JP3490270B2 - DC-DC switching converter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flyback type DC-DC switching converter which is compatible with an electronic device using a commercial power supply as an input and mainly reduces the generation of harmonic current and improves the power factor.

[0002]

2. Description of the Related Art Since there are a large number of electronic devices that directly rectify and smooth a commercial power source and use it as a power source, a harmonic current is generated in the commercial power source line or the instantaneous peak value of the current increases. As a result, there is a problem that the power factor of the electronic device is reduced, and a measure for suppressing harmonic current in the electronic device is desired. FIG. 4 is a circuit diagram showing a conventional flyback DC-DC switching converter. In FIG. 4, 1 and 2 are input terminals and 3 and 4
Is an output terminal, 5 is an error amplifier, 7 is a photo coupler, and 8
Is a pulse width modulation circuit. To the input terminals 1 and 2, the output of the pulsating current voltage of the full-wave rectifier circuit 10 connected to the commercial power supply AC is applied.

The primary winding L1 of the insulated flyback transformer 9 and the MOS transistor Q1 which is a switching element are connected in series, and one end of the primary winding L1 is connected to the input terminal 1
Connect to. The transistor Q1 is controlled by the output of the pulse width modulation circuit 8 applied to the gate. Comparator 6
The triangular wave oscillating circuit 22 constitutes the pulse width modulating circuit 8. A rectifier diode D1 and a smoothing capacitor C1 are connected to the secondary winding L2 of the flyback transformer 9, and further output terminals 3 and 4 are connected. The output terminals 3 and 4 are also the output terminals of a rectifying circuit formed by the rectifying diode D1 and the smoothing capacitor C1.

In the error amplifier 5 to which the feedback capacitor C2 is connected, the reference voltage V _REF1 is applied to one input terminal, and the other input terminal is a dividing resistor R between the output terminals 3 and 4.
2, connected to R3, detects an error from the reference set value of the output, and applies the error voltage to the pulse width modulation circuit 8 via the photo coupler 7. The error amplifier 5 is a capacitor C
2 has a low-pass characteristic of 10 Hz or less. The ON time of the transistor Q1 is controlled by the pulse width modulation circuit 8 based on the error voltage, so that the DC voltage set between the output terminals 3 and 4 is obtained. Note that this converter is called a current discontinuous mode flyback type, and it is necessary that the waveform of the current flowing through the switch element always stays within a triangular range.

Next, the technical problem in the converter of FIG. 4 will be described with reference to FIG. FIG. 5 is a current / voltage waveform diagram in FIG. 4, where I _PA is the current passing through the transistor Q1 from the primary winding L1 and V _PB is the pulse voltage applied from the pulse width modulation circuit 8 to the transistor Q1. Further, FIG. 5A shows a case where the load connected to the output terminals 3 and 4 is light, and FIG. 5C shows a case where the load is heavy.
(B) represents intermediate cases, respectively. FIG. 5 (a),
The horizontal axes of (b) and (c) represent a common time axis, but the time t
From 11 to time t12, the time of 1/2 cycle of the commercial power supply is shown. In FIG. 5, the period of the pulse voltage V _PB is shown to be about 1/150 of the actual period, and the width thereof is shown to be 150 times as large as possible for easy understanding. Also, the error amplifier 5 is 10H
By having a low-pass characteristic of z or less,
The width of the pulse voltage V _PB at the time of the cycle does not change.

The peak value I _PAP of the current I _PA is _calculated by the transformer 9
Is L, the instantaneous value of the pulsating current voltage of full-wave rectification, which is the input voltage applied between the input terminals 1 and 2, is V, and the pulse width of the pulse voltage V _PB is t. It I _PAP = V × t / L (1) Since the pulse width t is constant, the peak value I _PAP is proportional to the instantaneous value V, and the envelope indicated by the dotted line is the pulsating current voltage of the full-wave rectification that is the input. It will be proportional to the sine waveform. Also,
Although not shown in FIG. 4, the current I _PA is averaged by the small-capacity capacitor provided in the full-wave rectifier circuit 10 to form a chain line in FIG. 5, which is the input current I _{IN of the} converter.
Becomes This input current I _IN is 1/4 of the envelope level.
In the following, it is almost similar to the envelope curve, and the input current I _{IN of the} converter is proportional to the rectified pulsating flow, that is, the waveform of the commercial power source, and the suppression of the harmonic current and the improvement of the power factor are achieved. The dotted line continuous with the current I _PA represents the current I _S flowing through the secondary winding L2 of the flyback transformer 9. The average value of the current I _S becomes the output current value from the output terminals 3, 4 of the converter.

The following technical problems become apparent from FIG.
In order for the envelope of the peak value I _PAP of the current I _PA to be proportional to the pulsating current voltage of full-wave rectification, the current I _PA is as shown in FIGS.
It is necessary that the waveform be triangular. Figure 5
The fact that the current I _PA has a triangular shape in (a) and (b) means that the distribution angle of the current I _PA is 50% or less. Therefore, the input of the converter for converting the primary side of the transformer 9 to the secondary side. The current I _IN is an average value of the current I _PA indicated by the solid line, and is 1/4 or less of the envelope value of the peak value I _PAP . That is, the peak value I _PAP of the current passing through the switch element is 4 times or more the input current I _IN , and a large current rating of the switch element is required. Further, when the load is as shown in FIG. 5 (C) of large output becomes heavy is required, the current I _PA current I _PA1 since flow angle of the current I _PA spreads wider range of pulse voltage V _PB, current I _PA2 As described above, it starts from the residual value of the current I _S. Current I _PA
The shape is not a triangle (1) collapses the relationship type, the envelope of the peak value I _PAP of the current I _PA is no longer proportional to the sine wave of the pulsating flow voltage of the input, the average value of the current I _PA converter Since the input current I _{IN of is} also not proportional to the pulsating current voltage, it becomes difficult to suppress the harmonic current and the power factor also decreases. Further, the converter of FIG. 4 does not have the function of detecting the current I _PA or the peak value I _PAP to control the switch element, so that the width of the pulse voltage V _PB for controlling the switch element is changed by noise or the like to change the width of the switch element. Current I _PA flowing through the switch increases, and the switch element is easily damaged.

[0008]

The object of the present invention is to reduce the peak value of the current passing through the switch element, to suppress the generation of harmonics even when the current waveform is not triangular, and to switch due to noise or the like. An object of the present invention is to provide a DC-DC switching converter that solves the conventional technical problems such as the element not being damaged.

[0009]

SUMMARY OF THE INVENTION According to the present invention, a primary side winding of an insulation type flyback transformer and a switch element are connected in series, and a pulsating voltage for full-wave rectification of a commercial power source is connected to the primary side winding. In addition, in the flyback type DC-DC switching converter that obtains a set DC voltage from the output terminal of the rectifier circuit connected to the secondary winding of the flyback transformer by controlling the switch element, The start of the pulse to be turned on is performed in a fixed cycle, and the width of the pulse is the first voltage which is the voltage corresponding to the current passing through the switch element, and the sawtooth shape which rises at a constant slope in synchronization with the start. It is set by the time until the sum of the second voltage, which is the wave voltage, and the third voltage, which is the error voltage from the error amplifier connected to the output terminal, reaches the reference voltage. It is characterized in.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The start cycle of a pulse from a control circuit for turning on a switch element is fixed, and in the control circuit, a first voltage which is a voltage corresponding to a current passing through the switch element turned on by the pulse Voltage, a second voltage that is a sawtooth wave voltage that rises at a constant slope in synchronization with the start, and a third voltage that is an error voltage from an error amplifier connected to the output terminal, The pulse is extinguished when compared to the reference voltage and the sum voltage reaches the reference voltage.

[0011]

1 is a circuit diagram showing an embodiment of a DC-DC switching converter according to the present invention. The same parts as those in FIG. 4 are designated by the same reference numerals. To the input terminals 1 and 2, the output of the pulsating current voltage of the full-wave rectifier circuit 10 connected to the commercial power supply AC is applied. Between the input terminals 1 and 2, the primary winding L1 of the insulation type flyback transformer 9 and the MOS transistor Q which is a switching element.
1, the resistor R1 is connected in series, and the primary winding L
One end of 1 is connected to the input terminal 1. Transistor Q1
Are controlled by pulses from the control circuit 13 applied to the gate.

The control circuit 13 has a latch circuit 12, an oscillation circuit 21, a sawtooth wave generation circuit 11, and a comparator 6.
The output of the latch circuit 12 is applied to the gate of the transistor Q1. In the control circuit 13, the oscillation circuit 21 sets the latch circuit 12 at a constant cycle, and the sawtooth wave voltage rising from the set time at a predetermined slope is generated from the sawtooth wave generation circuit 11. The reference voltage V _REF2 is applied to one input terminal of the comparator 6, and the sum of the following three voltages is applied to the other input terminal.

The first voltage is the voltage generated by the resistor R1 and corresponds to the level of the current passing through the transistor Q1 from the input terminal 1 through the primary winding L1 and is converted into a voltage. The second voltage is a sawtooth wave voltage whose slope and rise are synchronized with the first voltage, and the third voltage
Is the error voltage from the error amplifier 5. Comparator 6
Is the reference voltage V _REF2 which is the sum of the first, second and third voltages.
And the latch circuit 12 is reset when the sum voltage reaches the reference voltage V _REF2 . As a result, the pulse that turns on the transistor Q1 generated when the latch circuit is set disappears. The start cycle of the first voltage is
It is determined by the set cycle of the latch circuit 12, and the cycle is the oscillation cycle of the oscillator circuit 21. As described above, the cycle of the second voltage is the same as the cycle of the first voltage.

A rectifier diode D1 and a smoothing capacitor C1 are connected to the secondary winding L2 of the transformer 9, and output terminals 3 and 4 are further connected. The error amplifier 5 connected to the feedback capacitor C2 has a reference voltage V at its non-inverting input terminal.
_REF1 is added, and the inverting input terminal is connected to the dividing resistors R2 and R3 between the output terminals 3 and 4, and the error from the actual set value of the DC voltage is detected, and the error voltage is passed through the photocoupler 7. It is added to the control circuit 13. The error amplifier 5 has a low-pass characteristic of 10 Hz or less due to the capacitor C2. Then, the width of the pulse for turning on the transistor Q1 is controlled by the control circuit 13 as described above, so that the DC voltage set between the output terminals 3 and 4 is obtained.

The operation of the converter of FIG. 1 will now be described in more detail with reference to FIG. FIG. 2 is a voltage waveform diagram in FIG. 1, where the horizontal axis is a common time axis.
From time t1 to time t10, 1/2 cycle time of the commercial power supply is shown. FIG. 2A shows a waveform of a pulsating current voltage V _IN obtained by full-wave rectification from a commercial power source in a 1/2 cycle, and FIG. 2B shows a waveform of a sawtooth voltage V _TE . FIG. 2C shows a voltage V _R corresponding to the current I _P passing through the transistor Q1 generated in the resistor R1 and a sawtooth voltage V.
_{The figure} shows how the sum of _TE and the error voltage V _ER reaches the reference voltage V _REF2 and is reset, and the voltage V _S corresponding to the current I _S flowing through the secondary side of the transformer is related to the voltage V _R. It is displayed. FIG. 2D shows the waveform of the pulse voltage V _P that turns on the transistor Q1. The error voltage V _ER during the 1/2 cycle of the commercial power supply does not change due to the low-pass characteristic of the error amplifier 5.

In FIG. 2C, the slope of the voltage V _R corresponding to the current I _P converted from the input terminal 1 to the output through the primary winding L1 of the transformer 9 and the transistor Q1 is expressed by the equation (1). The slope becomes steep when the instantaneous value of the full-wave rectified pulsating current voltage V _IN is high. At time t1 when the phase of the pulsating current voltage V _IN is 0 ° and the voltage V _IN is small, the rising slope of the voltage V _R is gentle, and _therefore the sum of the voltage V _R , the sawtooth wave voltage V _TE , and the error voltage V _ER . Is the reference voltage V _REF2
The time to reach time t2 strongly depends on the slope of the sawtooth voltage _VTE . Time t3 when the voltage V _IN slightly increases
Then, the rising slope of the voltage V _R slightly increases as compared with the case at the time t1, but the contribution of the slope of the voltage V _TE is still strong until the time t4 when the sum voltage reaches the reference voltage V _REF2 .

Next, at time t5 when the voltage V _IN is a little larger.
Then, the rising slope of the voltage V _R becomes steep, and the time until the time t6 when the sum voltage reaches the reference voltage REF2 depends on the slope of the voltage V _R rather than the voltage V _TE .
At time t7 near the phase where the pulsating current voltage V _IN is 90 ° and where the voltage V _IN is the largest, the slope of the voltage V _R becomes steeper, and the time t8 when the sum voltage reaches the reference voltage V _REF2.
Time of up to depends on the slope of the strong voltage V _R. Phase 90
Even from 0 ° to 180 °, the envelope shown by the dotted line connecting the peak values of the voltage V _R by the same series of operations corresponds to the envelope of the peak of the current I _P passing through the transistor Q1 and is the input. It approximates the pulsating current voltage V _IN for wave rectification.
The current I _IN represented by the alternate long and short dash line in FIG. 2C is the average value of the current I _P obtained by the small-capacity capacitor provided in the rectifier circuit 10 of the commercial power supply, which is not shown, and is the input current of the converter. It corresponds to I _IN and approximates the shape of voltage V _IN .

As described above, the time at which the sum of the three voltages of the voltage V _R corresponding to the current I _P passing through the transistor Q1, the sawtooth voltage V _TE , and the error voltage V _ER reaches the reference voltage V _REF2. the time until the effect of the inclined portion of the voltage V _R,
That is, the influence of the instantaneous value of the input pulsating current voltage V _IN is directly reflected from the equation (1), and as a result, the input current I _{IN of the} converter is increased.
Can be approximated to a sine wave of the input pulsating voltage V _IN . FIG. 3 is another voltage waveform diagram in FIG. 1, showing a case where the load connected to the output terminals 3 and 4 becomes heavy and the output current increases. Since the error voltage V _ER becomes smaller and the secondary current I _S remains when the phase of the voltage V _IN is 0 °, the waveform of the current I _IN is slightly distorted near the phase of 0 ° and 180 °. Close to the waves. In this way, the input current I _IN can be approximated to the sine wave of the pulsating current voltage V _IN of the commercial power source regardless of the load state, and the harmonic current can be suppressed and the power factor can be improved.

[0019]

As described above, the DC-DC of the present invention
The switching converter can make the input current of the converter substantially sinusoidal by controlling the on-time of the switch element connected in series to the primary winding of the flyback transformer. This is a first voltage corresponding to the current passing through the switch element for adjusting the width of the pulse that turns on the switch element, and a second voltage that is a sawtooth voltage whose slope does not change in synchronization with the start of the pulse. And the third voltage, which is the error voltage from the error amplifier,
This can be achieved by extinguishing the pulse when the summed voltage reaches the reference voltage.

The converter of the present invention suppresses the generation of harmonics regardless of the load condition and improves the power factor of the converter. However, in the process of approximating the input current to the input pulsating current voltage, it passes through the switch element. The current does not have to be triangular,
The distribution angle of the passing current can be 50% or more. And
Since the ratio of the peak current passing through the switch element to the average value, that is, the input current of the converter can be reduced, there is an advantage that the current rating of the switch element can be reduced. Also, even if the primary side current may start from the residual value of the secondary side current of the transformer, the current passing through the switch element does not necessarily have a triangular shape, so Both the envelope of the peak value and the input current of its average value have little deviation from the approximation of the sinusoidal wave of the input pulsating current voltage, and the suppression of harmonics and the power factor are less likely to decrease. Furthermore, since the peak current of the switch element is included in the voltage of the three sums and detected to eliminate the pulse,
Even if the peak width increases due to the pulse width being changed by noise or the like, this increase is detected and the pulse disappears, so that the switch element is not damaged. DC of the present invention
The DC switching converter does not require a circuit such as an active filter that suppresses harmonics, and
It is extremely practical because it suppresses harmonics and improves the power factor with only a single stage converter circuit and solves all the conventional technical problems.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a DC-DC switching converter of the present invention.

FIG. 2 is a voltage waveform diagram in FIG.

FIG. 3 is another voltage waveform diagram in FIG.

FIG. 4 is a circuit diagram of a conventional switching converter.

5 is a current and voltage waveform diagram of FIG.

[Explanation of symbols]

5 Error amplifier 6 comparator 9 Flyback transformer 13 Control circuit Q1 MOS transistor

Claims (3)

(57) [Claims] 1. A primary side winding of an insulation type flyback transformer and a switching element are connected in series, and a pulsating voltage for full-wave rectification of a commercial power source is applied to the primary side winding to control the switching element. As a result, in the flyback type DC-DC switching converter that obtains the set DC voltage from the output terminal of the rectifier circuit connected to the secondary winding of the flyback transformer, the start of the pulse that turns on the switch element is fixed. And the width of the pulse is a first voltage which is a voltage corresponding to a current passing through the switch element, and a second voltage which is a sawtooth wave voltage which rises at a constant slope in synchronization with the start. And DC-D, which is set by the time until the sum voltage of the third voltage, which is the error voltage from the error amplifier connected to the output terminal, reaches the reference voltage. C switching converter. 2. A primary side winding of an insulation type flyback transformer and a switching element are connected in series, and a pulsating current voltage of full-wave rectification of a commercial power source is applied to the primary side winding, and a switching element is controlled by a control circuit. Of the flyback type DC-DC that obtains a set DC voltage from the output terminal of the rectifier circuit connected to the secondary winding of the flyback transformer by controlling
In the switching converter, the start of the pulse from the control circuit that turns on the switch element has a fixed cycle, and the control circuit synchronizes the first voltage, which is the voltage corresponding to the current passing through the switch element, with the start. The sum voltage of the second voltage, which is the sawtooth wave voltage rising at a constant slope, and the third voltage, which is the error voltage from the error amplifier connected to the output terminal, is compared with the reference voltage, and the sum voltage is A DC-DC switching converter, wherein the pulse disappears when a reference voltage is reached. 3. The control circuit has an oscillating circuit, a sawtooth wave voltage generating circuit, a comparator and a latch circuit, and one input terminal of the comparator has a first voltage, a second voltage and a third voltage. The DC-DC switching converter according to claim 2, wherein a voltage that is the sum of the voltages is applied, and a reference voltage is applied to the other terminals.