JP4148760B2 - 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]
As a first problem, there 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 is particularly noticeable in switching power supplies with low voltage and large current output. Therefore, it is difficult to set an optimum dead time that avoids a through current and minimizes a decrease in conversion efficiency.
[0004]
The second problem is that the choke excitation period is limited from the viewpoint of high-speed response to sudden load changes. 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 evident from the fact that (load current-choke current) flows out of the output capacitor.
[0005]
FIG. 13 is a waveform showing the load current (A) and the ideal choke current (B). 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.
[0006]
The third problem is that the choke becomes large in the case of a large current output. 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.
[0007]
The fourth problem is that energy flows back 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 such a problem, it is necessary to add a protection circuit such as stopping driving of the synchronous rectification switch element in a light load.
[0008]
In order to solve these problems, the applicants previously proposed the circuits shown in FIGS. 8, 9, 10 and 11 (refer to the prior applications 1, 2, 3, and 4). FIG. 11 can solve the above four problems. However, there is a problem in that the number of parts is significantly increased as compared with the conventional forward converter. In particular, the need for four transformers whose outer dimensions tend to be large due to the need for insulation is an impediment to miniaturization of the power supply.
[0009]
FIG. 10 solves this problem, and only two transformers are required. This reduces the number of transformers and rectifiers on the secondary side, contributing to the miniaturization of the power supply. However, four primary windings of the transformer are still necessary, and the configuration of each transformer is more complicated than that in FIG. FIG. 9 shows a circuit in which there are two transformers and the configuration of each transformer is simplified as a countermeasure against these, which reduces the primary winding of the transformer and contributes to miniaturization of the transformer. However, this circuit needs to have different breakdown voltages for the high-side switch and the low-side switch on the primary side.
[0010]
FIG. 12 shows operation waveforms of the method of FIG. The input voltage was 75V, the output voltage was 1.5V, and the output current was 100A. Q1 and 4 are low-side switches, and Q2 and 3 are high-side switches. From this, it can be seen that a 100V breakdown voltage element is required as the high-side switch and a 200V breakdown voltage element is required as the low-side switch. This is because a voltage obtained by adding the choke voltage and the transformer flyback voltage is applied to the high-side switch, whereas a voltage obtained by adding the input voltage, choke voltage, and transformer flyback voltage to the low-side switch. This is due to the fact that is applied. For example, in the case of an input voltage of 75 V, an element with a withstand voltage of 100 V is required as a high side switch and an element with a withstand voltage of 200 V is required as a low side switch. For this reason, the low-side switch needs to be a high breakdown voltage element, and there is a problem that conduction loss increases.
[0011]
FIG. 8 solves this problem. Under the same conditions, it is possible to use a device with a withstand voltage of 100 V for both the high-side switch and the low-side switch. However, this method has a problem that it is not possible to use an element in which two switch elements are connected in series to form one package. It is obvious from FIG. 8 that the high-side switch and the low-side switch are not in series.
[0012]
[Prior Application 1]
Japanese Patent Application No. 2001-223213
[Prior Application 2]
Japanese Patent Application 2001-166187
[Prior Application 3]
Japanese Patent Application 2001-137001
[Prior Application 4]
Japanese Patent Application No. 2001-106829
[0013]
[Problems to be solved by the invention]
An object of the present invention is to provide a circuit that solves the above problems.
[0014]
[Means for Solving the Problems]
Input terminal, output terminal, first choke connected to input terminal, primary transformer primary winding, first switch element and second switch element in series circuit, first choke and first transformer A first rectifier element connected in parallel to the series circuit of the primary winding and the first switch element in parallel, and a parallel circuit in the series circuit of the primary winding of the first transformer, the first switch element and the second switch element A primary circuit of the second transformer connected to the third circuit, a series circuit of the third switch element and the fourth switch element, a series circuit of the first choke and the primary winding of the second transformer and the third switch element A second rectifying element connected in parallel to the first capacitor, a first capacitor connected to the output terminal, a second winding of the first transformer connected in parallel to the first capacitor and a third rectifying element in series. Circuit, connected in parallel to the first capacitor SUMMARY The switching power supply device including a second transformer secondary winding and a series circuit of a fourth rectifier element. In the case of synchronous rectification, a switch element may be provided in parallel with each of the third rectifier element and the fourth rectifier element.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment (basic circuit) of the present invention is shown in FIG. The embodiment shown in FIG. 1 is provided with two transformers 31 and 32 that insulate the primary and secondary sides. The first choke 2, the primary winding of the first transformer 31, the first switch element 12, and the second switch element 11 are connected in series to the input terminal. The first rectifier 22 is connected in parallel with the series circuit of the first choke 2, the primary winding of the first transformer 31 and the first switch element 12. In parallel with the primary winding of the first transformer 31 and the series circuit of the first switch element 12 and the second switch element 11, the primary winding of the second transformer 32, the third switch element 13, and the fourth A series circuit with the switch element 14 is connected. A second rectifying element 26 is connected in parallel with the series circuit of the first choke 2 and the primary winding of the second transformer 32 and the third switch element 13.
[0016]
The first capacitor 3 is connected to the output terminal, and the series circuit of the secondary winding of the first transformer 31 and the third rectifier element 21 is connected in parallel to the first capacitor 3, and the first capacitor 3 In parallel, the secondary circuit of the second transformer 32 and the series circuit of the fourth rectifying element 23 are connected.
[0017]
The switch elements 11 and 14 are main switches for performing output voltage 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 operating conditions of the switch elements 12 and 13 are that the switch element 12 is also on when the switch element 11 is on, the switch element 13 is on when the switch element 14 is on, and that the switch elements 11 and 14 are When both are off, either 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. In the following description of the operation based on the above conditions, 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, 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. In addition, you may use not only pulse width control but another control method.
[0018]
Next, the operation will be described. First, when the switch elements 11 and 12 are turned on, current flows from the input power source through the choke 2, the switch element 12, the primary winding of the transformer 31, and the route of the switch element 11, and at the same time, the rectifier element 21 becomes conductive. 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 route of the switch element 12, the primary winding of the transformer 31, and the rectifier element 22. The rectifying element 21 remains conductive. Since the voltage of the choke 2 is clamped by (output voltage × primary / secondary turns ratio of the transformer 31), the current decreases linearly. Next, when the switch elements 13 and 14 are turned on, current flows from the input power source through the choke 2, the switch element 13, 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 route of the switch element 13, the primary winding of the transformer 32, and the rectifier element 26. The rectifying element 23 remains conductive. Since the voltage of the choke 2 is clamped by (output voltage × primary / secondary turns ratio of the transformer 32), the current decreases linearly.
[0019]
In this circuit, it is not necessary to use a high breakdown voltage element for the low-side switch. This is because a voltage higher than the input voltage is not applied by the diodes 22 and 26 in FIG. When both the low-side switch and the high-side switch are turned off, the total voltage applied to the two elements is a voltage obtained by adding the input voltage, the choke voltage, and the transformer flyback voltage. As described above, since the applied voltage of the low side switch is the input voltage at the maximum, a voltage obtained by adding the remaining choke voltage and the transformer flyback voltage is applied to the high side switch. This is the same voltage as that of the conventional high side switch (FIG. 3). Therefore, for example, when designing with an input voltage of 36 V to 75 V, an element having a withstand voltage of 100 V can be used for both the low side switch and the high side switch. As a result, the conduction loss of the low-side switch is greatly reduced as compared with the case where an element with a withstand voltage of 200 V is used. Although the current flowing through the high-side switch increases, when the on-width of the low-side switch is wide, the conduction loss of the low-side switch is expected to be less than the increase in loss caused thereby. In addition, since the same parts can be used, there is an advantage that the types of parts are reduced.
[0020]
2 is a circuit diagram for the simulation in FIG. 1. The difference from FIG. 1 is that the secondary side is synchronous rectification (QD1, QD3 is added), and the rectifier elements D1, D3 are moved to the negative side for that purpose. The addition of the transformer reset winding and the diodes D4 and D5, and the diodes DQ1 to DQ4 are connected in parallel to the switch elements 1 to 4 assuming a MOSFET. Lm1 and Lm2 represent exciting inductances of the transformer.
[0021]
FIG. 3 is an operation waveform diagram of each part showing the simulation result of FIG. 2, and FIG. 3 shows an operation waveform 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. Q2 and Q3 were fixed at 50% duty as described above, and QD1 and QD3 were synchronized with Q2 and Q3, respectively. It can be seen from FIG. 3 that the operation is the same as that described above. FIG. 4 shows a simulation result when the input voltage is 75V. It can be seen that Q1, Q2, Q3, and Q4 all have a withstand voltage of 100V.
[0022]
In the circuit of FIG. 2, the problem of through current does not occur. This is because D2 and D6 prevent short-circuit current even if all the switching elements Q1, Q2, Q3, Q4, QD1, and QD3 are turned on simultaneously. Therefore, there is no problem that it is difficult to set an optimum dead time so as to minimize a decrease in conversion efficiency while avoiding a conventional through current. 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.
[0023]
It also solves the problem of large choke. 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. 2, since the transformer turns ratio is 16: 1, the average value of the choke current is 100/16 = 6.25A. The choke current is a combination of the currents Q2 and Q3 in FIG.
[0024]
In the circuit of FIG. 2, the excitation period and reset period of the choke can be changed from 0% to 100%. If the on-duty of Q1 and Q4 is 50%, the excitation period of the choke is 100%, and if the on-duty of Q1 and Q4 is 0%, the reset period of the choke is 100%. Therefore, when there is no delay in the control circuit, the choke current operation has the ideal waveform shown in FIG. 7, and there is no restriction on the high-speed response.
[0025]
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.
FIG. 5 shows a circuit in which the number of divisions is increased to three. By adding circuit blocks surrounded by dotted lines in this way, the number of divisions can be increased as much as possible.
Increasing the number of divisions has the advantage of reducing the ripple current of the choke. If the same ripple current is acceptable, the inductance value of the choke can be lowered, which is advantageous for high-speed response.
[0026]
Further, in the present invention, there is no problem that the energy flows backward. 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. In addition, what is necessary is just to connect a switch element in parallel with the rectifier elements 22 and 26 of 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, when a switch element connected in parallel to the switch element 11 and the rectifying element 22 is simultaneously turned on, a through current flows.
[0027]
The present invention has an advantage that a voltage surge generated is small because zero current switching is possible. In an actual circuit, there are parasitic inductances that are not intentionally inserted, such as transformer leakage inductance and wiring inductance, but this becomes a cause of voltage surge in a normal switching power supply. This is because the current that flows in the parasitic inductance loses the route that flows during switching and flows to the high impedance portion. In the present invention, this problem can be avoided by devising the timing for driving the high-side switch.
[0028]
FIG. 6 is a circuit diagram for simulation in which inductances LL1 and LL2 are added to FIG. LL1 and LL2 assume transformer leakage inductance. FIG. 7 is an operation waveform of each part showing the simulation result of FIG. Two cycles of operation waveforms are shown. The difference from FIG. 3 is that the period during which the high-side switch is turned on is changed from 0-50%, 50-100% to 0-55%, 50-105%.
[0029]
The high-side switches Q2 and LL1, Q3 and LL2 are connected in series, and the positive currents of Q2 and 3 are equal to the currents of LL1 and LL2. Therefore, when the currents of Q2 and 3 are positive, that is, when the currents are flowing through LL1 and LL2, when Q2 and 3 are turned off, the currents flowing through LL1 and 2 lose their roots, and a voltage surge is generated. In FIG. 6, this problem is avoided. That is, Q2 and 3 are turned off after the currents Q2 and 3 become zero. For this reason, a voltage surge does not occur.
[0030]
The reason why such an operation is possible is that a voltage that reduces the current is applied to LL1 and LL2. When the on period of the high side switch is extended, the low side switch connected to the transformer on the opposite side is turned on when the high side switch is on. When the low-side switch is turned on, the voltage of the choke Li is inverted, so that a reverse voltage is applied to LL1 and LL2.
[0031]
In the conventional example shown in FIG. 8, since the primary winding of the transformer 31 is inserted between the switch elements 11 and 12, it is not possible to use a component in which two switch elements are connected in series to form one package. However, this is possible in the embodiment shown in FIG. When such a component can be used, the wiring between the two switch elements can be made much shorter than when individual components are used. Then, the voltage surge generated at both ends of the switch element due to the inductance generated by the wiring can be suppressed, which contributes to the reduction in noise of the power supply device.
[0032]
If the voltage surge increases and exceeds the rated voltage of the device, the device will be damaged. If the wiring between the two switch elements is long, in order to avoid such a situation, a component that absorbs the voltage surge is separately required, resulting in an increase in the size of the power supply device. It is unnecessary.
[0033]
【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 low voltage high current output without problems of energy backflow. It is possible to use a switching power supply with a transformer having a simple configuration and a switch element in which two series switch elements are packaged as a primary switch element, which contributes to the miniaturization and cost reduction of the power supply.
[Brief description of the drawings]
FIG. 1 is a circuit diagram of an embodiment of the present invention (basic circuit).
2 is a circuit diagram for simulation. FIG. 3 is an operation waveform diagram of each part of FIG. 2. FIG. 4 is an operation waveform diagram of each part of FIG. 2. FIG. 5 is a circuit diagram of another embodiment of the present invention. Fig. 1 Circuit diagram for simulation [Fig. 7] Operation waveform diagram of each part of Fig. 6 [Fig. 8] Conventional circuit diagram [Fig. 9] Conventional circuit diagram [Fig. 10] Conventional circuit diagram [Fig. 11] Conventional circuit diagram 12 is an operation waveform diagram of each part of FIG. 9. FIG. 13 is a waveform diagram of load current and choke current.
1 Input Power Supply 2 Choke 3 Capacitor 4 Load 11-14 Switch Element 21-26 Rectifier 31-34 Transformer

Claims (6)

In a switching power supply unit with multiple transformers that isolate primary and secondary,
One end of the choke is connected to the positive side of the input terminal, one end of the primary winding of the transformer is connected to the other end of the choke, and a switch element is connected to the other end of each primary winding. A switching power supply device characterized in that a switch element and a rectifier element are connected to each of these, the switch element is connected to the negative side of the input terminal, and the rectifier element is also connected to one end of the choke. 2. The switching according to claim 1, wherein a rectifier element is connected in series to each of the secondary windings of the transformer, and the series circuit is connected in parallel to a first capacitor connected to an output terminal. Power supply. 3. The switching power supply device according to claim 1, wherein a rectifying element is connected in parallel to an arbitrary switching element among the switching elements. 4. The switching power supply device according to claim 1, wherein a MOSFET is used as the switch 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. 6. The switching power supply device according to claim 1, wherein any of the rectifying elements is a MOSFET.

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