Gate drive circuit for power switching device

Abstract: A gate drive circuit for a voltage-driven type power switching device, including a control device for generating a gate voltage for the voltage-driven type power switching device, and a correction device connected to the control device for receiving the gate voltage and for correcting an effect of a magnetic flux resulted from a main circuit current flowing in the voltage-driven type power switching device applied to the gate voltage to generate a corrected gate voltage. The corrected gate voltage is applied to a gate of the voltage-driven type power switching device.

Description: This invention relates to a gate drive circuit for a voltage-driven type power switching device which controls the switching on and off of the voltage-driven type power switching device. In order to switch power circuits by means of a control voltage, a insulated gate bipolar transistor (hereafter, abbreviated as IGBT) or a field effect transistor may be used as a switching device. Each of these devices has a large gate input impedance, and on-off switching control thereof can be effected by applying a gate voltage. A conventional gate drive circuit for an IGBT is shown in Figure 6. In Figure 6, a gate voltage V G output from a control device 2 for controlling the on/off switching of an IGBT 1 is supplied between the gate and emitter of IGBT 1 by means of a signal wire 3, such as a twisted pair wire or the like. In this case, the wiring is made such that the effect of the magnetic flux due to the main circuit current flowing in the conductor connected to the collector or emitter of IGBT 1 is made as small as possible. However, in reality, it is not possible completely to eliminate the effect of the magnetic flux resulted from the main circuit current due to the relative physical positioning. Therefore, there are cases where an induced voltage due to this magnetic flux is produced in signal wire 3, which has an adverse effect on a voltage v GE between the gate and emitter of IGBT 1. For example, an induced voltage v X is produced in one part of signal wire 3 by change in a magnetic flux  due to change in an emitter current i E of IGBT 1. There are cases where the polarity of this induced voltage v X is in the direction, as shown in Fig. 7 whereby voltage v GE applied between the gate and eimtter of IGBT 1 rises or falls when emitter current i E changes in the direction of an increase or a decrease, respectively. In this case, the following problems arise. Specifically, when the main circuit current flowing in the collector and emitter of IGBT 1 changes in the direction of an increase, if voltage V GE between the gate and emitter of IGBT 1 rises, an voltage v CE between the collector and emitter of IGBT 1 caused to fall, and thereby a positive feedback action is produced causing the main circuit current to further rise. Therefore, there is the danger that when there is a sudden increase in the main circuit current in IGBT 1 due to shorting of the load, for example, voltage v GE between the gate and emitter rises, to thereby cause the main circuit current to rise further, and as a result, IGBT 1 cannot be protected from an overcurrent and may be damaged. Furthermore, when the main circuit current flowing in IGBT 1 changes in the direction of a decrease, if voltage v GE between the gate and emitter falls, voltage v CE between the collector and emitter is caused to rise, and thereby a positive feedback effect is produced causing the main circuit current to further fall. Therefore, there is the danger that when gate voltage v G output from control device 2 is reduced at a prescribed rate of decrease and the main circuit current in ICBT 1 is reduced to zero, the main circuit current is caused further to fall suddenly due to the positive feedback action, and an excess voltage between the collector and emitter of IGBT 1 is produced by a back e.m.f. resulted from a floating inductance (not shown in drawing) present in the main circuit, and this may cause damage to IGBT 1. Furthermore, even if IGBT 1 is not damaged by the over current or overvoltage as described above, there is the risk that high-frequency oscillations may be produced in the main circuit current in IGBT 1 due to the aforementioned positive feedback action, which may adversely affect the life etc. of IGBT 1. SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide a gate drive circuit for a voltage-driven type power switching device which can perform a stable switching control of the voltage-driven type power switching device. Another object of this invention is to provide a gate drive circuit for a voltage-driven type power switching device which can reduce an adverse effect of the magnetic flux density due to the main circuit current flowing in the voltage-driven type power switching device applied to the gate voltage. Still another object of this invention is to provide a gate drive circuit for a voltage-driven type power switching device which can prevent an adverse effect of a positive feedback action by the magnetic flux density due to the main circuit current flowing in the voltage-driven type power switching device applied to the gate voltage. These and other objects of this invention can be achieved by providing a gate drive circuit for a voltage-driven type power switching device, including a control device for generating a gate voltage for the voltage-driven type power switching device, and a correction device connected to the control device for receiving the gate voltage and for correcting an effect of a magnetic flux resulted from a main circuit current flowing in the voltage-driven type power switching device applied to the gate voltage to generate a corrected gate voltage. The corrected gate voltage is applied to a gate of the voltage-driven type power switching device so that when the main circuit current increases the gate voltage is caused to fall, and when the main circuit current decreases the gate voltage is caused to rise. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 is a circuit diagram showing a gate drive circuit for a voltage-driven type power switching device according to an embodiment of this invention: Figure 2 is a waveform diagram for explaining the action of the embodiment shown in Figure 1; Figure 3 is a circuit diagram showing a gate drive circuit for a voltage-driven type power switching device according to another embodiment of this invention; Figure 4 is a circuit diagram showing a gate drive circuit for a voltage-driven type power switching device according to still another embodiment of this invention; Figure 5 is a circuit diagram showing a gate drive circuit for a voltage-driven type power switching device according to a further embodiment of this invention; Figure 6 is a circuit diagram showing one example of a conventional gate drive circuit for a voltage-driven type power switching device; and Figure 7 is a waveform diagram for explaining the action of the conventional gate drive circuit shown in Figure 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, the embodiments of this invention will be described below. An embodiment of this invention is shown in Fig. 1. In this Figure, IGBT 1, control device 2, and signal wire 3 are the same as in the prior art shown in Figure 6. In addition, however, there is provided a coupling wire 4 which is arranged to couple electromagnetically to a conductor connected to the collector of IGBT 1. This embodiment is constructed such that a gate voltage v G output from control device 2 is supplied between the gate and emitter of IGBT 1 through this coupling wire 4. In the aforementioned construction, when a gate voltage V G is output from the control device 2 and a current i C is flowing in the collector of IGBT 1, an induced voltage v Y corresponding to a rate of change di C /dt in this current i C is produced in coupling wire 4. This induced voltage v Y is added to gate voltage v G , and acts such that when current i C changes in the direction of an increase or a decrease, voltage V GE supplied between the gate and emitter of IGBT 1 falls or rises, as shown in Fig 2. Thereby, the positive feedback action of voltage v X induced in signal wire 3 is cancelled out, and furthermore, it is possible to apply a negative feedback action. Furthermore, coupling wire 4 can be implemented such that it is provided on a printed circuit board installed close to IGBT 1 and the coupling coefficient of the electromagnetic coupling can be adjusted. Thus, it is possible to reduce the positive feedback action, to cancel out the positive feedback action or apply negative feedback action, by the adjustment of the coupling coefficient of the electromagnetic coupling. Another embodiment of this invention is shown in Fig. 3. This Figure shows an arrangement including a coupling wire 4 with electromagnetic coupling to a conductor connected to the emitter of IGBT 1. Emitter current i E of IGBT 1 is virtually the same as collector current i C , and the similar operation and effect as that of the embodiment described above can be obtained. Still another embodiment of this invention is shown in Fig. 4. In this embodiment, an impedance element 5, such as a reactor, is connected in series to the emitter side of IGBT 1. The construction of this embodiment is such that gate voltage v G output from control device 2 is supplied via signal wire 3 between the gate of IGBT 1 and one terminal (non-emitter side) of impedance element 5. In the foregoing construction, when gate voltage v G is output and emitter current i E flows in IGBT 1, voltage drop v Y resulted from emitter current i E is produced in impedance element 5. This voltage v Y acts in a direction causing the voltage v GE applied between the gate and emitter of IGBT 1 to fall, thus acting as a negative feedback. Furthermore, when an inductance element is used as impedance element 5, voltage drop v Y corresponding to the rate of current change di E /dt in emitter current i E is produced in the inductance element, and a similar action to those in the previously described embodiments can be achieved. A further embodiment of this invention is shown in Fig. 5. The construction of this embodiment is such that the primary winding of a transformer 6 is connected in series to the emitter side of IGBT 1, and in supplying a gate voltage V G output from control device 2 between the gate and emitter of IGBT 1 by means of signal wire 3, the secondary winding of transformer 6 is inserted intermediately in this signal wire 3. In the foregoing construction, when emitter current i E is flowing in IGBT 1, induced voltage v Y corresponding to the rate of current change di E /dt of emitter current i E is produced in the secondary winding of transformer 6. A similar action to those in the previously described embodiments can be achieved. With these embodiments, it is possible to cancel out the previously described positive feedback action and moreover, to apply a negative feedback action. For example, when there is a sudden increase in the main circuit current in IGBT 1 due to shorting of the load or the like, it is possible to prevent IGBT 1 from being damaged by overcurrent by preventing that voltage v GE between the gate and emitter rises and the main circuit current rises further. Furthermore, it is possible to prevent IGBT 1 from being damaged by overvoltage by preventing that the excess voltage is produced between the collector and emitter of IGBT 1, when gate voltage v G is reduced at a prescribed rate of decrease and the main circuit current of IGBT 1 is caused to fall to zero. Moreover, the production of high-frequency oscillations in the main circuit current of IGBT 1 can also be prevented, and the operational reliability of IGBT 1 can be increased. According to the present invention, it is possible to provide a gate drive circuit for voltage-drive type power switching device whereby the voltage between the gate and emitter of a voltage-driven type power switching device can be prevented from the adverse effects by the magnetic flux due to the collector current or emitter current, and stable switching control can be performed. Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims: A gate drive circuit for a voltage-driven type power switching device (1), comprising: control means (2) for generating a gate voltage for said voltage-driven type power switching device, characterised by: correction means (4, 5, 6) connected to said control means for receiving said gate voltage and for correcting the effect of a magnetic flux resulting from a main circuit current flowing in said voltage-driven type power switching device by deriving a correction voltage from said main circuit current and applying said correction voltage to the gate of said voltage-driven type power switching device so that when said main circuit current increases said gate voltage is caused to fall, and when said main circuit current decreases said gate voltage is caused to rise. The gate drive circuit for a voltage-driven type power switching device according to claim 1 wherein: said correction means generates said correction factor by detecting said magnetic flux (). The gate drive circuit for a voltage-driven type power switching device according to any preceding claim, wherein: said correction means includes a coupling connection with an electromagnetic coupling to a conductor connected to the collector or the emitter of said voltage-driven type power switching device, and said gate voltage is applied to said gate of said voltage-driven type power switching device through said coupling connection to provide a corrected gate voltage. The gate drive circuit for a voltage-driven type power switching device according to claim 3, wherein: said correction means generates said corrected gate voltage by adjusting a coupling coefficient of said electromagnetic coupling between said conductor and said coupling connector. The gate drive circuit for a voltage-driven type power switching device according to claim 1, wherein: said correction means includes an impedance element (5) having one terminal connected in series to an emitter side of said voltage-driven type power switching device, and said gate voltage is applied between said gate and the other terminal of said impedance element, whereby said corrected gate voltage is applied between said gate and said emitter of said power switching device. The gate drive circuit for a voltage-driven type power switching device according to claim 5, wherein said impedance element includes an inductance element. The gate drive circuit for a voltage-driven type power switching device according to claim 1, wherein: said correction means includes a transformer (6) with a primary winding connected in series to the emitter of the switching device and a secondary winding connected between said control means and said gate of said voltage-driven type power switching device, said gate voltage being applied between said emitter and said gate through said secondary winding of said transformer, whereby said corrected gate voltage is applied between said gate and said emitter of said power switching device.

Inventors: KAWAKAMI KAZUTO, KAWAKAMI, KAZUTO, KAWAKAMI, KAZUTO, C/O INTEL.PROP.DIV. TOSHIBA COR.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1995
Main classification: H02M1
IPC codes: H02M1, H03K17, H02J1