Constant frequency DC / DC power converter

Abstract

A constant frequency DC / DC power converter includes an input terminal, an output terminal, a pulse wave generator electrically coupled to the input terminal, and a resonant rectification network electrically coupled to the pulse wave generator. The pulse wave generator is configured to generate a sequence of pulse voltages having a waveform of pulse waves. The resonant rectification network is configured to convert the pulse voltages to an intermediate voltage and to rectify the intermediate voltage. The resonant rectification network includes a transformer having a primary winding and a secondary winding, at least one DC blocking capacitor electrically coupled to the primary winding, a magnetizing inductor connected in parallel with the primary winding, a rectifier electrically coupled to the secondary winding, a filter capacitor electrically coupled to the rectifier, a resonant capacitor electrically coupled to the transformer, and at least one resonant inductor electrically coupled to the transformer. The filter capacitor is connected in parallel with the output terminal. The constant frequency DC / DC power converter further includes a control unit configured to vary a duty cycle of the pulse wave generator while keeping a frequency of the pulse wave generator constant. The constant frequency DC / DC power converter features lower rectifier losses, lower EMI, simple constant frequency control, and total lagging current drawn from the pulse wave generator.

Description

[0001] This application is a divisional application of the patent application filed on November 25, 2018, with application number 201880076359.3 and invention title "Constant Frequency DC / DC Power Converter". Technical Field

[0002] This invention relates to DC / DC converters, and more particularly, to constant frequency DC / DC power converters. Background Technology

[0003] A DC / DC converter transforms a DC voltage at one voltage level to another. These converters receive an input voltage from a DC power supply and produce a DC output voltage at their output. The input voltage provided by the DC power supply can vary widely. Furthermore, depending on the specific application of the converter, the desired DC output voltage can be variable within a certain range, rather than fixed to a certain value. For example, the range of DC output voltage can be from 250V to 410V (e.g., for a lithium-ion battery charger). Therefore, DC / DC converters are needed to provide output voltage regulation for operation from full load to no load over a wide input voltage range.

[0004] US 6,344,979 discloses a resonant converter, namely an LLC series resonant DC-DC converter. The converter can be implemented using a full-bridge or half-bridge structure and employs a series-connected inductor-inductor-capacitor (LLC) resonant circuit to create conditions for lossless switching of the semiconductor switches, while the load is connected in parallel with one of the inductors. To regulate the output voltage, the LLC series resonant DC-DC converter employs a variable switching frequency control method. One disadvantage of the LLC series resonant DC-DC converter is the loss of ZCS conditions for the rectifier when the switching frequency is higher than the resonant frequency, which leads to reverse recovery losses and therefore lower efficiency. Another disadvantage of the LLC series resonant DC-DC converter is the inherent lack of constant-frequency operation, which is valuable in optimizing electromagnetic interference (EMI) filters, which may be necessary to meet EMI regulation standards.

[0005] US 5,808,879 discloses a half-bridge ZVS PWM flyback converter. This circuit features ZVS operation of the primary switch and fixed-frequency operation. A major drawback of this converter is the voltage spike across the rectifier during turn-off due to parasitic elements such as stray capacitance of the rectifier and leakage inductance of the transformer. Another drawback is the high dv / dt switching of the rectifier, which results in higher EMI emissions.

[0006] Other publications relating to this subject include US 5,159,541 and US 5,959,850. Summary of the Invention

[0007] According to an embodiment of the present invention, a constant-frequency DC-DC power converter is provided, comprising: an input terminal and an output terminal; a pulse wave generator electrically coupled to the input terminal and configured to generate a sequence of pulse voltages having a pulse wave waveform; and a resonant rectifier network electrically coupled to the pulse wave generator and configured to convert the pulse voltages into an intermediate voltage and to rectify the intermediate voltage. The resonant rectifier network includes: (a) a transformer having a primary winding and a secondary winding; (b) at least one DC blocking capacitor electrically coupled to the primary winding; (c) a magnetizing inductor connected in parallel with the primary winding; (d) a rectifier electrically coupled to the secondary winding; (e) a filter capacitor electrically coupled to the rectifier and connected in parallel with the output terminal; (f) a resonant capacitor electrically coupled to the transformer; and (g) at least one resonant inductor electrically coupled to the transformer. The constant-frequency DC-DC power converter further includes a control unit configured to change the duty cycle of the pulse wave generator while maintaining a constant frequency of the pulse wave generator.

[0008] In some embodiments, at least one resonant inductor is a resonant inductor connected in series with the secondary winding, and a resonant capacitor is electrically coupled to the rectifier.

[0009] In some embodiments, at least one resonant inductor is a resonant inductor connected in series with the primary winding, and a resonant capacitor is electrically coupled to the rectifier.

[0010] In some embodiments, the resonant capacitor is electrically coupled to the rectifier, and at least one resonant inductor is a first resonant inductor connected in series with the primary winding and a second resonant inductor connected in series with the secondary winding.

[0011] In some embodiments, at least one resonant inductor is a resonant inductor connected in series with the primary winding, and a resonant capacitor is connected in parallel with the primary winding.

[0012] In some embodiments, the pulse wave generator includes a half-bridge inverter circuit with two switches connected in series with each other, the series combination of the two switches being connected in parallel with the input terminals, and each of the switches having a diode connected at its two ends.

[0013] In some embodiments, the pulse wave generator includes a full-bridge inverter circuit comprising two switches connected in series with each other and two other switches connected in series with each other, the series combination of the two switches being connected in parallel with the input terminal, the series combination of the other two switches being connected in parallel with the input terminal, and each of the switches having a diode connected at its two ends.

[0014] In some embodiments, at least one DC blocking capacitor is a first DC blocking capacitor; and a second DC blocking capacitor is connected in series with the first DC blocking capacitor, and the series combination of the first DC blocking capacitor and the second DC blocking capacitor is connected in parallel with the input terminal.

[0015] In some embodiments, the constant frequency DC-DC power converter further includes a voltage multiplier electrically coupled to the rectifier.

[0016] Additional features of the invention will be described below, forming the subject matter of the claims. Those skilled in the art will appreciate that they can readily use the disclosed concepts and specific embodiments as a basis for designing or modifying other structures for achieving the same purpose as the invention. Those skilled in the art should also recognize that such equivalent constructions do not depart from the spirit and scope of the broadest form of the invention. Attached Figure Description

[0017] The invention can be more clearly understood by referring to the following detailed description of non-limiting exemplary embodiments of the invention, in which:

[0018] Figure 1 This is a circuit diagram of a constant frequency DC / DC power converter according to an embodiment of the present invention;

[0019] Figure 2-5 This is a circuit diagram of a constant frequency DC / DC power converter according to an embodiment of the present invention;

[0020] Figure 6 The drive signal transmitted by the control unit and the voltage generated by the pulse wave generator are shown.

[0021] Figure 7 This is a circuit diagram of a constant frequency DC / DC power converter with a full-bridge inverter on the input side;

[0022] Figure 8 This is a circuit diagram of a constant frequency DC / DC power converter with a half-bridge inverter and two DC blocking capacitors on the input side.

[0023] Figure 9 A timing diagram illustrating the operation of a constant frequency DC / DC power converter according to an embodiment of the present invention is shown;

[0024] Figure 10 This is a circuit diagram of a constant frequency DC / DC power converter according to an embodiment of the present invention, wherein a voltage multiplier is incorporated.

[0025] Figure 11A timing diagram illustrating the operation of a constant-frequency DC / DC power converter according to an embodiment of the present invention is shown, wherein a voltage multiplier is incorporated; and

[0026] Figure 12 The electrical equivalent circuit of a pair of coupled inductors is shown.

[0027] The following detailed description of embodiments of the present invention refers to the accompanying drawings mentioned above. The dimensions of the parts and features shown in the drawings have been chosen for convenience of presentation or clarity and are not necessarily shown to scale or to indicate part specifications. Wherever possible, the same reference numerals will be used throughout the drawings and in the following description to refer to the same and similar elements. Detailed Implementation

[0028] Figure 1 A first embodiment is shown, having an input terminal 18 for connection to a DC input voltage Vin and an output terminal 20 for outputting a DC output voltage Vo. If an output load (not shown) is connected across output terminal 20, a DC output current Io can be drawn. The source of the DC input voltage Vin can be, for example, the output voltage of a power factor correction stage (not shown), which typically has a full-bridge rectifier circuit (not shown) that couples it to the mains. Other examples of sources of Vin include, but are not limited to, battery voltage, solar cell voltage, and fuel cell voltage. A pulse wave generator 10 is coupled to input terminal 18 to generate a sequence of pulsed voltages with a pulsed waveform. The pulse wave generator 10 is typically a half-bridge inverter, which includes a first switch 24 and a second switch 26. A full-bridge inverter can be used instead of a half-bridge inverter. Switches 24 and 26 are connected in series with each other, and their series combination is connected in parallel with input terminal 18. Switches 24 and 26 each have diodes 25 and 27 placed across them. If MOSFETs are used as switches 24 and 26, the integrated diodes inherent in them can be used as diodes 25 and 27. Capacitors 29 and 30 represent the inherent stray capacitances of switches 24 and 26, respectively. External buffer capacitors (not shown) can be added across switches 24 and 26 to reduce their turn-off switching losses.

[0029] Control unit 12 transmits drive signals Va and Vb to switches 24 and 26 respectively. Drive signals Va and Vb do not overlap and are complementary to each other (e.g., in...). Figure 6(As shown in the diagram). This means that if the duty cycle for switch 24 is D, then the duty cycle for switch 26 should be (1-D). There is a slight dead time between the falling transition of Vb and the rising transition of Va, during which capacitor 29 discharges and diode 25 conducts. Similarly, there is a slight dead time between the falling transition of Va and the rising transition of Vb, during which capacitor 30 discharges and diode 27 conducts. Therefore, switches 24 and 26 close and open complementaryly under zero-voltage switching (ZVS) conditions, thereby generating a pulse voltage Vg across switch 26. Considering that the forward voltages of diodes 25 and 27 are negligible, the pulse voltage Vg is equal to Vin when switch 24 is closed and switch 26 is open, and equal to zero when switch 24 is open and switch 26 is closed. Therefore, the pulse voltage Vg is approximately a pulse wave voltage waveform with period T, amplitude Vin, and duty cycle D. The frequency of the pulse wave generator 10, denoted by F, is the reciprocal of the period T. Therefore, the duty cycle of the pulse wave generator 10 is the same as the duty cycle of the pulse voltage Vg. Consequently, the generated current Ig is drawn from the pulse wave generator 10 into the resonant rectifier network 14. The generated current Ig lags behind the fundamental component of the pulse voltage Vg.

[0030] A resonant rectifier network 14 is coupled to a pulse generator 10 to convert the pulsed voltage into an intermediate voltage and to rectify the intermediate voltage. The resonant rectifier network 14 includes a DC blocking capacitor 28, a transformer 32, a magnetizing inductor 38, a resonant inductor 48, a resonant capacitor 46, a rectifier 44, and a filter capacitor 56. The transformer 32 includes a primary winding 34 and a secondary winding 36 magnetically coupled to each other. The turns ratio of the transformer 32, denoted by N, is the ratio of the number of turns in the primary winding 34 to the number of turns in the secondary winding 36. Figure 1 The embodiment employs a resonant circuit formed by a resonant inductor 48 and a resonant capacitor 46 on the secondary side of transformer 32; however, additional resonant inductors can be incorporated and / or other resonant circuits or combinations thereof can be used. The resonant inductor 48 can be implemented as an external component or as the leakage inductance of transformer 32. A DC blocking capacitor 28 is connected in series with the primary winding 34 on the primary side of transformer 32 to block the DC component of the pulse voltage Vg from transformer 32. A magnetizing inductor 38 is connected in parallel with the primary winding 34 and can be implemented as an external inductor or as the magnetizing inductor of transformer 32. The magnetizing inductor 38 conducts a magnetizing current Im, thereby storing energy in the form of DC magnetization.

[0031] The operation of rectifier 44 is similar to that of a Class E low dv / dt resonant rectifier circuit. When rectifier 44 is off, resonant capacitor 46 resonates with resonant inductor 48, thereby forming an intermediate voltage Vx to be rectified by rectifier 44. At the turn-on transition of rectifier 44, the initial current through it rises in a step change. During the turn-on period of rectifier 44, the voltage across resonant capacitor 46 is constant. The turn-off transition of rectifier 44 is at a low dv / dt and under zero-current switching (ZCS) conditions. The turn-on and turn-off transitions of rectifier 44 are asynchronous with the rise and fall transitions of pulse voltage Vg. Although Figure 1 In this embodiment, a diode is used as rectifier 44, but a switch can be used as a synchronous rectifier to serve as rectifier 44. A filter capacitor 56 (which filters the voltage ripple of the DC output voltage Vo) is connected in series with rectifier 44 and in parallel with output terminal 20.

[0032] Figure 9 The illustration shows Figure 1 The waveform of the operation of the constant frequency DC / DC power converter in the embodiment. Figure 9 The first (top) waveform shows the pulse voltage Vg across switch 26. Figure 9 The second and third waveforms represent the currents Ia and Ib flowing through switches 24 and 26, respectively. Figure 9 The fourth waveform shows the generated current Ig, which is drawn by the pulse wave generator 10. Figure 9 The fifth waveform represents the current Id flowing through rectifier 44. Figure 9 The sixth (bottom) waveform represents the reverse voltage Vdr across rectifier 44. Considering that the forward voltage of rectifier 44 is negligible, the intermediate voltage Vx is equal to the voltage difference (Vo - Vdr).

[0033] Now refer to Figure 9 describe Figure 1 The steady-state operation of the constant-frequency DC / DC power converter in this embodiment is as follows: The turn-off transition of rectifier 44 initiates a half-resonant interval τ, during which the reverse voltage Vdr across rectifier 44 rises sinusoidally and resonant capacitor 46 discharges. At the end of the half-resonant interval τ, resonant capacitor 46 begins to recharge, the current through it changes polarity from negative to positive, and the reverse voltage Vdr begins to drop from its peak level back to zero. As the reverse voltage Vdr drops, the pulse voltage Vg transitions to the level of Vin, thereby triggering a faster charging of resonant capacitor 46. When the reverse voltage Vdr reaches zero, rectifier 44 turns on and resonant capacitor 46 stops resonating. When rectifier 44 is on, the intermediate voltage Vx is clamped at the level of Vo. When the pulse voltage Vg transitions back to zero, current Id decreases with a negative linear slope, which determines the peak level of the reverse voltage Vdr in the next cycle of operation.

[0034] The adjustment and / or control of the DC output voltage Vo or DC output current Io can be achieved by fixing the frequency F of the pulse voltage Vg and changing the duty cycle D. Therefore, the control unit 12 changes the duty cycle of the pulse wave generator 10 while keeping the frequency F of the pulse wave generator 10 constant. The maximum output power, represented by Pmax, can be achieved under the following conditions:

[0035] for Figure 1 In the embodiment of the constant frequency DC / DC power converter, the half-resonant interval τ can be given by the following formula:

[0036]

[0037] Where Lr is the inductance of resonant inductor 48 and Cr is the capacitance of resonant capacitor 46. Typically, the output power of a constant-frequency DC / DC power converter can be reduced by decreasing the duty cycle D towards zero. The voltage transfer function (Vo / Vin as a function of D) can be specifically found for any load by duty cycle scanning. For constant DC output voltage Vo regulation, the peak level of the reverse voltage Vdr decreases as the output power decreases. For constant Vo and variable Vin operation, the peak level of the reverse voltage Vdr remains approximately the same for the same output power, regardless of the duty cycle D.

[0038] If possible Figure 9 Observedly, at the falling transition of the pulse voltage Vg, rectifier 44 turns on, generating a positive current Ig, and the sum of the currents (Ig + Im) is positive. At the rising transition of the pulse voltage Vg, the generated current Ig is negative, the intermediate voltage Vx rises positively, the current through resonant capacitor 46 is positive, and the sum of the currents (Ig + Im) is also positive. In other words, the DC bias of the magnetizing current Im is used to perform ZVS operation on both the rising and falling transitions of the pulse voltage Vg, resulting in lower circulating currents on switches 24 and 26. (This can also be seen from...) Figure 9 The observed approximately linear and non-sinusoidal waveform of the current Ig generated during the conduction of rectifier 44 is attributed to the large capacitance value of DC blocking capacitor 28, which decouples the DC component of pulse voltage Vg without participating in resonance.

[0039] Both switches 24 and 26 have conditions for their operation under ZVS; however, the ZVS conditions for switch 24 are more difficult to satisfy. While adjusting Vo, the following empirical formula can be satisfied (applicable only to...). Figure 1 The constant frequency DC / DC power converter in the embodiment ensures ZVS operation of switch 24 for all loads:

[0040]

[0041] in,

[0042]

[0043] Cb is the capacitance of the DC blocking capacitor 28, and Lm is the inductance of the magnetizing inductor 38. Considering that most magnetic cores have limited magnetic energy storage capacity, the inductance Lm of the magnetizing inductor 38 is preferably small enough that its core is not completely saturated. Saturation of the magnetizing inductor 38 can cause losses in the ZVS operation of both switches 24 and 26. Preferably, the capacitance Cb of the DC blocking capacitor 28 is large enough that the RMS value of the ripple voltage across the DC blocking capacitor 28 is lower than the RMS value of the pulse voltage Vg, because ZVS operation of switches 24 and 26 may be impossible with a smaller capacitance of the DC blocking capacitor 28.

[0044] The efficiency of a constant-frequency DC / DC power converter is affected by the half-resonant interval τ. The closer τ is to half the period T of the pulse voltage Vg, the higher the circulating current and voltage gain from the resonance of the resonant capacitor 46 and the resonant inductor 48. The farther τ is from half the period T, the smaller the turns ratio N required to achieve the desired output power, and the higher the DC magnetization of the magnetizing inductor 38. Therefore, trade-offs can be made between efficiency properties (such as current stress and / or voltage stress) on different components in a constant-frequency DC / DC power converter. Optimal operating efficiency can be obtained under the following conditions:

[0045] 0.5·T < 2·τ < 0.9·T

[0046] Figure 12 An equivalent model 100 of the electrical equivalent circuit as a pair of coupled inductors is shown. The equivalent model 100 includes a primary series inductor L1, a secondary series inductor L2, a parallel inductor Lμ, and an ideal transformer with a turns ratio M. Equations relating the terminal voltages (V1 and V2) to the terminal currents (I1 and I2) in the equivalent model 100 are given in the frequency domain by the following equation:

[0047]

[0048] Since these two equations contain four variables (L1, L2, Lμ, and M), it is possible to derive an infinite number of specific equivalent models from equivalent model 100. Therefore, Figure 1 The embodiment implements a particular equivalent model in which the primary series inductance L1 is set to zero.

[0049] Figure 2 It shows a configuration similar to, except that the resonant inductor 48 is connected in series with the primary winding 34. Figure 1 The embodiments of the present invention are alternative embodiments. Therefore, Figure 2 The embodiment implements another specific equivalent model, where the secondary series inductance L2 is set to zero. For Figure 2 In the embodiment of the constant frequency DC / DC power converter, the half-resonant interval τ is given by the following formula:

[0050] Figure 3 Another embodiment of the invention is shown, wherein an additional resonant inductor 49 is connected in series with the primary winding 34. The additional resonant inductor 49 can be implemented as an external component or as the leakage inductance of the transformer 32. Figure 3 In the embodiment of the constant frequency DC / DC power converter, the half-resonant interval τ is given by the following formula:

[0051]

[0052] Where La is the inductance of the additional resonant inductor 49.

[0053] Figure 4 It shows a configuration similar to, except that the resonant capacitor 46 is connected in parallel with the rectifier 44. Figure 1 Another embodiment of the present invention is described in the example. The resonant capacitor 46 may be implemented as an external component or as an inherent stray capacitance of the rectifier 44.

[0054] Figure 5 It shows a configuration similar to, except that the resonant capacitor 46 is connected in parallel with the primary winding 34. Figure 2 Another embodiment of the present invention. For Figure 5 In the embodiment of the constant frequency DC / DC power converter, the half-resonant interval τ is given by the following formula:

[0055]

[0056] Figure 7 An embodiment of the invention with a full-bridge inverter serving as a pulse wave generator 10 is shown. In addition to switches 24 and 26, the full-bridge inverter also incorporates two switches 64 and 66. Switches 64 and 66 are connected in series with each other, and their series combination is connected in parallel with input terminal 18. Switches 64 and 66 each have diodes 65 and 67 placed across them. Capacitors 69 and 70 are the respective inherent stray capacitances of switches 64 and 66. Switch 24 operates synchronously with switch 66, while switch 26 operates synchronously with switch 64.

[0057] Figure 8An embodiment of the invention with an additional DC blocking capacitor 31 is shown. The DC blocking capacitor 28 is connected in series with the additional DC blocking capacitor 31, and their series combination is connected in parallel with the input terminal 18.

[0058] Figure 10 An embodiment of the invention is shown, wherein the resonant rectifier network 14 is further incorporated into a voltage multiplier circuit formed by a first multiplier diode 51, a second multiplier diode 52, and a multiplier capacitor 53. When the rectifier 44 is in the off state, the multiplier capacitor 53 resonates with the resonant inductor 48. Therefore, the voltage multiplier circuit also rectifies the intermediate voltage Vx, reduces the DC magnetization of the magnetizing inductor 38, and reduces the peak level of the reverse voltage Vdr. The voltage multiplier circuit can be incorporated into the resonant rectifier network 14 by further cascading voltage multiplier circuits.

[0059] Figure 11 The illustration is shown. Figure 10 The waveform of the maximum power throughput operation of the constant frequency DC / DC power converter in the embodiment. Figure 11 The first (top) waveform shows the pulse voltage Vg across switch 26. Figure 11 The second waveform shows the generated current Ig. Figure 11 The third waveform represents the current Id flowing through rectifier 44. Figure 11 The fourth waveform represents the reverse voltage Vdr across rectifier 44. Figure 11 The fifth waveform represents the current Iw flowing through the multiplier capacitor 53. Figure 11 The sixth waveform represents the current Icr flowing through the resonant capacitor 46. Figure 11 The seventh (bottom) waveform represents the reverse voltage Vw across the multiplier diode 52.

[0060] Although the invention has been described with reference to this embodiment, it will be understood that various modifications thereto are possible within the principles outlined above and will be apparent to those skilled in the art, and therefore the invention is not limited to the preferred embodiment, but is intended to cover such modifications.

Claims

1. Constant frequency DC / DC power converter, including: Input terminals; Output terminals; A pulse wave generator, electrically coupled to the input terminal and including a half-bridge inverter or a full-bridge inverter, for generating a sequence of pulse voltages with a waveform having a pulse wave voltage. A resonant rectifier network, electrically coupled to the pulse wave generator and configured to convert the pulse voltage into an intermediate voltage and to rectify the intermediate voltage, the resonant rectifier network comprising: (a) A transformer having a primary winding, a secondary winding and a magnetizing inductor; (b) At least one DC blocking capacitor electrically coupled to the primary winding; (c) A resonant inductor connected in series with the primary winding; (d) A filter capacitor, which is connected in parallel with the output terminal and in series with the secondary winding; (e) A rectifier connected in parallel with the series combination of the filter capacitor and the secondary winding; and (f) A resonant capacitor connected in parallel with the rectifier or in parallel with the transformer; and A control unit configured to change the duty cycle of the pulse wave voltage while maintaining a constant frequency of the pulse wave voltage, such that a falling transition of the pulse wave voltage occurs when the rectifier is turned on and a rising transition of the pulse wave voltage occurs when the reverse voltage across the rectifier drops from its peak resonant position, thereby causing the output power to decrease if the duty cycle decreases and to increase if the duty cycle increases.

2. The constant frequency DC / DC power converter according to claim 1, wherein, The resonant inductor is implemented as the leakage inductance of the transformer.

3. The constant-frequency DC / DC power converter according to any one of claims 2, wherein, The resonant rectifier network also includes a voltage multiplier circuit.

4. The constant frequency DC / DC power converter according to claim 3, wherein, The rectifier is a diode.

5. The constant frequency DC / DC power converter according to claim 2, wherein, The rectifier is an active switch.

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