JP2001028877A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a forward type switching power supply in which an input higher-harmonic current and an inrush current are reduced. SOLUTION: The switching power supply has a rectifying means 2, an input- voltage detecting means 9, an output error voltage detecting means 12, a multiplier 17 outputting the product of the input-voltage detecting means and the output error voltage detecting means, a switching element 5, a switching-element current detecting means 18 detecting the current of the switching element 5, a comparator 19 comparing an output from the multiplier 17 and an output from the switching-element current detecting means 18 and sending an output when the output from the switching-element current detecting means 18 is made larger than the output from the multiplier 17, and an oscillation means 35. The switching element 5 is turned on by an output from the oscillation means, and turned off by the output from the comparator 19.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply which takes measures against harmonic current distortion and rush current.

[0002]

2. Description of the Related Art Conventionally, an input portion of a power supply is as shown in FIG. 8, and a commercial power supply 1 is full-wave rectified by a rectifier 2 which is a diode bridge via a power switch 49, and is smoothed by an electrolytic capacitor 3. I was Then, the input current waveform was as shown in FIG. 9, and a large inrush current reaching 100 A or more flowed, and after the current was stabilized, the waveform contained a large amount of harmonic current. As a countermeasure against the inrush current, a thermistor 50 having a large resistance when the temperature at the time of startup is low, and having a small resistance when the temperature during power supply is high is inserted. FIG. 10 shows a circuit diagram of a power supply called an active filter used for measures against harmonic current distortion. As described above, the conventional configuration is a self-excited boost type. First, the basic configuration of the self-excited boost switching power supply will be described. The commercial power supply 1 is full-wave rectified by a rectifier 2 that is a diode bridge, and a current is switched by a switching element 5 that is a MOSFET through a choke coil 4. When the switching element 5 is turned on, a current flows through the choke coil 4 to store energy. When the switching element 5 is turned off, the current passes through the high-speed switching diode 6 and is smoothed by the smoothing capacitor 7 to output a DC current to the load 8. I do. Next, a control method for taking measures against harmonic current distortion will be described. Reference numeral 9 denotes an input voltage detecting means, which divides a full-wave rectified input voltage by resistors 10 and 11. 12 is an output error voltage detecting means. The output voltage is divided by resistors 13 and 14, compared with a reference voltage 15, and an error is amplified by an amplifier 16. The amplifier 16 includes a low-pass filter (not shown) so as not to respond to a frequency much higher than the commercial frequency. The multiplier 17 outputs an output proportional to the input voltage and the output error voltage by inputting the outputs of the input voltage detecting means 9 and the output error means 12. Reference numeral 18 denotes a switching element current detecting means, which is a resistor connected to the source of the MOSFET 5.
Reference numeral 19 denotes a comparator which compares the output of the multiplier 17 with the output of the switching element current detecting means 18, and outputs an output to the RS flip-flop 20 when the output of the switching element current detecting means 18 becomes larger than the output of the multiplier 17. MOSF
Turn off the gate of ET5. 21 is a choke coil 4
The output voltage decreases when the choke coil finishes emitting the excitation energy. This is detected by the voltage detection circuit 22 and an output is output to the RS flip-flop 20 to turn on the gate of the MOSFET 5. The voltage detection circuit 22 includes a waveform shaping circuit 23 and a one-shot circuit 24. The capacitor 25 has a low capacity, and does not have a capacity enough to smooth the current of the commercial power supply for a half cycle only by smoothing the ripple of the switching current. FIG. 11 is a waveform diagram of the voltage of each part, and 26 is the output voltage of the input voltage detecting means 9.
It has a full-wave rectified waveform. An output voltage 27 of the switching element current detecting means 18 has a peak voltage similar to a full-wave rectified waveform. Reference numeral 28 denotes an output voltage of the sub winding 21. When the voltage drops, a switching element current starts to flow. When such a circuit is used, the input current from the commercial power supply 1 has a sine wave shape as shown in FIG. 12, which is a measure against harmonic current distortion. The output of the boost switching power supply is
Generally, if the input is AC100V, the output is DC30.
Change from 0V to 400V.

[0003]

As described above, the thermistor generates heat and causes a reduction in conversion efficiency. In addition, if the power is turned off and the power is turned on again immediately before the thermistor does not cool down, there is no effect of suppressing the rush current. Conventional power supplies for harmonic current countermeasures have been of a step-up type in which the primary and secondary are non-insulated. In general, it is said that a flyback type switching power supply of up to 50 W is suitable for an insulated switching power supply because of a large current ripple, and a forward type is more suitable for 50 W to 150 W. However, the configuration that detects that the excitation energy of the transformer has been released and starts to flow the current of the next switching cycle to the switching element has a problem that it cannot be applied to the forward type that originally does not store the excitation energy in the transformer. ing. An object of the present invention is to provide a forward-type power supply for preventing inrush current and harmonic current.

[0004]

According to the present invention, there is provided a rectifier for full-wave rectifying a commercial power supply as an input power supply, an input voltage detecting means for detecting a voltage of the input power supply, an output error voltage detecting means, An input voltage detection unit and a multiplier that receives an output of the output error voltage detection unit as an input and outputs a product; a switching element; a switching element current detection unit that detects a current of the switching element; an output of the multiplier; A comparator that compares the output of the switching element current detection means and outputs an output when the output of the switching element current detection means is greater than the output of the multiplier; and an oscillating means, wherein the switching element is turned on by the output of the oscillating means. It is a forward type switching power supply that is turned off by the output of the device.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention as defined in claim 1 is a rectifier for full-wave rectifying a commercial power supply as an input power supply, an input voltage detecting means for detecting a voltage of the input power supply, and an output error voltage detecting means. A multiplier that receives an output of the input voltage detection unit and the output of the output error voltage detection unit and outputs a product; a switching element; a switching element current detection unit that detects a current of the switching element; A comparator that compares the output with the output of the switching element current detection means and outputs an output when the output of the switching element current detection means is greater than the output of the multiplier; and
The switching element is turned on by the output of the oscillating means and turned off by the output of the comparator, so that even in a forward type switching power supply, the input current from the commercial power supply approaches a sine wave, and harmonic current countermeasures are taken. Can be done.

According to a second aspect of the present invention, an on-duty limiting means is provided, and the switching element is turned off by the output of the comparator or the output of the on-duty limiting means, so that the operating characteristic during a period when the voltage of the commercial power supply is low is improved. It's something to improve.

According to a third aspect of the present invention, the saturation of the switching transformer is prevented by setting the maximum value of the on-duty to 50% or less.

According to a fourth aspect of the present invention, an overcurrent of the switching element is prevented and an inrush current is suppressed by providing clipping means for clipping the output of the multiplier.

[0009]

FIG. 1 is a circuit diagram of a first embodiment of the present invention. Parts having the same configuration as the conventional boost type are given the same numbers. The commercial power supply 1 is rectified by the rectification means 2,
The ripple of the switching current is smoothed by a low-capacity capacitor 25 and full-wave rectified to obtain an input power. The input power supply switches current with the switching element 5 through the forward type switching transformer 29. When the switching element 5 is turned on, the smoothing capacitor 3 is passed through the smoothing coil 31 from the high-speed switching diode 30 on the secondary side.
2, a current flows. The off period of the switching element 5 is
A current flows from the high-speed switching diode 33 to the smoothing capacitor 32 through the smoothing coil 31. The voltage smoothed by the smoothing capacitor 32 is applied to the load 8 to output a DC current. Excitation energy left in the switching transformer 29 during the off period of the switching element 5 is regenerated from the high-speed switching diode 34 to the capacitor 28. Next, a control method for taking measures against harmonic current distortion will be described. The input voltage detector 9 that divides the full-wave rectified input voltage and the output error voltage detector 12 that amplifies the error of the output voltage from the target voltage are input to the multiplier 17 to obtain the input voltage and the output error voltage. The output is proportional to Oscillator 3
The RS flip-flop 20 is set by the one-shot circuit 36 driven by the output of the switching element 5, and the switching element current detecting means 18 flowing through the switching element 5
Is higher than the output voltage of the multiplier 17, the RS flip-flop 20 is reset by the comparator 19, and the switching element 5 is turned off.

FIG. 2 shows the voltage of each part. The output waveform of the multiplier 17 is a full-wave rectified waveform having a low voltage like 37 when the output voltage is high, and a high voltage like 38 when the output voltage is low. Reference numeral 39 denotes an output voltage of the switching element current detecting means 18 when the output of the multiplier 17 is 37. When the peak current is connected, it becomes a half sine wave.

(Embodiment 2) FIG. 3 is an enlarged view of a portion where the voltage of the input power supply in FIG. 2 is low. If the power supply voltage is low, the increase in current is small, the output of the switching element current detecting means 18 does not exceed the output of the multiplier 17 within the oscillation cycle of the oscillator 35, and the switching element continues to be turned on for a few cycles. An abnormal sound is generated because the frequency is disturbed.

FIG. 4 is a circuit diagram of a second embodiment of the present invention, in which this measure has been taken. The on-duty limiting means 41 is formed by inverting the output of the oscillator 35 by an inverter 40 and inputting the inverted signal to the RS flip-flop 20. Other parts are the same as those in FIG.
By providing the on-duty limiting means 41, even if the switching element current does not reach the specified value, the oscillator 35
Becomes low, the output of the RS flip-flop 20 becomes low, the switching element 5 is turned off, and the operation can always be performed at the frequency of the oscillating means 35, and the occurrence of abnormal noise is eliminated. Here, the maximum value of the on-duty of the switching element 5 is the duty of the oscillator 35. If the on-duty exceeds 50%, the reset current of the transformer may not completely flow during the off-period, but the saturation of the switching transformer can be prevented by setting the maximum value of the on-duty to 50% or less.

FIG. 5 shows a commercial power supply voltage waveform 42 and an input current waveform 43 from the commercial power supply of the switching power supply of the present invention. Although there is a period during which no current flows when the voltage of the commercial power supply is low, there is an effect that the harmonic current regulation value is sufficiently satisfied. The output voltage waveform output to the load 8 is shown at 44. Where the input voltage and current from the commercial power supply 1 are large, the input power is greater than the average value, so the output voltage rises and falls where the input voltage and current are small. Since the input power changes during one cycle of the commercial power supply, the output voltage includes ripples,
There may be no problem depending on the type of load such as a heater. The ripple can be reduced by increasing the capacity of the smoothing capacitor 32.

(Embodiment 3) FIG. 6 is a circuit diagram of a third embodiment of the present invention. By providing a clipping means 47 comprising a resistor 45 for clipping a voltage and a Zener diode 46 between the output of the multiplier 17 and the input of the comparator 19, the upper limit of the switching element current is determined.
When the load current becomes overcurrent or the output voltage of the switching power supply does not rise to the specified value when the voltage of the commercial power supply is low, the current flowing through the switching element 5 may exceed the rating of the element and break down. By providing the clip means 47, destruction can be prevented.

Although the output error voltage detecting means 12 is not insulated from the primary and the secondary in the above, it can be insulated by inserting a photocoupler 48 as shown in FIG. Since the capacitor 25 has a low capacitance, the switch 4
Even if the switch 9 is turned on, an inrush current of energy for welding the switch 49 does not flow. In addition, the output voltage is zero at startup and the error from the target voltage is maximum, but the peak current of the switching element 5 is limited by the clipping means 47, so that the rush current for charging the output capacitor 32 at startup is also suppressed. Is done.

[0016]

According to the first aspect of the present invention, the peak value of the switching element current is set to a waveform of the product of the input voltage and the output error voltage, so that the forward type switching power supply can be switched from the commercial power supply. The input current of the above can be approximated to a sine wave to take measures against harmonic currents.

According to a second aspect of the present invention, the on-duty limiting means is provided, and the switching element is turned off by the output of the comparator or the output of the on-duty limiting means to thereby improve the operation characteristics during a period when the voltage of the commercial power supply is low. It's something to improve.

According to a third aspect of the present invention, the saturation of the switching transformer is prevented by setting the maximum value of the on-duty to 50% or less.

According to a fourth aspect of the present invention, an overcurrent of the switching element is prevented and an inrush current is suppressed by providing clipping means for clipping the output of the multiplier.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a switching power supply according to a first embodiment of the present invention;

FIG. 2 is a waveform diagram of output voltages of a multiplier and a switching element current detecting means.

FIG. 3 is a waveform diagram of output voltages of a multiplier and a switching element current detecting means.

FIG. 4 is a circuit diagram of on-duty limiting means of a switching power supply according to a second embodiment of the present invention;

FIG. 5 is a diagram showing a commercial power supply voltage waveform, an input current waveform, and an output voltage waveform.

FIG. 6 is a circuit diagram of clipping means of a switching power supply according to a third embodiment of the present invention.

FIG. 7 is a circuit diagram of an insulation type output error voltage detection unit pressure.

FIG. 8 is a circuit diagram of a conventional input circuit of a power supply that does not take measures against harmonic currents.

FIG. 9 is an input current waveform diagram of the same.

FIG. 10 is a circuit diagram of a conventional switching power supply with harmonic current countermeasures.

FIG. 11 is a waveform chart of each part.

FIG. 12 is an input current waveform diagram of the same.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Commercial power supply 2 Rectifier 5 Switching element 9 Input voltage detecting means 12 Output error voltage detecting means 17 Multiplier 18 Switching element current detecting means 19 Comparator 20 RS flip-flop 29 Switching transformer 35 Oscillator 41 On-duty limiting means 47 Clipping means

Claims (4)

[Claims] 1. A rectifier for full-wave rectifying a commercial power supply as an input power, an input voltage detector for detecting a voltage of the input power, an output error voltage detector, the input voltage detector, and the output error. A multiplier that receives the output of the voltage detection unit as an input and outputs a product; a switching element; a switching element current detection unit that detects the current of the switching element; and an output of the multiplier and an output of the switching element current detection unit. A comparator that outputs an output when the output of the switching element current detection means is larger than the output of the multiplier; and an oscillation means, wherein the switching element is turned on by the output of the oscillation means and turned off by the output of the comparator. Switching power supply. 2. The switching power supply according to claim 1, further comprising an on-duty limiter, wherein the switching element is turned off by an output of the comparator or an output of the on-duty limiter. 3. The switching power supply according to claim 2, wherein the on-duty limiting means sets the maximum value of the on-duty to 50% or less. 4. The switching power supply according to claim 1, further comprising clip means for clipping an output of the multiplier.