JP3733986B2 - Output current direction discrimination method and inverter using the method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an inverter output current direction determination method and an inverter using the method, and more particularly to an output current direction determination method suitable for dead time compensation of a switching element used in the inverter, and a specific example for realizing the method. The present invention relates to an inverter incorporating a general circuit.
[0002]
[Prior art]
In recent years, PWM (Pulse Width Modulation) type inverters, which are popular inverters, are available when either one of the switching elements inserted in the upper arm of the inverter bridge or the switching element inserted in the lower arm is on. The switching element operates so as to be turned off. When both elements are switched on and off, both elements may be turned on due to the delay time of the switching element, which may cause a power supply short circuit. A period during which both elements are turned off (hereinafter referred to as “dead time”) is provided, and then the elements of the next turn-on order are finally turned on.
[0003]
However, when such a dead time is provided, an error occurs between the inverter voltage to be originally output and the actual inverter output voltage. As a result, the current of the motor is distorted, causing problems such as generation of vibrations and reduction in efficiency.
[0004]
Thus, dead time compensation for correcting the PWM signal in consideration of an error voltage due to dead time has been proposed so far. One example is described in JP-A-1-186172. This dead time compensation is a method of detecting the polarity of the motor current output from the inverter to the motor, and increasing or decreasing the duty of the PWM signal based on the detected value. It is where you are.
[0005]
However, in the dead time compensation, it is necessary to separately provide a means for discriminating the polarity of the current in order to discriminate the polarity of the motor current. A current transformer or the like is generally used as the current polarity discriminating means. On the other hand, recently, a demand for downsizing the inverter has advanced, and a circuit in which the inverter switching element, drive circuit, and protection circuit are mounted. Is realized by a hybrid integrated circuit, and miniaturization of peripheral components of the inverter is attempted, and this current polarity discrimination means is no exception and is required to be miniaturized. However, if the current transformer is reduced in size, its magnetic material is magnetically saturated, so there is a limit to downsizing, and the current detection sensitivity is easily affected by temperature changes, and the cost is increased. .
On the other hand, even if a current polarity discriminating means other than the current transformer is used, detecting the motor current causes an extra electric circuit to be inserted in the path of the motor current, resulting in a problem that the loss increases.
[0006]
As a dead time compensation that covers the above drawbacks, there is an invention described in Japanese Patent Laid-Open No. 7-7967. FIG. 5 is an inverter block diagram of the invention described in Japanese Patent Laid-Open No. 7-7967. In the figure, switching elements Q1 to Q6 constitute an inverter bridge, a DC power supply 16 is connected to the input side of the inverter bridge, and an induction motor 15 is connected to the output side of the inverter bridge. The switching elements Q1 to Q6 are controlled by the respective phase control circuits 11, 12, and 13. For example, regarding the U-phase control circuit 11, the U-phase control circuit 11 is driven by the output of the PWM control circuit 21, the drive circuits 1, 4, the current detection circuits 3, 6, and the overcurrent protection circuits 2, 5. And a current direction detection circuit 7. The other V-phase control circuit 12 and W-phase control circuit 13 have the same configuration. Freewheel diodes D1 to D6 are connected in antiparallel to the switching elements Q1 to Q6, respectively.
[0007]
Each of the switching elements Q1 to Q6 is a type of element in which a current sense terminal for overcurrent protection is arranged in addition to the emitter terminal, and resistors R1 to R6 are connected to the respective current sense terminals. Therefore, when a part of the emitter current flows on this side, this current is detected by the current detection circuits 3 and 6. One comparator is provided in the current detection circuit 3 for the upper arm, the current sense terminal is connected to one input terminal of the comparator, and a reference voltage corresponding to an overcurrent is applied to the other input terminal. A reference voltage source is connected. Therefore, the voltage of the current sense terminal is compared with the reference voltage in this comparator, and as a result of comparison, if the current sense terminal voltage does not exceed the reference voltage, it is not an overcurrent. This is output to the overcurrent protection circuit 2, and the overcurrent protection circuit 2 instructs the drive circuit 1 to turn off the switching element Q1. On the other hand, two comparators are provided in the current detection circuit 6 for the lower arm, and one of them performs the same overcurrent detection operation as the above comparator and outputs it to the overcurrent protection circuit 5. The other comparator has an element current detection function. That is, the current sense terminal is also connected to one input terminal of the second comparator, and a reference voltage is applied to the other input terminal of the comparator so as to detect whether or not current is flowing through the switching element. Yes. In this comparator, the voltage of the current sense terminal is compared with the reference voltage. As a result of comparison, if the current sense terminal voltage does not exceed the reference voltage, “no current” is detected. Output to the circuit 7.
[0008]
However, the invention described in the above-mentioned Japanese Patent Application Laid-Open No. 7-7967 uses a special element having a “current sense terminal for overcurrent protection” as the switching elements Q1 to Q6, so that a general-purpose switching element cannot be used. There was a drawback. Further, since the voltage of the switching elements Q1 to Q6 is viewed during the ON period of the switching element, the voltage is accurately detected and the presence or absence of current is detected by a comparator that operates at high speed, or the digital latch circuit The circuit becomes complicated and it is necessary to maintain the result, which is disadvantageous for cost reduction.
[0009]
[Problems to be solved by the invention]
The present invention solves these drawbacks, and can detect the output current direction with high reliability even with a detection circuit having a lower accuracy than the conventional one, and can be downsized, and uses a low loss method and the method thereof. It is to provide an inverter.
Furthermore, another problem of the present invention is that when current detection can be performed only within a limited period in the voltage phase, for example, the three-phase voltage is applied to the induction motor while modulating only two phases of the three-phase modulation of the inverter. In the so-called “two-phase modulation method” for supplying the current, current detection can be performed only for a limited period. In such a case, an inverter with good dead time compensation is provided.
[0010]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, an invention according to claim 1 relates to an inverter, wherein an inverter bridge in which a circuit composed of a switching element and a freewheel diode connected in reverse parallel to each other is inserted in each upper and lower arm of the inverter bridge. A drive circuit that drives each switching element of the inverter bridge, a PWM control circuit that supplies the drive circuit with a signal for alternately turning on and off the switching element so as to convert direct current to alternating current, and load current detection In an inverter having a circuit, and an on-delay correction circuit that causes the PWM control circuit to perform on-delay correction based on an output of the load current detection circuit,
The load current detection circuit includes a shunt circuit that shunts a part of the current flowing through the freewheel diode of the upper arm or the lower arm of the freewheel diode,
The shunt circuit is constituted by a series connection circuit of a base-emitter portion of a transistor and a high voltage diode, the series connection circuit is connected in parallel to the free wheel diode in the same polarity, and the remaining terminals of the transistor And a parallel circuit of a smoothing capacitor and a resistor between the power supply and the Vcc power supply .
[0011]
According to a second aspect of the present invention, in the inverter according to the first aspect , the on-delay correction circuit generates a first timing signal when the load current direction changes, and 180 degrees of the output voltage phase elapses from this point. Sometimes connected to an on-delay correction timing circuit for generating a second timing signal, the on-delay correction is performed by the first timing signal from the on-delay correction timing circuit, and the on-delay correction opposite to the above correction is performed by the second timing signal. It is characterized by doing.
[0014]
As described above, according to the present invention, only a part of the current flowing through the freewheeling diode of the inverter is shunted, and the direction of the output current of the inverter is determined not by the magnitude of the shunt current but only by the presence or absence of the shunt current. Therefore, there is no need to use a current polarity discriminating means which is limited in downsizing of a current transformer or the like as in the invention described in Japanese Patent Laid-Open No. 1-186172 and is easily affected by a temperature change. Since it is not necessary to use a special switching element having a “current sensing terminal for overcurrent protection” as in the invention described in Japanese Patent No. 7967, a general-purpose switching element can be used. The shunt circuit is composed of a series connection circuit of a diode component of the transistor and a high voltage diode, and the remaining end of the transistor is used for amplification. Since a parallel circuit of a smoothing capacitor and resistor is inserted between the Vcc power supply and the Vcc power supply, it can operate even with a very small current, can be downsized, has low loss, and is reliable even with a detection circuit with lower accuracy than before. It is possible to determine the direction of the output current with high characteristics.
[0015]
In addition, in order to make an accurate correction even during a period when the switching element on the opposite side is on for a long time and no current flows through the freewheeling diode when there should be a freewheeling diode current, such as two-phase modulation of the inverter, When the presence or absence of the freewheeling diode is detected or when it is no longer detected, the phase of the output voltage at that time is held, and during the 180-degree period of the output voltage phase from that phase, the output voltage is corrected to increase or decrease, and the remaining By performing the reverse correction for the period of 180 degrees, the on-delay correction can be performed easily and accurately.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS. FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is an equivalent circuit diagram when it is detected that a current flows through the free wheel diode FDu2 of the U-phase lower arm of FIG. FIG. 4 shows an example of the on-delay correction timing, and FIG. 4 shows an output current waveform in the presence or absence of on-delay correction in which the correction is switched between the 180 degree period and the remaining 180 degree period.
[0017]
In the block diagram of FIG. 1, the main circuit of the inverter of this embodiment is configured as follows. The output stage is a general three-phase inverter bridge, and the upper and lower arms of the U phase of this inverter bridge are composed of a series connection circuit of switching elements Su1 and Su2. At that time, the collector side of each transistor as a switching element is connected to a DC power source. The emitter side is connected to the plus side of E so as to be the minus side of the DC power source E. Similarly, the V-phase upper and lower arms are constituted by a series connection circuit of switching elements Sv1 and Sv2, and the W-phase upper and lower arms are constituted by a series connection circuit of switching elements Sw1 and Sw2. Then, the U phase of the induction motor M is connected to the connection point between the switching elements Su1 and Su2 of the series connection circuit, and similarly, the V phase of the induction motor M is connected to the connection point between the switching elements Sv1 and Sv2 with respect to the switching elements Sw1 and Sw2. The W phase of the induction motor M is connected to the connection point. Free wheel diodes FDu1 to FDw2 are connected in parallel to the switching elements Su1 to Sw2, respectively. As these switching elements Su1 to Sw2, a bipolar transistor (GBT), an insulated gate bipolar transistor (IGBT), a power MOS field effect transistor (POMSFET), and the like are used. If a circuit is used, a gate turn-off thyristor (GTO), a normal thyristor, or the like can also be used.
[0018]
These switching elements Su1 to Sw2 are controlled as follows.
Control values such as input / output current, input / output voltage, and output frequency of the inverter are detected by a detector, and the detected values are sent to the PWM control circuit PC. Since the PWM control circuit PC includes a PWM generator PG, a carrier generator CG, a voltage command device VI, and a voltage corrector VC, the detected value is sent to the voltage corrector VC of the PWM control circuit PC. The voltage corrector VC compares a predetermined reference value with the detected value, calculates an appropriate correction value, and outputs it to the voltage command device VI. This PWM generator PG compares the voltage command signal sent from the voltage command device VI with the carrier signal inputted from the carrier generator CG, creates a PWM signal based on the result, and outputs it to the drive circuit DR, The drive circuit DR outputs a duty on / off voltage signal according to the PWM signal to the gate terminal of the switching element. Accordingly, the switching element is turned on for a predetermined period, and as a result, the control target values of the inverter and the induction motor M are controlled to become the reference values.
[0019]
Next, how the direction of the output current of the inverter as shown in FIG. 1 is detected will be described with reference to FIG. FIG. 2 is an equivalent circuit diagram of the U-phase freewheeling diode FDu2 in FIG. 1. A block surrounded by a dotted line in FIG. 2 is a current detection circuit Dtu constituting the present invention, and the current detection circuit Dtu includes Su is a shunt circuit Su according to the present invention. This shunt circuit Su is constituted by a series connection circuit of a diode constituent part (base-emitter) of the transistor Tru and a high voltage diode HDu. This series connection circuit is connected in parallel with the same polarity to the freewheel diode FDu2, and a parallel circuit of a smoothing capacitor Cu, a resistor and Ru is inserted between the remaining terminal (collector terminal) of the transistor Tru and the Vcc power supply. Has been. The output terminal Ou is connected to the collector terminal of the transistor Tru, and the detection current a is sent from the output terminal Ou to the voltage corrector VC in the PWM control circuit PC. Further, the positive / negative of the U-phase current Iu is defined as the direction of the induction motor current indicated by the arrow in FIG. 2 (that is, the direction of the current flowing into the induction motor M from the output terminal of the inverter I), and the reverse direction is negative. The direction.
[0020]
When the current Iu flowing into the induction motor M is positive, the current flows to the freewheel diode FDu2 of the lower arm regardless of whether the PWM signal input to the switching element Su2 is on or off, and therefore no current flows to the switching element Su2.
On the other hand, when Iu is negative, the current flows to the freewheel diode FDu1 of the upper arm regardless of whether the PWM signal input to the switching element Su1 is on or off, so that no current flows to the switching element Su1.
Therefore, when a current flows through the free wheel diode FDu2, the current detection circuit Dtu according to the present invention operates as follows. If the current Iu is in the negative direction, even if the current flows from the induction motor M to the inverter I, the current detection circuit Dtu does not operate because the high-voltage diode HDu cuts the negative direction current. Therefore, the current detection signal a is output from the output terminal Ou as it is at a high potential of Vcc.
[0021]
Next, when the U-phase current Iu is in the positive direction as shown in the figure, the timing for the current to flow through the freewheel diode FDu2 occurs. At this time, a forward voltage is generated in the freewheel diode FDu2. On the other hand, a series connection circuit of the base and emitter of the transistor Tru and the high voltage diode HDu is connected in parallel to the freewheel diode FDu2, but the base and emitter of the transistor Tru are equivalently equivalent to a diode. As a result, a series connection circuit of two diodes is connected in parallel with the free wheel diode FDu2 with the same polarity. At this time, naturally, it is difficult for the current to flow in the series connection circuit of two diodes, but the forward voltage is zero if it does not flow at all, so the two diodes are equivalent to the forward voltage of the freewheel diode FDu2. The current flows to the level according to the diode characteristics. In FIG. 2, this corresponds to the base current Ib flowing from GND to the base of the transistor Tru to the emitter terminal. When the base current Ib flows, the collector current Ic shown in FIG. 2 is generated by the transistor action of the transistor Tru. When the collector current Ic flows through the resistor Ru, the voltage of the current detection signal output terminal Ou falls below the Vcc potential by the voltage drop of the resistor Ru, and a low potential current detection signal a is output from the current detection signal output terminal Ou. The Rukoto. Since the transistor Tru operates as a grounded-base amplifier, the current amplification factor is about 1, but it is possible to detect the presence or absence of a minute current by selecting a resistor Ru having a high resistance. Further, the effect of short-time switching can be prevented by the capacitor action of the capacitor Cu.
[0022]
The above description of the operation for the U phase applies to the other V and W phases as well, with the current detection signal b being output from the current detection signal output terminal and the current detection signal c being output from the current detection signal output terminal. Is done.
[0023]
The method of obtaining the output current direction signal A of the inverter using the current detection signal a obtained from the shunt circuit Su is, for example, the detection current signal a (whether a is “H” or “L”) and the gate of the switching element at that time This can be easily realized by taking the logic of the presence / absence of the PWM signal applied to the. The current direction signal A which is the discrimination result is sent into the voltage corrector VC of the PWM control circuit PC. The current direction signals B and C are similarly sent to the voltage corrector VC for the other V and W phases. In the voltage corrector VC, an on-delay correction signal is created based on the current direction signals A, B, and C of each phase, and the on-delay correction signal is output to the voltage command device VI to correct the voltage command signal. This voltage command signal is input to the PWM generator PG. Similarly, a carrier is also input from the carrier generator CG, and the PWM generator PG switches the output signal U1 that has been subjected to on-delay correction in consideration of + ΔV and −ΔV. The output signal U2 is sent to the drive circuit DRu1 of the element Su1, and the drive circuit DRu1 and DRu2 controls the switching elements Su1 and Su2 in an on / off period corrected for on-delay. Thus, the voltage corrector VC in the PWM control circuit PC also functions as the on-delay correction circuit OC.
[0024]
The above description of the operation for the U phase applies similarly to the other V and W phases, and the output signals V1 and V2 are respectively output from the PWM generator PG to the drive circuits DRv1 and DRv1 of the V-phase switching elements Sv1 and Sv2. And the output signals W1 and W2 are sent to the drive circuits DRw1 and DRw1 of the W-phase switching elements Sw1 and Sw2, respectively, and on-delay correction is performed.
Since the method of performing on-delay correction based on the current direction signals A, B, and C is well known, the description thereof is omitted here.
[0025]
The on-delay correction timing is more effective when performed as follows.
That is, the phase of the output voltage is captured at the time when the direction of the output current changes, and during this period of 180 degrees of the output voltage phase, correction is performed to increase or decrease the width of the on / off signal, and the remaining 180 degrees. During this period, correction opposite to the above correction is performed. FIG. 3 is a circuit example of an on-delay correction timing circuit OT for realizing this. V / f is created from the current rotational frequency f, and Vsin θ is created. The output of the Vsin θ block is output to the PWM block side, and θ at that time is added to the D input terminal of the latch block and one input terminal of the two AND circuits. An output from the current detection circuit Dtu is given to the set terminal of the latch block. A signal from the Q output terminal of the latch block is applied to the other input terminal of one AND circuit of the two AND circuits, and is added to the timing signal having a phase of 180 degrees to generate the signals of the two AND circuits. It is applied to the other input terminal of the other AND circuit. Of the two AND circuits, the former output is applied to the S input terminal of the RS flip-flop, the latter output is applied to the R input terminal of the RS flip-flop, and the output from the Q terminal of the RS flip-flop is an on-delay correction (+ ΔV). And -ΔV) as a switching signal. The on-delay correction circuit OC determines either the + ΔV or −ΔV correction amount for the first 180 ° period and the remaining + ΔV and −ΔV correction amounts in accordance with the switching timing of the on-delay correction timing circuit OT. Either one is output for the remaining 180 degrees, and the output from the Vsin θ block is added to the PWM block. In the PWM block, a correction that takes correction amounts + ΔV and −ΔV into consideration is performed and applied to the drive circuits DRu1 and DRu2 of the switching element to perform on-delay correction.
[0026]
As described above, when the current direction signal A is obtained using the current detection signal a obtained simply and accurately by the current detection circuit Dtu of the present invention, the current direction signal A changes (for example, rises). Then, θ is latched by the latch circuit at that time, and the phase of the output voltage at the time of switching in the current direction is known. Therefore, for the period of 180 degrees from this phase, for example, + ΔV is added to the output voltage, and the rest The period of 180 degrees is set to −ΔV.
[0027]
The result is shown in FIG. As can be seen from the figure, an error occurs only during the dead time period between the voltage command signal that the inverter should output and the PWM signal that is actually input to the inverter bridge, and when the induction motor current Iu is positive, the inverter The output voltage of the inverter becomes smaller than the target value. Conversely, when the induction motor current Iu is negative, the output voltage of the inverter becomes larger than the target value. Therefore, when the on-delay correction is not performed, the output voltage remains unchanged. As shown by a solid line, when the induction motor current Iu is negative, the inverter output voltage is larger than the target value, and when the induction motor current Iu is positive, the inverter output voltage waveform is smaller than the target value. I understand. However, if -ΔV is corrected in the 180-degree period and + ΔV is corrected in the remaining 180-degree period, a sine wave indicated by a dotted line can be obtained.
Thus, according to the present invention of claim 4, the on-delay correction is performed by detecting only the change point in the direction of the inverter output current and maintaining the output voltage phase at that time. Therefore, the on-delay correction can be easily and accurately performed by switching the correction between the period from the held phase up to 180 degrees and the remaining period.
[0028]
【The invention's effect】
As described above, in the present invention, the direction of the output current of the inverter is determined based on the presence / absence of a shunt current obtained by shunting a part of the current flowing in the free wheel diode of the upper arm or the lower arm among the free wheel diodes. Therefore, it is not necessary to use a current transformer or the like, and it is not necessary to use an element having a “current sense terminal for overcurrent protection”. Therefore, the detection circuit can be downsized, has low loss, and has lower accuracy than the conventional one. However, it is possible to determine the output current direction with high reliability at low cost.
[0029]
Specifically, the shunt circuit is configured by a series connection circuit of a diode component of the transistor and a high voltage diode, and this series connection circuit is connected in parallel to the free wheel diode in the same polarity, and the transistor Since a parallel circuit of a smoothing capacitor and a resistor is inserted between the remaining terminals and the Vcc power supply, an amplifying action can be obtained and a sufficient current detection signal can be obtained with a simple configuration.
[0030]
Further, since only the change point in the direction of the inverter output current is detected and the output voltage phase at that time is held, the on-delay correction is performed, so the period from the held phase to 180 degrees and the remaining period By switching the correction with and, the on-delay correction can be easily and accurately performed.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating an embodiment of the present invention.
FIG. 2 is an equivalent circuit diagram when it is detected that a current flows through the freewheeling diode of FIG.
FIG. 3 is a circuit example in which on-delay correction is started when the direction of the output current changes.
FIG. 4 shows an output current waveform in the presence or absence of on-delay correction in which the correction is switched between a 180-degree period and the remaining 180-degree period.
FIG. 5 is a block diagram of a conventionally known inverter performing on-delay correction.
[Explanation of symbols]
E: DC power supply M: Induction motor I: Inverter Su1, Su2: Switching element Sv1, Sv2: Switching element Sw1, Sw2: Switching element FDu1, FDu2: Free wheel diode FDv1, FDv2: Free wheel diode FDw1, FDw2: Free wheel diode DRu1, DRu2: Drive circuit DRv1, DRv2: Drive circuit DRw1, DRw2: Drive circuit PC: PWM control circuit VC: Voltage corrector VI: Voltage commander CG: Carrier generator PG: PWM generator U1 / U2: PWM signal V1 V2: PWM signal W1, W2: PWM signals a, b, c: current detection signals A, B, C: current direction signal Su: shunt circuit Dtu: current detection circuit Tru / Trv / Trw: transistor HDu / HDv / HDw : High voltage diode u · Rv · Rw: resistance Cu · Cv · Cw: Capacitor Ou: current detection signal output terminal OC: ON-delay correction circuit OT: ON-delay correction timing circuit

Claims (2)

An inverter bridge in which a circuit composed of a switching element and a freewheel diode connected in reverse parallel to each other is inserted in each upper and lower arm of the inverter bridge, a drive circuit for driving each switching element of the inverter bridge, and direct current to alternating current A PWM control circuit that supplies the drive circuit with a signal for alternately turning on and off the switching element so as to convert, a load current detection circuit, and an on-delay to the PWM control circuit based on the output of the load current detection circuit In an inverter having an on-delay correction circuit for performing correction,
The load current detection circuit includes a shunt circuit that shunts a part of the current flowing through the freewheel diode of the upper arm or the lower arm of the freewheel diode,
The shunt circuit is constituted by a series connection circuit of a base-emitter portion of a transistor and a high voltage diode, the series connection circuit is connected in parallel to the free wheel diode in the same polarity, and the remaining terminals of the transistor A parallel circuit of a smoothing capacitor and a resistor is inserted between the power supply and the Vcc power supply . The on-delay correction circuit is connected to an on-delay correction timing circuit that generates a first timing signal at the time of change in the load current direction and generates a second timing signal when the output voltage phase has passed 180 degrees from this point. 2. The inverter according to claim 1, wherein on-delay correction is performed with a first timing signal from an on-delay correction timing circuit, and on-delay correction opposite to the correction is performed with a second timing signal.

Images