JP2003299359A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively reduce ripples of input and output by limiting the maximum duty of a pulse driving signal to 50% or lower without incorporating a synchronous circuit or a phase shift circuit. <P>SOLUTION: Two units of forward converters 4A, 4B are connected in parallel to a resistor 3 that is a load, and the pulse driving signals the phases of which are shifted at 180° at the same frequency are fed to the switching elements 14, 34 of the forward converters 4A, 4B from a driving signal feeding means. A control IC 61 for feeding the pulse driving signal to each switching element of a push-pull converter is utilized as a driving-signals feeding means. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving signal supply means for supplying a pulse driving signal having the same frequency and a phase shifted by 180 ° to reduce input / output ripple current. To supply to each switching element of two forward converters. FIG. 2 is a circuit diagram of a switching power supply device having a conventional push-pull converter. In FIG. 1, reference numeral 101 denotes a transformer for insulating an input side and an output side. Drains of switching elements 104 and 105 each composed of a MOS FET are connected to one end of each of primary windings 102A and 102B divided by a center tap. Is done. The sources of the switching elements 104 and 105 are connected to each other, and an input power supply 107 and an input capacitor 1 are connected between the sources of the switching elements 104 and 105 and the center tap.
08 are connected respectively. Reference numeral 109 denotes a current transformer as a current detector for detecting the primary current of the transformer 101. Note that 111 and 112 are switching elements 104 and 1
Although the body diode is 05, an external diode may be used in some cases. On the other hand, on the secondary side of the transformer 101, diodes 115 and 116, each having an anode connected to one end of each of the secondary windings 103A and 103B divided by a center tap,
An output circuit 119 including a choke coil 117 having one end connected to a connection point between the cathodes of the diodes 115 and 116 and a smoothing capacitor 118 connected between the other end of the choke coil 117 and a center tap is provided. And transformer 1
01. The switching elements 104 and 105 connected in series with the primary windings 102A and 102B, respectively, are alternately switched.
The DC input voltage Vin from the input power supply 107 is applied to the primary winding 102A,
The voltage applied to the secondary windings 102A and 102B is alternately applied, and the voltage induced in the secondary windings 103A and 103B of the transformer 101 is rectified and smoothed by the output circuit 119. The output voltage Vo is supplied. [0004] Separately, as a feedback loop for stabilizing the output voltage Vo, an operational amplifier 123 for amplifying an error between the output voltage Vo and the reference voltage Vref from the reference power supply 122,
The error feedback signal obtained by the operational amplifier 123 is
Photocoupler 124 for insulated transmission to the primary side of transformer 101
And a control IC 125 for controlling a conduction width of a pulse drive signal to each of the switching elements 104 and 105 based on an error feedback signal from the operational amplifier 123. When the output voltage Vo increases, the control IC 125 reduces the conduction width of the pulse drive signal to each of the switching elements 104 and 105. Conversely, when the output voltage Vo decreases, the control IC 125 The output voltage Vo is stabilized by increasing the conduction width of the drive signal. However, the above configuration has a problem that the input and output ripple currents mainly due to the charging and discharging of the choke coil 117 are large, and the input capacitor 108 and the smoothing capacitor 118 having a large capacity have to be incorporated.
In order to deal with such a problem, for example, Japanese Patent Laid-Open No.
Japanese Patent No. 2966 discloses an oscillation as drive signal supply means for connecting n forward converters in parallel to a load and shifting the phase of a pulse drive signal supplied to a switching element of each forward converter by every 360 ° / n. A switching power supply device provided with a circuit has been proposed. However, in order to supply such a pulse drive signal to the switching element of each forward converter, a synchronous circuit for synchronizing the frequency of each pulse drive signal and a phase shift circuit for shifting the phase are provided in the oscillation circuit. It must be incorporated, and the circuit configuration becomes complicated. In addition, if the conduction width of the pulse drive signal to each switching element is expanded indefinitely according to the fluctuation of the output voltage Vo,
That is, the maximum duty of the pulse drive signal is 50% (0.
If the value exceeds 5), there is a problem that the transformer constituting the forward converter cannot be reset. SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to ensure that the maximum duty of a pulse drive signal is 50% or less without incorporating a synchronous circuit or a phase shift circuit. It is another object of the present invention to provide a switching power supply capable of effectively reducing input / output ripple. In order to achieve the above object, a switching power supply according to the present invention has two forward converters connected in parallel to a load, and the phases are shifted by 180 ° at the same frequency. A switching IC for supplying a pulse drive signal to each switching element of the push-pull converter as the drive signal supply means. It is characterized by using. According to the present invention, a control IC used for a push-pull converter supplies two pulse drive signals having the same frequency and a phase difference of 180 °, and the duty of each pulse drive signal is 50%. It is made by paying attention to the fact that it is designed not to exceed. In other words, for a circuit topology in which two forward converters are connected in parallel to a load, if a control IC for such a push-pull converter is incorporated as a drive signal supply means, a synchronous circuit and a phase shift circuit can be independently designed. Even if it is not, the ripple current components at the input and output of each forward converter are effectively canceled, and the capacitance of the input capacitor and the smoothing capacitor can be reduced. Moreover, since the duty of the pulse drive signal for the switching element of each forward converter is limited to 50% or less, the transformers constituting each forward converter are securely reset and do not saturate. Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In FIG. 1 showing the overall configuration of the circuit, reference numeral 1 denotes an input power source, 2 denotes an input capacitor connected between both ends of the input power source 1, and two forward converters connected in parallel to a resistor 3 as a load. 4A, 4B, input power 1
From the DC input voltage Vin. Each of the forward converters 4A and 4B has the same configuration. The configuration of the forward converter 4A will be described. A transformer 11 insulates between the input side and the output side.
A series circuit with the switching element 14 composed of a type FET is
It is connected between both ends of the input power supply 1. 15 is a transformer
11 is a current transformer as a current detector for detecting a primary side current. Reference numeral 16 denotes a body diode of the switching element 14, but an external diode may be used in some cases. 17 is the switching element 14
Parasitic capacitance, an external capacitor may also be used. The switching element 14 performs a switching operation by a pulse drive signal from a control IC 61 described later. On the other hand, on the secondary side of the transformer 11, a transformer
A rectifying switching element 21 as a rectifying element having a drain connected to a non-dot side terminal of the secondary winding 13 of FIG. 11 and a switching element connected between both ends of a series circuit including the secondary winding 13 and the rectifying switching element 21. A commutation switching element 22 as a current element and a choke coil 23 inserted and connected to one of the output voltage lines are provided. A resistor 24 is connected between the dot-side terminal of the secondary winding 13 and the gate of the rectifying switching element 21 so that the rectifying switching element 21 is turned on in synchronization with the turning on of the switching element 14.
When a pulse drive signal to the switching element 14 is applied to the primary winding of the insulating transformer 25 so that the commutation switching element 22 is turned on in synchronization with the switching element 14 being turned off, when the pulse drive signal is turned off, A drive signal is supplied from the secondary winding of the insulating transformer 25 to the gate of the commutation switching element 22. 26 and 27 are rectifying switching elements 21 and commutation switching elements 22 respectively.
Body diode. These rectifying switching elements 21 and commutation switching elements 22 are provided to reduce the internal loss of the rectifying elements and commutation elements, but diodes may be used instead. The same structure as that described above is provided in the forward converter 4B. That is, the forward converter 4B includes a transformer 31 and its primary winding 32 and secondary winding 33, a switching element 34, a current transformer 3
5, body diode 36 and parasitic capacitance 37 of switching element 34, rectifying switching element 41, commutation switching element 42, choke coil 43, resistor 44, insulating transformer 45 for signal transmission, and body diodes 46, 47
It comprises. Further, the smoothing capacitor 48 is common to each of the forward converters 4A and 4B, and has a DC output voltage Vo generated between both ends of the smoothing capacitor 48.
Is supplied to the resistor 3. As a feedback loop for stabilizing the output voltage Vo, the output voltage Vo and the reference voltage Vref from the reference power supply 51 are used.
An operational amplifier 52 that amplifies the error with the optical amplifier 53, a photocoupler 53 that insulates and transmits the error feedback signal obtained by the operational amplifier 52 to the primary sides of the transformers 11 and 31, and each switching element based on the error feedback signal from the operational amplifier 52 A control IC 61 is provided as drive signal supply means for controlling the conduction width of the pulse drive signal to 14 and 34. It should be noted that the control IC 125 for the switching elements 104 and 105 of the push-pull converter as shown in FIG. 2 is used as the control IC 61 for the circuit topology in this embodiment. That's what it means. Since the control IC 61 in this case is designed for a push-pull converter, it has a function of supplying two pulse drive signals necessarily having the same frequency and a phase difference of 180 °. And I for control
C61 is designed such that the duty of each pulse drive signal does not exceed 50% due to the characteristics of the push-pull converter. Further, the control IC 61 here is used to control the primary current (or the secondary current) of each of the forward converters 4A and 4B detected from the current transformers 15 and 35 via the respective diodes 62 and 63. A current mode control function is provided in which the point in time when the peak value reaches a signal level corresponding to the error feedback signal from the operational amplifier 52 is set as the off timing of the pulse drive signal to the switching element 14 or the switching element 34. Thereby, the current sharing of each forward converter 4A, 4B can be made equal. Next, the operation of the above configuration will be described. Each of the forward converters 4A and 4B applies a pulse drive signal having the same frequency and a phase shift of 180 ° from the control IC 61 to the gate of each of the switching elements 14 and 34. In the supplied relationship, when one switching element 14 is on, the other switching element 34 is off, and when one switching element 14 is off, the other switching element 34 is on. Become. Here, one forward converter 4A
When the switching element 14 is switched, the DC input voltage Vin from the input power supply 1
It is applied intermittently to the primary winding 12 of 11. Then, when the switching element 14 is turned on, a positive voltage is generated at the dot side terminal of the secondary winding 13, and a gate drive voltage is applied to the rectification switching element 26 via the resistor 24. Turn on. On the other hand, switching element
While the pulse drive signal is supplied to 14, the drive signal is not generated in the secondary winding of the insulating transformer 25, and the commutation switching element 27 does not turn on. Therefore, in this case, an output current flows from the secondary winding 13 of the transformer 11 to the resistor 3 via the choke coil 23, and energy is stored in the choke coil 23. When the pulse drive signal to the switching element 14 is interrupted, the switching element 14 is turned off, and a positive voltage is generated at the non-dot side terminal of the secondary winding 13.
At this time, the gate voltage of the rectification switching element 26 decreases, and the rectification switching element 26 turns off.However, a drive signal is generated in the secondary winding of the insulating transformer 25, so that the commutation switching element 27 turns on. I do. Therefore,
The energy previously stored in the choke coil 23 is supplied to the resistor 3 as an output current. When energy is stored in the choke coil 23 of the forward converter 4A, the choke coil 23
The current flowing in the choke coil 23 of the forward converter 4B emits energy during this period, and the current flowing in the choke coil 23 declines in this period. The output current to is almost free of ripple components. At the same time, from the input power source 1 to each forward converter 4A, 4
Since the input current to B has almost no ripple component, the input capacitor 2 and the smoothing capacitor 48 having a small capacitance can be used. On the other hand, an operational amplifier 52 constituting a feedback loop
Amplifies the error between the reference voltage Vref from the reference power supply 51 and the output voltage Vo, and supplies this to the control IC 61 as an error feedback signal. The control IC 61 starts the pulse drive signal of the switching element 14 or the switching element 34 at the pulse timing of the built-in oscillator, and then sets the peak of the current detection signal detected from the current transformers 15 and 35 via the diodes 62 and 63. When the value reaches a signal level commensurate with the error feedback signal, the pulse drive signal is turned off. This makes it possible to stabilize the output voltage Vo and equalize the current sharing between the forward converters 4A and 4B. As described above, according to the present embodiment, two forward converters 4A and 4B are connected in parallel to the resistor 3, which is a load, and a pulse drive signal having the same frequency and a phase shifted by 180 ° is driven. In a switching power supply unit that supplies switching elements 14 and 34 of each of the forward converters 4A and 4B from a signal supply unit, a control IC 61 that supplies a pulse drive signal to each switching element of a push-pull converter is used as a drive signal supply unit. I have. According to the present invention, the control IC 61 used for the push-pull converter supplies two pulse drive signals having the same frequency and a phase difference of 180 °, and the duty of each pulse drive signal is 50%. It was made by paying attention to the fact that it was originally designed not to exceed. That is, the two forward converters 4A and 4B
Is connected in parallel to the resistor 3,
The control IC 61 for such a push-pull converter is
If incorporated as a drive signal supply means for supplying a pulse drive signal to each of the switching elements 14 and 34, the ripple current component of the input and output of each of the forward converters 4A and 4B can be achieved without the need to separately design a synchronous circuit or a phase shift circuit. Is effectively canceled, the input capacitor 2 and the smoothing capacitor
48 capacity can be reduced. Moreover, the switching elements of the respective forward converters 4A and 4B
Since the duty of the pulse drive signal for 14 and 34 is limited to 50% or less, each of the forward converters 4A and 4A
The transformers 11 and 31 constituting B are reset without fail and do not saturate. In this embodiment, the peak currents flowing through the choke coils 23, 43 of the respective forward converters 4A, 4B are detected by the current transformers 15, 35, which are current detectors, and this detection signal and the output voltage Vo are taken into consideration. The control IC 61 controls the switching of the switching elements 14 and 34 based on the comparison result with the reference signal (error feedback signal). Thus, the control IC 61
Provided with the current mode control function, the forward converters 4A and 4B can equalize the current sharing. It should be noted that the present invention is not limited to the above embodiment, and various modifications are possible. According to the switching power supply of the present invention, without incorporating a synchronous circuit or a phase shift circuit,
Moreover, the maximum duty of the pulse drive signal is reliably 50%
By limiting to the following, input / output ripple can be effectively reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a switching power supply device showing one embodiment of the present invention. FIG. 2 is a circuit diagram of a switching power supply device showing a conventional example. [Description of Signs] 3 Resistance (load) 4A, 4B Forward converter 14, 34 Switching element 61 Control IC (drive signal supply means)

Claims (1)

Claims: 1. A switching element of each forward converter in which two forward converters are connected in parallel to a load and a pulse driving signal whose phase is shifted by 180 ° at the same frequency from a driving signal supply means. Wherein a control IC for supplying a pulse drive signal to each switching element of the push-pull converter is used as the drive signal supply means.