JP2018074720A - Voltage converter, step-down control method of voltage conversion circuit, and computer program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a voltage converter which can reduce a loss of a switching element when a so-called soft switching is performed by using an LC resonance and even when no zero-cross point exists in a resonance current, a step-down control method of a voltage conversion circuit, and a computer program.SOLUTION: A control portion in a voltage converter operates in a first mode flowing a resonance current by a first inductor and a capacitor by turning on a high-pressure side switching element, calculates a timing when the resonance current becomes minimum, operates in a second mode flowing a reflux current of a second inductor to a low-pressure side switching element or the capacitor by turning off the high-pressure side switching element at the calculated timing, and shifts to the first mode by turning on the high-pressure side switching element after the operation in the second mode.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a voltage conversion device that converts a DC voltage into a voltage, a step-down control method for a voltage conversion circuit, and a computer program.

  A DC-DC converter (hereinafter simply referred to as a converter) that steps up and down a DC voltage is widely used as a power source for in-vehicle devices and industrial devices. In response to the demand for miniaturization of power supplies, the operating frequency of converters tends to be increased in order to make it possible to use passive components such as inductors and capacitors having a small volume. On the other hand, another problem that the switching loss of the switching element that switches the current flowing through the inductor increases as the operating frequency increases.

  On the other hand, Patent Document 1 discloses a resonance current flowing in a resonance circuit composed of a resonance reactor (inductor) and a resonance capacitor (capacitor) connected in series to a transistor (switching element) that switches an input voltage. A step-down converter is disclosed in which a transistor is switched from on to off when it becomes 0 or less. By performing such zero current switching, the switching loss of the transistor that switches the current flowing through the inductor is reduced.

  The converter disclosed in Patent Document 1 is a transistor at the time when a time To = Tn · (1 + Zn · Io / Vi) / 2 during which a resonance current i of zero or more flows through a resonance reactor from the time when the transistor is turned on. The basic configuration is to switch from on to off. Also disclosed is a configuration in which the transistor is switched from on to off after a predetermined time when the resonance current i becomes equal to or less than zero after the resonance current i becomes equal to or less than the predetermined current.

JP 2002-58240 A

  However, the converter disclosed in Patent Document 1 is based on the premise that there is a period during which the resonance current i is less than or equal to zero and there is a zero cross point. For example, the output current increases and the zero cross point of the resonance current i increases. When it does not exist, the switching loss of the transistor cannot be reduced.

  The present invention has been made in view of such circumstances, and an object of the present invention is to perform a so-called soft switching using LC resonance, and even if a zero cross point does not exist in the resonance current, the switching element It is an object to provide a voltage conversion device, a voltage conversion circuit step-down control method, and a computer program capable of reducing loss of power.

  A voltage conversion device according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, the first inductor and the low-voltage side switching element. A voltage conversion circuit having a second inductor having one end connected to the connection point of the capacitor and a capacitor connected in parallel to the low-voltage side switching element, and turning on / off the high-voltage side switching element to the voltage conversion circuit A voltage conversion device including a control unit that performs step-down conversion, wherein the control unit turns on the high-voltage side switching element to operate in a first mode in which a resonance current is generated by the first inductor and the capacitor, and the high-voltage side Calculate the time from when the switching element is turned on until the resonance current becomes minimum. The high-voltage side switching element is turned off, and the low-voltage side switching element or the capacitor is operated in the second mode in which the return current of the second inductor flows. After the operation in the second mode, the high-voltage side switching element is turned on. The mode is shifted to the first mode.

  A step-down control method for a voltage conversion circuit according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, the first inductor, A voltage conversion circuit having a second inductor having one end connected to a connection point of the low-voltage side switching element, a capacitor connected in parallel to the low-voltage side switching element, and a control unit for turning on / off the high-voltage side switching element In the voltage conversion apparatus comprising: a control method in which the control unit performs step-down conversion in the voltage conversion circuit, wherein the high-voltage side switching element is turned on to operate in a first mode in which a resonance current from the first inductor and the capacitor flows. And calculate the time from when the high-voltage side switching element is turned on until the resonance current is minimized. The high-voltage side switching element is turned off when the calculated time has elapsed, and the low-voltage side switching element or the capacitor is operated in the second mode in which the return current of the second inductor flows, and after the operation in the second mode, The high-voltage side switching element is turned on to shift to the first mode.

  A computer program according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, and the first inductor and the low-voltage side switching element. A voltage converter comprising a second inductor having one end connected to a connection point, a capacitor connected in parallel to the low-voltage side switching element, and a control unit for turning on / off the high-voltage side switching element A first computer program for causing the voltage conversion circuit to perform step-down conversion in the control unit in the first mode in which the high-voltage side switching element is turned on and a resonance current from the first inductor and the capacitor is supplied to the control unit. And the high voltage side switching element is turned on. Calculating a time until the resonance current becomes minimum, and turning off the high-voltage side switching element when the calculated time elapses, and causing a reflux current of the second inductor to flow through the low-voltage side switching element or the capacitor. A step of operating in the second mode, and a step of turning on the high-voltage side switching element and shifting to the first mode after the operation in the second mode.

  In addition, this application is implement | achieved as a step-down control method of the voltage converter and voltage converter circuit which each have such a characteristic process part and step, or makes a computer perform the step corresponding to this characteristic process part For example, a part or all of the voltage conversion device can be realized as a semiconductor integrated circuit, or can be realized as another system including the voltage conversion device.

  According to the above, when so-called soft switching is performed using LC resonance, it is possible to reduce the loss of the switching element even when there is no zero cross point in the resonance current.

It is a block diagram which shows the structural example of the voltage converter which concerns on embodiment. It is a timing chart which shows an example of the waveform of each part at the time of the pressure | voltage fall operation of a voltage converter circuit. It is explanatory drawing which shows an example of the state D1 in the pressure | voltage fall operation | movement of a voltage converter circuit. It is explanatory drawing which shows an example of the state D2 in the pressure | voltage fall operation | movement of a voltage converter circuit. It is explanatory drawing which shows an example of the state D3 in the pressure | voltage fall operation | movement of a voltage converter circuit. It is explanatory drawing which shows an example of the state D4 in the pressure | voltage fall operation | movement of a voltage converter circuit. It is a timing chart which shows typically the waveform of each part after a voltage conversion circuit passes through state D4, changes to state D3 through states D1 and D2. It is a flowchart which shows the process sequence of CPU which makes a voltage conversion circuit carry out step-down conversion.

[Description of Embodiment of the Present Invention]
First, embodiments of the present invention will be listed and described. Moreover, you may combine arbitrarily at least one part of embodiment described below.

(1) A voltage converter according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, the first inductor, and the low-voltage A voltage conversion circuit having a second inductor having one end connected to a connection point of the side switching element, and a capacitor connected in parallel to the low voltage side switching element, and the voltage by turning on / off the high voltage side switching element A voltage converter including a control unit for step-down conversion in a conversion circuit, wherein the control unit turns on the high-voltage side switching element to operate in a first mode in which a resonance current is generated by the first inductor and the capacitor, Calculate the time from when the high-voltage side switching element is turned on until the resonance current becomes minimum, and the elapsed time The high-voltage side switching element is turned off, and the low-voltage side switching element or the capacitor is operated in the second mode in which the return current of the second inductor flows, and the high-voltage side switching element is turned on after the operation in the second mode. Then, the mode is shifted to the first mode.

(4) A step-down control method for a voltage conversion circuit according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, and the first A voltage conversion circuit having a second inductor having one end connected to a connection point between the inductor and the low-voltage side switching element, and a capacitor connected in parallel to the low-voltage side switching element, and turning on / off the high-voltage side switching element In the voltage conversion apparatus including the control unit, the control unit causes the voltage conversion circuit to step down the voltage conversion circuit, and the first switching element is turned on to flow a resonance current from the first inductor and the capacitor. Time from when the high-voltage side switching element is turned on until the resonance current becomes minimum When the calculated time elapses, the high-voltage side switching element is turned off and the low-voltage side switching element or the capacitor is operated in the second mode in which the return current of the second inductor flows, and the operation in the second mode is performed. Later, the high-voltage side switching element is turned on to shift to the first mode.

(5) A computer program according to an aspect of the present invention includes a first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, the first inductor and the low-voltage side A voltage conversion circuit having a second inductor having one end connected to a connection point of the switching element; a capacitor connected in parallel to the low-voltage side switching element; and a control unit for turning on / off the high-voltage side switching element. A computer program for causing the voltage conversion circuit to perform step-down conversion in the control unit in the voltage conversion device, wherein the high-voltage side switching element is turned on and a resonance current caused by the first inductor and the capacitor is supplied to the control unit. A step of operating in the first mode, and turning on the high-voltage side switching element; Calculating the time from when the resonance current becomes minimum, and turning off the high-voltage side switching element when the calculated time elapses to supply the return current of the second inductor to the low-voltage side switching element or the capacitor. A step of operating in the second mode of flowing, and a step of turning on the high-voltage side switching element and shifting to the first mode after the operation in the second mode.

  In this aspect, in the voltage conversion circuit, the high-voltage side switching element, the first inductor, and the low-voltage side switching element having capacitors connected to both ends are connected in this order. One end of the second inductor is connected to the connection point of the side switching element. When the control unit turns on / off the high-voltage side switching element, the voltage input to the high-voltage side of the voltage conversion circuit is stepped down and output to the low-voltage side.

  The control unit operates the voltage conversion circuit in a first mode in which the high-voltage side switching element is turned on while the low-voltage side switching element is turned off, and a resonance current is generated by the first inductor and the capacitor. In the first mode, the resonance current flows into the capacitor, the capacitor voltage starts to rise, and energy is stored in the second inductor. Thereafter, the resonance current is reversed, the voltage of the capacitor starts to decrease, and the current of the high-voltage side switching element decreases.

  The control unit calculates a time at which the resonance current flowing through the high-voltage side switching element is minimized in the first mode, turns off the high-voltage side switching element at the calculated time, and supplies the second voltage to the low-voltage side switching element or capacitor. The operation is performed in the second mode in which the return current of the inductor flows. Then, a control part turns on a high voltage | pressure side switching element in 2nd mode, and makes it transfer to 1st mode. Since the high-voltage side switching element is turned off when the resonance current flowing through the high-voltage side switching element is minimized, or when the voltage of the high-voltage side switching element is close to 0 V, it is expressed as voltage x current when the high-voltage side switching element is turned off. Switching loss is reduced.

(2) It is preferable that the said control part calculates time until the said resonance current becomes minimum based on the following formula | equation.
Tmi = (Lr1 / Vi) {Io-Vi (1-D) D / (2Lr2fs)}
+ (3/4) Tr
However,
Tmi: Time until the resonance current becomes minimum Lr1: Inductance of the first inductor Vi: Input voltage of the voltage conversion circuit Io: Output current of the voltage conversion circuit D: Target duty ratio of the high-voltage side switching element Lr2: inductance of the second inductor (Lr2 >> Lr1)
fs: switching frequency of the high-voltage side switching element Tr: 2π√ (Lr1Cr1)
Cr1: Capacitance of the capacitor

  In this aspect, the time from when the high-voltage side switching element is turned on until the resonance current is minimized is calculated based on the equation.

(3) The control unit is configured to detect a voltage of the low-voltage side switching element, and when the voltage of the low-voltage side switching element is lower than a predetermined voltage threshold in the second mode, It is preferable that the high voltage side switching element is turned on after the low voltage side switching element is turned on.

  In this aspect, the control unit turns on the low-voltage side switching element when the voltage of the low-voltage side switching element in the second mode is lower than the predetermined voltage threshold value, and shifts to the first mode. Before the switching element is turned on, the low voltage side switching element is turned off. Since the low voltage side switching element is turned on when the voltage of the low voltage side switching element is relatively low, the switching loss represented by the voltage × current when the low voltage side switching element is turned on is reduced.

[Details of the embodiment of the present invention]
Specific examples of a voltage conversion device, a voltage conversion circuit step-down control method, and a computer program according to embodiments of the present invention will be described below with reference to the drawings. In addition, this invention is not limited to these illustrations, is shown by the claim, and intends that all the changes within the meaning and range equivalent to a claim are included. In addition, the technical features described in each embodiment can be combined with each other.

(Embodiment)
FIG. 1 is a block diagram illustrating a configuration example of the voltage conversion device according to the embodiment. The voltage conversion apparatus includes a voltage conversion circuit 1 and a control unit 5 that controls voltage conversion by the voltage conversion circuit 1. The voltage conversion circuit 1 steps down the voltage supplied from the high-voltage side terminals H and G and outputs the voltage from the low-voltage side terminals L and G in parallel. A capacitor 41 is connected between the terminals H and G, and a relatively high voltage battery such as a lithium ion battery is connected to the outside. A capacitor 42 is connected between the terminals L and G, and a relatively low voltage battery such as a lead storage battery is connected to the outside. The voltage converter can perform a boosting operation in which the voltage supplied from the terminals L and G is boosted and output in parallel from the terminals H and G.

  The voltage conversion circuit 1 includes a first inductor L1, and a high-voltage side switching element Q1 and a low-voltage side switching element Q2 (hereinafter, high-voltage side and low-voltage side) connected in series between the terminals H and G via the first inductor L1. A switching element is also simply referred to as an SW element), a second inductor L2 having one end connected to a connection point between the first inductor L1 and the SW element Q2 (hereinafter, the first and second inductors are simply referred to as an inductor), and an SW element. And a capacitor C1 connected to both ends of Q2. The other end of the inductor L2 is connected to the terminal L. The SW element Q2 may be replaced with a diode whose anode is connected to the terminal G.

  The SW element Q <b> 1 has a drain connected to the terminal H and a gate connected to the control unit 5. The SW element Q <b> 2 has a source connected to the terminal G and a gate connected to the control unit 5. Here, the SW element is an N-channel MOSFET (Metal Oxide Field Effect Transistor), but is not limited to this, and other switching elements such as a bipolar transistor, an IGBT (Insulated Gate Bipolar Transistor), and the like. It may be.

  The voltage conversion circuit 1 further includes a SW element Q3 having a drain connected to a connection point between the SW element Q1 and the inductor L1, and a current sensor A1 that detects an output current. The SW element Q <b> 3 has a source connected to the terminal G and a gate connected to the control unit 5. The detection terminal of the current sensor A1 is connected to the control unit 5. In the present embodiment, the SW element Q3 may be replaced with a diode whose anode is connected to the terminal G.

  The control unit 5 includes a CPU (Central Processing Unit) (not shown), and controls the operation of each unit according to a control program stored in advance in a ROM (Read Only Memory), and performs processing such as input / output and calculation. . A computer program that defines the procedure of each process by the CPU may be loaded in advance into a RAM (Random Access Memory) using means (not shown), and the loaded computer program may be executed by the CPU. 5 may be configured by a dedicated hardware circuit.

  Control unit 5 turns on / off SW element Q1 in a timely manner. The control unit 5 also detects the voltage of the capacitor C1 (that is, the voltage of the drain of the SW element Q2), and turns on / off the SW element Q2 in a timely manner based on these detection results. The control unit 5 further captures the detection result of the current sensor A1 and uses it for calculating the length of the commutation period described later.

  FIG. 2 is a timing chart showing an example of the waveform of each part during the step-down operation of the voltage conversion circuit 1. The five timing charts shown in FIG. 2 all have the same time axis (t) as the horizontal axis, and the voltage of the SW element Q1, the inductor L1, the capacitor C1, the SW element Q2, and the inductor L2 in order from the top of the figure. A waveform and a current waveform are shown. The thick solid line in the figure indicates the voltage, and the thin solid line indicates the current. Particularly for the SW elements Q1 and Q2, the control voltage applied to the gate is indicated by a broken line. In the present embodiment, the direction from the terminal H to the terminal L is the current direction of the inductor L1, the inductor L2, and the SW element Q1. The direction from one end of the inductor L2 toward the terminal G is the current direction of the capacitor C1, and the opposite direction is the current direction of the SW element Q2.

  Hereinafter, the step-down operation of the voltage conversion circuit 1 in the states D1 to D4 illustrated in FIG. 2 will be described for each state. States D1 to D4 correspond to one switching cycle. In the explanatory diagrams from FIG. 3 to FIG. 6, illustration of the SW element Q3 is omitted. FIG. 3 is an explanatory diagram illustrating an example of the state D1 in the step-down operation of the voltage conversion circuit 1. State D1 corresponds to the first mode. The control unit 5 operates the voltage conversion circuit 1 in the first mode in which a sinusoidal resonance current flows through the inductor L1 and the capacitor C1 by turning on the SW element Q1 with the SW element Q2 turned off. In the first mode, a current flows from the terminal H via the SW element Q1 and the inductor L1, and a current flows from the terminal L via the inductor L2.

  In the state D1 shown in FIG. 3, while the voltage of the capacitor C1 rising in a sine wave is lower than the voltage between the terminals H and G, the current of the inductor L1 continues to increase in a sine wave, and the voltage of the capacitor C1 is increased to the terminals H and G. After becoming higher than the voltage between, the current of the inductor L1 starts to decrease. During this time, the voltage of the inductor L1 continues to decrease in a sine wave shape. During the state D1, the voltage of the capacitor C1 continues to rise with time.

  FIG. 4 is an explanatory diagram illustrating an example of the state D2 in the step-down operation of the voltage conversion circuit 1. State D2 also corresponds to the first mode. When the direction of the resonance current flowing in the capacitor C1 is reversed in the state D1 shown in FIG. 3 and the voltage of the capacitor C1 changes from rising to lowering, the voltage conversion circuit 1 becomes the state D2. In the state D2, the discharge current of the capacitor C1 flows to the terminal L side through the inductor L2. While the voltage of the capacitor C1 is higher than the voltage between the terminals H and G, the current of the inductor L1 continues to decrease.

  FIG. 5 is an explanatory diagram illustrating an example of the state D3 in the step-down operation of the voltage conversion circuit 1. State D3 corresponds to the second mode. In the state D2 shown in FIG. 4, when the sinusoidal current flowing through the SW element Q1 and the inductor L1 is minimized or the direction of the current is reversed and the current flows through the parasitic diode, the control unit 5 switches the SW element Q1. By turning off, the voltage conversion circuit 1 enters the state D3. As a result, the SW element Q1 is turned off when the current of the SW element Q1 is minimal or while the voltage of the SW element Q1 is close to 0V, so that the switching loss of the SW element Q1 is reduced. In the state D3, initially, the return current of the inductor L2 flows through the capacitor C1.

  When the control unit 5 turns off the SW element Q1 before the current of the inductor L1 becomes close to zero in the state D2, a surge voltage may be generated in the inductor L1. On the other hand, in the present embodiment, the SW element Q3 is connected between the connection point of the SW element Q1 and the inductor L1 and the terminal G. For this reason, the current flowing through the inductor L1 before the SW element Q1 is turned off continues to flow through the parasitic diode of the SW element Q3, and the surge voltage is suppressed. The control unit 5 may positively turn on the SW element Q3 from when the SW element Q1 is turned off to an appropriate time.

  FIG. 6 is an explanatory diagram illustrating an example of the state D4 in the voltage step-down operation of the voltage conversion circuit 1. In the state D3 shown in FIG. 5, after the voltage of the capacitor C1 and the SW element Q2 becomes lower than the predetermined voltage threshold, the control unit 5 turns on the SW element Q2, whereby the voltage conversion circuit 1 becomes the state D4. State D4 also corresponds to the second mode. As a result, the return current of the inductor L2 flows through the SW element Q2 having a low on-resistance. Further, since the SW element Q2 is turned on while the voltage of the SW element Q2 is relatively low, the switching loss of the SW element Q2 is reduced.

  In the state D4, when an appropriate time corresponding to the step-down ratio of the output voltage to the terminals L and G with respect to the input voltage from the terminals H and G has elapsed, the control unit 5 turns on the SW element Q1 after turning off the SW element Q2. As a result, the voltage conversion circuit 1 enters the state D1 shown in FIG. In this way, the state transition from state D1 to state D4 is repeated.

  Next, the above-described states D1 and D2 and the operation of the voltage conversion circuit 1 before and after that will be described in more detail. FIG. 7 is a timing chart schematically showing the waveforms of the respective parts from when the voltage conversion circuit 1 exits from the state D4 to when it switches to the state D3 through the states D1 and D2. The five timing charts shown in FIG. 7 all have the same time axis (t) as the horizontal axis, and in order from the top in the figure, the ON period of the SW element Q1, the effective ON period of the SW element Q1, The current of the element Q1, the current of the SW element Q2 (including the current of the parasitic diode), and the current of the inductor L2 are shown. The scale of the vertical axis is not necessarily the same. In particular, regarding the current of the SW element Q1, the case where the output current Io is relatively large and the case where it is small are indicated by a solid line and a one-dot chain line, respectively. Io is an average value of the output current of the voltage conversion circuit 1. ΔI is the amplitude of the ripple current included in the output current. The time from time t1 to t3 is the ON time Tmi of the SW element Q1.

  A period from time t1 to t3 shown on the horizontal axis in FIG. 7 corresponds to states D1 and D2 of the voltage conversion circuit 1, and a period after time t3 corresponds to states D3 and D4. The period from the state D1 through D2, D3, D4 to the next switching cycle state D1 corresponds to the current switching cycle. A period from time t0 to time t1 before time t1 represents a transitional state from state D4 to D1 in the previous switching cycle. During this time, the control unit 5 may have already turned off the SW element Q2, or may remain on until just before time t1. When the SW element Q2 is turned off, the current of the inductor L2, that is, the return current flowing from the source to the drain of the SW element Q2 in FIG. 6 described above, flows to the parasitic diode of the SW element Q2. The current in the inductor L2 gradually decreases from time t0 to time t1.

  The absolute value of the voltage of the SW element Q2 in the transient state is the on-voltage of the SW element Q2 or the on-voltage of the parasitic diode. That is, since the absolute value of the voltage of the SW element Q2 from time t0 to t1 can be regarded as substantially 0V, the voltage conversion circuit 1 transitions to the state D1 when the control unit 5 turns on the SW element Q1 at time t1. In this case, a current having a slope of dI / dt = Vi / Lr1 flows through the SW element Q1 and the inductor L1. However, Vi is an input voltage between the terminals H and G, and Lr1 is an inductance of the inductor L1.

  The return current of the inductor L2 flowing in the SW element Q2 is gradually replaced with the current having the above-described gradient that has started flowing in the SW element Q1 and the inductor L1 at time t1, so that the current of the SW element Q2 is changed from time t1 to time t2. In the meantime, it decreases linearly and becomes zero at time t2. As a result, during the period from time t1 to t2, the current of the inductor L2 gradually decreases with the same slope as the current flowing from time t0 to t1. Between time t1 and t2, since the return current of the inductor L2 corresponds to a commutation period in which the return current of the SW element Q2 is commutated from the SW element Q2, and the current of the inductor L2 continues to decrease, the time Ti during this time is It does not contribute to the duty ratio that determines the step-down ratio of the voltage conversion circuit 1. Therefore, the effective on-time of the SW element Q1 is a time Ton from time t2 to t3.

  After the commutation period, a sinusoidal resonance current flows through the inductor L1 and the capacitor C1 through the SW element Q1 from time t2 to time t3. The current of the inductor L1, that is, the current of the SW element Q1, becomes minimum at time t3 when indicated by a solid line, and becomes zero at time ta before time t3 when indicated by a one-dot chain line. In either case, the SW element Q1 is turned off at time t3. While the SW element Q1 is on, energy is injected into the inductor L2, and the current of the inductor L2 increases by ΔI. Thereafter, the current of the inductor L2 decreases by ΔI until the state D1 of the next switching period is reached.

  In FIG. 7, the current of the SW element Q1 is shown by a broken line when it is assumed that the SW element Q1 is kept on after time t3. In this case, if the time when the current of the SW element Q1 becomes the same as that at the time t2 is t4, the time from the time t2 to the time t4 corresponds to the resonance period Tr, and the time from the time t2 to the time t3, that is, the SW element The effective on-time Ton of Q1 corresponds to 3/4 of the resonance period Tr. The resonance period Tr is expressed by the following formula (1).

Tr = 2π√ (Lr1 / Cr1) (1)
However,
Lr1: Inductor Cr1 inductance Cr1: Capacitor C1 capacitance

  The current of SW element Q1 indicated by the alternate long and short dash line is less than or equal to zero from time ta to tb, and the loss of SW element Q1 is substantially zero at any point during this time, regardless of the SW element Q1 being turned off. It becomes. Therefore, in the present embodiment, the SW element Q1 is turned off at time t3 when the resonance current flowing through the SW element Q1 is minimized regardless of the magnitude of the output current Io.

  As described above, a current having a slope of dI / dt = Vi / Lr1 flows through the SW element Q1 from time t1 to time t2. The current It2 of the SW element Q1 at time t2 (“(It2)”, which is less than “It2” when indicated by the alternate long and short dash line, is expressed as “It2” in any case below) of the inductor L2 Equal to the current. Since the current of the inductor L2 at this time is Io−ΔI / 2, the following equation (2) is established.

It2 = Io−ΔI / 2
= (Vi / Lr1) Ti (2)
However,
Io: average value of output current ΔI: ripple current of output current of voltage conversion circuit 1 (peak to peak)
Vi: input voltage Lr1 of the voltage conversion circuit 1, inductance of the inductor L1, Ti: time from time t1 to t2.

  On the other hand, when the inductance of the inductor L1 is negligible with respect to the inductance of the inductor L2, the input / output voltage of the voltage conversion circuit 1 is expressed by the following equations (3) and (4) if the influence of the resonance current is ignored. ).

Vi−Vo = Lr2ΔI / Ton (3)
Vo / Vi = D (4)
However,
Vi: input voltage of voltage conversion circuit 1 Vo: output voltage Lr2 of voltage conversion circuit 1: inductance of inductor L2 ΔI: ripple current Ton of output current of voltage conversion circuit 1: effective on-time of SW element Q1 D: SW Effective duty ratio of switching element Q1

  Here, Ton is obtained by multiplying the switching period, which is the reciprocal of the switching frequency fs of the SW element Q1, by D, and Ton corresponds to (3/4) Tr as described above. Therefore, fs is expressed by the following formula (5 ). Furthermore, Ton of Formula (3) is represented by the following Formula (6) using fs.

fs = D / {(3/4) Tr} (5)
Ton = D (1 / fs) ... (6)
However,
fs: switching frequency of the SW element Q1 D: effective duty ratio of switching of the SW element Q1 Tr: resonance period Ton: effective on-time of the SW element Q1

  By applying Equations (4) and (6) to Equation (3) to eliminate Vo and Ton, the following Equation (7) is obtained.

ΔI = (Vi / Lr2) (1-D) D / fs (7)

  Next, by substituting equation (7) into equation (2), the following equation (8) is obtained.

Ti = (Lr1 / Vi) {Io-Vi (1-D) D / (2Lr2fs)} (8)

  On the other hand, the time Tmi from when the SW element Q1 is turned on until the current flowing through the SW element Q1 becomes the minimum, that is, the on-time of the switching element Q1 is the time obtained by adding Ton to Ti. Therefore, Tmi is expressed by the following equation (9) using the right side of equation (8) and (3/4) Tr. In this way, if Ti is calculated, it is possible to calculate Tmi, which is the time for which the SW element Q1 should be turned on, very easily.

Tmi = (Lr1 / Vi) {Io-Vi (1-D) D / (2Lr2fs)}
+ (3/4) Tr (9)

  When Vi is constant, Io is detected by the current sensor A1, and since Lr2 and Tr are constants, the effective duty ratio D, that is, the target duty ratio is substituted into Expression (9), and Expression (5) Tmi is calculated by substituting fs calculated in step (9) into equation (9). The off time Toff of the switching element Q1 can be determined by the following equation (10).

Toff = 1 / fs-Ton
= 1 / fs- (3/4) Tr (10)
However,
fs: switching frequency of SW element Q1 Tr: resonance period

  Below, operation | movement of the control part 5 mentioned above is demonstrated using the flowchart which shows it. The processing shown below is executed by a CPU (not shown) according to a control program stored in advance in a ROM (not shown). FIG. 8 is a flowchart illustrating a processing procedure of the CPU that causes the voltage conversion circuit 1 to perform step-down conversion. The process whose procedure is shown in FIG. 8 is activated when the voltage converter starts step-down conversion. The timing in the figure is performed using a timer (not shown). Note that the values of the resonance period Tr and (3/4) Tr are calculated in advance. The target duty ratio is calculated timely in other processes.

  When the process of FIG. 8 is started, the CPU turns off the low voltage side switching element Q2 (S11). Next, the CPU calculates an on-time Ti that does not contribute to the duty ratio according to equation (8) (S12). In this case, the output current Io detected by the current sensor A1, the target duty ratio (D), the switching frequency fs calculated by the equation (5), and other known constants are used.

  After that, the CPU calculates a time Tmi, that is, an on-time from when the high-voltage side switching element Q1 is turned on until the resonance current flowing through the high-voltage side switching element Q1 becomes the minimum, by Equation (9) (S13). Next, the CPU turns on the high voltage side switching element Q1 (S14), and causes the inductors L1 and L2 and the capacitor C1 to resonate. As a result, the voltage conversion circuit 1 is operated in the first mode. At this time, the CPU starts counting the on-time related to the switching element Q1 (S15).

  Thereafter, the CPU determines whether or not the on-time has elapsed (S16). When the on-time has not elapsed (S16: NO), the CPU waits until the on-time has elapsed. When the on-time has elapsed (S16: YES), the CPU turns off the high-voltage side switching element Q1 (S17) and operates the voltage conversion circuit 1 in the second mode. This causes the return current of the inductor L2 to flow through the capacitor C1 or the low-voltage side switching element Q2.

  Next, the CPU calculates the off time Toff by substituting the switching cycle fs calculated by Expression (5) into Expression (10) (S18). Then, the CPU starts counting the off time of the high-voltage side switching element Q1 by counting down the calculated off time Toff (S19).

  Thereafter, the CPU determines whether or not the off-time has elapsed (S20). When the off-time has elapsed (S20: YES), the CPU determines whether or not to end the step-down conversion (S21). When the step-down conversion is not terminated (S21: NO), the CPU shifts the process to step S11 in order to shift the voltage conversion circuit 1 to the next switching cycle. On the other hand, when the step-down conversion is ended (S21: YES), the CPU ends the process of FIG. 8 after turning off the low-voltage side switching element Q2 (S22).

  When the off time has not elapsed in step 20 (S20: NO), the CPU detects the voltage of the low-voltage side switching element Q2, and determines whether or not the detected voltage is lower than a predetermined voltage threshold (S23). . When the detected voltage is not lower than the predetermined voltage threshold (S23: NO), the CPU moves the process to step S20. On the other hand, when the detected voltage is lower than the predetermined voltage threshold (S23: YES), the CPU turns on the low-voltage side switching element Q2 (S24) and moves the process to step S20. The voltage threshold value here may be a positive value, or may be zero or a negative value less than or equal to zero.

  In the flowchart described above, the low-voltage side switching element Q2 is turned on / off. However, if the low-voltage side switching element Q2 is a simple diode, steps S23 and S24 may be skipped. Further, the execution order of each step is not strict. For example, the execution order of steps S14 and S15 may be reversed, and the execution order of steps S12, S13 and S14, S15 may be reversed.

  As described above, according to the present embodiment, in the voltage conversion circuit 1, the SW element Q1, the inductor L1, and the SW element Q2 having the capacitor C1 connected at both ends are connected in this order, and the inductor L1. One end of the inductor L2 is connected to the connection point of the SW element Q2. When the control unit 5 turns on / off the SW element Q1, the voltage input to the terminals H and G of the voltage conversion circuit 1 is stepped down and output to the terminals L and G.

  The control unit 5 causes the voltage conversion circuit 1 to operate in a first mode in which the SW element Q1 is turned on while the SW element Q2 is turned off and a resonance current from the inductor L1 and the capacitor C1 flows. In the first mode, the resonance current flows into the capacitor C1, the voltage of the capacitor C1 starts to rise, and energy is stored in the inductor L2. Thereafter, the resonance current is inverted, the voltage of the capacitor C1 starts to decrease, and the current of the SW element Q1 decreases. The control unit 5 calculates a time at which the resonance current flowing through the SW element Q1 is minimized in the first mode, turns off the SW element Q1 at the calculated time t3, and supplies the inductor L2 to the SW element Q2 or the capacitor C1. Is operated in the second mode in which a reflux current of 1 is passed. As a result, the SW element Q1 is turned off when the resonance current flowing through the SW element Q1 is minimized, or when the voltage of the SW element Q1 is close to 0V. Switching loss is reduced. Accordingly, when so-called soft switching is performed using LC resonance, it is possible to reduce the loss of the switching element even when there is no zero cross point in the resonance current.

  When the control unit 5 turns on the SW element Q1 in the second mode to shift to the first mode, the control circuit 5 turns on the SW element Q1 and then the return current of the inductor L2 is a series circuit of the SW element Q1 and the inductor L1. Is calculated by adding the time Ti until the commutation to ¾ and adding to a value ３ times the resonance period Tr (see equations (8) and (9)). ). Then, based on the effective on-time Ton of the SW element Q1, the off-time Toff of the SW element Q1 corresponding to the effective duty ratio is calculated (see Expression (10)). Therefore, by applying the target duty ratio to the effective duty ratio, it is possible to perform voltage conversion at a desired step-down ratio when so-called soft switching is performed using LC resonance.

  Further, according to the present embodiment, the time Tmi from when the SW element Q1 is turned on until the resonance current flowing through the SW element Q1 becomes minimum can be calculated based on the equation (9).

  Further, according to the present embodiment, when the voltage of the SW element Q2 in the second mode becomes lower than the predetermined voltage threshold, the control unit 5 turns on the SW element Q2 and switches the SW element Q2 to the first mode. The SW element Q2 is turned off before turning on the element Q1. The voltage threshold is set to 0 V or a voltage close to 0 V, for example. Since the SW element Q2 is turned on when the voltage of the SW element Q2 is relatively low, it is possible to reduce the switching loss represented by the voltage × current when the SW element Q2 is turned on.

DESCRIPTION OF SYMBOLS 1 Voltage conversion circuit 41, 42 Capacitor 5 Control part Q1 High voltage | pressure side switching element (SW element)
Q2 Low voltage side switching element (SW element)
Q3 SW element L1 First inductor (inductor)
L2 Second inductor (inductor)
C1 Capacitor A1 Current sensor H, L, G terminals

Claims (5)

A first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, and a second inductor having one end connected to a connection point of the first inductor and the low-voltage side switching element And a voltage conversion circuit having a capacitor connected in parallel to the low-voltage side switching element, and a voltage conversion device comprising a control unit for turning on / off the high-voltage side switching element and performing a step-down conversion to the voltage conversion circuit. And
The controller is
The high-voltage side switching element is turned on to operate in a first mode in which a resonance current from the first inductor and the capacitor flows.
Calculate the time from when the high-voltage side switching element is turned on until the resonance current is minimized,
When the calculated time has elapsed, the high-voltage side switching element is turned off, and the low-voltage side switching element or the capacitor is operated in a second mode in which a reflux current of the second inductor flows.
A voltage conversion device that turns on the high-voltage side switching element and shifts to the first mode after the operation in the second mode. The voltage conversion device according to claim 1, wherein the control unit calculates a time until the resonance current becomes a minimum based on the following expression.
Tmi = (Lr1 / Vi) {Io-Vi (1-D) D / (2Lr2fs)}
+ (3/4) Tr
However,
Tmi: Time until the resonance current becomes minimum Lr1: Inductance of the first inductor Vi: Input voltage of the voltage conversion circuit Io: Output current of the voltage conversion circuit D: Target duty ratio of the high-voltage side switching element Lr2: inductance of the second inductor (Lr2 >> Lr1)
fs: switching frequency of the high-voltage side switching element Tr: 2π√ (Lr1Cr1)
Cr1: Capacitance of the capacitor The controller is
The voltage of the low-voltage side switching element is detected,
When the voltage of the low-voltage side switching element is lower than a predetermined voltage threshold in the second mode, the low-voltage side switching element is turned on,
The voltage converter according to claim 1 or 2, wherein the high-voltage side switching element is turned on after the low-voltage side switching element is turned off. A first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, and a second inductor having one end connected to a connection point of the first inductor and the low-voltage side switching element And a voltage conversion circuit having a capacitor connected in parallel to the low-voltage side switching element, and a step-down to the voltage conversion circuit in the control unit in the voltage conversion device that turns on / off the high-voltage side switching element A control method for converting,
The high-voltage side switching element is turned on to operate in a first mode in which a resonance current from the first inductor and the capacitor flows.
Calculate the time from when the high-voltage side switching element is turned on until the resonance current is minimized,
When the calculated time has elapsed, the high-voltage side switching element is turned off, and the low-voltage side switching element or the capacitor is operated in a second mode in which a reflux current of the second inductor flows.
A voltage step-down control method for a voltage conversion circuit in which the high-voltage side switching element is turned on after the operation in the second mode to shift to the first mode. A first inductor, a high-voltage side switching element and a low-voltage side switching element connected in series via the first inductor, and a second inductor having one end connected to a connection point of the first inductor and the low-voltage side switching element And a voltage conversion circuit having a capacitor connected in parallel to the low-voltage side switching element, and a step-down to the voltage conversion circuit in the control unit in the voltage conversion device that turns on / off the high-voltage side switching element A computer program for converting,
In the control unit,
Turning on the high-voltage side switching element and operating in a first mode in which a resonance current is caused to flow by the first inductor and the capacitor;
Calculating a time from when the high-voltage side switching element is turned on until the resonance current is minimized;
Turning off the high-voltage side switching element when the calculated time elapses, and operating in a second mode in which a reflux current of the second inductor flows through the low-voltage side switching element or the capacitor;
And a step of turning on the high-voltage side switching element after the operation in the second mode to shift to the first mode.

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