Abstract: A resonant DC to DC converter circuit receiving an input voltage and capable of delivering multiple output voltages, while maintaining excellent cross-regulation with only one power transformer. The circuit of the invention uses trailing edge modulation, along with a new amplitude modulation arrangement which allows for the use of resonance with synchronous rectification and only a single power transformer. Thus, the circuit provides for DC to DC conversion, as well as an amplitude modulation arrangement.

Abstract: A controller circuit is specified, having a step-up controller, a resonant converter connected downstream of the step-up controller on the output side, a transformer, a rectifier, which rectifier is connected to the secondary winding of the transformer on the input side, and a CLL resonant circuit connected to the resonant converter and to the primary winding of the transformer, which CLL resonant circuit has a resonance capacitance and a first and a second resonance inductance. In order to reduce the switching losses, the CLL resonant circuit is embodied as a “T” circuit.

Abstract: The present invention relates to an oscillator that is capable of realizing duty balancing. The oscillator determines a switching frequency of a converter converting an input voltage according to a switching operation of switches to generate an output voltage. The oscillator determines a first half cycle of a duty signal determining the switching frequency by using a reference current according to a feedback signal corresponding to the output voltage. The oscillator senses a first half cycle period by using an output of the frequency setting unit, and determines the same period as the first half cycle as a second half cycle of the duty signal after the first half cycle.

Abstract: In an aspect, the present invention provides a high frequency switching power converter. The high frequency switching power converter may include a plurality of soft-switchable power cells flexibly connected to receive an input signal in series and provide an output. The high frequency switching power converter may further include a controller for configuring the flexible connection and for controlling the power cells to receive the input signal. In an embodiment, each of the plurality of power cells may be separately controllable by the controller. Further, a portion of the plurality of power cells may be arranged with parallel outputs. Additionally, at least one of the plurality of cells may include one or more switched capacitors. In another embodiment, the at least one of the plurality of cells may include at least one switched capacitor and a DC/DC regulating converter.

Abstract: This document discusses, among other things, apparatus and methods for controlling a converter. In an example, a voltage-controlled oscillator (VCO) can be configured to provide a first pulse train to control the converter. The frequency of the first pulse train of the VCO can be modulated using information indicative of an operating condition of the converter to maintain a desired DC voltage at an output. In an example, the VCO can include a frequency divider configured to provide a second pulse train to the output using information from the first pulse train.

Abstract: A power factor correct current resonance converter is disclosed which eliminates interference of switching operations between two cascade-connected converter circuits. The power factor correct current resonance converter includes a current resonance converter circuit having switches, a resonance capacitor, a resonance inductor, a transformer, diodes, a smoothing capacitor, and a control circuit. The power factor correct current resonance converter also includes a power factor correct converter circuit having a choke coil, a diode, a smoothing capacitor, and a switch. The switch is turned on or off in response to a voltage produced in a winding of the transformer. Thus, the switching operation of the power factor correct converter circuit is performed in synchronization with the switching operation of the current resonance converter circuit, so that interference of the switching operations is eliminated. In addition, since a dedicated control circuit is not required, the cost can be reduced.

Abstract: An embodiment common mode noise reduction apparatus comprises a common mode choke, a balance inductor, a first capacitor and a second capacitor. The common mode choke is placed between an input dc source and a primary side network of an isolated power converter. The balance inductor is coupled between an upper terminal of a primary winding of the isolated power converter and a negative terminal of the input dc source. The first capacitor is coupled between the upper terminal of a primary side of a transformer and an upper terminal of a secondary side of the transformer of the isolated power converter. The second capacitor is coupled between a lower terminal of the primary side of the transformer and a lower terminal of the secondary side of the transformer of the isolated power converter.

Abstract: The present invention relates to a duty adjuster circuit and a converter. The converter supplies output power according to a first output current and a second output current generated according to each switching operation of a first switch and a second switch. The duty adjuster circuit generates an adjuster current to compensate the difference between the peak of the first output current and the peak of the second output current to adjust duty of the first and second switches.

Abstract: A bi-directional DC-DC converter has a transformer for connecting a voltage type full bridge circuit connected to a first power source and a current type switching circuit connected to a second power source. A voltage clamping circuit constructed by switching elements and a clamping capacitor is connected to the current type switching circuit. The converter has a control circuit for cooperatively making switching elements operative so as to control a current flowing in a resonance reactor.

Abstract: The present invention is to provide a series resonant converter, which includes a transformer, two power switches connected to primary side of the transformer, a resonant control chip having two control pins connected to gates of the two power switches respectively, a resonant capacitor having one end connected to one end of the primary side and the other end connected to source of one power switch, a resonant inductor having one end connected to the other end of the primary side and the other end connected to a line between the two power switches, and at least one bypass resistor connected in parallel to the resonant capacitor, so as to allow voltage of the resonant capacitor to be rapidly released to ground when the converter is turned off and effectively lower inrush current of the resonant capacitor generated at an instant when the converter is turned on from off.

Abstract: An adaptive inductive ballast is provided with the capability to communicate with a remote device powered by the ballast. To improve the operation of the ballast, the ballast changes its operating characteristics based upon information received from the remote device. Further, the ballast may provide a path for the remote device to communicate with device other than the adaptive inductive ballast.

Abstract: A converter includes a rectifying circuit of an AC voltage of an AC power source into a rectified voltage, an input smoothing capacitor Ci, a first series circuit connected to the input smoothing capacitor and including a first and a second switching elements, a second series circuit connected to a connection point of the first and second switching elements and a first end of the input smoothing capacitor and including a primary winding of a transformer and a first capacitor Cri, a controller to alternately turn on/off the first and second switching elements, a rectifying-smoothing circuit (D1, Co) to rectify and smooth a high-frequency voltage of a secondary winding of the transformer into a DC output voltage, and a second capacitor connected between a connection point of the primary winding of the transformer and the first capacitor and a first end of the AC power source.

Abstract: A resonance-type power supply is provided in which no short circuit occurs and driving is performed with a constant switching period by performing switching control using a change of magnetic flux of a magnetic component as a trigger. When the change of magnetic flux of the transformer is detected, the first switching control signal is caused to transition to the Hi level. The detection voltage signal is A/D-converted, a first on-time is determined from the level thereof, and a second on-time is calculated by subtracting the first on-time from the constant switching period. When the first switching control signal is caused to transition to the Low level based on the first on-time, the change of magnetic flux of the transformer is detected, and the second switching control signal is caused to transition to the Hi level and is caused to transition to the Low level after the second on-time has elapsed.

Abstract: In a switching power supply apparatus, a comparator outputs a first determination criterion signal based on a saw-tooth wave signal whose level fluctuates with a constant period and a detection voltage signal. An inverter subjects the first determination criterion signal to reverse processing, and outputs a second determination criterion signal. The comparator outputs a first switching judgment-use signal from a monitor signal and a threshold value, and the comparator outputs a second switching judgment-use signal from the monitor signal and the threshold value. An AND circuit outputs the first switching control signal from the first determination criterion signal and the first switching judgment-use signal, and the AND circuit outputs the second switching control signal from the second determination criterion signal and the second switching judgment-use signal.

Abstract: The present invention relates to a DC/DC converter (1) with primary side (11) consisting of a resonant converter, which DC/DC converter (1) comprises a first and a second transformer (T1, T2), connected in series on the primary side (11) and on the secondary side (12) of the DC/DC converter. The secondary side (12) comprises an autotransformer (Tcd) consisting of a first and a second winding (Tcda, Tcdb) connected to a common center tap (Tcdc), where the first winding (Tcda) of the autotransformer (Tcd) is connected to the secondary winding (T1b) of the first transformer (T1), forming a first output connection point (P1), the second winding (Tcdb) of the autotransformer (Tcd) is connected to the secondary winding (T2b) of the second transformer (T2), forming a second output connection point (P2).

Abstract: In accordance with an embodiment, a switch controller for a switched-mode power supply includes an oscillator, an advance timing generator, and a dead zone generator. The advance timing generator generates an advance timing output pulse having a first pulse width that is asserted when the oscillator reaches a first phase. The dead zone generator produces a dead zone output having a second pulse width when the advance timing output pulse is de-asserted. This dead zone output pulse is coupled to a freeze input of the oscillator that freezes the phase accumulation of the oscillator when asserted. The controller also has a primary switch logic circuit that produces primary switch drive signals having a dead zone coincident with the dead zone output, and a secondary switch logic circuit that generates a secondary switch drive signal that is de-asserted when the advance timing output pulse becomes asserted.

Abstract: A passive lossless snubber cell for a switched-mode power converter comprises an energy absorbing circuit and an energy resetting circuit coupled to said energy absorbing circuit. The energy absorbing circuit is arranged to release energy stored in a snubber capacitor of the energy absorbing circuit to a storage capacitor of the energy resetting circuit through a resonant pathway of the snubber cell in response to a first switching action of a power converter transistor switch. The energy resetting circuit is arranged to release the energy stored therein to a part of a circuit of the power converter in response to a second switching action of the power converter transistor switch, the second switching action being a successive action to the first switching action. The passive lossless snubber cell has several advantages over existing snubbering techniques.

Abstract: A series resonant converter circuit that reduces power loss is provided that includes an inverter circuit having at least a pair of first and second switching elements that is connected between DC input terminals, a transformer connected to this inverter circuit, a resonant inductance means that are connected in series to a primary winding wire or a secondary winding wire of the transformer, a primary-side resonant capacitor that is connected in series to the resonant inductance means through the first or second switching element, first and second secondary-side resonant capacitors that are connected to each other in series between the DC input terminals, first and second unidirectional elements that are connected to each other in series between the DC output terminals, and a resonant inductance means that cooperates with resonant capacitance from the primary-side resonant capacitor and the first and second secondary-side resonant capacitors to resonate in series.

Abstract: There is provided a variable switching frequency type power supply that can achieve high efficiency by fixing a switching duty cycle to 50% and varying switching frequency. A variable switching frequency type power supply according to an aspect of the invention may include: a switching unit switching input power; a transformation unit converting a voltage level of the power switched by the switching unit into a predetermined voltage level; a rectification unit rectifying the power converted by the transformation unit; and a control unit fixing a switching duty cycle of the switching unit to a predetermined switching duty cycle, and varying switching frequency of the switching unit according to a voltage level of output power.

Abstract: The invention discloses a control circuit for an AC-DC converter. The control circuit includes a power control circuit for comparing an input current sensing signal generated by sensing an input current of the AC-DC converter and a power level control input and in response thereto generating a frequency modulation control signal, in which the frequency modulation control signal is used to control the output power of the AC-DC converter and suppress harmonics of the input current, and a square wave generator connected to the power control circuit for generating a driving signal used to drive the switch circuit of the AC-DC converter according to the frequency modulation control signal, in which the frequency of the driving signal is varied with the frequency modulation control signal, thereby suppressing harmonics of the input current and regulating the switching frequency the AC-DC converter, and regulating the output power of the AC-DC converter.

Abstract: A resonant switching power supply device includes: a first switching element and a second switching element which are configured to convert and adjust power. A capacitance of a first/second gate-drain capacitor existing between a gate and a drain of the first/second switching element and a resistance of a first/second gate resistor of the first/second switching element are set such that, in a period during which a resonance current flows by switching the second/first switching element, a gate-source voltage of the first/second switching element is lower than an ON threshold voltage of the first/second switching element due to the resonance current divided into the first/second gate-drain capacitor.

Abstract: An induction power system configured to drive a load includes a primary side circuit (PSC) and a secondary side circuit (SSC). The PSC has a main inductor configured to generate a current-induced magnetic field. The SSC has an induction electrification unit (IEU) and a power distribution unit (PDU). The IEU has a first inductor and a second inductor connected in series, and is adjacent to the main inductor to generate an induced AC. The PDU has a first capacitor, a second capacitor, and a switching device. The first capacitor and the first inductor are connected in series and generate a series resonance to supply a control power. The second capacitor is connected in parallel with the first inductor, the second inductor, and the first capacitor, and generates a parallel resonance, to provide a load power. When the switching device is turned on, the load power is supplied to the load.

Abstract: This invention relates to a synchronous rectifier for LLC resonant converter. This method allows simple drive method for the synchronous rectifier MOSFETS by using the transformer secondary winding voltage and one-shot vibrator. The synchronous rectifier MOSFETs are turned on by being triggered to the transformer secondary side winding voltage and turned off after predetermined time set by one shot vibrator. The predetermined time is set by the resonant period of the resonant network.

Abstract: A resonant, bi-directional, DC to DC voltage converter with loss-less (soft) switching having regulated output and capable of converting power between two, high-potential and low-potential DC voltage sources. The converter's semiconductor and magnetic components provide both, output regulation and soft switching in both (step-down and step-up) directions of power conversion which reduces total component count, cost and volume and enhances power conversion efficiency.

Abstract: A current mode resonant converter integrates current information from a first drive transistor to generate an integration signal. The integration signal is added to a sawtooth signal to generate a quasi-sawtooth signal. The quasi-sawtooth signal is compared to an error signal indicative of an output voltage of the resonant converter. The first drive transistor is switched OFF when the quasi-sawtooth signal reaches the level of the error signal. Once the first drive transistor is turned off, a second drive transistor is turned on for the same time duration same as the first drive transistor.

Abstract: A resonant converter includes a square wave generator including a first switch and a second switch, and generating a first square wave corresponding to an input voltage by alternately turning on/off the first and second switches; a resonator including a first coil of a primary coil of a transformer, and generating a resonance waveform corresponding to the first square wave; and an output unit including a second coil of a secondary coil of the transformer, and outputting a voltage corresponding to a current generated in the second coil corresponding to the resonance waveform.

Abstract: The present invention relates to a power factor corrector and a driving method thereof. The present invention includes a transformer including a first coil of a primary side transmitted with an input voltage and a second coil of a secondary side generating an output voltage. A power switch is connected to the first coil, and a capacitor removes a ripple of the output voltage. A diode is connected between the capacitor and the second coil. A resonance is generated between the capacitor and a leakage inductance of the transformer during a turn-on period of the power switch, and a resonance is generated between the capacitor and the magnetizing inductance of the transformer during the turn-off period of the power switch.

Abstract: An LLC resonant power regulator system (10) includes a transformer (22) comprising a primary inductor (20) and a secondary inductor (26) and an input resonant tank (18) comprising an input resonant capacitor, an input leakage inductor, and the primary inductor (20) connected in series. The system also includes an input stage (14) comprising a plurality of switches (16) that are controlled in response to a respective plurality of switching signals sweeping frequency to supply an input resonant current to the input resonant tank (18). Each of the respective plurality of switching signals can have a fixed duty cycle and a sweeping frequency. The system (10) further comprises an output resonant tank (24) comprising an output resonant capacitor, an output leakage inductor, and the secondary inductor (26) connected in series. The output resonant tank (24) can be configured to generate an oscillating output resonant current at an output.

Abstract: A power converter employing a control system configured to make multiple functional use of a circuit node therein and method of operating the same. In one embodiment, the power converter includes a power train including at least one power switch. The power converter also includes a control system including an opto-isolator circuit, including a resistor, configured to receive an output signal from the power converter and provide a feedback signal to a feedback node for the control system to provide a switch control signal for the at least one power switch. The control system also includes a current source configured to produce multiple voltage levels at the feedback node in accordance with the resistor, thereby enabling multiple functional uses of the feedback node.

Abstract: A resonant power converting circuit is provided, which includes a resonant converting unit, a control unit, a current detecting unit and a frequency modulation unit. The control unit outputs switching signals to the resonant converting unit to adjust an output thereof. The current detecting unit is configured to detect an output current of the resonant converting unit. The frequency modulation unit may adjust a lowest switching frequency of the control unit according to the detected output current so as to increase a gain of the resonant converting unit and an output stability of the resonant converting unit.

Abstract: A resonant power converting circuit is provided, which includes a resonant converting unit, a control unit, a voltage detecting unit and a frequency modulation unit. The control unit outputs switching signals to the resonant converting unit to adjust an output of the resonant converting unit. The voltage detecting unit is configured to detect an output voltage of the resonant converting unit. The frequency modulation unit may adjust a lowest switching frequency of the control unit according to the detected output voltage so as to increase a gain of the resonant converting unit and an output stability of the resonant converting unit.

Abstract: A circuit for transferring amplified resonant power to a load is disclosed. The circuit transfers amplified resonant power, which is generated in an inductor of a conventional transformer when serial or parallel resonance of a conventional power supply is formed, to a load through the conventional transformer. The circuit comprises: a power supply for producing and supplying voltage or current; a power amplifier for generating amplified resonant power using the voltage or current; and a power transferring unit for transferring the amplified resonant power to the load using a transformer.

Abstract: Provided is an inverter of a renewable energy storage system, which has an input port and an output port electrically insulated, and is compact and low-priced, while having a simplified circuit. The inverter includes a DC-DC converting unit connected to the DC link, and an inverting unit connected between the DC-DC converting unit and the power system, wherein the DC-DC converting unit is an unregulated DC-DC bus converter.

Abstract: A driving circuit includes a generator configured to generate a driving signal having plural levels of voltage at which a power switching device is turned on. The driving circuit also includes a switching controller configured to switch between the plural levels of voltage at which the power switching device is turned on. The driving circuit further includes a load current detector configured to output a load current detection signal to the switching controller, the load current detection signal indicating whether or not a current flowing through the power switching device exceeds a threshold, wherein the switching controller is configured to perform the switching based on the load current detection.

Abstract: A resonant conversion system is provided, in which a resonant converter receives an input voltage to generate an output voltage, and a buck converter provides the input voltage of the resonant converter, and controls the input voltage to perform an over-current protection process.

Abstract: A synchronous rectifying circuit for a switching power converter is provided. The synchronous rectifying circuit includes a power transistor, a diode, and a control circuit. The power transistor and the diode are coupled to a transformer and an output of the power converter for rectification. The control circuit generates a drive signal to switch on the power transistor once the diode is forward biased. The control circuit includes a monitor circuit. The monitor circuit generates a monitor signal an off signal to switch off the power transistor in response to a pulse width of the drive signal for generating an off signal to switch off the power transistor. The monitor circuit further reduces the pulse width of the drive signal in response to a change of a feedback signal. The feedback signal is correlated to an output load of the power converter.

Abstract: A current resonance power supply includes a transformer having a primary winding and a secondary winding, two switching elements connected to one end of the primary winding of the transformer and arranged in series, a resonance capacitor connected to the other end of the primary winding, and a voltage detection unit connected between the one end of the primary winding and the two switching elements and configured to detect that AC voltage input to a primary side of the transformer becomes lower, wherein operations of the switching elements are controlled based on a detection result of the voltage detection unit.

Abstract: In one embodiment, a control circuit for a series resonant switching power supply control system is configured to disable a power switch of secondary side of the system in response to a polarity reversal of the voltage across a primary side winding of the system.

Abstract: A power supply apparatus with a current-sharing function includes a conversion circuit, a square-wave generating circuit, a resonant circuit, and a rectifier-filter circuit. The conversion circuit has two transformers, and each of the transformers has a primary winding and two secondary windings. More particularly, two secondary windings of the different transformers are electrically connected in series and then the two in-series secondary windings are electrically connected in parallel. The square-wave generating circuit is used to switch a DC voltage into a pulsating voltage. The resonant circuit is electrically connected to the square-wave generating circuit, and having a first capacitor and the primary windings of the transformers.

Abstract: An improved structure for a switching power adaptor circuit, primarily providing a switching power adaptor circuit for a RCC (Ringing Choke Converter) system, in which a control circuit is mainly structured from a bipolar junction transistor, a stable voltage sensing circuit, an over current protection circuit and an over voltage protection circuit, thereby providing the present invention with multiple protection effectiveness, which is able to ensure that the voltage at secondary winding output terminals is maintained at a stable value, thus improving safety in the use of electrical appliance products, and achieving effectiveness to reduce size and lower manufacturing costs.

Abstract: A power conversion device is provided for converting a DC voltage to an alternating current corresponding to an AC voltage according to the AC voltage, which includes a power conversion unit and an output unit. The power conversion unit converts the DC voltage to a high-frequency current having two envelops corresponding to the waveform of the AC voltage. The output unit includes an inductive circuit, a full-wave rectifying circuit, an inverter circuit and a filter circuit. The inductive circuit provides two induced currents according to the high-frequency current, wherein one induced current and the high-frequency current are in phase, and the other induced current and the high-frequency current are in antiphase. The full-wave rectifying circuit full-wave rectifies the two induced currents. The inverter circuit alternatively transfers the two full-wave rectified induced currents, and thus output an output current. The filter circuit filters the output current to provide the alternating current.

Abstract: A DC-DC converter includes: a first series circuit in which a resonance reactor, a primary winding of a transformer, and a switching element are connected in series, the first series circuit being connected to both ends of a direct current power supply; a second series circuit in which a first rectifier, a current resonance capacitor, and a secondary winding of the transformer are connected in series, the second series circuit being connected to both ends of the direct current power supply; a rectifying/smoothing circuit having a second rectifier and a smoothing capacitor and connected to both ends of a series circuit of the current resonance capacitor and the secondary winding of the transformer; an output voltage detection circuit that detects an output voltage of the rectifying/smoothing circuit; and a control circuit that turns on and off the switching element based on an output voltage signal from the output voltage detection circuit.

Abstract: The present invention relates to a DC/DC converter (1) with primary side (11) consisting of a resonant converter, which DC/DC converter (1) comprises a first and a second transformer (T1, T2), connected in series on the primary side (11) and on the secondary side (12) of the DC/DC converter. The secondary side (12) comprises an autotransformer (Tcd) consisting of a first and a second winding (Tcda, Tcdb) connected to a common center tap (Tcdc), where the first winding (Tcda) of the autotransformer (Tcd) is connected to the secondary winding (T1b) of the first transformer (T1), forming a first output connection point (P1), the second winding (Tcdb) of the autotransformer (Tcd) is connected to the secondary winding (T2b) of the second transformer (T2), forming a second output connection point (P2), and the center tap (Tcdc) of the autotransformer (Tcd) is connected to the positive output (21) of the secondary side (12).

Abstract: A direct current (DC) power supply for varying an output voltage according to a load current variation is disclosed. The DC power supply includes an alternating current (AC)/DC conversion unit converting commercial AC power into DC power, a DC/DC conversion unit converting a voltage level of the DC power and outputting output power, and a control unit controlling conversion of the voltage level of the fed-back DC power according to a variation in a load current of the output power from the DC/DC conversion unit.

Abstract: An embodiment of the invention relates to an LLC power converter including a controller configured to regulate an output characteristic of the power converter by controlling a power converter switching frequency. In a first mode of operation, the controller turns off a secondary-side power switch earlier than a turn-off time of a primary-side power switch by a time difference that is controlled by a resistor coupled to an external circuit node. In a second mode of operation, the controller turns on a secondary-side power switch at substantially the same time as the primary-side power switch, and turns off the secondary-side power switch after a maximum on time that is a nonlinear function of a load current of the power converter. The nonlinear function is a substantially constant function of the load current for a value of the load current higher than a threshold value.

Abstract: The configurations of a parallel-connected resonant converter circuit and a controlling method thereof are provided in the present invention. The proposed circuit includes a plurality of resonant converters, each of which has two input terminals and two output terminals, wherein all the two input terminals of the plurality of resonant converters are electrically series-connected, and all the two output terminals of the plurality of resonant converters are electrically parallel-connected.

Abstract: A resonant converter is provided which may be used for supplying power to the primary conductive path of an inductively coupled power transfer (ICPT) system. The converter includes a variable reactive element in the resonant circuit which may be controlled to vary the effective inductance or capacitance of the reactive element. The frequency of the converter is stabilised to a nominal value by sensing the frequency of the converter resonant circuit, comparing the sensed frequency with a nominal frequency and varying the effective inductance or capacitance of the variable reactive element to adjust the converter frequency toward the nominal frequency.

Abstract: A current resonance power supply includes a current detecting unit detecting a current flowing through a primary side of a transformer and a current compensating unit compensating the current detected by the current detecting unit in accordance with a variation in voltage input into the primary side of the transformer. The current resonance power supply detects overcurrent on the basis of an output from the current compensating unit.

Abstract: Provided are methods, circuits, and systems for obtaining power from a power generator such as a photovoltaic cell or a fuel cell. The methods, circuits, and systems comprise converting substantially DC output power from the power generator into a high frequency AC voltage while rejecting or minimizing oscillations in the output power from the power generator; converting the high frequency AC voltage into a high frequency substantially sinusoidal voltage or current; and converting the high frequency substantially sinusoidal AC voltage or current into (i) a DC voltage or current, and (ii) a low frequency substantially sinusoidal AC voltage or current; wherein the high frequency substantially sinusoidal AC voltage or current is isolated from the DC voltage or current or the low frequency substantially sinusoidal AC voltage or current.

Abstract: A resonant circuit has two parallel resonant branches and one assistant inductor connecting the two resonant branches. Each resonant branch has a series capacitor and a series inductor connected in series. The other end of the series inductor is for connecting to a primary side of a transformer unit. The resonant circuit can provide two characteristic resonant frequencies fr and fm, so that a resonant power converter using the resonant circuit has an operating frequency fs that satisfies fs>fr or fm<fs<fr. The combination of the two resonant branches and the assistant inductor can reduce the frequency switch range of the resonant power converter within a specific voltage gain.