  FIG. 4  is a full-bridge type inverter which includes a first branch  416  and a second branch  418 . The first and second branches  416  and  418  are connected in parallel between the output terminal  412  of the voltage supply  402  and ground  414 . More particularly, the first branch  416  includes first and second series connected switches S 1  and S 2 , where a first terminal of the first switch S 1  receives the DC supply voltage from the output terminal  412  and a terminal of the second switch S 2  is connected to the ground  414 . The second branch  418  includes third and fourth series connected switches S 3  and S 4 , where a first terminal of the third switch S 3  receives the DC supply voltage from the output terminal  412  and a terminal of the fourth switch S 4  is connected to the ground  414 . The inverter  404  has a first output terminal  420  and a second output terminal  422  that provide the converted AC signal to the filter  406 . The first output terminal  420  is connected to a node between the first and second switches S 1  and S 2 , and the second output terminal  422  is connected to a node between the third and fourth switches S 3  and S 4 . 
     In normal operation, for example in a charging activity, in the first branch  416 , the first and second switches S 1  and S 2  are controlled by the controller  410  to be alternately closed. Similarly, in the second branch  418 , the third and fourth switches S 3  and S 4  are controlled by the controller  410  to be alternately closed. The switches S 1  and S 2 , or the switches S 3  and S 4 , are not closed simultaneously to avoid shorting the voltage supply  402 . The result, is that the voltage across the inverter circuit output is an (square-wave) AC signal, having a frequency corresponding to the switching frequency of the switches. The switches S 1  through S 4  of the wireless charger  400  of  FIG. 4  can be implemented as the MOSFETs as shown in  FIG. 1 , or can be other switches controllable to be conductive. 
     The filter  406  is connected to the inverter  404  to receive the AC signal via the first and second output terminals  420  and  422 . The filter  406  filters the AC signal and generates a filtered PWM (pulse-width modulated) signal. In the present disclosure, the filter  406  is a PI-filter (π-filter), which includes first and second inductors L 1  and L 2 , and a capacitor C 1 . The first inductor L 1  is connected to the second output terminal  422  of the inverter  404 , and the second inductor L 2  is connected to the first output terminal  420  of the inverter  404 . The first inductor L 1  has a first terminal  424  connected to the second output terminal  422  of the inverter  404  and a second terminal  426  connected to the resonant tank circuit  408  and to a first terminal of the capacitor C 1 . Similarly, the second inductor L 2  has a first terminal  428  connected to the first output terminal  420  of the inverter  404  and a second terminal  430  connected to the resonant tank circuit  408  and to a second terminal of the capacitor C 1 . Thus, the capacitor C 1  is connected between the second terminal  426  of the first inductor L 1  and the second terminal  430  of the second inductor L 2 . In other words, the first inductor L 1  and the second inductor L 2  are connected to opposite sides of the capacitor C 1 . According to an embodiment, the first inductor L 1  and the second inductor L 2  both have an inductance of 1 micro-Henry (μH), and the capacitor C 1  has a capacitance of 0.4 micro-Farad (μF). In other, non-limiting embodiments, the first inductor L 1  and the second inductor L 2  both have an inductance in a range between of 0.1 μH and 10 μH, and the capacitor C 1  has a capacitance in a range 0.1 μF to 2 μF. 
     The PI-filter  406  filters out harmonic components from the square-wave AC signal from the inverter  404  to produce a sinusoid-wave PWM signal, and provides the filtered sinewave PWM signal to the resonant tank circuit  408 . In alternative embodiments, the filter  406  can include more or fewer inductors. For example, the filter  406  can be implemented as an L-type filter which, as comparing with the PI-filter  406  shown in  FIG. 4 , includes the capacitor C 1  and only one of the first and second inductors L 1  and L 2 . The filter  406  enables the wireless charger  400  to be suitable for applications with high power, for example 10 W-60 W power solutions. The frequency of the PWM signal, as described above, is dependent on the switching frequency of the switches S 1  to S 4 . In applications according to the Qi protocol, the frequency of the PWM signal is between 105 kHz and 210 kHz. 
     Similar to the resonant tank circuit  106  of  FIG. 1 , the resonant tank circuit  408  of  FIG. 4  is connected to the filter  406  to receive the sine-wave PWM signal. To be more specific, the PWM signal of the filter  406  is provided across the capacitor C 1  at its first and second terminals (not labelled), and the resonant tank circuit  408  is connected across the capacitor C 1  of the filter  406  to receive the PWM signal. The resonant tank circuit  408  includes an inductor coil L 3  which radiates power. The radiated power may be used to charge a receiver device that is within the charging area of the wireless charger  400 . The wireless charger  400  can also use the inductor coil L 3  of the resonant tank circuit  408  to communicate with the receiver device, for example in the format of data packets according to the Qi standard. 
     The inductor coil L 3  of the resonant tank circuit  408  has a first terminal  432  coupled to the second terminal  426  of the first inductor L 1  of the filter  406 , and a second terminal  434  connected to the second terminal  430  of the second inductor L 2  of the filter  406 . The resonant tank circuit  408  also includes a capacitor C 2  connected between the second terminal  426  of the first inductor L 1  of the filter  406  and the first terminal  432  of the inductor coil L 3 . It can be understood that the capacitor C 2  is accordingly series connected with the inductor coil L 3 . 
     As described with reference to  FIG. 2 , in determination of the Q-factor, the first switch S 1  and the fourth switch S 4  are closed for a short time to issue an exciting current to trigger the resonant tank circuit  408  to oscillate. The time during which the first switch S 1  and the fourth switch S 4  are closed to issue such exciting current is shorter than a time the switches are closed in each cycle during the normal operation phase. After the exciting current is issued, if the wireless charger  400  operates similarly to the wireless charger  100  of  FIG. 1  to close the switches S 2  of the first branch  416  and S 4  of the second branch  418  and open the switches S 1  and S 3 , the voltage V r  will be difficult to be measured since neither the inductor coil L 3  nor the capacitor C 2  is connected to ground. Instead, the inductor coil L 3  and the capacitor C 2  are floating. 
     Furthermore, closing switches S 2  and S 4  will couple not only the inductor coil L 3  and the capacitor C 2 , but also the first inductor L 1  and the second inductor L 2  into a resonance network which starts resonating as triggered by the exciting current. Accordingly, these multiple resonant elements coupled into one resonance circuit will result multiple resonant frequencies of the resonance network, also referred to as “poles”. Referring to  FIG. 5  which is a current wave flowing through the inductor coil L 3  when coupled in the resonance network including the first and second inductors L 1  and L 2 , the signal wave is complicated and includes harmonic components, causing it difficult to determine the Q-factor. These harmonic components are particularly visible towards the left of the plot, at the start of the resonance, although it will be noted that they appear to damp out quicker than the fundamental resonance, such that the right side of the plot resembles a single harmonic, or single-pole, resonance. 
     Referring now to  FIG. 6  which shows signal sequence diagrams of the control signals applying from the controller  410  to the switches S 1  to S 4  according to the present disclosure. According to the embodiment, the controller  410  controls the first through fourth switches S 1  to S 4  to place the wireless charger  400  in a wireless charging phase before time t 0 . It can be seen from  FIG. 6  that during the wireless charging phase, the switches S 1  and S 2  are alternately closed, and the switches S 3  and S 4  are alternately closed, to provide PWM signal to the resonant tank circuit  408 . Starting from time t 0 , the wireless charger  400  enters a Q-factor determination phase: to determine the Q-factor, an exciting current is issued by closing the first switch S 1  and the fourth switch S 4  for a time period t 1  which is no longer than the switch-on time of the switches in each cycle of the wireless charging phase. 
     After the exciting current, also referred to as a current pulse, is issued, all of the switches S 1 , S 2 , S 3 , and S 4  are open for a time period t 2 . It can hereby be clear that because all switches are open, the inductor coil L 3 , the capacitor C 2  of the resonant tank circuit  408 , and the capacitor C 1  of the filter  406  connect into a resonance network which starts resonating as triggered by the exciting current. The resonance network includes only the inductor coil L 3 , the capacitor C 2  and the capacitor C 1 , which reduces the resonant elements inductors L 1  and L 2  from the resonance network, and removes the additional poles from the resonant signal. The simplified resonance network results in a simplified and clear resonant current I r  as shown in  FIG. 6 , and depicted in more details in  FIG. 7 , and makes it easier to determine the Q-factor. 
       FIG. 8  is a circuit diagram of a wireless charger  800  according to an embodiment of the present disclosure. Similar to the wireless charger  400  of  FIG. 4 , the wireless charger  800  of  FIG. 8  includes a voltage supply  802 , an inverter  804 , a filter  806 , a resonant tank circuit  808 , and a controller  810 , etc., all of which are similarly referred and labelled, and will not be described in detail herein. In addition to those, the wireless charger  800  includes a current sensor  840  coupled between the second terminal  834  of the third inductor coil L 3  of the resonant tank circuit  808  and the capacitor C 1  of the filter  806 . Accordingly, the current sensor  840 , the inductor coil L 3 , and the capacitor C 2  of the resonant tank circuit  808  are series connected. The current sensor  840  provides an output current I r  which is indicative of the current flowing through the third inductor coil L 3 . It is understood that, the output current I r  is indicative of the above-mentioned voltage V r , and can be further used for calculating the Q-factor by known equations and/or functions. In the embodiment depicted in  FIG. 8 , the current sensor  840  is a current transformer (CT) with a winding ratio of for example 1:100, wherein the primary windings of the CT  840  are coupled in the resonant tank circuit  808 , and the second windings of the CT  840  are coupled to provide the output current I r . As can be understand, the primary windings part of the current sensor  840  coupled into the wireless charger  800  is nearly a pure conductor, and almost introduces no pole into the resonant network. 
     During normal power transfer period (wireless charging phase), a power loss between a transmitted power from the wireless charger  800  and a received power by the receiver device is often used for determining if a foreign object is present within the charging area. The broadcasting power from the wireless charger  800  can also be calculated through the current I r  which is detected through the current sensor  840 . Accordingly, coupling the current sensor  840  into the wireless charger  800  further contributes in determining the power loss, and in the foreign object detection (FOD) during the power transfer period. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the subject matter (particularly in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “coupled” and “connected” both mean that there is an electrical connection between the elements being coupled or connected, and neither implies that there are no intervening elements. In describing transistors and connections thereto, the terms gate, drain and source are used interchangeably with the terms “gate terminal”, “drain terminal” and “source terminal”. Recitation of ranges of values herein are intended merely to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation, as the scope of protection sought is defined by the claims set forth hereinafter together with any equivalents thereof entitled to. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illustrate the subject matter and does not pose a limitation on the scope of the subject matter unless otherwise claimed. The use of the term “based on” and other like phrases indicating a condition for bringing about a result, both in the claims and in the written description, is not intended to foreclose any other conditions that bring about that result. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention as claimed. 
     Preferred embodiments are described herein, including the best mode known to the inventor for carrying out the claimed subject matter. Of course, variations of those preferred embodiments will become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventor expects skilled artisans to employ such variations as appropriate, and the inventor intends for the claimed subject matter to be practiced otherwise than as specifically described herein. Accordingly, this claimed subject matter includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed unless otherwise indicated herein or otherwise clearly contradicted by context.