JP5642621B2 - Switching power supply - Google Patents

Description

  The present invention relates to a full-bridge type switching power supply apparatus that stabilizes an output voltage by phase shift control.

  Conventionally, in a switching power supply device that handles power exceeding 1 kW, a full-bridge type power supply circuit that stabilizes an output voltage by phase shift control has been used from the viewpoint of high efficiency and fixed switching frequency. Yes.

  As a switching power supply device of this type, for example, as disclosed in Patent Document 1 as a conventional technique, the switching current flowing through the main switching element connected to the primary winding of the transformer is resonated (so-called current resonance) to generate power. There is a switching power supply device that performs conversion and stabilizes an output voltage by phase shift control. The resonance circuit that resonates the switching current is composed of a series circuit of a resonance inductor and a resonance capacitor, and is inserted at a position in series with the primary winding of the transformer.

  In the DC-DC converter disclosed in Patent Document 2, a resonance inductor for zero voltage switching (hereinafter referred to as ZVS) is inserted in a position in series with the primary winding of the transformer, and the energy discharging operation of the resonance inductor is performed. This assists the discharge of electric charges from the capacitors at both ends of the main switching element and enables the ZVS operation. In this DC-DC converter, first and second regenerative diodes are provided between the connection point of the primary winding and resonant inductor of the transformer and both ends of the input power supply, and the rectification is connected to the secondary winding of the transformer. Surge voltage generated at both ends of the diode is reduced.

JP 2006-197711 A International Publication WO2007 / 000830

  In the case of the switching power supply device according to the prior art of Patent Document 1, the ZVS operation of the main switching element is impossible depending on the state of the output current, and there is a problem that the switching loss of the main switching element increases and the efficiency is lowered. . In order to solve this problem, the switching power supply device according to the invention of Patent Document 1 is provided with an auxiliary resonance circuit using two capacitors and an auxiliary resonance inductor. However, by adding this auxiliary resonance circuit, at least two inductor elements having a relatively large outer shape (resonance inductor and auxiliary resonance inductor) are required, which is disadvantageous in terms of mounting space and cost.

  Further, in the case of the DC-DC converter of Patent Document 2, when the input power is turned on or when the output current changes sharply, the transformer is transiently DC-biased and magnetically saturated, and becomes a main switching element. There is a risk of circuit elements being damaged due to excessive electrical stress. Therefore, it is necessary to increase the outer shape of the transformer in order to provide a margin for magnetic saturation. In addition, the magnitude of the current flowing through the regenerative diode is not particularly considered, and depending on the position where the resonant inductor is connected, an excessive current flows through the regenerative diode and may be damaged.

  The present invention has been made in view of the above-described background art, can reliably perform the ZVS operation of the main switching element, and has a small size with high safety without applying excessive electrical stress to a specific circuit element. An object of the present invention is to provide a switching power supply apparatus.

The present invention is composed of a series circuit of main switching elements provided on the high side and the ground side, respectively, a first arm having a DC input source connected to both ends thereof, and provided on the high side and the ground side, respectively. A second circuit connected in parallel to the first arm, a primary winding and a secondary winding, and the primary winding of the first arm A transformer provided between a middle point and the middle point of the second arm, and connected to the secondary winding of the transformer, converts an AC voltage generated in the secondary winding into a DC output voltage. The rectifying / smoothing circuit that outputs the output voltage and the main switching elements are turned on and off to perform a first period in which an input voltage is applied to the primary winding in a positive direction, a second period in which no input voltage is applied, and a negative period. Applied in the direction A full-bridge type switching power supply comprising: a switching control circuit that performs phase shift control with a period of three periods and a fourth period during which no voltage is output again as one cycle, and stabilizes the output voltage of the rectifying and smoothing circuit A first capacitor that is inserted in a position in series with the primary winding and suppresses DC bias of the transformer, and is in a position in series with the primary winding and the first capacitor, and has one end A resonant inductor inserted at a position connected to the midpoint of the second arm, and an anode terminal connected to one end of the resonant inductor on the primary winding side, on the high side of the first and second arms A first regenerative diode having a cathode terminal connected to a connection point; and an anode terminal connected to a ground side connection point of the first and second arms; A second regenerative diode having a cathode terminal connected to one end of the primary winding side, and the switching control circuit is configured to turn on or off the main switching element that constitutes the first arm. The switching power supply device performs phase shift control in such a manner that a period is started and the second and fourth periods are started by turning on or turning off the main switching element constituting the second arm.

  The main switching element of the first and second arms is composed of an N-ch MOS type FET, a parallel capacitor and a parallel diode capable of conducting in the direction from the source terminal to the drain terminal between the individual drains and sources. Furthermore, the switching control circuit turns off the one main switching element that is turned on when the main switching elements on the high side and ground side of the first arm are complementarily turned on and off. After that, at the timing when the voltage across the other main switching element that is turned off decreases toward zero volts due to the resonance operation of the resonant inductor and the parallel capacitor, the other main switching element is turned on, The main switch on the high side and ground side of the second arm When the switching element is turned on and off in a complementary manner, after turning off one of the main switching elements that is turned on, the voltage across the other main switching element that is turned off is the resonance inductor, the parallel capacitor, The other main switching element may be controlled to be turned on at a timing when the resonance operation decreases to zero volts.

  It is preferable that the primary and secondary windings of the transformer are magnetically coupled so that a leakage inductance viewed from the primary winding side is smaller than an inductance of the resonant inductor. Further, a second capacitor for limiting a regenerative current flowing in the first and second regenerative diodes is inserted between a midpoint of the first and second regenerative diodes and one end of the resonant inductor on the primary winding side. May be.

  The rectifying / smoothing circuit has a full-wave rectifying method, a center tap method, or a current doubler method. Further, the rectifying unit included in the rectifying / smoothing circuit may be configured to perform synchronous rectification using a MOS FET.

  In the switching power supply device of the present invention, the ZVS operation of the main switching element can be reliably performed by the operation of the resonant inductor, and further, the first capacitor that suppresses the DC bias of the transformer is provided. Even in a transient state, it is possible to easily avoid magnetic saturation due to the DC bias of the transformer and prevent each circuit element from being damaged.

  In addition, since the surge voltage generated in the rectifying element of the rectifying and smoothing circuit is suppressed by the regenerative operation performed by the first and second regenerative diodes, it is possible to select a low withstand voltage semiconductor with a small conduction resistance or voltage drop as the rectifying element. Thus, power supply efficiency can be improved. Also, the first and second regenerative diodes can be obtained by inserting a resonant inductor at a position in series with the primary winding and the first capacitor and having one end connected to the midpoint of the second arm. In addition, since an excessive current does not flow, the safety is high.

1 is a circuit diagram showing a first embodiment of a switching power supply device of the present invention. It is a circuit diagram (a) and (b) which shows each modification of a position which provides the 1st capacitor. It is a circuit diagram which shows the structure of the rectification smoothing circuit of the switching power supply device of 1st embodiment. It is a time chart explaining the steady operation of the switching power supply device of a first embodiment. It is the voltage waveform (a) of both ends of the rectifier diode 38b of the switching power supply device of the first embodiment, and the voltage waveform (b) of both ends of the rectifier diode 38b when the first and second regenerative diodes are not provided. It is a circuit diagram of a switching power supply device of a comparative example in which a resonant inductor is inserted at an inappropriate position. It is a time chart explaining the steady operation of the switching power supply device of FIG. It is a circuit diagram (a), (b), and (c) which show each modification of a rectification smoothing circuit of a switching power supply device of a first embodiment. It is a circuit diagram which shows the 2nd capacitor added to the switching power supply device of 1st embodiment.

  Hereinafter, a first embodiment of a switching power supply device of the present invention will be described with reference to the drawings. As shown in FIG. 1, the switching power supply device 10 of the first embodiment converts a DC input voltage Vi input between input terminals 12a and 12b into a DC output voltage Vo, and outputs between the output terminals 14a and 14b. This is a power supply device that supplies an output voltage Vo and a current Io to a connected load 16. Here, the input and output terminals 12b and 14b are ground-side potentials of the input and output voltages Vi and Vo, and the input and output terminals 12a and 14a are positive-side potentials.

  A first arm 18, which is a series circuit of main switching elements 18 a and 18 b on the high side and ground side, is connected between the input terminals 12 a and 12 b, and further, in parallel with the first arm 18, the high side and ground side are connected. The second arm 20, which is a series circuit of the main switching elements 20a and 20b, is connected. Here, N-ch MOS FETs are used as the main switching elements 18a, 18b, 20a, and 20b, the drain terminals of the main switching elements 18a and 20a are the input terminal 12a, and the source terminals of the main switching elements 18b and 20b are the main terminals. Each is connected to the input terminal 12b. The connection point between the source terminal of the main switching element 18a and the drain terminal of the main switching element 18b is the midpoint of the first arm 18 (hereinafter referred to as the first midpoint 18c), and the source of the main switching element 20a. A connection point between the terminal and the drain terminal of the main switching element 20b is a midpoint 20c of the second arm 20 (hereinafter referred to as a second midpoint 20c). Further, in each chip of the main switching elements 18a, 18b, 20a, and 20b, there are formed circuit diodes (not shown), parasitic diodes in the direction from the source terminal to the drain terminal, and parasitic capacitors in parallel therewith.

  In this embodiment, the parallel diode and the parallel capacitor provided between the drain and source of each main switching element are composed of a parasitic diode and a parasitic capacitor, but further, a high-performance diode element is connected to the outside, A capacitor element for adjusting capacitance may be connected.

  A primary winding 22a of the transformer 22 is provided between the first middle point 18c and the second middle point 20c, and one end with a dot is connected to the first middle point 18c. The transformer 22 includes a primary winding 22a and a secondary winding 22b, which are closely magnetically coupled to each other. The leakage inductance viewed from the primary winding 22a side is greater than the inductance of a resonant inductor 26 described later. Is small enough.

  One end of the first capacitor 24 is connected to one end of the primary winding 22a on the second middle point 20c side, and a resonant inductor 26 is connected between the other end of the first capacitor 24 and the second middle point 20c. ing. The first capacitor 24 is a capacitor that suppresses the direct current bias of the transformer 22, and is set to a large capacitance value above a certain level, and has a sufficiently small impedance at the switching frequency. Therefore, the influence on the waveform shape of the switching current flowing in the main switching elements 18a, 18b, 20a, and 20b and the voltage generated in the primary winding 22a is small. The resonant inductor 26 assists the discharge of charges of parasitic capacitors (not shown) at both ends of the main switching elements 18a, 18b, 20a, and 20b by its energy discharging operation, and enables the ZVS operation.

  The series circuit including the primary winding 22a, the first capacitor 24, and the resonant inductor 26 may be configured so that one end of the resonant inductor 26 is connected to the second middle point 20c. For example, FIG. ), The arrangement of the primary winding 22a and the first capacitor 24 is reversed, or as shown in FIG. 2B, the divided first capacitor 24 is arranged on both sides of the primary winding 22a. It is also possible to adopt a configuration such as

  First and second regenerative diodes 28 a and 28 b are connected to one end of the resonant inductor 26. The first regenerative diode 28 a has an anode terminal connected to one end on the primary winding 22 a side of the resonant inductor 26, and a cathode terminal connected to a connection point on the high side of the first and second arms 18 and 20. The second regenerative diode 28 b has a cathode terminal connected to one end on the primary winding 22 a side of the resonant inductor 26, and an anode terminal connected to a connection point on the ground side of the first and second arms 18 and 20.

  A rectifying / smoothing circuit 30 is provided at both ends of the secondary winding 22b of the transformer 22 to convert the AC voltage generated in the secondary winding 22b into a DC output voltage Vo and output it to the output terminals 14a and 14b. The rectifying / smoothing circuit 30 has a full-wave rectifying configuration as shown in FIG. 3, and includes a rectifying unit 32 and a smoothing unit 34. The rectifier 32 includes rectifier diodes 36a and 36b each having an anode terminal and a cathode terminal connected to a dot-attached terminal of the secondary winding 22b, and an anode terminal and a cathode terminal at the other end of the secondary winding 22b. Are composed of rectifier diodes 38a and 38b connected to each other. The rectifier diodes 36a and 38a have their cathode terminals connected to each other, the rectifier diodes 36b and 38b have their anode terminals connected to each other, and a waveform obtained by full-wave rectifying the AC voltage of the secondary winding 22a between the two connection points. Is output. The smoothing unit 34 is a low-pass filter including a smoothing inductor 40 and a smoothing capacitor 42, and smoothes the full-wave rectified waveform output from the rectifying unit 32 and outputs a DC output voltage Vo.

  The main switching elements 18a, 18b, 20a, 20b are driven on and off by the switching control circuit 44. The switching control circuit 44 generates four types of drive pulses having a duty ratio of about 50% and different phases based on the output voltage Vo, and outputs them to the drive terminals of the main switching elements 18a, 18b, 20a, and 20b. To do. Then, by turning on and off the four main switching elements at different phases, a first period T11 in which the input voltage Vi is applied to the primary winding 22a in the positive direction (the dot-attached side is a high potential), the input voltage Vi Is provided in a second period T12 in which no voltage is applied, a third period T13 in which the negative direction (on the side opposite to the dots is at a high potential), and a fourth period T14 in which no voltage is output again are provided. The output voltage Vo is stabilized by performing phase shift control at a constant frequency. At this time, the switching control circuit 44 starts the first and third periods T11 and T13 by turning on or off the main switching elements 18a and 18b on the first arm 18 side, and the main switching element 20a on the second arm 20 side. , 20b is turned on or off, and the second and fourth periods T12 and T14 are started.

  Next, the steady operation of the switching power supply device 10 will be described based on the time chart of FIG. Here, the steady operation is a state in which a certain amount of time has elapsed after the input voltage Vi is input between the input terminals 12a and 12b, and a constant output voltage Vo and current Io are stably supplied to the load 16. Say. In FIG. 4, voltages Vg-18a, Vg-18b, Vg-20a, Vg-20b are drive pulses applied to the drive terminals of the main switching elements 18a, 18b, 20a, 20b, respectively. Each of the main switching elements that receives the signal is turned on when the level is high and turned off when the level is low. The voltage V-22a is a voltage of the primary winding 22a of the transformer 22 (a terminal without a dot is a reference potential). The voltages Vd-18a and Vd-20a are voltages between the drain and source of the main switching elements 18a and 20a (the source terminal is a reference potential), and the currents Id-18a and Id-20a are applied to the main switching elements 18a and 20a. This is a switching current that flows (the direction from the drain terminal to the source terminal is the positive direction). The voltages Vf-38b and Vf-36b are voltages across the rectifier diodes 38b and 36b (the anode terminal is a reference potential), and the currents If-38b and If-36b are currents flowing from the rectifier diodes 38b and 36b (from the anode terminal). The direction of the cathode terminal is the positive direction). The currents If-28a and If-28b are regenerative currents flowing through the first and second regenerative diodes 28a and 28b (the direction from the anode terminal to the cathode terminal is the positive direction).

  In the first period T11, the main switching elements 18a, 18b, 20a, and 20b are on, off, off, and on, respectively. At this time, since the impedance of the first capacitor 24 and the resonant inductor 26 is sufficiently smaller than the impedance at both ends of the primary winding 22a, a voltage substantially equal to the input voltage Vi is positive in the primary winding 22a as indicated by the voltage V-22a. Applied in the direction.

  The ZVS operation performed in a very short period immediately after the transition to the period T11 will be described later. In the subsequent period, the switching current is the DC input power supply, the input terminal 12a, the main switching element 18a, the primary winding 22a, the second winding. Secondary winding 22b, rectifier diode 36a, smoothing inductor 40, smoothing capacitor 42 and load 16, rectifier diode 38b, secondary winding 22b, primary winding 22a, first capacitor 24, resonant inductor 26, main switching element 20b, input It flows in the path of the terminal 12b. As shown in the waveforms of the currents Id-18a and If-38b, the switching current has a waveform that gradually increases with a slope substantially defined by the smoothing inductor 40, and simultaneously supplies the output current Io to the load 16, and at the same time, the smoothing inductor. 40 and the resonant inductor 26 store excitation energy. Further, the rectifier diodes 36b and 38a are off, and the voltage indicated by the waveform of the voltage Vf-36b, that is, a constant voltage generated in the direction of the dots is applied to the secondary winding 22b. In addition, the input voltage Vi is applied to both ends of the first regenerative diode 28a in the opposite direction, and the potential difference between the both ends of the second regenerative diode 28b becomes almost zero, so that the currents If-28a and If-28b do not flow.

  In the second period T12, the main switching elements 18a, 18b, 20a, and 20b are turned on, off, on, and off, respectively. When shifting from the period T11 to the period T12, the switching control circuit 44 first turns off the main switching element 20b, and then turns on the main switching element 20a when a short period Td (not shown) has elapsed, thereby turning the main switching element 20a on. The ZVS operation is realized.

  During the period T11, a parasitic capacitor (not shown) of the main switching element 20a is almost charged to the input voltage Vi. When the period 12 is started, first, the main switching element 20b is turned off, so that the current that releases the excitation energy of the resonant inductor 26 becomes the resonant inductor 26, the parasitic capacitor, the main switching element 18a, the primary winding 22a, the first It begins to flow in the path of the capacitor 24. The parasitic capacitor is discharged by this current, and the voltage Vd-20a of the main switching element 20a decreases. When the period Td elapses and the voltage Vd-20a becomes approximately zero volts, the main switching element 20a is turned on. With this operation, the ZVS operation of the main switching element 20a is performed. In the period after the period Td, the voltage V-22a of the primary winding 22a becomes substantially zero volts, and the current that the smoothing inductor 40 releases the excitation energy is the smoothing inductor 40, the smoothing capacitor 42 and the load 16, the rectifier diode 38b, the secondary The winding 22b, the primary winding 22a, the first capacitor 24, the resonant inductor 26, the main switching element 20a, the main switching element 18a, the primary winding 22a, the secondary winding 22b, and the rectifier diode 36a flow. As shown by the waveforms of currents Id-18a, Id-20a, and If-38b, this current has a waveform that gradually decreases with a slope substantially defined by the smoothing inductor 40, and supplies the output current Io to the load 16. Further, almost no current flows through the rectifier diodes 36b and 38a, and the potential difference between both ends becomes almost zero as shown by the waveform of the voltage Vf−36b.

  At this time, the resonant inductor 26 continuously performs the operation of releasing the excitation energy from the immediately preceding period Td. Therefore, the resonant inductor 26 is connected to both ends of the resonant inductor 26 from one end on the first middle point 18c side to one end on the second middle point 20c side. A slight voltage is generated in the direction, and a reverse voltage is applied to both ends of the first regenerative diode 28a. Further, the input voltage Vi is applied to the opposite ends of the second regenerative diode 28b in substantially the opposite direction. Therefore, currents If-28a and If-28b do not flow.

  In the third period T13, the main switching elements 18a, 18b, 20a, and 20b are turned off, on, on, and off, respectively. When shifting from the period T12 to the period T13, the switching control circuit 44 first turns off the main switching element 18a, and then turns on the main switching element 18b when a short period Td (not shown) elapses, whereby the main switching element 18b. The ZVS operation is realized.

During the period T12, the parasitic capacitor (not shown) of the main switching element 18b is almost charged to the input voltage Vi. When migrating to a period T 13, the first that the main switching element 18a is turned off, current that emits excitation energy in the resonant inductor 26, resonant inductor 26, a main switching element 20a, the input terminal 12a, a DC input power source, an input terminal 12b, the parasitic capacitor, the primary winding 22a, and the first capacitor 24 begin to flow. Due to this current, the parasitic capacitor is discharged, and the voltage across the main switching element 18b decreases. Then, when the period Td elapses and the voltage between both ends becomes approximately zero volts, the main switching element 18b is turned on. With such an operation, the ZVS operation of the main switching element 18b is stably performed.

  When the period Td elapses, a voltage substantially equal to the input voltage Vi is applied in the negative direction to the primary winding 22a as indicated by a voltage V-22a. Then, the switching current is changed to DC input power supply, input terminal 12a, main switching element 20a, resonant inductor 26, first capacitor 24, primary winding 22a, secondary winding 22b, rectifier diode 38a, smoothing inductor 40, smoothing capacitor 42. And the load 16, the rectifier diode 36b, the secondary winding 22b, the primary winding 22a, the main switching element 18b, and the input terminal 12b. This switching current increases with a steep slope as shown by the waveforms of the currents Id-20a and If-36b, and accumulates excitation energy in the resonant inductor 26.

  On the other hand, in the rectifier diode 38b, the current If-38b decreases to zero ampere with a steep slope and turns off, and the voltage indicated by the waveform of the voltage Vf-38b at both ends, that is, the direction opposite to the dot on the secondary winding 22b. A constant voltage generated is applied. At this time, a large surge voltage is not generated in the voltage Vf-38b due to the action of the first regenerative diode 28a.

  If the first regenerative diode 28a is not provided, when the voltage Vf-38b rises to a certain voltage, ringing due to resonance between the parasitic capacitance (not shown) of the rectifier diode 38b and the resonant inductor 26 occurs, and FIG. As shown in b), a large surge voltage is generated. This surge voltage is particularly large when a fast recovery diode that easily generates a recovery current is used as the rectifier diode 38b, and is generated even when a Schottky barrier diode or the like that hardly generates a recovery current is used. However, since the switching power supply 10 is provided with the first regenerative diode 28a, the voltage Vf-38b tends to exceed a certain voltage (voltage obtained by multiplying the input voltage Vi by the turns ratio of the transformer 22), and the primary winding 22a. If the voltage V-22a also exceeds the input voltage Vi, the first regenerative diode 28a becomes conductive, and the DC input power source is connected in parallel to both ends of the primary winding 22a. Then, when the regenerative current indicated by the waveform of the current If-28a flows toward the DC input power supply, the rise of the voltage V-22a is stopped and clamped to the input voltage Vi. Therefore, the voltage Vf-38b cannot rise beyond a certain voltage, and the surge voltage can be kept small as shown in FIG. Further, the rectifier diode 36a that is turned off at the same timing as the rectifier diode 38b does not generate a large surge voltage at both ends by the same operation.

  After the regenerative current flows through the first regenerative diode 28a, the switching current has a waveform that gradually increases with a slope substantially defined by the smoothing inductor 40, as shown by the waveforms of the currents Id-20a and If-36b. At the same time as the output current Io is supplied to 16, the excitation energy is stored in the smoothing inductor 40 and the resonant inductor 26. Further, the constant voltage shown in the waveform of the voltage Vf-36b is applied while the rectifier diodes 36b and 38a are off. In addition, the potential difference between both ends of the first regenerative diode 28a becomes almost zero, and the input voltage Vi is applied to both ends of the second regenerative diode 28b in the opposite direction, so that the currents If-28a and If-28b do not flow. .

  In the fourth period T14, the main switching elements 18a, 18b, 20a, and 20b are turned off, on, off, and on, respectively. When shifting from the period T13 to the period T14, the switching control circuit 44 first turns off the main switching element 20a, and then turns on the main switching element 20b when a short period Td (not shown) has elapsed, thereby switching the main switching element 20b. The ZVS operation is realized.

  During the period T13, the parasitic capacitor (not shown) of the main switching element 20b is almost charged to the input voltage Vi. When the period 14 is started, first, the main switching element 20a is turned off, so that the current that the resonant inductor 26 releases the excitation energy is the resonant inductor 26, the first capacitor 24, the primary winding 22a, the main switching element 18b, It begins to flow in the path of the parasitic capacitor. The parasitic capacitor is discharged by this current, and the voltage across the main switching element 20b decreases. The main switching element 20b is turned on when the voltage at both ends becomes zero volts after the period Td elapses. With this operation, the ZVS operation of the main switching element 20b is stably performed.

  In the period after the period Td, the voltage V-22a of the primary winding 22a becomes substantially zero volts, and the current that the smoothing inductor 40 releases the excitation energy is the smoothing inductor 40, the smoothing capacitor 42 and the load 16, the rectifier diode 36b, the secondary The winding 22b, the primary winding 22a, the main switching element 18b, the main switching element 20b, the resonant inductor 26, the first capacitor 24, the primary winding 22a, the secondary winding 22b, and the rectifier diode 38a flow. As shown by the waveform If-36b, this current has a waveform that gradually decreases with a slope substantially defined by the smoothing inductor 40, and supplies the output current Io to the load 16. Further, almost no current flows through the rectifier diodes 36a and 38b, and the potential difference between both ends becomes almost zero as shown by the waveform of the voltage Vf−38b.

  At this time, the resonant inductor 26 performs an operation of releasing the excitation energy continuously from the immediately preceding period Td. Therefore, the resonant inductor 26 is connected to both ends of the resonant inductor 26 from one end on the second middle point 20c side to one end on the first middle point 18c side. A slight voltage is generated in the direction of, and a reverse voltage is applied across the second regenerative diode 28b. Further, the input voltage Vi is applied to the opposite ends of the first regenerative diode 28a in the reverse direction. Therefore, currents If-28a and If-28b do not flow.

  In the first period T11 again, the main switching elements 18a, 18b, 20a, and 20b are turned on, off, off, and on, respectively. When shifting from the period T14 to the period T11, the switching control circuit 44 first turns off the main switching element 18b, and then turns on the main switching element 18a when a short period Td (not shown) elapses, whereby the main switching element 18a. The ZVS operation is realized.

  During the period T14, the parasitic capacitor (not shown) of the main switching element 18a is almost charged to the input voltage Vi. When the period 11 is started, first, the main switching element 18b is turned off, so that the current that the resonant inductor 26 releases the excitation energy is the resonant inductor 26, the first capacitor 24, the primary winding 22a, the parasitic capacitor, the input terminal. 12a, the DC input power supply, the input terminal 12b, and the main switching element 20b begin to flow, and this current discharges the parasitic capacitor, and the voltage across the main switching element 18a decreases. Then, when the period Td elapses and the voltage between both ends becomes approximately zero volts, the main switching element 18a is turned on. With such an operation, the ZVS operation of the main switching element 18a is stably performed.

  When the period Td elapses, a voltage substantially equal to the input voltage Vi is applied to the primary winding 22a in the positive direction as indicated by a voltage V-22a. Then, the switching current is changed to DC input power supply, input terminal 12a, main switching element 18a, primary winding 22a, secondary winding 22b, rectifier diode 36a, smoothing inductor 40, smoothing capacitor 42 and load 16, rectifier diode 38b, two The secondary winding 22b, the primary winding 22a, the first capacitor 24, the resonant inductor 26, the main switching element 20b, and the input terminal 12b start to flow. The switching current increases with a steep slope as shown by the waveforms of the currents Id-18a and If-38b, and the excitation energy is accumulated in the resonant inductor 26.

  On the other hand, in the rectifier diode 36b, the current If-36b decreases to zero ampere with a steep slope and is turned off, and the voltage indicated by the waveform of the voltage Vf-36b is generated at both ends, that is, in the direction of the dots in the secondary winding 22b. A constant voltage is applied. At this time, a large surge voltage is not generated in the voltage Vf-36b due to the action of the second regenerative diode 28b.

  If the second regenerative diode 28b is not provided, when the voltage Vf-36b rises to a certain voltage, ringing due to resonance between the parasitic capacitance (not shown) of the rectifier diode 36b and the resonant inductor 26 occurs, and a large surge voltage is generated. Will occur. However, since the switching power supply 10 is provided with the second regenerative diode 28b, the voltage Vf-36b tends to exceed a certain voltage (voltage obtained by multiplying the input voltage Vi by the turns ratio of the transformer 22), and the primary winding 22a. When the voltage V-22a exceeds the input voltage Vi, the second regenerative diode 28a becomes conductive, and the input DC power supply is connected in parallel to both ends of the primary winding 22a. Then, when the regenerative current indicated by the waveform of the current If-28b flows toward the DC input power supply, the rise of the voltage V-22a is stopped and clamped to the input voltage Vi. Therefore, the voltage Vf-36b cannot rise beyond a certain voltage, and the surge voltage is suppressed to a small value. Also, with respect to the rectifier diode 38a that is turned off at the same timing as the rectifier diode 36b, a large surge voltage is not generated at both ends by the same operation. The operation in the period T11 after the regenerative current flows is as described in the first period T11.

  Here, regarding the ZVS operation performed in each period Td of the periods T11 to T14, the function of a parasitic diode (parallel diode) provided between the drain and source of each main switching element will be described. Here, the length of the period Td is a uniform time due to the initial setting of the switching control circuit 44. On the other hand, for example, the rate at which the voltage Vd-20a of the main switching element 20a decreases depends on the frequency of resonance between the parasitic capacitor of the main switching element 20a and the resonant inductor 26, and tends to vary due to individual differences between the elements. . Therefore, for example, when the resonance frequency varies high, the voltage Vd-20a decreases quickly, reaches zero volts in the middle of the period Td, and further, the resonance operation continues to try to generate a voltage in the reverse direction. . However, when the voltage Vd-20a is going to increase in the reverse direction, a parasitic diode (not shown) between the drain and source of the main switching element 20a becomes conductive, and the voltage Vd-20a can be prevented from increasing in the reverse direction. As a result, it is maintained at substantially zero volts until the period Td ends, and the above-described ZVS operation can be performed reliably. Note that when the voltage Vd-20a does not reach zero volts during the period Td, the parasitic diode does not conduct and does not participate in the ZVS operation. The same applies to the parasitic diodes of the other main switching elements 18a, 18b, and 20b.

  Next, the operation of the switching power supply device 10 of the first embodiment is compared with the operation of a general switching power supply device 50. 6 includes a primary winding 22a, a first capacitor 24, and a resonant inductor 26 provided between the midpoint 18c of the first arm 18 and the midpoint 20c of the second arm 20. In the series circuit, one end of the resonant inductor 26 is configured to be connected to the first middle point 18c. The configuration is the same as that of the switching power supply device 10 of the present embodiment except that the position where the resonant inductor 26 is provided is different. Accordingly, the same reference numerals are given to the respective components and the description thereof is omitted.

  The steady operation of the general switching power supply device 50 is represented as shown in the time chart of FIG. The operations in the first and third periods T11 and T13 in FIG. 7 are substantially the same as the operations in the first and third periods T11 and T13 of the switching power supply device 10 described in FIG. A problem occurs in the operation of the four periods T12 and T14.

  In the second period T12, the main switching elements 18a, 18b, 20a, and 20b are turned on, off, on, and off, respectively. Immediately after shifting to the period T12, similarly to the above, the ZVS operation of the main switching element 20a is performed in the period Td (not shown).

  In the period after the period Td, the voltage V-22a of the primary winding 22a becomes substantially zero volts, the smoothing inductor 40 releases the excitation energy, and a current for supplying the output current Io to the load 16 flows. At this time, the second regenerative diode 28b does not conduct because the input voltage Vi is applied to both ends in the opposite direction. On the other hand, since the resonant inductor 26 performs the operation of continuously releasing the excitation energy from the immediately preceding period Td, the resonance inductor 26 has both ends extending from one end on the first midpoint 18c side to one end on the second midpoint 20c side. A slight voltage is generated, and a forward voltage is slightly applied to both ends of the first regenerative diode 28a, thereby enabling conduction. When the first regenerative diode 28a is turned on, the currents at which the smoothing inductor 40 and the resonant inductor 26 release the excitation energy are as shown by the waveforms of currents Id-18a, Id-20a, If-36b, If-38b, If-28a. In addition, each circuit element flows through a complicated route. In particular, as shown in the waveform of the current If-28, a large sawtooth current is generated in the first regenerative diode 28a.

  In the case of the switching power supply device 10 of the first embodiment described above, the current flowing through the first regenerative diode 28a is only a small regenerative current flowing during the third period T13, as shown by the waveform If-28a in FIG. An inexpensive diode element with a small current rating could be used. On the other hand, in the case of this general switching power supply device 50, the current flowing through the first regenerative diode 28a is a small regenerative current flowing during the first period T11 as shown by the waveform If-28a in FIG. In addition, since a large sawtooth current is also generated in the second period T12, an expensive diode element having a large current rating must be used as the first regenerative diode 28a.

  Also in the fourth period T14, a phenomenon similar to that when a large sawtooth current flows in the first regenerative diode 28a in the period T12 occurs in the second regenerative diode 28b. Therefore, an expensive diode element having a large current rating must also be used for the second regenerative diode 28b.

Thus, in the case of the general switching power supply device 50, the first and third periods T11 and T13 are started instead of the second arm 20 side starting the second and fourth periods T12 and T14. Since the resonant inductor 26 is provided on the arm 18 side, there arises a problem that an excessive current flows through the first and second regenerative diodes 28a and 28b as shown in FIG. This problem can also occur in the DC-DC converter of Patent Document 2 described in the background art, depending on the position where the resonant inductor is connected. Also in the case of the switching power supply device 10 of the first embodiment, in order to make this problem less likely to occur, the primary and secondary windings 22a and 22b of the transformer 22 are closely magnetically coupled, and the leakage inductance is that of the resonant inductor 26. It is preferable to make it sufficiently smaller than the inductance.

  As described above, the switching power supply device 10 of the first embodiment is connected to both ends of the rectifier diodes 36a, 36b, 38a, and 38b of the rectifying and smoothing circuit 30 by the regenerative operation performed by the first and second regenerative diodes 28a and 28b. Since the generated surge voltage can be suppressed, a voltage drop during conduction can be suppressed by using a low breakdown voltage diode element, and the power supply efficiency can be improved. Further, the resonance inductor 26 is inserted into a position in series with the primary winding 22a and the first capacitor 24, and one end thereof is connected to the midpoint 20c of the second arm 20, whereby the first and second A phenomenon in which an excessive current flows through the regenerative diodes 28a and 28b (a phenomenon in the periods T12 and T14 in FIG. 7) can be avoided, and it is safe to use an inexpensive diode element having a small current rating. In addition, the ZVS operation of the main switching elements 18a, 18b, 20a, and 20b is reliably performed by the operation of the resonant inductor 26, and the switching loss can be reduced.

  In addition, since the first capacitor 24 for suppressing the DC bias of the transformer 22 is provided, it is possible to easily avoid the transformer 22 from being DC biased and magnetically saturated during a transition such as input. Even without using a large transformer with a sufficient magnetic saturation, it is possible to prevent the circuit elements of each part from being damaged.

  Next, a modification of the rectifying / smoothing circuit provided in the switching power supply device of the present invention will be described with reference to FIG. As shown in FIG. 8A, the rectifying / smoothing circuit 52 of the first modification includes four rectifying diodes 36a, 36b, 38a, and 38b constituting the rectifying / smoothing circuit 30 as described above. The MOSFETs 54a, 54b, 56a, and 56b are used for synchronous rectification. According to the rectifying / smoothing circuit 52, the same effects as those of the rectifying / smoothing circuit 30 can be obtained, and the loss of the rectifying unit 32 can be further reduced.

  As shown in FIG. 8B, the rectifying / smoothing circuit 58 of the second modification divides the secondary winding 22b of the transformer 22 into two parts, and performs synchronous rectification by two N-ch MOS type FETs 60 and 62. This is a center tap type rectifying / smoothing circuit constituting the rectifying unit 63. The rectifying unit 63 may be composed of two diode elements. According to the rectifying / smoothing circuit 58, it is possible to obtain the same effects as the case of the rectifying / smoothing circuit 30 described above, and to further simplify the configuration of the rectifying unit 63.

  The rectifying / smoothing circuit 64 of the third modification performs synchronous rectification by two N-ch MOS type FETs 66 and 68 connected to both ends of the secondary winding 22b of the transformer 22 as shown in FIG. This is a current doubler type rectifying and smoothing circuit in which a rectifying unit 69 is configured and a smoothing unit 74 is configured by two smoothing inductors 70 and 72 and a smoothing capacitor 42. The rectifying unit 69 may be composed of two diodes. According to the rectifying / smoothing circuit 64, it is possible to obtain the same effects as those of the rectifying / smoothing circuit 30 described above, and to further reduce the ripple voltage of the smoothing capacitor 42.

  The switching power supply device of the present invention is not limited to the above embodiment. For example, a second capacitor for limiting the regenerative current flowing in the first and second regenerative diodes can be inserted between the midpoint of the first and second regenerative diodes and one end of the resonant inductor on the primary winding side. (For example, the second capacitor 66 shown in FIG. 9). By appropriately adjusting the capacitance of the second capacitor, the voltage stress due to the surge voltage applied to the rectifying element of the rectifying and smoothing circuit is also constant while suppressing the current stress due to the regenerative current flowing through the first and second regenerative diodes to a smaller level. The balance design of limiting to the following can be performed without loss.

  The switching control circuit that stabilizes the output voltage Vo is not limited to a configuration in which the output voltage Vo is directly monitored and controlled. For example, the switching control circuit indirectly uses a winding voltage of a transformer or a voltage of a smoothing inductor. Alternatively, the output voltage Vo may be monitored and controlled, or the input voltage Vi or the like may be monitored and feedforward control may be performed.

10, 50 Switching power supply 18 First arm 18a, 18b Main switching element 18c First middle point 20 Second arm 20a, 20b Main switching element 20c Second middle point 22 Transformer 22a Primary winding 22b Secondary winding 24 First Capacitor 26 Resonant inductor 28a First regenerative diode 28b Second regenerative diode 30, 52, 58, 64 Rectifier smoothing circuit 44 Switching control circuit 66 Second capacitor

Claims (6)

A first arm composed of a series circuit of main switching elements respectively provided on the high side and the ground side, and a DC input source connected to both ends thereof;
A second arm connected in parallel to the first arm, composed of a series circuit of other main switching elements respectively provided on the high side and the ground side;
A transformer having a primary winding and a secondary winding, wherein the primary winding is provided between a midpoint of the first arm and a midpoint of the second arm;
A rectifying / smoothing circuit connected to the secondary winding of the transformer and converting an alternating voltage generated in the secondary winding into a direct output voltage and outputting the direct current voltage;
By driving each main switching element on and off, a first period in which an input voltage is applied to the primary winding in a positive direction, a second period in which no input voltage is applied, and a third period in which a negative direction is applied. A full-bridge type switching power supply comprising: a switching control circuit that performs phase shift control with a period and a fourth period during which no voltage is output again as one cycle, and stabilizes the output voltage of the rectifying and smoothing circuit ,
A first capacitor that is inserted in a position in series with the primary winding and suppresses DC bias of the transformer;
A resonant inductor inserted in a position in series with the primary winding and the first capacitor and having one end connected to the midpoint of the second arm;
A first regenerative diode having an anode terminal connected to one end of the resonant inductor on the primary winding side and a cathode terminal connected to a high-side connection point of the first and second arms;
A second regenerative diode having an anode terminal connected to a connection point on the ground side of the first and second arms, and a cathode terminal connected to one end of the resonant inductor on the primary winding side;
The switching control circuit starts the first and third periods by turning on or turning off the main switching element constituting the first arm, and turns on or turning off the main switching element constituting the second arm. A phase shift control is performed in such a manner that the second and fourth periods are started. Between the midpoint of the first and second regenerative diodes and one end on the primary winding side of the resonant inductor, a second capacitor for limiting the regenerative current flowing in the first and second regenerative diodes is inserted. The switching power supply device according to claim 1. The main switching element of the first and second arms is composed of an N-ch MOS type FET, a parallel capacitor and a parallel diode capable of conducting in the direction from the source terminal to the drain terminal between the individual drains and sources. Is provided,
The switching control circuit includes:
When the main switching elements on the high side and the ground side of the first arm are complementarily turned on and off, the one main switching element that is turned on is turned off and then the other main switching that is turned off At the timing when the voltage across the element decreases toward zero volts due to the resonant operation of the resonant inductor and the parallel capacitor, the other main switching element is turned on,
When the main switching elements on the high side and the ground side of the second arm are complementarily turned on and off, the one main switching element that is turned on is turned off and then the other main switching that is turned off 3. The switching power supply device according to claim 1, wherein control is performed to turn on the other main switching element at a timing when a voltage across the element decreases toward zero volts due to a resonance operation of the resonant inductor and the parallel capacitor. .   The primary and secondary windings of the transformer are closely magnetically coupled so that a leakage inductance viewed from the primary winding side is smaller than an inductance of the resonant inductor. Switching power supply.   4. The switching power supply device according to claim 1, wherein the rectifying / smoothing circuit includes a full-wave rectifying method, a center tap method, or a current doubler method. 5.   The switching power supply device according to claim 5, wherein the rectifying unit included in the rectifying and smoothing circuit performs synchronous rectification using a MOS FET.

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