Abstract:
The present invention relates to a passive auxiliary circuit for driving power switches connected in series for much higher rating of a power system. The passive auxiliary circuit for an IGBT power switch connected in series with another, comprises two capacitors, which are connected in series across the IGBT; two resistors dividing a voltage applied across the IGBT, each resistor being connected in parallel with each capacitor; and a diode and a third resistor which are connected in series between a gate terminal of the IGBT and a node connecting the two capacitors, wherein the diode is placed such that its cathode is directed to the gate. This passive auxiliary circuit, whose structure is so simple, is able to distribute a supplied high voltage equally over series-connected IGBTs, to reduce additional power loss remarkably, and to relieve transient overvoltage like as a conventional active gate circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a passive auxiliary circuit for driving power switching elements connected in series for much higher rating of a power system. 
     2. Description of the Related Art 
     High voltage and current rating solid state switches are being used as devices for high voltage power system owing to development of solid state electronic device technology. Among solid state switches, an IGBT (Insulated Gate Bipolar Transistor) can operate at fast switching frequency and its driving circuit is so simple. In addition, the rating of an IGBT is higher than that of a MOSFET (Metal Oxided Semiconductor Field Effect Transistor). Therefore, an IGBT is replacing a thyristor and a GTO (Gate Turn Off) thyristor gradually in high-capacity power system as well as middle-capacity one. 
     However, an IGBT being used commonly at present has the rating of 3.3 kV/1200 A, so that, if the rating does not match the need for any several order kilovolt high voltage application such as a high-voltage pulse generator, a static var compensator, a high-voltage source inverter, and so on, several IGBTs should be connected in series for that need. Due to this reason, series connection methods of power semiconductor switches have been introduced in many papers recently. 
     When a plurality of power switches are to be connected in series for a high voltage application, it is very important to distribute the high voltage across all switches equally in order that an overvoltage exceeding a rating voltage of a single switch should not be applied across an arbitrary switch. Especially, since the characteristic difference is very large between each IGBT at the time when it is turned off, an IGBT may be destroyed if an overvoltage is applied due to the unbalance of voltage distribution. 
     To resolve such problems, various auxiliary circuits including passive elements only or active elements have been designed so far. FIG. 1 shows a passive snubber circuit composed of a resistor Rs, a capacitor Cs, and a diode Ds, and FIG. 2 shows a power circuit including an active gate driving circuitry. In the circuit of FIG. 2, each voltage across each switching element is detected and is compared with a predetermined reference, then, the gate driving circuitry is controlled based on the comparison result. 
     The voltage distributing method according to the snubber circuit of FIG. 1 has such drawbacks that the size of a circuit is increased since the capacitance of the snubber capacitor Cs should be increased if a switching element is for high voltage, and a power loss is much large in proportion to switching frequency. Therefore, the method using a passive snubber circuit is not applicable to higher-capacity and higher-frequency power circuit because of much power loss. 
     The method using an active gate driving circuitry of FIG.  2  has advantages over the method of a passive snubber circuit in that the switching power loss is negligible and its operation is more reliable. However, a power circuit including the active gate driving circuitry becomes more complex if it has to comprise more switching elements to be connected in series since additional circuits for detection, comparison, and control are added to each switching element, which causes a reliability and insulation problem of an overall power circuit. 
     Besides the passive and the active circuits shown in FIGS. 1 and 2, various technologies for series connection between solid state power switches has been presented, however they still have disadvantages that the circuit structure is complicated and power loss is not negligible. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a passive auxiliary circuit for series connection of a solid state power switch, which has simple structure composed of small-capacity passive elements and is able to distribute a supplied high voltage equally across a plurality of semiconductor switches connected in series in a high-rating power supplying system. 
     A passive auxiliary circuit for series-connection of a semiconductor power switch according to the present invention, comprises two capacitors, which are connected in parallel across the power switch; two resistors dividing a voltage applied across the power switch, each resistor being connected in parallel with each of the capacitors; and a diode and a third resistor which are connected in series between a driving terminal of the power switch and a node connecting the two capacitors, wherein the diode is placed such that its cathode is directed to the driving terminal. 
     Another passive circuit for series-connection of a semiconductor power switch according to the present invention, comprises two capacitors, which are connected in series across the power switch; two resistors dividing a voltage applied across the power switch, each resistor being connected in parallel with each of the capacitors; a third resistor connected between a driving terminal of the power switch and a node connecting the two capacitors; and a diode inserted into a branch between a current inflow terminal of the power switch and one of the capacitors, wherein the diode is placed such that its anode is directed to the current inflow terminal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the present invention. 
     In the drawings: 
     FIG. 1 shows series-connected power switches including a conventional passive snubber circuit; 
     FIG. 2 shows series-connected power switches including a conventional active gate driving circuitry; 
     FIG. 3 shows a passive auxiliary circuit for series connection of a solid state power switch according to the present invention; 
     FIGS. 4A to  4 D show operational circuit in associated with basic operational modes; 
     FIG. 5 shows waveforms at several concerning points of the circuit of FIG. 3 according to the operational modes classified as shown in FIGS. 4A to  4 D; 
     FIG. 6 shows series-connected power switches which the passive auxiliary circuits presented in FIG. 3 are added to; 
     FIG. 7 is another passive auxiliary circuit for series connection of a solid state power switch according to the present invention; and 
     FIG. 8 shows series-connected power switches which the passive auxiliary circuit presented in FIG. 7 are added to. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In order that the invention may be fully understood, preferred embodiments thereof will now be described with reference to the accompanying drawings. 
     FIG. 3 shows a passive auxiliary circuit  20  for series connection of a solid state power switch according to the present invention. An IGBT  30  is included in the switching circuit of FIG. 3 as a solid state switching device. The passive auxiliary circuit  20  consists of two capacitors C 1  and C 2 , three resistors R 1 , R 2  and Rg, and one diode D. The two capacitors C 1  and C 2  are connected in series between a drain and a source terminal of the IGBT  30 . Each of the two resistors R 1  and R 2  is connected in parallel with each of the two capacitors C 1  and C 2 . The other resistor Rg and the diode D are connected in series between a gate terminal of the IGBT  30  and a common dividing point G, and the diode D is placed such that its cathode is directed to the gate of the IGBT  30 . 
     It is preferable that the capacitance of the capacitor C 1  and the resistance of the resistor R 1  are much larger relatively to the capacitor C 2  and the resistor R 2 . In the embodiment, the capacitance of the capacitor C 1  is 100 [nF] whereas the capacitance of the other capacitor C 2  is 10 [nF], and the resistor R 1  is 30 [kΩ] whereas the resistor R 2  is 3 [kΩ]. The diode adopted in the embodiment has the rating of 1000 [V] and 1 [A]. 
     FIGS. 4A to  4 D shows several classified operational modes of the series-connected switching circuit of FIG. 3, and FIG. 5 shows waveforms at several concerning points according to the operational modes classified as shown in FIGS. 4A to  4 D. 
     To simplify the explanation of the operational modes, it is assumed that all components of FIG. 3 are ideal so that parasitic components are all zero. The operation of the passive auxiliary circuit  20  is described in detail only for turn-off of the IGBT  30  since this operation is same with turn-on mode of the IGBT  30 . 
     The switching circuit of FIG. 3 has a resistive and inductive load. Vs marked in FIGS. 4A to  4 D and  5  is a supplied voltage, Vgs is a gate-source voltage of the IGBT  30 , Vds is a drain-source voltage, Vc 1  is a divided voltage applied across the capacitor C 1 , and Vc 2  is a divided voltage across the capacitor C 2 . 
     FIG. 4A is the first mode M 1  in which the IGBT  30  has been turned on by a driving voltage applied to the gate from a basic drive circuit  10 . In the first mode M 1 , the voltage Vc 2  across the capacitor C 2  is charged up to a reverse voltage of Vc 1  across the capacitor C 1 , and the diode Da which the reverse voltage is applied to blocks the reverse voltage from being applied to the gate of the IGBT  30 . The divided voltage of the IGBT  30  is almost maintained by the capacitor C 1  while small voltage is discharged through the resistor R 1 . The operation of IGBT  31  is same as that of the IGBT  30 . 
     FIG. 4B is the second mode M 2  in which the IGBTs  30  and  31  begin to turn off from the first mode M 1  during the falling edge of the driving voltage from the basic drive circuit  10 . In the second mode M 2 , The current flowing through the IGBTs  30  and  31  is decreased to zero gradually since the driving voltage Vgs is changed to negative. At the same time, the current not being able to flow through the IGBT  30  starts to flow through the two capacitors C 1  and C 2  while charging the capacitor C 2  rapidly from negative to zero and the capacitor C 1  up to the divided voltage of the IGBT  30 . As a result, the voltage Vds between the drain and the source becomes Vc 1 +Vc 2 . In the case of IGBT  31 , the operation of auxiliary circuit is same. 
     FIG. 4C is the third mode M 3 , in which, it is assumed an overvoltage is applied to only the IGBT  30  at the lowest point of falling edge of the driving voltage from the basic drive circuit  10 . 
     Because the voltage of IGBT  31  is under the divided voltage, the voltage Vc 2  maintained to zero. Therefore, the IGBT  31  becomes to turn-off state. 
     This mode M 3  shows a noticeable feature of the present invention as follows. 
     In the third mode M 3 , the voltage Vds of the IGBT  30  is still increasing so that it exceeds the voltage Vc 1  across the capacitor C 1 . The current still charging the capacitors C 1  and C 2  is decreased gradually. After the voltage Vds across the IGBT  30  is higher than the divided voltage, the capacitor C 2  is charged to positive because the capacitance of C 1  is much larger than that of C 2 . The positive voltage of Vc 2  is then applied to the gate terminal of the IGBT  30 , which causes to increase the gate voltage above zero and acts to turn on the IGBT  30  since the gate voltage is the sum of the driving voltage from the basic drive circuit  10  and the voltage across the capacitor C 2 . Therefore, the increasing rate of the voltage Vds across the IGBT  30  being turned off becomes smooth, thereby suppressing possible overvoltage across the IGBT  30 . 
     FIG. 4D is the fourth mode M 4  in which the IGBT  30  has been turned off after the suppressed overvoltage of Vds on turning off disappears completely. In this mode M 4 , the current through the IGBT  30  and the voltage Vc 2  across the capacitor C 2  is reduced to zero. The gate voltage Vgs returns to and keeps zero after its positive state caused by the overvoltage of Vds is totally released, so that the IGBT  30  is kept turned off stably. The voltage across the IGBT  30  turned off is divided by the resistors R 1  and R 2 . 
     FIG. 6 shows a part of a power circuit including two series-connected solid state power switches S 1  and  32  which the passive auxiliary circuits presented in FIG. 3 are added to. In the power circuit of FIG. 6, a load composed of a resistor R and an inductor L is connected to a supplying voltage Vs and is driven through two series-connected IGBT switches S 1  and S 2 . Two passive auxiliary circuits  20  described in FIG. 3 are connected around each IGBT, and the two passive auxiliary circuits  20  are commonly connected at the connection point of the two switches S 1  and S 2 . Two basic drive circuits  10  drive individual gate of each IGBT switch, and a controlling circuit  40  generates switch control signals to be applied to the two basic drive circuits  20 . 
     FIG. 7 is another passive auxiliary circuit for series connection of a solid state power switch according to the present invention. The auxiliary circuit  50  presented in FIG. 7 is different from the circuit of FIG. 3 in that a diode D 1  is inserted into branch between the drain of an IGBT  30  and the connecting node of the capacitor C 1  and the resistor R 1  instead of being connected to the gate terminal. The diode D 1  is placed such that a freewheeling current can flow from the drain of the IGBT  30  to the connecting node. 
     In the auxiliary circuit  50  of FIG. 7, the diode D 1  connected to the drain can prevent a bottom capacitor C 2  from being charged in reverse and have the bottom capacitor C 2  discharged to zero when the IGBT  30  is turned on. Other operations of the auxiliary circuit  50  is same with the circuit  20  presented in FIG.  3 . 
     FIG. 8 shows a part of a power circuit including two series-connected solid state power switches S 1  and S 2  which the passive auxiliary circuits  50  presented in FIG. 7 are added to. In the power circuit of FIG. 8, a load composed of a resistor R and an inductor L is connected to a supplying voltage Vs and is driven through two series-connected IGBT switches S 1  and S 2 . Two passive auxiliary circuits  50  described in FIG. 7 are connected around each IGBT, and the two passive auxiliary circuits  20  are commonly connected at the connection point of the two switches S 1  and S 2 . Two basic drive circuits  10  and a controlling circuit  40  conduct same function as in the power circuit of FIG.  6 . 
     The aforementioned auxiliary circuit is simply constructed by passive elements which have very small power rating compared with main IGBT switches. Especially, an additional loss, which is caused from discharge of the bottom capacitor C 2  and power dissipation at the dividing resistors R 1  and R 2 , is almost negligible. That is, the loss is remarkably reduced compared with a conventional auxiliary circuit including snubbers or clampers since the capacitance C 2  is very small. 
     This passive auxiliary circuit operates similarly with a conventional active gate control method in the third mode M 3  described in the above. Moreover, The slope of rising and falling edge of an IGBT is almost same as using a basic drive circuit since a fast transient response for overvoltage is achieved. 
     This auxiliary circuit guarantees switching frequency up to several tens kHz for a main switch with little loss, and enables several tens kV voltage to be applied across all series-connected switches since the auxiliary circuit is easily connected each other in stack. 
     Although the preferred embodiment of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as recited in the accompanying claims.