JP2002330585A - Dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To constitutes a DC-DC converter, which has a wide voltage-control range, and has a linear-control characteristic. SOLUTION: Output of a smoothing filter of the forward converter is made as the output of the DC-DC converter, by having a constitution to PWM-control both converters with one semi-conductor switch: using a reactor having a plurality of windings as the transformer for a forward converter; the reactor is used also as a flyback converter; and the output of the flyback converter is injected into the circuit of the primary winding of the transformer or the secondary circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter for converting a DC voltage into another DC voltage.

[0002]

2. Description of the Related Art Bipolar transistors and power MOSs
D using a semiconductor switch for power conversion such as FET
C-DC converters have been recognized for their usefulness such as small size, light weight, and high efficiency, and are widely used as independent DC power supplies or as DC power supplies incorporated in various electronic devices. The DC voltage supplied from the DC-DC converter is also wide. As the low voltage, there is power supply to the semiconductor IC. 5V
Has become the standard voltage, but recently the supply of 3.5 V has been increasing. In the near future, there is a trend toward low voltage power supply to 1V and below. As the high voltage, there is a voltage of tens of thousands of volts for supplying power to a television / CRT or a display (CRT) of a personal computer.

The most characteristic function of a DC-DC converter is to control an output voltage by turning on and off an input voltage using a semiconductor switch. The effect of this switching is that the power loss in the process of power conversion is small. This means that the efficiency of power conversion is high,
In addition, the capability of the heat radiation system such as the cooling fins may be small, which leads to downsizing and weight reduction of the power supply device. It is also contributing to environmental issues that have recently been highlighted from the perspective of energy conservation. There is a strong demand for DC-DC converters to further enhance their effects. One approach to addressing this is to improve the use of components. This is expected to reduce the size and weight of parts, and also has the effect of reducing costs.

Next, this approach will be specifically described. These are the current status of DC-DC converters and are the problems they have. Due to the switching operation of the DC-DC converter, the input power is in an intermittent pulse train. The width and amplitude of each pulse are controlled by a DC-DC converter. The average value of this pulse train is the input power. The output voltage of the DC-DC converter is converted by controlling the width of the input pulse. If the voltage of the input DC power supply is constant, the pattern of the power pulse and the pattern of the flowing current are similar. Generally, in order to increase the utilization of the main circuit components of the DC-DC converter, the current pattern is made into a square wave as shown in FIG. The amplitude of the square wave current pulse is I and the width is D × T. Here, T is a switching cycle. D is the ratio of the period during which the semiconductor switch is on to the period T, that is, the period during which current is flowing, and is hereinafter referred to as the time ratio. The magnitude of D is 0 ≦ D ≦ 1. When the duty ratio D is expanded to the maximum, that is, the amplitude of the current D = 1 becomes the average current Iav. The relationship between the amplitude of the square-wave pulse current and the average current is I = Iav / D, and as the duty ratio D increases, the amplitude I of the input current decreases. Semiconductor switches, transformer windings, etc.
Is lower, that is, as the duty ratio D is increased, a component having a smaller current rating can be used. However, at present, there are restrictions on the circuit, and the duty ratio D cannot be sufficiently widened. This restriction is described below.

[0005] Fig. 8 shows a single-stone forward converter widely used among DC-DC converters. The switching control of the semiconductor switch 21 converts the voltage Ein of the DC power supply 1 which is the input power supply into an output voltage Eout in which the level is changed, the accuracy is improved, or the input and output are insulated, and the DC voltage 4 is supplied to the DC load 4. The power conversion process will be described below.
The input voltage Ein is turned on / off by the semiconductor switch 21 and applied to the primary winding np of the transformer 220. While the semiconductor switch 21 is on, the polarity of the voltage induced in the transformer 220 is as shown in the figure. Power input from the primary winding np is output from the secondary winding nf and applied to a smoothing filter including a reactor 27 and a capacitor 28. The excitation energy stored in the transformer 220 during the on-period of the semiconductor switch 21 is applied to the winding n during the off-period of the semiconductor switch 21.
The diode 24 is energized by a voltage having a polarity opposite to that shown in FIG.

The control device 3 turns on the semiconductor switch 21,
An off signal is generated to control the output voltage Eout. The output voltage Eout, which is the object of constant voltage control, is fed back and compared with a reference voltage 31 by an error amplifier 32 to generate a modulation signal corresponding to the error. The modulation signal is compared with a high-frequency triangular wave signal 33 by a comparator. The signal is converted into a pulse signal of which the pulse width is controlled by inputting the signal into a pulse signal, and if necessary, the semiconductor switch 21 is driven by insulation. The duty ratio D is controlled by changing the ON and OFF periods of the generated pulse signal. The configuration of the control device 3 is a general one, and has recently been put on the market as an IC-based control component.

The duty ratio D is restricted, and not all of the duty ratio D up to 1, which is the theoretical maximum value, can be used for control.
If the transformer 220 is an ideal transformer, it does not store the excitation energy. However, as an actual transformer, the electromagnetic energy required for excitation is stored in the magnetic core of the transformer. Must be released every cycle.
Accordingly, the excitation energy is released once per cycle, and the current waveform becomes a triangular pulse. Since the process of releasing the stored energy is performed while the semiconductor switch 21 is off, the off period (1-D) × T cannot be reduced to zero. The rest of the period required for exciting energy release can be used for controlling the width of the current pulse. As a result, the allowable variable range of the duty ratio D becomes narrow. Generally, an off-period ratio (1-D) of about 0.5 is secured, so that the range of the duty ratio that can be used for voltage control is 0 ≦ D ≦ 0.
It is limited to about 5.

The output voltage E of the DC-DC converter
The relationship between out and the duty ratio D is as follows. Eout = (nf / np) × D × Ein (1) The characteristic of the equation (1) is illustrated as a curve x1 in FIG. As shown in the figure, the upper limit of the practical range of the duty ratio D is limited to about 0.5. The slope of the curve is constant. That is, the output voltage Eout has a magnitude proportional to the duty ratio D. Since the upper limit of the duty ratio D is small, the maximum value of the output voltage Eout is low. In order to raise the output voltage Eout to a required level, it is necessary to design the ratio nf / np of the number of windings of the transformer 220 to be large, which results in an undesirable result of an increase in winding loss.

As another example of a DC-DC converter, there is a flyback converter using a reactor for storing energy shown in FIG. While the semiconductor switch 21 is on, a voltage of the illustrated polarity is induced, and the reactor 220
A current flows through the zero winding np, where electromagnetic energy corresponding to the excitation energy of the transformer is stored. When the semiconductor switch 21 is turned off, a voltage having a polarity opposite to that shown in the figure is induced.
The diode 24 is energized and the capacitor 23
Current flows to release the electromagnetic energy stored during the ON period. The voltage of the capacitor 23 becomes the output voltage Eout of the flyback converter. The control device 3 having the same configuration as the example in FIG. 8 can be used.

This flyback converter is a reactor 2
It operates without releasing all the energy stored in 200 every cycle. That is, it operates even if there is residual energy after one cycle. This control operation is referred to as a reactor current continuous mode (for example, “Basics of Switching Converter” by Harada and Ninomiya, pp. 48).
-52, Corona 1992, 2, 25 first edition). In order to allow a direct current to flow through this reactor, the maximum magnetic flux density of the core is designed to be low so as not to cause magnetic saturation. This reactor 220
When controlling so that a part of the excitation energy remains at 0, the output voltage changes according to the duty ratio. In the case of x1 in FIG. 2 in which the residual energy is made zero every one cycle, the voltage of the DC power source that emits the excitation energy is a constant value Ei.
n, which does not change with duty.
The output voltage E when the excitation energy remains
The relationship between out and the duty ratio D for turning on and off the semiconductor switch 21 is as shown in Expression (2). Eout = (nr / np) × D / (1-D) × Ein (2)

The voltage control characteristic of the equation (2) is represented by a curve x2 in FIG.
become that way. The upper limit of the duty ratio D can be used wider than the forward converter. However, the slope of the curve depends on the value of D. That is, there is a problem that control is difficult because the characteristics are nonlinear. For example, if the amplification factor of the voltage feedback control (the amplification factor of the error amplifier 32 in FIG. 8) is set high in order to increase the accuracy of the output voltage in the range where the duty ratio D is small, this amplification factor is high in the region where the duty ratio D is large. If it is too high, stable control cannot be performed. On the other hand, if the amplification factor is set low so that the control can be stably performed in the region where the duty ratio D is large, the amplification factor becomes insufficient in the region where the duty ratio D is small, and the accuracy of the output voltage cannot be obtained. Since the voltage control characteristic is non-linear as described above, it is difficult for the flyback converter to achieve both control operation stability and high-accuracy voltage, and if both are to be achieved, the control device becomes complicated. Furthermore, since current is passed through the reactor 2200 to temporarily store all of the energy to be converted as electromagnetic energy, the efficiency is low and the volume of the reactor 2200 increases. Therefore, the flyback converter is not suitable for a converter having a large power conversion capacity.

[0012]

SUMMARY OF THE INVENTION A DC-DC converter having a wide variable range of the duty ratio D for voltage control and having a linear relationship between the voltage and the duty ratio is constructed.

[0013]

SUMMARY OF THE INVENTION As a transformer of a forward converter, a reactor designed to have a low magnetic flux density in a core is used. The excitation energy stored in this transformer is released using a flyback converter. The flyback converter is operated in a so-called reactor current continuous operation mode in which excitation energy is always present in the reactor. By using this mode, the period required for energy release is reduced, and as a result, the maximum value of the duty ratio D that can be effectively used for voltage control is increased. Further, the relationship between the duty ratio D and the output voltage is made closer to a linear characteristic by making the winding used for the flyback converter among the plurality of windings constituting the transformer relatively small.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An exciting energy stored in a transformer of a forward converter is converted by a flyback converter using a reset winding to charge a capacitor. Then, the voltage of this capacitor is superimposed on the voltage of the DC power supply and input to the primary winding of the transformer, or is superimposed on the voltage of the secondary winding of the transformer. The flyback converter operates in a reactor current continuous operation mode. When the voltage of the capacitor is superimposed on the primary winding side of the transformer, the number of reset windings is designed to be smaller than the number of primary windings,
When the voltage of the capacitor is superimposed on the secondary winding side of the transformer, the number of reset windings is designed to be smaller than the number of secondary windings to make the control characteristics close to linear.

[0015]

FIG. 1 shows a first embodiment of the present invention. The multi-winding transformer 22 is used as a transformer of a forward converter and also as a reactor for storing electromagnetic energy of a flyback converter. Primary winding np and 2
The next winding nf functions as a transformer. That is, nf
During the period in which the current is flowing, the corresponding current is np
Flows to Further, the primary winding np and the reset winding nr function as a reactor. No current flows to nr and np at the same time, and the current flows to nr after the current of np becomes zero. Transformer primary winding np and capacitor 2
3 and the semiconductor switch 21 form a series circuit, and a voltage Ein from the DC power supply 1 is applied to the series circuit. Further, a series circuit of a winding nr of a transformer and a diode 24 is connected in parallel to the capacitor 23. A series circuit of the secondary winding nf of the transformer and the diode 25 is connected in parallel with a smoothing filter composed of a reactor 27 and a capacitor 28. A diode 26 is also connected to the smoothing filter in parallel. The load 4 is supplied with the voltage Eout of the capacitor 28, which is the output of the smoothing filter.

Next, the operation of the circuit will be described. When the semiconductor switch 21 is turned on, the sum of the input DC voltage Ein and the voltage Er of the capacitor 23 is applied to the winding np of the transformer, and a voltage having the illustrated polarity is induced in each winding. The voltage induced in the winding nf of the transformer is applied to the smoothing filter, and a current flows. The voltage of the winding nr is blocked by the diode 24 and no current flows. A current corresponding to the current flowing through the winding nf and a current for exciting the transformer 22 flow through the winding np. Electromagnetic energy is stored in the transformer 22 by this exciting current. These two currents are input from the DC power supply 1 via the semiconductor switch 21.

Next, when the semiconductor switch 21 is turned off,
The polarity of the voltage induced in each winding of the transformer 22 is opposite to that shown. The current from winding nf is blocked by diode 25 and does not flow. The electromagnetic energy stored in the transformer 22 is passed through the diode 24 and the capacitor 2
Charge 3 In addition, the diode 26 is energized and a circulating current of the smoothing filter flows, so that the continuity of the current flowing through the reactor 27 is maintained. Next, when the semiconductor switch 21 is turned on again, it returns to the initial state and enters the next cycle.

When a polar electrolytic capacitor is used as the capacitor 23 in FIG. 1, a diode 231 is provided in parallel with the electrolytic capacitor to prevent the capacitor 23 from being charged to the opposite polarity as shown. Since the capacitor 23 is charged to the opposite polarity by the current flowing through the semiconductor switch 21 during a transition period in which the capacitor 23 is not charged with the polarity shown in the figure, such as when the DC-DC converter is started, a diode 231 is provided. The charging current is bypassed to prevent this.

As the control device 3, the same control device as that of the related art can be used. The semiconductor switch 21 is turned on and off by a pulse width control signal generated by sensing the output voltage Eout to form a feedback loop.

The output voltage Eout of this converter has the following relationship with the duty ratio D for controlling ON / OFF of the semiconductor switch 21 as follows. Eout = 1 / {1−nr / np × D / (1−D)} × D × nf / np × E in (3) When nr / np≪1, {1−nr / np × D /
Equation (3) can be approximated as follows, where (1−D)｝ is 1, and Eout ≒ D × nf / np × Ein (4) The output voltage Eout within the range where this approximate expression holds holds the duty ratio Dout. It has a proportional size. This equation (4) is equivalent to equation (1)
However, the duty ratio D can be made larger than that of a general forward converter.

The solid line in FIG. 2 illustrates the relationship of equation (3). nr / np = 0.3 and output voltage Eo
ut is controlled by nf / np × Ein or less, that is, nf
When / out is set to 1 and Eout is controlled within the range of Ein, the range of the duty ratio D that can be controlled is 0 ≦ D
≦ 0.57. In the conventional forward converter, the range of the linear control is 0.5 at the maximum as shown by x1, whereas in the converter of the present invention, it is larger than this.
Furthermore, if the design is made as small as nr / np = 0.1, the duty ratio D
Extends to 0.72, and the characteristics also approach a straight line. As described above, by designing the ratio between the number of primary windings and the number of reset windings of the transformer 22 and nr / np to be small, the upper limit of the duty ratio D that can be used for control is widened, and the characteristics become nearly linear.

FIG. 3 shows a second embodiment of the present invention. If there is no need to insulate between the input and output DC voltages, the primary winding np of the transformer 22 in FIG. 1 can be used as the winding nf. The embodiment of FIG. 3 shows a case where a voltage higher than the input is output, and the windings np and nr of the transformer 22 are shown.
(Np + nr + nx) obtained by adding winding nx to winding n
Diverted as f. Similar characteristics can be obtained even if the winding nf is taken out of the reset winding nr or the intermediate tap of nr. Since the number of windings of the transformer can be reduced, the size of the transformer 22 can be reduced, and the loss of the winding can be reduced, thereby improving the efficiency. The voltage control characteristics are the same as in the first embodiment.
It becomes like the solid line.

FIG. 4 shows a third embodiment of the present invention. FIG.
The first embodiment has a configuration in which a capacitor 23 is connected in series with the primary winding np of the transformer 22 and the capacitor 23 is charged with the excitation energy extracted by the reset winding nr. Connects the capacitor 23 to the transformer 2
2 in series with the secondary winding nf of
Is charged with the transformer excitation energy obtained by the reset winding nr. The output voltage Eout of the DC-DC converter has the following relationship with the duty ratio D for controlling ON / OFF of the semiconductor switch 21 as follows. Eout = {1 + nr / nf × D / (1-D)} × D × nf / np × Ein (3) When nr / nf≪1, {1 + nr / nf × D /
Equation (3) can be approximated as follows, where (1-D)｝ is set to 1: Eout ≒ D × nf / np × Ein (4) The output voltage Eout in a range where this approximate expression holds is proportional to the duty ratio D Has the size of Equation (4) has the same characteristics as equation (1).

FIG. 2 shows the characteristics of the equation (3). As compared with the solid line which is the characteristic of the first embodiment, the maximum value of the usable duty ratio is increased and the linearity is also improved.
Further, as the ratio nr / nf of the winding of the transformer 22 is reduced, the allowable change range of the duty ratio D is increased, and the characteristic becomes closer to a linear characteristic.

FIG. 5 shows a fourth embodiment of the present invention. FIG.
In this embodiment, the secondary winding nf is configured using the reset winding nr.

In the third and fourth embodiments, DC-D
If it is not necessary to insulate between the input and output of the C converter, it goes without saying that all or a part of the windings np and nr can be shared similarly to the second embodiment.

In the second to fourth embodiments as in the first embodiment, the diode 23 is connected in parallel with the capacitor 23.
Needless to say, when 1 is provided, reverse charging of the capacitor 23 can be prevented when the DC-DC converter is started.

FIG. 6 shows an application example in which the present invention is combined with another power conversion circuit. FIG. 6A shows the DC-D of the present invention.
This is an AC-DC converter provided with a rectifier on the input side of the C converter. The voltage Eacin of the AC power supply 11 is converted to DC by a rectifier and used as an input Ein of a DC-DC converter. FIG. 6B shows a DC-AC converter in which an inverter is provided on the output side of the DC-DC converter of the present invention. Output voltage Eout of DC-DC converter
Is converted into an AC voltage Eacout by an inverter and output.

Although the semiconductor switch 21 is described using a bipolar transistor symbol in the circuit diagram of the embodiment, it goes without saying that a power MOSFET, an IGBT or the like can be used as the semiconductor switch 21.

[0030]

According to the DC-DC converter of the present invention, a reactor as a transformer constituting a forward converter is used, and there is no fear of transformer saturation. The flyback converter operates in continuous current mode using the reset winding of the transformer, and converts the excitation energy to charge the capacitor. As a result, the duty ratio of voltage control can be increased. The wide use of the duty ratio can increase the utilization of the semiconductor switch and the transformer. This has the effect of reducing the size, cost, and efficiency of the DC-DC converter. Voltage control characteristics by superimposing the voltage of this capacitor on the input voltage and applying it to the primary winding of the transformer, or by superimposing the voltage of this capacitor on the induced voltage of the secondary winding of the transformer and applying it to the smoothing filter Can be approximated to linear. This contributes to an improvement in the accuracy of the output voltage and an effect of improving reliability by stabilizing the voltage control. By designing the transformer winding ratio nr / np or nr / nf to be small, the duty ratio can be increased, and the characteristics of voltage control can be made more linear, and the effect can be enhanced.

The embodiment of the converter in which the voltage of the capacitor 23 constituting the flyback converter is applied to the primary winding np of the transformer has a feature that the circuit configuration on the secondary side is simplified. For example, it is effective when the secondary side wants to reduce circuit loss as much as possible with a low voltage and large current like a converter for converting Ein48V to 3.5V. Further, when high insulation is required, for example, when power is supplied to a cathode ray tube of a television at a high voltage of tens of thousands of volts, a high effect can be obtained by simply configuring the secondary side circuit.

The embodiment of the converter in which the voltage of the capacitor 23 constituting the flyback converter is applied to the secondary winding nf of the transformer has a feature that the circuit configuration on the primary side is simplified. For example, a high effect can be obtained when it is necessary to reduce the circuit loss by simplifying the primary-side low-voltage and large-current circuit, such as a converter for converting a battery voltage of 12 V for an automobile into 100 V for an electronic device.

[Brief description of the drawings]

FIG. 1 shows a first embodiment of the present invention.

FIG. 2 is a graph showing characteristics of an output voltage to a time ratio of an embodiment of the present invention.

FIG. 3 shows a second embodiment of the present invention.

FIG. 4 shows a third embodiment of the present invention.

FIG. 5 shows a fourth embodiment of the present invention.

FIG. 6 shows an application example of the present invention.

FIG. 7 shows a pattern of an input current of the DC-DC converter.

FIG. 8 shows a forward converter as a conventional example of a DC-DC converter.

FIG. 9 shows a flyback converter as a conventional example of a DC-DC converter.

[Explanation of symbols]

 Reference Signs List 1 DC power supply 11 AC power supply 21 Semiconductor switch 22, 220, 2200 Transformer 23 Transformer reset circuit capacitor 24, 25, 26 Diode 27 Smoothing circuit reactor 28 Smoothing circuit capacitor 3 DC-DC converter control device 4 DC load 41 AC load

Claims (4)

[Claims] 1. A voltage of a DC power supply (1) is inputted to a series circuit A of a winding np of a transformer having a plurality of windings and a semiconductor switch (21), and a winding nf of the transformer and a diode (25) are connected. Is connected in parallel with the diode (26), the series circuit of the reactor (27) and the capacitor (28) is connected in parallel with the diode (26), and the winding nr of the transformer and the diode (24) are connected. And capacitor (2
3) to form a closed loop, insert this capacitor (23) into the series circuit A, make the winding ratio of the transformer nr / np 0.3 or less, and feed back the voltage of the capacitor (28). A DC-DC converter for controlling the semiconductor switch (21) to output a DC voltage of the capacitor (28). 2. A voltage of a DC power supply (1) is inputted to a series circuit A of a winding np of a transformer having a plurality of windings and a semiconductor switch (21), and a winding nf of the transformer and a diode (25) are inputted. Is connected in parallel with the diode (26), the series circuit of the reactor (27) and the capacitor (28) is connected in parallel with the diode (26), and the winding nr of the transformer and the diode (24) are connected. And capacitor (2
3) to form a closed loop, insert this capacitor (23) into the series circuit B, make the transformer winding ratio nr / nf 0.3 or less, and feed back the voltage of the capacitor (28). A DC-DC converter for controlling the semiconductor switch (21) to output a DC voltage of the capacitor (28). 3. The DC-D according to claim 1, wherein all or some of the windings np and nr are used as the winding nf of the transformer having a plurality of windings.
C converter. 4. The DC-DC converter according to claim 1, wherein a diode (231) is provided in parallel with the capacitor (23).