JP2752182B2 - Demagnetization prevention circuit for CVCF transformer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a demagnetization prevention circuit for a CVCF transformer that supplies electric power via a transformer.

(Prior Art) FIG. 7 shows an example of a conventional CVCF transformer bias prevention circuit.
explain.

The voltage of the DC power supply 1 is converted into AC by the inverter bridge 2, applied to the output transformer 3, and the input current is detected by the current detector 4. On the secondary side of the output transformer 3, an AC voltage from which a harmonic component used for PWM control has been removed is output by the filter effect of the capacitor 5. The transformer 6 is provided for the purpose of detecting and controlling the output voltage.

The output V7 of the voltage reference generator 7 and the output V6 of the transformer 6 are compared and amplified by an amplifier 8 and output a control signal V8 so that the error is reduced. Current V4 detected by current detector 4
Is converted to V10 via the filter 10, the output V8 of the amplifier 8 is added by the adder 9, and the output of the triangular wave generator 12 is compared with the output of the comparator 11 to obtain a PWM signal. Control.

FIG. 8 shows a state where the output transformer 3 is magnetized.
The exciting current V4 of the output transformer 3 becomes positive / negative asymmetry due to the polarization of the transformer with respect to the output voltage V6.
As a result, the DC component V10 from which the fundamental wave component has been removed is detected, and acts on the adder 9 in a direction to reduce the DC component.

The main causes of such DC bias are variations in the DC component caused by the control circuit and the operating speed of the drive circuit and the transistor when the transistor of the inverter bridge 2 is PWM-controlled.

(Problems to be Solved by the Invention) However, in the conventional circuit described above, the DC component of the exciting current of the transformer is detected. There is a problem that the response of the prevention control cannot be made fast.

There is also a problem that the detection sensitivity is low because the detection is performed using the average value of the excitation current.

Therefore, there is a disadvantage that the maximum magnetic flux density of the transformer needs a margin and the transformer cannot be downsized.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a CVCF transformer bias prevention circuit which eliminates the above drawbacks, minimizes the delay caused by a filter, and has high sensitivity.

[Configuration of the Invention] (Means and Actions for Solving the Problems) In order to achieve the above object, the present invention provides an inverter that converts a DC voltage into an AC voltage and supplies power to a load via a transformer, In a CVCF device provided with a PWM control circuit for controlling an output voltage of an inverter according to a reference, a positive current flowing through a primary winding near the voltage reference or near the zero crossing of the output voltage, that is, near the maximum magnetic flux density of the transformer. And a means for correcting the above PWM control so that the current difference is reduced, thereby controlling the current near the maximum magnetic flux density to be symmetric to prevent the transformer from being demagnetized. I do.

(Example) An example of the present invention will be described with reference to FIG. The same parts as those in FIG. 7 are denoted by the same reference numerals, and description thereof will be omitted.

The output V7 of the voltage reference generator 7 is converted to a short-wave logic signal V14 via the polarity discrimination circuit 14.

At the rising and falling points of V14, the one-shot circuit 15 generates pulses of V15A and A15B, respectively, and the sample and hold circuits 16 and 17 sample and hold the input current V4 of the transformer 3 as V16 and V17, respectively. Adder 18
, And its output V18 is used as the input of the adder 9, and V18
The PWM is controlled in such a direction that becomes zero.

 The operation of the above embodiment will be described with reference to FIG.

The exciting current V4 of the transformer becomes maximum near the zero crossing of the voltage V6, and when the transformer is demagnetized, a positive and negative asymmetric waveform is obtained.
The voltage V6 is substantially in phase with the voltage reference V7, and the pulses V15A, V15 output from the one-shot circuit 15 at the time of the zero crossing.
By B, the sample and hold circuits 16 and 17 cause t _{1 of the} exciting current V4.
-T ₂ between the t ₃ -t ₄ between the values at each sample and hold, and updates the V16, V17. Since V16 is negative and V17 is positive, a deviation value is detected at the output V18 of the adder 18. Since the PWM is controlled so that this bias value V18 becomes zero,
V16 and V17 are controlled to be equal in magnitude. Therefore, the exciting current of the transformer is controlled to be positive / negative symmetric, and the DC component can be ignored.

According to the present embodiment, when the exciting current is symmetric, the deviation value V18 becomes zero and no AC component is generated, so that a very high-speed transformer demagnetization prevention control can be performed without the need for a filter element.

(Other Embodiments) As shown in the embodiment of FIG. 3, a one-shot circuit 19 generates a pulse signal V19 at the rise and fall of V14, and multiplies this signal V19 and current V4 by a multiplier 20. Signal V2 that selectively detects the transformer current at the zero crossing point
Get 0.

(See FIG. 4.) The signal V20 is averaged via the filter 21 and detected as a DC component V21. PWM so that this V21 becomes zero
The control is performed. This embodiment can be modified as shown in FIG. That is, a signal V22 having a phase difference of 90 ° is generated from the voltage reference V7 via the delay circuit 22, and a signal V23 obtained by full-wave rectifying this signal with the rectifier circuit 23 is obtained. This signal V23 and the exciting current V4
Is multiplied by the multiplier 20 to obtain a signal V20 in which the vicinity of the highest point (zero cross point) of the exciting current V4 is emphasized. (See FIG. 6.) It goes without saying that the same operation can be performed by using the output voltage V6 instead of the voltage reference V7.

In the above, analog control has been described, but digital control using a microprocessor can be similarly performed.

[Effects of the Invention] According to the present invention, the primary current of the transformer near the voltage zero crossing, that is, the current near the maximum magnetic flux density is emphasized and taken out, so that the detection density is good and the value is controlled so as to be symmetrical. By doing so, the transformer can be used without demagnetizing, so the CVCF transformer's demagnetization prevention circuit, which effectively uses the transformer's core and enables the use of a compact and economical transformer. Can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram of an embodiment of the present invention, FIG.
3, 5, 4, and 6 are diagrams showing another embodiment of the present invention and an operation explanatory diagram thereof, FIG. 7 is a conventional configuration diagram, and FIG.
The figure illustrates the operation. DESCRIPTION OF SYMBOLS 1 ... DC power supply 2 ... Inverter bridge 3 ... Output transformer 4 ... Current detector 5 ... Filter capacitor 6 ... Transformer 7 ... Voltage reference 8 ... Amplifier, 10,21 ... Filter 11 Comparator, 12 Triangular wave generator 13 Drive circuit, 14 Polarity discrimination circuit 15, 19 One-shot circuit 16, 17 Sample-hold circuit 18 Adder, 20 Multiplication Unit 22 Delay circuit, 23 Rectifier circuit

Claims (1)

(57) [Claims] 1. A CVCF device comprising: an inverter that converts a DC voltage to an AC voltage and supplies power to a load via a transformer; and a PWM control circuit that controls an output voltage of the inverter according to a voltage reference. The difference between the positive and negative currents flowing through the primary winding near the voltage reference or near the zero crossing of the output voltage, that is, where the magnetic flux density of the transformer is maximized, and thereby the PWM control is performed so that the current difference is reduced. A demagnetization prevention circuit for a CVCF transformer, comprising means for compensating for the deviation.