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Researchers Submit Patent Application, "Short-Circuit Protection Circuit", for Approval

July 30, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventor Shimizu, Naoki (Matsumoto-city, JP), filed on July 10, 2012, was made available online on July 17, 2014.

The patent's assignee is Fuji Electric Co., Ltd.

News editors obtained the following quote from the background information supplied by the inventors: "FIG. 3 is a diagram showing a three-phase inverter circuit. This inverter circuit includes six IGBTs 51 to 56 as voltage-driven type semiconductor devices, six FWDs (Free Wheeling Diode) 57 to 62, and a main power supply 63. An L load of a motor etc. is connected as a load 64 to the inverter circuit.

"Operation of the inverter will be described. Assume that the IGBT 51 and the IGBT 54 turn ON at a certain timing so that a current 71 is supplied from the main power supply 63 to the load 64. When the IGBT 51 and the IGBT 54 then turn OFF, the current 71 flowing into the load 64 passes through the FWDs 58 and 59 and flows back as a return current to the main power supply 63. When the IGBTs 51 to 56 turn ON and OFF sequentially in this manner, three-phase electric power is supplied to the load 64.

"When the IGBT 51 changes from ON to OFF and the IGBT 54 turns ON in the state in which a current 72 is flowing into the FWD 59, a series circuit of the FWD 59 and the IGBT 54 becomes an arm short-circuiting state instantaneously. This arm short-circuiting is cancelled when the FWD 59 is reversely recovered. However, when the IGBT 54 turns ON, the reverse recovery current of the FWD 59 is superimposed on a collector current of the IGBT 54 and flows. For this reason, as shown in FIG. 4, the collector current Ic of the IGBT 54 increases suddenly because the reverse recovery current Ir is superimposed on the collector current Ic of the IGBT 54 when the IGBT 54 is operated to be turned ON. Then, the collector current Ic of the IGBT 54 shifts to a steady state.

"FIG. 5 is a diagram for explaining an operation state of the IGBT at the time of short-circuiting. When a short-circuit current 66 flows, device breakdown occurs. In order to prevent this, an NLU (Non-Latch-Up) circuit is operated to restrain the short-circuit current. However, when the time in which a restraint current 67 flows is prolonged, device breakdown occurs. In order to prevent this, a blocking circuit is operated to block the restraint current 67 (blocking current 68) as soon as the period in which the restraint current 67 flows (NLU operation period) exceeds a predetermined time to (for example, about 2 .mu.s). In this manner, the device is protected from the short-circuit current. This circuit is a short-circuit protection circuit.

"FIG. 6 is a diagram showing a short-circuit protection circuit for an IGBT according to the background art. In addition to the short-circuit protection circuit, an IGBT drive circuit is also shown in FIG. 6. In addition, FIG. 6 coincides with a circuit of a part corresponding to a portion A in FIG. 3.

"The IGBT drive circuit driving the IGBT 56 includes a control power supply 86, a series circuit 92 of a p-channel MOSFET 80 and an n-channel MOSFET 81, and a drive circuit 82 driving gates of these MOSFETs 80 and 81. A contact point 87 between the p-channel MOSFET 80 and the n-channel MOSFET 81 is connected to a gate 56g of the IGBT 56. A control voltage Vgcc is applied to the gate 56g.

"An NLU circuit 94 includes a series circuit 93 of a p-channel MOSFET 83 and an n-channel MOSFET 84, and a drive circuit 85 driving these MOSFETs 83 and 84. A contact point 88 between the p-channel MOSFET 83 and the n-channel MOSFET 84 is connected to the contact point 87. When the NLU circuit 94 is operated, a control power supply voltage VCC (for example, about 15V) of the control power supply 86 is reduced. This reduced voltage is applied as the control voltage Vgcc (for example, about 1V) to the gate 56g of the IGBT 56.

"A detection circuit 91 is a circuit which activates the NLU circuit 94 upon detection of a short-circuit current. The detection circuit 91 includes an operational amplifier 75 and a reference power supply E. A high potential side of a sense resistor Rs is connected to a sense emitter 56se which is a current detection terminal of the IGBT 56. A low potential side of the sense resistor Rs is connected to a main emitter 56e. The high potential side of the sense resistor Rs is connected to a plus terminal of the operational amplifier 75. A minus terminal of the operational amplifier 75 is connected to a plus side of the reference voltage E. A minus side of the reference voltage E is connected to the low potential side of the sense resistor Rs. An output 78 of the operational amplifier 75 is connected to an input 79 of the drive circuit 85.

"In addition, a sense current Is flows into the sense resistor Rs series-connected to the sense emitter 56se of the IGBT 56 into which a main current is applied. The operational amplifier 75 having the plus terminal to which the high potential side of the sense resistor Rs is connected and the minus terminal to which the low potential side (GND) of the sense resistor Rs is connected through the reference voltage E detects whether the IGBT 56 is in a short-circuit state or not, based on the magnitude of the sense current Is, so that the output 78 of the operational amplifier 75 is inputted to the NLU circuit 94. In addition, a freewheeling diode 62 is anti-parallel connected to the IGBT 56.

"FIG. 7 shows operation waveform diagrams of respective portions in the circuit in FIG. 6 during turn-on operation, during steady operation, and during short-circuit operation. In FIG. 7, (a) is an operation waveform diagram of Vc, Ic and Vs, (b) is an operation waveform diagram of Vgcc and Vg, and is a diagram showing an output of a mask circuit. In FIG. 7, the three modes are illustrated together for convenience of explanation. Here, the reference sign VCC designates voltage of the control power supply 86; Vgcc, control voltage; Vg, gate voltage of the IGBT 56; Vc, collector voltage of the IGBT 56; Ic, collector current of the IGBT 56; Ie, emitter current of the IGBT 56; Is, sense current flowing into the sense emitter of the IGBT 56; Vs, sense voltage generated in the sense resistor Rs; L, L level; H, H level; and ton1, turn-on time. Ic is split into Ie and Is.

"First, a turn-on operation of the IGBT 56 will be described. When the p-channel MOSFET 80 in the IGBT drive circuit turns ON, the control voltage Vgcc equal to the voltage VCC (for example, about 15V) of the control power supply 86 is applied to the gate 56g of the IGBT 56.

"A gate current flows into the gate 56g of the IGBT 56 to charge gate capacitance (gate-emitter capacitance in this case). When the gate capacitance is charged, the gate voltage Vg rises. When the gate voltage vg rises to reach a gate threshold voltage, the collector current Ic rises but the collector voltage Vc begins to fall.

"In addition, the sense current Is which is about a several thousandth part of the collector current Ic rises, and a voltage at opposite ends of the sense resistor Rs into which the sense current Is flows, i.e. the sense voltage Vs also increases. When the gate voltage Vg reaches the gate threshold voltage, the collector voltage Vc decreases, mirror capacitance (gate-collector capacitance) of the IGBT 56 increases and the gate voltage vg shifts to a region where the gate voltage vg is substantially constant.

"In addition, when the sense voltage Vs increases to reach an operation threshold voltage Vo (which is determined based on the reference voltage E) where the sense voltage Vs can be regarded as a short-circuit current, the output 78 of the detection circuit 91 outputs an L level signal so that the NLU circuit 94 operates.

"The NLU circuit 94 turns ON the n-channel MOSFET 84 when the output 78 of the detection circuit 91 is on the L level. When the n-channel MOSFET 84 turns ON, a voltage of Vgcc is withdrawn to make the control voltage Vgcc of the contact point 87 lower than the control power supply voltage VCC, as indicated by the sign B. Incidentally, the current driving capability of the MOSFET 84 is set to be smaller than that of the MOSFET 80 so that the control voltage Vgcc can be prevented from being zero even when the MOSFET 84 turns ON.

"When the control voltage Vgcc of the contact point 87 is lower than the control power supply voltage VCC, the collector current Ic and the sense voltage Vs reach their peaks, then pass through the operation threshold voltage Vo and decrease so that the operation of the NLU circuit 94 is cancelled. Then, the collector current Ic and the sense voltage Vs become constant. After the collector voltage Vc decreases suddenly in the period of time in which the NLU circuit 94 is operating, the collector voltage Vc decreases gradually and then shifts to a steady on-state voltage.

"At a point of time (point C) in which the collector voltage Vc has become the steady on-state voltage which is sufficiently low, there is no change in the mirror capacitance any more, and the gate voltage Vg increases again to reach the control power supply voltage VCC (=the control voltage Vgcc) and then becomes constant.

"As described above, when the NLU circuit 94 operates to reduce the control voltage Vgcc applied to the gate 56g in the period of time in which the mirror capacitance increases and the gate voltage Vg becomes constant, the supply of the current to the gate of the IGBT 56 becomes insufficient so that the time required until the voltage Vg reaches a desired value becomes longer, the falling of the collector voltage Vc becomes gentler, and the turn-on time ton1 becomes longer. Thus, a turn-on loss increases.

"Next, a short-circuit operation of the IGBT will be described. When a short-circuit current flows into the IGBT 56 in the state in which the control power supply voltage VCC (=the control voltage Vgcc) is being applied to the gate 56g of the IGBT 56, the sense voltage Vs reaches the operation threshold voltage Vo so that the NLU circuit 94 operates. In addition, the collector voltage Vc increases toward a not-shown main circuit power supply voltage and then becomes a constant voltage. When the NLU circuit 94 operates, the control voltage Vgcc becomes lower than the control power supply voltage VCC and the collector current Ic (short-circuit current) is restrained. When the NLU operation continues for a predetermined period (for example, about 2 .mu.s), a not-shown blocking circuit built in the drive circuit 82 operates so that the collector current Ic is blocked. That is, when the not-shown blocking circuit operates, the operation of the NLU circuit 94 is cancelled. Simultaneously with this, the MOSFET 80 in the IGBT drive circuit turns OFF and the MOSFET 81 turns ON, so that the control voltage Vgcc decreases suddenly and the collector current Ic is blocked.

"In PTL 1, there has been disclosed a technique in which overcurrent detection is suspended in sync with an ON (OFF) signal command of a power semiconductor device for a predetermined period of time in order to prevent an overcurrent from being detected by mistake due to the sudden increase of a sense current when the power semiconductor device turns ON (OFF).

"In addition, in PTL 2, there has been disclosed a technique in which comparison between an operation threshold voltage used for a transient state and a current detection value is made in the transient state immediate after turning-on of an IGBT, with a leading edge of an input signal used as a trigger, in order to prevent an overcurrent state from being detected by mistake due to the rising of a current detection waveform in a transient period immediately after the turning-on."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventor's summary information for this patent application: "Technical Problem

"In FIG. 7, the ratio of the sense current Is to the collector current Ic is constant at the time of normal turn-on. However, when the collector current Ic increases to be close to a rated current, the ratio of the sense current Is to the collector current Ic becomes larger so that the sense voltage Vs becomes larger. It is presumed that this ratio becomes larger because the current flowing through the gate capacitance is also included in the current flowing into the sense resistor Rs. Therefore, when the IGBT 56 is operated to be turned ON with a current close to the rated current, the sense voltage Vs may increase to exceed the operation threshold voltage Vo operating the NLU circuit 94, as described above.

"When the sense voltage Vs exceeds the operation threshold voltage Vo, the NLU circuit 94 operates instantaneously so that the voltage supplied from the control power supply 86 decreases. As a result, the gate current supplied to the gate 56g (gate capacitance) of the IGBT 56 is supplied insufficiently. As a result, the turn-on time ton1 of the collector voltage Vc of the IGBT 56 becomes longer and the falling of the turn-on voltage delays so that a turn-on loss increases.

"In addition, PTL 1 and PTL 2 give no description about a technique in which a mask circuit is added to the NLU circuit 94 to prevent the NLU circuit from being operated at the time of turn-on so as to reduce a turn-on loss of an IGBT.

"In order to solve the foregoing problems, an object of the invention is to provide a short-circuit protection circuit which has a mask circuit which can prevent an NLU circuit from being operated during turn-on operation of an IGBT so as to reduce a turn-on loss of the IGBT.

"Solution to Problem

"In order to achieve the object, in a first aspect of the invention, there is provided a short-circuit protection circuit for preventing a voltage-controlled type semiconductor device from short-circuit breakdown. This short-circuit protection circuit has: an NLU circuit which changes a control voltage applied to a gate of the voltage-controlled type semiconductor device in order to prevent the voltage-controlled type semiconductor device from latch-up caused by a current flowing into the voltage-controlled type semiconductor device; and a mask circuit which sets the NLU circuit at a non-operating state when a current flowing into the voltage-controlled type semiconductor device is on a level to operate the NLU circuit during turn-on operation of the voltage-controlled type semiconductor device and a gate voltage of the voltage-controlled type semiconductor device is lower than the control voltage outputted to the gate of the voltage-controlled type semiconductor device from a control circuit of the voltage-controlled type semiconductor device when the NLU circuit operates.

"In addition, in a second aspect of the invention, the mask circuit performs mask operation in the state in which a voltage applied to the gate of the voltage-driven type semiconductor device is lower than the control voltage and lower than a first operation threshold voltage which is a voltage applied to the gate of the voltage-driven type semiconductor device when the NLU circuit operates, so that the mask circuit sets the NLU circuit at a non-operating state.

"In addition, in a third aspect of the invention, the mask circuit is provided with: a first comparison portion whose output turns to an H level when the voltage applied to the gate of the voltage-driven type semiconductor device is not lower than a first reference voltage set as the first operation threshold voltage; a second comparison portion whose output turns to an H level when a voltage on a high potential side of a current detection resistor series-connected to a current detection terminal of the voltage-driven type semiconductor device is not lower than a second reference voltage set as a second operation threshold voltage which is on a level in which a voltage generated in the current detection sense resistor can be regarded as short-circuiting of the voltage-driven type semiconductor device; and an AND circuit which takes a logical product of the output of the first comparison portion and the output of the second comparison portion.

"Advantageous Effects of Invention

"In the invention, a mask circuit is provided so that operation of an NLU circuit is suspended during turn-on operation of a voltage-controlled type semiconductor device. Accordingly, the voltage-controlled type semiconductor device can be turned ON with a sufficient gate voltage so that a turn-on loss can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

"[FIG. 1] A circuit diagram showing a short-circuit protection circuit according to an example of the invention.

"[FIG. 2] Operation waveform diagrams of respective portions in the circuit in FIG. 1 during turn-on operation, during steady operation, and during short-circuit operation, in which (a) is an operation waveform diagram of Vc, Ic, and Vs, (b) is an operation waveform diagram of Vgcc and Vg, and is a diagram showing an output of a mask circuit.

"[FIG. 3] A diagram of a three-phase inverter circuit.

"[FIG. 4] A waveform diagram of an IGBT 54 during turn-on operation.

"[FIG. 5] A diagram for explaining an operation state of the IGBT at the time of short-circuiting.

"[FIG. 6] A diagram of a short-circuit protection circuit for an IGBT according to the background art.

"[FIG. 7] Operation waveform diagrams of respective portions in the circuit in FIG. 6 during turn-on operation, during steady operation, and during short-circuit operation, in which (a) is an operation waveform diagram of Vc, Ic and Vs, (b) is an operation waveform diagram of Vgcc and Vg and is a diagram showing an output of a mask circuit."

For additional information on this patent application, see: Shimizu, Naoki. Short-Circuit Protection Circuit. Filed July 10, 2012 and posted July 17, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=3735&p=75&f=G&l=50&d=PG01&S1=20140710.PD.&OS=PD/20140710&RS=PD/20140710

Keywords for this news article include: Electronics, Fuji Electric Co., Fuji Electric Co. Ltd., Semiconductor.

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Source: Electronics Newsweekly


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