JPH0562792A - Inverter type x-ray high-voltage device - Google Patents

Abstract

PURPOSE:To provide an inverter protective device which can precisely detect an overcurrent at a high speed, gives no adverse effect to an inductance, and is small and inexpensive. CONSTITUTION:An inverter protective device is constituted of a small rewinding type coil 8 fitted to the terminal of a switching element 2, an AND circuit 21 calculating the logical product of the output from the coil 8, a comparing circuit 22 comparing the output from the AND circuit 21 and the preset value, and a counter 23 counting the output period to judge the output from the AND circuit 21 as an overcurrent. The output from the coil 8 is a differentiated value, thus the overcurrent can be detected before it becomes a large current, the overcurrent can be detected at a high speed, the load of the switching element 2 is small, thus the circuit can be safely stopped.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter type X-ray high voltage device, and more particularly to an inverter circuit thereof.

[0002]

2. Description of the Related Art An inverter 5 is used as an electric power converting means, and its configuration will be described with reference to FIG. The inverter 5 has a first series connection body composed of switching elements 2a and 2b, and a second series connection body composed of switching elements 2c and 2d connected in parallel to the first series connection body. .. The switching elements 2a and 2c are connected to the positive electrode of the DC power supply 1, and the switching elements 2b and 2d are connected to the negative electrode. Further, a driving circuit 12 is provided for driving the switching element 2, respectively.
Inverter circuits in recent years have tended to have higher power and higher frequencies, and have changed to large-capacity, high-speed switching elements. Along with this, the overcurrent also becomes a large current, and it is indispensable to detect the overcurrent promptly and turn off the switching element safely at high speed. Conventionally, as a means for detecting an overcurrent, there are a method of directly detecting an overcurrent using a current transformer, and a method of measuring a voltage between terminals of a switching element and detecting the overcurrent from the result.

In the method using a current transformer, the current transformer is attached to the terminals of each switching element, and a short circuit between the first and second switching elements or between the third and fourth switching elements is performed by an AND circuit. Check, if there is a short circuit then AN
A current signal is output from the D circuit, which is compared with a set value by a comparison circuit. When the current signal exceeds the set value, it is judged as an overcurrent and the inverter circuit is stopped.

Further, in the method of measuring the voltage between the terminals of the switching element, the voltage between the terminals of the switching element is divided by a known resistor, and the output of each divided voltage signal is input to a comparison circuit through an operational amplifier. .. Then, similar to the above, the set values are compared, and when the voltage signal exceeds the set value, it is determined that the current is an overcurrent and the inverter circuit is stopped.

That is, the method using a current transformer is to directly compare the measured current and the overcurrent, and the method for measuring the terminal voltage of the switching element is to indirectly measure the terminal voltage and the overcurrent. The so-called terminal voltage is compared with the voltage corresponding to the overcurrent.

[0006]

The method using the current transformer is high in speed because it directly detects the overcurrent, but the current transformer itself is large in size and difficult to mount. Circuit wiring becomes long. As a result, an increase in wiring inductance causes noise at the turn-on and turn-off of the switching element and an increase in the size of the energy absorption circuit. Further, in the method of measuring the voltage between terminals of the switching element, the voltage is measured and then calculated to detect the overcurrent. In this case, an error always occurs in a voltage measuring device, a calculation circuit, or the like, so that the more the measuring devices and circuits are passed, the different the obtained value becomes. In other words, the detection accuracy is low when detected indirectly, and the deviation from the actual detection point is large.Therefore, it is necessary to look at a considerable margin in the rating of the switching element. Was becoming. Furthermore, at the time of turn-on and turn-off, the change in the voltage between the terminals was so rapid that it was impossible to detect.

An object of the present invention is to provide an inverter type X-ray high voltage device which can detect an overcurrent directly at high speed and with high accuracy, does not adversely affect the main circuit due to an inductance, and is equipped with a small and inexpensive inverter protection device. To provide.

[0008]

A DC power supply for applying a DC voltage and a third and fourth switch in which a first and a second switch are connected in series and are connected in parallel to the first and the second switch. An inverter connected in parallel to the DC power supply, a drive circuit for operating each switch of the inverter, a load circuit operating by receiving the output voltage of the inverter, and an overcurrent flowing through the switch of the inverter. In an inverter type X-ray high voltage device having an inverter protection device for stopping an inverter when the inverter is turned on, a detector for detecting a current flowing through each switch of the inverter in the inverter protection device, and a logic of an output value of the detector The means for calculating the product, the means for comparing the output of this logical product with the reference value for judging the overcurrent of the current flowing through each switch, and the comparison means Is composed of a means for measuring the time flow of overcurrent exceeding standard value, the overcurrent exceeds said reference value,
Further, the inverter is stopped when the current flows for more than the specified time.

[0009]

[Operation] Attach a detector to each switch of the inverter,
A differential value of the current flowing through each switch is output by the detector, and the outputs of the detectors of the first and second switches and the outputs of the detectors of the third and fourth switches are ANDed. Then, it can be judged from the output of this logical product whether or not there is a short circuit, and this output is inputted to the comparison means and compared with a reference value to judge whether or not the current flowing through the inverter is an overcurrent. Further, of the outputs judged to be overcurrent by this comparison means, only the overcurrent which destroys the switch of the inverter, that is, the overcurrent which flows for a predetermined time or longer, is detected, and the inverter is stopped by this detection signal.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a circuit diagram of an inverter type X-ray high voltage device provided with an inverter protection device according to the present invention, which includes a DC power supply 1, an inverter 5, a drive circuit 12, and a load circuit 1.
6, an overcurrent detection coil 8, an AND circuit 21, a comparison circuit 22, and a counter 23.

The DC power supply 1 supplies a DC voltage,
For example, it is a combination of a commercial AC power supply and a thyristor rectifier circuit. The inverter 5 converts the output voltage of the DC power supply 1 into a high frequency AC voltage.
Four switching elements 2a to 2d made of T (gate insulating bipolar transistor) or the like are combined in a full-bridge type. In addition, the drive circuit 12
a to 12d are switching elements 2 of the inverter 5
It is for operating a to 2d, and one switching element is provided for each of the switching elements 2a to 2d.

The load circuit 16 is supplied with the voltage output from the inverter 5. In FIG. 1, a high voltage transformer 17 for boosting the output AC voltage of the inverter 5 and the output of the high voltage transformer 17 are converted to direct current. High-voltage rectifier 18 for conversion
And the output DC voltage of the high voltage rectifier 18 is applied to X
An example of a medical X-ray generator including a ray tube 19 is shown. In this case, the AC output of the inverter 5 is supplied to the series circuit composed of the high-voltage transformer 17 and the capacitor 20, and the leakage inductance of the high-voltage transformer 17 and the capacity of the capacitor 20 cause series resonance. A circuit is configured, and the waveform of the current flowing through the inverter 5 is sinusoidal so as to reduce the electromagnetic induction noise and the switching loss of the four switching elements 2 including the IGBT.

The overcurrent detecting coil 8 is wound around the terminal 2'of the IGBT of the switching element 2 of the inverter circuit as shown in FIG. 2, and the current flowing through the switching element 2 is constantly monitored. When a current flows through the switching element 2, the coil 8 is coiled by electromagnetic induction.
A voltage is induced at both terminals of. However, since this coil 8 is unwound as shown in FIG. 3, there is no adverse effect on the circuit due to self-induction. The output from the coil 8 is output as a differential value of the current flowing through the switching element 2.

AND the logical products of the outputs of the coils 8a and 8b
By using the circuit 21, the switching elements 2a and 2a
It is possible to determine whether b is arm short-circuited. The waveform at this time is shown in FIG. 4. When the inverter 5 is operating normally, the current I ₁ of the switching element 2a and the current I ₂ of the switching element 2b do not flow at the same time.
The outputs L ₁ and L _{2 of the} respective coils 8a and 8b are not output at the same time. However, when the arm is short-circuited, current _I'1 called switching element 2a and 2b is turned on, _I'2
If there is a part that flows at the same time, each coil 8
The outputs L' ₁ and L' _{2 of} a and 8b are also output at the same time. That is, when both of the switching elements 2a and 2b are turned on in some cases, it is determined that they are output by the AND circuit 21 and are short-circuited. Then, the output signal of the AND circuit 21 is input to the comparison circuit 22.

In the comparison circuit 22, the output signal from the AND circuit 21 is compared with a set value which is a reference for determining whether or not the current is an overcurrent, and when the output signal is larger than the set value, it is determined that the current is an overcurrent. A signal for turning off the switching element 2 is sent to the drive circuit 12 to stop the inverter circuit.

However, in this case, when the switching element 2 is turned on and off, the current changes rapidly, so that the differential value of the current momentarily exceeds the set value and is falsely detected as an overcurrent. In order to prevent the switching element 2 from being damaged by overcurrent, about 10 μs after the arm short circuit occurs.
While it is necessary to turn off the switching element 2 within that time, the change in the differential value of the current when the switching element 2 is turned on or turned off is 2 to 3 μs. Therefore, if the counter 23 detects only a value that has passed for about 5 μs or more, it is possible to detect only the overcurrent related to the breakdown of the switching element 2.

In this embodiment, the coil 8 is wound around the terminals 2'of all the switching elements 2, but only the terminals of the switching elements 2a and 2b, and the switching element 2 are also provided.
Even if it is wound only around the terminals c and 2d, the overcurrent can be similarly detected. Further, although the IGBT is used as the switching element 2, the same operation is performed by using a bipolar transistor or a power MOSFET.

[0018]

According to the present invention, the differential value of the current flowing through the switching element is detected and the logical product is taken. Only the current exceeding the set value for a certain period of time is regarded as the overcurrent, so the overcurrent itself is detected. Since the current value when the switching element is off is small at a higher speed than in the case of overcurrent, the load on the switching element is reduced. That is, the inverter can be stopped at high speed and safely even when an overcurrent flows due to an arm short circuit or the like when the inverter has high power and high frequency.

Since the coil used in the present invention is an unwinding type coil, it does not adversely affect the circuit due to self-induction, and since it is small, special consideration needs to be given to the installation location of the coil. No, it can be provided at a low price.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an inverter type X-ray high voltage device showing an embodiment of the present invention.

FIG. 2 shows a coil attached to a terminal of a switching element.

[Fig. 3] Structural drawing of coil

FIG. 4 is a waveform showing the current and coil output when the inverter is normal and when the arm is short-circuited.

FIG. 5 is a circuit diagram showing an inverter of a conventional device.

[Explanation of Codes] 1 DC power supply 2 Switching element 5 Inverter 8 Coil 12 Drive circuit 16 Load circuit 17 High voltage transformer 18 High voltage rectifier 19 X-ray tube 20 Capacitor 21 AND circuit 22 Comparison circuit 23 Counter

Claims (1)

[Claims] 1. A direct current power source for applying a direct current voltage, and a third and fourth switch in which first and second switches are connected in series and which are connected in parallel to the first and second switches. An inverter connected in parallel to the power supply, a drive circuit for operating each switch of the inverter, and a load circuit that operates by receiving the output voltage of the inverter,
An inverter type X-ray high voltage device having an inverter protection device for stopping the inverter when an overcurrent flows through the switch of the inverter, wherein the inverter protection device detects a current flowing through each switch of the inverter. And a means for obtaining the logical product of the output values of this detector, a means for comparing the output of this logical product with a reference value for judging the overcurrent of the current flowing through each switch, and a means for comparing the output value exceeding the reference value with this comparing means. An inverter type X-ray high-voltage device comprising: a means for measuring the time when the current flows, wherein the inverter is stopped when the overcurrent exceeds the reference value and flows for a predetermined time or more.