Zero voltage zero current three level DC-DC converter

[0002] This application claims priority to U.S. Provisional Application Serial No. 60/152,953, filed on Sep. 9, 1999, the entire contents of which are herein incorporated by reference.

[0003] 1. Field of the Invention

[0004] The present invention generally relates to dc-dc converters and, more particularly, to a three-level dc-dc converter which uses an auxiliary voltage source and flying capacitor to reduce the voltage stress across the power switches, improves the efficiency by reducing the circulating energy in the converter, and achieves zero voltage, zero current switching (ZVZCS).

[0005] 2. Description of the Prior Art

[0006] DC-DC power converters are required for many modern day applications. For example, such converters are required for high voltage, high power applications such as used in telecommunications, battery chargers, and uninterruptible power supplies with power factor correction.

[0007] The Power Factor Correction (PFC) stage is important stage for the future generation of three phase distributed power systems due to the IEC61000-3-2 Class A standard. The three-phase boost rectifier is an interesting option for this PFC stage, since it can easily comply with the aforementioned standard with simplicity, efficiency, reliability, and low cost. However, in order to reduce the harmonic distortion in this converter, its input to output voltage characteristic has to be increased. Therefore, the dc/dc step down second stage converter usually has to support voltage stress up to 800 Volts, and the need for high voltage devices is required. Unfortunately, these high voltage devices present a poor behavior in terms of conduction and switching losses, and also have a higher cost.

[0008] The so-called Zero Voltage Switching (ZVS) Three-Level (TL) dc/dc converters and the Dual Bridge dc/dc converter have overcome this problem. These dc/dc converters reduce the voltage stress across the power switch to half of the input voltage. Hence, power devices with better characteristics can be used. Also, these converters achieve zero voltage switching for the primary switches, which is a necessary characteristic in order to increase the efficiency in the converter.

[0009] Examples of three-level ZVS-PWM converters are presented by J. Renes Pinheiro et al., Three Level ZVS PWM Converter A New Concept in DC-to-DC Conversion, IEEE IECON Record, 1992, and J. Renes Pinheiro et al., Wide Load Range Three-Level ZVS-PWM DC-to-DC Converter, IEEE PESC Record, 1993.

[0010] FIG. 1 shows a prior art three level ZVS-PWM converter as discussed above. It comprises a main computation leg formed by four MOSFET switches (M₁, M₂, M₃, and M₄), four diodes (D₁, D₂, D₃, and D₄), and four capacitors (C₁, C₂, C₃, and C₄). The diodes D₁-D₄and the capacitors C₁-C₄are intrinsic elements of the MOSFET switches M₁-M₄. L_ris a commutation inductor which is comprised of an external inductor plus the leakage inductor of the isolation transformer T_r. The capacitors C_a1and C_a2and the inductor L_aform a commutation auxiliary circuit. The output stage is formed by rectifiers D_r1, and D_r2and by an output filter formed by L_rand C_f. R_ois the load and D_c1, and D_c2, are clamping diodes.

[0011] In order to insure the commutation ZVS at a wide load range , it is necessary to have enough energy to charge and discharge the intrinsic capacitances C₁-C₄. The switches of the converter operate under different conditions of commutation. The commutation of the outer switches M₁and M₄always takes place when the current through the primary transformer is equal to the output current reflected to the primary. Since the filter inductance current reflected to the primary is large, even for small primary current, such as the transformer magnetizing current, it is sufficient to perform the commutation. On the other hand, the inner switches M₁and M₃commutate when the transformer is short-circuited by the output rectifier. Thus, it is necessary to use an auxiliary inductor L_ato perform the commutation, since the resonant inductor L_ralone does not provide enough stored energy.

[0012] As can be seen from above, one of the drawbacks that these prior art converters present is the use of the energy stored in the transformer leakage inductance to achieve zero voltage switching for two of the switches. Hence, an additional resonant inductance has to be included in order to assure ZVS for almost the entire load range. This in turn increases the circulating energy, which increases the conduction losses in the converter. In addition, both a reduction in the effective duty cycle and a severe secondary parasitic ringing occur.

[0013] It is therefore an object of the present invention to provide a Three-level (TL) DC-DC converter that overcomes the drawbacks presented by the Zero Voltage Switching (ZVS). Three-level converter, such as high circulating energy, severe parasitic ringing on the rectifier diodes, and limited ZVS load range for the inner switches.

[0014] According to the invention, a three-level DC-to-DC converter is provided having zero-voltage and zero-current switching (ZVZCS). A primary (input) stage and a secondary (output) stage are separated by an insolation transformer. Four commutation switches on the primary side are connected in cascade fashion and are switched by phase-shift control. An input dc voltage source is connected between the first and fourth switches. Two capacitors are also connected between the first and fourth commutation switches and function to split the input voltage in half. The primary side of the isolation transformer is connected at one end between the two capacitors and at its second end between the second and the third commutation switches. Two clamping diodes are also provided; one between the first and second switches and the two capacitors and the other between the third and fourth switches and the and the two capacitors. A flying capacitor is connected at one end between the first and second commutator switches and at a second end between the third and fourth commutator switches.

[0015] The output stage comprises two rectifiers connected to either end tap of the secondary side of the isolation transformer and an output filter comprising an a resistor and capacitor connected at one end to the rectifiers and at a second end to the center tap of the isolation transformer. An auxiliary voltage source is additionally added between the rectifiers and the center tap of the isolation transformer.

[0016] In operation, the positive and negative charges on the plates of the flying capacitor constantly switch sides and the as the first and second, and third and fourth commutation switches switch on and off. In this manner the primary side achieves Zero Voltage Switching (ZVS) for the outer commutation switches. The flying capacitor is an important element to achieve ZVS for the switches. This flying capacitor provides a trajectory for the current in order to charge and discharge the parasitic capacitance of the switches S1 and S2, respectively. Also, this flying, capacitor allows operation of the converter with a phase-shift control. Without this flying capacitor, the zero voltage operation would not be achieved since the outer switches would be turned on with ¼ of the input voltage.

[0017] In addition, during freewheeling (i.e., when no power is being transferred from the primary side to the secondary side), the auxiliary power source is switched on to provide a voltage which is reflected back to the primary side in order to eliminate the circulating energy and also to achieve Zero Current Switching (ZCS) for the commutation switches. Of course, the proposed converter would operate just with ZVS if the auxiliary voltage source is not provided.

[0018] The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

[0019] FIG. 1 is a circuit diagram of a prior art three-level DC-to-DC converter:

[0020] FIG. 2A is a circuit diagram of the three-level DC-to-DC converter having flying capacitor and auxiliary secondary side voltage source according to the present invention;

[0021] FIG. 2B is circuit diagram showing an alternate arrangement for the output stage shown in FIG. 2A;

[0022] FIGS. 3A-3D are circuit diagrams of the three-level DC-to-DC converter shown in FIG. 2A where the alternate voltage source is realized by an auxiliary active circuit, first and second regenerative snubbers, and a saturable reactor and capacitor on the primary side, respectively;

[0023] FIGS. 4A-4B are the circuit diagram of the three-level DC-to-DC converter according to the present invention along with its principle waveform diagram, respectively.

[0024] Referring now to the drawings, and more particularly to FIG. 2A, there is shown the proposed Zero Voltage and Zero Current Switching (ZVZCS) Three-Level Dc/Dc converter according to the present invention. The converter uses a phase shift control with a flying capacitor, Css, in the primary side to achieve Zero Voltage Switching (ZVS) for the outer switches. Additionally, the converter uses an auxiliary voltage source, Vaux, to reset the primary current during the freewheeling stage in order to eliminate the circulating energy and also to achieve Zero Current Switching for the inner switches.

[0025] A primary (input) side 10 and a secondary (output) side 12 are separated by an insolation transformer 14. Four commutation switches (S₁, S₂, S₃, and S₄) on the primary side 10 are connected in cascade fashion and are switched by phase-shift controller 16. An input de voltage source V_inis connected between the first and fourth switches, S₁and S₄. Two capacitors C_in1, and C_in2are also connected between the first and fourth commutation switches, S₁and S₄and function to split the input voltage V_inin half. The primary side of the isolation transformer 14 is connected at one end between the two capacitors, C_in1and C_in2, and at its second end between the second and the third commutation switches, S₂and S₃. Two clamping diodes, D_c1, and D_c2, are also provided. Dc1 is connected between the first and second switches (S₁and S₂) and the two capacitors (C_in1and C_in2). D_c2is connected between the third an fourth switches (S₃and S₄) and the and the two capacitors (C_in1and C_in2). A flying capacitor C_ssis connected at one end between the first and second commutator switches (S₁and S₂) and at a second end between the third and fourth commutator switches (S₃and S₄).

[0026] The output stage 12 comprises two rectifiers, D_r1and D_r2, connected to either end tap of the secondary side of the isolation transformer 14 and an output filter comprising an a capacitor C_outand an inductor L_outconnected at one end to the rectifiers, D_r1and D_r2, and at a second end to the center tap 18 of the isolation transformer 14. The output load is represented by R_out. An auxiliary voltage source V_auxis additionally added between the rectifiers, D_r1and D_r2and the center tap 18 of the isolation transformer 14. FIG. 2B shows an alternate arrangement for the output stage using a non-center tapped transformer 14′ and four rectifying diodes D_r.

[0027] The auxiliary voltage source V_auxcan be implemented in different ways. FIGS. 3A-D shows the implementation of the proposed converter with an auxiliary active circuit (3A), first and second Regenerative Snubbers (3B) and (3C), and a Saturable Reactor and capacitor (3D).

[0028] In FIGS. 3A-D, like devices are given like reference numerals as those shown in FIG. 2A and therefore the detailed description is omitted. In FIG. 3A the auxiliary voltage source is shown as an active clamp comprising a clamp switch S_auxand capacitor C_aux. The capacitor C_auxstores a charge voltage which provides the auxiliary voltage source when the switch S_auxis closed by the controller 16.

[0029] In FIG. 3B, the auxiliary voltage source V_auxis provided by a regenerative snubber circuit comprising a capacitor C_rsand two additional rectifier diodes D_rs1and D_rs2. In the previous explanation, the primary current is reset by adding an auxiliary active circuit in the secondary side, which provides the necessary voltage source to reset the primary current during the freewheeling stage when S_auxis turned on. In this way, the ZVZCS condition for the primary switches and the active clamping of the voltage across the secondary diodes are achieved.

[0030] In FIG. 3B, the auxiliary voltage source V_auxis implemented with the capacitor C_rsand the rectifiers D_rs1and D_rs2. Its operating principle is similar to that one with the active auxiliary circuit. That is, when S₁or S₄is turned off, the capacitor C_rsworks as a temporary voltage source to reset the primary freewheeling current. That is, when S₁is turned off the reflected filter inductor current charges the parasitic capacitance of S₁and discharges the parasitic capacitance of S₄through the auxiliary flying capacitor C_ss. This action continues until the rectifier voltage in the secondary equals the auxiliary capacitor voltage V_crs, and then D_aux2is turned on. Then, a resonance between the leakage inductance and the parasitic capacitances of S₁and S₄takes place until the clamp diode D_c1and the anti-parallel diode of the switch S₄are turned on. During this time, the difference between the filter inductor current and the reflected leakage inductance current discharges C_rs. Once the anti-parallel diode of S₄starts conducting, S₄can be tamed on with zero voltage. The primary current freewheels through the clamp diode D_c1and S₂. The voltage across the auxiliary capacitor is reflected to the primary and applied to the leakage inductance. In this way, the primary current is reduced to zero. Also, the auxiliary clamp capacitor supplies energy to the output. This action ends when the primary current is reset to zero. In this case, this temporary voltage source is not controllable, which means that in every half period, the energy in the capacitor C_rsis discharged completely. Moreover, during the transition from the freewheeling stage to the active stage, these sources are reestablished in a resonant fashion trough the auxiliary diode D_rs1.

[0031] In FIG. 3C, the auxiliary circuit V_auxis also provided by a regenerative snubber comprising two auxiliary capacitors C_aux1and C_aux2and auxiliary diodes D_aux1-3. In FIG. 3C, the auxiliary voltage source needed to reset the current during the freewheeling stage is also provided by a passive lossless circuit comprising two auxiliary capacitors C_aux1and C_aux2, and three auxiliaries diodes D_aux1, D_aux2, and D_aux3. its operating principle is similar to the two previous described circuits. That is, when S₁or S₄is turned off, the capacitors C_aux1and C_aux2work as a temporary voltage source to reset the primary freewheeling current. Similarly to the previous case, when the lossless circuit is used, this temporary sources are not controllable, which means that in every half period, the snubber capacitors energy is discharged completely. Moreover, during the transition from the freewheeling stage to the active stage, these sources are reestablished in a resonant fashion.

[0032] In FIG. 3D, the auxiliary voltage source is provided by a saturable inductor L_son the primary side 10. FIG. 3D shows the ZVZCS Three-Level DC/DC converter using an auxiliary voltage source in the primary side. This auxiliary voltage source to achieve ZCS operation is implemented with a capacitor and a saturable inductor. If the capacitance is very low, the voltage level across it can be high enough to reset the primary current. Meanwhile, the saturable inductor is used to keep the current at zero during the freewheeling stage.

[0033] Referring to FIGS. 4A-D, and operational example is given for the case when the auxiliary voltage source is an active clamp circuit. In order to simplify the analysis of the converter, it is assumed that the circuit operation is in steady state, the output filter inductor L_outis large enough to be considered as a current source, all the devices are ideal, and the transformer magnetizing current is neglected.

[0034] The proposed converter has eight operation stages during a half switching cycle. As it was mentioned before, the circuit uses the well known phase shift control with turn on and turn off of nearly 50% duty cycle. The phase shift between S₁and S₂or S₃and S₄determines the operating duty cycle of the converter. The following explains the operation the circuit shown in FIG. 4A with reference to FIG. 4B.

[0035] [t0-t1]: During this stage, S₁and S₂are conducting and the input power is delivered to the output. At the beginning of this stage, the anti-parallel diode D_auxof the clamp switch S_auxstarts to conduct the resonant current between the leakage inductance, L_lk, and the junction capacitance of the output rectifier diodes D_r1and D_r2This stage ends at t1 when the current I_auxthrough the clamp capacitor, C_aux, reaches zero. The duration of this mode depends on the leakage inductance, the junction capacitance and the reverse recovery time of the rectifier diodes D_r1and D_r2.

[0036] [t1-t2]: In this stage, the input power continues delivering to the output. At t1, the current in the clamp capacitor C_auxreverses its polarity but the anti-parallel diode D_auxof the clamp switch S_auxblocks and the voltage across the secondary rectifier becomes: $V_{\mathrm{rec}}=\frac{V_{\mathrm{in}}}{n}$

[0037] [t2-t3]: At t2, S₁is turned off and the load current charges and discharges the parasitic capacitance of S1 and S4 through the flying capacitor, C_ssrespectively. Meanwhile, the clamp switch S_auxis turned on and the secondary voltage is held by the clamp capacitor, C_aux. This stage ends when the voltage across the parasitic capacitance of S1 reaches Vin/2 and the clamp diode D_c1begins to conduct. At the same time, the voltage across the parasitic capacitance of S₄reaches zero and its antiparallel diode, D₄, begins to conduct.

[0038] [t3-t4]: After D₄starts conducting, S₄can be turned on at t3 with ZVS. The primary current freewheels through the clamp diode, D_c1, and S₂. The auxiliary clamp capacitor C_auxvoltage is reflected to the primary side 10 and applied across the leakage inductance L_lk. In this way, the primary current is reduced linearly to zero with a slope equal to V_Caux*n/L_lk(where n is the turn ration of the transformer 14). Also, the auxiliary clamp capacitor C_auxsupplies energy to the output. This stage ends when the primary current is reset to zero.

[0039] [t4-t5]: The rectifier diodes D_r1and D_r2are turned off since the primary current is zero and S_auxis still on. During this stage, the output current still is supplied by the auxiliary clamp capacitor C_aux. As a result, the effective duty cycle seen in the secondary side is larger than the one seen in the primary side.

[0040] [t5-t6]: At t5, S_auxis turned off, the voltage applied to the output rectifiers D_r1and D_r2. immediately drops to zero, and the conventional freewheeling stage in the converter starts. During this time, there is no current freewheeling in the primary side. Therefore, the circulating energy in the primary side is reduced due to ZVZCS operation.

[0041] [t6-t7]: At t6, S₂is turned off with complete ZCS since the leakage current has been reset completely at t4. As a consequence, IGBTs can be used for S₂and S₃.

[0042] [t7-t8]: At t8, S₃is turned on and the primary voltage is applied to the primary side of the transformer. The primary current flows through S₃and S₄and begins to increase with a slope equal to V_in/(2*L_lk). At the end of this stage, the primary current reaches the reflected output current and a new half-switching cycle starts.

[0043] As can be seen from the above description, the present invention provides a novel Zero Voltage and Zero Current Switching (ZVZCS) Three-level (TL) DC/DC converter providing a reduction of the voltage stress across the power switch Lip to half the input voltage and an improvement in the efficiency by reducing the circulating energy in the converter during the freewheeling stage. Therefore, this converter can be used in applications where are needed high voltage and high power, such as the one used for telecommunication, battery charger and uninterruptible power supplies with a power factor pre-regulator.

[0044] While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.