Abstract:
Disclosed is a semiconductor apparatus having an IGBT, which includes a switch SWon  1  for supplying a gate current during an operation for turning on the gate of the IGBT, a switch SWoff  1  for discharging a gate capacitance during an operation for turning off the gate, a switch SWon  2  for increasing the gate current, a timer  14  for turning on the switch SWon  2  in conjunction with the turn-on of the switch SWon  1  and then maintaining the turn-on of the switch SWon  2  only for a first predetermined time, a switch SWoff  2  for increasing the discharge current during the gate turn-off operation, and a timer  15  for turning on the switch SWoff  2  in conjunction with the turn-on of the switch SWoff  1  and then maintaining the turn-on of the switch SWoff  2  only for a second predetermined time.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a semiconductor apparatus using an MOS-type device, designed to control a time-period between the receiving of an external signal and the actual action of the device itself so as to achieve improvement in response and efficiency, and reduction in EMI noise and switching loss.  
         [0002]     In a semiconductor apparatus having a tail-end circuit with a MOS-type device to be driven by an external signal, the MOS-type device has a relatively large delay time between the receiving of the external signal intended to turn off the device and the establishment of its actual OFF state. This delay time is caused by a capacitance around the gate of the device. Heretofore, in a circuit for switching the gate, a resistor inserted in series to the gate has been preset at a certain value to adequately control the charge and discharge of the gate. That is, the rise and fall times of a gate voltage have been adjusted in accordance with a CR time constant determined by a gate capacitance C and a gate resistance R.  
         [0003]     The respective timings of allowing current to start passing through the device and allowing the current to be cut off are dependent on a threshold voltage specific to the device. In particular, the turn-off action of the device is related to a switching speed specific to the device. Thus, the technique of controlling the gate charging and discharging times of the device only by a gate resistor set at a certain fixed value results in creation of another delay time in addition to the delay time due to the threshold voltage of the device, which leads to deterioration in efficiency.  
         [0004]     Further, if the charge and discharge of the gate is conducted using a gate resistor set at a fixed value, a switching speed during the turn-on of the device will become faster in a low current range than that in a rated current range. The resulting increased value dV/dt during the turn-on of a free-wheel diode (FWDi) connected in parallel to the device serves as a factor of causing the degradation in EMI noise. Otherwise, if the switching speed during the turn-on is lowered to reduce EMI noise, a switching loss will be increased.  
         [0005]     As above, there is a trade-off relationship between EMI noise and switching loss. For this reason, the gate resistor has heretofore been preset at a certain value selected based on a compromise point compatible with both EMI noise and switching loss.  
         [0006]     In order to allowing the gate of a device to be driven at low loss and low noise, there has been known a technique in which a plurality of MOSFETs each having a different turn-on resistance are used as switches for driving the gate of an IGBT, and these switches are sequentially combined such that the entire turn-on resistance is transitionally changed to vary a gate drive performance [see, for example, Japanese Patent Laid-Open Publication No. 2001-223571, “Gate Drive Circuit for Voltage Driven-type Semiconductor Device” (Paragraph [0005], FIG. 2), hereinafter referred to as Patent Publication 1].  
         [0007]     There has also been known a technique for noise reduction in which the value di/dt in a detected IGBT collector current is compared with a command value through a comparator, and a switch is turned on and off in accordance with the comparison result to instantaneously change a gate resistance [see, for example, Japanese Patent Laid-Open Publication No. 10-150764, “Gate Drive Circuit for use in Power Converter” (Paragraph [0008], FIG. 6), hereinafter referred to as Patent Publication 2].  
         [0008]     The circuit disclosed in Patent Publication 1 is a type of controlling a gate voltage. Further, the circuit involves a number of switches and a complicate control for sequentially turning on the switches depending on the characteristics of the IGBT.  
         [0009]     In the circuit disclosed in Patent Publication 2, if the gate resistance is sharply changed during a turn-on operation where a collector current flows at a value approximately equal to a comparison reference value of the comparator, the operation of the IGBT is likely to become unstable to cause oscillation in an output current therefrom.  
       SUMMARY OF THE INVENTION  
       [0010]     In view of the above circumstances, it is an object of the present invention to provide a semiconductor apparatus capable of stably operating a MOS-type device using a relatively simple configuration while facilitating improvement in response and efficiency, and reduction in EMI noise and switching loss.  
         [0011]     In order to achieve the above object, the present invention provides a semiconductor apparatus including a MOS-type device such as an IGBT, comprising a first switch for supplying a gate current during a gate turn-on operation for turning on the gate of the MOS-type device, and a second switch for discharging a gate capacitance during a gate turn-off operation for turning off the gate of the MOS-type device. The semiconductor apparatus further includes a third switch for increasing the gate current during the gate turn-on operation, first timer means for turning on the third switch in conjunction with the turn-on of the first switch and then turning off the third switch after a first predetermined time from the turn-on of the third switch, a fourth switch for increasing the discharge current during the gate turn-off operation, and second timer means for turning on the fourth switch in conjunction with the turn-on of the second switch and then turning off the fourth switch after a second predetermined time from the turn-on of the fourth switch.  
         [0012]     According to the present invention, in the gate turn-on operation, the gate current is increased only for the first predetermined time by turning on the first switch and turning on the third switch for the first predetermined time. Thus, the time-period allowing the gate to be turned on can be reduced. In the gate turn-off operation, the discharge current from the gate is increased only for the second predetermined time by turning on the second switch and turning on the fourth switch for the second predetermined time. Thus, the time-period allowing the gate to be turned off can be reduced. The semiconductor apparatus constructed as above can achieve reduction in switching loss and delay time.  
         [0013]     Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic circuit diagram of a semiconductor apparatus according to a first embodiment of the present invention.  
         [0015]      FIG. 2  is an operational time chart of the circuit in  FIG. 1 .  
         [0016]      FIG. 3  is a schematic circuit diagram of a semiconductor apparatus according to a second embodiment of the present invention.  
         [0017]      FIG. 4  is an operational time chart of the circuit in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     With reference to the drawings, an embodiment of the present invention will now be described.  
         [0019]     [First Embodiment] 
         [0020]      FIG. 1  shows a MOD-type power device and a drive system thereof in a tail-end circuit of a semiconductor apparatus according to a first embodiment of the present invention. For turning on the gate a power device (e.g. IGBT)  1  associated with a freewheel diode D, two switches SWon  1 , SWon  2  (or first and third switches) each composed of a transistor are inserted in series between a power supply VCC and the gate of the power device  1 . A constant current device  4  is connected in parallel with the switch SWon  2 .  
         [0021]     For turning off the gate of the power device  1 , a switch SWoff  1  (or second switch) composed of a transistor is connected between the gate and emitter of the power device  1  through a constant current device  5 , and a switch SWoff  2  (or fourth switch) composed of a transistor is connected between the gate and emitter of the power device  1  through a constant current device  7 .  
         [0022]     An input signal VIN is entered into a positive input terminal of a comparator  9  which is connected with a pull-up resistor  10 . A reference voltage source  11  is connected to a negative input terminal of the comparator  9 . An output signal of the comparator  9  is supplied to an input terminal of an amplifier  13  through an inverter  12 . A noninverted output of the amplifier  13  as a turn-on signal is supplied to the switch SWon  1  and concurrently entered into a timer  14 . In response to the input of the turn-on signal, the timer  14  (serving as first timer means) is operable to turn on the switch SWon  2 . After 100 ns from the turn-on of the switch SWon  2 , the timer  14  is operable to turn off the switch SWon  2 .  
         [0023]     An inverted output of the amplifier  13  as a turn-off signal is supplied to the switch SWoff  1  and concurrently entered into a timer  15 . In response to the input of the turn-off signal, the timer  15  (serving as second timer means) is operable to turn on the switch SWoff  2 . After 200 ns from the turn-on of the switch SWoff  2 , the timer  15  is operable to turn off the switch SWoff  2 .  
         [0024]     The operation of the above circuit will be described with reference to the time chart in  FIG. 2 . During the course that the input signal VIN from outside changes from an OFF signal (a signal for turning off the IGBT  1 ) of High Level to an ON signal (a signal for turning on the IGBT  1 ) of Low Level, upon falling to an input-ON threshold (1.5 V in  FIG. 2 ) (at Time T1), the switch SWon  1  is turned on, and simultaneously the switch SWon  2  is turned on through the timer  14 . At Time T1, the switch SWoff  1  is changed from ON state to OFF state (as can be understood from the later description, when the input signal VIN is in OFF state, the switch SWoff  1  is in ON state), and the switch SWoff  2  remains in OFF state.  
         [0025]     Thus, a gate current Ig is supplied through the switch SWon  1  and the switch SWon  2  which are turned on at Time T1, so that a gate charge is initiated. In this process, a large amount of gate current Ig supplied from both the switches SWon  1 , SWon  2  in ON state provides a shortened time-period required for a gate voltage to rise to a gate-ON threshold to allow the IGBT  1  to be turned on in a short time-period after the turn-on of the switches SWon  1 , SWon  2  (Time T1).  
         [0026]     After 100 ns from the turn-on of the switches SWon  1 , SWon  2  (Time T1), only the switch SWon  2  is turned off. While the gate current Ig is thereby reduced by about half, the remaining gate current is sufficient to keep the IGBT  1  in ON state. This time-period of 100 ns is set to be slightly longer than a delay time between the turn-on of the switches SWon  1 , SWon  2  and the switching of the IGBT  1  to ON state.  
         [0027]     Subsequently, during the course that the input signal VIN changes from the ON signal of Low Level to the OFF signal of High Level, upon rising to an input-OFF threshold (2.0 V in this embodiment) (at Time T2), the switch SWon  1  is turned off (the switch SWon  2  remains in OFF state). At Time T2, the switch SWoff  1  is turned on, and the switch SWoff  2  is simultaneously turned on through and the timer  15 . Upon separated from the power supply Vcc, the gate is connected to the emitter through the switches SWoff  1 , SWon  2 , so that a gate discharge (negative current in FIG.  2 ) is initiated. In this process, a large discharge current flowing through both the switches SWoff  1 , SWoff  2  in ON state provides a shortened time-period required for the gate voltage to fall to a gate-OFF threshold to allow the IGBT  1  to be turned off in a short time-period after the turn-on of the switches SWoff  1 , SWoff  2  (Time T2). After 200 ns from the turn-on of the switches SWoff  1 , SWoff  2  (Time T2), only the switch SWoff  2  is turned off. While the gate discharge current is thereby reduced by about half, the remaining gate discharge current is sufficient to keep the IGBT  1  in OFF state. This time-period of 200 ns is set to be slightly longer than a delay time between the turn-on of the switches SWoff  1 , SWoff  2  and the switching of the IGBT  1  to OFF state.  
         [0028]     In this way, the gate charge and discharge currents to the IGBT  1  can be varied so as to reduce the time-period between the receiving of the external input signal and the complete of the operation of the device to provide enhanced efficiency. In addition, the switching time-period (delay time) can be shortened to provide reduced switching loss.  
         [0029]     While the switches SWon  1  and SWon  2  in  FIG. 1  are connected in series with one another, they may be are a connected in parallel with one another in such a manner that the gate current Ig is increased by turning on the switch SWon  2 .  
         [0030]     [Second Embodiment] 
         [0031]     A second embodiment of the present invention is configured such that in accordance with the amount of the current flowing through an IGBT turned on according to a first turn-on operation, a gate charge current in a second turn-on operation subsequent to the first turn-on operation is varied to change a switching speed during turn-on of the gate.  FIG. 3  shows a tail-end circuit of a semiconductor apparatus according to the second embodiment.  
         [0032]     In the circuit of  FIG. 3 , the same component or element as that in  FIG. 1  is defined by the same reference numeral or code. As shown in  FIG. 3 , in addition to a turn-on circuit comprised of a switch SWon  1 , a switch SWon  2 , a constant current device  4  and a timer  14 , the circuit additionally includes a switch SWon  3 , a switch SWon  4 , a constant current device  21  and a timer  22 , as a second turn-on circuit. The following circuits are added in order to control this 2nd turn-on circuit.  
         [0033]     An IGBT  6  has a second emitter for detecting a collector current Ic. A voltage generated at a resistor  23  inserted in series to the second emitter circuit is entered into a positive input terminal of a comparator  24 . A reference voltage source  25  is connected to a negative input terminal of the comparator  24 , and an output of the comparator  24  is entered into a D terminal of a latch circuit  26 . Then, an output from a Q terminal of the latch circuit  26  is entered into a D terminal of a latch circuit  27 . The output of the aforementioned comparator  9  as a carry signal is entered into a C terminal of the latch circuit  26 , and concurrently entered into a C terminal of the latch circuit  27  through an inverter  28 .  
         [0034]     An output from a Q terminal of the latch circuit  27  is supplied to one of a pair of input terminals of an AND gate  29 , and the noninverted output of the aforementioned amplifier  13  is entered into the other input terminal of the AND gate  29 . Then, an output of the AND gate  29  as a drive signal is supplied to the switch SWon  3  and the timer  22 .  
         [0035]     The operation of the above circuit will be described with reference to the time chart in  FIG. 4 . The comparator  24  determines whether the collector current Ic flowing through the IGBT  1  turned on when an input signal VIN of a (n−1)-th pulse is an ON signal of Low Level is greater than one-half of a rated current, and then supplies the determination result to the latch circuit  26 .  
         [0036]     During the course that the (n−1)-th pulse input signal VIN changes from the ON signal of Low Level to an OFF signal (a signal for turning off the IGBT  6 ) of High Level, upon rising beyond an input-OFF threshold (at Time T3), the comparator  9  outputs the High Level signal. In response to this signal, the signal latched by the latch circuit  26  is supplied to the latch circuit  27 .  
         [0037]     Subsequently, during the course that input signal VIN changes from the OFF signal of High Level to the ON signal of Low Level, upon falling beyond an input-ON threshold and shifting from the (n−1)-th pulse to an n-th pulse (at Time T4), the switches SWon  1 , SWon  2  are turned on. In this process, if the collector current Ic is equal to or greater than 50% of the rated current at Time T3, the High Level signal will be supplied from the latch circuit  27  to the AND gate  29  to allow the switches SWon  3 , SWon  4  to be turned on.  
         [0038]     When all of the switches SWon  1  to SWon  4  are turned on in this way, a gate current Ig is increased up to 100% of a predetermined value, and the IGBT  6  is switched to ON state according to the gate current Ig. After 100 ns from Time T4, the switches SWon  2 , SWon  4  are turned off by the timers  14 ,  22 , and thereby the gate current Ig is reduced to 50% of the predetermined value.  
         [0039]     Otherwise, if the collector current Ic is less than 50% of the rated current at Time T3, the AND gate  29  keeps outputting the Low Level signal. Thus, only the switches SWon  1 , SWon  2  are in ON state, and thereby the gate current Ig is set at 50% of the predetermined value. The operation in this case is the same as that in  FIG. 1 .  
         [0040]     As above, in the high current (50% or more of the rated current) operation of the IGBT  6 , the gate current Ig is increased to provide a higher switching speed. Thus, as compared to the conventional semiconductor apparatus using a gate resistor set at a fixed value, a switching loss can be reduced. In the low current (less than 50% of the rated current) operation of the IGBT  6 , the gate current Ig is reduced to provide a lower switching speed. Thus, the value dV/dt during turning off the freewheel diode D is reduced to allow EMI noise to be effectively reduced.  
         [0041]     Further, when the high current operation of the IGBT  6  is detected, the gate current Ig is not immediately increased, but in a subsequent operation for turning on the IGBT  6 . Thus, the risk of the unstable operation of the IGBT and oscillation of output current as in Patent Publication 2 can be eliminated.  
         [0042]     Advantageous embodiments of the invention have been shown and described. It is obvious to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope thereof as set forth in appended claims.