JPH06209572A - Device for protecting switching element in power converter - Google Patents

Abstract

PURPOSE:To prevent thermal malfunction due to increase in switching frequency of a switching element (transistor) in a power converter where a waveform following control is performed by instant comparison with a hysteresis converter by feeding back an output waveform to a reference waveform. CONSTITUTION:An upper-limit frequency limiter circuit 20 for limiting so that the OFF time of control signal supplied to a transistor 5 is not equal to or less than the minimum OFF time is provided between a hysteresis converter 18 and the transistor 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a protection device for a switching element in a power converter in which waveform tracking control is performed.

[0002]

2. Description of the Related Art At present, when controlling a waveform of a power converter, follow-up control is performed by feeding back an output waveform to a reference waveform and making an instantaneous comparison. As a conventional example thereof, there is, for example, an input power factor adjusting device of a power conversion device disclosed in Japanese Patent Application No. 1-135749. In this conventional example, as shown in FIG. 4, the input current waveform is fed back and instantaneously compared with a reference sine wave (input voltage waveform) by a comparator to switch the chopper so that the input current waveform of the device becomes a sine wave. Waveform tracking control is performed by controlling. In FIG. 4, 1 is a rectifier which is a forward converter,
Reference numeral 2 is a chopper which is a DC power adjusting unit. Chopper 2
Is a DC reactor 3 connected to the DC output side of the rectifier 1, a diode 4, and a transistor 5 that is a switching element.
And a DC capacitor 6.

In FIG. 4, 7 is a transformer which is an AC voltage detector, and 8 is an AC / DC signal converter (AC / D).
C), 9 is a current transformer which is an alternating current detector, and 10 is current /
A voltage signal converter, 11 is an AC / DC signal converter (AC /
DC). 12 is a DC voltage detector (DC / DC) that detects the voltage of the DC capacitor 6 of the chopper 2 and converts it into a DC voltage signal, 13 is a first adder / subtractor, 14 is a deviation amplifier, and 15 is an output voltage of the deviation amplifier 14. Is a diode to prevent the voltage from becoming negative, 16 is a multiplier, 17 is a second
, And 18 is a hysteresis comparator.

In the apparatus shown in FIG. 4, when the transistor 5 of the chopper 2 is off, the DC capacitor 6 is in the rectifier 1
The current is charged to the polarity shown in the figure and a DC voltage V _DC is output. When the transistor 5 is turned on, the voltage V _CD of the DC capacitor 6 drops. Therefore, the output voltage and current of the chopper 2 are adjusted by adjusting the on / off cycle of the transistor 5.

FIG. 5 shows the voltage and current waveforms of the device of FIG. 4. The transformer 7 detects the AC input voltage of the rectifier 1 and outputs the AC voltage signal S1. The AC / DC signal converter 8 converts the AC voltage signal S1 into a DC voltage signal S2.
The DC voltage detection signal of the DC voltage detector 12 and the DC voltage (V
_{The DC} ) setting signal is added / subtracted by the first adder / subtractor 13 to obtain a deviation voltage signal. The deviation amplifier 14 amplifies the deviation voltage signal and outputs a DC voltage signal S3. Multiplier 16
Produces a voltage signal S4 by multiplying the DC voltage signal S2 from the AC / DC converter 8 and the signal S3 from the deviation amplifier 14. The current / voltage signal converter 11 converts the current detection signal into an AC voltage signal. This AC voltage signal is converted into a DC voltage signal S5 by the AC / DC signal converter 11. Second
The adder / subtractor 17 of (1) over-subtracts the DC voltage signals S4 and S5 and inputs the deviation signal to the hysteresis comparator 18.
The hysteresis comparator 18 outputs the square wave pulse signal S6 based on the deviation signal to the transistor 5 of the chopper 2.
As a control signal.

That is, according to FIG. 4, the input voltage of the rectifier 1 is detected and full-wave rectified by the AC / DC signal converter 8 to obtain the DC voltage signal S2. Further, the DC voltage output signal V _DC of the chopper 2 is detected, the deviation is compared with the DC setting voltage _VDC set, and the deviation amplifier 14 obtains the deviation voltage signal S3. The deviation voltage signal S3 and the DC voltage signal S
2 is multiplied by the multiplier 16 to obtain the DC voltage signal S4. Further, the input current of the rectifier 1 is detected, full-wave rectified by the AC / DC signal converter 11 to obtain a DC voltage signal S5, the signals S4 and S5 are matched, and the deviation signal thereof is input to the hysteresis comparator 18 and input. The transistor 5 of the chopper 2 is turned on and off so that the current matches the input voltage with a certain width (hysteresis width). When the transistor 5 is on, the input current increases, and when it is off, the input current decreases. Therefore, the input current is approximated to a sine wave to reduce the harmonic component, and the phases of the input voltage and the input current are matched to improve the input power factor.

[0007]

When performing the waveform tracking control by the conventional apparatus as shown in FIG. 4, the problem is the switching frequency of the switching element (transistor 5). Since this switching frequency is determined by the input power source impedance and load conditions, the switching frequency may exceed the specifications of the switching element depending on the conditions.

If this state continues, switching loss of the switching element increases, causing thermal runaway, resulting in destruction.
In order to prevent this thermal runaway, it is necessary to monitor and limit the switching frequency of the switching element.

The present invention has been made in view of the above points, and an object thereof is to provide a protection device capable of preventing a thermal runaway of a switching element in a power conversion device performing waveform tracking control. It is in.

[0010]

The present invention has a forward converter for converting AC power into DC power, and a switching element for adjusting the DC power of the forward converter, and the DC power is provided to a power converter at a subsequent stage. In a power conversion device having a DC power adjusting unit for supplying the power, a deviation amount between the output voltage of the DC power adjusting unit and a DC setting voltage is multiplied by a DC voltage signal corresponding to the AC input voltage of the forward converting unit. A multiplying means, a comparator for comparing a DC voltage signal corresponding to an AC input current of the forward converter and a multiplying signal by the multiplying means, and outputting the deviation as a control signal for the switching element;
A frequency limiting unit is provided between the comparator and the control terminal of the switching element, for limiting the frequency of the control signal to a predetermined value.

[0011]

If the limiting frequency of the frequency limiting means is set to the upper limit of the switching frequency of the switching element, the frequency will be limited even if the output signal of the comparator exceeds the upper limit of the switching frequency. Therefore, thermal runaway of the switching element can be prevented.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment in which the present invention is applied to the circuit of FIG. 1 is different from FIG. 4 in that an upper limit frequency limiter circuit 20 is provided on the output side of the hysteresis comparator 18 so as to limit the switching frequency of the control pulse supplied to the transistor 5, and the other parts are provided. Are configured the same as in FIG.

The upper limit frequency limiter circuit 20 is a circuit which prevents the width of the off time of the output pulse signal from becoming a predetermined value or less even if the width of the off time of the input pulse signal becomes small. For example, as shown in FIG. Composed. In FIG. 2, reference numeral 21 is a one-shot timer to which the output pulse of the hysteresis comparator 18 is input, and the minimum off-time (t _OFF min) is set by this one-shot timer. The relationship between the minimum off time and the maximum switching frequency fmax of the transistor 5 is fmax = 1 / (t _OFF mi
n). Reference numeral 22 denotes an AND circuit having an inverting input terminal. The output signal of the comparator 18 is input to one input terminal thereof, and the output signal of the one-shot timer 21 is input to the other input terminal thereof.

In the circuit configured as described above, a time chart of the output signal G1 of the hysteresis comparator 18, the output signal G2 of the one-shot timer 21, and the output signal G3 of the AND circuit 22 is shown in FIG.
The one-shot timer 21 outputs a pulse signal which is at a high level for a set time (minimum off time t _OFF min) when the input signal (G1) rises. As a result, the off period (high level) of the output pulse signal G1 of the comparator 18 becomes shorter, and the minimum off time becomes as shown in the period X in the figure.
Even if it becomes smaller than t _OFF min, the OFF period (low level) of the output pulse signal G3 of the AND circuit 22 is t _OFF mi
It cannot be less than n. As a result, the output signal frequency of the hysteresis comparator 18 is limited by the upper limit frequency limiter circuit 20, and the upper limit of the switching frequency of the transistor 5 can be limited. Therefore, thermal runaway of the transistor 5 can be prevented.

In the above embodiment, the upper limit of the switching frequency of the switching element is realized by setting the minimum off time, but it may be configured to set the maximum on time. The maximum switching time is determined by the minimum off time.

Although the switching element in the above embodiment is the transistor 5, the same operation and effect as described above can be obtained by using other switching elements. Further, the frequency limiting means is the upper limit frequency limiter circuit 20.
However, the circuit may be configured by another circuit having the same function.

[0017]

As described above, according to the present invention, since the frequency limiting means is provided, it is possible to prevent the thermal runaway due to the increase of the switching frequency of the switching element in the waveform tracking control of the power converter by a simple circuit. .

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram of a main part of an embodiment of the present invention.

FIG. 3 is a time chart for explaining the operation of the present invention.

FIG. 4 is a block diagram showing an example of a conventional input power factor adjusting device for a power converter.

5 is a voltage waveform diagram of essential parts of the device of FIG.

[Explanation of symbols]

 1 ... Rectifier 2 ... Chopper 5 ... Transistor 6 ... DC capacitor 7 ... Transformer 8 ... AC / DC signal converter 9 ... Current transformer 10 ... Current / voltage signal converter 11 ... AC / DC signal converter 12 ... DC voltage detection Device 13 ... First adder / subtractor 14 ... Deviation amplifier 17 ... Second adder / subtractor 18 ... Hysteresis comparator 20 ... Upper limit frequency limiter circuit 21 ... One shot timer 22 ... AND circuit with inverting input terminal

Claims (1)

[Claims] 1. A direct-current power adjustment unit that has a forward conversion unit that converts alternating-current power into direct-current power, and a switching element that adjusts direct-current power in the forward-conversion unit, and that supplies direct-current power to a power conversion device at a subsequent stage. In the power conversion device comprising: a multiplication means for multiplying the deviation between the output voltage of the DC power adjusting unit and the DC setting voltage by a DC voltage signal corresponding to the AC input voltage of the forward converting unit; A DC voltage signal corresponding to the AC input current of the section and a multiplication signal by the multiplication means, and a comparator that outputs the deviation as a control signal of the switching element; a control terminal of the comparator and the switching element; And a frequency limiting means for limiting the frequency of the control signal to a predetermined value, and a protection device for a switching element in a power conversion device. .