# Patent

## Title
High voltage current switch circuit

## Abstract

The present invention relates to a high voltage switch circuit, comprising an input port adapted to receive a pulse type input current and an output port, which can be used selectively to conduct an output current to a corresponding electrical load. The switch circuit comprises a buffer stage adapted to sense the input voltage at said input port and to provide a buffered voltage that follows said input voltage. The switch circuit comprises complementary switches electrically connected between said input port and said output port and a voltage level translator electrically connected with said switches, said buffer stage and a control terminal that provides a control signal. The voltage level translator provides suitable gate voltages at the gate terminals of said switches, so that the operation of these latter can be controlled by said control signal.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a National Phase filing under 35 U.S.C. § 371 of PCT/EP2013/052672 filed on Feb. 11, 2013; and this application claims priority to Application No. TV2012A000026 filed in Italy on Feb. 22, 2012; the entire contents of each application is hereby incorporated by reference.
The present invention relates to an electronic circuit of analog type, which can be of integrated type or realized with discrete components.
In particular, the present invention relates to a high voltage switch circuit for switching current signals, namely pulse type current signals, between an input port and a selectable output port.
In many applications, for example in the biomedical sector, multiplexers are used when it is necessary to supply a desired current signal to an output port, which can be activated selectively.
Italian patent application nr. MI2007A000595 describes a high voltage pulsed current generation circuit for a neuromuscular electrical stimulator. This document shows the use of a multiplexer having an input port adapted to receive a pulse type current signal, generated by a stimulation circuit, and a plurality of output ports, each of which can be selected to provide said current signal to a corresponding pair of stimulation electrodes.
Multiplexers, substantially of a type similar to the one described above, can also be used in biomedical applications of different type, such as in ultrasonic scanning apparatus.
Often, these devices do not have satisfactory galvanic isolation to ground of the output ports.
In many devices, even if intended for use, for example, in biomedical applications, in which the presence of effective galvanic isolation is a very important design requirement, it is possible to encounter the presence of non-negligible leakage currents to ground. The intensity of said leakage currents increases, generally not linearly, with the voltage at the terminals of the input/output port.
Many prior art multiplexers have high power consumption, both in stand-by mode and during operation, which increases significantly if high voltages are present at the terminals of the input/output ports.
Another example of electronic stimulation device is disclosed in U.S. Pat. No. 5,052,391.
In this document, an electronic circuit to provide high voltage, high rise-time, and charge balanced current pulses is disclosed.
This electronic circuit does not actually have current multiplexing functionalities but it basically provides for the splitting of one output for a parallel connection with a plurality of output channels.
Moreover, such a circuit shows relevant drawbacks in terms of weight and size, since multiple transformers have to be used to transfer the energy of a stimulation pulse to the corresponding output channels. The overall bulkness of such a system makes it difficult to use in portable stimulators.
Further, the output current supplied to each output port is often subject to relevant waveform distortions due to the currents adsorbed by the adopted switching devices.
The main object of the present invention is to provide a switch circuit, which is capable of overcoming the drawbacks described above.
A further object of the present invention is to provide a switch circuit that can be controlled by logic type signals to switch an input current towards a selectable output port.
A further object of the present invention is to provide a switch circuit, in which input and output terminals have high impedance to ground for voltages within the specification ranges.
A further object of the present invention is to provide a switch circuit having low quiescent and active power consumption for voltages within the specification ranges.
A further object of the present invention is to provide a switch circuit that can be supplied by high voltage power supplies.
A further object of the present invention is to provide a switch circuit that is easy to produce at industrial level, as an integrated circuit or as a discrete component circuit, at competitive costs with respect to prior art devices.
These objects, together with other objects that will be more apparent from the subsequent description and from the accompanying drawings, are achieved, according to the invention, by a high voltage switch circuit according to claim 1, proposed hereafter, and the related dependent claims which refer to preferred embodiments of the present invention.



Further characteristics and advantages of the present invention will be more apparent with reference to the description given below and to the accompanying figures, provided purely for explanatory and non-limiting purposes, wherein:
FIG. 1A, 1B illustrate block diagrams showing the general operation and structure of the high voltage switch circuit, according to the present invention;
FIG. 2 illustrates a block diagram of a multiplexer comprising the high voltage switch circuit, according to the present invention;
FIG. 3 illustrates a block diagram of the output stage included in an embodiment the high voltage switch circuit, according to the invention;
FIG. 4 schematically illustrates the buffer stage included in the high voltage switch circuit, according to the invention;
FIG. 5-6 illustrate in more details the circuit structure of the output stage of the high voltage switch circuit, in the embodiment shown in FIG. 3.



With reference to the aforesaid figures, the present invention relates to a high voltage switch circuit 1.
The high voltage switch circuit 1 is particularly adapted for use in a muscular or neuromuscular electrical stimulator and it will now be described with reference to such an implementation for simplicity of exposition.
However, this is not intended to limit in any way the scope of the present invention.
In fact, the switch circuit 1 can be used in different biomedical applications, for example in an ultrasonic devices, or in other types of devices in which it is necessary to selectively activate a plurality of current controlled ports, such as micro-electromechanical devices or systems (MEMS).
Referring to FIGS. 1A-1B, the switch circuit 1 comprises an input port IN adapted to receive an input current IIN.
The input current IIN is predefined and is generated by a current generator circuit 500, electrically connected with a pair of terminals (positive and negative) IN+ and IN− of the input port IN.
The input current IIN has a pulse type waveform, preferably of unipolar type.
The switch circuit 1 comprises an output port OI that can receive the input current IIN and conduct an output current ILI to a corresponding electrical load LI.
The switch circuit 1 is capable of directing the input current IIN to the output port OI, when this latter is selected to conduct the output current ILI to the load LI.
An input voltage VIN is present between the terminals IN+ and IN− of the input port IN, which is a function of the input current IIN and of the downstream equivalent impedance seen from the terminals of the input port IN.
The input voltage VIN can assume high values, for example values of a few hundreds of volts in an electrical stimulator.
The switch circuit 1 comprises an electronic buffer stage BUF that is electrically connected to the input port IN.
The buffer stage BUF is adapted to sense the input voltage VIN, at the terminals IN+, IN− of the input port IN, and to supply, at a buffer output BF, a buffered voltage VBUF, which follows the sensed input voltage VIN.
The switch circuit 1 comprises complementary switches T1, T2 that operate as current switches and are electrically connected between the input port IN and the output port OI.
The switch circuit 1 comprises a first control terminal K1 for providing a first control signal C1 of logic type (for example at 0V and 3.3V).
Preferably, the switch circuit 1 is operatively associated with an electronic control stage COM adapted to generate the control signal C1 and send it to the control terminal K1, connected thereto.
In some embodiments of the present invention, the control stage COM may be physically included in the switch circuit 1.
Preferably, the control stage COM may comprise a digital processing device, for example a microprocessor, or a shift register or another circuit of similar type.
The switch circuit 1 comprises a first voltage level translator A1 that is electrically connected with the buffer stage BUF, with the switches T1, T2 and with the control terminal K1.
The voltage level translator A1 is adapted to provide a first and second gate voltage VP1, VP2 respectively at a first and second gate terminal G1, G2 of the first and second switch T1, T2 to control said switches through the control signal C1.
Depending on the control signal C1, the switches T1, T2 enable or disable the flow of the input current IIN from the input port IN to the output port OI, thus providing or blocking a current path from the input port IIN to the output port OI for the input current IIN.
The connectivity between the input port IN and each output port O1 is determined by the control signal C1 that selects the output port O1 to receive the input current IIN.
The output current provided by the switches T1, T2 is thus equal to (IIN*C1) where C1 is a logic signal having logic values equal to 0 or 1.
The adoption of the voltage level translator A1 for providing the gate voltages VP1, VP2 is quite advantageous since it allows to properly set the voltage across the gate-source junctions of the switches T1, T2 in order to make it possible to control (in particular to turn on) said switches through the control signal C1. The switches T1, T2 are in fact transistors in which the voltage across the gate-source junction may vary, since they have the source terminals electrically connected with the terminals of the input port IN.
As shown in FIGS. 1B and 3, the switches T1, T2, the voltage level translator A1 and the input terminal K1 form an output circuit NET1 that is comprised in an electronic output stage MI of the switch circuit 1.
The output stage MI is electrically connected to the input port IN, the buffer stage BUF, the output port OI and preferably to the control stage COM.
According to a preferred embodiment of the present invention (FIG. 4), the buffer stage BUF comprises a circuit structure divided into two sections, substantially symmetrical with respect to ground.
Each of the aforesaid sections comprises a sensing circuit B1, B2 arranged in such a manner as to sense the voltage of a corresponding terminal IN+, IN− of the input port IN, and a voltage follower circuit F1, F2, arranged in such a manner that the voltage of the positive and negative terminals BF+, BF− of the buffer output BF follow the sensed voltage.
This solution makes it possible to maintain a high impedance to ground for the input port IN and the buffer output BF, the voltages at the terminals of which are floating with respect to ground.
The buffer output BF provides, between the terminals BF+, BF−, the buffered voltage VBUF that follows the variations of the voltage VIN at the terminals IN+, IN− of the input port IN.
This makes it possible to power the voltage level translator A1 with high voltages (VPP and VNN) that are different from VIN, thereby without introducing significant distortions (for example due to unwanted current absorptions) in the input current IIN, when this latter flows toward the output port OI.
In a first section, the buffer stage BUF preferably comprises a first sensing circuit B1 and a first follower circuit F1.
Preferably, the sensing circuit B1 is electrically connected with the positive terminal IN+ of the input port IN, with the sensing node S1 and with a first power supply VCC.
The sensing circuit B1 senses the voltage of the positive terminal IN+ of the input port IN and establishes an offset with respect to this voltage to compensate the threshold gate-source voltage of a transistor T9 of the voltage follower circuit F1 and prevent an unwanted conduction of the switch T1.
Preferably, the sensing circuit B1 comprises a Zener diode D21 and a capacitor Z21, connected in parallel between the positive terminal IN+ and the sensing node S1.
The diode D21 advantageously prevents from over-voltages at the sensing node S1 while the capacitor Z21 maintains the voltage offset with respect to the voltage of the positive terminal IN+.
Preferably, the sensing circuit B1 comprises a resistor R21 and a diode D24 electrically connected in series between the power supply VCC and the sensing node S1.
The voltage follower circuit F1 is electrically connected with the sensing node S1, with the positive terminal BF+ of the buffer output BF and with a second power supply VPP, which is a high voltage power supply.
In the voltage follower circuit F1, the voltage of the positive terminal BF+ substantially follows the voltage of the positive terminal IN+.
Preferably, the voltage follower circuit F1 comprises the transistor T9, for example an n-type enhancement mode MOSFET, connected between the power supply VPP and ground through the resistor R23.
The transistor T9 has the gate terminal connected with the sensing node S1, the drain terminal connected with the power supply VPP and the source terminal connected with the terminal BF+ and to a resistor R23, in turn connected with ground.
Operation of the first section of the stage BUF is now described in greater detail.
When there is no current flow towards the load LI (e.g. the input current IIN has no current pulses) the sensing node S1 is at a voltage approximately equal to VCC.
The voltage of the terminal BF+ is therefore approximately equal to VIN minus the voltage drop on the network composed of the circuit elements D21, Z21, D24 and R21 and the voltage VGSth(T9), i.e. the threshold gate-source voltage of the transistor T9.
The voltage of the sensing node S1 follows the voltage of the terminal IN+, so that the voltage of the terminal BF+ follows the voltage of the terminal IN+ and the switch T1 is in an OFF state.
If there is a current flowing toward the load LI (i.e. the switch T1 is in an ON state), the voltage of the terminal IN+ depends substantially on the voltage drop across said load. In this case, the voltage variations at the terminal IN+ are sensed by the sensing circuit B1 and followed by the voltage at the terminal BF+.
A second section of the buffer stage BUF preferably has a circuit structure substantially symmetrical to that of the first section described above, which comprises a second sensing circuit B2 and a second follower circuit F2.
Preferably, the sensing circuit B2 is electrically connected with the negative terminal IN− of the input port IN, with a second sensing node S2 and with a third power supply VDD.
The sensing circuit B2 senses the voltage of the negative terminal IN− of the input port IN and establishes a voltage offset with respect thereto to compensate the threshold gate-source voltage of a transistor T10 of the voltage follower circuit F2 and prevent an unwanted conduction of the switch T2.
Preferably, the sensing circuit B2 comprises a Zener diode D22 and a capacitor Z22, connected in parallel between the negative terminal IN− and the sensing node S2.
The diode D22 advantageously prevents from over-voltages at the sensing node S2 while the capacitor Z22 maintains the voltage offset with respect to the voltage of the negative terminal IN−.
Preferably, the sensing circuit B2 comprises a resistor R26, and a diode D23 electrically connected in series between the power supply VDD and the sensing node S2.
The follower circuit F2 is electrically connected with the sensing node S2 and with the negative terminal BF− of the buffer output BF.
In the follower circuit F2, the voltage of the negative terminal BF− substantially follows the voltage of the negative terminal IN−.
Preferably, the follower circuit F2 comprises the transistor T10, for example a p-type enhancement mode MOSFET, connected between a fourth power supply VNN, which is a high voltage power supply, and ground through the resistor R24.
In the transistor T10, the gate terminal is connected with the sensing node S2, the drain terminal is connected with the power supply voltage VNN and the source terminal of the transistor T10 is electrically connected with the terminal BF− and to a resistor R24, in turn connected with ground.
Operation of the second section of the stage BUF is substantially similar to that of the first section.
When there is no current flow toward the load LI (e.g. the input current IIN does not have current pulses), the sensing node S2 is at a voltage approximately equal to VDD.
The voltage of the terminal BF− is therefore approximately equal to VIN minus the voltage drop on the network composed of the circuit elements D22, Z22, D23 and R26 and the voltage VGSth(T10), i.e. the threshold gate-source voltage of the transistor T10.
The voltage of the sensing node S2 follows the voltage of the terminal IN−, so that the voltage of the terminal BF− follows the voltage of the terminal IN− and the switch T2 is in OFF state.
If there is current flow toward the load LI (i.e. the switch T2 is in ON state), the voltage of the terminal IN− depends substantially on the voltage drop across said load.
In this case, the voltage variations at the terminal IN− are sensed by the sensing circuit B2 and followed by the voltage at the terminal BF−.
The buffer stage BUF is thus capable to supply a buffered voltage VBUF that follows the input voltage VIN with a small power consumption and negligible distortions of the input current IIN.
The structure of the first output circuit NETT, in a preferred embodiment of the switch circuit 1 of the present invention (FIGS. 3 and 5), is now described in greater detail.
As mentioned above, the output circuit NETT comprises the switches T1, T2, the voltage level translator A1 and the control terminal K1.
Preferably, the output circuit NETT comprises a first output Y1 electrically connected with the output port OI.
The first output Y1 comprises a pair of terminals (positive and negative) Y1+, Y1− electrically connected with a pair of terminals (positive and negative) OI+, OI− the output port OI.
As shown in FIG. 3, the output Y1 is electrically connected with the output port OI in such a manner that the output current ILI, which is supplied by the output port OI to the corresponding electrical load LI, has a waveform with the same polarity as the input current IIN.
In this case, the terminals Y1+, Y1− of the output Y1 are electrically connected with the terminals OI+, OI− the output port OI with direct polarity, i.e. with the positive terminal Y1+ electrically connected with the positive terminal OI+ and the negative terminal Y1− electrically connected with the negative terminal OI+ the output port OI.
Of course, the output Y1 may be electrically connected with the output port OI in such a manner that the output current ILI has a waveform with reversed polarity with respect to the input current IIN.
The switch T1 is electrically connected between the positive terminal IN+ of the input port IIN and the positive terminal Y1+ of the output Y1 and the switch T2 is electrically connected between the negative terminal IN− of the input port IIN and the negative terminal Y1− of the output Y1.
The switches T1 and T2 are complementary and are preferably field effect transistors (J-FETs or MOSFET), respectively of p- and n-enhancement mode type.
Advantageously, the transistors T1 and T2 are arranged to have the drain terminals electrically connected with the terminals Y1+ and Y1− and the source terminals electrically connected with the terminals IN+ and IN−, respectively.
In this way, when the transistors T1 and T2 are in conduction state (switches T1 and T2 in ON state), the input current IIN can flow from the terminals of the input port IN to the terminals of the output Y1.
Instead, when the two transistors T1 and T2 are in cut-off state (switches T1 and T2 in OFF state), the passing of the input current IIN toward the output Y1 is prevented.
As mentioned above, the voltage level translator A1 is advantageously adapted to control the switches T1 and T2 through the control signal C1.
The voltage level translator A1 is electrically connected between the terminals (positive and negative) BF+, BF− of the buffer output BF and with the gate terminals G1, G2 of the switches T1 and T2.
The voltage level translator A1 comprises a first polarization circuit including the circuit series of the resistor R1, the third transistor T3, the resistor R2, the fourth transistor T4 and the resistor R3.
The transistors T3, T4 are preferably bipolar junction transistors (BJT), respectively of npn and pnp type, and are adapted to enable/prevent flow of a first polarization current IP1 along said first polarization circuit.
The transistors T3, T4 are arranged in such a manner to be controlled by the terminal K1, according to the state of the control signal C1.
Preferably, the transistor T3 has its collector terminal electrically connected with the resistor R1, which is in turn connected in series with the positive terminal BF+ of the buffer output BF, and is connected with the control terminal K1, at the base terminal thereof.
Instead, the transistor T4 has the base terminal connected to ground and the collector terminal electrically connected with the resistor R3, which is in turn connected in series with the negative terminal BF− of the buffer output BF.
The transistors T3 and T4 have their emitter terminals connected with the terminals of the resistor R2.
As an alternative, the transistors T3, T4 may have their base terminals connected to the ground and to the terminal K1, respectively.
Preferably, the voltage level translator A1 comprises a first circuit network to protect the switches T1 and T2 (in particular their gate terminals G1, G2) against over-voltages.
This protective network advantageously comprises first and second over-voltage protection elements D1 and D2 (preferably Zener diodes) that are respectively connected between the gate terminals G1, G2 of the transistors T1 and T2 and the terminals IN+ and IN− of the input port IN.
Preferably, the voltage level translator A1 also comprises some stabilizing circuit elements, such as the resistor R5 and the capacitor Z1, connected in parallel with the resistor R2, and the protection resistor R4 and R6, connected in series with the base terminals of the transistor T3 and T4, respectively.
Operation of the output circuit NET1 is now described in greater detail.
Let us assume that the output circuit NET1 is initially in a deactivated or stand-by state and the terminal K1 receives a control signal C1 at “low” logic level.
The transistors T3 and T4 are in the cut-off state and there is no flow of the polarization current IP1.
If the input current IIN does not have any current pulses, the voltage at the terminal BF+ is approximately VCC−VGS(T9) while the voltage at the terminal BF− is approximately VDD−VGS(T10), where VGS(T9) and VGS(T10) are the gate-source voltages of the transistors T9 and T10, respectively.
If the input current IIN has a current pulse, the voltage at the terminals BF+ and BF− increases up to VPP and VNN respectively.
In both cases, as there is no flow of the polarization current IP1, the voltage level translator A1 provides gate voltages VP1, VP2 to the gate terminals G1, G2, such as to maintain the switches T1 and T2 in the cut-off state.
From the above, it is evident how, with a control signal C1 at a “low” logic level, whatever the voltage VIN and the input current IIN (within the specification range of the circuit), the switches T1 and T2 remain in the OFF state and the input current IIN cannot flow toward the output Y1.
The output circuit NET1 is therefore maintained in deactivated or stand-by state.
When the terminal K1 receives a control signal C1 at “high” logic level, the transistors T3 and T4 are taken to conduction state and the polarization current IP1 can flow.
In this situation, before the switching of the transistors T3, T4 is completed, the voltage at the terminals BF+ and BF− initially tends to increase up to VPP and VNN respectively.
Due to the voltage drop across the resistors R1 and R3, which is determined by flow of the current IP1, the voltage level translator A1 provides gate voltages VP1, VP2 to the gate terminals G1, G2, such as to take the switches T1 and T2 to the conduction state (ON state).
The switches T1 and T2 are taken to the ON state and the input current IIN is free to flow toward the output Y1.
At this point, the voltage at the terminals BF+ and BF− depends substantially on the voltage across the load LI but the voltage drop across the resistors R1 and R3, due to flow of the current IP1, ensures that the gate terminals G1, G2 are always at voltages such as to maintain the switches T1 and T2 in conduction state.
Therefore, with a control signal C1 at a high logic level, whatever the voltage VIN and the input current IIN (within the specification range of the circuit), the switches T1 and T2 are always in ON state and the input current IIN can flow toward the output Y1.
From the above, it is apparent that the voltage level translator A1 provides a voltage level shifting of the control signal C1 to safely control the switches T1, T2, despite of the variations of the input voltage VIN, since these latter are constantly followed by the buffered voltage VBUF.
Given that the terminals of the output Y1 are preferably connected with direct polarity to the terminals of the output port OI, the output current IIL supplied to the electrical load LI, has a waveform with the same polarity as the input current IIN.
In other words, the condition ILI=IIN is obtained.
In this way, when the output circuit NET1 is enabled by the control signal C1 to transmit an input current IIN of pulse type toward the output port OI, the output current IIL has pulses with the same polarity and amplitude as the pulses of the input current IIN.
When the terminal K1 again receives a control signal C1 at “low” logic level, the transistors T3 and T4 return to the cutoff state and ideally there should be no flow of the polarization current IP1.
In this situation, in fact, the voltage level translator A1 supplies, respectively to the gate terminals G1, G2 voltages VP1, VP2 such as to take the transistors T1 and T2 to cut-off state (OFF state).
Regardless of this, due to the presence of stray capacitances between the gate terminals G1, G2 and the terminal IN+ of the input port IN, the transistors T1, T2 do not switch immediately but are taken to OFF state only when the input current IIN reaches zero, i.e. at the end of the input current pulse.
Based on the above, it can be observed that:

    
    
        activation of the output circuit NET1 is determined simply by the transition of the control signal C1 from a “low” logic level to a “high” logic level;
        deactivation of the output circuit NET1 is instead determined by transition of the control signal C1 to “low” logic level and by passage of the input current IIN through zero.
    
    


It is therefore evident how the output circuit NET1 behaves, from a functional viewpoint, in a manner substantially similar to that of a DIAC electronic device.
In an embodiment of the present invention, particularly suitable for use in a muscular or neuromuscular electrical stimulator, the switch circuit 1 comprises the fifth and sixth complementary switches T5, T6 that operate as current switches and that are electrically connected between the input port IN and the output port OI, in parallel with the switches T1, T2.
The switch circuit 1 comprises a second control terminal K2 for providing a second control signal C2 of logic type.
Preferably, the second control signal C2 is received from the control stage COM.
The switch circuit 1 comprises a second voltage level translator A3 that is electrically connected with the buffer stage BUF, with the switches T5, T6 and with the control terminal K2.
The voltage level translator A2 is adapted to provide a third and fourth gate voltage VP3, VP4 respectively at a third and fourth gate terminal G3, G4 of the switches T5, T6 in order to control these latter through the control signal C2.
Depending on the control signal C2, the switches T5, T6 can enable or disable the flow of the input current IIN from the input port IN to the output port OI, thereby providing or blocking a current path from the input port IIN towards the output port OI for the input current IIN.
The connectivity between the input port IN and each output port OI is determined by the control signal C2 and the output current provided by the switches T5, T6 is thus equal to (IIN*C2), where C2 is a logic signal having logic values equal to 0 or 1.
The adoption of the voltage level translator A3 for providing the gate voltages VP3, VP4 is quite advantageous since it allows to properly set the voltage across the gate-source junction of the switches T5, T6 in order to make it possible to control (in particular to turn on) them through the control signal C2.
As shown in FIGS. 3 and 6, the switches T5, T6, the voltage level translator A3 and the control terminal K2 form an output circuit NET2, which is comprised in an output stage MI of the switch circuit 1 and which is connected between the input port IN and the output port OI, as the output circuit NET1.
Referring to FIG. 6, the output circuit NET2 has a circuit structure similar to that of the circuit NET1, described above.
The output circuit NET2 comprises a second output Y2 electrically connected with the output port OI.
The second output Y2 comprises a pair of terminals (positive and negative) Y2+, Y2− electrically connected with the terminals OI+, OI− the output port OI.
Preferably, the output circuit NET2 is electrically connected with the output port OI in such a manner that the output current ILI, which is supplied by the output port OI to the corresponding electrical load LI, has a waveform with reverse polarity with respect to that of the input current IIN.
In this case, the terminals Y2+, Y2− of the output Y2 are electrically connected with the terminals OI+, OI− the output port OI with reverse polarity, i.e. with the positive terminal Y2+ electrically connected with the negative terminal OI− and the negative terminal Y2− electrically connected with the positive terminal OI+ of the output port OI.
Of course, the output circuit NET2 may be electrically connected with the output port OI in such a manner that the output current ILI has a waveform with direct polarity with respect to the input current IIN.
The switch T5 is electrically connected between the positive terminal IN+ of the input port IIN and the positive terminal Y2+ of the output Y2 and the switch T6 is electrically connected between the negative terminal IN− of the input port IIN and the negative terminal Y2− of the output Y2.
The switches T5 and T6 are complementary and preferably field effect transistors (FET or MOSFET), respectively of p- and n-port enhancement mode type.
Advantageously, the switches T5 and T6 are arranged in such a manner as to have the drain terminals electrically connected with the terminals Y2+ and Y2− and the source terminals electrically connected with the terminals IN+ and IN−, respectively.
In this way, when the transistors T5 and T6 are in conduction state (switches T5 and T6 in ON state), the input current IIN can flow from the terminals of the input port IN to the terminals of the output Y2.
Instead, when the transistors T5 and T6 are in the cut-off state (switches T5 and T6 in OFF state), the passage of the input current IIN toward the output Y2 is prevented.
Preferably, the voltage level translator A3, adapted to control the transistors T5 and T6, is electrically connected between the terminals (positive and negative) BF+ and BF− of the buffer output BF and with the gate terminals G3, G4 of the switches T5 and T6.
The voltage level translator A3 advantageously comprises a second polarization circuit formed by the circuit series consisting of the resistor R11, the seventh transistor T7, the resistor R12, the eighth transistor T8 and the resistor R13.
The transistors T7 and T8 are preferably bipolar junction transistors (BJT), respectively of npn and pnp type, and are adapted to enable/prevent flow of a second polarization current IP2 along said second polarization circuit.
Preferably, the transistor T7 has its collector terminal electrically connected with the resistor R11, in turn connected in series with the positive terminal BF+ of the buffer output BF, and is connected with the terminal K2, at the base terminal thereof.
The transistor T8 has the base terminal connected with ground and the collector terminal electrically connected with the resistor R13, in turn connected in series with the negative terminal BF− of the buffer output BF.
The transistors T7 and T8 have their emitter terminals connected to the terminals of the resistor R12.
As an alternative, the transistors T7, T8 may have their base terminals connected to the ground and to the terminal K2, respectively.
Preferably, the voltage level translator A3 comprises a second circuit network to protect the gate terminals of the transistors T5 and T6 against over-voltages.
This protective network advantageously comprises third and fourth over-voltage protection elements D10 and D11 (preferably Zener diodes) that are respectively connected between the gate terminals G3, G4 of the transistors T5 and T6 and the terminals IN+ and IN− of the input port IN.
Preferably, the voltage level translator A3 also comprises some stabilizing circuit elements, such as the resistor R15 and the capacitor Z10, connected in parallel with the resistor R12, and the protection resistor R14 and R16 connected in series with the base terminals of the transistors T7 and T8, respectively.
Operation of the output circuit NET2 is similar to that of the output circuit NET1.
Let us assume that the output circuit NET2 is initially in a deactivated state and the terminal K2 receives a logic control signal C2 at “low” level. The transistors T7 and T8 are in the cut-off state and there is no flow of the polarization current IP2.
In this situation, in the presence or absence of pulses of the input current IIN, the gate terminals of the switches T5 and T6 are always at gate voltages VP3, VP4 such as to maintain them in a cut-off state.
Therefore, with a control signal C2, at a low logic level, the switches T5 and T6 remain in the OFF state and the input current IIN cannot in any case flow toward the output Y2.
The output circuit NET2 is therefore maintained in a deactivated or stand-by state.
When the terminal K2 receives a logic control signal C2 at “high” level, the transistors T7 and T8 switch to conduction state and the polarization current IP2 can flow.
In this situation, due to the voltage drop across the resistors R11 and R13, determined by the flow of the current IP2, the gate terminals G3, G4 of the transistors T5 and T6 are polarized at gate voltages VP3, VP4 such as to take the transistors T5 and T6 to conduction state.
The switches T5 and T6 are then taken to the ON state and the input current IIN is free to flow toward the output Y2.
At this point, the voltage at the terminals BF+ and BF− depends substantially on the voltage across the load LI but the voltage drop across the resistors R11 and R13, due to circulation of the current IP2, ensures that the gate terminals G3, G4 are always at gate voltages VP3, VP4 such as to maintain the transistors T5 and T6 in conduction state.
Therefore, with a control signal C2 at high logic level, whatever the voltage VIN and the input current IIN (within the specification range of the circuit), the switches T5 and T6 are always in the ON state and the input current IIN can in any case flow toward the output Y2.
From the above, it is apparent that the voltage level translator A3 provides a voltage level shifting of the control signal C3 to safely control the switches T5, T6, despite of the variations of the input voltage VIN, since these latter are constantly followed by the buffered voltage VBUF.
Given that the terminals of the output Y2 are preferably connected with reverse polarity to the terminals of the output port OI, the output current IIL supplied to the electrical load LI, will have a waveform with reverse polarity with respect to the input current IIN.
In other words, the condition ILI=−IIN is obtained.
In this way, when the output circuit NET2 is enabled by the control signal C2 to transmit a pulse type input current IIN toward the output port OI, the output current IIL has pulses with the same amplitude but with reverse polarity with respect to the pulses of the input current IIN.
When the terminal K2 once again receives a control signal C2 at “low” logic level, the transistors T7 and T8 are again taken to the cutoff state and ideally there should be no flow of the polarization current IP2.
In this situation, the voltage level translator A3 supplies, respectively to the gate terminals of the transistors T5 and T6, gate voltages VP3, VP4 such as to take the same transistors T5 and T6 to the cut-off state.
Regardless of this, due to the presence of stray capacitances between the gate terminals of the transistors T5, T6 and the terminal IN− of the input port, the transistors T5, T6 do not switch immediately but are taken to the cut-off state only when the input current IIN reaches zero, i.e. at the end of the input current pulse.
On the basis of the above, it can be observed that:

    
    
        activation of the output circuit NET2 is determined simply by transition of the control signal C2 from a “low” logic level to a “high” logic level;
        deactivation of the output circuit NET2 is determined by transition of the control signal C2 at “low” logic level and by passage of the input current IIN through zero.
    
    


Therefore, also the output circuit NET2 behaves, from a functional viewpoint, in a manner substantially similar to that of a DIAC electronic device.
Referring to FIG. 2, the high voltage switch circuit 1, which does not have multiplexing functionalities per se, is particularly suitable for implementation in a multiplexer 100.
The multiplexer 100 comprises a common input port IN and a plurality of output ports OI, each of which can be selected by logic control signals.
The multiplexer 100 receives an input current IIN at the input port IN and it directs it towards the selected output ports OI.
The multiplexer 100 thus implements a multiplexing function of the type 1→N, with N>1, for the current signals received at the input port IN.
The adoption of the high voltage switch circuit 1 in a multiplexer 100 is particularly advantageous for use in an electrical stimulator.
In this case, each of the mentioned output ports can be electrically connected with a pair of stimulation electrodes and the output current, supplied by each output port, is the current effectively injected by the electrodes during stimulation, while the electrical load connected to each output port typically consists of the impedance offered by the stimulation electrodes and by the portion of the patient's body affected by the stimulation current.
The multiplexer 100 comprises a common buffer stage BUF (as described above), which senses the voltage VIN between the terminals of the input port IN and provides, at the buffer output BF, a buffered voltage VBUF that substantially follows the input voltage VIN.
The multiplexer 100 comprises a plurality of output stages MI, each of which is electrically connected with the input port IN, the common buffer stage BUF and a corresponding output port OI.
Each of the output stages MI comprises the output circuit NET1 and preferably also the output circuit NET2, as described above.
Preferably, the control terminals K1 (and possibly K2) of each output stage MI are electrically connected with a common control stage COM that may be physically included in the multiplexer 1.
It is apparent that the common buffer stage BUF and each of the output stages MI form a switch circuit 1, according to the invention, which is electrically connected between the input port IN and the corresponding output port OI (FIG. 2).
The operation of the multiplexer 100 is now briefly described.
Normally, the output stages MI are maintained in a deactivated state.
Therefore, the control signals sent by the control stage COM are normally maintained at a “low” logic level.
To direct the input current IIN toward any desired output port OI, the control stage COM must activate the output circuit NET1 (or optionally the output circuit NET2) of the output stage MI, which is operatively associated with the chosen output port OI.
The control signal C1 (or possibly C2) sent to the output circuit NET1 (or possibly NET2) of the output stage MI, is therefore taken to “high” logic level, enabling the input current IIN to flow toward the output port OI.
If the output Y1 (or possibly Y2) of the output circuit NET1 (or NET2) is connected with direct polarity to the output port OI, the output current IIL has the same waveform as the input current IIN.
If the output Y1 (or possibly Y2) of the output circuit NET1 (or NET2) is connected with reverse polarity to the output port OI, the output current IIL has a waveform with pulses of opposite polarity with respect to the input current IIN.
It can be noted how by suitably managing the output circuit NET1 (or NET), one or more pulses of the input current IIN can be “neutralized”, simply by maintaining the control signals C1 (or C2) in the “low” logic state. The pulses of the input current IIN thus “neutralized”, do not appear, with direct or reverse polarity, in the output current IIL.
In this case, the output current IIL has a different time distribution of the pulses, with respect to the input current IIN.
Operation of the output stage MI, as adjusted by the control signals C1, C2, can be summarized in the following exemplificative table:
















C1
C2
MI
ILI









0
0
OFF
0



0
1
Circuit NET2 ON (reversing)
−IIN



1
0
Circuit NET1 ON (not reversing)
IIN



1
1
Short circuit
0






(VIN=0)










From the table above it is evident how the output current from each output stage MI is equal to (IIN*C1) or (IIN*C2), where C1, C2 are logic signals that assume the logic values 0 or 1.
The multiplexer 1 is therefore not only capable of reversing the polarity of the pulses of the input current IIN (for example, by alternately activating the switching circuits NET1 and NET25 where both are present) but also of modifying the waveform of this latter.
The use of the switching circuits NET1 and (optionally) NET2 in each output stage MI, with the functionalities described above, is particularly useful in the case in which the multiplexer is used in a muscular or neuromuscular electrical stimulator.
For different biomedical applications or for other scopes of use of the multiplexer 1, the output stages MI may however have different structure and functionality and comprise only the output circuit NET1.
It has been seen in practice how the high voltage switch circuit 1, according to the present invention, allows the set objects to be achieved.
With respect to prior art devices, the switch circuit 1 has improved functionalities, in terms of reduction of dissipated power and high impedance of inputs/outputs.
The switch circuit 1 ensures effective high impedance of inputs and outputs. It is arranged in such a manner that the voltages present between the terminals of the input port IN, of any output port OI and of the buffer output BF are virtually floating with respect to ground.
An advantage of the switch circuit 1 is the absence of working point bias currents for the active elements (transistors). This substantially reduces the power consumption to what is caused by leakage currents in the transistors.
Additional power consumption in active state is caused by charging/discharging of the parasitic capacitances trough the polarization currents IP1, IP2. This can be minimized through reducing the time periods during which the control signals C1, C2 are at a logic level commanding the ON state for the switches T1, T2, T5, T6. This is basically a design factor that depends on the parasitic capacitances, mainly in said switches.
The switch circuit 1 is particularly suitable for operating in the presence of high voltages to the terminals of the input port IN or of the output port.
For this purpose, it is sufficient to select in the most appropriate manner the type of transistor of each output stage.
The switch circuit 1 is characterized by considerable flexibility of use.
It is particularly suitable for use in biomedical applications, such as in a muscular or neuromuscular electrical stimulator.
In this application, the use of the output circuits NET1 and NET2 for the output stage MI, according to the description above, enables simple and effective adjustment of the polarity and time distribution of the output current ILI, at each output port OI.
However, the switch circuit 1 can be easily integrated in other biomedical applications, for example in ultrasonic devices or micro-electromechanical devices or systems (MEMS).
The switch circuit 1 has a simple structure and is easy and inexpensive to produce at industrial level, with manufacturing techniques using discrete or integrated components.

## Claims

1. A high voltage switch circuit characterized in that it comprises:
a constant current generator circuit adapted to provide a predefined constant current;
an input port, which has a positive terminal and a negative terminal, which are electrically connected to the constant current generator circuit, so that said input port receives the predefined constant input current;
an output port, which has a positive terminal and a negative terminal;
a first output having a positive terminal and a negative terminal, which are electrically connected with the terminals of said output port;
a first switch and a second switch that operate as current switches and are field effect transistors of complementary type, said first switch having a source terminal electrically connected to the positive terminal of said input port to receive the predetermined constant current from the constant current source and a drain terminal electrically connected to the positive terminal of the first output, said second switch having a source terminal electrically connected to the negative terminal of said input port and a drain terminal electrically connected to the negative terminal of said first output;
a buffer stage that is electrically connected to the positive and negative terminals of said input port and that has a buffer output having a positive terminal and a negative terminal, said buffer stage comprising:
first and second sensing circuits configured to sense the voltages of the positive and negative terminals of said input port, respectively;
first and second follower circuits respectively configured to provide voltages at the positive and negative terminals of said buffer output, which follow the voltages sensed by said first and second sensing circuit at the positive and negative terminals of said input port, respectively;

a first control terminal for providing a first control signal of logic high or logic low directly to a first voltage level translator;
the first voltage level translator being electrically connected with the terminals of said buffer output, with first and second gate terminals of said first and second switch and with said first control terminal, said first voltage level translator being configured to provide a first and second gate voltage at said first and second gate terminals to control said first and second switch through said first control signal, said first and second switch, controlled by said first control signal, enabling or disabling the flow of the constant input current from said input port to said output port, depending on said first control signal;
wherein said first voltage level translator comprises a third transistor and a fourth transistor to provide said first and second gate voltages, said third and fourth transistor being electrically connected with said first control terminal and to ground, respectively, or vice versa, so as to be controlled by said first control terminal according to the state of said first control signal.

2. The high voltage switch circuit, according to claim 1 characterized in that said first voltage level translator comprises a first over-voltage protection element that is electrically connected with the first gate terminal of said first switch and the positive terminal of said input port and a second over-voltage protection element that is electrically connected with the second gate terminal of said second switch and the negative terminal of said input port.

3. The high voltage switch circuit, according to claim 1, characterized in that said buffer stage comprises:
said first sensing circuit electrically connected with the positive terminal of said input port, with a first sensing node and with a first power supply, said first sensing circuit being configured to sense the voltage of the positive terminal of said input port and establish a voltage offset with respect thereto;
said first voltage follower circuit electrically connected with said first sensing node, with the positive terminal of the buffer output and with a second power supply, said first voltage follower circuit being configured to provide a voltage at the positive terminal of the buffer output, which follows the voltage of the positive terminal of said input port;
said second sensing circuit electrically connected with the negative terminal of said input port, with a second sensing node and with a third power supply, said second sensing circuit being configured to sense the voltage of the negative terminal of said input port and establish a voltage offset with respect thereto;
said second voltage follower circuit electrically connected with said second sensing node, with the negative terminal of said buffer output and with a fourth power supply, said second voltage follower providing a voltage at the negative terminal of the buffer output, which follows the voltage of the negative terminal of said input port.

4. The high voltage switch circuit, according to claim 1, characterized in that it is operatively associated with or it comprises a control stage that outputs said first control signal.

5. The high voltage switch circuit, according to claim 1, characterized in that it comprises:
a second output having a positive terminal and a negative terminal, which are electrically connected with the terminals of said output port;
a fifth switch and a sixth switch that operate as current switches and are field effect transistors of complementary type, said fifth switch having a source terminal electrically connected to the positive terminal of said input port and a drain terminal electrically to the positive terminal of the second output, said sixth switch having a source terminal electrically connected to the negative terminal of said input port and a drain terminal electrically connected to the negative terminal of said second output;
a second control terminal for providing a second control signal;
a second voltage level translator electrically connected with the terminals of said buffer output, with third and fourth gate terminals of said fifth and sixth switch and with said second control terminal, said second voltage level translator being configured to provide a third and fourth gate voltage at said third and fourth gate terminals to control said fifth and sixth switch through said second control signal, said fifth and sixth switch, controlled by said second control signal, enabling or disabling the flow of an input current from said input port to said output port, depending on said second control signal.

6. The high voltage switch circuit, according to claim 5, characterized in that said first output is electrically connected with said output port with a direct polarity and said second output is electrically connected with said output port with an inverse polarity, or vice versa.

7. The high voltage switch circuit, according to claim 5, characterized in that said second voltage level translator comprises a seventh transistor and an eighth transistor to provide said third and fourth gate voltages, said seventh and eighth transistor being electrically connected with said second control terminal and to ground, respectively, or vice versa, so as to be controlled by said second control terminal according to the state of said second control signal.

8. The high voltage switch circuit, according to claim 5, characterized in that said second voltage level translator comprises a third over-voltage protection element that is electrically connected with the third gate terminal of said fifth switch and the positive terminal of said input port and a fourth over-voltage protection element that is electrically connected with the fourth gate terminal of said sixth switch and the negative terminal of said input port (IN).

9. A high voltage current multiplexer characterised in that it comprises a high voltage switch circuit, according to claim 1.

10. A muscular or neuromuscular electrical stimulator characterized in that it comprises a high voltage switch circuit according to claim 1.

11. An ultrasonic device characterized in that it comprises a high voltage switch circuit (1), according to claim 1.

12. A micro-electromechanical device characterized in that it comprises a high voltage switch circuit according to claim 1.