JPH0667117B2 - Overcurrent protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention is directed to the destruction of a switching element, which is a constituent element of an inverter, due to an excessive current due to malfunction or overload in an inverter used in an inverter, an air conditioner, or the like. The present invention relates to an overcurrent protection circuit for preventing.

<Prior Art> A conventional overcurrent protection circuit of an inverter used for this type of application has a configuration as shown in FIG. That is, the DC motor M and the four switching elements Q1 to Q1 including the power transistors for driving the DC motor M1.
Q4 and flywheel diodes D1 to D4 connected in antiparallel to the switching elements Q1 to Q4, respectively.
And the drive circuits DR1 to DR4 that drive two of the switching elements Q1 to Q4 in pairs to form a single-phase inverter. The first and third switching elements Q1 and Q3 are connected to the positive electrode side of the inverter DC power supply P1, and the second and fourth switching elements Q2 and Q4 are connected via the current detection resistor R1. Is connected to the negative side of the DC power source P1 and is connected to each drive circuit D
First to second switching elements Q1 and Q2 and third and fourth switching elements Q by R1 to DR4
By turning on 3 and Q4 alternately,
A reverse current is alternately supplied from the DC power source P1 to the DC motor M. In addition, each drive circuit DR of the first and third switching elements Q1 and Q3 on the positive electrode side of the DC power source P1
1, DR3 are supplied with power from the driving power supplies P2, P3, respectively, while the driving circuits DR2, DR4 of the second and fourth switching elements Q2, Q4 on the negative side of the DC power supply P1 are respectively supplied. , Is shared by the single drive power source P4 and is supplied with power. Further, two diodes D5, D6, D7, connected in series in the forward direction are provided between the first and third switching elements Q1 and Q3 on the positive electrode side and the drive circuits DR1 and DR3.
D8 and electrolytic capacitor C connected in parallel to this series circuit
A reverse bias circuit configured to apply a reverse bias voltage to each of the switching elements Q1 and Q3 at the time of off operation by each of the drive circuits DR1 and DR3 is connected. Similarly, between the common connection point of the second and fourth switching elements Q2 and Q4 on the negative electrode side of the DC power supply P1 and the common connection point of these drive circuits DR2 and DR4, two are connected in series in the forward direction. Reverse bias for selectively applying a reverse bias voltage to each switching element Q2, Q4 at the time of off operation by each drive circuit DR2, DR4, which is composed of a plurality of diodes D9, D10 and an electrolytic capacitor C3 connected in parallel to this series circuit. The circuit is connected. These reverse bias circuits are for shortening the off time of the switching elements Q1 to Q4.

The current of the inverter flows through the above-mentioned current detection resistor R1, and the resistor R1 performs current-voltage conversion, and the converted detection voltage is compared with the set reference voltage by the overcurrent detection circuit CP configured by the comparison amplifier. When the detected voltage becomes equal to or higher than the set reference voltage, that is, the current flowing through the switching elements Q1 to Q4 due to the malfunction of the inverter, the overload of the DC motor M, the output short circuit, or the like is up to a value close to the current rated withstanding value. At the time of increase, the power switch transistor Q5 of both drive circuits DR2 and DR4 is turned off by being output from the overcurrent detection circuit CP, and the power supply P4 of the drive circuits DR2 and DR4 of the second and fourth switching elements Q2 and Q4.
Is cut off, the current loop of the inverter is opened, and the switching elements Q1 to Q4 are protected from destruction due to an excessive current exceeding the rating. The excessive current detection circuit CP is supplied with power from the driving power source P5.

<Problems to be Solved by the Invention> By the way, the overcurrent protection circuit includes five power sources P1 to P1.
5 is required, the structure is complicated, and the running cost is high. Therefore, for example, the sixth
As shown in the figure, the second and fourth switching elements Q
The power supply P4 of the driving circuit of 2, Q4 is set to the overcurrent detection circuit CP.
When connected to and shared with each other, the supply voltage Vp of the overcurrent detection circuit CP is the diodes D9 and D1 charged in the electrolytic capacitor C3 in order to flow the reverse bias current.
When the forward voltage of 0 is 2VF, the resistance value of the current detection resistor R1 is r, and the current flowing through this resistor R1 is I, V
It is represented by c = Vp−2VF + I × r. Here, the switching element Q1 to Q is applied to the current I flowing through the resistor R1.
Since it contains switching noise due to 4, the overcurrent detection circuit CP malfunctions. Therefore, in the circuit configuration as shown in FIG. 6, the power source P4 is connected to the overcurrent detection circuit CP.
Cannot be shared with other power sources.

<Objects of the Invention> The present invention has been made in view of the above conventional problems, and uses a power supply for a drive circuit of a switching element as a power supply for an overcurrent detection circuit without superposing switching noise of a switching element of an inverter. An object of the present invention is to provide an overcurrent protection circuit having a circuit configuration that can be shared.

<Means for Solving Problems> In order to achieve the above-mentioned object, an overcurrent protection circuit of the present invention is an inverter including a plurality of switching elements provided in pairs and a drive circuit for each switching element. A current detection resistor is connected to the negative side of the DC power supply, and the current flowing through this resistor is converted into a voltage value proportional to it for detection, and an overcurrent detection circuit is activated when this detection voltage exceeds a set value. By shutting off the power source of the drive circuit of the switching element arranged on the negative side of the DC power source by the overcurrent protection circuit for protecting the switching element from being destroyed by an overcurrent exceeding its current withstand capacity, the current The detection resistor is reverse-biased to the reverse-bias circuit that applies a reverse-bias voltage to the negative-side switching element at the time of off-operation by its drive circuit. In this configuration, the resistance value is set to be small enough to allow a current to flow, and the reverse bias circuit is inserted and connected, and the power source of the drive circuit for the switching element on the negative electrode side is commonly connected to the overcurrent detection circuit. is there.

<Operation> With the above configuration, when the negative side switching element of the inverter power supply is turned off by the drive circuit,
Reverse bias current flows through the switching element through the current detection resistor that is connected to the reverse bias circuit.
Since a resistor having a small resistance value is used as the current detecting resistor, it does not affect the reverse bias current. Further, since the current detection resistor is inserted and connected in the reverse bias circuit, power is supplied to the excessive current detection circuit from the power supply of the drive circuit of the switching element on the negative side through the reverse bias circuit.
Since no current detection resistor is used, the switching noise of each switching element of the inverter will not be superimposed on the power supplied to the excessive current detection circuit.

<Example> Hereinafter, one example of the present invention will be described in detail with reference to the drawings.

In FIG. 1 showing an embodiment of the present invention, the same or equivalent parts as those in FIG. 5 are designated by the same reference numerals. Then, the power source P5 of the overcurrent detection circuit CP in FIG.
Is removed, the power supply P4 of the drive circuits DR2 and DR4 of the second and fourth switching elements Q2 and Q4 is connected to the overcurrent detection circuit CP, and the current detection resistor R1 is
The circuit configuration in which the reverse bias circuits of the second and fourth switching elements Q2 and Q4 are inserted and connected is different from that of FIG. 5, and a reverse bias current is supplied to the reverse bias circuit as the current detecting resistor R1. It has a small resistance value.

Since the basic operation of protecting the switching elements Q1 to Q4 from destruction due to excessive current due to driving and malfunction of the DC motor M in this embodiment is the same as in FIG.

Next, only the operation different from FIG. 5 will be described. First,
To facilitate understanding, the operation of each drive circuit DR1 to DR4 will be described.
All have the same configuration, the first drive circuit D is shown in FIG.
R1 is shown, and as shown in FIG. 4 (a), when an ON current flows from the control circuit (not shown) to the light emitting diode LD and the light emitting diode LD emits light, this is received by the photodiode FD. The signal is received in an electrically insulated state from the control circuit, and this signal is amplified by the amplifier A to turn on one output transistor Q6 and turn off the other output transistor Q7. Therefore, the base current flows from the power source P to the first switching element Q1 via the one output transistor Q6 and the base resistor R2, and the first switching element Q1 is turned on. At this time, as shown in FIG. 3, the two diodes D5 and D5 of the reverse bias circuit are
The forward voltage VF of D6 charges the electrolytic capacitor C1. Next, as shown in FIG. 4 (a), the light emitting diode LD
When the input current to the transistor Q is turned off, one output transistor Q6 is turned off by the amplifier A and the other transistor Q6 is turned off.
7 is turned on. Therefore, a reverse bias voltage of 2VF is applied between the base and the emitter of the first switching element Q1 by the charging potential of the electrolytic capacitor C1, and FIG. 4 (b).
As shown in, from the + side of the electrolytic capacitor C1, the emitter and base of the first switching element Q1, the resistor R3,
Electrolytic capacitor C via the other output transistor Q7
A reverse bias current flows to one side of 1 to turn off the first switching element Q1.

In the present invention, the current detecting resistor R1 of such a reverse bias circuit is inserted and connected. However, the current detecting resistor R1 has a smaller resistance value, that is, the resistor R3 has a smaller resistance than the resistor R3 of the reverse bias circuit. By using the one having a small value, there is no influence on the reverse bias current during the off operation of the second and fourth switching elements Q2, Q4.

Moreover, by connecting the current detection resistor R1 to the reverse bias circuit, as is apparent from FIG. 2, the drive circuits D for the second and fourth switching elements Q2 and Q4 are respectively formed.
Since the current detection resistor R1 does not exist in the power supply circuit that shares the power supply P4 of R2 and DR4 as the power supply of the overcurrent detection circuit CP, each switching element Q1 of the inverter
The switching noise of Q4 to Q4 does not overlap with the power supplied to the excessive current detection circuit CP.

It should be noted that the present invention is not limited to the above embodiment, and various embodiments can be considered without departing from the scope of the claims. For example, the above embodiment illustrates a single-phase inverter. However, it goes without saying that the same can be applied to the case of a three-phase inverter.

<Effects of the Invention> As described in detail above, according to the overcurrent protection circuit of the present invention,
The current detection resistor of the inverter is connected to the reverse bias circuit of the switching element on the negative side of the power source in the inverter by using a resistor with a small resistance value that allows the reverse bias current to flow. Since the power supply is commonly connected to the overcurrent detection circuit, the power supply for driving the switching element on the negative side of the inverter can be shared with the power supply for the overcurrent detection circuit of the inverter, simplifying the configuration and reducing costs. Can be achieved.

Moreover, by using a resistor with a sufficiently small resistance value as the current detection resistor, the reverse bias current to the switching element is not affected, and the switching noise of the inverter does not affect the power supplied to the overcurrent detection circuit. Is not superposed, the malfunction does not occur in the excessive current detection circuit.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of an embodiment of an overcurrent protection circuit of the present invention, FIG. 2 is an electric circuit diagram of a part of FIG. 1, and FIG. 3 is an electric circuit diagram of a drive circuit block of FIG. 4, FIG. 4 is a timing chart of FIG. 3, FIG. 5 is a block diagram of a conventional circuit, and FIG. 6 is a partial electric circuit diagram of FIG. Q1 to Q4 ... Switching elements DR1 to DR4 ... Drive circuit P1 ... Inverter DC power supply P4 ... Negative side switching element drive circuit power supply R1 ... Current detection resistor CP ... Excessive current detection circuit

Claims (1)

[Claims] 1. A current detecting resistor is connected to a negative side of a DC power source of an inverter having a plurality of switching elements provided in pairs and a drive circuit for each switching element, and a current flowing through the resistors is Is detected by converting it into a voltage value proportional to, and when the detected voltage exceeds a set value, the overcurrent detection circuit shuts off the power supply of the drive circuit of the switching element arranged on the negative side of the DC power supply. In an overcurrent protection circuit configured to protect the switching element from destruction due to an excessive current exceeding its current withstanding capacity, the current detection resistor is reversely connected to the switching element on the negative electrode side at the time of off operation by the drive circuit thereof. If the reverse bias circuit that applies the bias voltage has a small resistance value that allows the reverse bias current to flow, and if it is inserted and connected to this reverse bias circuit, Moni, wherein the negative electrode side power supply of the driving circuit of the switching element, the overcurrent protection circuit, characterized in that connected in common to said overcurrent detecting circuit.