DESCRIPTION

TECHNICAL FIELD

The present invention relates to systems and methods for overvoltage protection in a wireless power transfer receiver.

BACKGROUND ART

Systems for Wireless Power Transfer, in which a transmitting device, connected to an electrical power source, emits an electromagnetic wave that is received by a receiver capable of using the electrical power carried by the wave to charge a battery or, in general, power a load, are becoming increasingly popular nowadays.

One of the most widely felt problems with these systems is overvoltages, which can damage the load.

The Qi protocol, described in the Technical Report "The Qi Wireless Power Transfer System," Version 1.2.3, released by the Wireless Power Consortium in 2017, is currently the industry standard for Wireless Power Transfer. The protocol generally stipulates that overvoltage protection must be implemented at the transmitter, while the receiver takes no action in this regard. Thus, in the event of a defect at the transmitter, overvoltages can be generated at the receiver and damage to its load. The same standard, however, also provides a solution that provides for disconnecting, by acting on a switch, the receiver load if the received voltage exceeds a predefined threshold.

A solution that provides for disconnecting the load in case of overvoltages is described in the article by Beom W Gu, et al. "Receiver-side Self-Harvesting Protection Circuit in Wireless Power Transfer Systems," In Proc, of IECON 2020 The 46th Annual Conference of the IEEE IndustrialElectronics Society. IEEE, 3866-3871. In this paper, the use of bidirectional switches is proposed for implementing a cut-off circuit, which limits the voltage at the load to a certain maximum threshold value. Similarly, an overvoltage-protection system is also proposed in the patent US 9,640,976 which provides for disconnecting the load if the voltage level exceeds a predefined safety threshold.

Classical methods for overvoltage prevention using Zener diodes or dummy loads are not always possible due to the very limited space in the circuits of devices using these technologies, such as smartphones, so other solutions have been devised over time.

The first class of solutions is applied to wireless charging systems that involve a receiver working in resonance with the transmitter. For these systems, it is known to act on the resonant frequency of the receiver to de-tune it from the transmitter and thus reduce the received power.

Such a solution is described in the article by Nachiket Desai et al. "An actively detuned wireless power receiver with public key cryptographic authentication and dynamic power allocation.", IEEE Journal of Solid-State Circuits 53, 1 (2017), 236-246. In this paper, two inductors are coupled to the circuit that receives the radio signal, and the reduction of the received signal power is achieved by adjusting the load connected to the second inductor. The solution proposed here requires authentication of the transmitter, and thus requires a cryptographic exchange step that is potentially subject to attack.

Similarly, an overvoltage protection device for a receiver for use in resonant wireless power transmission systems is described in U.S. Patent 8,929,043. The receiver includes a resonant circuit for receiving a radio signal transmitted by a transmitter. The overvoltage protector acts on the resonant frequency of the receiver circuit so as to de-tune it, so as to reduce the power of the received signal, in the event of a overvoltage.

Other solutions to the overvoltage problem have been proposed in some patents, such as US 9,130,369 and US 11,056,878.

In the patent US 9,130,369, a protection circuit is described that provides for limiting the voltage at the load to a certain maximum value in the event of a received voltage that could lead to overvoltages on the load.

In the patent US 11,056,878, an overvoltage protection system is proposed in which the receiver receiving a voltage above a certain threshold informs the transmitter so that the transmitter limits the transmitted power. The paper by LEE BYUNGHUN ET AL: "Robust Self-Regulated Rectifier for Parallel-Resonant Rx Coil in Multiple-Receiver Wireless Power Transmission System," IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS, PISCATAWAY, NJ, USA, vol. 9, no. 3, July 16, 2019, pages 3812- 3821 , describes a Low Dropout Regulator (LDO), which is a device that regulates its own voltage according to the voltage drop at its ends.

US 2015/207338 discloses a wireless charging device. This patent application addresses the problem of replacing a diode bridge with an H-bridge in order to reduce losses and make charging more efficient.

US 2017/256989 discloses a wireless charging circuit provided with an impedance adjustment module to the load. Such a module may consist of a variable resistor, or a combination of resistor, capacitor, and transistor. The purpose of this module is to maintain the supply voltage Vout of the load at a desired value.

Known solutions, although effective in preventing overvoltages, can nevertheless have some drawbacks. In particular, solutions that involve disconnecting the load can cause damage to the load itself. Consider, in fact, the case where the load is a battery that needs to be charged. The continuous connection and disconnection of the battery reduce its life.

Solutions involving de-tuning the load may not avoid the overvoltage since even varying the frequency of the receiver the received power may cause the overvoltages.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the limits of the prior art.

In particular, it is an object of the invention to protect a receiver load of a wireless power transmission system from overvoltages by limiting or preventing disconnection of the load.

These and further objects are achieved by a method and system for overload protection of a load in a wireless transmitted power receiver, incorporating the features of the appended claims. In a first aspect, the invention is directed to a method for overvoltage protection of a load in a wireless transmitted power receiver. The method provides for receiving an electromagnetic wave via a receiving circuit and measuring an electric load current, generated by receiving the electromagnetic wave, flowing in a load electrically connected to the receiving circuit. The method provides for estimating by a predictive method at least a future value of the load current, and varying a resistance value of a variable resistor connected in series with the load, so that the voltage drop across the load, in the case of load current equal to the estimated value, does not exceed a critical threshold value.

By means of this solution, a resistor in series with the load is dynamically acted upon, thus preventing the risk of overvoltage at the load. Such dynamic action makes it possible to improve power transfer to the load by preventing the risks of overvoltages.

Advantageously, the predictive method used to estimate the future value of the load current makes use of an autoregressive model, specifically an autoregressive moving average model (ARIMA).

In an advantageous embodiment, the method provides for estimating a plurality of future values of the load current, and varying the variable resistance such that the voltage drop across the load, in the case of a load current equal to any of the estimated future values of the load current, does not exceed the critical threshold value.

The estimation of a plurality of future values of the load current allows greater protection from overvoltage risks in case of incorrect estimation.

In one embodiment, the method provides for checking whether the value of the measured load current causes a voltage drop across the load exceeding the critical threshold value, and storing in a memory area both the value of the measured load current and an exceeding information indicating whether the voltage at the load exceeds the critical threshold value. The method then provides for estimating future values of the load current based on the measurement taken, on a plurality of stored current values, and on a plurality of stored exceeding information.

By using not only the stored current measurements but also the stored exceeding information, i.e., the historical data, it is possible to improve the estimation of future values of the load current and make the system more robust. In one embodiment, the variable resistor is varied by an iterative algorithm that provides for increasing or decreasing its resistance by a predetermined amount. Advantageously, at each iteration, such an algorithm attempts to reduce the variable resistance to maximize the energy transfer to the load, but avoids doing so or even provides for increasing it if future values of the estimated load current predict a risk of overvoltage if the value of the resistance is reduced.

According to a further aspect, the invention is directed to an overvoltage protection system and to a wireless power receiver incorporating such a system.

Further features and objects of the present invention will appear clearer from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described here below with reference to some examples, provided for explanatory and non-limiting purposes, and illustrated in the accompanying drawings. These drawings illustrate different aspects and embodiments of the present invention and, where appropriate, similar reference numbers illustrating similar structures, components, materials, and/ or elements in different figures are indicated by similar reference numbers.

Figure 1 illustrates a system for wireless energy transfer according to the present invention.

Figure 2 illustrates a flow chart of a method for overvoltage protection in a receiver of a wireless power transfer system.

Figure 3 illustrates a flowchart of a variant of the method of Figure 2.

Figure 4 illustrates a flowchart of a further variant of the method of Figure 2.

Figure 5 illustrates the receiver of the system of Figure 1 according to the first embodiment of the invention.

Figure 6 illustrates a variant of the circuit of Figure 5.

DETAILED DESCRIPTION OF THE INVENTION

While the invention is susceptible to various modifications and alternative constructions, certain embodiments provided for explanatory purposes are described in detail below. It should be understood, however, that there is no intention to limit the invention to the specific embodiment illustrated, but, on the contrary, the invention is intended to cover all modifications, alternative constructions, and equivalents that fall within the scope of the invention as defined in the claims.

In the following description, therefore, the use of "e.g.," "etc.," "or" means nonexclusive alternatives without limitation unless otherwise indicated; the use of "also" means "including but not limited to" unless otherwise indicated; the use of "includes/ includes" means "includes/includes but not limited to" unless otherwise indicated.

The term "overvoltage" indicates a condition in which an electrical system or part of it, such as a load, is at a voltage above a threshold value for which it was designed.

With reference to Figure 1, a system 1 for wireless energy transfer, comprising a transmitter 2 and a receiver 3, is illustrated. The transmitter 2, in a per se known manner, is connected to a power source 4 and emits an electromagnetic field B that is received by the receiver 3 connected to a load 5, such as a battery.

In the example in Figure 1, system 1 is of inductive type, i.e., energy transfer is accomplished by electromagnetic coupling between two coils (6 and 7) that form a transformer. An alternating current is circulated in coil 6 of the transmitter, and this generates a variable electromagnetic field B that passes through coil 7 of the receiver into which a current I is induced, which, eventually rectified, supplies the load 5.

For the purposes of the present invention, however, the system 1 for wireless energy transfer could be of another type, for example, it could be of the resonant inductive coupling type (which as known uses circuits that resonate at the same frequency to improve energy transfer) or of the capacitive type, or of another known type. The receiver may thus be equipped with an antenna, winding or other receiving circuit suitable for receiving wireless energy and supplying a current intended to power a load. For the purposes of the present invention, what is relevant is that some power is transferred wirelessly from a transmitter 2 to a receiver 3 that is equipped with a overvoltage protection system 30 comprising at least one variable resistor 301 placed in series with the load and a controller 302 configured to control the variable resistor 301 in such a way as to prevent overvoltages to the load. According to one embodiment, controller 302 is configured to implement an overvoltage control method as described below with reference to Figure 2.

Controller 302 measures (step 2001) the current Io flowing at the load and stores the value in a memory area (step 2002).

Controller 302 then predicts (step 2003) at least one future value of current Ii. More preferably, controller 302 predicts a plurality of future current values, for example, future current values Ii, I2, 13 at future time instants Ti, T2, T3, with Ti < T ₂ < T ₃ .

The prediction can be made by any predictive method per se known, such as by the use of an autoregressive model, specifically an autoregressive integrated moving average (ARIMA) model, which, based on the current measured current value Io and based on historical current values (e.g., Ii, I2, 13) previously measured by the controller 302 and stored in the aforementioned memory area, estimates future current values Ii, I ₂ , 13. Alternatively, an ARMA (Autoregressive Moving Average) or a machine learning method can be used.

The controller 302, then, calculates (step 2004) a new value of the variable resistor that allows the maximum possible voltage to be maintained at the load while avoiding exceeding a threshold value beyond which damaging overvoltages to the load occurs. This threshold value will depend on the type of load and can be static or dynamic, that is, it can be fixed or time-varying. In the case of batteries, where charging that rises according to certain charging curves is preferable, this threshold will preferably be time-varying.

In the simplest case where the controller predicts a single future value of current Ii, then the new value of the variable resistor is chosen in such a way as to cause a voltage drop across the variable resistor 301 such that the voltage at the load results equal to the predetermined or expected threshold value for the corresponding instant of time.

In the case where the controller predicts multiple future current values Ii, I2, 13, then the new value of the variable resistor is chosen in such a way as to cause a voltage drop on the variable resistor, such that with any of the estimated values Ii, I2, I3, the voltage at the load always results less than or equal to the predetermined or expected threshold value for the corresponding time instant. Having calculated the new value of the variable resistor 301, the controller 302 checks the variable resistor (step 2005) so that it assumes the newly calculated resistance value.

In one embodiment shown in Figure 3, the method (step 2020) provides for testing whether the value of the measured load current results in a voltage at the load exceeding the critical threshold value, and provides for storing in a memory area both the measured value of the load current (step 2002) and an exceeding information (step 2020) indicating whether the voltage at the load exceeds the critical threshold value. The method then provides for (step 2030) estimating future values of the load current based on the measurement made in step 2002, a plurality of stored current values, and a plurality of stored exceeding information.

With reference to Figure 4, a variant of the method of Figure 2, that can be performed by the controller to protect the load from overvoltages, is illustrated. As illustrated below, this method essentially provides for varying the variable resistance by means of an iterative algorithm that increases or decreases the resistance by a predetermined amount. Although not illustrated, this algorithm can also be applied to the method of Figure 3, i.e., in other words, a step of storing the overshoot information can be added to the method of Figure 4 described below, which is used to estimate future current values at the load.

The method of Figure 4 begins at step 3000 by setting the value of variable resistor 301 (hereafter referred to as R301) to its maximum value. As an alternative to setting it to its maximum value, it is possible to use another value that is still close, say 15% less, than the maximum value. As with the method in Figure 2, the controller 302 measures the current Io at the load (step 3001), stores it in the designated memory area (step 3002), and predicts one or more future values of the load current (step 3003), e.g., values Ii, I2, 13.

At this point, instead of calculating the new value of the variable resistor so that the voltage at the load does not exceed a critical threshold value, the method of Figure 3 provides for (step 3004) reducing the variable resistor by a predetermined amount AR.

Before varying the value of the variable resistor 301, the controller performs an initial check (step 3005) to see if the new value of the variable resistor could, with the previously estimated future current values Ii, h, I3, cause a voltage drop at the load greater than the predetermined threshold.

If the new value of the resistor, obtained after decreasing AR, does not cause overvoltages, then the method provides for storing the new value of the variable resistor and controlling it (step 3006) to set it to the new value.

If the new value of the resistor, obtained after decreasing AR, could cause overvoltages with the estimated current values Ii, I2, 13, then the method provides for checking whether with the current value of the variable resistor (step 3007) a voltage drop at the load above the critical threshold could occur in the future. If not, the value of the variable resistor is kept unchanged (step 3008) and the method is repeated from steps 3001 onward. In the positive case, on the other hand, it is provided to increase the resistance by a fixed value AR (step 3009).

Figure 5 illustrates an embodiment of receiver 3 equipped with an overvoltage protection system 30 suitable for implementing the overvoltage protection methods described above.

The receiver 3 includes a winding 7 at the ends of which the induced current I _ac generated by the variable magnetic field B flows.

Receiver 3 further includes a rectifier 303 capable of rectifying the alternating current I _ac at the ends of winding 7. In the example shown here, rectifier 303 includes a diode 3030 in series with winding 7, a capacitor 3031 and a resistor 3032 in parallel with winding 7. Other types of rectifiers, such as diode bridges, can be used as an alternative to the one described here.

The rectified current I is sent to load 5. A switch 304 and variable resistor 301 are connected in series between rectifier 303 and load 5; in this way, switch 304, variable resistor 301 and load 5 are passed through by the same rectified current I. Variable resistor 301 is preferably a Digitally Controllable Resistor (DCR) that is preferably initialized to its maximum value and then controlled as described above. Alternatively, such a variable resistor may include a generic potentiometer as long as it can be automatically controlled by a microcontroller in controller 302.

Controller 302 measures the current I directed to the load and controls the value of variable resistor 301 to protect the load from overvoltages. For this purpose, in the example described here, the controller 302 is a computer system (preferably a chip or circuit board comprising multiple electronic components) that comprises a current meter 3020, a comparator 3021, and a controller 3022.

The current meter 3020 measures the rectified current I at fixed instants of time, e.g., set by a system clock, and outputs the Ampere value of the measured signal, e.g., the current value Io. The current measurement can be made in any way per se known, for example, the current measurement can be obtained by reading the voltage at the ends of a low-value resistor placed in series with the variable resistor 301 and preferably before the switch 304. Given the possibility of making measurements in different ways, a generic connection between current meter

3020 and the branch of the circuit carrying the current to load 5 is schematized in the example in Figure 5.

The current value Io measured by meter 3020 is given as input to both comparator

3021 and controller 3022.

Comparator 3021, based on the current value measured by meter 3020, the current value of the variable resistor 301, and the characteristics of the load - in particular to its resistance- , calculates whether the current I could cause an overvoltage at load 5, that is, whether it could result in a voltage drop (VL) at the load above a maximum threshold (VLTH) allowed by the load.

Comparator 3021 then returns a binary value (0 or 1) indicating whether load 5 can support the measured current Io. For example, the comparator returns the value 0 if the current can be supported (i.e., if VL VLTH), while it returns the value 1 if it cannot be supported (i.e., if VL > VLTH).

The output of comparator 3021 is given as input to both switch 304 and controller 3022.

In case the measured current Io is such that an overvoltage is generated at the load (i.e., if VL > VLTH), and thus the output of the comparator is equal to 1, switch 304 is opened, so that load 5 is protected. This intervention is used to protect the load should the overvoltage protection methods described above fail.

Controller 3022, which receives as input the current value provided by meter 3020 and the output of comparator 3021, is equipped with a memory unit (such as a register) in which it stores the two input time series for a window of time preferably adjustable by the user through a suitable user interface of receiver 3.

Controller 3022 is then configured to control variable resistor 301 by setting it to a new value determined as described above with reference to the methods of Figure 2 (steps 2003 to 2005) or Figure 3 (steps 3003 to 3008).

The new resistance value determined by controller 3022 is sent to comparator 3021, which will use this value to assess whether the load current can cause overvoltages.

In light of the above, it is clear how the invention allows the voltage at the load to be controlled dynamically by acting on the variable resistance while avoiding repeated disconnection of the load. The switch 304 provided in the example of Figure 5 serves to provide safety in the event that the estimate of future current values is wrong and enough current may reach the load to cause a voltage drop above a maximum allowable threshold for the load itself. Although this is not advisable, the switch 304 could also be omitted.

It is therefore clear that the solutions described above by way of example may be modified within the limits of the invention as reflected in the appended claims.

For instance, Figure 6 shows a variant of the receiver of Figure 5, wherein the controller 302 is connected to a branch of the circuit parallel to the one wherein the load is connected.

In this example, the meter 3020 measures a current I that is a fraction of the II current flowing to the load since the same voltage falls on the two branches. In detail, in the circuit in Figure 6, the two currents I and II are related by the relationship

Where Rcon is the value of the resistance in input to the controller 302, R301 is the value of variable resistance 301 and RL is the resistance value of load 5.

From the measurement of the current I _c , the controller 3022 and the comparator 3021 can calculate the measurement of the load current and perform the same operations as described above with reference to the circuit in Figure 5. For the purposes of the present invention, the measurement of the current Ic should also be understood as an indirect measurement of the load current. Both indirect measurement of the load current (e.g., Fig. 6) and direct measurement of the load current (Fig. 5) fall under the concept of measuring the load current. Regarding the methods described, it is clear to the person skilled in the art that some of the steps can be carried out in parallel or in a different order from that described without thereby changing the claimed invention.