JP5176922B2 - DC-DC converter and control method thereof - Google Patents

Description

  The present invention relates to a DC-DC converter and a control method thereof, and more specifically, switching loss of a DC-DC converter having a function of boosting and outputting an output voltage of a low-voltage power supply by on / off control of a power semiconductor switching element. It relates to reduction technology.

  A so-called booster that boosts the output voltage of a low-voltage power supply by combining the operation of storing electromagnetic energy in the inductor and the operation of releasing electromagnetic energy stored in the inductor by repeatedly turning on and off the power semiconductor switching element. A circuit configuration of a chopper type DC-DC converter is known.

  In such a type of DC-DC converter, an inductor is required as a circuit element. However, there is a trade-off relationship between an increase in switching frequency for downsizing the inductor and an increase in switching loss due to the increase in frequency. . For this reason, so-called soft switching such as zero current switching (ZCS) or zero voltage switching (ZVS) is possible so that switching loss can be suppressed even if the power semiconductor switching element is turned on and off at a high frequency. Application is in progress.

  For example, in Japanese Patent Application Laid-Open No. 2007-43852 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2005-261059 (Patent Document 2), an auxiliary current path for realizing soft switching is added to the configuration of a normal boost chopper circuit. A circuit configuration and control method of a DC-DC converter including a circuit element group to be formed is described. Japanese Patent Laid-Open No. 2007-336769 (Patent Document 3) discloses that in a DC-DC converter including a plurality of element converters, one element converter is controlled by hard switching while the remaining element converters are controlled by soft switching. Thus, a control method is described in which high efficiency is obtained over a wide output range and the stability of the output voltage is enhanced.

Furthermore, in Japanese Patent Laid-Open No. 5-336732 (Patent Document 4), the gate resistance value of an IGBT (Insulated Gate Bipolar Transistor) is variable, and the gate resistance value is normally set small to reduce the switching loss. A configuration of an IGBT gate circuit is described in which the gate drive speed is variably controlled so that the gate resistance value is set to be large when overvoltage is applied.
JP 2007-43852 A JP 2005-261559 A JP 2007-336769 A JP-A-5-336732

  In the circuit configurations of the DC-DC converters described in Patent Documents 1 and 2, as a main switching element, a resonance current is caused to flow through an auxiliary current path by on / off control of an auxiliary power semiconductor switching element. The turn-on of the power semiconductor switching element can be zero voltage switching. That is, the efficiency of the DC-DC converter can be improved because the power loss of the current consumption is smaller due to the formation of the auxiliary current path than the power loss due to hard switching of the main switching element.

  For this reason, in the light load region where the output power is small, there is a concern that the power consumption for flowing the resonance current through the auxiliary current path becomes larger than the switching loss reduction effect of the main switching element. The application of soft switching may reduce the overall efficiency of the DC-DC converter.

  The present invention has been made to solve such a problem, and an object of the present invention is to improve the efficiency of a DC-DC converter over a wide operating range including a light load region. .

  A DC-DC converter according to the present invention is a DC-DC converter for performing DC voltage conversion between a low-voltage power supply and a power supply wiring, and includes a main inductor, first and second main switching elements, A second main rectifying element; an auxiliary resonance circuit provided corresponding to at least one main switching element of the first and second main switching elements; and a control circuit. The main inductor is connected between the low voltage power source and the first node. The first main switching element is connected between the power supply line and the first node, and the second main switching element is connected between the reference voltage line and the first node. The first main rectifying element is connected in antiparallel with the first switching element, and the second main rectifying element is connected in antiparallel with the second main rectifying element. The auxiliary resonant circuit includes a capacitor connected in parallel with at least one main switching element, a series-connected auxiliary switching element and auxiliary inductor connected in parallel to the capacitor, and an on-state current of the auxiliary switching element. And an auxiliary rectifying element connected in series with the auxiliary switching element so as to block current in the reverse direction. The control circuit is configured to control the duty ratio of at least one of the main switching elements, and includes a mode selection unit and a timing control unit. The mode selection unit includes a first operation mode in which current is generated in the auxiliary resonance circuit by on / off control of the auxiliary switching element and at least one main switching element is turned on / off, and at least one main in a state where the auxiliary switching element is fixed off. The second operation mode for turning on / off the switching element is selected according to the output level of the DC-DC converter. The timing control unit controls on / off of the first and second main switching elements and the auxiliary switching elements. In particular, the timing control unit sets the switching period of at least one main switching element so that the current direction of the main inductor is maintained during the off period of at least one main switching element when the first operation mode is selected. On the other hand, when the second operation mode is selected, the switching period of at least one of the main switching elements is set so that the off period of at least one of the main switching elements is maintained until the current direction of the main inductor is reversed. .

  A DC-DC converter control method according to the present invention is a DC-DC converter control method for performing DC voltage conversion between a low-voltage power supply and a power supply wiring, and the DC-DC converter includes the main inductor, The first and second main switching elements, the first and second main rectifying elements, and the auxiliary resonance circuit. The control method includes a first operation mode in which current is generated in the auxiliary resonant circuit by on / off control of the auxiliary switching element and at least one main switching element is turned on / off, and at least one of the auxiliary switching elements is fixed in an off state. A step of selecting a second operation mode for turning on and off the main switching element according to an output level of the DC-DC converter, a selection result of the first and second operation modes, and at least one of the main switching elements; And controlling on / off of the first and second main switching elements and the auxiliary switching elements based on the duty ratio control. In particular, the controlling step sets the switching period of at least one main switching element so that the current direction of the main inductor is maintained during the off period of at least one main switching element when the first operation mode is selected. On the other hand, when the second operation mode is selected, the switching period of at least one of the main switching elements is set so that the off period of at least one of the main switching elements is maintained until the current direction of the main inductor is reversed. .

  According to the DC-DC converter and the control method thereof, when the first operation mode is selected, switching loss is reduced by applying soft switching by operating the auxiliary resonance circuit for at least one of the first and second main switching elements. it can. In addition, when the second operation mode is selected, the current direction of the main inductor is reversed and the current flows through the main rectifier element, and then the main switching element is turned on so that the auxiliary resonance circuit is not activated. Switching can be applied. Therefore, the operation mode is selected according to the output level of the DC-DC converter according to the relationship between the power loss due to hard switching of the main switch element and the power consumption due to the operation of the auxiliary resonance circuit. Efficiency can be improved over a wide operating range including the load region.

  Preferably, the timing control unit or the controlling step includes at least one main switching within an ON period of the auxiliary switching element provided within a period in which the current direction of the main inductor is maintained when the first operation mode is selected. While the element is turned on, at the time of selecting the second operation mode, at least one main switching element is turned on after reversal of the current direction of the main inductor.

  Preferably, the mode selection unit or the selecting step selects the first operation mode when the output power is equal to or greater than a predetermined value, and selects the second operation mode when the output power is smaller than the predetermined value. To do.

  In this way, the selection of the first and second operation modes can be simplified, and the DC-DC converter can be operated with high efficiency over a wide operation range.

  More preferably, in the mode selection unit or the selecting step, the driving speed of the control electrode at the turn-on time is set to be higher than the driving speed at the turn-off time in the first and second operation modes and the state where the auxiliary switching element is fixed off. One of the operation modes is selected according to the output power of the DC-DC converter from the third operation mode in which at least one of the main switching elements is turned on / off so as to reduce the power consumption. The timing control unit or the controlling step includes the step of controlling at least one of the main switching elements so that the current direction of the main inductor is maintained during the off period of at least one of the main switching elements when the third operation mode is selected. Set the switching period.

  In this way, when the second operation mode is applied, the auxiliary inductor is used in the operation region (at the time of extremely low power output) in which the current ripple of the main inductor becomes relatively large and the switching frequency is excessively increased. After the circuit is stopped, the third operation mode for reducing the turn-on loss can be applied to the first and second main switching elements by performing hard switching with the turn-on / off speed controlled. Therefore, by properly using the first to third operation modes, it becomes possible to reduce the switching loss even at the time of extremely low power output, and to improve the efficiency of the DC-DC converter over a wider operation range. Can do.

  The mode selection unit or the selecting step selects the first operation mode when the output power is larger than the first reference value, and sets the first operation mode when the output power is smaller than the second reference value smaller than the first reference value. While the third operation mode is selected, the second operation mode is selected when the output power is between the first and second reference values.

  In this way, the selection of the first to third operation modes can be simplified, and the DC-DC converter can be operated with high efficiency over a wide operation range.

  According to the present invention, the efficiency of the DC-DC converter can be improved over a wide operation range including a light load region.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following, the same or corresponding parts in the drawings are denoted by the same reference numerals, and the description thereof will not be repeated in principle.

(Embodiment 1)
FIG. 1 is a circuit diagram showing a circuit configuration of a DC-DC converter 100 according to Embodiment 1 of the present invention.

  Referring to FIG. 1, DC-DC converter 100 according to the first embodiment includes a main converter circuit 110 and an auxiliary resonance circuit 120.

  The main converter circuit 110 outputs a voltage Vo obtained by boosting the voltage Vi corresponding to the output voltage of the battery BAT, which is a “low voltage power supply”, between the power supply wiring PL connected to the load 200 and the reference voltage wiring GL. It has a configuration of a so-called non-insulated current bidirectional (step-up / step-down) converter that can charge the low-voltage power supply by stepping down voltage Vo of line PL and reference voltage line GL to voltage Vi.

  As load 200, for example, an AC motor driven through an inverter circuit is applied. Application to a hybrid vehicle that travels by engine output and / or motor output, an electric vehicle that travels only by motor output, and the like can be given as typical application examples of the DC-DC converter according to the embodiment of the present invention. In this case, for example, the output voltage (voltage Vi) of the low-voltage power supply (battery BAT) is set to about 200V, while the voltage Vo to be supplied to the load 200 is set to about 500V.

  The main converter circuit 110 includes a capacitor C0, an inductor L1 as a “main inductor”, power semiconductor switching elements Q1 and Q2 as “main switching elements”, and diodes D1 and D2 as “main rectifier elements”. Including.

  Capacitor C0 is connected between the positive terminal and the negative terminal of battery BAT to smooth voltage Vi. Inductor L1 is connected between the positive terminal of battery BAT and node N1 (first node). A current sensor 112 for detecting the current of the inductor L1 (inductor current IL) is arranged.

  As the power switching elements (hereinafter also simply referred to as “switching elements”) Q1 and Q2, in the present embodiment, an IGBT (Insulated Gate Bipolar Transistor) is illustrated, but by drive control of the control electrode (gate or base), As long as the switching elements can control the turn-on and turn-off, voltage-driven switching elements (MOS-FET, IGBT, etc.), current-driven switching elements (bipolar transistors, etc.), and various switching elements can be applied.

  Switching element Q1 is connected between power supply line PL and node N1. The collector of switching element Q1, which is an IGBT, is connected to power supply line PL, while the emitter is connected to node N1. The switching element Q2 is connected between the reference voltage line GL and the node N1. The collector of switching element Q2 is connected to node N1, while the emitter is connected to reference voltage line GL. Further, on / off of the switching elements Q1 and Q2 is controlled by the voltage drive of the control electrode (gate) by the control circuit 150. Diodes D1 and D2 are connected in antiparallel to switching elements Q1 and Q2.

  Auxiliary resonance circuit 120 includes capacitors C1 and C2 connected in parallel to switching elements Q1 and Q2, inductors L2 and L3 connected in series between nodes N1 and N2, and a series connection between power supply line PL and node N2, respectively. Switching element Q3 and diode D3 connected, and diode D4 and switching element Q4 connected in series between node N2 and reference voltage line GL are included.

  The switching elements Q3 and Q4 are provided as “auxiliary switching elements”, and on / off of the switching elements Q3 and Q4 is controlled by the voltage drive of the control electrode (gate) by the control circuit 150. Diodes D3 and D4 as “auxiliary rectifying elements” are connected with a polarity that prevents currents in the opposite direction to auxiliary currents Irp # and Irp generated when switching elements Q3 and Q4 are turned on, respectively.

  The inductor L2 is electromagnetically coupled to the inductor L1 so that an electromotive force is induced with a reverse polarity at a node N1 side terminal of the inductor L1 and a node N1 side terminal of the inductor L2. The electromagnetic coupling is realized, for example, by configuring a transformer with the inductor L1 and the inductor Lr.

  As can be understood from FIG. 1, the configurations and operations of the main converter circuit 110 and the auxiliary resonance circuit 120 that constitute the DC-DC converter 100 are the same as those in Patent Documents 1 and 2 described above. Therefore, in main converter circuit 110, basically, switching elements Q1 and Q2 are controlled by control circuit 150 so as to be alternately turned on and off. In terms of the operating principle, when operating as a step-up converter, the switching element Q1 may be fixed to OFF and on / off control (duty control) of the switching element Q2 may be executed. When operating as a step-down converter, the switching element Q2 On / off control (duty control) of the switching element Q1 may be executed after fixing is turned off.

  In the boosting operation for boosting the voltage Vi from the battery BAT to the voltage Vo to the power supply wiring PL, the main converter circuit 110 converts the electromagnetic energy accumulated in the main inductor L1 by the conduction of the switching element Q2 into the switching element Q1 and It operates so as to be supplied to the power supply wiring PL via the antiparallel diode D1. Further, during the step-down operation in which the voltage Vo of the power supply wiring PL is stepped down to the voltage Vi to the battery BAT, the main converter circuit 110 converts the electromagnetic energy accumulated in the main inductor L1 by the conduction of the switching element Q1 to the switching element Q2 and the reverse. It operates to supply to the low voltage power supply BAT via the parallel diode D2.

  Control circuit 150 includes a duty control unit 152, a timing control unit 154, a driver 156, and an operation mode selection unit 158. The control circuit 150 comprehensively shows the control elements of the DC-DC converter 100. Regarding the duty control unit 152, the timing control unit 154, and the operation mode selection unit 158 corresponding to each control function part, It can be realized by either software processing by execution of a predetermined program or hardware processing by construction of a dedicated electronic circuit. The driver 156 is usually constructed by an electronic circuit (hardware).

  Duty control unit 152 controls the duty ratio of switching elements Q1 and Q2 based on voltage Vo or voltage command value Vor of voltage Vi and detected values of voltage Vi and voltage Vo. The duty ratio is generally indicated by the ratio of the ON period of switching elements Q1 and / or Q2 to a predetermined switching period. For example, the command duty of switching element Q2 is set during the step-up operation, and the command duty of switching element Q1 is set during the step-down operation.

  The operation mode selection unit 158 selects the operation mode of the DC-DC converter 100 according to the output level (typically, output power Po) of the DC-DC converter 100, as will be described in detail below. In the following, output power Po is exemplified as a quantitative evaluation index of the output level of DC-DC converter 100, but output level evaluation for selecting an operation mode may be performed based on other various quantities.

  Then, the timing control unit 154 turns on / off the switching elements Q1 to Q4 according to the operation mode selected by the operation mode selection unit 158, the inductor current IL detected by the current sensor 112, and the command duty from the duty control unit 152. A pulse signal for controlling each of the above is generated.

  For example, the pulse signal is set to a logic high level (hereinafter also referred to as H level) in each of the on-periods of the switching elements Q1 to Q4, while the logic low level (hereinafter also referred to as L level) in each of the off-periods. To be set).

  The driver 156 drives the gate voltages Vg1 to Vg4 of the switching elements Q1 to Q4 according to the pulse signal from the timing control unit 154.

  Next, the operation of the DC-DC converter 100, particularly the difference in operation in each operation mode, will be described in detail.

  First, the operation of the DC-DC converter 100 in the normal operation mode in which the auxiliary resonance circuit 120 is operated will be described with reference to FIG. Here, the step-up operation of the DC-DC converter 100 will be described as an example.

  Referring to FIG. 2, switching elements Q1, Q2 as main switching elements are exclusively turned on / off. For example, in the timing control unit 154 (FIG. 1), a DC voltage corresponding to the command duty from the duty control unit 152 and a carrier wave (triangular wave or sawtooth wave) having a predetermined frequency corresponding to the switching frequency of the switching elements Q1 and Q2. Based on the voltage comparison, it is possible to generate a pulse signal that defines on / off of the switching elements Q1, Q2.

  Here, when the switching elements Q1 and Q2 are turned off, the increase in the collector-emitter voltage is suppressed by the capacitors C1 and C2 connected in parallel to the switching elements Q1 and Q2, so that zero voltage switching can be applied. Further, during the boosting operation, the diode D1 is normally turned on by commutation in response to the OFF period of the switching element Q2, so that the switching element Q1 is turned on by zero voltage switching with a slight voltage applied to the collector and emitter. be able to.

  Therefore, it is necessary to take measures against the turn-on loss of the switching element Q2 during the boosting operation, and in the normal operation mode, zero voltage switching is applied to the turn-on of the switching element Q2 by the auxiliary resonance circuit 120.

  As shown in FIG. 2, in the normal operation mode during the boosting operation, after the switching element Q1 is turned off, the switching element Q4 is turned on to generate the auxiliary current Irp (FIG. 1) in the auxiliary resonance circuit 120. As a result, since the diode D2 is turned on, the switching element Q2 can be turned on while a slight voltage is applied to the collector and the emitter, so that the power loss can be suppressed by applying soft switching (zero voltage switching). Can be planned. Then, after the switching element Q2 is turned on, the auxiliary current Irp is extinguished by turning off the switching element Q4.

  On the other hand, as described above, during the boosting operation, switching element Q1 has a low switching loss both in turn-on and turn-off, so that switching element Q3 in auxiliary resonance circuit 120 is fixed in the off state.

  Although not shown, during the step-down operation of the DC-DC converter 100, the switching elements Q1 and Q2 which are main switching elements are exclusively turned on / off, while in the auxiliary resonance circuit 120, the switching element Q4 is fixed in the off state. At the same time, switching element Q3 is turned on for a certain period of time so as to generate auxiliary current Irp # (FIG. 1) prior to turn-off of switching element Q1.

  The auxiliary resonant circuit 120 shown in FIG. 1 includes a switching element Q4 for generating an auxiliary current Irp for soft switching of the switching element Q2 during the boost operation, and a soft switching of the switching element Q1 during the step-down operation. The auxiliary resonance circuit 120 may be configured to generate an auxiliary current for only one of the switching elements Q1 and Q2, although the switching element Q3 for generating the auxiliary current Irp # is provided. Is possible. As an example, in the case of a configuration corresponding to soft switching of only the switching element Q2, the arrangement of the switching element Q3 and the diode D3 can be omitted and the node N2 and the power supply line PL can be separated.

  As described above, in the normal operation mode of the DC-DC converter 100, the auxiliary resonance circuit 120 operates so that at least one of the switching elements Q1, Q2 is soft-switched. Specifically, the auxiliary current Irp or Irp # is generated by the on / off control of the switching elements Q3 and / or Q4, thereby reducing the switching loss of the main switching element.

FIG. 3 shows the relationship between the output power and efficiency of the DC-DC converter.
Referring to FIG. 3, when switching elements Q1 and Q2 are switched in main converter circuit 110 without operating auxiliary resonance circuit 120, the efficiency characteristic with respect to output power Po of DC-DC converter 100 is indicated by dotted line 500. As shown.

  On the other hand, when the auxiliary resonant circuit 120 is operated and the switching elements Q1 and Q2 are soft-switched, the efficiency characteristic with respect to the output power Po of the DC-DC converter 100 is as indicated by a solid line 510.

  That is, in the region where the output power Po is relatively high, the power loss suppression effect by applying soft switching exceeds the power consumption by the auxiliary resonance circuit 120, and therefore the efficiency is higher when the auxiliary resonance circuit 120 is operated. On the other hand, as the output power Po decreases, the efficiency of the DC-DC converter 100 rapidly decreases due to an increase in the ratio of power consumption by the operation of the auxiliary resonant circuit 120 to the output power Po.

  In the light load region where the output power Po is lower than the predetermined value (for example, Po <Pa region in FIG. 3), the power consumption in the auxiliary resonance circuit 120 exceeds the power loss suppression effect by applying soft switching. Thus, it is understood that the operation of the auxiliary resonant circuit 120 decreases the efficiency of the DC-DC converter 100 instead.

  Therefore, in the DC-DC converter 100 according to the embodiment of the present invention, the operation of the auxiliary resonance circuit 120 is stopped while the main converter circuit 110 is in the normal operation mode in order to cope with the light load region as described above. Introduce a new operation mode that operates with different current behavior. Hereinafter, such an operation mode is referred to as a “light load operation mode”. That is, the operation mode selection unit 158 illustrated in FIG. 1 is a normal operation mode in which the auxiliary resonance circuit 120 is operated according to the output power Po of the DC-DC converter 100 and a light operation in which the operation of the auxiliary resonance circuit 120 is stopped. It is configured to select one of the load operation modes.

  FIG. 4 shows an operation waveform diagram of the DC-DC converter in the light load operation mode. In FIG. 4, as in FIG. 2, the operation waveform during the boosting operation will be described as an example.

  Referring to FIG. 4, in the light load operation mode, switching elements Q3 and Q4 are fixed to OFF, and the operation of auxiliary resonance circuit 120 is stopped. That is, auxiliary current Irp (Irp #) is not generated.

  The switching elements Q1 and Q2 are alternately turned on and off while providing a predetermined dead time according to the command duty by the duty control unit 152. At this time, the switching element Q2 has a direction of the inductor current IL at the time of turn-off. Until inversion, control is performed so that the off period (that is, the on period of the switching element Q1) is maintained until IL <0 in FIG.

  When the inductor current IL <0, the diode D2 shown in FIG. 1 becomes conductive. Under this state, the switching element Q2 can be turned on with a slight voltage applied to the collector and the emitter. . That is, the switching element Q2 can be soft-switched (zero voltage switching) without operating the auxiliary resonant circuit 120.

  1 determines the turn-on timing of the switching element Q2 (turning off the switching element Q1) based on the output of the current sensor 112, and also turns off the switching element Q2 (turning on the switching element Q1). About timing, it determines so that the instruction | command duty by the duty control part 152 may be reflected. That is, in the light load operation mode, the switching frequency of the switching elements Q1, Q2 is not constant, but changes according to the behavior of the inductor current IL.

  On the other hand, in the normal operation mode shown in FIG. 2, the on / off periods of the switching elements Q1 and Q2 are set so as to maintain the direction of the inductor current IL during the on or off period, and the switching element Q1. , Q2 is basically fixed in switching frequency. In general, an appropriate switching frequency is set in advance in consideration of a trade-off relationship between an increase in switching loss due to higher frequencies and a reduction in inductor size, noise generation due to switching in an audible frequency band, and the like.

  Further, the operation of the main converter circuit 110 in the light load operation mode will be described in detail with reference to FIG.

  FIG. 5A shows an operating state during a period in which electromagnetic energy is accumulated in the inductor L1 when the switching element Q2 is turned on and the switching element Q1 is turned off. During this period, the capacitor C1 is charged.

  From this state, as shown in FIG. 5B, when the switching element Q2 is turned off, the capacitor C2 is charged while the voltage at the node N1 is changed, while the capacitor C1 is discharged. Since the rise of the collector-emitter voltage of the switching element Q2 is suppressed by the charging operation of the capacitor C2 at this time, the turn-off of the switching element Q2 is zero voltage switching, and the switching loss is suppressed.

  When the charging / discharging of the capacitors C1 and C2 is completed, as shown in FIG. 5C, the diode D1 becomes conductive, so that the energy stored in the inductor L1 is supplied to the power supply line PL. As a result, the collector-emitter voltage of the switching element Q1 becomes a slight value.

  As shown in FIG. 5D, when the dead time ends, zero voltage switching (ZVS) can be realized by turning on the switching element Q1 with the diode D1 turned on. As shown in FIG. 5 (e), the ON state of the switching element Q1 is a state in which current is supplied from the power supply line PL to the inductor L1 via the switching element Q1, that is, a state in which the inductor current IL is inverted. Continue until

  Then, after the inversion of the inductor current, the switching element Q1 is turned off as shown in FIG. Also at this time, the capacitor C1 is charged while the capacitor C2 is discharged along with the voltage change of the node N1. Since the rise of the collector-emitter voltage of the switching element Q1 is suppressed for the charging operation of the capacitor C1 at this time, the switching element Q1 is turned off to zero voltage switching, and the switching loss is suppressed.

  When the charging / discharging of the capacitors C1 and C2 is completed, as shown in FIG. 5G, the diode D2 is turned on to secure a path for the inverted inductor current. Thereby, the collector-emitter voltage of the switching element Q2 becomes a slight value.

  Furthermore, when the dead time ends, as shown in FIG. 5 (h), the switching element Q2 is turned on, but the voltage between the collector and the emitter is almost zero, so zero voltage switching (ZVS) is applied. can do. Further, when the switching element Q2 is turned on, power is supplied from the battery BAT to the inductor L1, so that the inductor current is inverted again (IL> 0), and the circuit state of FIG. 5A is reproduced.

  As described above, in the light load operation mode, even when the operation of the auxiliary resonance circuit 120 is stopped, the switching loss of the switching elements Q1 and Q2 can be suppressed by controlling the polarity of the inductor current IL to be reversed.

  In the step-down operation, the polarity of the inductor current IL is reversed in FIG. 4 so that Iav <0, and the ON period of the switching element Q1 changes from IL <0 at the time of turn-on to IL> 0. Will be maintained until. Also, the operation of the auxiliary resonance circuit 120 is stopped by fixing the switching elements Q3 and Q4 to OFF.

  In DC-DC converter 100 according to the first embodiment, the operation mode of DC-DC converter 100 is selected according to the flowchart shown in FIG. The flowchart shown in FIG. 6 can be executed by software processing by executing a predetermined program stored in advance in the control circuit 150 shown in FIG. 1 or by hardware processing by a dedicated electronic circuit (hardware).

  Referring to FIG. 6, control circuit 150 compares output power Po of DC-DC converter 100 with reference value Pa in step S100. The output power Po may be set based on a command value of the DC-DC converter 100 or may be set based on a detected value of voltage / current.

  The reference value Pa of the output power is a comparison of the efficiency characteristics of the DC-DC converter 100 when the normal operation mode is applied (solid line 510) and when the auxiliary resonance circuit 120 is stopped (dotted line 500) as shown in FIG. Can be set in advance. Note that since the reference value Pa in FIG. 3 may change according to operating conditions (for example, switching frequency) other than the output power of the DC-DC converter 100, the reference value Pa is variable according to the operating conditions. It is good also as a control structure set to.

  When output power Po ≧ Pa (when YES is determined in S100), control circuit 150 selects the normal operation mode in step S110, while when Po <Pa (when NO is determined in S100). In step S120, the light load operation mode is selected. As described above, in the normal operation mode, the switching period of the switching elements Q1, Q2 is fixed to an appropriate value (for example, about several kHz), while in the light load operation mode, the switching period of the switching elements Q1, Q2 is , And changes depending on the time required until the inductor current IL is inverted. As described above, the processing in steps S100 to S120 corresponds to the function of the operation mode selection unit 158 in FIG.

  In step S150, the control circuit 150 selects the main switching element based on the selected operation mode (normal operation mode or light load operation mode) and the duty ratio (command duty) controlled by the duty control unit 152. The on / off of the switching elements Q1, Q2 is controlled. As described above, in the light load operation mode, the detected value of the inductor current IL is reflected in the on / off timing of the switching elements Q1 and Q2.

  On the other hand, the switching elements Q3 and Q4 in the auxiliary resonance circuit 120 are fixed to off when the light load operation mode is selected, and respond to whether the operation is step-up operation or step-down operation when the normal operation mode is selected. Then, one of the switching elements Q3 and Q4 is turned on in a pulsed manner in accordance with the on timing of the switching elements Q1 and Q2.

  As described above, according to the DC-DC converter 100 according to the first embodiment, the main current is reversed after the inductor current IL is inverted even in a light load region where the efficiency is lowered due to the operation of the auxiliary resonance circuit 120. Soft switching can be applied without operating the auxiliary resonance circuit 120 by setting the circuit operation state to turn on and off the switching element.

  Therefore, as shown by solid lines 510 and 520 in FIG. 7, the efficiency is improved over a wide operation range including a light load region by appropriately selecting both operation modes according to the output power of the DC-DC converter 100. can do.

(Embodiment 2)
In the DC-DC converter 100 according to the first embodiment, in the light load operation mode in which the operation of the auxiliary resonance circuit 120 is stopped, a current ripple is generated in order to control the on / off of the switching elements Q1, Q2 according to the inversion of the inductor current IL. Since it increases, there is a concern that this causes an increase in power loss in the inductor L1. Further, since the average current of the inductor current IL decreases as the output power decreases, the switching frequency of the switching elements Q1, Q2 accompanying the inversion of the inductor current IL is shortened, so that the switching frequency is increased. There is also concern that the switching loss will increase.

  In the second embodiment, a new operation mode (hereinafter also referred to as “extremely light load operation mode”) for dealing with such a further light load region (hereinafter also referred to as “extremely light load region”) is introduced. The configuration of the DC-DC converter will be described.

  FIG. 8 is a circuit diagram showing a configuration of DC-DC converter 100 # according to the second embodiment of the present invention.

  Referring to FIG. 8, DC-DC converter 100 # according to the second embodiment further includes gate drive control circuits GC1, GC2 in addition to the configuration of DC-DC converter 100 according to the first embodiment shown in FIG. It differs in the point to prepare.

  The gate drive control circuit GC1 is provided corresponding to the control electrode (gate) of the switching element Q1, and the gate drive control circuit GC2 is provided corresponding to the control electrode (gate) of the switching element Q2.

  The gate drive control circuit GC1 includes a resistance element Rl1 connected between the output node of the driver 156 and the control electrode (gate) of the switching element Q1, and a diode Dg1 and a resistance element Rs1 connected in parallel with the resistance element Rl1. And a set of The gate drive control circuit GC2 includes a resistance element Rl2 connected between the output node of the driver 156 and the control electrode (gate) of the switching element Q2, and a diode Dg2 and a resistance element Rs2 connected in parallel to the resistance element Rl2. And a set of The resistance values of the resistance elements Rs1 and Rs2 are designed to be smaller than the resistance values of the resistance elements Rl1 and Rl2.

  As a result, in each of the switching elements Q1 and Q2, at the turn-on time when the gate voltage is increased from the L level to the H level, the gate voltage is driven via the relatively high resistance resistance element Rl1 or Rl2. At the turn-off time when the voltage is lowered from the H level to the L level, the gate voltage is driven via the relatively low resistance resistor element Rs1 or Rs2. That is, due to the arrangement of the gate drive control circuits GC1 and GC2, each of the switching elements Q1 and Q2 has a relatively low switching speed when turned on, that is, a drive speed of the control electrode, while a switching speed when turned off, that is, As a result, the drive speed of the control electrode is increased.

  The circuit configuration of other parts of DC-DC converter 100 # is the same as that of DC-DC converter 100 of FIG.

  Further, in DC-DC converter 100 # according to the second embodiment, operation mode selection unit 158 further introduces an extremely light load operation mode in addition to the normal operation mode and the light load operation mode described in the first embodiment. . That is, the operation mode selection unit 158 selects one operation mode from the normal operation mode, the light load operation mode, and the extremely light load operation mode according to the output power Po.

  The timing control unit 154 controls the switching elements Q1 to Q4 as shown in FIG. 9 when selecting the extremely light load operation mode.

  FIG. 9 is an operation waveform diagram illustrating an operation in the extremely light load operation mode in DC-DC converter 100 # according to the second embodiment.

  Referring to FIG. 9, switching elements Q3 and Q4 are fixed to OFF in the extremely light load operation mode as in the light load operation mode. That is, the operation of the auxiliary resonance circuit 120 is stopped.

  Furthermore, the switching elements Q1 and Q2 are exclusively turned on and off alternately with a predetermined dead time according to a predetermined switching frequency. The switching elements Q1 and Q2 are turned on / off before the polarity of the inductor current IL is reversed, as in the normal operation mode.

  That is, in the extremely light load operation mode, the switching elements Q1 and Q2 are hard-switched. At this time, the presence of the capacitors C1 and C2 suppresses the switching loss at the time of turn-off, but at the time of turn-on, there is a concern about the inrush current due to the charges accumulated in the capacitors C1 and C2. More specifically, during the step-up operation, it is necessary to cope with the inrush current due to the accumulated charge of the capacitor C2 when the switching element Q2 is turned on. On the other hand, during the step-down operation, due to the accumulated charge of the capacitor C1 when the switching element Q1 is turned on. Need to deal with inrush current.

  Therefore, when selecting the extremely light load operation mode, driver 156 drives the control electrode (gate) of switching element Q1 and / or Q2 via gate drive control circuit GC1 and / or GC2. As a result, in switching elements Q1 and / or Q2 in which the control electrode is driven via the gate drive control circuit, the drive speed of the control electrode at the time of turn-on is slower than in the normal operation mode. As a result, the rate of decrease in voltage (collector-emitter voltage) and the rate of increase in current (collector current) Ic at turn-on can be suppressed, so that switching loss at turn-on can be suppressed. Further, at the time of turn-off, since the driving speed of the control electrode is the same as that in the normal operation mode, the switching loss does not increase.

  For example, as shown in FIG. 10, in the region of Po <Pa, the efficiency characteristic 520 (a combination of the solid and dotted lines) when the light load operation mode described in the first embodiment is applied, and the main converter circuit A reference value Pb for switching between the light load operation mode and the extremely light load operation mode can be set by comparison with the efficiency characteristic 500 (dotted line) when the 110 is hard-switched.

  That is, in the region of Po <Pb, by selecting an extremely light load operation mode and applying hard switching using the gate drive control circuits GC1 and / or GC2, the characteristics shown by the solid line 530 in FIG. Thus, DC-DC converter 100 # can be controlled.

  That is, according to DC-DC converter 100 # in accordance with the second embodiment, the operation mode of DC-DC converter 100 # is selected according to the flowchart shown in FIG. The flowchart shown in FIG. 11 can also be executed by software processing by executing a predetermined program stored in advance in the control circuit 150 shown in FIG. 8 or by hardware processing by a dedicated electronic circuit (hardware).

  Referring to FIG. 11, control circuit 150 compares output power Po of DC-DC converter 100 # with reference value Pa at step S100 similar to FIG. 6, and if Po <Pa (NO in S100) At the time of determination), the process proceeds to step S105, and the output power Po is further compared with the reference value Pa. As described above, the output power Po may be set based on the command value, or may be set based on the detected value of voltage / current.

  When the output power Po ≧ Pa (when YES is determined in S100), the control circuit 150 selects the normal operation mode in step S110. On the other hand, when Po <Pb (when NO is determined in S105), the control circuit 150 selects the extremely light load mode in step S130 and when Pb ≦ Po <Pa (when YES is determined in S105). In step S120, the light load mode is selected. Even in the extremely light load mode (S130), the switching cycle of switching elements Q1, Q2 is fixed to an appropriate value (for example, the same frequency as in the normal operation mode).

  As described above, the processing in steps S100 to S130 corresponds to the function of the operation mode selection unit 158 in FIG. Also, the reference value Pb of the output power can be set in advance based on the comparison between the efficiency characteristic 520 when the light load operation mode is applied and the efficiency characteristic 500 when hard switching as shown in FIG. Reference value Pb may also be variably set according to operating conditions other than the output power of DC-DC converter 100 #.

  Further, in step S150 #, control circuit 150 selects the selected operation mode (normal operation mode, light load operation mode, or extremely light load mode) and the duty ratio (command duty) controlled by duty control unit 152. Based on the above, in the light load operation mode, the inductor current is further reflected to control the on / off of the switching elements Q1, Q2.

  The switching elements Q3 and Q4 in the auxiliary resonance circuit 120 are fixed to OFF when the light load operation mode and the extremely light load operation mode are selected, and when the normal operation mode is selected, either the step-up operation or the step-down operation is performed. In response to one of the switching elements Q3 and Q4, one of the switching elements Q3 and Q4 is turned on in a pulsed manner in accordance with the on timing of the switching elements Q1 and Q2.

  As described above, according to DC-DC converter 100 # according to the second embodiment, a further light load region (extremely light load) in which efficiency is lowered when the light load region described in the first embodiment is applied. Even in the load region), the efficiency can be improved by applying an extremely light load operation mode for performing hard switching with control electrode (gate) drive speed control. As a result, the efficiency can be improved over a wider operating range than the DC-DC converter 100 according to the first embodiment.

  In the first and second embodiments, the operation mode selection in the region where the output power Po> 0 has been described. The same operation mode selection can be applied in the region where the output power Po <0. In this case, the operation mode can be selected so that the light load mode and the ultra light load mode are applied from the normal operation mode as | Po | becomes smaller.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a circuit diagram which shows the circuit structure of the DC-DC converter 100 by Embodiment 1 of this invention. FIG. 3 is an operation waveform diagram in a normal operation mode of the DC-DC converter 100 according to the first embodiment. 3 is a conceptual diagram showing a relationship between output power and efficiency of a DC-DC converter 100. FIG. FIG. 4 is an operation waveform diagram in the light load mode of the DC-DC converter 100 according to the first embodiment. It is a circuit diagram explaining the circuit operation | movement of the main converter circuit part in light load mode. 3 is a flowchart illustrating a processing procedure for selecting an operation mode in DC-DC converter 100 according to the first embodiment. It is a conceptual diagram which shows the efficiency characteristic with respect to the output power of the DC-DC converter 100 by Embodiment 1. FIG. It is a circuit diagram which shows the structure of DC-DC converter 100 # by Embodiment 2 of this invention. FIG. 11 is an operation waveform diagram in the extremely light load mode of DC-DC converter 100 # according to the second embodiment. It is a conceptual diagram which shows the efficiency characteristic with respect to the output electric power of DC-DC converter 100 # by Embodiment 2. FIG. FIG. 10 is a flowchart illustrating a processing procedure for operation mode selection in DC-DC converter 100 # according to the second embodiment.

Explanation of symbols

  100, 100 # DC-DC converter, 110 main converter circuit, 112 current sensor, 120 auxiliary resonance circuit, 150 control circuit, 152 duty control unit, 154 timing control unit, 156 driver, 158 operation mode selection unit, 200 load, 500 , 510, 520, 530 Efficiency characteristics, BAT battery (low voltage power supply), C0 capacitor, C1, C2 capacitor (auxiliary resonance circuit), D1, D2 diode (main rectifier element), D3, D4 diode (auxiliary rectifier element), Dg1 , Dg2 diode (gate drive control circuit), GC1, GC2 gate drive control circuit, GL reference voltage wiring, IL inductor current, Irp, Irp # auxiliary current (auxiliary resonance circuit), L1 inductor (main inductor), L2, L3 inductor (Auxiliary inductor), N1, N2 node, Pa, Pb reference value, Q1, Q2 power semiconductor switching element (main switching element), Q3, Q4 power semiconductor switching element (auxiliary switching element), Rl1, Rl2, Rs1 , Rs2 resistive element, Vi, Vo voltage, Vor voltage command value, Vg1-Vg4 gate voltage.

Claims (10)

A DC-DC converter for performing DC voltage conversion between a low-voltage power supply and a power supply wiring,
A main inductor connected between the low-voltage power source and the first node;
A first main switching element connected between the power supply wiring and the first node;
A first main rectifying element connected in anti-parallel with the first switching element;
A second main switching element connected between a reference voltage line and the first node;
A second main rectifying element connected in anti-parallel with the second main switching element;
An auxiliary resonance circuit provided corresponding to at least one main switching element of the first and second main switching elements,
The auxiliary resonant circuit is
A capacitor connected in parallel with the at least one main switching element;
An auxiliary switching element and an auxiliary inductor connected in series connected in parallel to the capacitor;
An auxiliary rectifying element connected in series with the auxiliary switching element so as to prevent a current in a direction opposite to a current when the auxiliary switching element is on,
A control circuit for controlling a duty ratio of the at least one main switching element;
The control circuit includes:
On-off control of the auxiliary switching element generates a current in the auxiliary resonant circuit and turns on / off the at least one main switching element, and the at least one main with the auxiliary switching element fixed off A mode selection unit that selects a second operation mode for turning on and off the switching element according to the output level of the DC-DC converter;
A timing control unit that controls on / off of the first and second main switching elements and the auxiliary switching elements;
The timing control unit is configured to switch the at least one main switching element so that a current direction of the main inductor is maintained during an off period of the at least one main switching element when the first operation mode is selected. While the period is set, when the second operation mode is selected, the at least one main switching element is maintained so that the off period of the at least one main switching element is maintained until the current direction of the main inductor is reversed. A DC-DC converter that sets the switching period of the element.   The timing control unit is configured to switch the at least one main switching element within an on period of the auxiliary switching element provided in a period in which the current direction of the main inductor is maintained when the first operation mode is selected. 2. The DC-DC converter according to claim 1, wherein, when the second operation mode is selected, the at least one main switching element is turned on after the current direction of the main inductor is reversed when the second operation mode is selected.   The mode selection unit selects the first operation mode when the output power is greater than or equal to a predetermined value, and selects the second operation mode when the output power is smaller than the predetermined value. The DC-DC converter according to claim 1 or 2. The mode selection unit is configured to lower the drive speed of the control electrode at the time of turn-on than the drive speed at the time of turn-off in the state where the first and second operation modes and the auxiliary switching element are fixed off. The third operation mode for turning on or off the at least one main switching element is selected according to the output power of the DC-DC converter,
The timing controller switches the at least one main switching element so that the current direction of the main inductor is maintained during the off period of the at least one main switching element when the third operation mode is selected. The DC-DC converter according to claim 1, wherein the period is set.   The mode selection unit selects the first operation mode when the output power is larger than a first reference value, and the output power is smaller than the second reference value smaller than the first reference value. 5. The DC-DC converter according to claim 4, wherein the third operation mode is sometimes selected while the second operation mode is selected when the output power is between the first and second reference values. . A method of controlling a DC-DC converter for performing DC voltage conversion between a low-voltage power supply and a power supply wiring,
The DC-DC converter
A main inductor connected between the low-voltage power source and the first node;
A first main switching element connected between the power supply wiring and the first node;
A first main rectifying element connected in anti-parallel with the first switching element;
A second main switching element connected between a reference voltage line and the first node;
A second main rectifying element connected in anti-parallel with the second main switching element;
An auxiliary resonance circuit provided corresponding to at least one main switching element of the first and second main switching elements,
The auxiliary resonant circuit is
A capacitor connected in parallel with the at least one main switching element;
An auxiliary switching element and an auxiliary inductor connected in series connected in parallel to the capacitor;
An auxiliary rectifying element connected in series with the auxiliary switching element so as to prevent a current in a direction opposite to a current when the auxiliary switching element is on,
The control method is:
On-off control of the auxiliary switching element generates a current in the auxiliary resonant circuit and turns on / off the at least one main switching element, and the at least one main with the auxiliary switching element fixed off Selecting a second operation mode for turning on and off the switching element according to the output level of the DC-DC converter;
Based on the selection results of the first and second operation modes and the duty ratio control of the at least one main switching element, the first and second main switching elements and the auxiliary switching element are turned on / off. The step of controlling,
The controlling step includes switching the at least one main switching element so that a current direction of the main inductor is maintained during an off period of the at least one main switching element when the first operation mode is selected. While the period is set, when the second operation mode is selected, the at least one main switching element is maintained so that the off period of the at least one main switching element is maintained until the current direction of the main inductor is reversed. A method for controlling a DC-DC converter, wherein a switching cycle of an element is set.   In the controlling step, when the first operation mode is selected, the at least one main switching element is turned on within an ON period of the auxiliary switching element provided in a period in which the current direction of the main inductor is maintained. 7. The method of controlling a DC-DC converter according to claim 6, wherein, when the second operation mode is selected, the at least one main switching element is turned on after the current direction of the main inductor is reversed when the second operation mode is selected.   The selecting step selects the first operation mode when the output power is equal to or greater than a predetermined value, and selects the second operation mode when the output power is smaller than the predetermined value. The method for controlling a DC-DC converter according to claim 6 or 7. In the selecting step, the driving speed of the control electrode at the time of turn-on is made lower than the driving speed at the time of turn-off under the state in which the first and second operation modes and the auxiliary switching element are fixed off. Selecting one of the operation modes according to the output power of the DC-DC converter from the third operation mode in which the at least one main switching element is turned on and off.
The controlling step includes switching the at least one main switching element so that a current direction of the main inductor is maintained during an off period of the at least one main switching element when the third operation mode is selected. The method for controlling a DC-DC converter according to claim 6, wherein the period is set.   The selecting step selects the first operation mode when the output power is larger than a first reference value, and the output power is smaller than the second reference value smaller than the first reference value. 10. The DC-DC converter according to claim 9, wherein sometimes the third operation mode is selected while the second operation mode is selected when the output power is between the first and second reference values. Control method.

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