Active-clamp current-source push-pull DC-DC converter

[0001] This application claims the benefit of Korean Patent Application No. 10-2006-0035324, filed on Apr. 19, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

[0002] The present invention relates to an active-clamp current-source push-pull DC-DC converter, and more particularly, to a current-source push-pull DC-DC converter comprising the diodes on a secondary side of a transformer to be zero-current switched using a series-resonant full-wave rectifier, and the active clamp circuit on a primary side of the transformer providing a discharge path of the energy stored in the leakage inductances to reuse energy of leakage inductance.

[0003] The present invention also relates to an active-clamp current-source push-pull DC-DC converter increasing the power conversion efficiency of the current-source push-pull DC-DC converter even for a wide input voltage range and reducing a switch voltage stress as compared to a conventional current-source push-pull circuit by operating even for a duty ratio below 0.5 by flowing a current of an input inductor through capacitors of the active clamp circuit when both main switches are off.

[0004] As well known to those of ordinary skill in the art, when an input voltage is low and an output voltage is high for 1˜2 KW DC-DC converters, a current-source push-pull DC-DC converter illustrated in FIG. 1 is generally used to attain a high efficiency. An operation of the current-source push-pull DC-DC converter illustrated in FIG. 1 will now be described.

[0005] Two switches S₁ and S₂ have the same duty ratio and a phase difference of half a period. The duty ratio is T_on/T_s, where T_s denotes a switching time and T_on denotes a conduction time. Since the duty ratio of the switches S₁ and S₂ is always greater than 0.5, a state where all of the switches S₁ and S₂ are off does not exist. When all the switches S₁ and S₂ are on, a voltage over a primary winding of a transformer T₁ is 0, and energy is stored in an input inductor L. When only one of the switches S₁ and S₂ is on, an input voltage V_i and the energy stored in the input inductor L is transferred to the secondary side of the transformer T₁. This is similar to an operation of a boost converter, ripples of an input current are few, and the conduction time of the switches S₁ and S₂ are long, thereby having a small switch conduction loss. Thus, the conventional current-source push-pull DC-DC converter illustrated in FIG. 1 is suitable when the input voltage V_i is low and the input current is high. In FIG. 1, it will be understood by those of ordinary skill in the art that C_d denotes a capacitor for removing an AC component, which can be included in the DC input voltage V_i, C_o denotes a capacitor for removing an AC component, which can be included in an output voltage Vo, and D₁ and D₂ denote diodes for constructing a center-tap full-wave rectifier with a secondary winding of the transformer T₁. Thus, detailed description of the components C_d, C_o, D₁ and D₂ is omitted in the present specification.

[0006] However, since the duty ratio of the switches S₁ and S₂ is always greater than 0.5 in the conventional current-source push-pull DC-DC converter, a period of time in which the switches S₁ and S₂ are on at the same time exists. Thus, a range of the use of the duty ratio is narrow, and thereby, a maximum voltage over the switches S₁ and S₂ is higher than the maximum input voltage, and it is difficult for the conventional current-source push-pull DC-DC converter to operate for wide input range. In addition, a passive snubber circuit mainly used to prevent a switching component from being damaged due to energy stored in a leakage inductance of the transformer T₁ reduces the converter power conversion efficiency.

[0007] The present invention provides a current-source push-pull DC-DC converter comprising the diodes on a secondary side of a transformer to be zero-current switched using a series-resonant full-wave rectifier, and the active clamp circuit on a primary side of the transformer providing a discharge path of the energy stored in the leakage inductances to reuse energy of leakage inductance.

[0008] The present invention also provides a current-source push-pull DC-DC converter using an active clamp circuit for increasing a power conversion efficiency of the converter even for a wide input voltage range and reducing a switch voltage stress as compared to a conventional current-source push-pull circuit by operating even for a duty ratio below 0.5 by flowing a current of an input inductor through capacitors of the active clamp circuit when both main switches are off.

[0009] According to an aspect of the present invention, there is provided a current-source push-pull DC-DC converter comprising a primary circuit and a secondary circuit divided by a transformer, wherein: the primary circuit, which is a current-source push-pull circuit, comprises an input inductor L, two main switches S₁ and S₂, and an active clamp circuit comprising two sub-switches S₃ and S₄ and clamp capacitors C_c1 and C_c2; and the secondary circuit is a full-wave output rectifier.

[0010] The full-wave output rectifier may be a series-resonant full-wave rectifier and comprise two diodes D₁ and D₂ and resonant capacitors C_r1 and C_r2.

[0011] The full-wave output rectifier may be a center-tap full-wave rectifier and comprise two diodes D₁ and D₂.

[0018] The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. However, the terminology described below is defined considering functions in the present invention and may vary according to a user or application. Thus, the definitions should be understood based on all the contents of the specification.

[0019] In the description below, if the elements in the present invention are the same as elements in the prior art, reference numerals used in the prior art are used as present in the prior art, and their detailed description is omitted.

[0020] FIG. 1 is a circuit diagram of a conventional current-source push-pull DC-DC converter.

[0021] Referring to FIG. 1, the active-clamp current-source push-pull DC-DC converter according to the present invention solves the problems of the conventional current-source push-pull DC-DC converter illustrated in FIG. 1 by applying an active clamp circuit to a primary side of a transformer.

[0022] FIG. 2 is a circuit diagram of an active-clamp current-source push-pull DC-DC converter according to an embodiment of the present invention.

[0023] Referring to FIG. 2, a primary side of the active-clamp current-source push-pull DC-DC converter, i.e., a primary side of a transformer T, includes an input inductor L, two main switches S₁ and S₂, and an active clamp circuit 110. A circuit forming the primary side of the transformer T is an active-clamp current-source push-pull circuit 100. The active clamp circuit 110 included in the active-clamp current-source push-pull circuit 100 includes two sub-switches S₃ and S₄ and clamp capacitors C_c1 and C_c2.

[0024] A secondary side of the active-clamp current-source push-pull DC-DC converter is a series-resonant full-wave rectifier 200 and includes two diodes D₁ and D₂ and resonant capacitors C_r1 and C_r2.

[0025] The transformer T electrically isolates the primary side and the secondary side of the active-clamp current-source push-pull DC-DC converter. An output voltage V_o of the active-clamp current-source push-pull DC-DC converter is adjusted by adjusting duty ratios of the main switches S₁ and S₂ by being fed back to an output voltage control circuit 300, which is widely known to those of ordinary skill in the art.

[0026] FIGS. 3A and 3B are circuit diagrams of a center-tap full-wave rectifier 200a and a full-bridge full-wave rectifier 200b, which can be used on the secondary side of the active-clamp current-source push-pull DC-DC converter illustrated in FIG. 2, according to an embodiment of the present invention. The series-resonant full-wave rectifier 200 on the secondary side of the active-clamp current-source push-pull DC-DC converter illustrated in FIG. 2 can be replaced by the center-tap full-wave rectifier 200a or the full-bridge full-wave rectifier 200b, each having a different characteristic as described below.

[0027] The center-tap full-wave rectifier 200a illustrated in FIG. 3A has a structure including a minimum number of elements D₁ and D₂. However, since a voltage stress applied to each of the diodes D₁ and D₂ is high, the center-tap full-wave rectifier 200a is suitable when the output voltage V_o is relatively low.

[0028] The full-bridge full-wave rectifier 200b illustrated in FIG. 3B has a structure including four diodes D₁, D₂, D₃, and D₄. Thus, a voltage applied to each of the diodes D₁, D₂, D₃ and D₄ is very low. However, since more diodes are included in the full-bridge full-wave rectifier 200b, the full-bridge full-wave rectifier 200b is suitable when the output voltage V_o is high.

[0029] The series-resonant full-wave rectifier 200 used on the secondary side of the active-clamp current-source push-pull DC-DC converter illustrated in FIG. 2 has a relatively simple structure. Thus, a voltage applied to each diode of the series-resonant full-wave rectifier 200 is equal to the output voltage V_o, and each diode is zero-current switched, thereby reducing a loss due to a diode reverse recovery current. Accordingly, the series-resonant full-wave rectifier 200 illustrated in FIG. 2 has a very high power conversion efficiency for a converter in which relatively high power conversion is required for a high output voltage.

[0030] FIGS. 4A and 4B illustrate operational waveforms of the active-clamp current-source push-pull DC-DC converter illustrated in FIG. 2 in a non-overlapping mode and in an overlapping mode, according to an embodiment of the present invention.

[0031] Referring to FIGS. 2, 4A, and 4B, the main switch S₁ and a sub-switch S₃ are complementarily operated by gate driving signals V_gs1 and V_gs3 in a pre-set switching period T_s. The main switch S₂ and a sub-switch S₄ are operated the same as the main switch S₁ and the sub-switch S₃. The main switches S₁ and S₂ have the same duty ratio (D=T_on/T_s) and have a phase difference of half a period. In FIGS. 4A and 4B, V_gs2 and V_gs4 denote gate driving signals of the main switch S₂ and the sub-switch S₄, respectively.

[0032] Unlike the conventional current-source push-pull DC-DC converter illustrated in FIG. 1, the active-clamp current-source push-pull DC-DC converter according to an embodiment of the present invention operates even when the duty ratio of the main switches S₁ and S₂ is below 0.5. That is, when the duty ratio of the main switches S₁ and S₂ is below 0.5, the active-clamp current-source push-pull DC-DC converter operates in the non-overlapping mode as illustrated in FIG. 4A, and when the duty ratio of the main switches S₁ and S₂ is greater than 0.5, the active-clamp current-source push-pull DC-DC converter operates in the overlapping mode as illustrated in FIG. 4B. In general, the active-clamp current-source push-pull DC-DC converter operates in the overlapping mode when an input voltage V_i is low and in the non-overlapping mode when the input voltage V_i is high, and both modes are automatically changed.

[0033] Referring to FIG. 4A, when the active-clamp current-source push-pull DC-DC converter operates in the non-overlapping mode according to an embodiment of the present invention while one of the main switches S₁ and S₂ is on, energy is charged in the input inductor L and the clamp capacitor C_c1 or C_c2, and leakage inductances L_ik1 and L_ik2 of the transformer T, and the resonant capacitors C_r1 and C_r2 of the series-resonant full-wave rectifier 200 are series-resonant, thereby transferring energy to the secondary side of the transformer T. When both of the main switches S₁ and S₂ are off, a current, which had flowed through the input inductor L and the leakage inductances L_ik1 and L_ik2 of the transformer T, flows through the clamp capacitors C_c1 and C_c2 resulting in energy not being transferred from the primary side to the secondary side of the active-clamp current-source push-pull DC-DC converter.

[0034] A current i_L flowing through the input inductor L is a ripple current having a frequency corresponding to twice a switching frequency f_s, and in general, this high frequency reduces an inductance of the input inductor L. The currents i_D1 and i_D2 flowing through the diodes D₁ and D₂, respectively, reach a zero current due to a series-resonance before the main switches S₁ and S₂ are off. This indicates that the diodes D₁ and D₂ are zero-current switched, and thus, a switching loss due to the diode reverse recovery characteristic does not exist.

[0035] Referring to FIG. 4B, when the active-clamp current-source push-pull DC-DC converter operates in the overlapping mode according to an embodiment of the present invention while both of the main switches S₁ and S₂ are on, energy is charged in the input inductor L, and the inductor current i_L is divided and flows through the main switches S₁ and S₂. In this case, energy is not transferred from the primary side to the secondary side of the active-clamp current-source push-pull DC-DC converter. When one of the main switches S₁ and S₂ is on and the other one is off, the clamp capacitor c₁ or c₂, the leakage inductances L_ik1 and L_ik2 of the transformer T, and the resonant capacitors C_r1 and C_r2 of the series-resonant full-wave rectifier 200 are series-resonant, thereby transferring energy to the secondary side of the transformer T.

[0036] The inductor current i_L illustrated in FIG. 4B is a ripple current having a frequency corresponding to twice the switching frequency f_s. The output diode currents i_D1 and i_D2 illustrated in FIG. 4B reach a zero current due to a series-resonance before the main switches S₁ and S₂ are off. This indicates that the diodes D₁ and D₂ are zero-current switched, and thus a switching loss due to the diode reverse recovery characteristic does not exist.

[0037] As described above, the active-clamp current-source push-pull DC-DC converter according to an embodiment of the present invention, which is illustrated in FIG. 2, can reuse energy of the leakage inductances L_ik1 and L_ik2 by the active clamp circuit 110 on the primary side providing a discharge path of the energy stored in the leakage inductances L_ik1 and L_ik2, and includes the diodes D₁ and D₂ on the secondary side being zero-current switched using the series-resonant full-wave rectifier 200. In addition, since the current i_L of the input inductor L can flow through the clamp capacitors C_c1 and C_c2 of the active clamp circuit 110 when both of the main switches S₁ and S₂ are off, the active-clamp current-source push-pull DC-DC converter, according to an embodiment of the present invention, operates even for a duty ratio below 0.5, thereby having a high power conversion efficiency even for a wide input voltage range and having a lower switch voltage stress than the conventional current-source push-pull DC-DC converter.

[0038] In the active clamp circuit 110 of FIG. 2, the clamp capacitors C_c1 and C_c2 can be combined into a single clamp capacitor shared by the sub-switches S₃ and S₄, and the single clamp capacitor operates the same even if the single clamp capacitor is connected to a voltage source. In addition, as shown in sub-switch current waveforms of FIGS. 4A and 4B, since the sub-switches S₃ and S₄ have a small amplitude of currents and are zero-voltage switched, a small capacity of switch can be used for the sub-switches S₃ and S₄.

[0039] As described above, in an active-clamp current-source push-pull DC-DC converter according to embodiments of the present invention, the energy of leakage inductances can be reused by not only diodes on a secondary side of a transformer being zero-current switched using a series-resonant full-wave rectifier but also an active clamp circuit on a primary side of the transformer providing a discharge path of the energy stored in the leakage inductances.

[0040] In addition, by operating even for a duty ratio below 0.5 by flowing a current of an input inductor through capacitors of the active clamp circuit when both main switches are off, a power conversion efficiency can be increased even for a wide input voltage range, and a switch voltage stress can be reduced compared to a conventional current-source push-pull circuit.

[0041] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.