Abstract:
It is presented a gate driver circuit for driving an electric switch, the switch being arranged to control a main current using a gate signal. The gate driver circuit comprises: a non-linear capacitor means having a lower capacitance when an applied voltage is under a threshold voltage and a higher capacitance when an applied voltage is over the threshold voltage, wherein the non-linear capacitor is arranged to be connected between a high voltage connection point of the switch and a connection point for the gate signal; a current change rate sensor, the current change rate sensor being configured to detect changes in a main current of the electric switch and to control a gate signal of the electric switch depending on the current change; a gate buffer; and at least one current source, arranged to drive the gate buffer. The current change rate sensor is connected to control the current source to thereby control the gate signal of the electric switch.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates generally to driving electric switches, and more particularly to driving electric switches for power conversion use. 
       BACKGROUND 
       [0002]    Power switches, such as insulated gate bipolar transistors (IGBTs) can be used to control electric power transfer to an appliance such as a motor. Often such switching is used to effect pulse width modulation (PWM) to control frequency and/or current transferred to the appliance. 
         [0003]    Stray inductance in emitter and collector terminal connections of such-switches are often relatively large due to the mechanical design. This typically leads to overshoots of the voltage across the switch. 
         [0004]    One solution to this problem is to select switches which have ratings that are sufficient to withstand the overvoltages resulting from such overshoots. However, such over-dimensioning of components is unnecessarily expensive and may require more physical volume. 
         [0005]    Other solutions include limiting the switching speed or times, and thereby the overshoots, e.g. by using a relatively large gate resistor in series with a voltage stiff driver stage buffer. The gate charge current of the switch is then limited. The drawback with this solution is that if low overshoots in voltage and current (the current overshoot being a result of diode recovery current) shall be accomplished the switching times need to be slowed down considerably more than optimum. 
         [0006]    US2002-0070772 presents an active resistance controlled to modify a drive signal provided to a gated device such as an insulated gate bipolar transistor (IGBT). The active resistance is between an input lead that receives an input drive signal, such as from a conventional gate driver IC, and an output lead at which an output drive signal is provided to the device&#39;s gate. The active resistance is controlled in response to a feedback signal that includes information about the output drive signal, so that the output drive signal is a modified version of the input drive signal. To reduce di/dt and hence control EMI emission, the output drive signal can include turn-on and turn-off transitions where the input drive signal includes steps. Such a solution is complicated and can be liable to ill-effects if controlled incorrectly. 
         [0007]    Hence, there is a need for a robust solution of how to reduce voltage overshoots when driving power switches. 
       SUMMARY 
       [0008]    An object of the present invention is thus to reduce voltage overshoots when switching electric power components. 
         [0009]    According to a first aspect of the invention, it is presented a gate driver circuit for driving an electric switch, the switch being arranged to control a main current using a gate signal. The gate driver circuit comprises: a non-linear capacitor means having a lower capacitance when an applied voltage is under a threshold voltage and a higher capacitance when an applied voltage is over the threshold voltage, wherein the non-linear capacitor is arranged to be connected between a high voltage connection point of the switch and a connection point for the gate signal; a current change rate sensor, the current change rate sensor being configured to detect changes in a main current of the electric switch and to control a gate signal of the electric switch depending on the current change; a gate buffer; and at least one current source, arranged to drive the gate buffer. The current change rate sensor is connected to control the current source to thereby control the gate signal of the electric switch. The main current is to be construed as the main current through the switch. 
         [0010]    Using the non-linear capacitor and the current change rate sensor according to the invention, overshoots are reduced. The feedback regulated, gate charge current control makes it possible to achieve higher switching speed while still maintaining the above mentioned low overshoots. 
         [0011]    The at least one current source may comprise two current sources arranged on either side of the buffer. 
         [0012]    The current change rate sensor may be configured to reduce a discharging of a gate of the switch when a large negative current change rate of the main current is detected. 
         [0013]    The current change rate sensor may be configured to reduce a charging of a gate of the switch when a large positive current change rate of the main current is detected. 
         [0014]    The current change rate sensor may be realized by a device for measuring a voltage over stray inductance. 
         [0015]    The non-linear capacitor means may be implemented using a ceramic capacitor, such as an X7R capacitor, in series with an NPO/C0G capacitor with a biased mid-point. 
         [0016]    The non-linear capacitor means may be implemented using two linear capacitors in series with a zener diode parallel to one of the linear capacitors. The zener diode may be implemented as two or more zener diodes connected in series. The zener diodes could be replaced by any suitable transient voltage suppressor. 
         [0017]    The gate driver circuit may further comprise a gate buffer; and two current sources arranged to drive the gate buffer; wherein the current change rate sensor is connected to control the current sources to thereby control the gate signal of the electric switch. 
         [0018]    The gate driver circuit may further comprise a first resistor connected to the buffer and the non-linear capacitor means in one end and the first resistor arranged to be connected to the connection point for the gate signal in the other end. 
         [0019]    The gate driver circuit may further comprise a second resistor connected in series with the non-linear capacitor means. 
         [0020]    The gate driver circuit may be arranged to drive an insulated-gate bipolar transistor. 
         [0021]    A second aspect of the invention is a switch assembly comprising a gate driver circuit according to the first aspect and an electric switch, wherein the gate driver circuit is arranged to drive the electric switch. 
         [0022]    The switch may be an insulated-gate bipolar transistor. 
         [0023]    A third aspect is a switch system comprising two switch assemblies according to second aspect, for each phase. 
         [0024]    The switch system may comprise six switch assemblies for use with a three phase appliance. 
         [0025]    For each phase: the two switch assemblies may be connected to a higher DC voltage and a lower DC voltage, respectively, and there may be a motor connection point between the two switch assemblies. 
         [0026]    The switch system may further comprise a controller connected to all gate driver circuits. 
         [0027]    It is to be noted that any feature of the first, second and third aspects may, where appropriate, be applied to any other aspect. 
         [0028]    Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0029]    The invention is now described, by way of example, with reference to the accompanying drawings, in which: 
           [0030]      FIG. 1  is a schematic diagram of an environment where a transistor driver according to an embodiment of the present invention can be applied, 
           [0031]      FIG. 2  illustrates one leg of the system of  FIG. 1 , and one gate driver circuit in more detail, 
           [0032]      FIG. 3  is a schematic graph showing the dynamics of the gate driver circuit and the lower switch of  FIG. 2  when the lower switch is changed from a closed to an open state, 
           [0033]      FIG. 4  is a schematic graph showing the dynamics of the gate driver circuit and the lower switch of  FIG. 2  when the lower switch is changed from an open to, a closed state, and 
           [0034]      FIG. 5  is a schematic graph showing the dynamics of the gate driver circuit and the lower switch of  FIG. 2  in a fault situation, e.g. a short circuit. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0035]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description. 
         [0036]      FIG. 1  is a schematic diagram of an environment where a transistor driver according to an embodiment of the present invention can be applied. 
         [0037]    A three-phase motor  50  is powered through three AC (Alternating Current) inputs  32   u - w.  To allow control of frequency and power supplied to the motor  50 , a bridge leg inverter system is used where the inputs  32   u - w  are switched between a positive DC voltage  30  and a negative DC voltage  31 . The difference in voltage between the positive and the negative DC voltages  30 ,  31  typically ranges between 300 and 900 volts. The positive and negative DC voltages can be symmetrical or asymmetrical about zero. The DC voltages  30  and  31  can in turn be created from a rectified AC source or from a DC source. The switching is performed in switches  21   u  and  22   u  for a u-phase, in switches  21   v  and  22   v  for a v-phase and in switches  21   w  and  22   w  for a w-phase. The switches  21   u - w,    22   u - w  can be insulated-gate bipolar transistors (IGBT) or any other suitable switch. 
         [0038]    Each switch  21   u - w,    22   u - w  has a corresponding gate drive circuit  1   u - w,    2   u - w,  respectively. Each gate drive circuit  1   u - w,    2   u - w  is controlled from an output (typically a digital output) of a controller  40  to effect pulse width modulation (PWM) as desired from the controller  40 . The controller  40  can thus control the frequency and power supplied to the motor  50 . 
         [0039]      FIG. 2  illustrates one leg of the system of  FIG. 1 , and one gate driver circuit in particular. The leg can be any of the three legs of  FIG. 1  since all three legs have the same configuration in this regard. 
         [0040]    There is an upper switch  22  and a lower switch  21 , connected to a positive DC voltage  30  and a negative DC voltage  31 , respectively. The upper switch  22  is driven by an upper gate driver circuit  2  and the lower switch  21  is driven by a lower gate driver circuit  1 . As is known in the art, the switches control a main current passing through a collector and emitter of the switches  21 ,  22 . The two gate driver circuits  1 ,  2  have the same configuration but for clarity only the lower gate driver circuit is shown in detail in  FIG. 2 . The controller  40  sends low voltage control signals to the two gate driver circuits  1 ,  2  to control the switching of the switches  21 ,  22 . Each gate driver circuit  1 , 2  is connected to a positive DC power supply  10  and a negative DC power supply  12 . The positive DC power supply  10  can, for example, have a voltage of 5 to 30 volts, e.g. 22 volts. The negative DC power supply  12  can, for example, have a voltage of −5 to −30 volts, e.g. −7 volts. The control signal from the controller can for example vary between −30 to +30 volts, e.g. −5 to +15 volts. It is to be noted that the parameters specified above are only examples and any voltage suitable for the particular case can be selected. 
         [0041]    Now the details of the lower gate driver circuit  1  will be discussed. The lower gate driver circuit  1  comprises a buffer  15  which is connected to the controller  40 , to react to a signal from the controller. The buffer  15  is driven by two current sources  13 ,  14 . While two current sources are presented in this embodiment, it is also possible to use only one current source. The output of the buffer  15  is connected, via a resistor  18  to the a connection point for a gate signal of the lower switch  21 . The resistor  18  can optionally be integrated in the switch  21 . 
         [0042]    Furthermore, a current sensor  23  is connected to a current change rate (di/dt) sensor  16 , which in turn is connected to the current sources  13 ,  14  for control. In other words, current changes affect the current sources  13 ,  14  which then affects the signal out of the buffer  15  which drives the gate. The dynamics of this will be explained in more detail below, for example when reducing overshoots on switching. In this embodiment, the current of the current sources could then follow a formula of I=I nom +k*dI ce /dt, where I nom  is a current not affected by the current derivative, k is a constant and I ce  is a main current going through the collector and emitter of the switch  21 . The current change rate (di/dt) sensor  16  can for example be implemented by measuring a voltage over an inductor connected to the main current. The inductor can be a specific component or a stray inductance. 
         [0043]    Also, there is a connection from a high voltage connection point of the switch  21  via a resistor  17  and a non-linear capacitor  19  to the output of the buffer  15  affecting the gate of the switch  21  (via the resistor  18 ). The high voltage of the switch is typically the collector in case an IGBT switch is used and a drain in case a MOSFET switch is used. The non-linear capacitor  19  is such that the capacitance is relatively low when supplied with a voltage which is lower than a threshold voltage, and relatively high when supplied with a voltage which is higher than a threshold voltage. As an example, if the switch  21  normally has a voltage across it of about 850 volts when it is open, the threshold voltage of the non-linear capacitor can be about 600 volts, where the non-linear capacitor has a capacitance of about 300 pF (picofarads) for voltages significantly lower than 600 volts and about 1000 pF for voltages significantly higher than 600 volts. The numbers above are only an example and it is to be noted that the component parameters and voltages can be of any value suitable for the application. The threshold voltage of the non-linear capacitor, however, should be lower than the normal voltage across an open switch  21 . As will be explained in more detail below, this then reduces overshoots when switching the switch  21 . The non-linear capacitor can for example be realized using a ceramic capacitor such as an X7R capacitor in series with an NPO/C0G capacitor with a suitable biased mid-point, or two linear capacitors in series with a zener diode parallel to one of them. 
         [0044]      FIG. 3  is a schematic graph showing the dynamics of the gate driver circuit  1  and the lower switch  21  of  FIG. 2  when the lower switch  21  is changed from a closed to an open state. It is to be noted that only the time is common for all parameters in the graph. In other words, the relative vertical distance of the various parameters are not to be compared. For example, the absolute voltage measurements vary greatly from one voltage to another. 
         [0045]    In the graph, S g  denotes a gate control signal from the controller, V g  denotes the voltage of the gate of the switch  21  relative the emitter of the switch, I g  denotes the current going to the gate of the switch  21 , I ce  denotes the current passing through the collector and the emitter of the switch  21  and V ce  denotes the voltage difference between the collector and the emitter of the switch  21 . 
         [0046]    The signal S g  from the controller  40  goes from high to low at time T 0 . This results in a I g  going negative to discharge the gate of the switch  21 . Consequently, the voltage V g  of the switch  21  decreases. 
         [0047]    At time T 1 , the gate of switch  21  is sufficiently depleted for the switch to start the transition from the closed to the open state. Consequently, the voltage V ce  across the collector and emitter of the switch starts to increase. The increase in V ce  charges the capacitor  19 , whereby the gate current I g  increases slightly. Hence, this causes the magnitude of the gate current I g  to decrease. 
         [0048]    At time T 2 , the voltage V ce  across the switch  21  has increased to a threshold level  60 . This means that the threshold level of the capacitor  19  has been reached whereby the capacitance of the capacitor  19  increases. Consequently, more of the negative gate current instead goes to the capacitor  19 . Hence, the gate current I g  increases even more. This reduces the overshoot of V ce  significantly. 
         [0049]    At time T 3 , the current I ce  starts to decrease. This is detected by the current change rate sensor  16 , which controls the current generators  13 ,  14 . Hence, the current generators  13 , 14  limit the maximum gate current I g . 
         [0050]    At time T 4 , the current I ce  has dropped to about zero. The voltage Vg is now stable at a low level and the gate current I g  is on its way back to zero. The overshoot of V ce  has now ended and V ce  is stable. 
         [0051]      FIG. 4  is a schematic graph showing the dynamics of the gate driver circuit  1  and the lower switch  21  of  FIG. 2  when the lower switch  21  is changed from a closed to an open state. 
         [0052]    A gate signal S g  from the controller  40  goes from low to high at time T 0 . This results in an increase in I g  to charge the gate of the switch  21 . Consequently, the voltage V g  of the switch  21  increases. 
         [0053]    At time T 1 , the gate of switch  21  is sufficiently charged for the current I ce  to start to increase. The current change rate sensor  16  reacts to the increase in I ce , controlling the current sources which thereby limits Ig, whereby I g  decreases. Also, due to stray inductances, V ce  drops somewhat at this time. 
         [0054]    At time T 2 , I ce  peaks and starts to decrease. Due to the current change rate sensor  16 , the current sources are controlled whereby the gate current I g  is increased. The voltage V ce  now starts to decrease. 
         [0055]    At time T 3 , the voltage V ce  has dropped to a level for when the switch is fully open. The current I ce  has now dropped to its end level and, the voltage V g  now increases again. The current I g  continues to flow until the gate is fully charged and drops off subsequently. 
         [0056]      FIG. 5  is a schematic graph showing the dynamics of the gate driver circuit I and the lower switch  21  of  FIG. 2  in a fault situation, e.g. a short circuit. V ce  denotes the voltage difference between the collector and the emitter of the switch  21 , S 1  denotes a signal from an over current protection device indicating an over current and I m  denotes a current going to the motor at point  32  of  FIG. 2 . 
         [0057]    At time T 0 , the short circuit occurs, whereby the voltage V ce  drops sharply. The current I m  starts to increase, indicating a fault. 
         [0058]    At the time T 1 , the switch goes out of saturation and the voltage V ce  increases. Somewhat after time T 1 , the signal S 1  drops as a response to the fault. This turns off the switch  21 , whereby the voltage V ce  increases further. This increase is limited by the current change rate (di/dt) control. 
         [0059]    At T 2 , the current I m  has returned to its lower value. V ce  drops to a lower value. 
         [0060]    The benefits with this gate drive circuit are lower voltage overshoots in collector-emitter voltage at switching and thereby lower EMC emission. This is especially applicable to high power drives where the stray inductances in the connections inside and to the switch/power semiconductor are substantial. By reducing the voltage overshoots, the power semiconductor can be used for higher DC voltages and thereby higher power can be converted for a drive/inverter with a given IGBT module voltage rating. 
         [0061]    It is to be noted that while the embodiments above show the invention embodied in a bridge-leg topology, the present invention can be applied to any situation where overshoots are desired to be avoided. 
         [0062]    The inventive concepts presented herein can be applied generally for power conversion devices, particularly in inverter for electric motor control, e.g. for hybrid or fully electrically powered vehicles. 
         [0063]    The gate drive circuit described above protects the switch against excessive voltages or currents in all operating states. This particularly includes turning off the switch at a short-circuit situation when the collector current can become as high as ten times the normal operating current without voltage over-shoots reaching above the IGBT breakdown voltage. 
         [0064]    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 patent claims.