JP4716598B2 - Switching power supply - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply device that has excellent high-speed response to sudden load changes and is suitable for low-voltage, large-current output.
[0002]
[Prior art]
In a switching power supply with a low voltage output, there are many examples in which a synchronous rectification method is applied to improve conversion efficiency. However, for example, in the case of a forward converter, there are some problems.
[0003]
The first problem is a problem of through current.
If the switch element on the flywheel side is conductive when the transformer is excited, there is no element that blocks the current, and the input power supply is short-circuited. The current flowing at this time is a through current, and when the through current flows, there is a problem that the element is stressed and the conversion efficiency is lowered.
In order to avoid this, it is necessary to provide a so-called dead time in which the switch element on the flywheel side is turned off earlier than the timing at which the transformer is excited. However, if the dead time is set too long, the merits of the synchronous rectification method are diminished and the conversion efficiency is lowered. If the dead time is set too short, there is an increased risk of through current flowing due to component variations.
This problem is particularly noticeable in a switching power supply with a low voltage and large current output.
As described above, there is a problem that it is difficult to set an optimum dead time that avoids a through current and minimizes a decrease in conversion efficiency.
[0004]
As a second problem, there is a problem that the excitation period of the choke is restricted from the viewpoint of high-speed response to sudden load change.
In order to minimize fluctuations in the output voltage when the load current changes suddenly, the choke current must quickly follow the load current. This is apparent from the fact that (load current-choke current) flows out of the output capacitor.
FIG. 6 is a waveform showing a load current (A) and an ideal choke current (B) corresponding thereto. This is a waveform when the duty at which the choke is excited is 100%. In addition, there is no delay in the control circuit.
However, in the forward converter, since the excitation period of the transformer and the excitation period of the choke are equal, the duty with which the choke is excited cannot be set to 100% due to the constraints of the transformer. Since there is a risk of saturation of the transformer and the applied voltage of the switch element increases, 50 to 60% is a practical maximum duty.
Therefore, the actual choke current operation has a waveform as shown in (C). Compared with the ideal waveform (B), it can be seen that the response is clearly delayed.
As described above, since the duty with which the choke is excited is limited, there is a problem that the high-speed response to a sudden load change is also limited.
[0005]
As a third problem, in the case of a large current output, there is a problem that the choke becomes large. This is because the output current and the choke current are equal except for the ripple.
In recent years, the power supply voltage of a microprocessor has been steadily decreasing and the current consumption has been increasing. Therefore, a choke that can carry a large current has become necessary. However, there is also a demand for downsizing, but if the choke is forcibly downsized, the direct current resistance increases and the conduction loss increases, so that the choke tends to increase in size.
[0006]
As a fourth problem, there is a problem that energy flows backward from the output to the input. This is because the state where the choke current becomes negative is established by the synchronous rectification.
If there is a backflow of energy, problems may occur during parallel operation. For example, when operating synchronously rectified forward converters in parallel, if there is a difference in the output voltage setting, energy flows from the higher setting voltage to the lower setting voltage, and in a converter with a lower setting voltage, energy is output from the output to the input. It begins to flow. There is a protective fuse at the input of each converter, and if the reverse fuse of the converter is blown for some reason, the converter is damaged. This is because there is no place for energy and the voltage on the primary side rises.
In order to avoid the above problems, for example, it is necessary to add a protection circuit such as stopping driving the synchronous rectification switch element at a light load.
[0007]
In order to solve the above problems, the applicant of the present application previously proposed the circuit shown in FIG. (Japanese Patent Application No. 2001-106829) According to this circuit, the above four problems can be solved. However, as compared with the original forward converter, the number of parts is greatly increased, and in particular, since insulation is required, four transformers whose outer dimensions tend to be large are necessary.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a circuit system that solves the above problems.
[0009]
[Means for Solving the Problems]
A series circuit of a primary winding of the first transformer and a first switch element is connected in parallel to the input power source, and a first rectifier element and a first capacitor are connected in parallel to the secondary winding of the first transformer. In a switching power supply device in which a series circuit is connected and a load is connected in parallel to the first capacitor, a first choke is inserted in series into the primary winding of the first transformer, and is parallel to the first choke. Is connected to a series circuit of the second rectifier element, the tertiary winding of the first transformer, and the second switch element, and is connected in parallel to the series circuit of the tertiary winding of the first transformer and the second switch element. A third winding of the second transformer and a series circuit of the third switch element are connected, and the primary winding of the first transformer and the primary winding of the second transformer in parallel with the series circuit of the first switch element; Connect the series circuit of the fourth switch element, Serial to solve the problems by connecting a series circuit of a second transformer secondary winding and the third rectifying element in parallel with the first capacitor.
In the case of synchronous rectification, a switch element may be provided in parallel with each of the first rectifier element and the third rectifier element.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The basic circuit configuration of the present invention is shown in FIG. In the figure, the same reference numerals as those in the conventional example indicate equivalent parts. In FIG. 1, the circuit configuration on the primary side is the same as that of the conventional example (FIG. 2).
The secondary side has the same circuit configuration as the four circuits are two.
Therefore, in the present invention, the conventional transformers 31 and 32 are integrated, and the transformers 33 and 34 are integrated. For this reason, the driving method of each switch element is the same as that of the conventional example.
[0011]
The switch elements 11 and 14 are main switches for performing pulse width control, and are alternately turned on. The switch elements 12 and 13 are auxiliary switches for securing the route through which the reset current of the choke 2 flows and the reset conditions of the transformers 31 and 32. The switch elements 12 and 13 may have a fixed ON width. However, as an operating condition, when both the switch elements 11 and 14 are off, either the switch element 12 or 13 needs to be on. Moreover, it is necessary to determine the OFF period of the switch elements 12 and 13 so as to ensure the reset condition of the transformers 31 and 32.
Based on the above conditions, in the following description of the operation, it is determined that the switch element 12 is turned on at 0% to 50% of one cycle and the switch element 13 is turned on at 50% to 100% of one cycle. Further, it is assumed that the switch element 11 is turned on between 0% and 50%, and the switch element 14 is turned on between 50% and 100% to apply pulse width control.
[0012]
Next, the operation will be described.
When the switch element 11 is turned on, current flows from the input power source through the choke 2, the primary winding of the transformer 31, and the route of the switch element 11, and at the same time, the rectifier element 21 is conducted. Since a voltage of (input voltage−output voltage × primary / secondary turns ratio of the transformer 31) is applied to the choke 2, the current increases linearly.
When the switch element 11 is turned off, the reset current of the choke 2 flows through the rectifier element 22, the tertiary winding of the transformer 31, and the route of the switch element 12. The rectifying element 21 remains conductive. Since the voltage of the choke 2 is clamped by (output voltage × secondary / third winding ratio of the transformer 31), the current decreases linearly.
Next, when the switch element 14 is turned on, current flows from the input power source through the choke 2, the primary winding of the transformer 32, and the route of the switch element 14, and at the same time, the rectifier element 23 becomes conductive. Since a voltage of (input voltage−output voltage × primary / secondary turns ratio of the transformer 32) is applied to the choke 2, the current increases linearly.
When the switch element 14 is turned off, the reset current of the choke 2 flows through the rectifier 22, the tertiary winding of the transformer 32, and the route of the switch element 13. The rectifying element 23 remains conductive. Since the voltage of the choke 2 is clamped by (output voltage × secondary / third winding ratio of the transformer 32), the current decreases linearly.
[0013]
A circuit diagram for simulation is shown in FIG. The difference from Fig. 1 is that the secondary side is synchronous rectification (QD1 and QD3 are added), and therefore the rectifier elements D1 and D3 are moved to the negative side, the transformer reset winding n3 and diodes D4 and D5 And that diodes DQ1 to DQ4 are connected in parallel to the switching elements Q1 to Q4. Lm1 and Lm2 represent exciting inductances of the transformer.
[0014]
FIG. 5 is an operation waveform diagram of each part by simulation, and FIG. 5 shows operation waveforms of two cycles.
The input / output conditions of the simulation circuit are an input voltage of 48V, an output voltage of 1.5V, and an output current of 100A.
As described above, Q2 and 3 are fixed to 50% duty, and QD1 and QD3 are synchronized with Q2 and Q3, respectively.
It can be seen from FIG. 5 that the operation is the same as that described above.
[0015]
In the circuit of FIG. 4, the problem of the through current mentioned as the first problem of the conventional method does not occur. This is because D2 prevents short-circuit current even if all the switch elements Q1, Q2, Q3, Q4, QD1, and QD3 are turned on simultaneously.
Therefore, there is no problem that it is difficult to set an optimal dead time in the conventional method so as to avoid a through current and minimize a decrease in conversion efficiency. In the first place, since dead time is unnecessary, the merit of the synchronous rectification method is not reduced and the conversion efficiency is not lowered, resulting in higher efficiency.
[0016]
It also solves the problem of increasing the size of the choke, which was cited as the third problem of the conventional system. This is due to the fact that the choke current is converted into the turns ratio by the transformer because the choke exists on the primary side. For this reason, the choke current is greatly reduced, and an increase in the choke size can be avoided.
For example, in FIG. 4, since the transformer turns ratio is 16: 1, the average choke current is 100/16 = 6.25A.
The choke current is a combination of the currents Q1 to Q4 in FIG.
[0017]
In the circuit of FIG. 4, both the excitation period and reset period of the choke 2 can be changed from 0% to 100%. If the on-duty of Q1 and Q4 is 50%, the excitation period of choke 2 is 100%. If the on-duty of Q1 and Q4 is 0%, the reset period of choke 2 is 100%. This solves the second problem of the conventional circuit. Therefore, when there is no delay in the control circuit, the choke current operation has the ideal waveform shown in FIG. 6, and there is no restriction on the high-speed response.
[0018]
Although FIG. 1 shows a circuit in which the choke current is divided into two and flows through the transformers 31 and 32, the present invention is not limited to two. Any number of divisions is possible.
A circuit in which the number of divisions is increased to three is shown in FIG. By adding circuit blocks surrounded by dotted lines in this way, the number of divisions can be increased as much as possible.
There is an advantage that the ripple current of the choke is reduced by increasing the number of divisions. If the same ripple current is acceptable, the inductance value of the choke can be lowered, which is advantageous for high-speed response.
[0019]
Next, the fourth problem of the conventional system that the energy flows backward is not a problem in the present invention. This is because there is a rectifying element in the route through which the choke reset current flows, and the choke current does not flow backward. Therefore, no energy flows from the output to the input.
On the other hand, when there is a request to reverse the energy, a switch element may be connected in parallel to the rectifying element 22 in FIG. However, since a through current may flow, it is necessary to pay attention to the drive timing of each switch element. For example, in the case of FIG. 1, a through current flows when the switch elements 11 and 12 and the switch element connected in parallel to the rectifying element 21 are simultaneously conducted.
[0020]
【The invention's effect】
As described above, according to the present invention, there is no problem of through-current due to synchronous rectification, excellent high-speed response to sudden load change, suppression of choke enlargement, and no problem of energy backflow, for low-voltage high-current output The switching power supply can be realized with a smaller number of parts, which contributes to miniaturization and cost reduction of the power supply.
[0023]
[Brief description of the drawings]
FIG. 1 Basic circuit of the present invention FIG. 2 Conventional circuit FIG. 3 Application circuit of the present invention FIG. 4 Simulation circuit of FIG. 1 FIG. Waveform of load current and choke current [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Input power supply 2 Choke 3 Capacitor 4 Load 11-14 Switch element 21-25 Rectifier 31-34 Transformer

Claims (5)

A series circuit of a primary winding of the first transformer and a first switch element is connected in parallel to the input power source, and a first rectifier element and a first capacitor are connected in parallel to the secondary winding of the first transformer. In a switching power supply device in which a series circuit is connected and a load is connected in parallel to the first capacitor, a first choke is inserted in series into the primary winding of the first transformer, and is parallel to the first choke. Is connected to a series circuit of the second rectifier element, the tertiary winding of the first transformer, and the second switch element, and is connected in parallel to the series circuit of the tertiary winding of the first transformer and the second switch element. A third winding of the second transformer and a series circuit of the third switch element are connected, and the primary winding of the first transformer and the primary winding of the second transformer in parallel with the series circuit of the first switch element; Connect the series circuit of the fourth switch element, Switching power supply apparatus characterized by connecting a series circuit of a second transformer secondary winding and the third rectifying element in parallel to the serial first capacitor. It consists of a transformer primary winding and a series circuit of switch elements, a transformer secondary winding and a series circuit of rectifier elements, and a transformer tertiary winding and a series circuit of switch elements. A circuit block with both ends of the first input terminal, both ends of the series circuit of the transformer tertiary winding and switch element as the second input terminal, and both ends of the series circuit of the transformer secondary winding and rectifier element as the output terminal Arbitrary numbers are provided, and each circuit block has an output terminal connected to the first capacitor, and a first input terminal connected to a primary circuit of the first transformer and a series circuit of a first switch element. The switching power supply device according to claim 1, wherein the second input terminal is connected to a series circuit of a tertiary winding of the first transformer and a second switch element. The switching power supply device according to claim 1, wherein a rectifying element is connected in parallel with each of the switch elements. 4. The switching power supply device according to claim 3, wherein a MOSFET is used instead of the parallel circuit of each switch element and rectifier element. 5. The switching power supply device according to claim 1, wherein a switching element is connected in parallel to an arbitrary rectifying element among the rectifying elements.

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