Abstract:
A solid state relay and a method for controlling a signal path between an AC-signal output and a load in a power amplifier assembly are disclosed. The relay comprises a first and a second MOSFET having a common gate junction, a common source junction and wherein and wherein a drain terminal of a first MOSFET and a drain terminal of a second MOSFET form relay terminals. The solid state relay further comprises a control circuit comprising a positive side comprising a first controlled current generator configured to provide a first control current to the gate junction, and a negative side comprising a current mirror circuit configured to sink a second current from the source junction. Hereby, a generic solid state speaker relay has been disclosed. The relay performs up to the most stringent demands regarding pop/click on high quality products. It can be used to ground wire break, hot wire break and BTL (Bridge Tied Load) break. The design is rather tolerable to different MOSFETs and very competitive in quality and price.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to loudspeaker relays, and more specifically to solid state loudspeaker relay. 
       BACKGROUND 
       [0002]    Solid state switches or relays have been used for decades for various audio applications, an example of this type of circuitry is disclosed, e.g. in U.S. Pat. No. 4,438,356. Moreover, the use of solid state relays for mute purposes is not new. It has for example been used for muting hybrid amplifiers in order to avoid excessive pop/click. A known application is in the B&amp;O (Bang &amp; Olufsen) product “BeoLab5” where it was used to mute the ICEpower™ amplifier to suppress pop/click. 
         [0003]    Although a significant improvement, present prior art designs still suffer from some drawbacks. For example, one concern is that the known designs result in a minor pop/click at the speaker(s) coming from the control circuit. This pop/click is actually perceivable, which has been the case for all solid state designs done up till now. 
         [0004]    The prior art designs often rely on intelligent SW control for timing. The power up/down signal/no signal conditions must be under SW control to avoid pop/click and even worse to avoid stressing the solid state circuit (in some cases). This is a major weakness and might result in that SW errors cause defects in the HW. 
         [0005]    As mentioned, the different solutions known in the prior art typically suffer from a remaining pop/click which is a little too significant at least for really high quality perception. There is therefore a need for a solution that reduces the remaining pop/click to an acceptable level for residual noise in high quality products like B&amp;O (Bang &amp; Olufsen) equipment. The most used solution today is to break the ground connection when not BTL (Bridge Tied Load). However, there is no generic solution for the two modes. 
         [0006]    Thus, there is furthermore a need for a solution that is generic and can be used in ground wire break, hot wire break and BTL (Bridge Tied Load) mode break. 
         [0007]    There have been some attempts to alleviate some of the above-discussed drawbacks in solid state relays, an example of this can be found in U.S. Pat. No. 4,682,061 which discloses a MOSFET switch control. The design presented in U.S. Pat. No. 4,682,061 described a balanced control circuit in the form of two current generators that outbalance each other. The current generators are realized by means of diodes which are known to act as voltage independent current generators when applied with reverse voltage. However, a solution of this type is very sensitive to tolerances of the diodes, which inherently can be very sensitive to temperature and voltage variations. Moreover, matching diodes in order to provide matched current sources is extremely difficult and it is not uncommon that variations in leakage current can differ by a factor of 1000, thus making it more or less impossible to control the time constants which are dependent on the leakage currents. Moreover, the use of an opto-coupled transistor in order to control the time constant of the “mute” is a rather costly solution. 
         [0008]    Accordingly, there is a need for a cost-efficient and robust circuit design that takes care of the problems mentioned above and comes up with a sort of generic solution that can also be used in other applications than the ones we see today. 
       SUMMARY 
       [0009]    It is therefore an object of the present invention to provide a system for an audio amplifier assembly which alleviates all or at least some of the above- discussed drawbacks of the presently known systems. 
         [0010]    This object is achieved by means of a solid state relay and a method for controlling a signal path between an AC-signal (Alternating Current Signal) output and a load in a power amplifier assembly as defined in the appended claims. 
         [0011]    According to a first aspect of the present invention, there is provided a solid state relay for controlling a signal path between an AC-signal output and a load in a power amplifier assembly, wherein the relay comprises a first MOSFET and a second MOSFET, wherein a source terminal of the first MOSFET is connected to a source terminal of the second MOSFET, thereby forming a source junction, wherein a gate terminal of the first MOSFET is connected to a gate terminal of the second MOSFET, thereby forming a gate junction; and wherein a drain terminal of the first MOSFET and a drain terminal of the second MOSFET form relay terminals. The relay further comprises a control circuit including a positive side comprising a controlled current generator configured to provide a first control current to the gate junction from a first voltage source for actuating the MOSFETs and thereby to control the signal path; and a negative side comprising a current mirror circuit configured to sink a second current from the source junction. 
         [0012]    Hereby a robust and cost-efficient solid state relay is provided, with high tolerances. Controlling a signal path in this context is to be understood as to switch an AC-signal on/off in the signal path. In other words, to provide a bi-directional switch in said signal path, such as, e.g. for “muting-action” in an audio system. Moreover, the solid state relay according to the first aspect is very effective in avoiding plops/noise when enabling an amplifier when going from “off to on” and from “on to off”. Primarily this is obtained by the control circuit that all current delivered to the switching circuit is of removed, such that no “ghost current” is left as a noisy signal. 
         [0013]    The first control current creates a first gate source voltage that forces the two semiconductors (MOSFETs) to switch into an on state to act as an active relay state, and simultaneously a second controlled current generator (the current mirror) is sinking current out of the common sources, the second current generator sinking current. Moreover, the present invention is based on the realization that, by using open collectors, the control circuit tolerances are transferred to resistors and thereby it is to a great degree independent of tolerances in the semiconductors. Further, by providing a control circuit operational simply by controlling the matched controlled current generator and current mirror, the control circuit is made rather cost-efficient (most expensive components are the MOSFETs), unlike solutions utilizing e.g. an optocoupled transistor. Even further, the need for a protective zener diode between the gate junction and the source junction is diminished since the control current is well-controlled. 
         [0014]    In one exemplary embodiment the first voltage source has a higher voltage level than the AC-signal intended to be controlled by the solid state relay. Accordingly, it can also be said that the first voltage source has a higher voltage level than the signal which is the target to be switched off. 
         [0015]    In another exemplary embodiment the current mirror circuit is connected to a second voltage source having a lower voltage level than the AC-signal intended to be controlled by said solid state relay, and wherein the current mirror circuit is configured to sink the second current from the source junction to the second voltage source. Accordingly, it can also be said that the second voltage source has a lower voltage level than the signal which is the target to be switched off. 
         [0016]    Moreover, the present invention allows for the power supply of the switching arrangement (power amplifier assembly) to be the same as the power supply for the amplifier. In other words, in yet another exemplary embodiment, the first voltage source comprises (or is formed by) a positive rail of a power supply of the power amplifier assembly, and the second voltage source comprises (or is formed by) a negative rail of the power supply. 
         [0017]    According to another aspect of the present invention there is provided a method for controlling a signal path between an AC-signal output and a load in a power amplifier assembly, the method comprising: 
         [0018]    providing a pair of MOSFETs having a common gate junction, a common source junction and wherein and wherein a drain terminal of a first MOSFET and a drain terminal of a second MOSFET form relay terminals; 
         [0019]    supplying a control current to the gate junction in order to actuate the pair of MOSFETs and thereby control the signal path; and 
         [0020]    sinking a second current from the source junction via a current mirror connected to the source junction. It could also in some contexts be seen as if the second current is sunk into a current mirror. 
         [0021]    With this aspect of the present invention, similar advantages and preferred features are present as in the previously discussed first aspect of the invention. 
         [0022]    These and other features of the present invention will in the following be further clarified with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    For exemplifying purposes, the invention will be described in closer detail in the following with reference to embodiments thereof illustrated in the attached drawings, wherein: 
           [0024]      FIG. 1  shows a prior art example which illustrates a schematic drawing of a solid state relay used with a hybrid amplifier. 
           [0025]      FIG. 2 a    shows another prior art example which illustrates a rough schematic drawing of a solid state relay. 
           [0026]      FIG. 2 b    shows a graph with the resulting current signal of the solid state relay illustrated in  FIG. 2   a.    
           [0027]      FIG. 3 a    shows another prior art example which illustrates a rough schematic drawing of the solid state relay from  FIG. 2 a    in an exemplary situation. 
           [0028]      FIG. 3 b    shows a graph with the resulting current signal of the solid state relay illustrated in  FIG. 3   a.    
           [0029]      FIG. 4 a    shows a rough schematic drawing of a solid state relay in accordance with an embodiment of the present invention. 
           [0030]      FIG. 4 b    shows a rough schematic drawing of a solid state relay in accordance with another embodiment of the present invention. 
           [0031]      FIG. 5  shows a schematic drawing of a solid state relay in an audio amplifier circuit in accordance with yet another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    In the following detailed description, some embodiments of the present invention will be described. However, it is to be understood that features of the different embodiments are exchangeable between the embodiments and may be combined in different ways, unless anything else is specifically indicated. Even though in the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well known constructions or functions are not described in detail, so as not to obscure the present invention. 
         [0033]      FIG. 1  illustrates an example of a solid state relay utilizing n-channel MOSFETs  104 ,  106  as known in the art. A control current generated in the solid state relay  110  passes through the two identical resistors  103 ,  105 . This current is alien in the signal path and results in pop/click at the output  102 . Furthermore, the MOS-FETs  104 ,  106  are oftentimes chosen so that V DS, MAX (maximum voltage across the drain and source terminal) is smaller than the supply voltage. This means if the associated amplifier swings to the supply while the control circuit is in a muted state, i.e. while the MOS-FETs  104 ,  106  are OFF, one MOSFET may be forced into avalanche and might be hurt. 
         [0034]    The pop/click coming from the amplifier is limited by two times the resistance of each resistor  103 ,  105  (e.g. 1 kΩ). If the amplifier swings to full range the residual pop/click will be rather annoying so the raw amplifier itself must perform pretty well regarding pop/click as the mute performance of the solid state relay  110  is rather limited. 
         [0035]    A more general prior art realization of a solid state relay for audio implementations can be seen in  FIG. 2 a   . A speaker is indicated by the load  202 . A current generator  201  supplies a current which drives the gate circuit  210 . This current must return to the power supply  201  somehow. The most low impedance branch is through the bottom (in reference to the layout presented in the figure) MOSFET  206  if it is switched on. If the two MOSFETs  104 ,  106  are exactly the same, the simulated current signals at two points  211 ,  212  in the circuit are presented in  FIG. 2   b.    
         [0036]      FIG. 2 b    illustrates the generated current signal  211   a  (solid line) and the current signal  212   a  (dashed line) passing through the speaker  202  in the circuit presented in  FIG. 2 a   , where the y-axis and the x-axis indicate current and time respectively. During charge up, half of the current passes through the speaker, as indicated by signal  212   a . The opposite will occur if we discharge using an unbalanced source. 
         [0037]      FIG. 3 a    illustrates a scenario where the bottom MOSFET  206  is “late” because of tolerances in the V GS  (gate-source voltage) threshold. A worst case situation can be understood if we simulate completely without the bottom MOSFET  206 , as indicated in the circuit presented in  FIG. 3 a   . The resulting simulation of the circuit in  FIG. 3 a    is shown in  FIG. 3 b   . The simulation parameters and “measurement points  211 ,  212 ” are chosen to be the same as in the previous example in reference to  FIG. 2 a    and  FIG. 2   b.    
         [0038]    As illustrated in  FIG. 3 b   , all of the control current passes through the speaker  202 , according to the circuit design in  FIG. 3 a   . In reality the resulting signals would probably be something in between these two results ( FIG. 2 b    and  FIG. 3 b   ). 
         [0039]    The underlying concept of the operation of the solution provided by the present invention is illustrated in  FIG. 4 a    and  FIG. 4   b.    
         [0040]      FIG. 4 a    illustrates a schematic illustration of an exemplary embodiment of the present invention. In the figure a current source  401  delivers the needed control current (to the gate junction  407   a ) for switching the MOSFETs  404 ,  406  on. Clearly no control current can run between the source terminals of the MOSFETs  404 ,  406 , the control current is instead pulled out from the source junction  407   b  back to the current/signal generator  401 , so no noisy pop/click can occur in a connected speaker (not shown). The resistor  405  serves to control the voltage V AS , in more detail it serves to control the time-constants and to discharge the V GS  in the mute/off state, i.e. when the MOSTFETs  404 ,  406  are switched off. The zener diode  406  is an additional safety measurement serving as back-up for the resistor  405  in case an error occurs in the current control circuit. Moreover the zener diode  406  may be used to change the de-mute/mute (on/off) time constants, e.g. if the current generator  401  would supply twice the voltage needed to turn on the MOSFETs  404 ,  406  as controlled by the resistance  405 . This way a de-mute (turn on) time constant of half the mute (turn off) constant may be achieved. 
         [0041]      FIG. 4 b    serves to illustrate that the current source ( 401  in  FIG. 4 a   ) can be divided into two  401   a ,  401   b  and be moved to the supply wires acting exactly the same way as the single source  401 . Other components and their functions are analogous to the circuit in  FIG. 4   a.    
         [0042]      FIG. 5  shows a control circuit  500  in accordance with an exemplary embodiment of the invention. The control circuit  500  may be seen as a practical realization of a circuit performing the concept shown in  FIGS. 4 a  and 4 b   . Specific values are given to certain currents, resistances and voltages in order to further elucidate the inventive concept, this should however not be considered limiting, and the skilled artisan readily understands that the values given may be any other values depending on the intended application. 
         [0043]    The relay function is switched on by pulling out a well-controlled current of 110 μA through the resistor  501 , i.e. the control input or control current. This can for example be done by means of a transistor and a zener diode (not shown), as known in the art. The topology is chosen so the relay is switched off initially and stays off until energy is applied to the input control circuit under the presence of supply voltage. It&#39;s applicable to use the same supply voltage as the power amplifier. Thus, a positive supply rail  540   a  of the power amplifier may be connected to a common node of the top 3 resistors ( 502   a ,  502   b ,  502   c ). For the sake of brevity, the power amplifier assembly is in this schematic drawing represented by the signal generator  550  in order to elucidate the operation of the relay circuit, i.e. to control a sinusoidal signal across a load  515 . However, the skilled artisan readily understands how a relay circuit would be implemented in an audio amplifier assembly, in accordance with the inventive concept. 
         [0044]    This 110 μA current generates a voltage across a resistor  502  of 5.2 V. By means of a transistor  504  this voltage is transferred to a current  504   a  of 460 μA. Next, a precise copy of this current  504   a  is to be made and sunk on the negative side  511 . The current is monitored by a transistor  503  and transmitted, as indicated by  503   a , to the Wilson current mirror formed by transistors  505 - 507 . Other current mirror configurations may alternatively be used, e.g. a Widlar current mirror. However, the Wilson current mirror is preferable in terms of precision/accuracy. 
         [0045]    In some situations, there can be rather big differences in the Early voltage on these NPN  505 - 507  and PNP high voltage transistors  503 - 504 . By having transistor  506  with a higher early voltage than transistors  503 ,  504  (e.g. −21 V versus −161 V), then the complete control circuit  500  can be adjusted (fine-tuned) to zero balance by trimming the direct current (DC) through the monitor chain ( 503 - 511 - 505 ). Now a copy of the current  504   a    460 uA is pulled out by the transistor  506  so the control circuit  500  is in balance. 
         [0046]    Unfortunately the transmission from the positive rail  540   a  to the negative rail  540   b  takes some time. This delay will result in a small unbalance as the positive current generator current  504   a  only can disappear through the speaker  515  and/or ground. The energy in this error is so small that it hardly can be heard, but it can be measured and it is observed in simulations. This error is removed (nominal only) by adding a capacitor  512  on the negative side. The capacitor  512  may not remove the time difference but it generates an equal opposite current injection on the negative rail as on the positive but a little later (far below 1 us). So the sum becomes zero. The result is a single positive/negative current swing around a few MHz which will have no audible effect. This happens during mute as well as un-mute (or de-mute), i.e. during the turning off and on of the MOSFETs  513 ,  514 . The MOSFETs are illustrated as N-channel MOSFETs in this particular exemplary embodiment together with their inherited diode as indicated in the figure; however P-channel MOSFETs may alternatively be used. 
         [0047]    An advantage of this drive circuit is that we are rather independent of the MOSFETs  513 ,  514 . So even if the MOSFETs  513 ,  514  vary by differences in capacitances and/or threshold voltages (in comparison to each other) this will have no effect in the pop/click noise transferred from the control circuit  500 . The only performance coming from the MOSFETs  513 ,  514  is their on-resistance resulting in heat, distortion and reduced damping (output resistance) and unbalance in capacitances resulting in distortion during the transition between mute/un-mute. 
         [0048]    References  551 ,  552 ,  553  and  554  are Schottky diodes placed to handle inductive currents from the speaker  515  that might stress (Avalanche break down) the MOSFETs  513 ,  514  during mute (i.e. when the MOSFETs are off) with signal appearance. 
         [0049]    The bottom (in reference to the illustrated layout) Schottky diodes  553 ,  554  are optional for BTL operation or breaking the hot side of the amplifier. 
         [0050]    The design may further comprise two resistors  518 ,  519  connected over the MOSFETs  513 ,  514 , i.e. each resistor between the source and drain of each MOSFET. The job is to discharge the V DS  (drain-source voltage). An exemplary scenario may be that the mute action happens while output is negative −60 V. Now the bottom MOSFET  514  will end up being charged to 60 V across its drain and source terminals. A later un-mute will transfer this energy to a pop/click if there are no resistors present. The values of the resistors  518 ,  519  are to be chosen not to diminish the muting action obtained by the MOSFETs  513 ,  514  themselves too much. The resistances  518 ,  519  can generally be increased up to 1 MΩ (mega ohm), for e.g. a IRF540N HEXFET Power MOSFET, for further reduction in mute action, then the MOSFETs  513 ,  514  take over. This is due to the drain-source capacity present in the MOSFETs  513 ,  514  in the off-state, which can be seen as an impedance that decreases with frequency. This impedance limits the reduction of the audio-signal intended to be “muted”. Thus, if the two resistors  518 ,  519  across the MOSFETs  513 ,  514  would have a much higher value than the residual impedance of the MOSFETs  513 ,  514  then there would be no beneficial audio signal reduction. If the output from the amplifier swings to ±60V the resultant current in the speaker will be limited by the two resistors  518 ,  519  that are connected over the MOSFETs  513 ,  514 . The resistor  523 ,  524  connected to the gates of each MOSFET  513 ,  514  are there to remove parasitic oscillation in switching. 
         [0051]    The diode  521  is placed to increase robustness against handling and measurement. Basically the voltage across the resistor  522  is determined by the input control current and the circuit will function without the diode  521  anyway. 
         [0052]    V GS  is activated by the current generators which send 460uA which would create 15V. This is limited to 13V by the (zener) diode  521 . This way mute and de-mute takes nearly the same time. It is possible to make the time-constants different. Imagine if the resistor  522  is three times higher. Now the discharge of the capacitor  525  is increased by a factor of 3, but the charge is controlled by the 460 uA and therefore unchanged. Thereby, mute action can be made softer (slower). 
         [0053]    The capacitor  525  is chosen so small that it makes nearly no delay in the mute process. When used for breaking the ground connection the capacitor  525  can be chosen to 100 pF, but optionally the capacitor  525  can be chosen to be much bigger (i.e. having a larger capacitance). 
         [0054]    This could be the case if the amplifier has so much DC offset that we want to soften the mute action in order to make it acceptable. Another reason for using a big capacitor  525  is when used in BTL or hot wire break. In the illustrated design, a big capacitance acts in conjunction with the diodes  526 ,  527  connected to each side of the capacitor  525  to boot strap the control voltage beyond the limit of the supply voltage. Moreover, the capacitor  525  is often chosen in accordance with the low frequency bandwidth. 
         [0055]    The resistors  531 ,  532  are chosen to increase robustness. They have no operating action in the circuitry. The figure further shows a set of scaling resistors  502   a - e  which are used to set the current magnitudes in the current generating part of the control circuit  500 . 
         [0056]    The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word “comprising” does not exclude the presence of other elements or steps than those listed in the claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.