Abstract:
Described is a rectification circuit to generate a direct current at an output of the rectification circuit subject to an alternating voltage at an input of the rectification circuit. The rectification circuit comprises: coupling means at the input to receive the alternating voltage from a galvanically decoupled electronic subsystem; a first switch arranged between the coupling means and the output to block current in a first direction and to conduct current in a second direction, wherein a resistance of the first switch is adjustable; a first modulation unit to receive encoded information; mapping the encoded information to a first modulation state, wherein each modulation state specifies a resistance value and/or a temporal evolution of the resistance value; adjusting the resistance of the first switch, thereby modulating the current conducted by the first switch according to the first modulation state.

Description:
1. TECHNICAL FIELD 
       [0001]    The present disclosure relates to isolated electrical systems coupled e.g. by a transformer/inductor system, a piezo transformer, a capacitor or other isolation means. In particular, the present disclosure relates to a method and system for communicating across isolation means in isolated electrical systems. 
       2. BACKGROUND 
       [0002]    Many electrical applications require a galvanic isolation of electrical subsystems. By way of example, wireless charging applications require the transfer of energy from a mains power supply to an electronic device comprising a battery, without any wired coupling, between a charging unit (receiving power from the mains power supply) and the electronic device. The energy is typically transferred from the charging unit to the electronic device through inductive coupling. The charging unit may use an induction coil (the primary side of a transformer) to create an alternating electromagnetic field. A second induction coil (the secondary side of the transformer) in the electronic device takes energy from the electromagnetic field and converts the energy into an electrical current to supply power to charge the battery of the electronic device. The first and the second induction coils form an electrical transformer, when arranged in proximity with each other. The distances between the first and second induction coil can typically be increased when using resonant inductive coupling. 
         [0003]    The charging unit (comprising the first induction coil) and the electronic device (comprising the second induction coil) are one particular example for an isolated electronic system comprising a first subsystem (e.g. the charging unit) and a second subsystem (e.g. the electronic device), wherein the first and second subsystems are galvanically isolated. It may be desirable to provide communication means between the first and the second subsystems, e.g. from the second subsystem to the first subsystem and vice versa. In the context of wireless charging such communication means may be used e.g. to inform the charging unit of the charging status of the battery comprised in the electronic device. The present disclosure describes a system and a method for enabling communication between the subsystems of an isolated electronic system in an efficient manner. The communication system and method described herein make use of the already available hardware components and can therefore be implemented in a cost efficient manner. 
       SUMMARY 
       [0004]    According to an aspect a rectification circuit is described. The rectification circuit may be configured to generate a direct current at an output of the rectification circuit subject to an alternating voltage at an input of the rectification circuit. The rectification circuit may comprise a half-wave rectifier (configured to generate a direct current during a first phase, e.g. the positive or negative phase, of the alternating voltage) or a full-wave rectifier (configured to generate a direct current during a first and second phase, e.g. the positive and the negative phase, of the alternating voltage). 
         [0005]    The rectification circuit may comprise coupling means at the input, configured to receive the alternating voltage from a galvanically decoupled electronic subsystem. The coupling means may e.g. comprise an induction coil of a transformer. The transformer may be configured to induce the alternating voltage at the coupling means using an alternating current through a corresponding induction coil of the transformer at the decoupled electronic subsystem. Alternatively the coupling means may e.g. comprise a piezo electric element which is coupled to another piezo electric element at the decoupled electronic subsystem. As such, an alternating voltage at the piezo electric element of the decoupled electronic subsystem may be coupled to the piezo electric element of the coupling means. 
         [0006]    The rectification circuit may comprise a first switch arranged between the coupling means and the output of the rectification circuit. The first switch may be part of a rectifier (e.g. a half-wave or full-wave rectifier). The first switch may be configured to block current in a first direction and to conduct current in a second direction, opposite to the first direction. The first direction may correspond to a first (e.g. a negative) polarization of the voltage across the first switch and the second direction may correspond to a second (e.g. a positive) polarization of the voltage across the first switch. As a result of the alternating voltage, the polarization of the voltage across the first switch may alternate between the first and the second direction at the alternation frequency of the alternating voltage. 
         [0007]    The alternating voltage may comprise a first phase (e.g. a negative half-wave of the alternating voltage) and a second phase (e.g. a positive half-wave of the alternating voltage). During the first phase, the voltage across the first switch may be in the first polarization and in the second phase, the voltage across the first switch may be in the second polarization. The first phase and the second phase may alternate at twice the alternation frequency. The first switch may be configured to block current during the first phase and configured to conduct current during the second phase. The current which is conducted by the first switch (i.e. the current during the second phase) may contribute to the direct current. In particular, the current which is conducted by the first switch may correspond to the direct current during the second phase. 
         [0008]    The rectification circuit may comprise a second switch arranged between the coupling means and the output of the rectification circuit. The second switch may be configured in a similar manner than the first switch. However, the second switch may be arranged such that the polarization of the voltage across the second switch is opposed to the polarization of the voltage across the first switch. As such, the second switch may conduct current during the first phase of the alternating voltage, thereby contributing to the direct current during the first phase. Furthermore, the second switch may be configured to block current during the second phase. As indicated above, the second switch may be arranged such that the polarization of the voltage across the second switch is opposed to the polarization of the voltage across the first switch. By doing this, it can be ensured that the current provided by the second switch has the same direction as the current provided by the first switch. As such, the direct current provided by the rectification circuit may correspond to the current through the first switch (during the second phase) and to the current through the second switch (during the first phase). 
         [0009]    Typically, the first switch is coupled to a first output port of the coupling means and the second switch is coupled to a second output port of the coupling means, wherein the alternating voltage is provided between the first and second output port of the coupling means. Furthermore, the first switch and the second switch are typically coupled to a same output port at the output of the rectification circuit. Typically, the output of the rectification circuit comprises two output ports, wherein a load voltage is provided across the two output ports. By coupling the first and second switches as outlined above, it can be ensured that the voltage drop across the first and second switch is opposed to each other and that the current provided by the first switch within the second phase has the same direction as the current provided by the second switch within the first phase, thereby providing a direct current during the first and second phase. 
         [0010]    The first and/or second switch may comprise any one or more of: a diode, and a transistor e.g. a MOS (Metal Oxide Semiconductor) transistor. By way of example, the first and/or second switch may be MOS transistors comprising a body diode. A resistance of the first switch (and/or the second switch) when conducting current may be adjustable. This may be achieved e.g. by adjusting a drive voltage to the first/second switch (notably by adjusting a voltage applied to the gate of the first/second switch). By way of example, a MOS transistor may be operated as a body diode (have a maximum resistance), at a low gate voltage (having a medium resistance) and at a high gate voltage (having a minimum resistance). 
         [0011]    The rectification circuit may comprise a first modulation unit configured to receive encoded information. The encoded information may be information to be transmitted from the rectification circuit to the decoupled electronic subsystem. By way of example, the rectification circuit may be comprised within an electronic device comprising a battery, and the decoupled electronic subsystem may comprise a charging unit for charging the battery of the electronic device. The encoded information may be related to a charging status of the battery of the electronic device. The rectification unit or the electronic device may comprise an encoding unit configured to encode the information to be transmitted to the decoupled electronic subsystem, thereby providing the encoded information. The encoded information may comprise an error detection and/or error correction code. 
         [0012]    The first modulation unit may be configured to map the encoded information to a first modulation state from a plurality of different modulation states of the resistance of the first switch. A modulation state of the resistance of the first switch typically specifies a resistance value and/or a temporal evolution of the resistance value of the resistance of the first switch. In other words, the modulation state may specify a resistance value at a particular time instance and/or a sequence of resistance values at a corresponding sequence of time instances. 
         [0013]    The first modulation unit may be configured to adjust the resistance of the first switch according to the first modulation state, thereby modulating the current conducted by the first switch according to the first modulation state. In particular, the first modulation unit may be configured to adjust the resistance of the first switch, when the first switch is conducting current (e.g. during the second phase of the alternating voltage, i.e. when the voltage across the first switch has a second polarization). As a result of the modulation of the resistance, the current conducted by the first switch is modulated, i.e. the direct current is modulated. Typically, the current through the first switch is derived from current through the coupling means (e.g. from the induction coil comprised within the coupling means). This means that the modulated current through the first switch also affects the current through the coupling means. By consequence, the current through the coupling means is modulated according to the first modulation state. 
         [0014]    The first modulation unit may be configured to adjust the resistance of the first switch according to any one of the plurality of modulation states. A modulation state of the plurality of modulation states may comprise any one or more of: adjusting the resistance to one or more of a plurality of resistance values (e.g. for amplitude modulation); and periodically adjusting the resistance of the first switch between a first and a second resistance value of the plurality of resistance values at one of a plurality of adjustment frequencies (e.g. for frequency modulation). 
         [0015]    The plurality of modulation states may be selected such that an average of the direct current in a pre-determined time interval remains substantially constant. This may be important in order to ensure a continuous and constant energy flow towards a load coupled to the rectification circuit. By way of example, the rectification circuit may be a solid state lighting (SSL) device, e.g. a LED (Light Emitting Diode) or a OLED (organic LED). Such SSL devices typically require a continuous energy flow, in order to ensure a constant (flickerless) emission of light. This may be achieved by always and only performing frequency modulation (even in a default state, when no information is transmitted). It should be noted that in order to ensure a flickerless emission of light, the continuous energy flow should be continuous with respect to variations of light intensity which are at frequencies visible to the human eye (e.g. frequencies below 400 Hz). Intensity variations above such frequencies (e.g. variations due to the modulation of the current described herein) are typically not visible to the human eye. Consequently, frequency modulation should be performed at modulation frequencies above the frequencies of intensity variations which are visible to the human eye. 
         [0016]    In particular, the first modulation unit may be configured to periodically adjust the resistance of the first switch between the first resistance value and the second resistance value at a first frequency of the plurality of adjustment frequencies, thereby providing the first modulation state. The first modulation unit may be further configured to periodically adjust the resistance of the first switch between the first resistance and the second resistance at a second frequency of the plurality of adjustment frequencies, thereby providing a second modulation state. Typically the first and second frequencies are higher than the alternation frequency of the alternating voltage. The average direct current during the second phase of the alternating voltage (i.e. the direct current during the phase when the first switch is conducting) may be substantially the same in the first modulation state and in the second modulation state. 
         [0017]    The rectification unit may further comprise a second modulation unit configured to receive encoded information and to map the encoded information to a third modulation state from a plurality of different modulation states of the resistance of the second switch. In a similar manner to the first switch, each of the plurality of modulation states may specify a resistance value and/or a temporal evolution of the resistance value of the resistance of the second switch. The second modulation unit may be configured to adjust the resistance of the second switch according to the third modulation state of the resistance of the second switch, thereby modulating the current conducted by the second switch according to the third modulation state of the resistance of the second switch. 
         [0018]    According to another aspect, a galvanically decoupled system is described. The galvanically decoupled system may comprise a first subsystem configured to generate a varying current, e.g. an alternating current. Furthermore, the system may comprise a second subsystem comprising a rectification unit according to any of the aspects outlined in the present document. The second subsystem (notably the rectification circuit) may be configured to receive an alternating voltage derived from the varying current (e.g. via a transformer). Furthermore, the second subsystem may be configured to modulate a resistance of the rectification unit according to a first modulation state derived from encoded information. The first subsystem may comprise modulation sensing means configured to detect the first modulation state e.g. from the varying current. 
         [0019]    By way of example, the first subsystem may comprise a half bridge comprising a high side switch and a low side switch. The high side and low side switches may be opened and closed in an opposed and periodic manner (at the alternation frequency), thereby generating an alternating current. In such a case, the modulation sensing means may comprise a resistor to measure a current through the high side switch and/or the low side switch. The second subsystem may further comprise a decoding unit configured to determine the encoded information from the first modulation state. 
         [0020]    The first subsystem may comprise a first induction coil of a transformer and the second subsystem may comprise a second induction coil of the transformer. The transformer may be configured to inductively couple the alternating current across the first and second subsystems. In such a case, the modulation sensing means may comprise an auxiliary induction coil of the transformer configured to sense the first modulation state from the voltage across the auxiliary induction coil. 
         [0021]    According to another aspect, a method for communicating encoded information from a second subsystem to a first subsystem is described. The first and second subsystems may be galvanically decoupled. The method comprises receiving an alternating voltage from the first subsystem at the second subsystem. The method proceeds in blocking current in a first direction and conducting current in a second direction, opposite to the first direction, to provide a direct current. The current may be conducted in the second direction via an adjustable resistance. The method further comprises the step of receiving encoded information and of mapping the encoded information to a first modulation state from a plurality of different modulation states of the adjustable resistance. Each of the plurality of modulation states may specify a resistance value and/or a temporal evolution of the resistance value of the adjustable resistance. The method proceeds in adjusting the adjustable resistance according to the first modulation state, thereby modulating the conducted current in the second direction according to the first modulation state. 
         [0022]    It should be noted that the methods and systems including its preferred embodiments as outlined in the present disclosure may be used stand-alone or in combination with the other methods and systems disclosed in this document. Furthermore, all aspects of the methods and systems outlined in the present disclosure may be arbitrarily combined. In particular, the features of the claims may be combined with one another in an arbitrary manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The invention is explained below in an exemplary manner with reference to the accompanying drawings, wherein 
           [0024]      FIG. 1  illustrates the circuit diagram of an example isolated electrical system comprising a full wave rectifier; 
           [0025]      FIG. 2  illustrates the circuit diagram of an example isolated electrical system comprising a full wave rectifier with a center tap transformer; 
           [0026]      FIG. 3  shows an example circuit diagram illustrating the functional principle of a modulated rectifier; 
           [0027]      FIG. 4  shows an example of a modulated current; 
           [0028]      FIG. 5  shows the circuit diagram of an example rectification circuit comprising an example modulation unit; 
           [0029]      FIG. 6   a  shows another circuit diagram of an example isolated electrical system comprising a full wave rectifier; 
           [0030]      FIG. 6   b  shows an example implementation of current modulation means; 
           [0031]      FIG. 7  shows a circuit diagram of an example isolated electrical system comprising a piezo transformer; 
           [0032]      FIG. 8  shows a circuit diagram of an example isolated electrical system comprising a capacitor; and 
           [0033]      FIG. 9  shows a circuit diagram of an example isolated electrical system comprising a flyback converter. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]      FIG. 1  shows the circuit diagram of an example isolated electrical system  100 . In particular,  FIG. 1  shows the circuit diagram of an example wireless charging system  100  comprise a first subsystem  140  (i.e. the charging unit) and a second subsystem  150  (i.e. the electronic device). The charging unit  140  comprises a high side switch Q 1   101  and a low side switch Q 2   102  which are arranged to generate an alternating current at the capacitor C 1   105  and the first induction coil P 1  of the transformer  106 . The capacitor C 1   105  may be used to eliminate a DC (direct current) voltage across the first induction coil P 1  of the transformer  106 . 
         [0035]    The high side switch Q 1   101  and the low side switch Q 2   102  form a half bridge which may be used to generate an AC (alternating current) voltage at a pre-determined frequency (typically referred to as the converter frequency). The converter frequency may be in the range of 20 kHz to several MHz. The high side and the low side switches  101 ,  102  may be transistors, e.g. MOS transistors such as PMOS or NMOS transistors. The capacitor C 1   105  and the first induction coil P 1  of the transformer  106  may form an LC circuit having a pre-determined resonance frequency which depends on the capacitance C of the capacitor  105  and the inductance L of the first induction coil. As such, the charging unit  140  may be configured to perform resonant inductive coupling, thereby increasing the possible distances between the charging unit  140  and the electronic device  150 . The higher distances may be due to the higher voltages which may be generated when using resonant inductive coupling, thereby compensating for the higher distances. 
         [0036]    The AC voltage generated by the half bridge leads to an alternating current in the first induction coil P 1  of the transformer  106 , which is coupled to the second induction coil S 1  of the transformer  106  comprised within the electronic device  150 . Consequently, the alternating current in the second induction coil S 1  of the transformer  106  is derived from the alternating current in the first induction coil P 1 . Both alternating currents are typically proportional to each other, wherein the proportionality factor typically depends on the coupling efficiency of the transformer  106 . 
         [0037]    In addition to the second induction coil S 1  of the transformer  106 , the electronic device  150  comprises a rectifier  110  comprising the switches D 1   111 , D 2   112 , D 3   113 , D 4   114  and a smoothening capacitor C 2   115 . The switches  111 ,  112 ,  113 ,  114  may be diodes which automatically switch between the on-state and the off-state based on the voltage across the diodes. In particular, if the voltage across the diode is negative and the diode is operated in its reverse-biased mode, the diode is in the off-state. On the other hand, if a positive voltage greater than the threshold voltage of the diode (also referred to as the diode voltage V D ) is applied to the diode, i.e. if the diode is operated in its forward-biased mode, then the diode is in the on-state. Alternatively, some or all of the switches  111 ,  112 ,  113 ,  114  may be implemented as transistors, e.g. MOS transistors, thereby providing a so called active rectifier. The transistors may be switched between the on-state and the off-state in synchronization with the alternating current (i.e. in sync with the converter frequency), in order to provide a full-wave rectification of the alternating current. The capacitor  115  which is arranged in parallel to, the load may be used to smoothen the rectified current. The rectifier  110  provides an output voltage Vout to the load. 
         [0038]    The electronic device  150  and in particular the rectifier  110  comprise a modulation unit  120  configured to modulate the voltage drop across the rectifier  110 , and by consequence to modulate the current provided by the rectifier  110 . The modulation of the current provided by the rectifier  110  leads to a modulation of the alternating current at the second induction coil of the transformer  106 . This modulation of the alternating current is inductively coupled to the first induction coil of the transformer  106 , thereby impacting the current through the switches  101 ,  102  of the half bridge. The charging unit  140  comprises modulation sensing means  103 ,  104  configured to sense the modulation on the current through the switches  101 ,  102  of the half bridge. In other words, the modulation sensing means  103 ,  104  are configured to provide an indication of the modulation performed in the rectifier  110  at the output  107 ,  108  of the modulation sensing means  103 ,  104 . In the illustrated example, the modulation sensing means  103 ,  104  are implemented as shunt resistors  103 ,  104 , wherein the outputs  107 ,  108  provide the voltage drop across the shunt resistors  103 ,  104  as an indication of the current through the high side switch  101  and/or the low side switch  104 . 
         [0039]    The functional principle of the modulation unit  120  and the modulation sensing means  103 ,  104  will be illustrated in further detail in the context of  FIGS. 3 ,  4  and  5 . Overall, it may be stated that the modulation unit  120  may be used to modulate a voltage drop across the rectifier  110 , in order to induce a modulation on the load current. This modulation of the load current is coupled to the alternating current in the charging unit  140  via the first and second induction coils of the transformer  106 . The modulation of the alternating current in the charging unit  140  can be measured at the outputs  107 ,  108  of the current sensing means  103 ,  104 . In particular, the modulation of the positive half-wave of the alternating current can be measured as a modulated voltage drop across the resistor  103  and the modulation of the negative half-wave of the alternating current can be measured as a modulated voltage drop across the resistor  104 . 
         [0040]    As such, the modulation unit  120  and the current sensing means  103 ,  104  can be used to provide a communication path from the electronic device  150  to the charging unit  140 . The electronic device  150  may comprise an encoding unit  170  configured to encode information (e.g. a state of charging of a battery comprised within the electronic device  150 ). The encoded information may be mapped to one or more modulation states of the modulation unit  120 , wherein the modulation unit  120  modulates the current according to the one or more modulation states. The one or more modulation states are detected in a decoding unit  180  within the charging unit  140  which demodulates the sensed voltage at the outputs  107 ,  108  and which decodes the encoded information. 
         [0041]    It should be noted that in principle encoded information may be communicated from the electronic device  150  to the charging unit  140  during the positive and negative half-waves for the half bridge comprising the switches  101 ,  102 . However, due to a relatively high voltage difference between the high side switch  101  and ground during a positive half-wave, the current sensing means  103  may not be able to provide reliable measurements of the modulation. Hence, it may be preferable to restrict the communication of encoded information to the negative half-wave, when the low-side switch  102  is closed (and the high-side switch  101  is open). 
         [0042]      FIG. 2  shows the circuit diagram of another example isolated system  200  (e.g. an example wireless charging system  200 ). The wireless charging system  200  comprises a charging unit  240  with the high side switch Q 3   201  and the low side switch Q 4   202  configured to generate an alternating current through the first induction coils P 1 , P 2  of the transformer  206 . In the illustrated example, the transformer  206  is implemented as a center tap transformer  206  (also referred to as a push pull transformer) comprising an upper pair of induction coils P 1 , S 1  and a lower pair of induction coils P 2 , S 2 . The first upper and lower induction coils P 1 , P 2  (on the primary side of the transformer  206 ) are coupled in the center of the transformer  206  to the supply voltage Vcc. 
         [0043]    The electronic device  250  is coupled to the charging unit  240  via the second upper and lower induction coils S 1 , S 2  (on the secondary side) of the transformer  206 . The electronic device  250  comprises a rectifier  210  comprising two switches D 1   211 , D 2   212  which provide full-wave rectification of the coupled alternating current. In the illustrated example, the two switches  211 ,  212  are implemented as diodes, i.e. as automatic switches. Alternatively or in addition, some or all of the switches  211 ,  212  may be implemented as active switches, such as transistors (e.g. MOS transistors). Furthermore, the rectifier  215  comprises a smoothening capacitor C 2   215  arranged in parallel to the output of the rectifier  210 . 
         [0044]    In a similar manner to  FIG. 1 , the isolated system  200  comprises a modulation unit  220  configured to modulate the current within the rectifier  210 . For this purpose, the two switches  211 ,  212  may be implemented as transistors comprising a body diode. The modulation unit  220  may be configured to vary the resistance of the switches  211 ,  212 , thereby varying the voltage drop across the rectifier  210 , thereby modulating the current within the rectifier  210 . The modulation of the current within the rectifier  210  may be sensed by modulation sensing means  203 ,  204  within the charging unit  240 . In the illustrated example, the modulation sensing means  203 ,  204  are implemented as shunt resistors R Shunt 2   203 , R Shunt 3   204  which transform the current through the high side and/or low side switches  201 ,  203  into a voltage drop which can be provided at the outputs  207 ,  208 . Other sensing means may e.g. make use of current mirrors. Further examples for sensing means comprise a coil system for frequency monitoring, a Hall sensor or a current transformer, which changes its behavior as a function of the current. 
         [0045]    Furthermore, the electronic device comprises an encoding unit  270  configured to encode information which is to be communicated to the charging unit  240 . The encoded information may be e.g. a sequence of bits. The encoding unit  270  may make use of error detection and/or error correction schemes when determining the encoded information. Examples for error detection schemes are parity bits, checksums, cyclic redundancy checks. Examples for schemes which also allow for error correction are e.g. error-correcting codes. As such, the encoded information may comprise redundant information, which may be used for error detection and/or error correction. 
         [0046]    The encoded information may be passed to the modulation unit  220  which may be configured to assign the encoded information (e.g. a sequence of bits) to one or more modulation states and/or to a sequence of modulation states. Furthermore, the modulation unit  220  may be configured to apply the modulation (in accordance to the modulation states) to the current within the rectifier  250 , thereby communicating the encoded information to the current sensing means  203 ,  204 . At the output  207 ,  208  of the current sensing means, the modulated current/voltage may be passed to a demodulation and decoding unit  280 . The demodulation and decoding unit  280  may be configured to demodulate the modulated current/voltage. For this purpose, the demodulation and decoding unit  280  may apply one or more filters (e.g. bandpass filters), in order to detect a frequency modulation of the modulated current/voltage. Alternatively or in addition, the demodulation and decoding unit  280  may apply one or more comparators, in order to determine an amplitude modulation of the modulated current/voltage. As such, the demodulation and decoding unit  280  may be configured to determine the one or more modulation states and/or the sequence of modulation states from the modulated current/voltage. The detected modulation states may be mapped to the encoded information which may then be decoded, in order to provide the charging unit  140  with the information transmitted from the electronic device  250 . 
         [0047]      FIG. 3  shows a circuit diagram which illustrates the functional principle of rectifier current modulation for communication purposes across an isolated electronic system  300 . The isolated system  300  comprises a first subsystem  340  (e.g. a charging unit) and a second subsystem  350  (e.g. an electronic device comprising a battery). Typically, the first subsystem  340  and the second subsystem  350  are isolated e.g. using a galvanic isolation such as a transformer (as shown e.g. in  FIGS. 1 and 2 ). Such an isolation between the first and second subsystems  340 ,  350  is not shown in  FIG. 3  for simplicity reasons. 
         [0048]    The first subsystem  340  comprises a voltage source  301  providing an alternating voltage. This alternating voltage creates an alternating current which is passed to the second subsystem  350  (e.g. via inductive coupling). The alternating current is rectified using one or more switches  311 ,  312 . In the illustrated example, a first diode  311  and a second diode  312  are used. The two diodes  311 ,  312  are arranged in series, thereby providing a half-wave rectification of the alternating current. The electronic device  350  comprises a smoothening capacitor  316  which is arranged in parallel to the load  330  of the rectifier. The load  330  may be e.g. a battery comprised within the electronic device  350 . 
         [0049]    The second subsystem  350  comprises a modulation unit  320 . In the illustrated example, the modulation unit  320  comprises a modulation switch  323  which is arranged in parallel to the second diode  312  and which is configured to bypass the second diode  312  when the modulation switch  323  is in the on-state. By doing this, the modulation switch  323  can be used to modulate the voltage drop across the series of diodes  311 ,  312 , when the two diodes  311 ,  312  are operated in their forward-biased mode. As indicated above, a diode  311 ,  312  is operated in its forward-biased mode by applying a positive voltage across the diode  311 ,  312 , wherein the positive voltage is greater than the threshold voltage of the diode  311 ,  312 . If the modulation switch  323  is in the off-state, the voltage drop across the first and second diodes  311 ,  312  is the sum of the first threshold voltage across the first diode  311  and the second threshold voltage across the second diode  312 . On the other hand, if the modulation switch  323  is in the on-state, the voltage drop across the first and second diodes  311 ,  312  corresponds to the first threshold voltage (if assuming that the voltage drop across the modulation switch  323  is negligible or small compared to the second threshold voltage). Hence, by switching the modulation switch  323  between the on-state and the off-state, the voltage drop across the rectifier diodes  311 ,  312  can be modulated. This also results in a modulation of the output voltage across the capacitor  316  and across the load  330 , because the sum of the voltage drop across the rectifier diodes  311 ,  312  and the output voltage corresponds to the voltage provided by the voltage source  301  (when neglecting the voltage drop at the sensing resistor  303 ). A modulation of the output voltage leads to a modulation of the current through the load, and consequently to a modulation of the current provided by the rectifier. Consequently, the modulation switch  323  can be used to modulate the current through the rectifier diodes  311 ,  312 . The current through the rectifier diodes  311 ,  312  is (inductively) coupled to the first subsystem  340 , such that a modulation of the current through the rectifier diodes  311 ,  312  is coupled back to the first subsystem  340 . It is proposed in the present disclosure to use this backward coupling of modulation of the current as communication means between the second subsystem  350  and the first subsystem  340 . 
         [0050]      FIG. 3  illustrates an example control circuit for controlling the modulation switch  323 . The control circuit comprises a first voltage source  321 , a second voltage source  324  and an inverter  322 . By appropriately controlling the first and second voltage sources  321 ,  324 , the modulation switch  323  may be switched between the on-state and the off-state, thereby modulating the current in the rectifier and thereby communicating information from the second subsystem  350  to the first subsystem  340 . As indicated above, the information may be encoded into a sequence of bits (e.g. comprising error detection and/or error correction). The sequence of bits may be used to control the modulation switch  323 , thereby transmitting the encoded information from the second subsystem  350  to the first subsystem  340 . 
         [0051]    The first subsystem  340  comprises a shunt resistor  303  (also referred to as a sensing resistor  303 ), as well as a low pass filter comprising a resistor  304  and a capacitor  305 . The low pass filter may be used to reduce or remove oscillations of the current through the shunt resistor  303 .  FIG. 4  shows a modulated current  400  through the shunt resistor  303  (or a modulated voltage drop at the shunt resistor  303 ). It can be seen that during the negative wave of the alternating voltage provided by the voltage source  301 , the current  401  is zero (as the rectifier diodes  311 ,  312  are operated in reverse-biased mode). On the other hand, during a positive wave of the alternating voltage, the rectifier diodes  311 ,  312  are operated in forward-biased mode, thereby allowing for a current flow  402 . This current flow can be modulated by means of the modulation unit  320 , i.e. by modulating the voltage drop across the rectifier diodes  311 ,  312 . 
         [0052]    Hence, a modulation within the rectifier of the second subsystem  350  can be sensed by the modulation sensing means (i.e. at the shunt resistor  303 ) of the first subsystem  303 . The modulation within the rectifier may be achieved by modulating the voltage drop across one or more of the switches  311 ,  312  which are in the on-state. This modulation of the voltage drop leads to a modulation of the current provided by the rectifier, i.e. to a modulation of the current provided to the load  330 . The modulated current in the second subsystem  350 , in particular the modulations on the current in the second subsystem  350 , are coupled back to the current within the first subsystem  340  (e.g. via a transformer  106 ,  206 ) and can therefore be measured at the first subsystem  340  using modulation sensing means  303 . As indicated above, the first subsystem  340  may further comprise a demodulation unit for extracting the modulation state (or a sequence of modulation states) from the sensed current/voltage. Subsequently, the (sequence of) modulation states may be mapped to the encoded information, which may then be decoded by a decoding unit. 
         [0053]    The modulation of the voltage drop across the rectifier of the second subsystem  350  can be used to signal information from the second subsystem  350  to the first subsystem  340 . By way of example, the second subsystem  350  can inform the first subsystem  340  of the charging status of a battery  330  comprised in the second subsystem  350 . In another example, the second subsystem  350  may comprise a solid-state lighting (SSL) device such as alight emitting diode (LED) or an Organic LED (OLED) device. The SSL device may use the modulation within the rectifier to provide the first subsystem  340  (e.g. a power supply) with information regarding its illumination status (e.g. regarding the intensity of the emitted light). In general terms, the voltage drop across the rectifier within the second subsystem  350  may be modulated in a pre-determined manner, in order to provide information from the second subsystem  350  to the first subsystem  340 , even though the first and second subsystems  340 ,  350  are (galvanically) isolated from each other. 
         [0054]    The modulation of the voltage drop across the rectifier (and consequently the modulation of the current) may comprise amplitude modulation. Amplitude modulation may be achieved by controlling the total voltage drop across the rectifier to be at a limited number of N voltage values. Alternatively or in addition, the modulation of the voltage drop across the rectifier may comprise frequency modulation, by varying the voltage drop across the rectifier between a first amplitude and a second amplitude at a pre-determined number M of frequencies. Alternatively or in addition, the modulation of the voltage drop across the rectifier may comprise modulation at a pre-determined number L of duty cycles, by varying the ratio of the length of the voltage drop at the first amplitude and the length of the voltage drop at the second amplitude according to L pre-determined ratio values. Alternatively or in addition, the modulation may be changed along the time line, e.g. at a pre-determined number Q of modulation states/second. The above mentioned modulation schemes may be combined, thereby providing a maximum number of N times M times L states, of which Q states can be implemented (and communicated) per second. Hence, the modulation of the voltage drop across the rectifier can be used to encode up to log(N*M*L*Q) bits per second (wherein “*” is the multiply operator and “log” is the logarithm-to-the-base-two operator) of information from the second subsystem  350  to the first subsystem  340 . 
         [0055]    By way of example, the modulation unit  120 ,  220  may be configured to change the resistance of the rectifier  110 ,  210  (i.e. to change the voltage drop across the rectifier  110 ,  210 ) between a first and a second resistance value (i.e. N=2). As such, the modulation unit  120 ,  220  provides N=2 modulation states, which may be used to transmit log(N)=1 bit of information from the second subsystem  350  to the first subsystem  340 . If the frequency of the alternating current is f=1 MHz, and if the modulation unit  120 ,  220  is configured to implement a different modulation state for each cycle of the alternating current, then log(N)*f=1 Mbit/second of encoded information can be transmitted from the modulation unit  120 ,  220 . As indicated above, the encoded information can comprise error detection and/or error correction coding. 
         [0056]    Alternatively or in addition, the modulation unit  120 ,  220  may be configured to perform frequency modulation. A first modulation state may comprise the periodic change between the first and second resistance values at a first frequency (higher than the frequency of the alternating current) (e.g. f1=10 MHz). The second modulation state may comprise the periodic change between the first and second resistance values at a second frequency (different from the first frequency) (higher than the frequency of the alternating current) (e.g. f2=5 MHz), i.e M=2. The modulation unit  120 ,  220  may be configured to implement a different modulation state for each cycle of the alternating current, thereby providing a transmission rate of log(M)*f=1 Mbit/second. 
         [0057]    It should be noted that the communication of information from the second subsystem  350  to the first subsystem  340  via modulation of the rectifier current typically requires a synchronization of the modulation unit  120 ,  220  and the modulation sensing means  103 ,  203 , and/or a synchronization of the encoding unit  170  and the decoding unit  180 ). In particular, it should be ensured that the encoded information is modulated onto the current at the rectifier  110 ,  210  at the same rate, as it is demodulated at the first subsystem  340 . For this purpose, the first and second subsystem should make use of a common clocking. The common clocking may be derived from the frequency/cycle rate of the alternating current generated by the first subsystem  340  (e.g. by the high side/low side switches  101 ,  102  comprised within the first subsystem  340 ). The frequency/cycle rate of the alternating current may be detected independently at the first and at the second subsystem. 
         [0058]      FIG. 5  shows the circuit diagram of an example electronic device  500  (i.e. of an example second subsystem  500 ). The electronic device  500  is coupled to a first subsystem (e.g. to a charging unit) via a transformer  506 . The second induction coil (also referred to as the secondary side) of the transformer  506  is typically integrated within the electronic device  500 . Furthermore, the electronic device  500  comprises an active rectifier  510  comprising four switches  511 ,  512 ,  513 ,  514  which correspond to the diodes  111 ,  112 ,  113 ,  114 , respectively, of the rectifier  510  of  FIG. 1 . The active rectifier  510  also comprises a smoothening capacitor  515 . 
         [0059]    During a positive half-wave of the alternating voltage supplied across the transformer  506 , the second high side switch  514  and the first low side switch  511  are in the on-state, whereas the first high side switch  513  and the second low side switch  512  are in the off-state. This phase of the active rectifier  510  may be referred to as the positive half-wave phase. During the positive half-wave phase, the voltage drop across the active rectifier  510  comprises the voltage drop across the switches which are in on-state, i.e. the voltage drop across the second high side switch  514  and the first low side switch  511 . During a negative half-wave of the alternating voltage supplied across the transformer  506 , the first high side switch  513  and the second low side switch  512  are in the on-state, whereas the second high side switch  514  and the first low side switch  511  are in the off-state (referred to as the negative half-wave phase of the active rectifier  510 ). During the negative half-wave phase, the voltage drop across the active rectifier  510  comprises the voltage drop across the switches which are in on-state, i.e. the voltage drop across the first high side switch  513  and the second low side switch  512 . 
         [0060]    In the illustrated example of  FIG. 5 , the high side switches  513 ,  513  are implemented as P channel MOS (metal-oxide semiconductor) FETs (field effect transistors), whereas the low side transistors are implemented as N channel MOS FETs. It should be noted that transistors (and in particular MOS FETs) typically comprise so called body diodes between the drain and the source of a P-channel transistor and between the source and the drain of an N-channel transistor. These body diodes are automatically activated, if the voltage across the transistor exceeds the threshold voltage (also referred to as the diode voltage) of the body diode of the transistor. This has at least two consequences. Firstly, this means that by exploiting the drain-to-source body diodes of the P-channel transistors (i.e. the first and second high side transistors  513 ,  514 ) and by exploiting the source-to-drain body diodes of the N-channel transistors (i.e. the first and second low side transistors  511 ,  514 ), the rectifier  510  can be operated in an automatic mode (e.g. during a start up phase), without the need of a controlled switching of the switches  511 ,  512 ,  513 ,  514 . Secondly, the diode voltage of any of the switches  511 ,  512 ,  513 ,  514  may be used to modulate the voltage drop across the rectifier  510 . In particular, each of the switches  511 ,  512 ,  513 ,  514  of the rectifier  510  may be operated in an active mode, thereby providing a reduced transistor voltage drop V S , or in a passive mode, thereby providing the (higher) diode voltage drop V D . 
         [0061]    The rectifier  510  of  FIG. 5  comprises a first modulation unit  520  configured to modulate the voltage drop across the rectifier  510  during the positive half-wave phase and a second modulation unit  527  configured to modulate the voltage drop across the rectifier  510  during the negative half-wave phase. The first modulation unit  520  comprises an operational amplifier  522  and a voltage source  521  configured to provide an activating gate voltage to the first low side switch  511 . The voltage source  521  may be adjustable, thereby adjusting the gate voltage to the first low side switch  511 . By adjusting the gate voltage, the resistance of the first low side switch  511  can be adjusted, thereby adjusting the voltage drop across the first low side switch  511 . Hence, the operational amplifier  522  and the voltage source  521  are configured to modify the voltage drop across the first low side switch  511 . 
         [0062]    Furthermore, the first modulation unit  520  comprises a NAND (Not AND) gate  523 . The NAND gate  523  is configured to apply an activating gate voltage to the second high side switch  514 , if an activating gate voltage is applied to the first low side switch  511  AND if a voltage is applied to the second input port of the NAND gate  523 . If no voltage is applied to the second input port, the second high side switch  514  is operated as a body diode with a voltage drop corresponding to the diode voltage V D . On the other hand, if a voltage is applied to the second input port of the NAND gate  523 , then an activating gate voltage is applied to the second high side switch  514 , thereby putting the second high side switch  514  in the on-state, at a reduced (transistor) voltage drop V S . 
         [0063]    The first modulation unit  520  is configured to modulate the voltage drop across the rectifier  510  during the positive half-wave phase of the rectifier  510 . The voltage drop may be modified by
       operating the second high side switch  514  as a body diode (leading to a diode voltage V D ) or as a switched-on transistor (leading to a possibly adjustable transistor voltage V S ). Typical examples are V D  in the range of 0.7V and V S  in the range of 0.2V.   operating the first low side switch  511  as a body diode (leading to a diode voltage V D ) or as a switched-on transistor (leading to a possibly adjustable transistor voltage V S ).   adjusting the gate voltage applied to the first low side switch  511  and/or to the second high side switch  514  (not shown), thereby adjusting the transistor voltage V S , e.g. between ½ and 1 times V S .       
 
         [0067]    Table 1 shows example modulation states and corresponding example voltage drops across the rectifier  510  during the positive half-wave phase. Table 1 indicates for each state, the gate voltage applied to the first low side switch  511  and the gate voltage applied to the second high side switch  514  and the resulting voltage drop across the rectifier  510 . It should be noted that Table 1 assumes the same voltage drop V D , V S  across the (P-channel) second high side transistor  514  and across the (N-channel) first low side transistor  511 . Typically, the on-resistance of P-channel and N-channel transistors differ significantly. Hence, the appropriate selection of P-channel and N-channel transistors provides a further parameter for implementing a different number N of voltage drops across the rectifier  510 , i.e. for implementing different amplitude modulation states. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 State 
                 1 st  low side switch 
                 2 nd  high side switch 
                 Total Voltage drop 
               
               
                   
               
             
             
               
                 1 
                 No gate voltage 
                 No gate voltage 
                 2*V D   
               
               
                 2 
                 Full gate voltage 
                 No gate voltage 
                 V D  + V S   
               
               
                 3 
                 “half” gate voltage 
                 No gate voltage 
                 V D  + 1/2*V S   
               
               
                 4 
                 Full gate voltage 
                 Full gate voltage 
                 2*V S   
               
               
                 5 
                 “half” gate voltage 
                 Full gate voltage 
                 3/2*V s   
               
               
                   
               
             
          
         
       
     
         [0068]    The second modulation unit  527  is designed in an analogous manner to the first modulation unit  520 , i.e. comprising an operational amplifier  525 , a voltage source  524  and a NAND gate  526 . The second modulation unit  527  is configured to modulate the voltage drop across the rectifier  510  during the negative half-wave phase of the rectifier  510  (as outlined in the context of the first modulation unit  520 ). 
         [0069]    By modulating the voltage drop across the rectifier  510  (i.e. by varying the resistance of the rectifier  510 ) (during the positive waveform phase and/or during the negative waveform phase), the current provided by the rectifier  510  may be modulated. This current modulation is coupled via the transformer  506  back to the first subsystem, e.g. to a charging unit, and may be sensed within the first subsystem. Hence, the modulation of the voltage drop across the rectifier  510  can be used to provide a communication path from the second subsystem  500  via the galvanic isolation  506  to the first subsystem. The modulation of the voltage drop does not require any additional components, such as additional resistors or capacitors, for modulating the load current. The modulation of the voltage drop and by consequence the modulation of the load current is performed using the switches  511 ,  512 ,  513 ,  514  already available within the rectifier  510 . Furthermore, the modulation of the voltage drop across the rectifier  510  based on the control of the transistors  511 ,  512 ,  513 ,  514  comprised within the rectifier  510  allows for a fast modulation, thereby enabling a high number of modulation states/second, i.e. thereby enabling a fast communication speed between the second subsystem and the first subsystem. 
         [0070]    Overall, the functional principle of the rectifier  510  of  FIG. 5  can be described as follows. If no gate signals (gate voltages) are applied to the gates of the transistors  511 ,  512 ,  513 ,  514 , the transistors  511 ,  512 ,  513 ,  514  act as diodes (due to their body diodes). Such an automatic (passive) operations of some or all of the switches  511 ,  512 ,  513 ,  514  of the rectifier  510  may be used e.g. during a start up phase of the rectifier  510  (e.g. when initially coupling the rectifier  510  to a charging unit via the transistor  506 ). 
         [0071]    The active rectifier  510  may be controlled using the operational amplifiers  522 ,  525  of the first and second modulation units  520 ,  527 , respectively. The level for activation of the switches  511 ,  512 ,  513 ,  514  (i.e. switch-on or on-state) may be e.g. −100 mV. The drain/source voltage across the switches  511 ,  512 ,  513 ,  514  may be stabilized at −100 mV. These values may be forced by a regulation loop of the operational amplifiers  522 ,  525 . The P channel high side transistors  513 ,  514  may be activated in parallel to the respective N channel low side transistors  512 ,  511 . As indicated above, the voltage supplied by the voltage sources  521 ,  524  may be used to adjust the gate voltage applied to the respective low side transistors  511 ,  512 , thereby adjusting the voltage drop across the respective low side transistors  511 ,  512 , i.e. thereby performing amplitude modulation on the current provided by the rectifier  510 . It should be noted that the P channel high side transistors  513 ,  514  are operated in a digital manner in the example of  FIG. 5 . In an alternative embodiment, the P channel high side transistors  513 ,  514  may be controlled using separate operational amplifiers (similarly to the low side transistors  512 ,  511 ), in order to provide additional states for amplitude modulation. 
         [0072]    The modulation may be performed using the modulation units  520 ,  527  by modulating the drain/source voltage across the low side transistors  511 ,  512  and/or by modulating the voltage across the high side transistors  513 ,  514  (transistor switched on—no diode voltage, transistor switched off—diode voltage). The transistors  511 ,  512 ,  513 ,  514  may be operated in a regulated mode, in order to adjust the voltage drop to system requirements (e.g. the modulation amplitude). As already indicated above, other combinations (than those shown in  FIG. 5 ) for modulating the amplitude of the voltage drop across the rectifier  510  are possible (e.g. by using the body diodes of the low side transistors  511 ,  512 ). 
         [0073]    The modulation of the voltage drop across the rectifier  510  may be performed within a positive and/or a negative waveform phase. This means that each phase (or pulse) of the alternating voltage provided to the rectifier  510  across the transistor  506  may be used to modulate the voltage drop, thereby communicating information from the electronic device  150  to a first subsystem connected to the primary side (i.e. to the first induction coil) of the transistor  506 . 
         [0074]    It should be noted that an alternative or an additional option for modulating the voltage drop across the rectifier  510  could be a short circuit of the transformer  506  by switching on all the four switches  511 ,  512 ,  513 ,  514  of the rectifier  510  at the same time for a short period of time. An additional decoupling diode function may be used for such an option, if the capacitor  515  should not be discharged. This is illustrated in  FIG. 6   a  which shows the charging unit  140  of  FIG. 1  in combination with an electronic device  650  comprising an active rectifier  610 . The operational amplifiers  620  and  621  are used as modulation units for the transistors Q 3  and Q 7 , respectively. During a positive half wave of the alternating voltage, the transistors Q 6  and Q 7  of the rectifier  610  are in the on-state (or the body diodes of the transistors Q 6  and Q 7  are forward-biased). The transistor Q 7  may be controlled using the operational amplifier  621 , thereby controlling the resistance of the transistor Q 7  (thereby varying the voltage drop across the active rectifier  610 ). Alternatively or in addition, the transistor Q 3  may be controlled using the operational amplifier  621 . The transistor Q 3  could be controlled to provide a direct link across the transformer  106  via a variable resistance (provided by the on-resistance of the transistor Q 3 ), even during a positive half wave of the alternating voltage, i.e. even though the body diode of Q 3  is reverse biased. Hence, the transistor Q 3  could be used during a positive half wave of the alternating voltage, in order to modulate the current, thereby providing alternative or additional modulation means. As indicated above, the diode  622  may be used to ensure that the capacitor C 2  is not discharged at time instants when the transistor Q 3  is closed (even though the body diode of Q 3  is reverse biased). 
         [0075]    In other words, the transistors of the rectifier  610  may be used as a current source, thereby providing a modulation of the current within the rectifier  610 . As discussed above, the transistor Q 7  may be acting as an active diode (during a first half wave of the alternating voltage) and the transistor Q 3  may act as an additional current source (by closing the transistor Q 3  in a controlled manner during the first half wave of the alternating voltage). As a consequence, a higher current may be added to the system, thereby modulating the current within the rectifier  610  at an increased amplitude. 
         [0076]    If all the transistors of the rectifier  610  are switched on, a short circuit will typically occur at the transformer  106 . This may be used for protection of the electronic device  650  in emergency conditions (e.g. over load). Furthermore, the short circuit may be used as a means for modulating the current within the rectifier  610 . During short circuit situations, the diode D 1   622  is acting as a decoupling element. In a short circuit situation, the voltage across the capacitor C 2  will be decoupled by the diode D 1   622 . 
         [0077]    As an additional option, the active rectifier may be used as a current source, if the reference of the OPAMP (operational amplifier) will be positive. In other words, the active rectifier  610  may comprise a current source for modulation purposes. By way of example, the transistor Q 3  and the operational amplifier  620  in  FIG. 6   a  may be replaced by a current source to modulate the current within the active rectifier  610 . An example current source  680  is illustrated in  FIG. 6   b . The example current source  680  comprises a current mirror comprising the input transistor Q 2   682  and the output transistor Q 1   681 . The output transistor Q 1   681  may be the transistor Q 3  of  FIG. 6   a . As such, the current through the output transistor Q 1   681  (i.e. the current through the transistor Q 3  of  FIG. 6   a ) may be controlled by a current source  683  via the current mirror  682 ,  681 . The dimensions (length/width) of the transistors  681 ,  682  determine a gain of the current mirror  681 ,  682 . 
         [0078]    It should be noted that the modulation of the voltage drop at the rectifier  510  (and consequently the modulation of the current provided by the rectifier  510 ) causes variations of the load current. Such variations of the load current may be disadvantageous for loads requiring a stable load current, as is the case e.g. for SSL devices. For such devices it may be beneficial to perform a modulation of the voltage drop across the rectifier  510  and consequently a modulation of the load current which ensures a constant average current, regardless the modulation state. In other words, the average load current should be independent of the different modulation states. This can be achieved e.g. by performing frequency modulation, at different frequencies. Regardless the frequency, the same average current may be provided to the load. A first state (at a first modulation frequency) may be regarded as the default state where no information is transmitted from the electronic device  500  to a first subsystem connected to the primary side of the transformer  506 . The other states (using different modulation frequencies) may be used to transmit information from the electronic device  500  to the first subsystem. 
         [0079]      FIG. 7  illustrates a circuit diagram of a decoupled system comprising a charging unit  740  and an electronic device  750 . The coupling between the two subsystems  740 ,  750  is performed using a piezo transformer  706 , thereby providing a galvanic decoupling between the two subsystems  740 ,  750 . The capacitor C 1   705  may be used, if the piezo transformer  706  cannot handle a DC voltage. The resistor R 1   708  may be used to define the ground for the piezo transformer  706 , so that the voltage after the capacitor C 1   705  oscillates between positive and negative. The inductance L 1   707  may be used for removing high current spices, because the piezo transformer  706  typically has a relatively large input capacitor. The components  705 ,  707 ,  708  are optional. 
         [0080]      FIG. 8  illustrates a circuit diagram of a decoupled system comprising a charging unit  840  and an electronic device  850 . The coupling/decoupling between the two subsystems  840 ,  850  is performed using the capacitors  805  and  806 . That is, the C 1   805  and C 3   806  are used as decoupling elements. The coil L 1   807  may be used for peak reduction and may be used to generate a time constant without losses. Such an isolated system may make use of high voltage capacitors with a relative large size and relatively small capacitive values. The capacitors C 1   805 , C 3   806  may have an isolation voltage of around 1000V or more. 
         [0081]      FIG. 9  illustrates a circuit diagram of a decoupled system comprising a first subsystem  940  and a second subsystem  950 . The first and second subsystems  940 ,  950  jointly form a flyback converter with the coil P 1  within the first subsystem  940  and the coil S 1  within the second subsystem  950  forming a transformer  906 . The transformer  906  comprises an auxiliary coil (or auxiliary winding) S 2   903  which functions as a modulation sensing means. The first subsystem  940  comprises a switch Q 2   902  which is configured to generate an alternating voltage at the primary coil P 1  of the transformer  906 . Furthermore, the first subsystem  940  may comprise a shunt resistor  905 . The second subsystem  950  comprises the secondary coil S 1  of the transformer  906  and a rectifier  110 . In the illustrated example of a flyback converter, the rectifier  110  comprises a diode D 6   911 . The diode D 6   911  may be implemented as an active switch, e.g. a transistor, thereby allowing for a modulation of the voltage drop across the rectifier  110 . Furthermore, the second subsystem  950  comprises a modulation unit  120  and an encoding unit  170 . The modulation unit  120  may be configured to control the voltage drop across the rectifier  110  (e.g. by modulating a resistance of the switch  911 ). In order to sense the modulation, the first subsystem  903  may comprise the auxiliary coil S 2   903  of the transformer  906  which is configured to sense the voltage modulations within the second subsystem  950 . As such, the auxiliary coil S 2   903  (in combination with an optional resistor R 3   904 ) forms modulation sensing means of the first subsystem  940 , which are configured to sense a modulation of the voltage in the second subsystem  950 . The sensed modulation may be analyzed by the decoding unit  180 . 
         [0082]    In a similar manner to the examples described in the previous figures, encoded information may be transmitted from the second subsystem  950  to the first subsystem  940  via the galvanic decoupling provided by the transformer  906 . Hence, the communication concept outlined in the present disclosure can also be applied to a flyback converter. In this case the voltage at the auxiliary winding S 2   903  may be used to sense the modulation. Flyback converters may be used e.g. in low power LED applications and in the main plug of a charger. 
         [0083]    In the present disclosure a method and system for performing fast modulation at a second subsystem of a decoupled system, for communication with a first subsystem of the decoupled system, have been described. The method and system make use of an active rectifier which is modulated, wherein the modulation may be performed in sync with the converter frequency, i.e. with the frequency of an alternating voltage provided by the first subsystem to the second subsystem. The method and system described in the present disclosure allow for a high communication speed with no additional components required in the signal path at the second subsystem. The losses incurred by the method and system described herein can be adjusted by adjusting the modulation amplitude, i.e. by adjusting the amplitude of the modulation of the voltage drop across the rectifier. A minimum required modulation amplitude may depend on the sensitivity of the modulation sensing means used within the first subsystem and on the (inductive) coupling parameters of a transformer used between the first and the second subsystems. 
         [0084]    In the present document, the term “couple” or “coupled” refers to elements being in electrical communication with each other, whether directly connected e.g., via wires, or in some other manner. 
         [0085]    It should be noted that the description and drawings merely illustrate the principles of the proposed methods and systems. Those skilled in the art will be able to implement various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and embodiment outlined in the present disclosure are principally intended expressly to be only for explanatory purposes to help the reader in understanding the principles of the proposed methods and systems. Furthermore, all statements herein providing principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.