Isolated DC / DC Converter and Control Method for Isolated DC / DC Converter

Abstract

In an isolated DC / DC converter that performs intermittent operation using a transformer and an inverter, the bias magnetization of the transformer is suppressed. 【Solution means】The inverter 2 has its DC side connected to the first DC voltage source 1. The primary winding of the transformer 3 is connected to the AC side of the inverter 2. A rectifier 4 is connected to the secondary winding of the transformer 3. The second DC voltage source 5 is connected to the DC side of the rectifier 4. The isolated DC / DC converter performs pulse density control by intermittent operation. The inverter 2 is set to a switching state in which the primary winding of the transformer 3 is short-circuited during the inverter off period. The switching states during the inverter operation periods immediately before and after the inverter off period are made the same. Control is performed so that the sum of the pulse widths during the inverter operation periods immediately before and after the inverter off period matches the normal pulse width during the inverter operation period.

Description

Technical Field

[0001] The present invention relates to a power transmission technology in an isolated DC / DC converter using a transformer and an inverter, and particularly to a control method for intermittent operation in which a period of voltage output by the inverter and a period of non-voltage output are repeated.

Background Art

[0002] As a circuit for generating another isolated DC voltage source from a DC voltage source, an isolated DC / DC converter using the insulation of a transformer 3 as shown in FIG. 1 is known. As shown in FIG. 1, an inverter 2 is provided on the primary side of the transformer 3, and a rectifier 4 is provided on the secondary side of the transformer 3. The output voltage of the inverter 2 is applied to the primary side of the transformer 3, and its voltage waveform is pulsed.

[0003] As a control method for the inverter 2 that varies the power transmitted from the primary side to the secondary side, there are a method of varying the pulse width (duty) of the pulse voltage that is the inverter output voltage, and a method of performing intermittent operation by providing a period during which the output of the inverter 2 is stopped while the pulse width remains constant and varying the pulse density.

[0004] In the method of controlling the pulse density among these two methods, the power that can be transmitted per pulse is determined. When the ratio of the inverter operation period to the inverter stop period is small, the pulse density is low, and on average, the power that can be transmitted from the primary side to the secondary side is small.

[0005] On the other hand, when the ratio of the inverter operation period is large, the pulse density is high, and on average, the power that can be transmitted from the primary side to the secondary side is large. The difference in the transmitted power due to the difference in the pulse density is shown in FIG. 2. Note that the inverter output voltage is either +V1, 0, or -V1 with respect to the voltage V1 of the first DC voltage source 1 in FIG. 1. The inverter output voltage is ±V1 during the inverter operation period and 0 during the inverter stop period.

[0006] In the configuration of Fig. 1, when the average value of the voltage applied to transformer 3 is not zero, the exciting current of transformer 3 continues to increase or decrease, resulting in a magnetic bias phenomenon where the core of transformer 3 becomes magnetically saturated. To suppress this, Patent Documents 1 to 4 have taken the following measures.

[0007] Patent Document 1: Controls the pattern of the inverter output voltage so as to enter the inverter pause period at the timing when the exciting current becomes zero.

[0008] Patent Documents 2 and 3: Provide a mechanism for measuring the voltage waveform applied to the transformer and the magnetic flux of the transformer core, and control the pulse width of the inverter output voltage according to the measured values.

[0009] Patent Document 4: Switches the positive and negative of the inverter output voltage for each operation period.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0011] In the case of Patent Document 1, since the timing when the exciting current becomes zero occurs only twice per pulse period, it is difficult to perform high-precision control to match the inverter operation period to a desired time.

[0012] In the case of Patent Documents 2 and 3, an additional mechanism for measuring voltage and magnetic flux is required, and furthermore, since the pulse width is controlled simultaneously with the pulse density, the control becomes complicated.

[0013] As in Patent Document 1, a waveform example when the inverter is controlled to enter the inverter stop period at the timing when the excitation current becomes 0 in the configuration of FIG. 1 is shown in FIG. 3. As described above, since the timing when the excitation current becomes 0 occurs only twice per pulse period, there are timing constraints. If the inverter enters the stop period at a timing when the excitation current does not become 0 while ignoring this constraint, DC may be superimposed on the excitation current and bias magnetization may occur as shown in FIG. 4.

[0014] In Patent Document 4, the positive and negative of the inverter output voltage are switched and output for each operation period. When the time widths of two consecutive operation periods are the same, the magnetic fluxes of the transformer can be canceled out for every two operation periods. However, when the time widths are different for each operation period, the magnetic fluxes of the transformer cannot be canceled out and the magnetic fluxes remain, and there is a risk that a bias magnetization phenomenon in which the iron core of the transformer becomes magnetically saturated may occur due to the accumulation of these magnetic fluxes.

[0015] From the above, in an isolated DC / DC converter that performs intermittent operation using a transformer and an inverter, suppressing the bias magnetization of the transformer becomes an issue.

Means for Solving the Problem

[0016] The present invention has been devised in view of the above-described conventional problems, and one aspect thereof is an isolated DC / DC converter to which pulse density control by intermittent operation is applied, including a first DC voltage source, an inverter having a DC side connected to the first DC voltage source, a transformer having a primary winding connected to an AC side of the inverter, a rectifier connected to a secondary winding of the transformer, and a second DC voltage source connected to a DC side of the rectifier. The inverter is in a switching state in which the primary winding of the transformer is short-circuited during the inverter stop period, the switching states of the inverter operation periods immediately before and after the inverter stop period are the same, and the sum of the pulse widths of the inverter operation periods immediately before and after the inverter stop period is controlled to match the normal pulse width during the inverter operation period.

[0017] Also, as one aspect, the inverter includes a first switching element and a second switching element connected in series between one end and the other end of the first DC voltage source, and a third switching element and a fourth switching element connected in series between one end and the other end of the first DC voltage source. A connection point between the first and second switching elements and a connection point between the third and fourth switching elements are connected to the primary winding of the transformer, which is a single-phase full-bridge circuit. During the inverter off period, the first and third switching elements are turned on, the second and fourth switching elements are turned off, or the second and fourth switching elements are turned on, and the first and third switching elements are turned off.

[0018] Also, as another aspect, the inverter includes a first switching element and a second switching element connected in series between one end and the other end of the first DC voltage source, a third switching element and a fourth switching element connected in series between one end and the other end of the first DC voltage source, and a fifth switching element and a sixth switching element connected in series between one end and the other end of the first DC voltage source. A connection point between the first and second switching elements, a connection point between the third and fourth switching elements, and a connection point between the fifth and sixth switching elements are connected to the primary winding of the transformer, which is a three-phase full-bridge circuit. During the inverter off period, the first, third, and fifth switching elements are turned on, the second, fourth, and sixth switching elements are turned off, or the second, fourth, and sixth switching elements are turned on, and the first, third, and fifth switching elements are turned off.

Advantages of the Invention

[0019] According to the present invention, in an isolated DC / DC converter that performs intermittent operation using a transformer and an inverter, it is possible to suppress the bias magnetization of the transformer.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0021] Hereinafter, Embodiments 1 and 2 of the isolated DC / DC converter according to the present invention will be described in detail with reference to FIGS. 5 to 8.

[0022] [Embodiment 1] FIG. 5 shows the circuit configuration of an isolated DC / DC converter to which the control method of the present Embodiment 1 is applied. First, the circuit configuration of the isolated DC / DC converter in the present Embodiment 1 will be described based on FIG. 5.

[0023] An inverter 2 of a single-phase full-bridge circuit is connected to the first DC voltage source 1. Specifically, the first and second switching elements S1 and S2 of the U phase are connected in series between one end and the other end of the first DC voltage source 1. Also, the third and fourth switching elements S3 and S4 are connected in series between one end and the other end of the first DC voltage source 1.

[0024] The connection point of the first and second switching elements S1 and S2 is connected to one end of the primary winding of the transformer 3. The connection point of the third and fourth switching elements S3 and S4 is connected to the other end of the primary winding of the transformer 3.

[0025] A single-phase rectifier 4 is connected to the secondary winding of the transformer 3. Specifically, one end of the secondary winding of the transformer 3 is connected to the connection point of the first and second diodes D1 and D2 of the rectifier 4, and the other end of the secondary winding of the transformer 3 is connected to the connection point of the third and fourth diodes D3 and D4 of the rectifier 4.

[0026] The first and third diodes D1 and D3 are connected to one end of the second DC voltage source 5, and the second and fourth diodes D2 and D4 are connected to the other end of the second DC voltage source 5. A load 6 is connected in parallel to the second DC voltage source 5.

[0027] The inverter 2 connected to the first DC voltage source 1 is a single-phase full-bridge circuit, and the phase voltage Vuv between the U phase and the V phase of the single-phase full-bridge circuit is applied to the primary winding of the transformer 3. By connecting a rectifier 4 to the secondary winding of the transformer 3, it is converted into DC. Thereby, a second DC voltage source 5 insulated from the first DC voltage source 1 at the input part of the inverter 2 can be generated.

[0028] The inverter 2 of the single-phase full-bridge circuit has four switching states (states 1 to 4) as shown in Table 1. Although two switching states are described for state 4, since the inverter output voltage becomes 0 in either switching state, they are treated as one switching state without distinction.

[0029]

Table 1

[0030] During the inverter operation period, the output voltage of the inverter 2 is alternately switched between +V1 and -V1 by alternately switching between state 1 and state 2. When switching, it is generally necessary to insert a dead time, and the dead time corresponds to state 3. On the other hand, during the inverter rest period, there are two methods: state 3 or state 4.

[0031] When applying state 3 as the inverter off period, during the period of state 3, the inverter 2 operates as a rectifier. When switching from the inverter operation period to the inverter off period as state 3 with the exciting current not being zero, the exciting current flows into the first DC voltage source 1 or the second DC voltage source 5, so that the energy charged in the exciting inductance of the transformer 3 is discharged to the first DC voltage source 1 or the second DC voltage source 5, and the exciting current decays.

[0032] Here, when the inverter off period is short and the next inverter operation period occurs before the energy charged in the exciting inductance of the transformer 3 is completely discharged, as shown in Fig. 6, a DC component is superimposed on the exciting current and magnetic bias occurs.

[0033] When applying state 4 as the inverter off period, when switching from the inverter operation period to the inverter off period with the exciting current not being zero, the exciting inductance of the transformer 3 is short-circuited by the switching elements of the inverter 2 on the primary side. As a result, ignoring the circuit losses, the exciting current during the inverter operation period is maintained as shown in Fig. 4.

[0034] In the first embodiment, state 4 is applied during the inverter off period to maintain the exciting current. Then, the switching states of the inverter operation periods immediately before and after the inverter off period are made the same, and the sum of the pulse widths of the inverter operation periods immediately before and after the inverter off period is made to match the normal pulse width during the inverter operation period.

[0035] Thereby, the change in the exciting current can be suppressed within the same range as when the inverter is turned off at the timing when the exciting current becomes zero as shown in Fig. 3. In this method, there is no need for the constraints on the inverter operation period as in Patent Document 1, or the additional voltage and magnetic flux measurement mechanisms as in Patent Documents 2 and 3. Also, there is no need to match the time widths of two consecutive operation periods which is a constraint in Patent Document 4.

[0036] The operation pattern derivation method according to the first embodiment is summarized below. A waveform example is shown in Fig. 7.

[0037] (1) Inverter operation period During the inverter operation period, when ignoring the dead time and setting the pulse period as Tpulse, the operations of alternately repeating the states 1 and 2 in Table 1 for 0.5×Tpulse (0.5 cycle) each are performed. As a result, the inverter output voltage becomes a constant operation with a duty ratio of 50%.

[0038] When switching from the inverter operation period to the inverter standby period, based on the switching state (state 1 or state 2) in the immediately preceding inverter operation period and the output time (Tbefore) in that state, the shortage of the output time for 0.5 cycle, "Tshort", is calculated and recorded by the following formula (1).

[0039] [Equation]

[0040] (2) Inverter standby period During the inverter standby period, by setting the state as state 4 in Table 1, the exciting current during the immediately preceding inverter operation period is maintained. State 4 is that the first and third switching elements S1, S3 are on, the second and fourth switching elements S2, S4 are off, or the second and fourth switching elements S2, S4 are on, and the first and third switching elements S1, S3 are off.

[0041] (3) Inverter operation period The first switching state at the time of resuming inverter operation is the same as the state in the (1) immediately preceding inverter operation period of the (2) inverter standby period. After that, the operations of alternately repeating states 1 and 2 are resumed again. At this time, the time (pulse width) of the first switching state at the time of resuming is set as "Tshort" calculated and recorded in the "(1) Inverter operation period".

[0042] As a result, when operating without intermittent operation and with the inverter output voltage fixed at a duty ratio of 50%, an inverter off period for maintaining the exciting current is inserted into the exciting current waveform. As a result, without restricting the length of the inverter operation period, it is possible to confine the variation range of the exciting current to the same as that in the case of continuous inverter operation periods, and thus suppress bias magnetization.

[0043] As described above, according to the first embodiment, by applying the switching state (state 4 in Table 1) capable of maintaining the exciting current during the inverter off period, it is no longer necessary to end the inverter operation period at the timing when the exciting current in Patent Document 1 becomes zero, and bias magnetization can be suppressed in any inverter operation period.

[0044] Further, when the inverter operation period ends, by calculating and recording the pulse width required in the next inverter operation period, the pulse width in the next inverter operation period can be adjusted without information on the applied voltage of the transformer 3 or the magnetic flux information of the transformer 3, and a mechanism for measuring additional voltages and magnetic fluxes as in Patent Documents 2 and 3 becomes unnecessary. Also, it is not necessary to match the time widths of two consecutive operation periods, which is a constraint in Patent Document 4.

[0045] (Regarding the condition where circuit losses can be ignored) Assuming that the primary-side equivalent series resistance component associated with the windings and circuit element wirings of the transformer 3 is ESR1, the primary-side winding inductance is L1, and the primary-side current is I1, since the secondary side is substantially open during the period corresponding to the inverter off period in state 4, the following equation (2) holds.

[0046]

Equation

[0047] Therefore, when the fluctuation of the secondary-side voltage can be ignored, assuming that the current immediately after the transition of the off period is I0 at t = 0, the following equation (3) holds.

[0048]

Equation

[0049] That is, as a condition for ignoring the above attenuation, it is necessary that the rest period t is sufficiently smaller than L1 / ESR1 corresponding to the primary time constant. Conversely, even if L1 divided by the rest period t is sufficiently larger than the primary equivalent series resistance component ESR1, it becomes a condition for ignoring circuit losses such that the attenuation can be ignored. It is only necessary that the primary equivalent series resistance component ESR1 is sufficiently small to the extent that this condition is satisfied. This condition is satisfied in normal designs.

[0050] [Embodiment 2] The difference between Embodiment 1 and Embodiment 2 in this embodiment is that the transformer 3, the inverter 2, and the rectifier 4 are changed from single-phase to three-phase. The circuit system in this Embodiment 2 is shown in FIG. 8.

[0051] Specifically, the first and second switching elements S1 and S2 of the U phase are connected in series between one end and the other end of the first DC voltage source 1. Also, the third and fourth switching elements S3 and S4 are connected in series between one end and the other end of the first DC voltage source 1. Also, the fifth and sixth switching elements S5 and S6 are connected in series between one end and the other end of the first DC voltage source 1.

[0052] The connection points of the first and second switching elements S1 and S2, the connection points of the third and fourth switching elements S3 and S4, and the connection points of the fifth and sixth switching elements S5 and S6 are connected to the transformer 3.

[0053] The secondary winding of the transformer 3 is connected to the connection points of the first and second diodes D1 and D2, the connection points of the third and fourth diodes D3 and D4, and the connection points of the fifth and sixth diodes D5 and D6.

[0054] The first, third, and fifth diodes D1, D3, and D5 are connected to one end of the second DC voltage source 5, and the second, fourth, and sixth diodes D2, D4, and D6 are connected to the other end of the second DC voltage source 5. A load 6 is connected in parallel to the second DC voltage source 5.

[0055] Let the line-to-line voltages between the U phase and the V phase, between the V phase and the W phase, and between the W phase and the U phase be Vuv, Vvw, and Vwu, respectively.

[0056] In the case of the three-phase inverter in FIG. 8, as shown in Table 2, there are eight switching states (States 1 to 8). Note that although two switching states are described for State 8, since the inverter output voltage becomes 0 in either switching state, they are treated as one switching state without distinction. State 8 is when the first, third, and fifth switching elements S1, S3, and S5 are on and the second, fourth, and sixth switching elements S2, S4, and S6 are off, or when the second, fourth, and sixth switching elements S2, S4, and S6 are on and the first, third, and fifth switching elements S1, S3, and S5 are off.

[0057] During the inverter operation period, pulses with a duty ratio of 50% are output in each phase constituting the inverter 2 under the condition that the timing is shifted by 1 / 3 of the pulse period between phases (phase difference of 120 degrees). As a result, an operation is performed in which States 1 to 6 in Table 2 are output in order by 1 / 6 of the pulse period each.

[0058]

Table 2

[0059] Next, the operation during the inverter rest period will be described.

[0060] In the single-phase full-bridge circuit of Embodiment 1, in State 4 in Table 1, the exciting inductance of the transformer 3 is short-circuited by the switching elements of the inverter 2 on the primary side.

[0061] Similarly, in the case of the three-phase inverter of this Embodiment 2, in State 8 in Table 2, the exciting inductance of the transformer 3 is short-circuited by the switching elements of the inverter 2 on the primary side.

[0062] In this way, similar to Embodiment 1, by selecting State 8 in Table 2 during the inverter pause period, the exciting current during the inverter pause period can be maintained. And the first switching state at the time of resuming inverter operation is the same as the switching state during the inverter operation period immediately before the inverter pause period. Then, States 1 to 6 are output in order by 1 / 6 of the pulse period each. At this time, the time of the first switching state at the time of resumption is set as "Tshort" calculated and recorded during the inverter operation period immediately before the inverter pause period.

[0063] Therefore, it becomes a state in which an inverter pause period for maintaining the exciting current is inserted into the exciting current waveform when the inverter output voltage is operated with a constant duty without performing intermittent operation. As a result, without restricting the length of the inverter operation period, it is possible to keep within the same change range of the exciting current as in the case of a continuous inverter operation period, and it is possible to suppress bias magnetization.

[0064] As described above, according to the second embodiment, even in the case of a three-phase inverter, the same operational effects as those of the first embodiment are obtained.

[0065] As described above, in the present invention, although the description has been made in detail only for the specific examples described, it is obvious to those skilled in the art that various modifications and corrections are possible within the scope of the technical idea of the present invention, and it is natural that such modifications and corrections belong to the scope of the claims.

Explanation of Signs

[0066] 1... First DC voltage source 2... Inverter 3... Transformer 4... Rectifier 5... Second DC voltage source 6... Load S1~S6... First to sixth switching elements D1~D6... First to sixth diodes

Claims

1. An isolated DC / DC converter that applies pulse density control through intermittent operation, A first DC voltage source; an inverter having a DC side connected to the first DC voltage source; a transformer having a primary winding connected to an AC side of the inverter; a rectifier connected to a secondary winding of the transformer; a second DC voltage source connected to the DC side of the rectifier; Equipped with The inverter is During the inverter off period, a switching state is set to short-circuit the primary winding of the transformer, The switching states of the inverter operation periods immediately before and after the inverter pause period are made the same, an isolated DC / DC converter controlling a sum of pulse widths during the inverter operation periods immediately before and after the inverter idle period so as to match a pulse width during normal operation during the inverter operation period.

2. The inverter is a single-phase full-bridge circuit including first and second switching elements connected in series between one end and the other end of the first DC voltage source, and third and fourth switching elements connected in series between one end and the other end of the first DC voltage source, wherein a connection point between the first and second switching elements and a connection point between the third and fourth switching elements are connected to the primary winding of the transformer, 2. The isolated DC / DC converter according to claim 1, wherein, during the inverter pause period, the first and third switching elements are turned ON and the second and fourth switching elements are turned OFF, or the second and fourth switching elements are turned ON and the first and third switching elements are turned OFF.

3. The inverter is a three-phase full bridge circuit including first and second switching elements connected in series between one end and the other end of the first DC voltage source, third and fourth switching elements connected in series between one end and the other end of the first DC voltage source, and fifth and sixth switching elements connected in series between one end and the other end of the first DC voltage source, wherein a connection point between the first and second switching elements, a connection point between the third and fourth switching elements, and a connection point between the fifth and sixth switching elements are connected to the primary winding of the transformer, 2. The isolated DC / DC converter according to claim 1, wherein, during the inverter pause period, the first, third and fifth switching elements are turned ON and the second, fourth and sixth switching elements are turned OFF, or the second, fourth and sixth switching elements are turned ON and the first, third and fifth switching elements are turned OFF.

4. A first DC voltage source; an inverter having a DC side connected to the first DC voltage source; a transformer having a primary winding connected to an AC side of the inverter; a rectifier connected to a secondary winding of the transformer; a second DC voltage source connected to the DC side of the rectifier; A control method for an isolated DC / DC converter using pulse density control by intermittent operation, comprising: The inverter is During the inverter off period, a switching state is set to short-circuit the primary winding of the transformer, The switching states of the inverter operation periods immediately before and after the inverter pause period are made the same, a control method for an isolated DC / DC converter, comprising controlling a sum of pulse widths during the inverter operation periods immediately before and after the inverter idle period so as to match a pulse width during normal operation during the inverter operation period.

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