DC / DC converter

Abstract

In a DC / DC converter 1, a control unit 11 controls the duty ratio of switching elements Q1 to Q4 so that a detection value of an output voltage of a power conversion unit 10 maintains a target value. The control unit 11 refers to a preset correspondence relationship between an output current of the power conversion unit 10 and an addition value to be added to the duty ratio to determine the addition value corresponding to the detection value of the output current of the power conversion unit 10, and adds the determined addition value to the duty ratio determined on the basis of the detection value of the output voltage.

Description

DC / DC Converter

[0001] The present disclosure relates to a switching-type DC / DC converter.

[0002] A DC / DC converter is used to charge and discharge a storage battery. In a general switching-type DC / DC converter, feedback control is performed so that the difference between the detected value and the target value of the output voltage becomes zero so that the output voltage falls within a desired voltage range. With only such constant voltage control, when the output voltage fluctuates due to a sudden load change, the constant voltage control cannot follow, and the fluctuation range of the output voltage becomes large.

[0003] Regarding the control of a DC / DC converter, Patent Document 1 discloses a method of approximating the relationship between the command value of control and the set current value by a polynomial, substituting the set current value into the polynomial to obtain the command value for feedforward control, and multiplying the command value by a compensation coefficient to obtain the control amount of a pulse width modulation signal. This method relates to constant current control and not to constant voltage control.

[0004] Patent Document 2 discloses a method of suppressing the fluctuation of output when the control mode shifts from a non-linear control mode to a linear control mode. In this method, a constant current circuit is used to calibrate the feedforward term.

[0005] Patent Document 3 discloses a method of performing feedforward control using the inductance estimation value and the internal resistance estimation value of a reactor so that the output value of a voltage control unit and the input current of a capacitor are in a proportional relationship, and a method of feeding back the current flowing through an output-side reactor to a current target value obtained by adding an output current to the output value of the voltage control unit by a current deviation calculation unit. This method is a method of controlling the output current itself.

[0006] Japanese Unexamined Patent Application Publication No. 2018-125938, Japanese Unexamined Patent Application Publication No. 2010-259280, Japanese Unexamined Patent Application Publication No. 2024-030750

[0007] To reduce the fluctuation range of the output voltage due to load changes, one option is to increase the capacitance of the output capacitor. However, increasing the capacitance of the capacitor leads to an increase in the component mounting area and component costs.

[0008] This disclosure is made in view of these circumstances, and its purpose is to provide a technology for reducing output voltage fluctuations during load changes in a DC / DC converter at low cost.

[0009] To solve the above problems, a DC / DC converter in one aspect of the present disclosure includes at least one switching element, a power conversion unit including an inductor capable of controlling the voltage of DC power, a voltage detection unit for detecting the output voltage of the power conversion unit, a current detection unit for detecting the output current of the power conversion unit, and a control unit for controlling the duty cycle of the switching element so that the detected value of the output voltage of the power conversion unit maintains a target value. The control unit refers to a pre-set correspondence between the output current of the power conversion unit and an addition value to be added to the duty cycle, determines an addition value corresponding to the detected value of the output current of the power conversion unit, and adds the determined addition value to the duty cycle determined based on the detected value of the output voltage.

[0010] According to this disclosure, fluctuations in the output voltage of a DC / DC converter during load fluctuations can be reduced at low cost.

[0011] This figure illustrates the configuration of a DC / DC converter according to an embodiment. This figure shows an example of the configuration of a control unit according to a comparative example. This figure shows an example of the configuration of a control unit according to an embodiment. This figure shows an example of approximating the output current and duty cycle sum using a logarithmic function. This figure shows an example of approximating the output current and duty cycle sum using a square root function. This figure shows the waveform of the output voltage during load fluctuations when only the voltage feedback control shown in Figure 2 is applied. This figure shows the waveform of the output voltage during load fluctuations when both the voltage feedback control and current feedforward control shown in Figure 3 are applied.

[0012] Figure 1 is a diagram illustrating the configuration of a DC / DC converter 1 according to an embodiment. The DC / DC converter 1 is an H-bridge type step-up / step-down DC / DC converter and comprises a power conversion unit 10 and a control unit 11. A DC power supply SB1 is connected to the input side of the DC / DC converter 1, and a DC bus 2 is connected to the output side.

[0013] In this embodiment, we assume that the DC power supply SB1 is a battery, and that the DC power supply SB1 and the DC / DC converter 1 are unitized as an energy storage system. This energy storage system is used as a backup power supply system for a data center. Load 3 is a collective term for the numerous servers and storage devices installed in the data center. Load 3 is connected to DC bus 2. In this embodiment, we assume a 48V DC bus 2. In addition to the DC / DC converter 1 of the energy storage system and load 3, DC bus 2 is also connected to a DC / DC converter 5 connected to the commercial power grid 4 and an emergency generator (not shown).

[0014] The AC / DC converter 5 converts the AC power supplied from the commercial power grid 4 into DC power while stepping it down, and outputs it to the DC bus 2. For example, it converts the AC voltage of 200-240V supplied from the commercial power grid 4 into a DC voltage of 48V.

[0015] The emergency generator is a generator that supplies power to load 3 in the event of a power outage in the commercial power grid 4, and can be a diesel generator or a gas turbine generator, for example. It takes several tens of seconds to several minutes from the time a power outage in the commercial power grid 4 is detected until the emergency generator starts up. The energy storage system can continue to supply backup power to load 3 during that time.

[0016] The DC power supply SB1, a storage battery, includes multiple (e.g., 9 to 14) cells connected in series. These cells can be lithium-ion, nickel-metal hydride, lead-acid, or the like. In this specification, we will assume the use of lithium-ion cells (nominal voltage: 3.6–3.7V). Note that multiple cells may be connected in parallel in the series stage of each cell to increase capacity.

[0017] The power conversion unit 10 includes a first smoothing capacitor C1, an inductor L1, a first switching element Q1 to a fourth switching element Q4, a second smoothing capacitor C20, a first current sensor A1 to a second current sensor A2, and a first voltage sensor V1 to a second voltage sensor V2. For example, electrolytic capacitors can be used for the first smoothing capacitor C1 and the second smoothing capacitor C2. MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors) can be used for the first to fourth switching elements Q1 to Q4.

[0018] A first smoothing capacitor C1 is connected between the positive and negative wires of the DC power supply SB1. A first arm, including a first switching element Q1 and a second switching element Q2 connected in series, is connected in parallel to the DC power supply SB1 and the first smoothing capacitor C1. A second smoothing capacitor C2 is connected between the positive and negative wires of the DC bus 2. A second arm, including a third switching element Q3 and a fourth switching element Q4 connected in series, is connected in parallel to the DC bus 2 and the second smoothing capacitor C2.

[0019] The first switching element Q1 functions as a high-side switching element for stepping down voltage, the second switching element Q2 functions as a low-side switching element for stepping down voltage, the third switching element Q3 functions as a high-side switching element for stepping up voltage, and the fourth switching element Q4 functions as a low-side switching element for stepping up voltage.

[0020] An inductor L1 is connected between the midpoint of the first arm (the connection point between the first switching element Q1 and the second switching element Q2) and the midpoint of the second arm (the connection point between the third switching element Q3 and the fourth switching element Q4).

[0021] For each of the first switching element Q1 to the fourth switching element Q4, a first diode D1 and a fourth diode D4 are formed or connected in antiparallel as body diodes. When N-channel MOSFETs are used for the first switching element Q1 to the fourth switching element Q4, parasitic diodes formed from source to drain can be used as the first diode D1 and fourth diode D4. When IGBTs are used for the first switching element Q1 to the fourth switching element Q4, external diodes are connected as the first diode D1 and fourth diode D4.

[0022] The first voltage sensor V1 detects the input voltage Vin of the power conversion unit 10 of the DC / DC converter 1 and outputs it to the control unit 11. The second voltage sensor V2 detects the output voltage Vout of the power conversion unit 10 of the DC / DC converter 1 and outputs it to the control unit 11. The first voltage sensor V1 and the second voltage sensor V2 are configured, for example, as a resistive voltage divider circuit.

[0023] The first current sensor A1 detects the input current Iin of the power conversion unit 10 of the DC / DC converter 1 and outputs it to the control unit 11. The second current sensor A2 detects the output current Iout of the power conversion unit 10 of the DC / DC converter 1 and outputs it to the control unit 11. The first current sensor A1 and the second current sensor A2 may each be composed of a shunt resistor and an amplifier that amplifies and outputs the voltage across the shunt resistor, or they may be composed of a Hall element and an amplifier that converts the magnetic field generated in the Hall element into a voltage.

[0024] The control unit 11 is composed of multiple analog elements and a microcontroller, and by controlling the first switching element Q1 to the fourth switching element Q4, it can perform bidirectional boost or buck operation of the DC / DC converter 1.

[0025] In step-down mode, the control unit 11 fixes the third switching element Q3 in the ON state and the fourth switching element Q4 in the OFF state, and controls the ON / OFF state of the first switching element Q1 and the second switching element Q2. The control unit 11 makes the first switching element Q1 and the second switching element Q2 operate in a complementary manner. When the first switching element Q1 is ON and the second switching element Q2 is OFF, power is transmitted to the output side while charging the inductor L1 from the DC power supply SB1. When the first switching element Q1 is OFF and the second switching element Q2 is ON, the DC power supply SB1 and the inductor L1 are disconnected, and only the power stored in the inductor L1 is transmitted to the output side.

[0026] The output voltage Vout in step-down mode is given by the following equation (Equation 1). The duty cycle D is the ratio of the on-period (the period during which the first switching element Q1 is on and the second switching element Q2 is off) in a unit period. Vout = D・Vin ... (Equation 1)

[0027] The control unit 11 can increase the output voltage Vout by increasing the duty cycle D, and decrease the output voltage Vout by decreasing the duty cycle D.

[0028] In boost mode, the control unit 11 fixes the first switching element Q1 in the ON state and the second switching element Q2 in the OFF state, and controls the ON / OFF state of the third switching element Q3 and the fourth switching element Q4. The control unit 11 makes the third switching element Q3 and the fourth switching element Q4 operate in a complementary manner. When the fourth switching element Q4 is ON and the third switching element Q3 is OFF, power transmission from the input side to the output side is stopped, and the DC power supply SB1 charges the inductor L1. When the fourth switching element Q4 is OFF and the third switching element Q3 is ON, power is transmitted to the output side from both the DC power supply SB1 and the inductor L1.

[0029] The output voltage Vout in boost mode is given by the following equation (Equation 2). The duty cycle D is the ratio of the on-period (the period during which the fourth switching element Q4 is on and the third switching element Q3 is off) in a unit period. Vout = 1 / (1 - D) * Vin ... (Equation 2)

[0030] The control unit 11 can increase the output voltage Vout by increasing the duty cycle D, and decrease the output voltage Vout by decreasing the duty cycle D.

[0031] In this embodiment, constant voltage control is the basis. That is, the control unit 11 adaptively controls the duty cycle D of the first switching element Q1 to the fourth switching element Q4 so that the detected value of the output voltage Vout of the power conversion unit 10 is maintained at a target value (for example, 48V).

[0032] Figure 2 shows an example of the configuration of the control unit 11 according to the comparative example. The control unit 11 according to the comparative example includes a subtraction unit 111, a phase compensation unit 112, a first amplification unit 113, a duty cycle calculation unit 114, and a PWM generation unit 115.

[0033] The subtraction unit 111 subtracts the output voltage Vout detected by the second voltage sensor V2 from the target output voltage Vref of the DC / DC converter 1 to calculate the deviation err between the target value Vref and the detected value. The phase compensation unit 112 adjusts the pole and zero position of the feedback loop to perform phase compensation of the feedback loop. The first amplification unit 113 amplifies the phase-compensated deviation err with a predetermined gain. The duty cycle calculation unit 114 performs PI control on the deviation err amplified with a predetermined gain to calculate the duty cycle D.

[0034] The PWM generation unit 115 compares the calculated duty cycle D with the carrier wave and generates a PWM signal to drive the first switching element Q1 to the fourth switching element Q4. At each period of the carrier wave, the PWM generation unit 115 switches the PWM signal to OFF when a significant signal is input from the duty cycle calculation unit 114.

[0035] In step-down mode, the driver (not shown) for the first switching element Q1 generates a drive signal for the first switching element Q1 based on the PWM signal generated by the PWM generation unit 115 and drives the first switching element Q1. The driver (not shown) for the second switching element Q2 generates a drive signal for the second switching element Q2 based on a PWM signal with the opposite phase to the PWM signal generated by the PWM generation unit 115 and drives the second switching element Q2. If the switching element is a MOSFET, the driver generates a gate-source voltage and applies it between the gate and source of the MOSFET.

[0036] In boost mode, the driver (not shown) for the fourth switching element Q4 generates a drive signal for the fourth switching element Q4 based on the PWM signal generated by the PWM generation unit 115 and drives the fourth switching element Q4. The driver (not shown) for the third switching element Q3 generates a drive signal for the third switching element Q3 based on a PWM signal with the opposite phase to the PWM signal generated by the PWM generation unit 115 and drives the third switching element Q3.

[0037] Figure 3 shows an example of the configuration of the control unit 11 according to the embodiment. The control unit 11 according to the embodiment further includes an addition value determination unit 116, a second amplification unit 117, and an addition unit 118, in addition to the configuration of the control unit 11 according to the comparative example shown in Figure 2.

[0038] When the power demanded by load 3 increases and the output current Iout of the power conversion unit 10 increases, the output voltage Vout decreases. If the power demanded by load 3 increases sharply, it is necessary to rapidly increase the duty cycle D.

[0039] The relationship between the output voltage Vout and the change in the duty cycle D in response to load fluctuations is basically proportional. The higher the output voltage Vout, the more proportionally the change in the duty cycle D in response to load fluctuations increases. In contrast, the relationship between the output current Iout and the change in the duty cycle D in response to load fluctuations is nonlinear. In the light load region where the output current Iout is small, the change in the duty cycle D in response to load fluctuations is relatively large, while in the heavy load region where the output current Iout is large, the change in the duty cycle D in response to load fluctuations is relatively small.

[0040] In this embodiment, the correspondence between the output current Iout of the power conversion unit 10 and the duty cycle addition value ΔD to be added to the duty cycle D generated by voltage feedback control is derived in advance, and the correspondence between the output current Iout and the duty cycle addition value ΔD is put into a table and stored.

[0041] The designer observes the fluctuations in the output voltage Vout by increasing the output current Iout from each output current Iout at a predetermined current slew rate (e.g., 1 [A / μs]) within the range in which the output voltage Vout of the power conversion unit 10 is maintained near the target value Vref. The designer sets various duty cycle addition values ​​ΔD for each output current Iout and observes the fluctuations in the output voltage Vout. For each output current Iout, the designer selects the duty cycle addition value ΔD that minimizes the fluctuation of the output voltage Vout when the output current Iout increases as the optimal duty cycle addition value ΔD.

[0042] The designer can comprehensively derive and table the relationship between the output current Iout and the duty cycle ΔD through experimentation or simulation. Alternatively, the designer may discretely derive the relationship between the output current Iout and the duty cycle ΔD through experimentation or simulation, and use multiple samples of the output current Iout and duty cycle ΔD as a dataset to approximate a nonlinear function of the output current Iout and duty cycle ΔD. The approximated nonlinear function will be a curve in which the slope of increase in the duty cycle ΔD becomes gentler as the output current Iout increases. For example, the designer can approximate a logarithmic function or a square root function by regressing the dataset containing multiple samples of the output current Iout and duty cycle ΔD using the least squares method.

[0043] FIG. 4 is a diagram showing an example when the output current and the duty addition value are approximated by a logarithmic function (LOG10). FIG. 5 is a diagram showing an example when the output current and the duty addition value are approximated by a square root function (SQRT). The minimum value (0) of the output current on the horizontal axis corresponds to a load of 0%, and the maximum value (4096) corresponds to a load of 100%. The duty addition value on the vertical axis is the value before PI control. The logarithmic function shown in FIG. 4 or the square root function shown in FIG. 5 is tabulated and held in the addition value determination unit 116.

[0044] The addition value determination unit 116 determines a duty addition value corresponding to the detected value of the output current Iout detected by the second current sensor A2 by referring to a table describing the relationship between the output current Iout and the duty addition value. The second amplifier 117 amplifies the determined duty addition value with a predetermined gain. This amplification process is introduced as a normalization process for matching the duty ratio D generated by voltage feedback control with the scale.

[0045] The addition unit 118 adds the duty addition value amplified by the second amplifier 117 to the deviation err amplified by the first amplifier 113. The duty calculation unit 114 performs PI control on the deviation err to which the duty addition value is added to calculate the duty ratio D. The PWM generation unit 115 compares the calculated duty ratio D with a carrier wave to generate a PWM signal for driving the first switching element Q1 - the fourth switching element Q4.

[0046] FIG. 6 is a diagram observing the waveform of the output voltage Vout during load variation when only the voltage feedback control shown in FIG. 2 is applied. FIG. 7 is a diagram observing the waveform of the output voltage Vout during load variation when the voltage feedback control and the current feedforward control shown in FIG. 3 are applied. The waveforms in FIGS. 6 and 7 are schematic depictions of the actually observed output current Iout and output voltage Vout with an oscilloscope.

[0047] The waveforms shown in FIGS. 6 and 7 indicate the waveforms when the output current Iout is changed as 24 [A] → 96 [A] → 24 [A] at a slew rate of 1 [A / μs]. As shown in FIG. 7, it can be confirmed that by adding the current feedforward control according to the present embodiment, the fluctuation of the output voltage Vout is suppressed.

[0048] As described above, according to the present embodiment, the duty addition value for the output current is previously held in a table, and the feedforward control according to the detected value of the output current is added to the feedback control according to the detected value of the output voltage. Thereby, the control responsiveness during load fluctuation can be improved, and the fluctuation of the output voltage can be suppressed. That is, by predicting and superimposing in advance the increment of the duty ratio during load increase from the detected value of the output current, the responsiveness during load fluctuation can be improved. As shown in FIG. 4 or FIG. 5, by setting the duty addition value at light load to be large and the duty addition value at heavy load to be small, the responsiveness can be improved over the entire region.

[0049] In addition, since the fluctuation of the output voltage can be suppressed without increasing the capacitance of the second smoothing capacitor C2, an increase in the component mounting area and an increase in the component cost can be suppressed. That is, the fluctuation of the output voltage of the DC / DC converter 1 during load fluctuation can be reduced at low cost.

[0050] The above is an explanation of the present disclosure based on the embodiments. It is understood by those skilled in the art that the embodiments are examples, and various modifications are possible for each combination of the components and each processing process, and such modifications are also within the scope of the present disclosure.

[0051] In the embodiments described above, an H-bridge type step-up / step-down DC / DC converter was used as an example. However, the control according to this disclosure can be applied to any DC / DC converter that includes at least one switching element and an inductor, and has a power conversion unit capable of controlling the voltage of DC power. For example, when applied to a unidirectional step-down converter, the second switching element Q2 is replaced with a diode, and the third switching element Q3 and the fourth switching element Q4 are omitted. Similarly, when applied to a unidirectional step-up converter, the third switching element Q3 is replaced with a diode, and the first switching element Q1 and the second switching element Q2 are omitted. Furthermore, the control according to this disclosure can also be applied to isolated flyback converters and forward converters.

[0052] The energy storage system described above can be applied to purposes other than data center backup. For example, it can be used to charge electricity generated by a solar power generation system and discharge it to the commercial power grid 4 at a predetermined time. In that case, it is not necessary to step down the voltage to 48V, so it can be used as an energy storage system that stores electricity at a voltage close to the voltage of the commercial power grid 4.

[0053] The embodiments may be specified by the following items.

[0054] [Item 1] A DC / DC converter (1) comprising: a power conversion unit (10) including at least one switching element (Q1-Q4) and an inductor (L1) capable of controlling the voltage of DC power; a voltage detection unit (V2) for detecting the output voltage of the power conversion unit (10); a current detection unit (A2) for detecting the output current of the power conversion unit (10); and a control unit (11) for controlling the duty cycle of the switching element (Q1-Q4) so ​​that the detected value of the output voltage of the power conversion unit (10) maintains a target value, wherein the control unit (11) refers to a pre-set correspondence between the output current of the power conversion unit (10) and an addition value to be added to the duty cycle, determines an addition value corresponding to the detected value of the output current of the power conversion unit (10), and adds the determined addition value to the duty cycle determined based on the detected value of the output voltage. According to this, fluctuations in the output voltage of the DC / DC converter (1) during load fluctuations can be reduced at low cost. [Item 2] The DC / DC converter (1) according to Item 1, characterized in that the correspondence is described by a nonlinear function in which the slope of increase of the added value becomes gentler as the output current of the power conversion unit (10) increases. With this, the optimal added value according to the output current can be determined. [Item 3] The DC / DC converter (1) according to Item 2, characterized in that the nonlinear function is a logarithmic function or square root function generated by regression on a dataset obtained by experiment or simulation so as to minimize fluctuations in the output voltage of the power conversion unit (10) when the output current of the power conversion unit (10) increases. With this, the relationship between the output current and the added value can be made into a table from a relatively small dataset.[Item 4] The DC / DC converter (1) according to Item 1, characterized in that the power conversion unit (10) includes a first switching element (Q1) and a second switching element (Q2) connected in series, a first arm connected in parallel to a DC power supply and a first smoothing capacitor (C1), a second arm including a third switching element (Q3) and a fourth switching element (Q4) connected in series, a DC bus (2) connected in parallel to a second smoothing capacitor (C2), and the inductor (L1) connected to the midpoint of the first arm and the midpoint of the second arm. According to this, fluctuations in the output voltage of the H-bridge type step-up / step-down DC / DC converter (1) during load fluctuations can be reduced at low cost.

[0055] This disclosure is applicable to switching-type DC / DC converters.

[0056] SB1 DC power supply, 1 DC / DC converter, 2 DC bus, 3 load, 4 commercial power grid, 5 AC / DC converter, 10 power conversion unit, 11 control unit, 111 subtraction unit, 112 phase compensation unit, 113 first amplification unit, 114 duty cycle calculation unit, 115 PWM generation unit, 116 sum value determination unit, 117 second amplification unit, 118 summing unit, L1 inductor, Q1-Q4 switching elements, D1-D4 diodes, C1-C2 smoothing capacitors, A1-A2 current sensors, V1-V2 voltage sensors.

Claims

1. A DC / DC converter comprising: at least one switching element; a power conversion unit including an inductor capable of controlling the voltage of DC power; a voltage detection unit for detecting the output voltage of the power conversion unit; a current detection unit for detecting the output current of the power conversion unit; and a control unit for controlling the duty cycle of the switching element so that the detected value of the output voltage of the power conversion unit maintains a target value, wherein the control unit refers to a pre-set correspondence between the output current of the power conversion unit and an addition value to be added to the duty cycle, determines an addition value corresponding to the detected value of the output current of the power conversion unit, and adds the determined addition value to the duty cycle determined based on the detected value of the output voltage.

2. The DC / DC converter according to claim 1, characterized in that the correspondence is described by a nonlinear function in which the slope of the increase in the sum value becomes gentler as the output current of the power conversion unit increases.

3. The DC / DC converter according to claim 2, characterized in that the nonlinear function is a logarithmic or square root function generated by regression on a dataset obtained by experiment or simulation, such that the fluctuation of the output voltage of the power converter is minimized when the output current of the power converter increases.

4. The DC / DC converter according to claim 1, wherein the power conversion unit includes a first switching element and a second switching element connected in series, a first arm connected in parallel to a DC power supply and a first smoothing capacitor, a second arm connected in parallel to a DC bus and a second smoothing capacitor, a third switching element and a fourth switching element connected in series, and the inductor connected to the midpoint of the first arm and the midpoint of the second arm.

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