JP2016144326A - Resonance type dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resonance type DC-DC converter, capable of reducing a conduction loss of a rectification part to be applied a synchronous rectification system, and realizing a low loss and high performance.SOLUTION: An insulation type DC-DC converter comprises: an inverter, a transformer, and a rectification part. In a resonance type DC-DC converter in which a resonance current is made to flow in a primary side of a transformer Trby using a resonance phenomenon by a capacitor Cor the like of an output side of the inverter 20, a rectification part 40A comprises a rectifier cell having a MOSFETs SWto SWformed by a wide band gap semiconductor of an Sic and a diodes Dto Dformed by a silicon semiconductor connected in reverse parallel. Those rectifier cells turn on the MOSFETs SWto SWin a period when the current has to be made to flow in a forward direction of the diodes Dto D, and the current of rectifier cells is separated into a channel of the MOSFETs SWto SWand the diodes Dto D.SELECTED DRAWING: Figure 1

Description

  In the present invention, DC input power is converted into AC power by an inverter, and this AC power is converted to DC power by a rectifier through a transformer and output, thereby insulating the input side from the output side. The present invention relates to an insulating DC-DC converter that performs direct current conversion, and relates to a resonant DC-DC converter that uses an electrical resonance phenomenon to reduce loss.

  As is well known, a power converter includes a DC-DC converter that converts the magnitude of a DC voltage, an AC-AC converter that converts the magnitude and frequency of an AC voltage, and a DC-AC that converts a DC voltage into an AC voltage. There are converters (so-called inverters).

As an example of the DC-DC converter, a circuit shown in FIG. 3 is known.
3, the 10 DC power source E and the input terminal 1a, an input unit consisting of 1b, the high-frequency inverter 21 is composed of a capacitor _{C 1} and the semiconductor switching element _Q 1 to Q ₄ for smoothing, 30 consists of a transformer Tr ₁ insulating parts, 41 rectifier consisting of diodes _D 1 to D ₄ and the capacitor _{C 3} for smoothing, 50 is an output unit consisting of output terminals 2a, 2b, these output terminals 2a, a load R are connected between 2b ing.

In this DC-DC converter, direct current power input from the direct current power source E is converted into high frequency alternating current power by the inverter 21 and then transmitted from the primary side to the secondary side by the transformer Tr ₁ of the insulating unit 30. Then, it is converted into DC power by the rectifier 41 and output to the load R. Thereby, it can be made to function as an insulation type DC-DC converter which performs direct current-direct current conversion, insulating the input side and the output side.
As is well known, in the transformer Tr ₁ , the primary side and the secondary side are obtained by converting the electric energy of the AC power input to the primary side thereof into magnetic energy, transmitting it to the secondary side, and outputting it as electric energy. AC power is transmitted while being electrically insulated from each other.

As the rectifying elements in the rectifying unit 41 in FIG. 3, diodes D _{1 to} D ₄ that are semiconductor devices having a rectifying action are basically used.
Here, when a current flows through the diode in the forward direction, Joule heat proportional to the product of the current and the voltage across the diode is generated, and power is consumed. A loss is obtained by integrating the power consumption with respect to time, and is referred to as a conduction loss or conduction loss of a diode or a rectifying element.

  Incidentally, there is a voltage offset in the forward voltage of the diode. This voltage offset is not found in MOSFETs or the like which are semiconductor switching elements, and is approximately 0.5 to 1.5 [V], depending on the type of diode, and has temperature dependence. There are many cases. The forward current of the diode rises when the voltage across the diode is equal to or greater than the offset voltage, and thereafter flows as the voltage drop of the sum of the offset voltage and the voltage corresponding to the product of the differential resistance and current. . For this reason, in the diode, compared to a MOSFET or the like having the same differential resistance, a voltage drop when the same forward current flows is increased, and as a result, a conduction loss is increased.

Therefore, as described in Patent Document 1, a MOSFET is used as an element instead of the diode constituting the rectifying unit, and the MOSFET is turned on during the flow period to the original diode so that a current flows through the channel. A method called “synchronous rectification” has been proposed. At this time, a current flows through the MOSFET in a direction opposite to that at the time of the normal forward flow, and thus may be referred to as “reverse conduction”.
FIG. 4 is a circuit configuration diagram of the DC-DC converter described in Patent Document 1 to which the synchronous rectification method is applied. 4, SW _{a to} SW _d are MOSFETs, L is an inductor, C ₄ and C ₅ are capacitors, and the other components are denoted by the same reference numerals as those in FIG.

Next, FIG. 5 relates to the rectifying unit of the insulated DC-DC converter, and instead of the diodes D _{1 to} D ₄ of the rectifying unit 41 in FIG. A configured example is shown. In FIG. 5, the MOSFET body is represented by symbols SW ₁ to SW ₄ , and the parasitic diodes of the MOSFET are represented by symbols BD ₁ to BD ₄ , but in the following, the entire MOSFET is also represented by symbols SW _{1 to} SW _{4 in} some cases. Will be used.
Note that the MOSFETs SW _{1 to} SW ₄ in FIG. 5 are all Si-MOSFETs configured using silicon (Si) -based semiconductors.

In the circuit shown in FIG. 5, as MOSFET of the upper and lower arms do not simultaneously turned on, for example, MOSFET _SW 2, SW ₃ is to respectively turn on the _SW 1, SW ₄ with a predetermined dead time from OFF, SW ₂ SW ₁ and SW ₄ are turned off earlier than the timing at which SW ₃ is turned on by a dead time. The dead time is set to a minimum time with a likelihood that the MOSFETs SW ₂ , SW ₃ and SW ₁ , SW ₄ do not turn on at the same time. The MOSFETs SW ₁ , SW ₄ Compared to the reverse conduction time, it is very short.

The parasitic diodes BD ₁ and BD _{4 in} FIG. 5 are provided in anti-parallel to the original flow direction of the MOSFETs SW ₁ and SW ₄ , and the flow direction of the parasitic diodes BD ₁ and BD ₄ is rectified in FIG. It is equal to the flow direction of the diodes D ₁ and D ₄ of the part 41. These parasitic diodes are also called built-in diodes and body diodes, and current flows through the parasitic diodes during the above-described dead time.
As described above, the MOSFET has reverse conduction characteristics and can effectively use the parasitic diode, so that it can be said to be an element suitable for synchronous rectification.

  In addition, as described in Non-Patent Document 1 described later, it is known that in an isolated DC-DC converter, the conduction loss can be reduced by performing synchronous rectification using a MOSFET instead of a diode as a rectification unit. In some cases, it has been put to practical use.

  By the way, in recent years, in the field of power semiconductor devices, research and development of devices using wide band gap semiconductors such as silicon carbide (SiC) semiconductors and gallium nitride (GaN) semiconductors have been actively carried out, and commercialization has already started. ing. Wide bandgap semiconductors are known to be superior to conventional silicon (Si) semiconductors in that high breakdown voltage devices can be fabricated with low on-resistance and high temperature operation is possible. This is an advantage resulting from a large forbidden bandwidth (band gap) of a wide band gap semiconductor and a large dielectric breakdown electric field.

  If this type of wide band gap semiconductor is used, a high breakdown voltage element can be manufactured with low on-resistance as described above, and conduction loss can be reduced. SiC-based MOSFETs are known to have a higher switching speed than Si-based MOSFETs, and switching loss is also small. For this reason, it can be used even under conditions where the operating frequency of the switching element is high, and passive components such as capacitors and inductors in peripheral circuits can be miniaturized as the frequency increases. It becomes possible to increase the power capacity.

  A Schottky barrier diode (SBD), which is a kind of diode using a SiC-based semiconductor, has also been put into practical use. In the case of conventional Si-based semiconductors, SBDs cannot be made for physical reasons in the field of high breakdown voltage, and diodes using PN junctions are provided exclusively. There is a problem that reverse recovery loss, which is a kind of switching loss, is large.

  On the other hand, SiC-based SBDs are known as low-loss diodes because theoretically no reverse recovery phenomenon occurs. For this reason, in the field of general-purpose inverters and the like that require the use of a pair in which switching elements and diodes (freewheeling diodes) are connected in antiparallel, in order to improve power conversion efficiency, SiC-MOSFET and SiC-SBD are used. It is considered ideal to use an “all-SiC” device with pairs connected in antiparallel, and recently commercially available (“all-SiC” is expressed as “All-SiC” or “ Sometimes referred to as “full SiC”).

  However, SiC-MOSFETs are finally in the stage of practical use, while SiC-SBD is less technically difficult to realize than SiC-MOSFETs, so it is popular before SiC-MOSFETs. I am doing. For this reason, in the field of general-purpose inverters and the like, a “hybrid” device is devised by a pair in which IGBTs and SiC-SBDs, which are Si-based switching elements, are connected in antiparallel, and this hybrid device includes SiC-MOSFET and SiC- It has been put to practical use in advance of all SiC devices based on SBD.

Therefore, in the insulation type DC-DC converter, it has been attracting attention that SiC-SBD is applied as the diode of the rectifying unit instead of the conventional Si-based diode. Loss can be achieved. Further, when the synchronous rectification method is adopted, the use of SiC-MOSFETs as the MOSFETs SW _{1 to} SW ₄ as in the rectification unit 43 shown in FIG. 6 reduces the conduction loss and the switching loss, and the power conversion. Improvement in efficiency is expected.

  Recently, as an all-SiC device, an element in which a pair in which SiC-MOSFET and SiC-SBD are connected in antiparallel is bundled in one package, or a pair in which SiC-MOSFET and SiC-SBD are connected in antiparallel is two. Half-bridge modules (also called 2-in-1 modules) in which circuit elements connected in series are bundled in one package have been commercialized.

For example, FIG. 7, the rectifying section 44 ol SiC half bridge module 44X, an example in which a 44Y, these modules 44X, 44Y are MOSFET having a parasitic diode _{BD 1 ~BD 4 (SiC-MOSFET} ) SW and ₁ to SW _4, is constituted by a Schottky barrier diode _SBD 1 _{~SBD 4} is a SiC-SBD. Hereinafter, the Schottky barrier diode is also referred to as an “SB diode”.
This type of half-bridge module is highly versatile because the upper and lower arms for one phase of the inverter and the rectifying unit can be configured by a single package module. Furthermore, since it is easy to handle and is considered advantageous for simplification and miniaturization, all-SiC half-bridge modules will become a product category that is highly available and advantageous in terms of cost in the popularization period of SiC devices. It is thought to go.

  By the way, the inverter provided in the insulation type DC-DC converter converts DC power into AC power by the switching action of the switching element. When the switching element is switched, that is, turned off or turned on, the There is a period in which neither current is zero, and power proportional to the product of voltage and current is consumed during that period. This power consumption integrated over time is a loss, which is called a switching loss.

As a type of insulated DC-DC converter, as shown in FIG. 8, a resonant element is inserted between the output side of the full bridge circuit composed of switching elements Q _{1 to} Q ₄ and the primary side of the transformer Tr ₁ to generate a high frequency. 2. Description of the Related Art A resonant DC-DC converter in which a switching loss is suppressed by configuring a resonant inverter 20 and performing switching using an electrical resonance phenomenon is known.
In Figure 8, there is shown an example using a capacitor C ₂ as a resonant element, the resonant element comprises at least a capacitor, it may further comprise an inductor.

This type of resonance type DC-DC converter adjusts the inductance of an inductor, which is a transformer and a resonance element, and the capacitance of a capacitor, which is a resonance element, so that an oscillating current (resonance current) is generated after the switching element of the inverter is turned on. ) Is generated. For this reason, if the switching element is turned off at the timing when the half-cycle oscillating current ends, ideally, switching is performed under the condition that no current flows at the time of turn-on and turn-off. Disappears. This operation is called soft switching.
Incidentally, FIG. 9, which the concept of soft switching at the resonant DC-DC converter, indicated switching element, as an output voltage and current waveforms of the voltage and current waveforms and DC-DC converter of a switching element Q ₁ in FIG. 8, for example , and the by the current of the switching element Q ₁ is switched to a period of zero, allowing soft switching.

  Also in the above-described resonant DC-DC converter, an SB diode such as SiC-SBD is applied as the rectifying unit in place of the Si-based diode, similarly to the above-described (non-resonant) isolated DC-DC converter. An example, an example of performing synchronous rectification by a Si-MOSFET provided with a parasitic diode as shown in FIG. 10, an example of performing synchronous rectification by an SiC-MOSFET provided with a parasitic diode as shown in FIG. As shown in FIG. 12, an example using an all SiC device, in particular, an all SiC half bridge module 44X, 44Y can be considered.

JP 2010-4726 A (paragraphs [0023] to [0025], [0033] to [0035], FIG. 1 etc.)

Fuji Jiho, Vol. 85, no. 3, pp. 226-230 “Highly efficient front-end power supply conforming to 80 PLUS”, published in March 2012

  When the synchronous rectification method is applied to the rectifying unit 42 of the resonant DC-DC converter shown in FIG. 10, a current flows through the parasitic diode of the rectifying unit 42 during the dead time. As described above, the dead time is set to the minimum time, and at the operating frequency intended for the power converter mainly composed of a conventional Si-based power semiconductor element, the dead time is in the reverse conduction period of the MOSFET of the rectifier 42. It is very short compared. For this reason, it is considered that the characteristics of the parasitic diode of the MOSFET were not particularly problematic.

However, as shown in FIG. 11, when synchronous rectification is performed by the rectification unit 43 using a MOSFET made of a wide band gap semiconductor such as SiC, the switching period is shortened by increasing the operating frequency, but the dead time is long. Can't change much.
For this reason, the ratio of the dead time becomes larger than the reverse conduction period of the MOSFET of the rectifying unit 43, and the period occupied by the dead time per unit time increases in proportion to the operating frequency. That is, the period during which current flows through the parasitic diode of the MOSFET becomes longer.
In general, a parasitic diode of a MOSFET is inferior in current-voltage characteristics and has a larger voltage drop than a diode designed and manufactured as a rectifying component that is not a parasitic element. When using, there is a possibility that the effect of the synchronous rectification of reducing the conduction loss in the rectification unit 43 is not sufficiently exhibited.

On the other hand, as shown in FIG. 12, when an all SiC device such as the all SiC half bridge modules 44X and 44Y is used for the rectifying unit 44, the SiC- excellent in current-voltage characteristics as compared with the parasitic diode of the MOSFET. Since the SB diodes SBD _{1 to} SBD ₄ as the SBD are connected in parallel to the SiC-MOSFET, the conduction loss is reduced as compared with the case where the diode flowing in the dead time is only the parasitic diode of the SiC-MOSFET. Is done. However, although the offset voltage of the SiC-SBD is smaller than that of the parasitic diode of the SiC-MOSFET, it is relatively large, so that there is still room for improvement from the viewpoint of reducing the conduction loss.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a resonant DC-DC converter that can reduce the loss of current flowing through a rectifying unit to which a synchronous rectification method is applied, thereby reducing the loss and increasing the efficiency.

In order to solve the above problems, the invention according to claim 1 is directed to an inverter that converts DC input power into AC power, a transformer that insulates and transmits AC power output from the inverter, and output from the transformer. A rectifying unit that converts alternating current power into direct current power and supplies the load to a load, and an insulated DC-DC converter comprising:
In a resonance type DC-DC converter in which a resonance current is caused to flow to the primary side of the transformer using an electrical resonance phenomenon by a resonance element provided on the output side of the inverter,
The rectifying unit is
A plurality of rectifying elements each including a FET made of a wide band gap semiconductor and a diode made of a silicon-based semiconductor connected in antiparallel to the FET,
The rectifier element turns on the FET during a period in which a current should flow in the forward direction of the diode, thereby diverting the current flowing through the rectifier element to the channel of the FET and the diode.

  According to a second aspect of the present invention, in the resonant DC-DC converter according to the first aspect, the rectifying unit includes a bridge whose input side is connected to the secondary side of the transformer and whose output side is connected to the load. The rectifier element is connected to each side of the circuit.

  The invention according to claim 3 is the resonant DC-DC converter according to claim 1 or 2, wherein the FET is a MOSFET made of a wide band gap semiconductor.

  The invention according to claim 4 is the resonant DC-DC converter according to any one of claims 1 to 3, wherein a Schottky barrier diode is connected in parallel to the diode made of the silicon-based semiconductor. And

  The invention according to claim 5 is the resonant DC-DC converter according to claim 4, wherein the Schottky barrier diode is made of a wide band gap semiconductor.

  The invention according to claim 6 is the resonant DC-DC converter according to claim 5, wherein the FET is formed of a MOSFET made of a wide band gap semiconductor of the same type as the Schottky barrier diode, and the MOSFET and the shot A pair of key barrier diodes connected in antiparallel is configured as a package.

  According to the present invention, it is possible to reduce the current loss in the rectification unit to enhance the synchronous rectification effect, and to realize a low-loss, high-efficiency, small-sized, resonant DC-DC converter with high power density.

1 is a circuit configuration diagram of a resonant DC-DC converter according to a first embodiment of the present invention. It is a circuit block diagram of the resonance type DC-DC converter which concerns on 2nd Embodiment of this invention. It is a circuit block diagram of a well-known insulation type DC-DC converter. It is a circuit block diagram of the prior art described in patent document 1. FIG. It is a circuit block diagram at the time of applying a synchronous rectification system to the rectification part of a well-known insulation type DC-DC converter. It is a circuit block diagram at the time of applying a synchronous rectification system and applying SiC-MOSFET to the rectification | straightening part of a well-known insulation type DC-DC converter. It is a circuit block diagram at the time of applying a synchronous rectification system to the rectification part of a well-known insulation type DC-DC converter, and applying an all SiC half bridge module. It is a circuit block diagram of a well-known resonance type DC-DC converter. It is a conceptual diagram of the soft switching in a resonance type DC-DC converter. It is a circuit block diagram at the time of applying a synchronous rectification system to the rectification part of a well-known resonance type DC-DC converter. It is a circuit block diagram at the time of applying a synchronous rectification system and applying SiC-MOSFET to the rectification part of a well-known resonance type DC-DC converter. It is a circuit block diagram at the time of applying a synchronous rectification system and applying an all SiC half bridge module to the rectification part of a well-known resonance type DC-DC converter.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings showing the embodiments, the same or corresponding circuit elements are denoted by the same reference numerals.
Further, the configuration of the present invention is not limited in any way by each embodiment, and in particular, in the circuit according to each embodiment, an example in which a load such as a DC power source is connected to the input side and a resistor is connected to the output side is shown. However, the configuration of the DC power supply and the type of load are not particularly limited. Furthermore, the present invention also includes a case where another power conversion circuit is connected to the input side or the output side to form a product as an integrated resonance type DC-DC converter as a whole or a part of the product. Is included.

Moreover, although the input / output of each embodiment is basically a direct current, it does not exclude the possibility of ac pulsation / fluctuation components and voltage level fluctuations.
Furthermore, the main circuit portion (circuit portion between the input and output terminals) of each embodiment shows a typical configuration in a simplified manner, and other circuit elements such as a protection circuit or alternative parts are present. It does not exclude the case of using.

First, a first embodiment of the present invention will be described with reference to FIG.
FIG. 1 is a circuit configuration diagram of the resonant DC-DC converter according to the first embodiment, and is configured in the same manner as in FIG. 8 except for the rectifying unit 40A.

That is, as shown in FIG. 1, the resonant DC-DC converter according to the present embodiment includes an input unit 10, a high frequency resonant inverter 20, an insulating unit 30, a rectifying unit 40 </ b> A, and an output unit 50. through the transformer Tr _1, the high-frequency resonant inverter 20 to the primary side rectifying portion 40A are connected to the secondary side.
In the input unit 10, the positive and negative electrodes of the DC power source E are connected to the input terminals 1a and 1b, respectively.

The high-frequency resonant inverter 20 is a single-phase full-bridge inverter in which semiconductor switching elements Q ₁ , Q ₂ , Q ₃ , and Q ₄ made of MOSFETs are connected via a smoothing capacitor C ₁ connected between input terminals 1a and 1b. Connected to make up. A connection point between the switching elements Q ₁ and Q _{2 and} a connection point between the switching elements Q ₃ and Q ₄ are connected to the primary side of the transformer Tr ₁ via a capacitor C ₂ as a resonance element.

MOSFETs SW ₁ , SW ₂ , SW ₃ , SW ₄ are connected to the rectifier 40A so as to constitute a single-phase full-bridge converter.
In the present embodiment and the second embodiment to be described later, a case where a SiC-MOSFET is used as the MOSFETs SW ₁ , SW ₂ , SW ₃ , SW ₄ will be described, but the wide band gap semiconductor is not limited to the SiC system, The type of FET is not limited to MOSFET. However, in each embodiment, for the purpose of performing synchronous rectification, the switching element of the rectification unit needs to use an FET capable of reverse conduction, and in this respect, the use of a MOSFET is generally assumed. Incidentally, as elements other than MOSFET, it is conceivable to use HEMT (High Electron Mobility Transistor), or a cascode circuit that achieves normally-off by combining HEMT and normally-off switching element.

The MOSFETs SW ₁ , SW ₂ , SW ₃ , and SW _{4 in} FIG. 1 include parasitic diodes BD ₁ , BD ₂ , BD ₃ , and BD ₄ that are antiparallel to the original flow direction (forward direction) of the element, respectively. ing.
Further, the MOSFETs SW ₁ , SW ₂ , SW ₃ , SW ₄ have diodes (anti-parallel diodes) D ₁ , D ₂ , D ₃ , D ₄ that are also anti-parallel to the original flow direction of the elements, respectively. It is connected. These diodes D ₁ , D ₂ , D ₃ , and D ₄ are Si-based diodes.
Further, the output side of the rectifier 40A is connected to the capacitor C ₃ for smoothing the output terminal 2a of the output unit 50, the load R through the 2b are connected.

Resonant DC-DC converter having the above configuration, the switching operation and electrical resonant operation of high-frequency resonant inverter 20 upon which converts DC power to AC power, transmitted as magnetic energy to the secondary side from the primary side of the transformer Tr ₁ Further, by converting to DC power by the rectifier 40A and outputting it, DC-DC conversion is performed while insulating the input side and the output side.
Here, similarly to FIG. 8, a capacitor C ₂ as a resonance element is inserted in the output portion of the high frequency resonance inverter 20, and soft switching is performed using an electrical resonance phenomenon to suppress switching loss. Designed to be As described above, the resonant element, it may further comprise an inductor in addition to the capacitor C _2.

In the present embodiment, by the capacitance of the inductance and the capacitor C ₂ of the transformer Tr _1, it is adjusted so that the vibration current is generated from the time of turning on the switching elements of the inverter 20. Then, by turning off the switching element at the timing when the half-cycle oscillating current finishes flowing, ideally switching is performed under the condition that no current flows at the time of turn-on and turn-off. Disappears.

Further, during the period corresponding to the flow period to the diodes D ₁ and D ₄ in FIG. 8, the MOSFETs SW ₁ and SW ₄ of the rectifying unit 40A in FIG. 1 are turned on, and the diodes D ₂ and D ₃ in FIG. By turning on the MOSFETs SW ₂ and SW ₃ during a period corresponding to the current passing period, synchronous rectification is performed by causing a current due to reverse conduction to flow through the channel of each MOSFET.

Hereinafter, the operation and the conduction loss of the rectifying unit 40A according to each period and condition in the synchronous rectification will be considered together with comparison with the case where the synchronous rectification is not applied and the case where the synchronous rectification is applied according to the related art.
In the following description, the operations of the MOSFETs SW ₁ and SW ₄ and the parasitic diodes BD ₁ and BD ₄ , and the diodes D ₁ and D ₄ will be described, but the MOSFETs SW ₂ and SW ₃ and the parasitic diodes BD ₂ , The operation is the same for BD ₃ and diodes D ₂ and D ₃ .

A period of MOSFET _SW 1 of the rectification section 40A, SW ₄ are reverse conducting on, following a voltage the absolute value of the drain-source voltage of the MOSFET _SW 1, SW ₄ are turned on in the diode _D 1, _{D 4} In the period, the reverse conduction path by the MOSFETs SW ₁ and SW ₄ becomes most of the current path. The loss of the rectifying unit 40A during this period is equivalent to that in the case of the synchronous rectification according to the prior art (FIGS. 11 and 12), but the diode D having a voltage offset in the forward voltage as in the case of not applying the synchronous rectification in FIG. Compared with the case where the current flows only through ₁ and D ₄ , the current loss of the rectifying unit 40 </ b> A is significantly reduced in the present embodiment because there is no voltage drop corresponding to the voltage offset.

In addition, in a period in which the absolute value of the drain-source voltage of the MOSFETs SW ₁ and SW ₄ of the rectifying unit 40A exceeds the voltage at which the diodes D ₁ and D ₄ are turned on, in addition to the reverse conduction of the MOSFETs SW ₁ and SW ₄ Since current is also shunted to the diodes D ₁ and D ₄ , the resistance value of the rectifying unit 40A is greatly reduced.
In the synchronous rectification operation of the present embodiment and the prior art (FIGS. 11 and 12), shunting of the MOSFETs SW ₁ and SW _{4 to} the parasitic diodes BD ₁ and BD ₄ may occur, but these parasitic diodes BD ₁ , The BD ₄ has inferior current-voltage characteristics, and has a larger offset voltage and resistance than the diodes D ₁ and D ₄ designed and manufactured as products that are not parasitic elements, so that the resistance value of the rectifying unit 40A is reduced. The contribution of is small. Therefore, the loss of the rectifying unit 40A in the above-described period of the present embodiment is greatly reduced not only when the conventional synchronous rectification is not applied (FIG. 8) but also when the synchronous rectification is applied (FIG. 11). become.

Further, in the synchronous rectification operation of the prior art (FIG. 12), shunting to the SB diodes SBD ₁ and SBD ₄ which are SiC-SBDs can occur, and these SB diodes SBD ₁ and SBD ₄ are MOSFET SW ₁ , The current-voltage characteristics are superior to the parasitic diodes BD ₁ and BD ₄ of SW ₄ , but the offset voltage is larger than that of the Si-based diodes D ₁ and D ₄ in this embodiment, so that the resistance value of the rectifier is reduced. The contribution to is small. Therefore, the loss of the rectification unit 40A in the above period of the present embodiment is reduced even when the synchronous rectification according to the conventional technique (FIG. 12) is applied.

  As described above, according to the present embodiment, it is possible to reduce the conduction loss of the rectifying unit 40A even in an operating condition in which the load current is light with respect to the rated current or in an operating condition in which the load current is heavy. This advantage can be enjoyed independently of the switching frequency.

Next, an operation of dead time in which the MOSFETs SW ₁ and SW ₄ of the rectifying unit 40A are turned off and not reversely conducted will be described.
In the dead time when the synchronous rectification of the conventional technique (FIG. 11) is applied, the current flows exclusively through the parasitic diodes BD ₁ and BD ₄ of the MOSFETs SW ₁ and SW ₄ of the rectification unit 43. On the other hand, at the operating frequency of the power converter mainly composed of Si-based MOSFETs as shown in FIG. 10, the dead time is very small compared to the reverse conduction period of the MOSFETs SW ₁ and SW ₄ of the rectifying unit 42. The characteristics of BD ₁ and BD ₄ were not particularly problematic.

  However, when the frequency of the DC-DC converter is increased using a MOSFET made of a wide band gap semiconductor element as in this embodiment, the ratio of the dead time period to the reverse conduction period of the MOSFET of the rectifier 40A is large. Thus, the ratio of the dead time per unit time increases in proportion to the operating frequency. In other words, the period during which a MOSFET having a larger voltage drop than a normal diode is passed through a parasitic diode becomes longer, and the effect of reducing the conduction loss in the rectifying unit during synchronous rectification cannot be sufficiently obtained.

Further, in the dead time when the synchronous rectification of the conventional technique (FIG. 12) is applied, current flows to the SB diodes SBD ₁ and SBD ₄ which are SiC-SBDs. However, these SB diodes SBD ₁ and SBD ₄ have excellent current-voltage characteristics compared to the parasitic diodes BD ₁ and BD ₄ of the MOSFETs SW ₁ and SW ₄ , but have a larger offset voltage than the Si-based diodes. Large voltage drop and large conduction loss.

On the other hand, in the present embodiment, the majority of the current path in the dead time is excellent in the current-voltage characteristics, and in particular, the Si-type diodes D ₁ and D ₄ with a small offset voltage and therefore a small voltage drop are used. Even if the proportion of dead time increases as the frequency increases, an increase in the flow loss in the rectification unit can be suppressed.

By the way, when rectification by a diode is generally performed in this type of field, a reverse recovery phenomenon that occurs when a Si-based diode transitions from an on-state that is flowing forward to an off-state due to a high voltage in the reverse direction. As a result, a large loss occurs. On the other hand, SiC-SBD is known as a low-loss diode because a reverse recovery phenomenon does not theoretically occur.
Also, while SiC-MOSFETs are finally in the process of practical use, SiC-SBD is less technically difficult to implement than SiC-MOSFETs, so it has become popular prior to SiC-MOSFETs. There is a background. For this reason, the replacement of a portion where a Si-based diode has conventionally been used with SiC-SBD has become common.

  As described above, in the field of general-purpose inverters, in order to improve power conversion efficiency, it is ideal to use an “all-SiC” device with a pair of SiC-MOSFET and SiC-SBD connected in antiparallel. Recently, it has been on the market. On the other hand, due to the difference in the development and commercialization time of SiC-SBD and SiC-MOSFET, a “hybrid” device consisting of a pair of Si-based IGBT and SiC-SBD connected in anti-parallel is SiC-MOSFET and SiC-SBD. Has been put into practical use in advance of the all SiC device.

In an isolated DC-DC converter, it is conceivable to use SiC-SBD as a rectifier diode in place of a conventional Si-based diode. In this case, reverse recovery loss does not occur and low loss is generally achieved. It is thought that it becomes.
However, in the resonance type DC-DC converter as in the present embodiment, soft switching is performed as described above, and so-called zero current switching or zero voltage switching is performed, so that synchronous rectification is performed in the rectification unit 40A. the diode D _1, a result of stopping the flow of forward current of D ₄ and a transition to the oFF state by the high voltage is temporally separated, theoretically, the diode D _1, D ₄ is a Si-based The reverse recovery phenomenon does not occur in spite of the diode.
Therefore, without worrying about the occurrence of reverse recovery loss, it is possible to use a Si-based diode having an offset voltage lower than that of SiC-SBD and having a small voltage drop, and in the rectifying unit 40A compared to the case of using SiC-SBD. Loss can be reduced.

  As described above, according to the present embodiment, since the conduction loss of the rectifying unit 40A can be reduced and suppressed, the loss and efficiency of the resonant DC-DC converter are reduced, and the cooling body is reduced in size accordingly. This also contributes to the miniaturization of passive elements through the increase of the operating frequency. Further, particularly under heavy load conditions, the current flowing through the rectifying unit 40A is divided by the reverse conduction operation of the SiC-MOSFET and the flow through the Si-based diode, so that the conventional synchronous rectification is applied (FIGS. 11 and 12). ) SiC-MOSFET and the diode when synchronous rectification is not applied (FIG. 8), the burden of the element can be reduced and the reliability can be improved, and the cost can be reduced by adopting the element with reduced rating. You can also plan.

In general, according to the present embodiment, particularly when applying a wide band gap semiconductor element such as a SiC system to increase the frequency, it is possible to obtain low loss, high efficiency, and advantages associated therewith.
That is, when the resonance type DC-DC converter is increased in frequency, the effect of the synchronous rectification that reduces the conduction loss in the rectification unit is large, and the low loss and high efficiency are achieved. -A DC converter can be realized. In the present embodiment, even when a conventional Si-based element is used for an inverter or the like or a relatively low operating frequency is applied, an effect of improving the efficiency can be obtained.

Next, a second embodiment of the present invention will be described.
FIG. 2 is a circuit configuration diagram of a resonant DC-DC converter according to the second embodiment. In this embodiment, the rectifier 40B includes an SiC half-bridge module 40X including SiC MOSFETs SW ₁ and SW ₂ and SiC SB diodes SBD ₁ and SBD ₂ , and SiC MOSFETs SW ₃ and SW _4. And all-SiC half-bridge module 40Y incorporating SiC-based SB diodes SBD ₃ and SBD ₄ and Si-based diodes D ₁ and D connected in reverse parallel to MOSFETs SW ₁ , SW ₂ , SW ₃ and SW ₄ , respectively. ₂ , D ₃ , and D ₄ .
That is, in the second embodiment, the SB diodes SBD ₁ , SBD ₂ , SBD ₃ , and SBD ₄ are connected in parallel to the Si diodes D ₁ , D ₂ , D ₃ , and D ₄ in the first embodiment, respectively. It corresponds to a thing.

  Recently, as the all-SiC device described above, a device in which a pair of SiC-MOSFET and SiC-SBD connected in antiparallel is bundled in one package, or the SiC-MOSFET and SiC-SBD used in this embodiment. A half-bridge module (also referred to as a 2-in-1 module) in which circuit elements in which two pairs connected in reverse parallel are connected in series in one package has been commercialized. In particular, since the half-bridge module can realize the upper and lower arms for one phase with one package module, it has high versatility, is easy to handle, and is advantageous for simplification and miniaturization. For this reason, it is considered that the all-SiC half-bridge module will become a product category that is highly available and advantageous in terms of cost in the popularization period of SiC devices.

The basic configuration and operation of the second embodiment are the same as those of the first embodiment. As a characteristic point related to the second embodiment, in the dead time in the synchronous rectification of the rectification unit 40B, flow to the SiC-based SB diodes SBD ₁ and SBD ₄ may also occur, but the SB diodes SBD ₁ and SBD ₄ Since the flow to the Si-based diodes D ₁ and D _{4 having} a lower offset voltage is preferentially performed, the voltage drop and the conduction loss of the rectifying unit 40B are reduced as compared with the case where there is no Si-based diode. . When the voltage exceeds the offset voltage of the SB diodes SBD ₁ and SBD ₄ , the voltage is divided by the SiC-based SB diodes SBD ₁ and SBD ₄ and the Si-based diodes D ₁ and D _4. Resistance value is reduced.

  As in the first embodiment described above, if the rectifying unit 40A is configured such that only the Si-based diode is connected in reverse parallel to the SiC-MOSFET and does not include the SiC-based SB diode, the circuit is simplified. -While downsizing and cost reduction may be achieved, by using half-bridge modules 40X and 40Y that are assumed to have high versatility and availability through larger-scale mass production as in the second embodiment, In some cases, it can be expected that the cost will be reduced, and that simplification and miniaturization will be achieved. Therefore, in view of these circumstances, it is desirable to select and apply the first embodiment or the second embodiment as appropriate.

  The present invention is a so-called isolated DC-DC converter, which is a resonant DC-DC converter that reduces loss by using an electrical resonance phenomenon, such as a server power supply, a railway vehicle or electric vehicle power supply system, solar power The present invention can be used for various power conversion devices constituting a power generation system and the like.

1a, 1b: input terminals 2a, 2b: output terminal 10: input unit 20: high frequency resonant inverter 21: high frequency inverter 30: insulating units 40A, 40B, 41-44: rectifying units 40X, 40Y, 44X, 44Y: all SiC half bridge module 50: output section _Q 1 to Q _4: semiconductor switching element Tr _1: trans _SW 1 _{to SW} 4: MOSFET
D _{1 to} D ₄ : Diodes BD _{1 to} BD ₄ : Parasitic diodes SBD _{1 to} SBD ₄ : Schottky barrier diodes C ₁ , C ₂ , C ₃ : Capacitor E: DC power supply R: Load

Claims (6)

An inverter that converts DC input power into AC power, a transformer that insulates and transmits AC power output from the inverter, and a rectifier that converts AC power output from the transformer into DC power and supplies the load to a load An isolated DC-DC converter comprising:
In a resonance type DC-DC converter in which a resonance current is caused to flow to the primary side of the transformer using an electrical resonance phenomenon by a resonance element provided on the output side of the inverter,
The rectifying unit is
A plurality of rectifying elements each including a FET made of a wide band gap semiconductor and a diode made of a silicon-based semiconductor connected in antiparallel to the FET,
The rectifying element is configured to divert current flowing through the rectifying element to the channel of the FET and the diode by turning on the FET during a period in which current should flow in the forward direction of the diode. Resonant type DC-DC converter. The resonant DC-DC converter according to claim 1,
The rectifying unit is
A resonant DC-DC converter comprising an input side connected to a secondary side of the transformer and an output side connected to each side of a bridge circuit connected to the load. . The resonant DC-DC converter according to claim 1 or 2,
A resonant DC-DC converter, wherein the FET is a MOSFET made of a wide band gap semiconductor. The resonant DC-DC converter according to any one of claims 1 to 3,
A resonant DC-DC converter, wherein a Schottky barrier diode is connected in parallel to the diode made of a silicon-based semiconductor. The resonant DC-DC converter according to claim 4,
A resonant DC-DC converter, wherein the Schottky barrier diode is made of a wide band gap semiconductor. The resonance type DC-DC converter according to claim 5,
The FET is formed of a MOSFET made of the same type of wide band gap semiconductor as the Schottky barrier diode, and a pair in which the MOSFET and the Schottky barrier diode are connected in antiparallel is formed as a package. Type DC-DC converter.

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