JP3226084B2 - Power transistor overcurrent limiting circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for limiting a current flowing through a power transistor to protect the power transistor from an overcurrent generated due to a load short circuit or the like, and more precisely, a power transistor connected to each arm of a bridge circuit. And an overcurrent limiting circuit provided with each power transistor to protect the power transistor.

[0002]

2. Description of the Related Art As is well known, in order to drive various loads such as a motor, a power transistor is connected to a two-phase or three-phase bridge circuit to form, for example, an inverter device. In general, a drive circuit is provided on the power transistor side, and an overcurrent protection circuit is provided in association with each power transistor so as not to be damaged or deteriorated by a large overcurrent flowing when a load is short-circuited. As is well known, a configuration example of a bridge circuit will be described first with reference to FIG. 5 and a conventional example in which an overcurrent protection circuit is incorporated will be described with reference to FIG.

FIG. 5 shows a three-phase circuit in which three series circuits of an upper arm 11 and a lower arm 12 each including a power transistor 1 between a power supply potential point V and a reference potential point E are connected in parallel. 3 shows a bridge circuit 10 as an inverter device, and three output terminals u, derived from respective interconnection points of the upper and lower arms.
A load 3, which is, for example, a three-phase motor, is connected to v and w. The power transistor 1 is an insulated gate bipolar transistor (IGBT) in the example shown in the figure, and ordinary freewheeling diodes 2 are respectively connected in anti-parallel, and a drive circuit 30 is provided on each input side as a gate. The drive circuit 30 drives the corresponding power transistor 1 on and off according to a control command given from a command circuit 50 of the inverter device shown in FIG.

FIG. 6 shows only two phases of the bridge circuit 10 shown in FIG. The IGBT as the power transistor 1 of each of the arms 11 and 12 includes an auxiliary emitter 1a for current detection, and a current detection resistor 1b is connected to the auxiliary emitter 1a. Protection transistor
The base 21 receives the voltage drop of the resistor 1b due to the current of the auxiliary emitter 1a and turns on when the voltage drop exceeds the base-emitter voltage when an overcurrent occurs.
Short-circuits the gate with the emitter to shut off the overcurrent by its off operation.

[0005] A drive circuit 30 provided for each power transistor 1 generally has a gate of the power transistor 1 in accordance with a control command Ls received from a command circuit 50 of the inverter device shown in a lower portion of the figure. The operation control power supply voltage Vs for the operation is individually received by the arm 11 on the power supply potential point V side and commonly received by the arm 12 on the reference potential point E side. The bridge circuit 10 operates under a high voltage, and the reference potential at which the command circuit 50 operates is a power supply potential point V
In general, the control command Ls is transmitted to each drive circuit 30 by using a photocoupler in the form of an optical signal indicated by a thin line in the figure.

When the protection transistor 21 is turned on and the corresponding power transistor 1 is turned off, the overcurrent is cut off and the voltage drop of the resistor 1b is reduced, so that the protection transistor 21 is turned off and the power transistor 1 is turned off. Is turned on. If the on / off operation is repeated, the power transistor 1 may be damaged. Therefore, once the protection transistor 21 is turned on, the drive circuit 30 detects it and outputs the overcurrent signal Lo. Is sent to the command circuit 50, and the control command Ls is lost accordingly. This overcurrent signal Lo is also transmitted from each drive circuit 30 to the command circuit 50 in the form of an optical signal using a photocoupler.

[0007]

In the above-described conventional bridge circuit 10 shown in FIG. 5, the protection transistor 21 prevents the overcurrent of a predetermined value or more from flowing through each power transistor 1, and responds to the ON operation thereof. By transmitting the overcurrent signal Lo from the corresponding drive circuit 30 to the command circuit 50, harmful repetition of the ON / OFF operation of the power transistor 1 can be avoided. However, the control command Ls between each drive circuit 30 and the command circuit 50 can be avoided. In addition, since two photocouplers are provided for transmitting the overcurrent signal Lo, there is a problem that the entire circuit configuration of the inverter device and the like becomes complicated and expensive.

[0008] Of course, the photocoupler itself is not so expensive, but the light emitting side and the light receiving side require ancillary circuits respectively, and the current consumption of the light emitting side circuit becomes considerably large. Is disadvantageous when integrating the In addition, the photocoupler tends to be a weak point in the entire circuit in terms of operation reliability, and there is a problem that the lifetime cannot be compensated for as long as a normal electronic circuit.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to restrict an overcurrent flowing through each power transistor of a bridge circuit to a predetermined value or less while controlling a command between a drive circuit and a command circuit. Another object of the present invention is to reduce the necessity of insulating a signal transmission path through a photocoupler or the like as much as possible.

[0010]

SUMMARY OF THE INVENTION According to the overcurrent limiting circuit of the present invention, it is an object of the present invention to protect a bridge-connected power transistor from overcurrent by using one of the bridge circuits.
Side arm, the reference potential point side arm and the other side arm.
That over-current limit value of the one power belonging to the side arm transistor of the power supply potential point side arm set lower than for the power transistors belonging to the other side arm, a command circuit for providing a control command to each power transistor in the bridge circuit At a potential substantially common to the power transistors belonging to the one-sided arm.

The overcurrent limit value of the power transistor belonging to the one arm is set to 50 to 80%, more preferably 60 to 70% of the overcurrent limit value for the power transistor belonging to the other arm. Good. The simplest way to make the overcurrent limit values different for the power transistors on one side and the other side is to make a difference between their overcurrent detection values.
In the case of a gate control type transistor such as an IGBT, it is advantageous to make the overcurrent limit different depending on the resistance of the gate resistor connected to the transistor. The overcurrent limiting circuit according to the present invention detects an overcurrent since the current flowing through the power transistor and the voltage drop between the main terminals thereof exceed a predetermined limit value, and performs the limiting operation and the cutoff operation. Good.

The control command by the command circuit according to the above configuration is directly applied to the power transistor of one arm operating at substantially the same potential, and a photocoupler or the like is used for the power transistor of the other arm. In this case, it is advantageous to use an arm on one side as a reference potential point side arm. Further, an overcurrent detection circuit is provided in association with a current limiting circuit or a driving circuit for the power transistor belonging to the one side arm, and when the current limiting circuit operates, an overcurrent signal is supplied to the command circuit, and the command circuit is provided. Based on this, it is preferable to cause the corresponding power transistor to designate an off operation by a control command or to eliminate the on command.

[0013]

In the present invention, the power transistors of each arm are controlled so that the current flowing in one arm of the bridge always flows through a load to one of the other arms not connected in series with the other arm. Paying attention to the fact that substantially the same current flows through the arm even if there is a slight difference in timing or phase, one side arm
The overcurrent limit value of the power transistor of the reference potential point side arm is set lower than that of the power supply potential point side arm which is the other arm , and the role of overcurrent limitation is mainly An overcurrent is reliably limited to a desired value by operating a command circuit that gives a control command to each power transistor in the bridge and that applies a control command to each power transistor in the bridge on the same potential as the power transistor in the one arm. However, the necessity of transmitting a command or a signal between the command circuit and each power transistor through an insulating path is reduced to less than half of the conventional case.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a main part circuit diagram showing an example of a bridge circuit incorporating the overcurrent limiting circuit of the present invention. FIG. 2 shows a case where the current limiting values of one arm and the other arm are made different by the gate resistance of a power transistor. FIG. 3 is a characteristic diagram showing the dependence of the overcurrent limit value on the gate resistance. FIG. 3 is a circuit diagram showing an example of an overcurrent limit circuit and a drive circuit for the power supply potential point side arm.
FIG. 4 is a circuit diagram showing an example of the overcurrent limiting circuit and the driving circuit for the reference potential point side arm. Note that the embodiment is directed to the three-phase bridge circuit of FIG. 5 described above, and the power transistor for each arm is an IGBT. However, the present invention is not limited to such a specific mode, but is applicable to a wider range. It can be implemented in various modes.

FIG. 1 shows two phases in the three-phase bridge circuit 10 in the same manner as in FIG. 6 described above, and the corresponding parts are denoted by the same reference numerals, so that the description of the overlapping parts will be omitted as appropriate. It shall be. The provision of an auxiliary emitter 1a for each power transistor 1, which is an IGBT, and the connection of a current detection resistor 1b to the current detection resistor 1b are the same as in FIG. I do. In this embodiment, the current limiting circuit 20 receives the voltage drop of the resistor 1b between the base and the emitter in the same manner as in the prior art, and the collector is connected to the power transistor 1
, And a gate resistor 22 inserted and connected between the protection transistor 21 and the driving circuit 30.

Each power transistor 1 is connected to an output terminal u of a bridge circuit 10 in an arm 11 on the power supply potential point V side on the upper side of the figure.
Operate independently on the potentials V and v, but the lower reference potential point E
Since all of the side arms 12 operate on a common potential,
In this embodiment, the command circuit 50 is operated on the same potential as the power transistor 1 of the arm 12. In response to this, the control command Ls is transmitted in the form of an optical signal from the command circuit 50 to the power transistor 1 of the arm 11 on the power supply potential point V side in the same manner as in the related art. Transistor 1
Transmits the control signal Ss in the form of a normal electric signal. Further, a circuit for detecting that the current limiting circuit 20 has been operated based on a change in the gate voltage of the power transistor 1 is incorporated in each drive circuit 30 of the arm 12 so that the overcurrent signal So can be converted to a command circuit 50 in the form of a normal electric signal. To give to.

FIG. 2 shows the operating characteristic C of the current limiting circuit 20 as a function of the resistance Rg of the gate resistor 22 on the horizontal axis and the overcurrent limiting value on the vertical axis.
This is indicated by the dependency of Is. As shown, as the resistance value Rg increases, the overcurrent limit value Is decreases. Therefore, in this embodiment, the resistance value Rg for the power transistor 1 is set to 10Ω in the arm 11 on the side of the power supply potential point V, and to the reference potential point E. 20 on side arm 12
By setting to Ω, the overcurrent limit value Is by the current limiting circuit 20 on the arm 12 side is set to 100A which is considerably lower than 150A on the arm 11 side.

In the bridge circuit 10 of FIG. 1 in which the overcurrent limiting value Is is set by the current limiting circuit 20, the arm
Of course, the overcurrent flowing through the power transistor 1 on the 12 side is limited to the lower overcurrent limit value Is of 100 A, and the overcurrent is also limited to the corresponding value on the arm 11 side. While safely protecting all of the power transistors 1 from overcurrent, the number of control commands Ls to be transmitted by an optical signal from the command circuit 50 to each drive circuit 30 is reduced, and the overcurrent signal So is transmitted in the reverse direction. Need not be communicated via

The overcurrent limit of the bridge circuit 10 shown in FIG. 1 is mainly set at the reference potential point E side arm 12 having a low limit value as described above. There is a possibility that the power supply potential point V side arm 11 through arm 12 equal to or higher than current from accidental <br/> disturbance of the arm between the operation sequence of the power transistor 1, the arm 11 side of the current limiting circuit 20 such prediction In the event of an impossible accident or disturbance of the operation of the bridge circuit, the power transistor 1 serves to safely protect the power transistor 1.

FIG. 3 shows a configuration example of the current limiting circuit 20 and the driving circuit 30 used in the arm 11 on the power supply potential point V side in FIG. In the example of the current limiting circuit 20, an overcurrent of the power transistor 1, which is an IGBT, is detected from its collector-emitter voltage. 1 is divided into an on-operation voltage Vs1 and an off-operation voltage Vs2 for the power transistor 1, and an intermediate point between both voltages is connected to the emitter of the power transistor 1. The protection transistor 21 in the current limiting circuit 20 has a collector and an emitter connected between the gate of the power transistor 1 and the negative terminal of the off-operation voltage Vs2, and has its base connected via a diode 23 and a Zener diode 24. And a resistor 25 is connected between the base and the emitter of the transistor 1. Further, a capacitor 26 is connected between the interconnection point of the diodes 23 and 24 and the emitter of the protection transistor 21.

The output of the drive circuit 30 is complementary transistors 31 and 32 for receiving both voltages Vs1 and Vs2, and their interconnection point is connected to the gate of the power transistor 1 via a gate resistor 22. As described above, the control command Ls which is an optical signal
Is transmitted to the phototransistor 33 on the protection circuit 30 side via a photocoupler, and its collector potential is given to the base of the associated transistor 34, and the collector potential is further connected to the common connection base of the transistors 31 and 32 in the output section. , And the interconnection point of the above-mentioned diodes 23 and 24 of the voltage limiting circuit 20.

When the power transistor 1 is turned off by the control command Ls, the phototransistor 33 is turned off, the next transistor 34 is turned on, and the transistor of the drive output section is turned on.
Since the transistor 31 is turned off and the transistor 32 is turned on, the off operation voltage Vs2 is applied in the reverse bias direction between the gate and the emitter of the power transistor 1 to keep it in the off state. In this state, the capacitor 26 in the voltage limiting circuit 20 is discharged, and the base of the protection transistor 21 is set at the same potential as the emitter by the resistor 25. Conversely, control command
When Ls is turned on, the phototransistor 33 is turned on, the next transistor 34 is turned off, and the transistor 31 in the drive output section is turned on and the transistor 32 is turned off. This time, the ON operation voltage Vs1 is applied between the emitters in the positive bias direction to perform the ON operation, whereby a current flows to the load of the bridge circuit 10.

In this ON state, the voltage applied to the capacitor 26 is the sum of the OFF operation voltage Vs2, the low collector-emitter voltage of the power transistor 1 and the forward voltage of the diode 23, and is higher than the Zener breakdown voltage of the Zener diode 24. Since it is low, the protection transistor 21 does not operate. However, if an overcurrent flows through the power transistor 1 due to a short circuit on the load side of the bridge circuit 10 or the like, the voltage between the collector and the emitter of the power transistor 1 becomes extremely high.
Exceeds the Zener breakdown voltage of the Zener diode 24, the protection diode 21 is turned on, and the overcurrent flowing through the power transistor 1 is limited to a predetermined value. In this case, the gate drive voltage applied from the drive circuit 30 to the power transistor 1 is divided by the gate resistor 22 and the on-resistance of the protection transistor 21 to be reduced, so that the current limit value is
Can be set more easily.

FIG. 4 shows a configuration example of a current limiting circuit 20 and a drive circuit 30 suitable for the arm 12 on the reference potential point E side in FIG. 4, the overcurrent of the power transistor 1 is supplied to the current limiting circuit 20 in the same manner as in FIG.
1a, and the driving circuit 30 detects the command circuit 50 of FIG.
Both the subjected to control command Ss in the form of electrical signals from,
The overcurrent signal circuit 40 is incorporated in association with them, and the overcurrent signal So is output to the command circuit 50 in the form of an electric signal.

The current limiting circuit 20 shown in FIG.
3 in that the voltage drop of the current detection resistor 1b is
The difference is that overcurrent detection accuracy is improved over the case of FIG. 3 by operating the protection transistor 21 with the output of the result compared with Vt. Further, the driving circuit 30 has a simple configuration corresponding to the output section of FIG. 3 comprising a pair of complementary transistors 31 and 32, and the control command Ss is transmitted from the command circuit 50 to the common base of the transistors 31 and 32. Is to receive it directly.

Further, the overcurrent signal circuit 40 includes a comparator
The comparator 41 detects that the gate voltage of the power transistor 1 has fallen below a predetermined reference value Vr by the comparator 41 to detect that an overcurrent has occurred and that the current limiting circuit 20 has operated. The high output is output by the AND gate 42 when the control command Ss is in the high state, that is, only when the power transistor is in the on state.
It is output to the command circuit 50 as So.

In the current limiting circuit 20 shown in FIGS. 3 and 4, the overcurrent state of the power transistor 1 is detected from the collector-emitter voltage and the current of the auxiliary emitter 1a. It is desirable to unify the overcurrent detection methods on the 11 and 12 sides. As described with reference to FIG. 2, in the present invention, the overcurrent limit value is different between the arm 11 on the power supply potential point V side and the arm 12 on the reference potential point E side in FIG. The value is the higher overcurrent limit value.
It is better to set within the range of 50-80%, and further 60-70%
It is desirable to set in the range.

Further, in the embodiment, the overcurrent of the power transistor 1 is limited by the limiting operation of the current limiting circuit 20 and the limit value is set by the resistance value of the gate resistor 22. Of course, it can be appropriately set by means other than the gate resistance. In some cases, the level of the overcurrent detection by the current limiting circuit 20, for example, the overcurrent of the power transistor 1 can also be determined by the threshold Vt set in the comparator 27 in FIG. The limit value can be set for each of the arms 11 and 12.

[0029]

As described above, according to the present invention, in the bridge circuit, the power transistor is controlled so that the current flowing in one arm of the bridge circuit flows through the load to one of the other arms. Paying attention to the point where substantially the same current flows through both arms, the overcurrent limit value for the power transistor in the reference potential point side arm which is one arm of the bridge is set to the other arm .
The following effects are obtained by setting a lower level for the power supply potential point side arm and operating a command circuit that gives a control command to each power transistor of the bridge circuit at a potential substantially common to the power transistor of the one side arm. Can be

(A) By operating the command circuit on the same potential as the power transistor in the one-side arm in which the overcurrent limit value is set low, the role of limiting the overcurrent is mainly given to the one-side arm. However, the number of insulation paths for transmitting commands and signals between the command circuit and the power transistor can be reduced to less than half of the conventional case. (b) The other arm has a higher limit value than the one arm but has an overcurrent limiting function, so an unexpected overcurrent may flow due to an unexpected accident on the load side or disturbance of the operation sequence between the arms of the power transistor. In this case, all the power transistors in the bridge circuit can be safely protected.

The embodiment of the present invention in which one arm having a low overcurrent limit value is used as the reference potential point side arm is advantageous in that the overall configuration of the bridge circuit can be simplified. An embodiment in which the current limit value is set to 50 to 80% of the overcurrent limit value of the other arm optimizes the sharing of the overcurrent limit value of the both arms to sufficiently take advantage of the above (a) and (b). It is advantageous in showing it. Power transistor
In the case of a gate-controlled transistor such as an IGBT, the embodiment in which the overcurrent limit value is set by the gate resistance has advantages in that the setting is easy and the overcurrent limit operation is ensured.

Further, the control command of the command circuit is directly given to the power transistor of one arm and the power transistor of the other arm is given via a photocoupler. Number
There is an advantage that can be reduced. Further, an embodiment in which an overcurrent signal circuit is provided in association with the current limiting circuit for the power transistor in the one-sided arm to give the overcurrent signal to the command circuit, the corresponding power transistor is supplied from the command circuit to the corresponding power transistor. By commanding the OFF operation, all power transistors in the bridge circuit can be protected very reliably.

[Brief description of the drawings]

FIG. 1 is a main part circuit diagram showing an example of an overcurrent limiting circuit according to the present invention;

FIG. 2 is a characteristic diagram showing the dependence of the overcurrent limit value on the gate resistance when the current limit value of the power supply potential point side arm and the current limit value of the ground potential point side arm differ depending on the gate resistance of the power transistor. It is.

FIG. 3 is a circuit diagram showing a specific configuration example of an overcurrent limiting circuit and a drive circuit of a power supply potential point side arm.

FIG. 4 is a circuit diagram showing a specific configuration example of an overcurrent limiting circuit and a driving circuit of a reference potential point side arm.

FIG. 5 is a configuration circuit diagram of a three-phase bridge circuit as an example to which the overcurrent limiting circuit according to the present invention is applied;

FIG. 6 is a main part circuit diagram of a bridge circuit in which a conventional overcurrent limiting circuit is incorporated.

[Explanation of symbols]

 Reference Signs List 1 power transistor or IGBT 1a power transistor auxiliary emitter 1b current detection resistor 10 bridge circuit 11 bridge circuit power supply point side arm 12 bridge circuit reference potential point side arm 20 current limiting circuit 21 protection transistor 22 gate resistor 30 drive Circuit 40 Overcurrent signal circuit 50 Command circuit E Reference potential point Is Overcurrent limit value Ls Control command by optical signal Rg Resistance value of gate resistance So Overcurrent signal by electric signal Ss Control command by electric signal u, v Bridge circuit output Terminal V Power supply potential point Vs Operating power supply voltage for drive circuit

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03K 17/00-17/70

Claims (5)

(57) [Claims] 1. A current limiting circuit provided for each power transistor to protect a power transistor connected in each arm of the bridge circuit from overcurrent, the current limiting circuit being one arm of the bridge. one side earth of the power supply potential point side arm is a reference potential point side arm and the other arm
The overcurrent limit value for the power transistor belonging to the other arm is set lower than that for the power transistor belonging to the other arm , and a command circuit for giving a control command to each power transistor in the bridge circuit is a power transistor belonging to one arm. An overcurrent limiting circuit for a power transistor, wherein the overcurrent limiting circuit is operated on an operation reference potential substantially common to the above. 2. The circuit according to claim 1, wherein the overcurrent limit value for the power transistor belonging to one arm is set to 50 to 80% of the overcurrent limit value for the power transistor belonging to the other arm. An overcurrent limiting circuit for a power transistor. 3. The circuit according to claim 1, wherein the power transistor is a gate-controlled transistor, and an overcurrent limit value for the transistor is set by a value of a gate resistance. Current limit circuit. 4. The circuit according to claim 1, wherein the control command of the command circuit is directly given to the power transistor belonging to one arm and is given to the power transistor belonging to the other arm via a photocoupler. An overcurrent limiting circuit for a power transistor. 5. The circuit according to claim 1, further comprising an overcurrent detection circuit provided in addition to a current limiting circuit for a power transistor belonging to one arm, and then providing an overcurrent signal to a command circuit. An overcurrent limiting circuit for a power transistor, characterized in that the off-operation is designated by a control command to a corresponding power transistor based thereon.