JP2006223021A - Switching power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply device capable of increasing the holding time of an output voltage by relieving the limit of a maximum duty Dmax. <P>SOLUTION: When an input voltage Vi has dropped and has reached a low input, a low-input voltage detector 32 detects the low-input state to turn off a switch device 31. When a reset current Ir runs during an OFF period Toff, a Zener voltage Vz occurs at a Zener diode 30, and a voltage Vds between a drain and a source becomes a total (Vds=Vi+Vz) of the input voltage Vi and the Zener voltage Vz. Accordingly, the voltage Vds between the drain and the source becomes at least the input voltage Vi and a reset voltage Vr of at least the input voltage Vi can be applied to a primary winding 10a of a transformer 10. Thus, by relieving the limit of the maximum duty Dmax, the holding time of an output voltage Vo can be increased. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a switching power supply device that converts input power into output power having a desired voltage value by controlling a duty ratio (on / off time ratio) of the switching element.

  As a conventional switching power supply device, a so-called cascade forward converter of a two-stone forward type as disclosed in Patent Document 1 is widely known. FIG. 3 shows a circuit configuration of a general cascade forward converter.

  Reference numeral 2 denotes a DC power source for inputting an input voltage Vi to the cascade forward converter 1. The drain of the switching element 6 made of, for example, a MOS type FET is connected to the positive side thereof, while the negative side is made of, for example, a MOS type FET. The source of the switching element 7 is connected. The source of the switching element 6 is connected to the dot side of the primary winding 10a, and the drain of the switching element 7 is connected to the non-dot side of the primary winding 10a. A diode 8 is connected between the drain of the switching element 6 and the non-dot side of the primary winding 10a, the drain side of the switching element 6 is the cathode of the diode 8, and the non-dot side of the primary winding 10a is the anode of the diode 8. It has become. Similarly, a diode 9 is connected between the source of the switching element 7 and the dot side of the primary winding 10a. The source side of the switching element 7 is the anode of the diode 9, and the dot side of the primary winding 10a is the cathode of the diode 9. It has become. Switching control means 11 for supplying a pulse drive signal is connected to the gates of the switching elements 6 and 7. The switching control means 11 variably controls the pulse conduction width of the pulse drive signal supplied to the switching elements 6 and 7 according to the fluctuation of the output voltage Vo in order to stabilize the output voltage Vo. Reference numeral 5 denotes a capacitor for removing the pulsating flow of the input voltage Vi and is connected in parallel to the DC power source 2.

  The secondary winding 10b of the transformer 10 includes a rectifier diode 15, a flywheel diode 16, a choke coil 17, a smoothing capacitor 18 for rectifying and smoothing the induced voltage induced in the secondary winding 10b. A rectifying / smoothing circuit comprising: More specifically, the anode of the rectifier diode 15 is connected to the dot side of the secondary winding 10b, the anode of the flywheel diode 16 is connected to the non-dot side of the secondary winding 10b, and the cathode of the rectifier diode 15 The cathode of the flywheel diode 16 is connected. A choke coil 17 and a smoothing capacitor 18 are connected in an inverted L shape between both ends of the flywheel diode 16, and a pair of terminals for supplying the output voltage Vo to the load 21 between both ends of the smoothing capacitor 18. Output terminals 19 and 20 are provided.

  In the cascade forward converter 1, the switching elements 6 and 7 are simultaneously turned on / off, whereby the output voltage Vo is output to the load 21. That is, when the cascade forward converter 1 is operated, a pulse drive signal synchronized by the switching control means 11 is supplied to the gates of the switching elements 6 and 7, respectively, and the switching elements 6 and 7 are simultaneously switched to thereby input from the DC power source 2. The voltage Vi is intermittently applied to the primary winding 10a of the transformer 10. Then, the voltage induced in the secondary winding 10b of the transformer 10 is rectified by the rectifier diode 15 and the flywheel diode 16, and then smoothed by the choke coil 17 and the smoothing capacitor 18, between the output terminals 19 and 20. It is output as a DC output voltage Vo to the connected load 21.

FIG. 4 is a waveform diagram showing the drain-source voltage Vds of the switching element 7 on the low side and the reset current Ir flowing through the diode 8 on the high side when the cascade forward converter 1 is operating. . In the on period Ton in which the switching elements 6 and 7 are on, the drain-source voltage Vds is substantially 0 V, that is, the drain-source of the switching elements 6 and 7 is conducted, and the input voltage Vi is transferred from the DC power source 2 10 is applied to the primary winding 10a. On the other hand, in the off period Toff in which the switching elements 6 and 7 are off, a voltage due to the magnetic energy stored in the transformer 10 is generated between the primary windings 10a, and the drain-source voltage Vds rises rapidly. When the drain-source voltage Vds reaches the input voltage Vi, the diodes 8 and 9 are turned on, and a path extending from the negative side of the DC power source 2 to the diode 9 → the primary winding 10 a → the diode 8 → the positive side of the DC power source 2. A reset current Ir flows, and the magnetic energy of the transformer 10 is regenerated in the DC power source 2. At this time, the drain-source voltage Vds is clamped to the input voltage Vi. When the magnetic energy of the transformer 10 is regenerated by the reset current Ir, the reset current Ir decreases with time as the magnetic energy decreases. Go. When the reset current becomes 0, the drain-source voltage Vds gradually decreases. Finally, the drain-source voltage Vds becomes 1 of the input voltage Vi due to the balance of the parasitic capacitances of the diodes 8 and 9. It will decrease to around / 2.
Japanese Unexamined Patent Publication No. 7-177741

  However, in the conventional switching power supply device such as the cascade forward converter 1, the maximum duty Dmax (maximum ON ratio in one cycle) of the pulse drive signal supplied to the switching elements 6 and 7 is normally limited to 50%. For this reason, there is a problem that the rated value of the output voltage Vo cannot be maintained when the input voltage Vi is low as shown in FIG. 5 (or when the input is shut off). In general, in a converter such as the cascade forward converter 1, the voltage applied to the transformer 10 is unidirectional. Therefore, if the magnetic flux of the transformer 10 is not reset during the off period Toff of the switching elements 6 and 7, the core causes magnetic saturation, Overcurrent flows through the primary circuit. Therefore, it is necessary to satisfy the reset condition Vi · Dmax <Vr (1−Dmax) derived from the ET product (product of applied voltage and application time) representing the magnetic flux density of the transformer. Vr is a reset voltage generated in the primary winding 10a when the switching elements 6 and 7 are turned off. In the cascade forward converter 1, the reset voltage Vr becomes the drain-source voltage Vds in the off period Toff of the switching elements 6 and 7, but the drain-source voltage Vds is clamped by the input voltage Vi as described above. The reset voltage Vr cannot be made higher than the input voltage Vi. Therefore, when Vr = Vi under the reset condition, Dmax <0.5, and the maximum duty Dmax is limited to 50%. In order to maintain the output voltage Vo at the time of low input, it is necessary to increase the duty, but the duty cannot be increased to 50% or more, and it is difficult to maintain the output voltage Vo.

  SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a switching power supply device that can relax the maximum duty limit and extend the output voltage holding time.

  In the switching power supply device according to claim 1 of the present invention, a transformer having a primary winding and a secondary winding, the primary winding, a first switching element connected to one end of the primary winding, and the primary winding And a positive electrode side serves as the first switching element, while a negative electrode side serves as the second switching element and supplies an input voltage to the series circuit. A DC power source, a first rectifying element that bypasses current from the other end of the primary winding to the positive side of the DC power source, and a second that bypasses current from the negative side of the DC power source to one end of the primary winding. The input voltage is intermittently applied to the primary winding by turning on and off the first switching element and the second switching element. In the switching power supply apparatus for extracting an output voltage from a winding, a clamp voltage value of a reset voltage generated in the primary winding when the first switching element and the second switching element are turned off is set to a desired voltage as the input voltage. Clamp voltage shift means for shifting to a voltage value obtained by adding is provided.

  The present invention has been made in view of the fact that the maximum duty limit can be relaxed by increasing the reset voltage, and the clamp voltage shift means shifts the clamp voltage value of the reset voltage to a voltage value higher than the input voltage. As a result, the reset voltage can be made equal to or higher than the input voltage, so that the maximum duty of the switching element is not limited to 50%, and the holding time of the output voltage can be extended even at low input.

  In the switching power supply device according to the second aspect of the present invention, the clamp voltage shifting means is a constant voltage element.

  In this way, the clamp voltage value of the reset voltage can be easily adjusted to a predetermined value with a simple circuit configuration, and damage due to the reset voltage exceeding the withstand voltage of the switching element can be prevented.

  According to a third aspect of the present invention, a switching element that is turned on when the input voltage is steady and turned off when the input voltage is low is connected in parallel to the clamp voltage shift means.

  In this way, when the input voltage does not require a large duty, the clamp voltage shift means is short-circuited by turning on the switch element, so that the loss in the clamp voltage shift means can be reduced.

  According to the first aspect of the present invention, it is possible to provide a switching power supply device that can relax the limitation on the maximum duty and extend the holding time of the output voltage.

  According to the second aspect of the present invention, it is possible to relax the limit of the maximum duty by raising the reset voltage within a range where the switching element is not damaged.

  According to the third aspect of the present invention, it is possible to suppress the useless loss by causing the constant voltage element to function only at the time of low input.

  Hereinafter, preferred embodiments of a switching power supply device according to the present invention will be described with reference to the accompanying drawings. In addition, the same code | symbol is attached | subjected to the same location as a prior art example, and since description of a common part overlaps, it abbreviate | omits as much as possible.

  FIG. 1 shows a circuit configuration of a cascade forward converter 1 in this embodiment. Also in this embodiment, the basic configuration as a switching power supply device is the same as the conventional one. That is, a series of a switching element 6 on the high side, a primary winding 10a of the transformer 10 and a switching element 7 on the low side is provided between both ends of the DC power supply 2 that inputs the input voltage Vi to the cascade forward converter 1. The circuit is connected, and the capacitor 5 is connected in parallel. Switching control means 11 for supplying a pulse drive signal is connected to the gates of the switching elements 6 and 7. The secondary winding 10b of the transformer 10 includes a rectifier diode 15, a flywheel diode 16, a choke coil 17, a smoothing capacitor for rectifying and smoothing the induced voltage induced in the secondary winding 10b. And a pair of output terminals 19 and 20 for supplying an output voltage Vo to the load 21 are provided between both ends of the smoothing capacitor 18.

  Also in this embodiment, between the drain of the switching element 6 and the non-dot side of the primary winding 10a, the drain side of the switching element 6 is the cathode of the diode 8, and the non-dot side of the primary winding 10a is the anode of the diode 8. The diode 8 is connected, and the source side of the switching element 7 is the anode of the diode 9 and the dot side of the primary winding 10a is the diode 9 between the source of the switching element 7 and the dot side of the primary winding 10a. In this embodiment, a diode 9 and a switching element 31 are connected in parallel between the drain of the switching element 6 and the cathode of the diode 8. A circuit is inserted.

  The Zener diode 30 has an anode connected to the drain of the switching element 6 and a cathode connected to the cathode of the diode 8, and generates a predetermined Zener voltage Vz when a reset current Ir flows through the diode 8 as will be described later. The switch element 31 is, for example, a transistor or a relay, and performs an on / off operation in response to an on / off signal from the low input voltage detection means 32. The low input voltage detection means 32 is connected between both ends of the DC power source 2 and monitors the voltage of the input voltage Vi. In normal times, the low input voltage detection means 32 outputs an ON signal to the switch element 31, while the input voltage Vi is a predetermined value. An OFF signal is output to the switch element 31 at the time of low input that has decreased to a value. That is, when the input voltage Vi is high, the switch element 31 is turned on (conductive), and the Zener diode 30 is short-circuited. On the other hand, when the input voltage Vi is low, the switch element 31 is turned off (opened). Will work.

  Next, the operation of the cascade forward converter 1 will be described with respect to the above configuration.

  When the cascade forward converter 1 is operated, a pulse drive signal synchronized by the switching control means 11 is supplied to the gates of the switching elements 6 and 7, respectively, and the switching elements 6 and 7 are simultaneously switched, whereby the input voltage Vi is supplied from the DC power source 2. Is intermittently applied to the primary winding 10a of the transformer 10. Then, the voltage induced in the secondary winding 10b of the transformer 10 is rectified by the rectifier diode 15 and the flywheel diode 16, and then smoothed by the choke coil 17 and the smoothing capacitor 18, between the output terminals 19 and 20. It is output as a DC output voltage Vo to the connected load 21.

  Since the Zener diode 30 is short-circuited at normal times when the input voltage Vi is high, the reset operation of the transformer 10 is the same as in FIG. 4 shown in the conventional example. That is, in the off period Toff, the reset voltage Vr (drain-source voltage Vds) is generated by the magnetic energy stored in the transformer 10 in the on period Ton, and the negative side of the DC power supply 2 → the diode 9 → the primary winding 10a → A reset current Ir flows through the path from the diode 8 to the positive electrode side of the DC power supply 2, and the magnetic energy of the transformer 10 is regenerated in the DC power supply 2. At this time, since the drain-source voltage Vds which is the reset voltage Vr is clamped to the input voltage Vi, the maximum duty is limited to 50%.

  When the input voltage Vi decreases to a predetermined value for some reason and the input is low, the low input voltage detection unit 32 detects the low input state and outputs an off signal to the switch element 31. The switch element 31 is turned off by this off signal, and the reset current Ir flows to the Zener diode 30. When the reset current Ir flows during the off period Toff, a Zener voltage Vz is generated in the Zener diode 30. This Zener voltage Vz is generated in a direction overlapping the input voltage Vi with respect to the non-dot side of the primary winding 10a. Therefore, as shown in FIG. 2, the drain-source voltage Vds is the sum of the input voltage Vi and the Zener voltage Vz (Vds = Vi + Vz). That is, the Zener diode 30 operates to shift the clamp voltage value for clamping the reset voltage Vr from the input voltage Vi to a voltage value Vi + Vz that is a higher voltage value. Accordingly, the drain-source voltage Vds becomes equal to or higher than the input voltage Vi, and the reset voltage Vr equal to or higher than the input voltage Vi can be applied to the primary winding 10a of the transformer 10. Here, the reset condition Vi · Dmax <Vr (1−Dmax) is considered. When the conditional expression is modified for Dmax, Dmax <(Vi + Vz) / (2Vi + Vz) is expressed using Vr = Vds = Vi + Vz. In the case of FIG. 2, since Vi = 270V and Vz = 80V, Dmax <350 / 620≈0.565, and theoretically, the maximum duty Dmax can be widened to about 56.5%. The actual duty in FIG. 2 is about 53.7%. Therefore, it is possible to relax the limitation on the maximum duty Dmax that is up to 50% and extend the holding time of the output voltage Vo long.

  The withstand voltage of the switching elements 6 and 7 in the present embodiment may be the input voltage Vi in normal times, but greatly changes at low input. That is, since the voltage stress of the switching element 6 does not change, the same breakdown voltage may be used. However, since a voltage of Vds = Vi + Vz is applied to the switching element 7, a breakdown voltage corresponding to the voltage is required. Therefore, the Zener diode 30 having the Zener voltage Vz corresponding to the breakdown voltage of the switching element 7 must be selected.

  As a modification of the present embodiment, there may be considered one that linearly controls the clamp voltage value of the reset voltage Vr using, for example, a transistor or the like instead of the Zener diode 30. In short, any clamp voltage shifting means for shifting the clamp voltage value of the reset voltage Vr may be used as long as it can generate a desired voltage through the reset current Ir. Even in this case, it is necessary to suppress the drain-source voltage Vds, which is the reset voltage Vr, to a level that does not damage in consideration of the breakdown voltage of the switching element 7. In that respect, when the Zener diode 30 is used as the clamp voltage shift means as in this embodiment, the clamp voltage value of the reset voltage Vr can be easily adjusted to the predetermined value Vi + Vz with a simple circuit configuration. Damage due to the reset voltage Vr exceeding the withstand voltage of the switching element 7 can be prevented.

  In this embodiment, the switch element 31 is connected in parallel to the Zener diode 30 so that the restriction on the maximum duty Dmax is relaxed only at the time of low input. However, the maximum duty Dmax is 50% or more even in the normal time. If it is necessary to provide the switch element 31, the switch element 31 may not be provided. In this case, however, the duty can be increased, but the loss in the diode 8 and the Zener diode 30 increases. In other words, the loss in the Zener diode 30 can be reduced by providing the switch element 31 and short-circuiting the Zener diode 30 when the input voltage is steady.

  As described above, in this embodiment, the transformer 10 having the primary winding 10a and the secondary winding 10b, the primary winding 10a, the first switching element 6 connected to one end of the primary winding 10a, and the primary winding. A DC circuit that includes a second switching element 7 connected to the other end of the line 10a and a positive electrode side that serves as the switching element 6, while a negative electrode side that serves as the switching element 7 and supplies the input voltage Vi to the series circuit. 2, a diode 8 as a first rectifying element that bypasses the reset current Ir from the other end of the primary winding 10 a to the positive side of the DC power supply 2, and a reset current from the negative side of the DC power supply 2 to one end of the primary winding 10 a And a diode 9 as a second rectifier element that bypasses Ir, and the input voltage Vi is intermittently applied to the primary winding 10a by turning on and off the switching element 6 and the switching element 7. In addition, in the cascade forward converter 1 that is a switching power supply device that extracts the output voltage Vo from the secondary winding 10b, the clamp voltage value of the reset voltage Vr generated in the primary winding 10a when the switching elements 6 and 7 are turned off is determined as the input voltage. A Zener diode 30 is provided as a clamp voltage shift means for shifting to a voltage value obtained by adding a Zener voltage Vz as a desired voltage to Vi.

  The present invention has been made by paying attention to the fact that the limit of the maximum duty Dmax can be relaxed by increasing the reset voltage Vr. The Zener diode 30 sets the clamp voltage value of the reset voltage Vr to a voltage value higher than the input voltage Vi. Since the reset voltage Vr can be made equal to or higher than the input voltage Vi by shifting to, the maximum duty Dmax of the switching elements 6 and 7 is not limited to 50%, and the holding time of the output voltage Vo is reduced even at low input. Can be extended for a long time. Therefore, it is possible to provide a switching power supply device that can relax the restriction on the maximum duty Dmax and extend the holding time of the output voltage Vo for a long time.

  In the cascade forward converter 1 of this embodiment, the clamp voltage shift means is a Zener diode 30 as a constant voltage element.

  In this way, the clamp voltage value of the reset voltage Vr can be easily adjusted to the predetermined value Vi + Vz with a simple circuit configuration, and damage due to the reset voltage Vr exceeding the withstand voltage of the switching element 7 can be prevented. it can. Therefore, the reset voltage Vr can be increased within a range where the switching element 7 is not damaged, and the limitation on the maximum duty Dmax can be relaxed.

  Further, in the cascade forward converter 1 of this embodiment, a switching element 31 that is turned on when the input voltage Vi is steady and turned off when the input voltage is low is connected in parallel to the Zener diode 30.

  In this way, when the input voltage Vi does not require large duty, the Zener diode 30 is short-circuited when the switch element 31 is turned on, so that the loss in the Zener diode 30 can be reduced. Therefore, the Zener diode 30 is allowed to function only at the time of low input, and useless loss can be suppressed.

  In addition, this invention is not limited to the said Example, It can change in the range which does not deviate from the meaning of this invention. The clamp voltage shift means only needs to be able to shift the relative clamp voltage value viewed from one end of the primary winding 10a, which is the reference potential of the reset voltage Vr. For example, a Zener on a path through which the reset current Ir flows, such as the low side. A diode 30 may be provided. When the Zener diode 30 is provided on the low side, for example, the cathode of the Zener diode 30 may be connected to the negative side of the DC power supply 2 and the anode of the Zener diode 30 may be connected to the anode of the diode 9.

It is a circuit diagram which shows the structure of the switching power supply device in 1st Example of this invention. FIG. 6 is a waveform diagram showing a reset voltage Vr and a reset current Ir when the switching power supply device is at a low input (input voltage Vi = 270 V). It is a circuit diagram which shows the structure of the switching power supply device in a prior art example. FIG. 4 is a waveform diagram showing a reset voltage Vr and a reset current Ir when the switching power supply device is normal (input voltage Vi = 300 V). FIG. 6 is a waveform diagram showing a reset voltage Vr and a reset current Ir when the switching power supply device is at a low input (input voltage Vi = 270 V).

Explanation of symbols

1 Cascade forward converter (switching power supply)
2 DC power supply 6 Switching element (first switching element)
7 Switching element (second switching element)
8 Diode (first rectifier)
9 Diode (second rectifier)
10 transformer
10a Primary winding
10b Secondary winding
30 Zener diode (clamp voltage shift means, constant voltage element)
31 Switch element

Claims (3)

A transformer having a primary winding and a secondary winding; a primary switching element connected to one end of the primary winding; and a second switching element connected to the other end of the primary winding. A series circuit consisting of
A DC power supply for supplying an input voltage to the series circuit, with a positive electrode side serving as the first switching element and a negative electrode side serving as the second switching element;
A first rectifying element that bypasses current from the other end of the primary winding to the positive electrode side of the DC power supply; and a second rectifying element that bypasses current from the negative electrode side of the DC power supply to one end of the primary winding; A switching circuit for intermittently applying the input voltage to the primary winding and extracting the output voltage from the secondary winding by turning on and off the first switching element and the second switching element. In the power supply apparatus, a clamp that shifts a clamp voltage value of a reset voltage generated in the primary winding when the first switching element and the second switching element are turned off to a voltage value obtained by adding a desired voltage to the input voltage A switching power supply comprising a voltage shift means. 2. The switching power supply device according to claim 1, wherein the clamp voltage shift means is a constant voltage element. 3. The switching power supply device according to claim 1, wherein a switching element that is turned on when the input voltage is steady and turned off when the input voltage is low is connected in parallel to the clamp voltage shift means.

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