JP7501341B2 - 3-phase LLC converter - Google Patents

Description

The present invention relates to a three-phase LLC converter.

In the three-phase LLC converter described in Patent Document 1, an example is shown in which the primary windings are interleaved in a star connection and the resonant capacitor is divided.

However, doing so distorts the current flowing through each phase, causing harmonic currents to flow through the transformer. There is no mention of a method to avoid these harmonic currents by splitting the resonant capacitor and passing the harmonic currents through the common potential of the input, thereby reducing losses caused by harmonic currents in the transformer.

US Patent Application Publication No. 2011/0007527

However, changing the division ratio of the resonant capacitor changes the current balance of each phase. For this reason, there is an optimal division ratio for achieving current balance depending on the load conditions, etc.

The objective of the present invention is to provide a three-phase LLC converter that can obtain a division ratio of the resonant capacitor that can optimize the current balance of each phase even if the component constants of the resonant circuit vary.

In order to solve the above problems, the three-phase LLC converter according to the present invention has a switch circuit in which a first switch and a second switch are connected in series to both ends of a DC power source, a resonant circuit consisting of a resonant reactor, a first resonant capacitor, a second resonant capacitor, and a primary winding of a transformer, one end of which is connected to a connection end to which the first switch and the second switch are connected, a control circuit that alternately turns on and off the first switch and the second switch, and a rectifying and smoothing circuit that rectifies and smoothes the voltage of the secondary winding of the transformer, and is operated in three phases with a phase difference of 120°. The present invention is characterized in that the three LLC resonant converters are provided, one end of the first resonant capacitor is connected to one end of a series circuit of the resonant reactor and the primary winding of the transformer, and the other end is connected to a neutral point common to each phase, and the second resonant capacitor is connected to one end of the first resonant capacitor and the other end is connected to a common potential of the input, and when the resonant capacitor Cr is divided into the first resonant capacitor (1-α)Cr and the second resonant capacitor αCr, the division ratio α is set to 0.3≦α≦0.8 based on the resonant circuit impedance and the load constant.

According to the present invention, when the resonant capacitor Cr is divided into the first resonant capacitor (1-α) Cr and the second resonant capacitor αCr, the division ratio α is verified and its range is set based on the constants of the resonant reactor, the resonant capacitor Cr, and the load, so that the current balance of each phase can be optimized.

1 is a circuit diagram of a three-phase LLC converter according to a first embodiment of the present invention. This is a simplified equivalent three-phase circuit shown in terms of impedance in comparison with FIG. This is a diagram plotting the effective resonant current value of each phase when an error of ±10% is applied to the resonant constant based on the circuit constant and α is changed. FIG. 1 is a circuit diagram of a three-phase LLC converter in which only the phase 1 circuit is operated and the two phase 2 and 3 circuits are stopped. FIG. 13 is a diagram showing a frequency increment Δf versus division ratio α when switching from three-phase operation to one-phase operation. This is a circuit diagram of a control unit that uses an LLC-IC as a control core and obtains three-phase gate signals using a frequency divider. FIG. 13 is a graph showing the efficiency of the prototype with natural cooling versus output power. FIG. 8(a) shows the waveform of the resonant current in one-phase operation, and FIG. 8(b) shows the waveform of the resonant current in three-phase operation. 9A shows the waveforms of the resonant current and the output voltage when switching from one-phase operation to three-phase operation, and FIG 9B shows the waveforms of the resonant current and the output voltage when switching from three-phase operation to one-phase operation. FIG. 11 is a diagram showing a three-phase LLC converter according to a second embodiment in which the secondary side of a transformer is star-connected. FIG. 11 is a circuit diagram of a star connection of a divided Cr of a three-phase LLC converter according to a third embodiment of the present invention. FIG. 11 is a circuit diagram for triple frequency excitation of a three-phase LLC converter according to a third embodiment of the present invention. FIG. 13 is a diagram illustrating a primary side resonant current and a secondary side rectified current of a three-phase LLC converter according to a third embodiment. FIG. 13 is a circuit diagram of a three-phase LLC converter according to a fourth embodiment. FIG. 13 is a circuit diagram for triple frequency excitation of a three-phase LLC converter according to a fourth embodiment. FIG. 13 is a circuit diagram for detecting a phase abnormality in a three-phase LLC converter according to a fourth embodiment. FIG. 13 is a waveform diagram of a resonance current and a detection resistor voltage for detecting a phase abnormality in a three-phase LLC converter according to a fourth embodiment. FIG. 13 is a circuit diagram of a three-phase LLC converter according to a fifth embodiment. FIG. 13 is a circuit diagram of a three-phase LLC converter according to a sixth embodiment.

The following describes in detail the three-phase LLC converter according to an embodiment of the present invention with reference to the drawings.

First Embodiment
Fig. 1 is a circuit diagram of a three-phase LLC converter according to a first embodiment of the present invention. The three-phase LLC converter shown in Fig. 1 includes three half-bridge type LLC resonant converters whose input and output sides are connected in parallel and operated in three phases with a phase difference of 360°/3.

The input side of the three-phase LLC resonant converter is connected in parallel to the DC power supply Vin, and the output side is connected in parallel to output the output voltage Vo.

The three-phase LLC resonant converter has a switch circuit in which a switch Q11 and a switch Q12 are connected in series across both ends of a DC power supply Vin, a resonant circuit consisting of a resonant reactor Lr1, one end of which is connected to the connection end where the switch Q11 and the switch Q12 are connected, a first resonant capacitor (1-α) Cr1, a second resonant capacitor αCr1, and a primary winding P1 of a transformer T1, a control circuit 10 that alternately turns on and off the switch Q11 and the switch Q12, and rectifier circuits D1a and D1b that rectify and smooth the voltage of the secondary windings S1a and S1b of the transformer T1.

The three-phase LLC resonant converter also includes a switch circuit in which a switch Q21 and a switch Q22 are connected in series across both ends of a DC power supply Vin, a resonant circuit consisting of a resonant reactor Lr2, one end of which is connected to the connection end where the switch Q21 and the switch Q22 are connected, a first resonant capacitor (1-α) Cr2, a second resonant capacitor αCr2, and a primary winding P2 of a transformer T2, a control circuit 10 that alternately turns on and off the switch Q21 and the switch Q22, and rectifying and smoothing circuits D2a and D2b that rectify and smooth the voltage of the secondary windings S2a and S2b of the transformer T2.

The three-phase LLC resonant converter further includes a switch circuit in which a switch Q31 and a switch Q32 are connected in series across both ends of a DC power supply Vin, a resonant circuit consisting of a resonant reactor Lr3, one end of which is connected to the connection end where the switch Q31 and the switch Q32 are connected, a first resonant capacitor (1-α) Cr3, a second resonant capacitor αCr3, and a primary winding P3 of a transformer T3, a control circuit 10 that alternately turns on and off the switch Q31 and the switch Q32, and rectifying and smoothing circuits D3a and D3b that rectify and smooth the voltage of the secondary windings S3a and S3b of the transformer T3.

Switches Q11, Q21, and Q31 and switches Q12, Q22, and Q32 are made of MOSFETs. The control circuit 10 operates switches Q11 and Q12, switches Q21 and Q22, and switches Q31 and Q32 in three phases with a phase difference of 360°/3.

The first resonant capacitor (1-α) Cr1 has one end connected to the series circuit of the resonant reactor Lr1 and the primary winding P1 of the transformer T1, and the other end connected to the neutral point K common to each phase. The second resonant capacitor αCr1 has one end connected to the series circuit of the resonant reactor Lr1 and the primary winding P1 of the transformer T1, and the other end connected to the common potential of the input. Here, the common potential of the input is ground.

The first resonant capacitor (1-α) Cr2 has one end connected to a series circuit of the resonant reactor Lr2 and the primary winding P2 of the transformer T2, and the other end connected to a neutral point K common to each phase. The second resonant capacitor αCr2 has one end connected to a series circuit of the resonant reactor Lr2 and the primary winding P2 of the transformer T2, and the other end connected to the common potential of the input.

The first resonant capacitor (1-α) Cr3 has one end connected to a series circuit of the resonant reactor Lr3 and the primary winding P3 of the transformer T3, and the other end connected to a neutral point K common to each phase. The second resonant capacitor αCr3 has one end connected to a series circuit of the resonant reactor Lr3 and the primary winding P3 of the transformer T3, and the other end connected to the common potential of the input.

That is, the resonant capacitor is divided into first resonant capacitors (1-α)Cr1, (1-α)Cr2, (1-α)Cr3 and second resonant capacitors αCr1, αCr2, αCr3.

Each LLC resonant converter forms a first resonant circuit with the resonant reactors Lr1, Lr2, Lr3 of each phase, the primary windings P1, P2, P3 of the transformers T1, T2, T3, and the first resonant capacitors (1-α)Cr1, (1-α)Cr2, (1-α)Cr3. Furthermore, a second resonant circuit is formed with the resonant reactors Lr1, Lr2, Lr3 of each phase, the primary windings P1, P2, P3 of the transformers T1, T2, T3, the second resonant capacitors αCr1, αCr2, αCr3.

In addition, although input G is shown as the negative side in FIG. 1, input G may be the positive side. In other words, the common potential of the input may be the positive side of the input.

The primary windings P1, P2... of each transformer T1, T2... are star-connected via the first resonant capacitors (1-α)Cr1, (1-α)Cr2, (1-α)Cr3..., and the currents flowing through each phase are balanced. In addition, the second resonant capacitors αCr1, αCr2, αCr3 are connected to the series circuit of the primary windings P1, P2... and resonant reactors Lr1, Lr2... of each converter, and to the input G.

α is the division ratio when the resonant capacitor Cr is divided into the first resonant capacitor (1-α) Cr1 and the second resonant capacitor αCr1, and 0≦α≦1. When α=0, it is a three-phase LLC converter with an electrically floating neutral point K. When α=1, it is three independent LLC converters. If there is no error in all the resonant components, the average voltage of the neutral point K is zero, and the resonant frequency fr is expressed by equation (1).

Figure 2 is a simplified equivalent three-phase circuit shown in impedance in comparison with Figure 1. As shown in equation (2), the divided resonant capacitor Cr is defined by CrN connected to the neutral point K and Crg connected to the power line. The turns ratio of the transformer T is N (= number of primary turns np / number of secondary turns ns). The equivalent three-phase circuit shown in Figure 2 is expressed by equation (2).

The matrix is defined by equation (3).

The circuit network in Figure 2 is applied with KVL (Kirchhoff voltage loop) and KCL (Kirchhoff current loop) to obtain equations (4) and (5) and equation (6) for the total current of each phase.

(Equilibrium state)
Next, in equation (6), assuming that there is no error in the resonant components, let Z1 = Z2 = Z3 = Z, ZC1 = ZC2 = ZC3 = ZC. When α = 0, the neutral point K is in an electrically floating state, and equation (7) is obtained.

The input voltages have equal amplitudes and are phase shifted by 120°, so the relationship between the input voltages is shown in equation (8).

From equations (7) and (8), the three-phase currents of the balanced circuit can be calculated as equation (9).

In other words, it can be simply considered as a single-phase equivalent model. Also, when α = 1, equation (10) represents the state in which the three LLC converters are operating completely independently without interfering with other phases.

The calculation result is equal to equation (9). As described above, in a balanced state, this is equivalent to single-phase operation.

(Unbalanced state)
Consider an appropriate division ratio α of the resonant capacitor Cr in an unbalanced state. For example, assume that there is an error in the resonant inductor Lr or the excitation inductance Lm in phase 1, but no error in other components. Therefore, the impedance Z1 of phase 1 is replaced with Z', and equation (14) in the unbalanced state is obtained from equations (11), (12), and (13).

If equation (14) is equal to equation (10), current balance is established. Find equation (16) as the division ratio α that obtains a physically finite value from equation (15).

Normalizing using equation (17) gives a complex number.

Since the division ratio α is a real number, we extract the real part to obtain equation (18). As described above, the proposed three-phase LLC converter can obtain a division ratio α of the resonant capacitor Cr that can balance the current.

Next, the topology of Figure 1 is used to confirm with a SCAT circuit simulator. Figure 3 plots the effective resonant current value of each phase when an error of ±10% is applied to the resonant constant and α is changed based on the following circuit constants. Specifically, it shows the effective resonant current values of phase 1, phase 2, and phase 3 by the simulator for the division ratio α when an error is applied to the resonant inductor Lr. Phase 1: Lr +10%, Phase 2: Lr +0%, Phase 3: Lr -10%.

The circuit constants are: input voltage Vin 380V, output voltage Vo 48V, output current 42A, output power Wo 2000W, turns ratio N=Np/Ns 4, magnetic inductance 285μH. In FIG. 3(a), the resonant inductance Lr is 25μH, and the resonant capacitor Cr is 200nF. In FIG. 3(b), the resonant inductance Lr is 50μH, and the resonant capacitor Cr is 100nF. In FIG. 3(c), the resonant inductance Lr is 100μH, and the resonant capacitor Cr is 50nF.

The split ratio α calculated by equation (18) is shown by the dotted line. In FIG. 3(a), the split ratio α is 0.399. In FIG. 3(b), the split ratio α is 0.559. In FIG. 3(c), the split ratio α is 0.723.

α matches well with the simulator results. Compared to the electrically floating neutral point α = 0, the Cr split ratio α obtained with equation (18) has better balance characteristics, demonstrating the validity of the calculation accuracy. In addition, α is expressed in a form that includes Lm, Lr, Cr, and Rac, so it is possible to find the optimal split ratio α even if an error occurs in any of the components.

Here, let us consider the physical meaning of the division ratio α. The division ratio α, which is expressed as the impedance ratio or impedance division ratio as in equation (16), is characterized by the addition of a minus sign. This indicates that α is derived from negative impedance. This shows that the magnitude of the phase current is reversed at a certain point as shown in Figure 3 depending on the circuit parameters and operating frequency of equation (2), and the intersection point at that time is the division ratio α that achieves current balance. The gain curves drawn by LLC converters with different circuit impedances are different, and therefore power is concentrated in the circuit with the high gain, resulting in an unbalanced state. As the current is concentrated, the circuit has a high Q, the gain decreases, and the rate of gain decrease increases, so there is a point where the circuit to which power is concentrated is switched. This is the division ratio α that achieves current balance. From this, it is possible that it is not possible to find a division ratio α that achieves current balance depending on the values of Lm, Lr, Cr, and Rac.

The state of α>1 is shown as a case in which it is not possible to obtain a split ratio α that achieves current balance. As Q increases, α approaches 1, making current balance more difficult. It is realistic to set Q and determine α at a heavy load. In order to suppress uneven component temperature rise, it is desirable to achieve current balance at a heavy load. For the above reasons, in order to achieve current balance, it is desirable for the split ratio α to be in the practical design range of 0.3≦α≦0.8, as shown in Figures 3(a) to (c).

In Reference 1, the neutral point α is in an electrically floating state at α = 0, and the current balance is maintained by the constraint that the total current at the neutral point is zero and by adjusting the voltage applied to the transformer according to the charge and discharge voltage of the resonant capacitor voltage connected to the neutral point.

In this invention, the circuit impedance divided by the capacitor becomes negative impedance, so the voltage applied to the transformer is adjusted to maintain current balance.

(Phase switching)
A three-phase LLC converter can easily switch phases by simply stopping the circuits. In Fig. 4(a), the control circuit 10 operates only the circuit of phase 1 and stops the two circuits of phases 2 and 3. When viewed from phase 1, the resonant capacitor network can be treated as one resonant capacitor Crphase1, as shown in Fig. 4(b). The resonant capacitor Crphase1 is expressed by equation (19).

The frequency increment Δf for the frequency equation (1) during three-phase operation is expressed by equation (20).

Figure 5 shows the frequency increment Δf versus division ratio α. When α = 0, the frequency becomes infinite. When α = 1, it is an independent LLC converter that is not connected to other phases. Here, as in Figure 3(b), it is divided into α = 0.5, CrN:Crg = 47nF:47nF, so it operates in single phase with a frequency increase of about 15% compared to three-phase operation.

Figure 6 is a diagram showing the detailed internal configuration of the control circuit 10. In the control circuit 10, the LLC-IC 101 serves as the control core, and a frequency divider 102, six OR circuits 103a-103e, and six AND circuits 104a-104e are used to generate three-phase gate signals Ho1-Ho3 (gate signals for Q11, Q21, and Q31) and Lo1-Lo3 (gate signals for Q12, Q22, and Q32).

In addition, the error amplifier 105 amplifies the error voltage between the output voltage Vo and the reference voltage, and the LLC-IC 101 generates a control signal consisting of a high level Ho and a low level Lo based on the error voltage from the error amplifier 105 so that the output voltage becomes a predetermined voltage, and outputs the control signal to the frequency divider 102 via the OR circuit 106.

The control circuit 10 has a burst function (intermittent operation) to increase the microload efficiency (light load efficiency). In addition, the frequency divider 102 generates gate signals for three phases.

When the voltage based on the current flowing through the output resistor Ro becomes equal to or greater than the reference voltage Vr, the hysteresis comparator 107 outputs a high level to the 2nd and 3rd phase AND circuits 104b, 104c, 104d, and 104e, and outputs a gate signal to the 2nd and 3rd phases, thereby performing a 3-phase operation. When the voltage based on the output current flowing through the output resistor Ro becomes less than the reference voltage Vr, the hysteresis comparator 107 outputs a low level to the 2nd and 3rd phase AND circuits 104b, 104c, 104d, and 104e, and performs a 1-phase operation of phase 1. In other words, the hysteresis comparator 107 switches from 3-phase operation to 1-phase operation when the output current is small under a light load, thereby improving efficiency. The hysteresis comparator 107 switches between 1-phase operation and 3-phase operation by performing a hysteresis operation based on the output current and the reference voltage.

When the output power reaches, for example, 422 W during single-phase operation, the operation is switched from single-phase operation to three-phase operation. In contrast, when the output power reaches 220 W during three-phase operation, a hysteresis operation is performed to switch from three-phase operation to single-phase operation. In other words, an output power of 422 W is a heavy load, and an output power of 220 W is a light load, so by performing hysteresis operation, switching between heavy and light loads can be performed efficiently.

Figure 7 shows the efficiency of the prototype with natural cooling versus output power. Figure 7 shows the efficiency curve of the prototype with natural cooling, with a DC voltage Vin of 380 V and an output voltage of 48 V.

The efficiency of the three-phase main circuit peaked at 99.57%. The overall efficiency, including the power of the controller and gate drive circuit, peaked at 99.41%. As shown by the efficiency of the main circuit, three-phase operation achieves a higher peak efficiency. Single-phase operation has the disadvantage of a larger magnetic path in the magnetic coupling transformer, but three-phase operation effectively reduces iron loss by canceling the magnetic flux of the magnetic components mentioned above. And, despite being 5W/cc, it is fanless and heat dissipation finless.

Figure 8 (a) shows the resonant current waveform in single-phase operation, and Figure 8 (b) shows the resonant current waveform in three-phase operation. Figure 9 (a) shows the resonant current and output voltage waveforms when switching from single-phase operation to three-phase operation. Figure 9 (b) shows the resonant current and output voltage waveforms when switching from three-phase operation to single-phase operation. As can be seen from Figures 8 (a), 8 (b), 9 (a), and 9 (b), the automatic current is shared well, and output voltage fluctuations are kept to around 1%.

Second Embodiment
The load using DC voltage may be used for applications requiring high voltage, such as a motor for an automobile, etc. The second embodiment is optimal for such applications.

10 is a circuit diagram of a three-phase LLC converter according to a second embodiment of the present invention. In the three-phase LLC converter according to the second embodiment shown in FIG. 10, the secondary sides of transformers T1, T2, and T3 are star-connected.
The series circuit of the capacitors C1 and C2 is connected to the output voltage Vo side, and one end of each of the secondary windings S1, S2, and S3 is connected to the connection end of the capacitors C1 and C2.

The series circuit of switches Q41 and Q42 is connected to the output voltage Vo side, and the other end of the secondary winding S1 is connected to the connection end of switches Q41 and Q42. The series circuit of switches Q51 and Q52 is connected to the output voltage Vo side, and the other end of the secondary winding S2 is connected to the connection end of switches Q51 and Q52. The series circuit of switches Q61 and Q62 is connected to the output voltage Vo side, and the other end of the secondary winding S3 is connected to the connection end of switches Q61 and Q62.

In addition, it is also possible to use only capacitor C1 or only capacitor C2.

The above configuration allows the control circuit 10 to switch between one-phase and three-phase operation.

In a conventional half-bridge LLC converter, the transformer has a primary winding Np and a secondary winding Ns, and the output voltage Vo=(Vin·Ns)/(2·Np).
As shown in FIG. 10, when the transformer secondary winding is star-connected in a three-phase multiphase operation, the switching square waves of the two phases are added together.
Vo={Vin·(Ns/2)·2}/(2·Np)
As a result, the number of turns of the secondary winding of the transformer of the conventional half-bridge LLC converter can be reduced to half.

In addition, when the two-phase and three-phase circuits are stopped under light load conditions, the capacitors C1 and C2 and the switches Q41 and Q42 perform double voltage rectification, and the output voltage Vo can be obtained in the same way as with three-phase operation.

In this way, with the three-phase LLC converter according to the second embodiment, when outputting a high voltage voltage, it is possible to miniaturize the transformer without increasing the number of turns on the secondary side, both under light and heavy loads.

In the first embodiment, the rectifier circuit of the secondary winding is a parallel connection of full-wave rectifiers using a center tap, but in the second embodiment, the full-wave rectifiers may be star-connected and at least one of the capacitors C1 and C2 may be connected.

Third Embodiment
11 is a circuit diagram of a star-connected split Cr of a three-phase LLC converter according to a third embodiment of the present invention. As shown in FIG. 11, for each of the resonant reactors Lr1, Lr2, and Lr3, the resonant capacitor Cr is divided into a first resonant capacitor (8/9)×Cr and a second resonant capacitor Cr/9. The three first resonant capacitors (8/9)Cr are star-connected.

By adjusting the division of the resonant capacitor Cr, it is possible to excite triple harmonic currents due to the interconnection of the three-phase LLC converter, suppress the current peaks of the elements, and reduce the effective current. For simplicity, the division of Cr for a star-connected three-phase LLC converter is shown in Figure 11.

The frequency of the resonant current of a three-phase LLC converter is expressed by the following equation.
fr=1/{2π(Lr×Cr) ^1/2 }
On the other hand, the neutral point has a power source that is synthesized and excites a triple frequency component as shown in Fig. 12. If the neutral point is considered as another power source, a resonant circuit of Cr and Lr connected to the power line can be seen. If Cr connected to the power line is set as follows, the triple resonant frequency is tuned.
3fr=3/{2π(Lr×Cr) ^1/2 }
= 1 / {2π (Lr × Cr / 9) ^{1 / 2} }
Therefore, the division ratio is 1/9. Therefore, in order to obtain a triple resonant frequency, the division ratio α is
It is preferable that 0.09<α≦0.13.

Therefore, to maintain the switching frequency fr, the capacitor connected to the neutral point should be 8 x Cr/9. Setting the capacitance of the capacitor in this way excites the third harmonic and reduces the peak current. As a result, as shown in Figure 13, the peaks of the primary resonant current and secondary rectified current are lowered, and the effective current can be reduced. The waveform shown by the thick solid line is when the third harmonic current is superimposed, and the waveform shown by the thin solid line is when the third harmonic current is not superimposed.

When the third harmonic current is superimposed, the copper loss in the resonant reactor Lr and the transformer winding current can be reduced.

Fourth Embodiment
Fig. 14 is a circuit diagram of a three-phase LLC converter according to a fourth embodiment. As shown in Fig. 14, the first resonant capacitor (1-α)Cr/3 is delta-connected. Specifically, one end of the first resonant capacitor (1-α)Cr/3 is connected to one end of a series circuit of the resonant reactor Lr and the primary winding of the transformer, and the other end is connected to one end of the first and second resonant capacitors of the adjacent phase.

The second resonant capacitor αCr is connected to one end of the first resonant capacitor (1-α)Cr/3, and the other end is connected to the common potential of the input.

When the resonant capacitor Cr is divided into a first resonant capacitor (1-α)Cr/3 and a second resonant capacitor αCr, the division ratio α is set to 0.3≦α≦0.8 based on the resonant circuit impedance and the load constant.

The capacitance of the delta-connected capacitor can be 1/3 of that of the star-connected capacitor, and the current flowing through the first resonant capacitor (1-α)Cr/3 is 1/√3, making it possible to reduce the size. In this case as well, it is possible to switch between one-phase operation and three-phase operation. The frequency during one-phase operation is expressed by equation (19), and the amount of frequency change is expressed by equation (20).

Figure 15 shows a circuit for obtaining a triple harmonic. In this case, the first resonant capacitor 8/9Cr shown in Figure 11 is further reduced to 1/3, so that the first resonant capacitor shown in Figure 15 is 8/27Cr.

As shown in Figure 16, a capacitor Cs with a capacitance of about tens to hundreds of times smaller than the Cr capacitor network is connected in a polygonal fashion (star connection) from the connection point of the Cr capacitor network. Capacitor Cs is connected to the power line via a detection resistor Rs with a capacitance of tens to hundreds of ohms. This allows phase abnormalities to be detected.

As shown in FIG. 17, when the three phases are operating normally, the Rs voltage generated across the detection resistor Rs by combining the three-phase resonant currents is small. However, when a phase abnormality such as a missing phase occurs, the balance of the three-phase resonant currents is lost and a large voltage is generated across the detection resistor Rs. This makes it possible to immediately detect the occurrence of a phase abnormality on a pulse-by-pulse basis.

Fifth Embodiment
When the input range is wide, particularly when the input voltage is low, or when the output voltage is increased, the boost period for charging the resonant capacitor increases, and the frequency decreases.

In other words, the peak current of the output current increases, and the effective current value also increases. This can result in a loss of switch current margin or an increase in the effective current value at low input voltages, resulting in a decrease in efficiency.

Figure 18 is a circuit diagram of a three-phase LLC converter according to the fifth embodiment. The three-phase LLC converter according to the fifth embodiment differs from the three-phase LLC converter according to the first embodiment shown in Figure 1 in the following configuration.

The first resonant capacitor (1-α) Cr has one end connected to one end of a series circuit formed by the resonant reactors Lr1 to Lr3 and the primary windings P1 to P3 of the transformers T1 to T3, and the other end connected to the first neutral point K common to each phase.

The second resonant capacitor βCr is connected to one end of the first resonant capacitor (1-α)Cr and the other end is connected to ground.

The third resonant capacitor (α-β) Cr is connected to one end of the first resonant capacitor (1-α) Cr, and the other end is connected to the second neutral point M common to each phase and one end of the third switch SW. The other end of the third switch SW is connected to ground.

The anode of diode D is connected to the second neutral point M, and the cathode of diode D is connected to the positive electrode of the DC power supply Vin. Diode D functions as a regenerative diode when switch SW is off.

When the resonant capacitor Cr is divided into a first resonant capacitor (1-α) Cr, a second capacitor (α-β) Cr, and a third resonant capacitor βCr, the division ratio α is set based on the resonant circuit impedance and the load constant.

The value of β is not specified. It is sufficient that β>0. In other words, the sum of the resonant capacitor (1-α)Cr, the second capacitor (α-β)Cr, and the third resonant capacitor βCr is 1.

The control circuit 11 includes a first control unit 12 and a second control unit 13. The first control unit 12 has the same functions as the control circuit 10 shown in FIG. 1, and alternately turns on and off the switches Q11, Q21, and Q31 and the switches Q12, Q22, and Q32.

The second control unit 13 is connected to the positive electrode of the DC power supply Vin and turns the third switch SW on and off depending on the input voltage of the DC power supply Vin.

The three-phase LLC converter according to the fifth embodiment configured in this manner allows harmonic currents to flow through the first neutral point K and the second neutral point M, thereby reducing the peak value of the resonant current. Specifically, the third switch SW is turned on at low input voltages when the peak value of the resonant current is large, and the third switch SW is turned off at other input voltages. That is, the second control unit 13 can maintain high efficiency over a wide input range by turning the third switch SW on and off depending on the input voltage of the DC power supply Vin.

Sixth Embodiment
Fig. 19 is a circuit diagram of a three-phase LLC converter according to a sixth embodiment. The three-phase LLC converter according to the sixth embodiment is a combination of the three-phase LLC converter according to the fourth embodiment shown in Fig. 14 and the three-phase LLC converter according to the fifth embodiment shown in Fig. 18.

The three-phase LLC converter according to the sixth embodiment provides the effects of the three-phase LLC converter according to the fourth embodiment and the effects of the three-phase LLC converter according to the fifth embodiment shown in FIG. 18.

In other words, the capacitance of a delta-connected capacitor can be 1/3 of that of a star-connected capacitor, and the current flowing through the first resonant capacitor (1-α) Cr/3 is 1/√3, making it possible to reduce the size.

In addition, high efficiency can be maintained over a wide input range by turning the third switch SW on and off depending on the input voltage of the DC power supply Vin.

Vin DC power supplies Q11, Q12, Q21, Q22, Q31, Q32 Switch SW Switches (1-α) Cr1, (1-α) Cr2, (1-α) Cr3 First resonant capacitors αCr1, αCr2, αCr3 Second resonant capacitors Lr1, Lr2, Lr3 Resonant reactors T1, T2, T3 Transformers P1, P2, P3 Primary windings S1a, S1b, S2a, S2b, S3a, S3b Secondary windings D1a, D1b, D2a, D2b, D3a, D3b, D Diode Ro Output resistor 10 Control circuit 101 LLC-IC
102 Frequency dividers 103a to 103f, 106 OR circuits 104a to 104f AND circuits 107 Hysteresis comparator 105 Error amplifier

Claims (8)

a switch circuit in which a first switch and a second switch are connected in series to both ends of a DC power supply;
a resonant circuit including a resonant reactor, one end of which is connected to a connection end to which the first switch and the second switch are connected, a first resonant capacitor, a second resonant capacitor, and a primary winding of a transformer;
a control circuit that alternately turns on and off the first switch and the second switch;
a rectifying and smoothing circuit that rectifies and smoothes the voltage of the secondary winding of the transformer, and three LLC resonant converters are provided that are operated in three phases having a phase difference of 120°;
the first resonant capacitor has one end connected to one end of a series circuit of the resonant reactor and the primary winding of the transformer, and the other end connected to a neutral point common to each phase;
the second resonant capacitor is connected to one end of the first resonant capacitor and to a common potential of the input;
A three-phase LLC resonant converter characterized in that, when a resonant capacitor Cr is divided into the first resonant capacitor (1-α) Cr and the second resonant capacitor α Cr, a division ratio α is set to 0.3≦α≦0.8 based on a resonant circuit impedance and a load constant. The three-phase LLC resonant converter according to claim 1, characterized in that the control circuit switches from three-phase operation to one-phase operation when the load is light. The three-phase LLC resonant converter according to claim 2, characterized in that the control circuit is provided with a hysteresis comparator that switches between the one-phase operation and the three-phase operation by performing a hysteresis operation based on the output current and a reference voltage. a switch circuit in which a first switch and a second switch are connected in series to both ends of a DC power supply;
a resonant circuit including a resonant reactor, one end of which is connected to a connection end to which the first switch and the second switch are connected, a first resonant capacitor, a second resonant capacitor, and a primary winding of a transformer;
a control circuit that alternately turns on and off the first switch and the second switch;
a rectifying and smoothing circuit that rectifies and smoothes the voltage of the secondary winding of the transformer, and three LLC resonant converters are provided that are operated in three phases having a phase difference of 120°;
the first resonant capacitor has one end connected to one end of a series circuit of the resonant reactor and the primary winding of the transformer, and the other end connected to a neutral point common to each phase;
the second resonant capacitor is connected to one end of the first resonant capacitor and to a common potential of the input;
A three-phase LLC resonant converter characterized in that, when a resonant capacitor Cr is divided into the first resonant capacitor (1-α) Cr and the second resonant capacitor α Cr, a division ratio α is set based on a resonant circuit impedance and a load constant, a harmonic current is passed through the neutral point, and a peak value of the resonant current is reduced. The three-phase LLC resonant converter described in claim 4, characterized in that the harmonic current flowing through the neutral point is three times, and the division ratio is 0.09<α≦0.13. a switch circuit in which a first switch and a second switch are connected in series to both ends of a DC power supply;
a resonant circuit including a resonant reactor, one end of which is connected to a connection end to which the first switch and the second switch are connected, a first resonant capacitor, a second resonant capacitor, and a primary winding of a transformer;
a control circuit that alternately turns on and off the first switch and the second switch;
a rectifying and smoothing circuit that rectifies and smoothes the voltage of the secondary winding of the transformer, and three LLC resonant converters are provided that are operated in three phases having a phase difference of 120°;
the first resonant capacitor has one end connected to one end of a series circuit of the resonant reactor and a primary winding of the transformer, and the other end connected to one end of the first and second resonant capacitors of an adjacent phase;
the second resonant capacitor is connected to one end of the first resonant capacitor and to a common potential of the input;
A three-phase LLC resonant converter characterized in that, when a resonant capacitor Cr is divided into the first resonant capacitor (1-α)Cr/3 and the second resonant capacitor αCr, a division ratio α is set to 0.3≦α≦0.8 based on a resonant circuit impedance and a load constant. a switch circuit in which a first switch and a second switch are connected in series to both ends of a DC power supply;
a resonant circuit including a resonant reactor, one end of which is connected to a connection end to which the first switch and the second switch are connected, a first resonant capacitor, a second resonant capacitor, a third resonant capacitor, and a primary winding of a transformer;
a control circuit that alternately turns on and off the first switch and the second switch;
a rectifying and smoothing circuit that rectifies and smoothes the voltage of the secondary winding of the transformer, and three LLC resonant converters are provided that are operated in three phases having a phase difference of 120°;
the first resonant capacitor has one end connected to one end of a series circuit of the resonant reactor and the primary winding of the transformer, and the other end connected to a first neutral point common to each phase;
the second resonant capacitor is connected to one end of the first resonant capacitor, and the other end is connected to a second neutral point common to each phase and to one end of a third switch;
the third resonant capacitor is connected to one end of the first resonant capacitor and to a common potential of the input, and the other end of the third switch is connected to the common potential of the input;
When the resonant capacitor Cr is divided into the first resonant capacitor (1-α) Cr, the second resonant capacitor (α-β) Cr, and a third resonant capacitor βCr, division ratios α and β are set based on a resonant circuit impedance and a load constant;
The three-phase LLC resonant converter according to claim 1, wherein the control circuit turns the third switch on and off in response to an input voltage of the DC power source. a switch circuit in which a first switch and a second switch are connected in series to both ends of a DC power supply;
a resonant circuit including a resonant reactor, one end of which is connected to a connection end to which the first switch and the second switch are connected, a first resonant capacitor, a second resonant capacitor, a third resonant capacitor, and a primary winding of a transformer;
a control circuit that alternately turns on and off the first switch and the second switch;
a rectifying and smoothing circuit that rectifies and smoothes the voltage of the secondary winding of the transformer, and three LLC resonant converters are provided that are operated in three phases having a phase difference of 120°;
the first resonant capacitor has one end connected to one end of a series circuit of the resonant reactor and a primary winding of the transformer, and the other end connected to one end of the first and second resonant capacitors of an adjacent phase;
the second resonant capacitor is connected to one end of the first resonant capacitor, and the other end is connected to a neutral point common to each phase and to one end of a third switch;
the third resonant capacitor is connected to one end of the first resonant capacitor and to a common potential of the input, and the other end of the third switch is connected to the common potential of the input;
When the resonant capacitor Cr is divided into the first resonant capacitor (1-α)Cr/3, the second resonant capacitor (α-β)Cr, and the third resonant capacitor βCr, division ratios α and β are set based on a resonant circuit impedance and a load constant;
The three-phase LLC resonant converter according to claim 1, wherein the control circuit turns the third switch on and off in response to an input voltage of the DC power source.

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