JPH04101670A - Protective detection circuit for current type inverter - Google Patents

Abstract

PURPOSE:To realize reliable protective detection of individual switching element with an appropriate margin by detecting input and output voltages of a current type inverter, rectifying thus detected voltages, making the difference therebetween and canceling the load voltage part thereby detecting the ON voltage of the switching element. CONSTITUTION:A comparator 91 in an excessive element ON voltage judging element SH 9 is fed with a signal produced by subtracting the output voltage of an input voltage detector 6 from the output voltage of an output voltage detector 7 and then rectifying the difference. Consequently, the SH 9 extracts and detects only the ON voltages of the elements 41a, 41b and the like in an inverter 4. As a result, ON voltage of transistor increases abruptly from the vicinity of rated current Ic1 thus enabling overcurrent detection of individual element. Since the ON voltage Vce increases abruptly even if the base current Ib is insufficient to cause a current flow over the DC amplification factor of the transistor, abnormality including malfunction of base drive circuit can be detected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a protection detection circuit for a current source inverter, and particularly to one for detecting overcurrent of individual switching elements.

[Conventional technology]

Figure 4 is a circuit diagram showing the basic main circuit configuration of a current source inverter disclosed in, for example, Japanese Patent Publication No. 58-21495 and Japanese Patent Application Laid-Open No. 59-14373. converter transformer connected to a frequency power supply,
(2) is a converter which is a forward conversion circuit, C3) is a DC reactor, (2) is a current source inverter which is an inverse conversion circuit, and a transistor (41a) is a switching element.
~ (44a) and diodes (41b) ~ (44b). (5) is a load consisting of an induction heating coil (51) and a capacitor (52) connected in parallel with the induction heating coil (51).

Next, the operation of this inverter (2) will be explained mainly. The converter [21] receives an ignition pulse signal from a control circuit (not shown), performs six-phase full-wave rectification at a predetermined phase angle, and converts the three-phase open frequency AC from the converter transformer (1) into a DC output. The DC reactor (3) smoothes ripples in the DC output current from the converter (2).

The inverter (a) receives a base current from a control circuit (not shown), turns on and off transistors (4]a), etc., and outputs high-frequency alternating current at a frequency that matches the resonance frequency of the load (9). Note that transistors (41a), etc. The diode (41b) and the like connected in series with the transistors are used to prevent reverse current from flowing through the transistors when the transistors are switched.

FIG. 5 shows the waveforms of the input voltage Vdc and output voltage Vac of this inverter (A), and FIG. 6 shows the energization mode of each element during periods A to E shown in FIG. However, in FIG. 6, Vdt indicates the on-voltage of the transistor and the diode, and period B is the commutation mode.

Further, in order to maintain the output voltage VaC at a predetermined set value, this voltage is detected and the output voltage of the converter (2) is controlled by a feedback control circuit (not shown).

A protection detection circuit is provided which detects the output voltage and current and, when these values exceed predetermined set values, cuts off the circuit or the like to protect the inverter (a).

[Problem to be solved by the invention]

Conventional overcurrent protection detection for current source inverters determines whether or not there is an overcurrent from the detected value of the inverter's output current itself, as described above, and the set value is based on the standard characteristic value of the switching element. Therefore, it was insufficient in terms of reliably protecting each switching element with an appropriate margin. That is, the characteristics of individual switching elements are not necessarily constant, and the allowable overcurrent value changes due to fluctuations in the base current and the like.

If the protection operation level is set by taking into consideration these characteristic variations and variations in usage conditions in advance, the set value will inevitably become low and there will be an excessive margin, which will be uneconomical.

In particular, self-extinguishing transistors, which have recently been increasingly used as switching elements in current-source inverters, have lower overcurrent capability and insufficient base current than high-speed switching thyristors, which have traditionally been widely used. The above-mentioned problems become even more serious because of the large change in characteristics due to the loss of the base current or the like.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a circuit that can protect and detect each switching element with an appropriate margin and with reliability.

[Means and operations for solving the problem] A protection detection circuit for a current source inverter according to the present invention detects the input and output voltages of a current source inverter, rectifies each of these detected voltages, and converts both of these rectified voltages. The overcurrent of the switching element is detected from the difference.

By taking the difference between the two rectified voltages, the portion corresponding to the load voltage is canceled out, and the on-voltage of the switching element is detected. Then, the overcurrent state of the switching element is determined based on whether this on-voltage is within a safe operation range.

〔Example〕

FIG. 1 is a circuit diagram showing a protection detection circuit for a current source inverter according to an embodiment of the present invention. In FIG. . (6) is an input voltage detector that detects the input voltage Vdc of the inverter (A).
1), a capacitor (62), and a resistor (63). c71 is the output voltage Va of the inverter (A)
The rectifier bridge (7I) is an output voltage detector that detects c.
and a resistor (72).

(to) is the output voltage Vac, specifically the output voltage detector (7
) is an output overvoltage detector that outputs a "1" signal when the output voltage exceeds a predetermined set value, (9) is the difference between the output voltage Vac and the input voltage Vdc, specifically the output voltage detector (7) If the value obtained by subtracting the output voltage of the input voltage detector (6) from the output voltage of the input voltage detector (6) and further rectifying it exceeds a predetermined set value, ``1'' will occur.
An element-on voltage over-discriminator that outputs a "1" signal, (lO) is an abnormality memory transmitter consisting of an OR gate (101), a flip-flop (+02) and a photocoupler (103), and outputs an output over-voltage discriminator (8) and an element-on When an abnormality, that is, a 1'' signal is input from any of the voltage over-discriminators (9), a necessary signal is transmitted to a protection operation circuit (not shown) that performs circuit breakage, etc.

(11) is an insulated DC power supply that supplies a control voltage to the terminals 1 to 10).

Next, the operation will be explained. However, the operation of the main circuit explained in Figures 4 to 6 is exactly the same this time as well, and the output overvoltage discriminator itself is the same as the one used conventionally, so below we will mainly focus on the following. The operation of the element on-voltage excessive discriminator (9) will be mainly explained.

Comparator (91) of element on-voltage excessive discriminator (9)
As described above, the signal input to one of the input terminals is the result obtained by subtracting the output voltage of the input voltage detector (6) from the output voltage of the output voltage detector port, which is further rectified. Therefore, when this is determined for each mode of periods A to E shown in FIGS. 5 and 6, the results are as follows.

Period A] 1e+2Vdtl l e l l-2
Vdt period B IIed±2Vdtl -1e l l
# 2Vdt period Cl 12Vdt-el-lel
1=2Vdt period D 2Vdt period E 11e+2Vdtl l e l l =
2Vdt However, Vdt is the total ON voltage of the transistor and diode, e is the output (load) $ pressure, ed is the voltage of the capacitor (62) in the input voltage detector (6), and commutation period 1? This capacitor (6
2). Further, the voltage drop of the diode in the element on-voltage excessive discriminator (9) is ignored as it is sufficiently small.

In this way, the element on-voltage excessive discriminator (9) extracts and detects only the on-voltage of each element (41a) (41b), etc. of the inverter (a). Therefore, for example, as shown in FIG.
) has a characteristic of rapidly increasing from around the rated current (Ic, ), and this can be utilized to detect overcurrents corresponding to individual elements.

In addition, for example, as shown in FIG. 3, the base current (Ib)
Even if there is a shortage of current and a current exceeding the DC amplification factor of the transistor flows, the on-voltage (Vce) rises rapidly, making it possible to detect abnormalities including defects in the base drive circuit.

When looking at each transistor during inverter operation, it is necessary to withstand the blocking voltage applied when it is off.
When it is on, it must withstand the current flowing through it. In this embodiment, the output overvoltage discriminator (8) is responsible for the former overvoltage protection detection, and the element on voltage overvoltage discriminator (9) is responsible for the latter overcurrent protection detection, so that protection can be detected in all operating regions. This means that it will definitely be done. In particular, the latter overcurrent detection is configured to detect the on-voltage of the element rather than simply detecting the output current, so it is possible to perform appropriate protection detection that matches the characteristics of the individual element being detected, and the base current It becomes possible to determine whether or not the transistor element is truly in a dangerous state, including the effects of shortage and the like.

Note that the element on-voltage excessive discriminator (9) in the above embodiment
), a comparator (91) was used as the discriminator, but a Zener diode or the like may also be used. In addition, the photocoupler (10
3) may utilize a pulse transformer or the like.

Furthermore, in the above embodiment, an output overvoltage discriminator (8) is provided to perform overvoltage protection detection at the same time, but it is not necessarily necessary from the purpose of the invention. Therefore, it is possible to perform overcurrent protection detection appropriate to the characteristics and usage conditions of each switching element.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a protection detection circuit for a current source inverter according to an embodiment of the present invention, FIGS. 2 and 3 are schematic characteristic diagrams of transistors related to the operation of the present invention, and FIG.
The figure shows the basic main circuit F of a current source inverter! FIG. 5 is a waveform diagram showing the input and output voltages of the inverter, and FIG. 6 is a diagram showing the energization mode of each element. In the figure, (2) is a current source inverter, (41a) etc. are transistors with switching elements, (6) is an input voltage detector, (7) is an output voltage detector, and (9) is an element on-voltage excessive discriminator. , Vdc is the input voltage, and Vac is the output voltage. Note that the same reference numerals in each figure indicate the same or corresponding parts. Agent Patent Attorney Masuo Ookimi Figure 1 Figure 2 Figure 3 Figure 6 (A) (C) (D) (E)

Claims (1)

[Claims] Protection detection for a current source inverter in which the input and output voltages of the current source inverter are detected, each of these detected voltages is rectified, and overcurrent of the switching elements of the current source inverter is detected from the difference between these two rectified voltages. circuit.