Dc-dc converter with extended light load operating range and method of use thereof

Abstract

A DC-DC converter includes a bridge circuit; a resonant tank coupled to an output of the bridge circuit; a rectifier circuit having an output and an input coupled to the resonant tank; and an output filter coupled to the output of the rectifier circuit. A controller is coupled to the bridge circuit, the resonant tank, and an output of the DC-DC converter. The controller is configured to control a switching frequency of the bridge circuit to operate the DC-DC converter as a boost converter and a buck converter. When the DC-DC converter is operating as a buck converter, the controller controls the rectifier circuit to selectively block the transfer of energy from the rectifier circuit to the converter output based on an output current of the DC-DC converter being greater than its current demand, thereby reducing the output current of the DC-DC converter and extending its operating range.

Description

Technical Field

[0001] This invention generally relates to a DC-DC converter, and more particularly to a DC-DC converter having an extended operating range for light loads. Background Technology

[0002] A DC-DC converter, which transforms an input DC voltage into an output DC voltage, is used as part of a battery charging system, such as in electric vehicles, where the converter is powered by a large, high-voltage battery that requires recharging from the grid. For personal safety and to avoid the risk of electric shock, current isolation from the grid to the high-voltage battery is required. Magnetic coupling coils meet this current isolation requirement. To transfer energy in this magnetic circuit, the DC source is excited by a transistor bridge circuit. The DC source is provided by a power factor correction (PFC) circuit, which converts the AC grid voltage to a 400V DC source. The grid voltage limits this DC source to a worst-case voltage of 375V.

[0003] The battery voltage range can be any from 200V to 500V, depending on the series and parallel combination of individual battery cells. Battery charging systems typically require a constant DC current to the battery pack. A separate battery controller monitors the battery voltage and individual cell temperature, and governs the charging current appropriate for the specific battery state of charge.

[0004] One limitation of some resonant converter topologies is the voltage range, especially under low load conditions. For some battery charging systems, the turns ratio and inductance ratio design parameters mitigate this limitation. Generally speaking, the transformer turns ratio sets an approximate minimum voltage capability, while the ratio between the magnetizing inductance and the series resonant inductance determines the gain of the tank circuit for maximum battery voltage charging. For a 2:1 primary-to-secondary turns ratio, a 400VDC link source can operate down to a certain battery charging state and maintain zero-voltage switching (ZVS) in the bridge transistor, even for relatively low currents. This is because the primary-side current at this operating point is dominated by the magnetizing current. The low magnetizing inductance used to set the tank gain intentionally increases the magnetizing current. However, as the switching frequency increases above the resonant frequency of the tank circuit, the peak magnetizing current decreases, and ZVS is lost. This is the buck operating region, and the resonant tank transfer function also flattens, requiring switching frequencies higher than the bridge transistor's capability. Figure 1The diagram illustrates the normalized transfer function of the resonant tank circuit as a function of the switching frequency, showing both boost and buck operating regions. Below the resonant frequency, the DC-DC converter operates in the boost region or in boost mode, where the resonant tank circuit operates in discontinuous conduction mode (DCM), and the zero-voltage switch (ZVS) is maintained as long as the energy in the resonant tank is not depleted. It is desirable to reduce this boost range because energy losses in power transfer increase as the switching frequency decreases. Furthermore, the hold-up time on the capacitors of the resonant tank circuit eventually becomes a limitation because the ZVS is lost, and the transfer function also becomes non-monotonic. This low switching frequency operating point sets the maximum charging voltage capability. If a decision is made to reduce the minimum battery voltage or current required by the battery charging system beyond the operating range of the resonant tank circuit, the resonant tank circuit may need to be redesigned. And this increased range may only be needed to support rare conditions where the battery is in a deeply depleted state of charge. Summary of the Invention

[0005] Ideally, the battery charger should support an extended voltage range while also providing a wide range of charging currents. Since AC current can increase battery cell temperature, it is desirable to minimize the AC current component within the charging current and maximize the DC current. It is also desirable for current-isolated DC-DC converters to have high efficiency and power density.

[0006] The example embodiment overcomes the shortcomings of known DC-DC converters and meets the need for a DC-DC converter with an extended operating range. When the DC-DC converter is part of a battery charging system, the extended operating range allows the battery to be charged even when it is in a deeply depleted state.

[0007] According to an example embodiment, the DC-DC converter includes a bridge circuit having at least one circuit branch with a pair of switching transistors. A resonant tank circuit is coupled to the output of at least one circuit branch of the bridge circuit. A rectifier circuit has at least one output terminal and at least one input terminal coupled to the resonant tank circuit, and the rectifier circuit rectifies the signal at the output of the resonant tank circuit. An output filter includes at least one input terminal coupled to at least one output terminal of the rectifier circuit, and the output of the filter circuit is the output of the DC-DC converter. A controller has a first input terminal coupled to the output of the DC-DC converter, a second input terminal coupled to the resonant tank circuit, and a first output terminal connected to the control terminals of the switching transistors of the bridge circuit. The controller is configured to control the switching frequency of the bridge circuit to operate the DC-DC converter as a boost converter and a buck converter. When the DC-DC converter is operating as a buck converter, the controller is configured to control the rectifier circuit based on the output current of the DC-DC converter being greater than its current demand, to selectively block energy transfer from the rectifier circuit to the output filter. This energy transfer barrier reduces the output current of the DC-DC converter and thus extends its operating range.

[0008] The DC-DC converter further includes a power transistor connected to the rectifier circuit along at least one current path in the rectifier circuit. The controller includes a second output terminal coupled to a control terminal of the power transistor. The controller turns off the power transistor for a period of time based on the output current of the DC-DC converter and based on a setpoint current level, said setpoint current level being based on the current demand of the DC-DC converter's output current.

[0009] When the power transistor is off, the resonant tank circuit is in discontinuous conduction mode (DCM). The controller turns off the power transistor in response to the current level in the resonant tank circuit reaching a first predetermined current value to maintain zero-voltage switching. After the power transistor is turned off, the controller turns the power transistor back on when the output current of the DC-DC converter reaches the setpoint current level. The controller controls the power transistor to turn on while maintaining zero-current switching.

[0010] In one aspect, the resonant tank circuit includes an LLC resonant tank circuit comprising a capacitor and a first inductor, the capacitor having a first terminal coupled to an output of at least one circuit branch. The DC-DC converter further includes a transformer having a primary winding and a secondary winding, wherein the primary winding forms part of the resonant tank circuit, and the first inductor is coupled between the capacitor and the primary winding of the transformer.

[0011] In one aspect, the bridge circuit includes a full-bridge circuit, and the at least one circuit branch includes a pair of circuit branches. The rectifier includes a full-wave rectifier connected to the end terminals of the secondary winding.

[0012] The DC-DC converter includes a sensing resistor coupled to the output terminal of the DC-DC converter. The sensing resistor is coupled to a third input terminal of the controller. Attached Figure Description

[0013] The various aspects of the present invention will now be explained in detail with reference to the accompanying drawings and exemplary embodiments, wherein:

[0014] Figure 1 This is a graph illustrating the boost and buck operating regions of a conventional DC-DC converter, which is a resonant tank circuit transfer function based on the switching frequency.

[0015] Figure 2 This is a schematic diagram of a DC-DC converter according to an example embodiment;

[0016] Figure 3 The diagram shows the operation in normal buck mode at the first output voltage. Figure 2 A collection of waveforms from a DC-DC converter;

[0017] Figure 4 The diagram illustrates the operation in extended buck mode at the first output voltage. Figure 2 A collection of waveforms from a DC-DC converter;

[0018] Figure 5 It is the same as operating in normal pressure reduction mode. Figure 2 The set of waveforms associated with the DC-DC converter; and

[0019] Figure 6 The diagram illustrates the operation in extended buck mode at the second output voltage. Figure 2 A collection of waveforms from a DC-DC converter. Detailed Implementation

[0020] The following description of one or more example embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or use. The same reference numerals are used to identify the same or similar elements in the drawings and throughout the detailed description. For clarity, elements are not shown to scale unless otherwise specified.

[0021] The example embodiments generally relate to a DC-DC converter that has an extended operating range without increasing the modulation frequency. Specifically, the example embodiments allow the DC-DC converter to operate as both a buck converter and a boost converter. Regarding operation as a buck converter, the DC-DC converter selectively and temporarily blocks energy transfer to the converter's output, thereby reducing the converter's output current, causing the DC-DC converter to operate in a discontinuous conduction mode, and thus operating within an extended operating range.

[0022] Figure 2 This is a block diagram of a DC-DC converter 10 according to an example embodiment. The DC-DC converter 10 converts an input DC voltage into an output DC voltage. The DC-DC converter 10 is configured to operate as a boost converter to provide a voltage at its output that is increased or boosted relative to the received input voltage, and as a buck converter to provide a DC voltage at its output that is lower than the received DC input voltage. One application of the DC-DC converter 10 is in a battery charging system for charging one or more batteries in a motor vehicle. The batteries in such a motor vehicle can provide propulsion. When used as part of such a motor vehicle battery charging system, the DC-DC converter 10 receives at its input the output of a PFC and / or an AC-DC converter (not shown) that provides a connection to an AC power source.

[0023] In the illustrated embodiment, the DC-DC converter 10 includes an input filter circuit 12 coupled across the input terminals of the DC-DC converter 10. The input filter circuit 12 is a capacitor that smooths the signal received at the input of the DC-DC converter 10. It should be understood that the input filter circuit 12 may have different configurations and / or include additional electrical or electronic components. A bridge circuit 14 is connected to the input filter circuit 12 and coupled across the input of the DC-DC converter 10. The bridge circuit 14 is illustrated as a full-bridge circuit having a first circuit branch 14A and a second circuit branch 14B. Each of the first circuit branch 14A and the second circuit branch 14B includes a high-side switching transistor and a low-side switching transistor connected between the input terminals of the DC-DC converter 1. Control terminals of each high-side transistor and low-side transistor are coupled to receive an input control signal. In an alternative embodiment, the bridge circuit 14 is a half-bridge circuit comprising a single circuit branch.

[0024] The DC-DC converter 10 further includes a resonant tank path 16 having input terminals coupled to the output terminals of the bridge circuit 14, such that a first circuit branch 14A carries current through the resonant tank path 16 in a first direction, and a second circuit branch 14B carries current through the resonant tank path in the opposite second direction. Figure 4 In the example embodiment illustrated, the resonant tank path 16 is a resonant LLC tank path including a capacitor 16A and an inductor 16B connected in series. The transformer 18 includes a primary winding 18A connected in series with the capacitor 16A and the inductor 16B to form the resonant tank path 16, and a secondary winding 18B coupled to the primary winding 18A. The transformer 18 provides current isolation between the AC power supply and the output terminals of the DC-DC converter 10.

[0025] Although the resonant tank path 16 is an LLC tank path as shown, it should be understood that the resonant tank path 16 can have different implementations. For example, the resonant tank path 16 can be an LCC or CLL resonant tank path. These different implementations can include the resonant tank elements of the transformer 18. In one example embodiment, the inductor 16B is part of the transformer 18, and in particular, the inductor 16B is the leakage inductance of the transformer 18 as part of an LLC, LCC, or CLL resonant tank implementation.

[0026] Continue to refer to Figure 2 The rectifier circuit 20 has an input terminal coupled to the secondary winding 18B. The rectifier circuit 20 rectifies the voltage appearing across the secondary winding 18B and provides a rectified signal at its output. In the illustrated example embodiment, the rectifier circuit 20 is a full-wave rectifier circuit with an input terminal connected to an end of the secondary winding 18B. Each end of the secondary winding 18B is coupled between a pair of diodes 21A, 21B connected in series. Specifically, each end of the secondary winding 18B is coupled to the anode of diode 21A and the cathode of diode 21B in the series-connected diode pair, wherein the cathodes of diode 21A are connected together at one output terminal of the rectifier circuit 20, and the anodes of diode 21B are connected together at a second output terminal. It should be understood that the rectifier circuit 20 can be connected along different portions of the secondary winding 18B. Furthermore, the rectifier circuit 20 can be a full-wave rectifier circuit, but has any of several different known implementations. It should be further understood that rectifier 20 can be implemented as a half-wave rectifier, wherein, for example, a single diode is coupled to the end of the secondary winding 18B.

[0027] The DC-DC converter 10 further includes an output filter circuit 22 for filtering unwanted ripple and noise from the output of the rectifier circuit 20. The output filter circuit 22 is connected across the output capacitor 24 of the DC-DC converter 10. Figure 2As shown, the output filter circuit 22 includes an input terminal coupled to the output terminal of the rectifier circuit 20. In one implementation, the output filter circuit 22 includes a first-order LC filter comprising an inductor 22A connected in series with a capacitor 22B, wherein the output of the DC-DC converter 10 is connected across the capacitor 22B. It should be understood that the output filter circuit 22 can be implemented differently and / or use different and / or additional electrical or electronic components to perform the signal filtering function.

[0028] The DC-DC converter 10 further includes a transistor 26 connected to and / or deployed within the rectifier circuit 20. The transistor 26 is connected in the current path of each circuit path having diodes 21A and 21B connected in series. Figure 4 In the example embodiment shown, transistor 26 is connected between diode 21B and ground in each rectifier circuit path. It should be understood that, alternatively, transistor 26 may be connected elsewhere in the current path of rectifier circuit 20.

[0029] The DC-DC converter 10 further includes a current measurement circuit 40 that senses the current on the primary side of the transformer 18 and provides a sensed current signal 41 at its output. Figure 2 In one example implementation illustrated, the current measurement circuitry includes an inductor 40A connected to the rectifier circuitry 40B. The DC-DC converter 10 further includes a sensing resistor 30 disposed between the negative output terminal of the DC-DC converter 10 and ground. By providing an ungrounded terminal of the sensing resistor 30 to the controller 28, the controller 28 is able to determine the output current of the DC-DC converter 10.

[0030] The DC-DC converter 10 includes a controller 28 configured to control the DC-DC converter 10 in various operating modes. For example... Figure 2 As shown, controller 28 includes: a first input terminal coupled to the positive output terminal of DC-DC converter 10 via filter circuit 29A; and a second input terminal coupled to the negative output terminal of DC-DC converter 10 via differential amplifier circuit 29C and filter circuit 29B. Generally, based in part on the output of DC-DC converter 10, controller 26 generates control signals for controlling bridge circuit 14 at output terminals 28A-28B (which are coupled to the control or gate terminals of switching transistors in circuit branches 14A and 14B of bridge circuit 14). In one embodiment, the control signals at output terminals 28A-28B are pulse width modulation (PWM) signals. Output terminals 28C-28F are connected to circuit branches 14A and 14B via signal conditioning and / or level shifting circuit 27.

[0031] exist Figure 2 In the example embodiment illustrated, controller 28 includes analog-to-digital converter (ADC) circuit 32 that receives a filtered voltage across each output terminal of DC-DC converter 1 and generates a digital output signal. The output of ADC circuit 32 is provided to compensation / filter block 34, which filters the digital output of ADC circuit 32 to limit the amount of overshoot and undershoot. The output of compensation / filter block 34 is provided to frequency modulator circuit 36, which also receives the output of comparator 38, which receives an output current signal 41 from current measurement circuit 40 at its input. Frequency modulator circuit 36 ​​may utilize an enhanced pulse width modulation (ePWM) topology. Based on the output current of DC-DC converter 1, as measured across sensing resistor 30, amplified by differential amplifier 29C, and filtered by filter 29B, frequency modulator circuit 36 ​​generates a control signal at its output terminal, which is applied to the control terminals of the high-side and low-side switches of circuit branches 14A and 14B of bridge circuit 14.

[0032] The controller 28 can be implemented as a microcontroller, including one or more processor circuits and a memory (non-volatile, volatile, or both) storing instructions that, when executed by the one or more processor circuits, control the DC-DC converter 10 to provide a DC voltage output. In one implementation, the controller 28 is a digital signal processor circuit. The controller 28 controls the operation of the DC-DC converter 10, causing the converter to operate in boost and buck modes or within an operating range. Operating in both boost and buck modes provides a relatively wide operating range for the output of the DC-DC converter 10.

[0033] Figure 3 The diagram illustrates the operation of DC-DC converter 1 as a buck converter in a CCM. The waveform above represents the voltage of the control signal generated by frequency modulator circuit 36 ​​and applied to two circuit branches 14A and 14B of bridge circuit 14. The waveform below includes the current in the primary winding 18A, the current in the secondary winding 18B, and the output current at the output of DC-DC converter 1. Turning on one circuit branch 14A or 14B while turning off the other branch causes the current in the primary winding 18A to rise or fall. Rectifying the current in the secondary winding 18B using rectifier circuit 20, the current at the output of DC-DC converter 1 is approximately 2A DC at an output voltage of 185V.

[0034] In addition to its normal operation as a boost converter and buck converter, controller 28 controls DC-DC converter 10 within an extended operating range. This extended operating range better ensures efficient charging of vehicle batteries that have already been significantly discharged when DC-DC converter 10 is used as part of a battery charging system for charging the batteries of electric vehicles.

[0035] Specifically, when the DC-DC converter 10 is operating as a buck converter, where the converter's output voltage is less than the voltage at the converter's input, transistor 26 remains on, and the DC-DC converter 10 is in continuous conduction mode (CCM). However, if the output current required by the load of the DC-DC converter 10 is at least a predetermined amount less than the current supplied by the DC-DC converter 10 at the converter output, as seen across the sensing resistor 30 and determined by the controller 28, then the controller 28 operates the DC-DC converter 10 in discontinuous conduction mode (DCM), in which a reduced current is supplied at the output terminals of the DC-DC converter 10. This is accomplished by selectively turning transistor 26 on and off during this period in DCM.

[0036] Figure 4 The diagram illustrates the operation of DC-DC converter 10 as a buck converter in this extended operating mode. This operation occurs when the output current required by the load of DC-DC converter 10 is at least a predetermined amount less than the current supplied by DC-DC converter 10 at the converter output. For the full-bridge circuit 24 (300kHz), as... Figure 3 The DC-DC converter 1 illustrated herein operates as a buck converter in the CCM and as follows: Figure 4 The converter illustrated operates as a buck converter in a DCM at the same output voltage (185V) and the same switching frequency. During operation, transistor 26 is selectively turned on and off, and the amount of time transistor 26 is off is adjusted cycle-by-cycle by controller 28 until the output DC current of DC-DC converter 1 equals the setpoint current value, which can be based on the output current demand. In a given cycle, when the primary side of DC-DC converter 1 reaches the desired or predetermined current, transistor 26 is synchronously turned off to maintain zero-voltage switching (ZVS). The transistor remains off for the desired modulation time. This off-time is set by controller 28 and is based on the output current of DC-DC converter 1 and the output current setpoint. After being turned off, the current in the secondary side of transformer 18—that is, the current through the secondary winding 18B—is zero, therefore turning on transistor 26 results in zero-current switching (ZCS), and switching losses are reduced or minimized.

[0037] Figure 4The topmost waveform is the voltage at the control terminal of transistor 26, and the middle waveform set is the control signal of the two circuit branches 14A and 14B of bridge circuit 14. Figure 4 The following waveform set includes the current in the primary winding 18A, the current in the secondary winding 18B, and the output current of the DC-DC converter 1. When the desired / predetermined current is reached on the primary side of transformer 18, transistor 26 is turned off to maintain ZVS. Specifically, transistor 26 is turned off when one of the active circuit branches 14A and 14B is turned off. In response to transistor 26 being turned off, the current in the secondary winding 18B of transformer 18 becomes zero, and no energy is transferred from rectifier circuit 20 to output filter circuit 22. Transistor 26 remains off for a predetermined time, during which time transistor 26 is turned on by controller 28. Transistor 26 remains on until the next active circuit branch 14A or 14B is turned off. See again. Figure 4 The current in the secondary winding 18B is non-zero only when transistor 26 is turned on. Therefore, selectively turning off transistor 26 reduces the output current of DC-DC converter 1. Figure 4 When the output voltage of the DC-DC converter 10 is 185V, the output current of the converter is 0.5A, which is much lower than that of the DC-DC converter 10. Figure 3 The DC-DC converter 10 shown in the diagram has a 2A converter output current under CCM, thus providing an extended operating range for the converter. This extended operating range extends the output voltage of the DC-DC converter 10 and allows charging of batteries or other devices at relatively low output voltage and current levels (which, conversely, would be unavailable when maintaining AVS or ACS), thereby reducing or minimizing switching losses.

[0038] To achieve the same low charging current (0.5A) at the output of the DC-DC converter 10 as provided in extended operating mode, a switching frequency of 385kHz is required using normal (standard) CCM. Figure 5 As shown in the diagram. Here, for ZVS, DC-DC converter 1 has only 3.4A on the primary side of transformer 18, compared to ZVS, Figure 4 The extended buck operating mode has 4.8A on the primary side.

[0039] Figure 6 The diagram illustrates the operation of DC-DC converter 1 in extended operating mode at a 150V output, far exceeding the frequency range of the normal (standard) CCM for a buck converter. Here, extended operating mode operation reduces the converter's output current to 1.3A while maintaining a ZVS of 7A on the primary side of transformer 18.

[0040] In the example embodiment, during normal boost operation, transistor 26 is on 100% of the time, and power consumption is a manageable 25W. Therefore, the penalty for inserting this extended range transistor 26 is an efficiency of approximately 0.45%. Due to the additional magnetizing current, the efficiency drop at the maximum battery voltage (i.e., approximately 475V) is about 0.5% lower than the peak efficiency (near the resonant frequency). With transistor 26 in place, the efficiency drop is reduced by 0.5% by decreasing and / or minimizing the boost region and using a larger voltage range available in the extended buck region.

[0041] Example embodiments have been described in an illustrative manner herein, and it should be understood that the terminology used is intended to be descriptive rather than limiting. It is obvious that many modifications and variations of the invention are possible in light of the above teachings. The above description is merely exemplary in nature, and therefore, variations may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A DC-DC converter, comprising: A bridge circuit includes at least one circuit branch having at least two switching transistors; A resonant tank circuit, coupled to the output of at least one circuit branch of the bridge circuit; A rectifier circuit having at least one output terminal and at least one input terminal coupled to the resonant tank circuit, the rectifier circuit rectifying the signal at the output of the resonant tank circuit; An output filter circuit having at least one input terminal coupled to the at least one output terminal of a rectifier circuit, the output of the output filter circuit including the output of a DC-DC converter; A controller has a first input terminal coupled to the output of the DC-DC converter, a second input terminal coupled to the resonant tank circuit, and a first output terminal connected to the control terminal of the switching transistor of the bridge circuit. The controller is configured to control the switching frequency of the bridge circuit to operate the DC-DC converter as both a boost and buck converter, and when the DC-DC converter is operating as a buck converter, based on the output current of the DC-DC converter being greater than its current demand, to control the rectifier circuit to selectively prevent energy transfer from the rectifier circuit to the output filter. This energy transfer prevention reduces the output current of the DC-DC converter and extends its operating range. A power transistor is connected to the rectifier circuit along at least one current path in the rectifier circuit, wherein the controller includes a second output terminal coupled to a control terminal of the power transistor, the controller turning off the power transistor for a period of time based on the output current of the DC-DC converter and based on a setpoint current level, the setpoint current level being based on the current demand of the output current of the DC-DC converter. The controller turns off the power transistor in response to the current level in the resonant tank circuit reaching a first predetermined current value to maintain zero-voltage switching.

2. The DC-DC converter of claim 1, wherein the resonant tank circuit is in discontinuous conduction mode (DCM) when the power transistor is turned off.

3. The DC-DC converter of claim 1, wherein after the power transistor is turned off, the controller turns on the power transistor when the output current of the DC-DC converter reaches the setpoint current level.

4. The DC-DC converter according to claim 1, wherein the controller controls the power transistor to turn on while maintaining zero current switching.

5. The DC-DC converter of claim 1, wherein the resonant tank circuit comprises an LLC resonant tank circuit, the LLC resonant tank circuit comprising a capacitor and a first inductor, the capacitor having a first terminal coupled to the output of the at least one circuit branch, and the DC-DC converter further comprising a transformer having a primary winding and a secondary winding, the primary winding forming part of the resonant tank circuit, and the first inductor coupled between the capacitor and the primary winding of the transformer.

6. The DC-DC converter of claim 1, wherein the bridge circuit comprises a full-bridge circuit, and the at least one circuit branch comprises a pair of circuit branches.

7. The DC-DC converter of claim 1, further comprising a transformer having a primary winding and a secondary winding, wherein the primary winding forms part of a resonant tank circuit, and wherein the rectifier comprises a full-wave rectifier connected to the end terminals of the secondary winding.

8. The DC-DC converter of claim 1, further comprising a sensing resistor coupled to an output terminal of the DC-DC converter, the sensing resistor being further coupled to a third input terminal of the controller.

9. A method for controlling a DC-DC converter, the DC-DC converter comprising a resonant tank circuit, a bridge circuit having at least one output terminal coupled to an input terminal of the resonant tank circuit, and a rectifier circuit coupled to the output terminal of the resonant tank circuit, the method comprising: Based on the output of the DC-DC converter, the switching frequency of the bridge circuit is controlled so that the DC-DC converter can operate as both a buck converter and a boost converter. During the period when the DC-DC converter operates as a buck converter, selectively preventing energy transfer from the rectifier circuit to the output of the DC-DC converter extends the operating range of the DC-DC converter as a buck converter; and The power transistor is turned off for a period of time based on the output current of the DC-DC converter and based on a setpoint current level, the setpoint current level being based on the current demand of the output current of the DC-DC converter, the power transistor being connected to the rectifier circuit along at least one current path in the rectifier circuit, wherein the controller includes a second output terminal coupled to a control terminal of the power transistor, wherein the power transistor is turned off in response to the current level in the resonant tank circuit reaching a first predetermined current value to maintain zero-voltage switching.

10. The method of claim 9, wherein the energy blocking is performed in response to the output current of the DC-DC converter being greater than the current demand for the output current.

11. The method of claim 10, wherein the energy blocking ends in response to the current level at the output of the DC-DC converter reaching a setpoint current level, the setpoint current level being based on the current demand.

12. The method of claim 11, wherein the DC-DC converter includes a transistor coupled within a current path of a rectifier circuit, and the energy blocking includes selectively turning off the transistor to open a current path in the rectifier circuit.

13. The method of claim 12, wherein the resonant tank circuit comprises a transformer having a primary winding and a secondary winding, wherein when the transistor is turned on, the current path is closed, and the rectifier circuit rectifies the signal appearing across the secondary winding and transfers energy to the output of the DC-DC converter.

14. The method of claim 13, wherein the transistor is turned off to maintain zero-voltage switching on the primary side of the transformer and remains off for a period of time based on the setpoint current level and the output current of the DC-DC converter.

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