JPH04265666A - Two-transistor forward converter provided with feedback diode - Google Patents

Abstract

PURPOSE:To realize high conversion frequency for operating a two-transistor forward converter provided with a feedback diode and to reduce the size thereof. CONSTITUTION:Primary winding 14 of a transformer 4 is connected, respectively, through switch elements 2, 3 with plus and minus sides of a DC input power supply 1 and first and second feedback diodes 5, 6 are connected, respectively, with the opposite ends of the transformer 4. The primary winding 14 is provided with a center tap 16 which is then connected through a circuit including at least a capacitor 17 with the minus side, for example, of the DC input power supply 1. Voltage across the capacitor 17 is maintained constant even if two switch elements 2, 3 are turned ON, OFF with some phase shift thus suppressing the reset voltage, minimizing the reset interval of the transformer 4 and realizing high speed operation.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-stone forward converter with a feedback diode that is effective for downsizing.

0002

[Prior Art] A converter is a converter that converts a DC voltage into an AC voltage by switching a switch element, steps it up or down using a transformer, and then rectifies and smoothes it to convert it back into a DC voltage. This is a power converter that realizes the conversion of Conventionally, as this type of converter, a two-stone forward converter with a feedback diode shown in FIG. 8 using two switching elements is known.

In the figure, 1 is a DC input power supply, 2 is a first switching element, 3 is a second switching element, 4 is a transformer, and 5 is a first switching element. Diode, 6 is a second diode, 7 is an output rectifying element, 8 is an output flywheel element, 9 is an output smoothing choke coil, 10 is an output smoothing capacitor, 11 is a load, 12 is a first switching element drive circuit,
13 is a second switch element drive circuit, 14 is a transformer 4
15 is the primary winding of the transformer 4, and 15 is the secondary winding of the transformer 4.

In connection of the primary winding 14 side of the transformer 4 in the above, the beginning of the winding is connected to the positive terminal of the DC input power supply 1 through the first switch element 2, and the end of the winding is connected to the positive terminal of the DC input power supply 1 through the first switch element 2. 3 is connected to the negative terminal of the DC input power supply 1. A switch element drive signal D is connected to the inputs of the first and second switch element drive circuits 12 and 13, and each output is connected to the corresponding first switch element drive circuit 12, 13.
and connected to the gate terminals of the second switch elements 2 and 3. The first diode 5 for feedback is connected in the forward direction from the negative side of the DC input power supply 1 toward the beginning of the primary winding 14 of the transformer 4, and the second diode 6 for feedback is connected from the negative side of the DC input power source 1 to the beginning of the primary winding 14 of the transformer 4. It is connected in the forward direction from the winding end of the secondary winding 4 toward the positive side of the DC input power source 1. On the other hand, in the connection on the secondary winding 15 side of the transformer 4, the winding start of the secondary winding 15 → output rectifying element 7 → output smoothing choke coil 9 → output smoothing capacitor 10 →
A circuit is formed at the end of the winding of the secondary winding 15, and the load 1
1 is connected in parallel to the output smoothing capacitor 10. Further, the output flywheel element 8 is connected between the connection point between the output rectifying element 7 and the output smoothing choke coil 9 and the end of the winding of the secondary winding 15.

The operation of the conventional circuit constructed as described above will be explained with reference to FIGS. 9 and 10. FIG. 9 shows an operation waveform diagram of the conventional example, in which (a) shows the waveform of the switch element drive signal D, which controls the on/off switching of the switch elements 2 and 3 at a constant cycle. Moreover, (b) is the waveform of the voltage v1 of the first switching element 2, and (c) is the waveform of the voltage v1 of the first switching element 2.
(d) shows the waveform of the voltage vt of the primary winding 14 of the transformer 4. Vi in the figure is the voltage of the DC input power supply 1. Further, FIG. 10 shows an equivalent circuit of FIG. 1 for explaining the reset operation of the transformer 4. Here, 21 is the output capacitance of the first switch element 2, 31 is the output capacitance of the second switch element 3, and 41 is the excitation inductance of the transformer 4. Figure 9
, each switch element 2, 3, each drive circuit 12, 1
The circuit operation when the primary windings 14 of the transformer 3 and the transformer 4 have the same characteristics will be described below.

First, when the two switch elements 2 and 3 are turned on simultaneously, power is supplied to the secondary winding 15 of the transformer 4, and excitation energy proportional to the on period is accumulated in the excitation inductance 41 of the transformer 4. be done. next,
When the two switch elements 2 and 3 are both turned off, the positive side of the DC input power supply 1 → output capacitance 21 → transformer primary winding 1
4→Output capacitor 31→A current proportional to the load current flows through the negative side route of the DC input power source 1, and the output capacitor 21 and the output capacitor 31 are rapidly charged, and voltages v1 and v2 are generated. The voltage of the sum of v1 and v2 is the voltage Vi of the DC input power supply 1
When it becomes equal to , the transformer 4 starts a reset operation. Next, the excitation current of the transformer 4 stored during the on period is changed from the end of the winding of the primary winding 14 of the transformer 4 to the output capacity 3.
1 → DC input power supply 1 → output capacitor 21 → flow through the route of the beginning of winding of the primary winding 14 of the transformer 4, and the switch element 2,
The output capacitors 21 and 23 of the transformer 4 are further charged, and the excitation inductance 41 of the transformer 4 generates a reset voltage vt due to the resonance of the series circuit of the excitation inductance 41, the output capacitor 21, and the output capacitor 31. When the voltage of the output capacitor 21 and the voltage of the output capacitor 31 exceed the voltage of the DC input power supply 1 and the first diode 5 and the second diode 6 are turned on, the voltage applied to the switch elements 2 and 3 and the reset voltage are It becomes equal to the voltage of the DC input power supply 1. Next, after the excitation inductance 41 releases all of the excitation energy, the output capacitance 2
1 and 31 start discharging. As this discharge progresses, the voltage of the primary winding 14 of the transformer 4 becomes 0, the sum of the voltages of the switching elements 2 and 3 becomes equal to the voltage of the DC input power supply 1, and the voltage remains constant until the transformer is turned on again. maintain. In this series of operations, v1 and v2 are always equal.

[0007]

[Problems to be Solved by the Invention] However, in the conventional two-stone forward converter described above, in reality, there is a difference in the off timing of the two switch elements, and
Since there is an imbalance in the output capacitance of each switching element, the above-mentioned operation is not exhibited. This and the problems caused by this will be described below.

In the above conventional example, each switch element 2,
3. When there is an imbalance in the characteristics of each drive circuit 12, 13 and the primary winding 14 of the transformer 4, the circuit operates as follows. Assuming that the first switch element 2 turned off slightly earlier than the second switch element 3, FIG.
In, the positive side of DC input power supply 1 → output capacity 21 →
A current proportional to the load current flows through the route from the primary winding 14 of the transformer 4 to the switch element 3 to the negative side of the DC input power source 1, and the output capacitor 21 is rapidly charged to generate a voltage v1. Next, when the second switch element 3 is turned off with a slight delay, the current flow is as follows: the positive side of the DC input power supply 1 → the output capacitor 21 → the primary winding 14 of the transformer 4 → the output capacitor 31 → the DC input power supply 1, the output capacitor 31 is charged, and as the voltage v2 is generated, the output capacitor 21 is further charged. Then, when the voltage of the sum of v1 and v2 becomes equal to the voltage of the DC input power source 1, the transformer 4 starts a reset operation. Here, the initial voltage value v1 (t1) of the output capacitor 21 and the output capacitor 31 at the start of reset,
When comparing v2(t2), v which is the voltage of the output capacitor 21
1 (t1) is higher.

Next, in the period from t1 to t2 in FIG.
The excitation current of the transformer stored during the ON period is routed from the end of the primary winding 14 of the transformer 4 → the output capacitor 31 → the DC input power supply 1 → the output capacitor 21 → the beginning of the winding of the primary winding 14 of the transformer 4. and the output capacitance of the switch element 21
, 31 are further charged. The rise in reset voltage during this period is the sum of the rise in voltage v1 and the rise in voltage v2. Next, at time t2 in FIG. 9, since the initial voltage value v1 (t1) of the output capacitor 21 at the start of reset was higher than the initial voltage value v2 (t2) of the output capacitor 31, the first switch element 2 The voltage exceeds the voltage of the DC input power source before the voltage of the second switch element 3, and the first diode 5 is turned on. Next, in the period from t2 to t3 in FIG. 9, the first diode 5 is turned on, so that the rise in the reset voltage becomes equal to the rise in v2. Next, after the excitation inductance releases all of the excitation energy, the output capacitance 21,3
1 starts discharging and the reset voltage decreases. Here, the maximum value of the reset voltage may not reach the voltage of the DC input power source 1 because the rise in the reset voltage was small during the period from t2 to t3, and the reset voltage is suppressed to a low value.

As described above, the two switch elements 2 and 3
In actual operation, there is a slight difference in the off timing of
Since the voltages v1 and v2 of the circuit in FIG. 10 are not equal and the reset voltage vt is suppressed to a low level, the reset period of the transformer 4 becomes longer. Furthermore, even when there is an imbalance in the characteristics of each switch element 2, 3 and the primary winding 14 of the transformer 4, the reset voltage vt is similarly suppressed to a low level, so that the reset period of the transformer 4 becomes longer. As described above, in the conventional example, it is necessary to allow for a long reset period, which hinders the progress of increasing the conversion frequency, which is effective for downsizing the converter.

The present invention has been made to solve the above problems, and its object is to provide a two-stone forward converter with a feedback diode that can increase the conversion frequency by minimizing the reset period of the transformer. Our goal is to provide the following.

0012

Means for Solving the Problems In order to achieve the above object, in the two-stone forward converter with feedback diode of the present invention, a first switching element is connected to one end of the primary winding of the transformer, and the first switching element is connected to one end of the primary winding of the transformer. A second switch element is connected to the other end of the primary winding of the transformer, and a series circuit of the first switch element, the primary winding of the transformer, and the second switch element is connected to a DC input power source. , the cathode of the first diode is connected to one end of the primary winding of the transformer and the first diode.
, the anode of the first diode is connected to the negative side of the DC input power supply, and the anode of the second diode is connected to the other end of the primary winding of the transformer and the second Connect to the connection point of the switch element of
a two-stone feedback diode with a cathode of the second diode connected to the positive side of the DC input power supply, switching the first and second switching elements to perform rectification and smoothing on the secondary winding side of the transformer; In the forward converter, a center tap is provided in the primary winding of the transformer, and a capacitor or at least a capacitor or a series-connected capacitor is connected between the center tap and either the positive or negative terminal of the DC input power source. It is characterized by connecting a circuit containing a resistor and a resistor.

[0013]

[Operation] In the two-stone forward converter with feedback diode of the present invention, a center tap is provided in the primary winding of the transformer, and this center tap is connected to a capacitor or a circuit including at least a capacitor or a series-connected capacitor and a resistor to connect the input DC power supply. It is connected to the positive side or the negative side to equalize the voltages across the first and second switching elements regardless of their on or off states. This prevents the reset voltage of the transformer from being suppressed to a low level even if there is a difference in the on/off timing of the two switch elements, thereby eliminating an increase in the reset time. This eliminates the need for margins due to variations in reset time, which was required in the past, and enables high-speed operation.
Enables higher conversion frequencies.

[0014]

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing the configuration of a first embodiment of the present invention. In the figure, 1 is a DC input power supply, 2 is a first switch element, and 3 is a second switch element.
, 4 is a transformer, 5 is a first diode, 6 is a second diode, 7 is an output rectifying element, 8 is an output flywheel element, 9 is a choke coil for output smoothing, and 10 is for output smoothing. A capacitor, 11 is a load, 12 is a first switch element drive circuit, 13 is a second switch element drive circuit, 14 is a primary winding of the transformer 4, 15 is a secondary winding of the transformer 4, 16 is a transformer 4 The center tap 17 of the primary winding 14 is a capacitor.

Primary winding 1 of transformer 4 in this embodiment
In the connection on the 4th side, the winding start of the primary winding 14 is
The end of the winding is connected to the negative terminal of the DC input power supply 1 through the second switch element 3, and the center tap 16 of the primary winding 14 is connected to the capacitor. Connect to the negative terminal side of the DC input power supply 1 through 17. Further, the first diode 5 has a cathode connected to a connection point between the winding start of the primary winding 14 of the transformer 4 and the first switch element 2, and an anode connected to the negative side of the DC input power source 1. The second diode 6 has an anode connected to a connection point between the end of the primary winding 14 of the transformer 4 and the second switch element 3, and a cathode connected to the positive side of the DC power supply 1. Semiconductor switch elements such as FETs (field effect transistors) are suitable as the switch elements 2 and 3, and in that case, the outputs of the switch element drive circuits 12 and 13 are connected to the gate terminals of the respective switch elements 2 and 3. The same switch element drive signal D is connected to the inputs of the switch element drive circuits 12 and 13. In this way, in this embodiment, FIG.
The difference from the conventional example is that a center tap 16 is provided on the primary winding 14 of the transformer 4, and a capacitor 17 is connected between the center tap 16 and the negative terminal of the DC input power source 1. . On the other hand, the secondary winding 1 of the transformer 4
The connection on the 5 side is similar to the conventional example shown in FIG. That is, a circuit is formed in which the winding start of the secondary winding 15 → the output rectifying element 7 → the output smoothing choke coil 9 → the output smoothing capacitor 10 → the winding end of the secondary winding 15, and the load 11 is used for output smoothing. Connect in parallel to capacitor 10. Further, the output flywheel element 8 is connected between the connection point between the output rectifying element 7 and the output smoothing choke coil 9 and the end of the winding of the secondary winding 15.

The operation and effects of the first embodiment configured as above will be described with reference to FIGS. 2 and 3.

FIG. 2 is a diagram showing an example of the operating waveforms of this embodiment, and (a) is the waveform of the switch element drive signal D, which controls the on/off of the switch elements 2 and 3 at a constant cycle. In addition, (b) is the waveform of the voltage v1 of the first switch element 3, (c) is the waveform of the voltage v2 of the second switch element 3, (
d) shows a change in the voltage vt of the primary winding 14 of the transformer 4. Here, Vi in the figure is the voltage of the DC input power supply 1. Further, FIG. 3 shows an equivalent circuit diagram of FIG. 1 for explaining the reset operation of the transformer 4. Among the reference numerals in the figure, the same components as in FIG. 1 are indicated by the same reference numerals. Here, 21 is the output capacitance of the first switching element 2, and 31 is the second
41 is the excitation inductance of the transformer 4.

First, the first switch element 2 and the second switch element 2 in FIG.
During the period when both switch elements 3 are on, the voltage between the center tap 16 and the negative side of the DC input power source 1 is 1/2 of the voltage of the DC input power source 1. Therefore, the capacitor 17 is 1/2 of the voltage Vi of the DC input power supply 1.
voltage.

Next, when the first switch element 2 is turned off before the second switch element 3, the voltage of the capacitor 17 is equal to the voltage between the center tap 16 and the transformer 4.
It is applied between the end of the next winding 14 and the capacitor 17
is discharged with a current proportional to the load current, and the electrostatic energy of the capacitor 17 is supplied to the load. Conversely, when the second switch element 3 turns off before the first switch element 2, the positive side of the DC input power supply 1 → the output capacitor 21 → the start of winding of the primary winding 14 of the transformer 4 → The energy of the DC input power source 1 is supplied to the load through a closed circuit consisting of the center tap 16 of the primary winding 14 of the transformer 4 → the capacitor 17 → the negative side of the DC input power source 1, and
Capacitor 17 is charged with a current proportional to the load current. Here, the capacitance of the capacitor 17 is the switching element 2,
Even if the voltage is charged and discharged with a current proportional to the load current during the period when the drive timing of 3 is shifted, if the voltage is at least a value that can be considered to be approximately constant, the negative side of the DC input power supply 1 and the center tap 16 When both switching elements 2 and 3 are on, the voltage between DC input power supply 1 and
The voltage Vi is maintained at 1/2 of the voltage Vi.

Next, when both switch elements 2 and 3 are turned off, the excitation current of the transformer 4 stored during the on period mainly flows from the end of the primary winding 14 of the transformer 4 to the output capacitance 3.
1 → Input power supply 1 → Switch element 2 → Flows through the route of the beginning of winding of the primary winding 14 of the transformer 4, and the switch elements 2 and 3
Since the output capacitors 21 and 31 of are charged, the voltages v1 and v
2 occurs. Here, since the voltage Vc of the capacitor 17 is still Vi/2, the following formula, v1-vt1=V
i-Vi/2=Vi/2...(1), v2-vt2=Vi
/2...(2) holds true. Since the windings where vt1 and vt2 are generated have the same number of turns, the voltages are also equal and vt
1=vt2 (3). Therefore (1), (2), (
From equation 3), v1=v2. This relationship is maintained even if the output capacitances 21 and 31 of the switching elements 2 and 3 have different values.

Therefore, even if there is a difference in the off timing of the two switch elements 2 and 3, the reset voltage increases.
Since the voltage is always the sum of the rise in v1 and the rise in v2, the reset voltage is not suppressed to a low level. As described above, since the capacitor 17 always makes the voltage of the switching element 2 and the voltage of the switching element 3 equal, and the reset voltage is not suppressed to a low level, the reset time does not increase and the conversion frequency can be increased.

Next, a second embodiment of the present invention will be explained. FIG. 4 is a circuit diagram showing its configuration. As for the members constituting this embodiment, 18 is a resistor, and the symbols of other members are the same as in the first embodiment in FIG. It is similar to This second embodiment differs from the first embodiment in that the connection between the center tap 16 of the transformer 4 and the negative side of the DC input power supply 1 is made by a series circuit of a capacitor 17 and a resistor 18. be. The other connection configurations are the same as in the first embodiment. The resistor 18 mentioned above is for suppressing the rush current flowing into the capacitor 17 during the period when both the switch elements 2 and 3 are on. Therefore, since this embodiment basically operates in the same manner as the first embodiment, the capacitor 17 always maintains the voltage of the switching element 2 and the voltage of the switching element 3, regardless of the state of the switching elements 2 and 3. Since the reset voltage is not suppressed to a low level, the reset time does not increase, and the conversion frequency can be increased to a high frequency.

Next, a third embodiment of the present invention will be described. FIG. 5 is a circuit diagram showing its configuration. The members constituting this embodiment are the same as those of the first embodiment shown in FIG. This third
The difference between the embodiment and the first embodiment is that the capacitor 17
, center tap 16 of transformer 4 and DC input power supply 1
This is the point connected to the positive side of . The other connection configurations are the same as in the first embodiment. Even when the center tap 16 is connected in this manner, the operation can be performed in the same manner as in the first embodiment. That is, in this embodiment as well, the capacitor 17 always makes the voltage of the switching element 2 equal to the voltage of the switching element 3 regardless of the state of the switching elements 2 and 3, and since the reset voltage is not suppressed to a low level, the reset time is not increased. Therefore, the conversion frequency can be increased to a higher frequency.

Next, a fourth embodiment of the present invention will be described. FIG. 6 is a circuit diagram showing its configuration. The structural members of this embodiment are the same as the members with the same reference numerals in the first embodiment of FIG. This embodiment differs from the first embodiment in that the first diode 5 is omitted. In the operation according to this embodiment, during the period in which the two switch elements 2 and 3 are off, the excitation inductance (not shown;
The output capacitance of the switching element 2 (not shown, corresponding to 21 in FIG. 3) and the output capacitance of the switching element 3 (not shown, corresponding to 31 in FIG. 3) are charged by the excitation energy release of the switching element 2 (corresponding to 41 in FIG. 3). . Then, when the voltage of the output capacitor 31 exceeds the voltage of the DC input power source, the second diode 6 is turned on, and the voltage applied to the second switch element 3 is suppressed to the voltage of the DC input power source 1. Here, in this embodiment as well, as in the first embodiment, the capacitor 17 is connected to the switch element 2,
3 always makes the voltage of the switching element 2 equal to the voltage of the switching element 3 in any state, so when the second diode 6 turns on, the voltage applied to the second switching element 3 alone Therefore, the voltage applied to the first switch element 2 is also suppressed to the voltage of the DC input power source 1. Therefore, in this embodiment as well, the reset voltage is not suppressed to a low level, so the reset time does not increase, the conversion frequency can be increased, and the circuit can be simplified by omitting one diode.

Next, a fifth embodiment of the present invention will be described. FIG. 7 is a circuit diagram showing its configuration. The structural members of this embodiment are the same as the members with the same reference numerals in the first embodiment of FIG. This embodiment differs from the first embodiment in that the second diode 6 is omitted. Even in this case, it is possible to operate in the same manner as in the fourth embodiment described above. That is, since the reset voltage is not suppressed to a low level, the reset time does not increase, the conversion frequency can be increased, and the circuit can be simplified by omitting one diode.

In the third, fourth, and fifth embodiments, it is possible to insert a resistor in the capacitor 17 to block rush current, as in the second embodiment. Also,
Even if either the first diode 5 or the second diode 6 is omitted from the second embodiment, the same operation as the fourth or fifth embodiment is possible. As described above, the present invention can be applied in various ways and can take various embodiments in accordance with the gist thereof.

[0028]

[Effects of the Invention] As is clear from the above explanation, the two-stone forward converter with feedback diode of the present invention can reduce the reset voltage Since the voltage is not suppressed to a low value, there is no increase in reset time, the conversion frequency can be increased, and the converter can be made smaller.

[0029] Furthermore, according to the invention of claim 2 of the present invention,
In particular, the circuit configuration can be simplified, and the advantage of further miniaturization can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

[Fig. 2] A diagram showing an example of operation waveforms in the first embodiment.

[Figure 3] The above first
Equivalent circuit diagram of the embodiment of

FIG. 4 is a circuit diagram showing a second embodiment of the present invention.

FIG. 5 is a circuit diagram showing a third embodiment of the present invention.

FIG. 6 is a circuit diagram showing a fourth embodiment of the present invention.

FIG. 7 is a circuit diagram showing a fifth embodiment of the present invention.

[Figure 8] Circuit diagram showing a conventional example

[Fig. 9] A diagram showing an example of operation waveforms of the above conventional example.

[Figure 10] Equivalent circuit diagram of the above conventional example

[Explanation of symbols]

1... DC input power supply, 2... First switch element, 3... Second
Switch element, 4...Transformer, 5...First diode, 6...Second diode, 7...Output rectifying element, 8...Output flywheel element, 9...Output smoothing choke coil, 10...Output smoothing capacitor , 11... Load, 12... First switch element drive circuit, 13... Second switch element drive circuit, 14... Primary winding of transformer 4, 15... Secondary winding of transformer 4, 16... Transformer 4 Center tap of primary winding 14, 17... capacitor, 18... resistor.

Claims (2)

[Claims] 1. A first switch element is connected to one end of a primary winding of a transformer, a second switch element is connected to the other end of the primary winding of the transformer, and the first switch element and A series circuit of the primary winding of the transformer and the second switch element is connected to a DC input power source, and a cathode of the first diode is connected to one end of the primary winding of the transformer and the first switch element. the anode of the first diode is connected to the negative side of the DC input power supply, and the anode of the second diode is connected to the other end of the primary winding of the transformer and the second switch element. the cathode of the second diode is connected to the positive side of the DC input power supply, and the first and second switch elements are switched to perform rectification and smoothing on the secondary winding side of the transformer. In the two-stone forward converter with a feedback diode, a center tap is provided in the primary winding of the transformer, and a capacitor or at least a capacitor is connected between the center tap and either the positive or negative terminal of the DC input power source. Or a two-stone forward converter with a feedback diode, which is characterized by connecting a circuit containing a series-connected capacitor and a resistor. 2. A two-stone forward converter with a feedback diode according to claim 1, wherein either the first diode or the second diode is omitted.