Controller for resonant converter

Abstract

The invention relates to a controller for a resonant converter. The controller is configured to: compare the measured current signal to a positive current threshold; comparing the measured current signal to a negative current threshold; and setting a power reduction mode to active if either or both of the following occur: the measured current signal is less than the positive current threshold at the end of a previous high side switching half-cycle; and at the end of a previous low-side switching half-cycle, the measured current signal is greater than the negative current threshold. When the power reduction mode is active: the controller gradually reduces a value of a reduced power signal. When the power reduction mode is inactive: the controller gradually increases the value of the reduced power signal. The controller sets an upper voltage threshold and a lower voltage threshold based on the lower of: i) the reduced power signal; and ii) a power setting signal.

Description

Technical Field

[0001] The present disclosure relates to controllers and associated methods for resonant converters. Background Art

[0002] There are technical bottlenecks in resonant converters. Summary of the invention

[0003] According to a first aspect of the present disclosure, a controller for a resonant converter is provided, wherein the resonant converter is used to supply electric energy from a supply source to a load, and the resonant converter includes:

[0004] a first switch and a second switch, the first switch and the second switch being connected in series with each other between a supply source and a reference terminal, wherein when the first switch is closed and the second switch is open, the resonant converter has a high-side switching half-cycle, and wherein when the first switch is open and the second switch is closed, the resonant converter has a low-side switching half-cycle;

[0005] a resonant tank electrically connected to the first switch and the second switch, wherein the resonant tank includes a resonant capacitor;

[0006] The controller is configured as:

[0007] receiving a measured current signal representative of a current flowing in the resonant tank;

[0008] receiving a measured voltage signal representative of a voltage at a predetermined point in the resonant tank;

[0009] receiving a power setting signal defining a requested power level for the load;

[0010] comparing the measured current signal to a positive current threshold;

[0011] comparing the measured current signal to a negative current threshold;

[0012] The power reduction mode is set to active if either or both of the following occur:

[0013] At the end of the previous high-side switching half cycle, the measured current signal is less than a positive current threshold; and

[0014] At the end of the previous low-side switching half cycle, the measured current signal is greater than the negative current threshold;

[0015] The power reduction mode is set to Inactive if both of the following occur:

[0016] At the end of the previous high-side switching half cycle, the measured current signal is not less than a positive current threshold; and

[0017] At the end of the previous low-side switching half cycle, the measured current signal is not greater than the negative current threshold;

[0018] When the power reduction mode is active: gradually decreasing the value of the reduced power signal;

[0019] When the power reduction mode is inactive: gradually increasing the value of the reduced power signal;

[0020] setting the upper and lower voltage thresholds based on the lower of: i) the reduced power signal; and ii) the power setting signal;

[0021] In response to the measured voltage signal exceeding an upper voltage threshold, opening the first switch and closing the second switch;

[0022] In response to the measured voltage signal falling below a lower voltage threshold, the second switch is opened and the first switch is closed.

[0023] Advantageously, such a controller may adjust the levels of the upper and lower voltage thresholds such that the controller will automatically transition the resonant converter from being controlled based on the level of the measured current signal to being controlled based on the level of the measured voltage signal.

[0024] In one or more embodiments, the controller is configured to:

[0025] When the power reduction mode is inactive: gradually increasing the value of the reduced power signal until the reduced power signal is greater than the power setting signal.

[0026] In one or more embodiments, the controller is configured to:

[0027] When the power reduction mode is inactive: gradually increasing the value of the reduced power signal until the reduced power signal is a predetermined amount greater than the power setting signal.

[0028] In one or more embodiments, the controller is configured to:

[0029] When power reduction mode is inactive:

[0030] gradually increasing the value of the reduced power signal until the reduced power signal is a predetermined amount greater than the power setting signal; and

[0031] The value of the reduced power signal is maintained at a predetermined amount greater than the power setting signal until the power reduction mode becomes active.

[0032] In one or more embodiments: the predetermined amount is any of: a predetermined proportion of the power setting signal; or a predetermined absolute amount greater than the power setting signal.

[0033] In one or more embodiments, the controller further comprises a timeout timer, wherein the controller is configured to:

[0034] When the power reduction mode is active, starting a timeout timer; and

[0035] If the timeout timer reaches the end value, corrective action is taken.

[0036] In one or more embodiments, the controller is configured to:

[0037] When the power reduction mode is active: gradually decreasing the value of the reduced power signal at a first rate of change;

[0038] When the power reduction mode is inactive: gradually increasing the value of the reduced power signal at a second rate of change; and

[0039] The first change rate is higher than the second change rate.

[0040] In one or more embodiments, the controller is further configured to:

[0041] in response to the measured current signal falling below a positive current threshold at the end of a high-side switching half cycle, opening the first switch and closing the second switch;

[0042] In response to the measured current signal exceeding a negative current threshold at the end of the low-side switching half-cycle, the second switch is opened and the first switch is closed.

[0043] In one or more embodiments, the controller is configured to:

[0044] In response to the measured voltage signal exceeding an upper voltage threshold, the first switch is opened, a delay period is waited, and then the second switch is closed.

[0045] In one or more embodiments, the controller is configured to:

[0046] In response to the measured voltage signal falling below a lower voltage threshold, the second switch is opened, a delay period is waited, and then the first switch is closed.

[0047] In one or more embodiments, the controller is further configured to:

[0048] receiving a protected power signal defining a power level based on a time delay between a change in state of one of the switches and a subsequent zero crossing of the measured current signal; and

[0049] The upper voltage threshold and the lower voltage threshold are set based on the lower of: i) a reduced power signal; ii) a power setting signal; and iii) a protected power signal.

[0050] In one or more embodiments, the protected power signal corresponds to a power level at which the measured voltage signal will cross: an upper voltage threshold during a high-side switching half cycle; and a lower voltage threshold during a low-side switching half cycle.

[0051] Also disclosed is a resonant converter comprising any controller disclosed herein.

[0052] According to another aspect of the present disclosure, there is provided a method of operating a resonant converter, wherein the resonant converter is used to supply electrical energy from a supply source to a load, the resonant converter comprising:

[0053] a first switch and a second switch, the first switch and the second switch being connected in series with each other between a supply source and a reference terminal, wherein when the first switch is closed and the second switch is open, the resonant converter has a high-side switching half-cycle, and wherein when the first switch is open and the second switch is closed, the resonant converter has a low-side switching half-cycle; and

[0054] a resonant tank electrically connected to the first switch and the second switch, wherein the resonant tank includes a resonant capacitor;

[0055] The methods include:

[0056] receiving a measured current signal representative of a current flowing in the resonant tank;

[0057] receiving a measured voltage signal representative of a voltage at a predetermined point in the resonant tank;

[0058] receiving a power setting signal defining a requested power level for the load;

[0059] comparing the measured current signal to a positive current threshold;

[0060] comparing the measured current signal to a negative current threshold;

[0061] The power reduction mode is set to active if either or both of the following occur:

[0062] At the end of the previous high-side switching half cycle, the measured current signal is less than a positive current threshold; and

[0063] At the end of the previous low-side switching half cycle, the measured current signal is greater than the negative current threshold;

[0064] The power reduction mode is set to Inactive if both of the following occur:

[0065] At the end of the previous high-side switching half cycle, the measured current signal is not less than a positive current threshold; and

[0066] At the end of the previous low-side switching half cycle, the measured current signal is not greater than the negative current threshold;

[0067] When the power reduction mode is active: gradually decreasing the value of the reduced power signal;

[0068] When the power reduction mode is inactive: gradually increasing the value of the reduced power signal;

[0069] setting the upper and lower voltage thresholds based on the lower of: i) the reduced power signal; and ii) the power setting signal;

[0070] In response to the measured voltage signal exceeding an upper voltage threshold, opening the first switch and closing the second switch;

[0071] In response to the measured voltage signal falling below a lower voltage threshold, the second switch is opened and the first switch is closed.

[0072] Although the present disclosure allows for various modifications and alternative forms, the details of the present disclosure have been shown in the drawings by way of example and will be described in detail. However, it should be understood that other embodiments other than the described specific embodiments are also possible. All modifications, equivalents and alternative embodiments falling within the spirit and scope of the appended claims are also encompassed.

[0073] The above discussion is not intended to represent every example embodiment or every implementation within the scope of the current or future technical solution set. The drawings and the following detailed description also illustrate various example embodiments. Considering the following detailed description in conjunction with the accompanying drawings, various example embodiments can be more fully understood. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] One or more embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0075] Figure 1 A resonant converter is shown as part of the overall power supply;

[0076] Figure 2 shows the basic principle of capacitor voltage control for a resonant converter, where the resonant capacitor is on the ground side (shunt);

[0077] Figure 3a An example embodiment of a resonant converter is shown, the resonant converter including a controller according to the present disclosure;

[0078] Figure 3b Shows Figure 3a Additional details of the controller;

[0079] Figure 4 Another example embodiment of a controller according to the present disclosure is shown; and

[0080] Figure 5 An example embodiment of a method of operating a resonant converter according to the present disclosure is shown. DETAILED DESCRIPTION

[0081] The resonant power converter may use a half cycle by half cycle switchoff mechanism that involves sensing the resonant capacitor voltage (Vcap) and comparing Vcap to a power reference level for closed loop operation. This may be referred to as capacitor voltage control. Instead of directly controlling the switch by forcing the switching frequency, the switching frequency is generated by the Vcap control mechanism.

[0082] Figure 1 A resonant converter is shown as part of the overall power supply, which in this example is used in an adapter. The adapter includes the following blocks:

[0083] ●Active bridge controller (power rectification);

[0084] PFC controller section (power factor correction using boost converter architecture);

[0085] The resonant LLC controller section (driving half-bridge stage plus LLC resonant tank including tapped winding transformer); and

[0086] ● SR controller (synchronous rectifier stage for generating the rectified output voltage Vout).

[0087] The resonant converter can be sized using the first harmonic approximation. In addition, using the capacitor voltage control method to control the output power provides the following advantages:

[0088] ●Possibility to use a dedicated low power mode.

[0089] • Much better dynamics, since the delivered power follows the set point of the resonant tank variable almost immediately. This simplifies the control loop design.

[0090] A resonant converter may have a resonant capacitor in series with the half-bridge node, and information for controlling the switches may be derived from the voltage across the resonant capacitor (the side not connected to the half-bridge node).

[0091] This method actually senses the voltage across the transformer, and therefore the DC component of the sensed signal is zero. In some applications, the high voltage signal at the resonant capacitor / transformer node should be attenuated to a low voltage signal that can be processed by an integrated circuit (IC). For this purpose, a capacitive voltage divider can be used, because a resistive voltage divider will produce undesirable low-pass filtering. Due to the fact that the DC component is zero, no information about the DC component is needed, so a capacitive voltage divider (which cannot transmit a DC component) is sufficient.

[0092] As an alternative, the resonant capacitor can also be placed in the ground branch in series with the primary winding of the transformer, e.g. Figure 1 As shown in .

[0093] Figure 2 shows the basic principle of capacitor voltage control for a resonant converter, where the resonant capacitor is on the ground side (shunt). That is, the resonant capacitor is shunted between ground and the voltage source (Vbus) in order to reduce the ripple current in the ground path and the supply path.

[0094] FIG. 2 also shows the timing control principle of the switch using this embodiment.

[0095] The resonant converter of Fig. 2 includes a controller (not shown) that operates a first controllable switch and a second controllable switch arranged in series. These switches are connected between a supply source (Vbus) and a reference terminal (ground). The controller controls the first high-side switch using a signal marked as gh. The controller controls the second low-side switch using a signal marked as gl. The voltage at the half-bridge node (node ​​between the high-side switch and the low-side switch) is marked as Vhb.

[0096] The controller receives a measured voltage signal (Vcr) representing the voltage at a predetermined point in the resonant tank. In this example, the measured voltage signal (Vcr) represents the voltage across the resonant capacitor. As shown in FIG. 2, the controller compares the measured voltage signal (Vcr) with an upper voltage threshold (VCRH) and a lower threshold (VCRL). Based on the comparison, the switch half cycle ends. More specifically, in response to the measured voltage signal (Vcr) exceeding the upper voltage threshold (VCRH), the second low-side switch is closed, and the first high-side switch is disconnected. In response to the measured voltage signal (Vcr) falling below the lower voltage threshold (VCRL), the second low-side switch is disconnected, and the first high-side switch is closed. It should be understood that in an alternative embodiment, the polarity of the signal can be reversed, so that: in response to the measured voltage signal (Vcr) falling below the lower voltage threshold, the second low-side switch is closed, and the first high-side switch is disconnected; and in response to the measured voltage signal (Vcr) exceeding the upper voltage threshold, the second low-side switch is disconnected, and the first high-side switch is closed. In this example, a non-overlap time is implemented after opening one of the switches and before closing the other switch. In this way, there is a slight delay between the end of one half cycle and the beginning of the next half cycle. For simplicity, the decay of the Vcr voltage for the controller IC is not included in Figure 2.

[0097] For the graph shown in Figure 2, the resonant converter uses an input voltage Vbus of 400V, and therefore Vhb switches between 400V and 0V. For this example:

[0098] ΔV = 360-40 = 320V;

[0099] VCRH = Vbus / 2 + ΔV / 2 = 200 + 160 = 360 V; and

[0100] VCRl=Vbus / 2-ΔV / 2=200-160=40V.

[0101] gh and gl have an amplitude of 1V, which is a convenient representation of true = 1 and false = 0.

[0102] The arrangement of Figure 2 also takes into account the DC component of the resonant capacitor. The DC component is defined by the input voltage to the half bridge (Vbus) times the duty cycle of the half bridge voltage (typically Vbus / 2 at 50% duty cycle operation).

[0103] In practice, the capacitive divider cannot represent the DC component of the resonant capacitor due to leakage, and the resistive divider cannot reliably attenuate the high frequency portion of the Vcr signal due to parasitic capacitance. In the case of a resistive and capacitive divider in parallel, it is difficult to maintain the same attenuation ratio due to component tolerances. The DC component across the capacitive divider (and optionally the parallel resistive divider) can be restored in a practical circuit by adding a so-called symmetry loop.

[0104] The symmetry loop compares the measured duty cycle to the expected value of 50% and corrects the capacitive divider DC component so that a 50% duty cycle is achieved. The switch, together with the voltage divider, produces a DC component at the voltage divider. This DC component, together with the vcrh level and the vcrl level, defines the duty cycle and symmetry / asymmetry. The DC symmetry loop senses the duty cycle and adjusts the DC value so that a 50% duty cycle occurs with a symmetrical half cycle. The symmetry loop can rely on completing each half cycle by Vcr crossing one of the thresholds (VRCH or VCRL) so that a direct correlation is maintained with the divided resonant capacitor voltage. This includes the DC level and the difference between the two threshold levels (VRCH or VCRL).

[0105] We calculate below the converted power of the resonant converter of FIG. 2 as a function of ΔV, which is the difference between two threshold levels VCRH and VCRL (these thresholds may also be referred to as vcaph and vcapl).

[0106] ●ΔV across Cr between the start and end of each half cycle => charge difference

[0107] ○Power = average current × input voltage = I × Vbus

[0108] ○I=Q×Fswitch

[0109] ○Q=Cr×ΔV

[0110] ○Power = Cr × ΔV × Fswitch × Vbus can be set according to the switch cycle

[0111] It can be seen that the converted energy per switching cycle is proportional to ΔV. However, this does not guarantee that this energy can be achieved.

[0112] As ΔV is increased, the result is that the resonance becomes larger. However, larger resonance also means that the resonant current becomes closer to zero at the end of each half cycle. The situation where the resonant current becomes zero at the completion of the half cycle is called the 'capacitive mode limit'. Since the dv / dt of Vcr is proportional to the resonant current, it is not possible to achieve more power because this would mean that VCRH and VCRL from Figure 2 could not be reached because the peak value of the Vcr signal (when the resonant current crosses zero) is less than VCRH or VCRL or both, and after this moment, Vcr will start to return to zero.

[0113] A controller for a resonant converter may control the operation of a switch based on the resonant current. For example, two current thresholds may be used: a positive threshold (Iend_pos) and a negative threshold (Iend_neg). Iend_pos and Iend_neg may represent predefined levels of the resonant current, such that: when the resonant current is closer to zero than the level defined by Iend_pos, the high-side half cycle is completed; and when the resonant current is closer to zero than the level defined by Iend_neg, the low-side half cycle is completed. The result is a half cycle completed based on a comparison between the resonant current and the current threshold. This ensures that the system cannot enter a capacitive mode region, which may be destructive to the converter.

[0114] It has been discovered that, in some cases, a controller for an LLC resonant converter (i.e., an LLC resonant converter in which the resonant tank includes two inductors and one capacitor) may get stuck in asymmetric operation of the half-bridge. When this happens, the switches (which can be viewed as half-bridge (HB) MOSFETs) may hard switch, get hot, and possibly get damaged. Furthermore, in applications for adapters and PC power supplies, this asymmetric drift is detected and considered an undesirable mode of operation.

[0115] It may be important that the converter is able to recover from a dynamic asymmetric situation. For example, a situation where the system settles in an undesirable asymmetric solution, where one or both half cycles are ended by the resonant current crossing a current threshold, should be avoided. In addition, if a symmetrical loop is present, it should be able to recover a 50% duty cycle. This can be achieved by the following operations: two half cycles are ended due to Vcap control, and if necessary, the symmetrical loop adapts the DC component of the divided version of the Vcr signal so that when Vcap control occurs for both half cycles, the duty cycle changes. However, when only one of the half cycles is ended by Vcap control, this results in an open loop situation for the symmetrical loop, because when the Vcap mechanism is unable to complete a half cycle, varying the DC component has no effect on completing the half cycle.

[0116] The examples disclosed herein can address these issues by detecting the end of one of the two half cycles through the resonant converter crossing a current threshold, and in response, dialing back the power level (by reducing ΔV) until Vcap control takes over again. The result is that the system can recover towards a symmetrical situation, with a desired predefined distance between the actual power and the maximum power at the capacitive mode limit.

[0117] Figure 3a An example embodiment of a resonant converter 300 is shown, which includes a controller 301 according to the present disclosure. Figure 3b Shows Figure 3a The resonant converter 300 is used to supply electrical energy from a supply source (Vs) to a load at the secondary side of a transformer.

[0118] The resonant converter includes a first switch 302 and a second switch 303 connected in series with each other between a supply source (Vs) 304 and a reference terminal (ground in this example) 305. When the first switch 302 is closed and the second switch 303 is open, the resonant converter 300 has a high-side switching half-cycle. When the first switch 302 is open and the second switch 303 is closed, the resonant converter also has a low-side switching half-cycle. The resonant converter 300 also has a resonant tank electrically connected to the first switch 302 and the second switch 303. In this example, the resonant tank includes a resonant capacitor (Cr) and two inductors (Lr and Lm, where the resonant inductor Lr is generally defined by the extended inductance of a transformer), one of the two inductors being the primary winding of the transformer. The transformer has a secondary winding connected to the secondary side of the resonant converter 300. Therefore, Figure 3a The resonant converter 300 is an LLC converter (because the resonant tank includes two inductors and one capacitor), but it should be understood that the examples disclosed herein can be used with different types of resonant converters having different types of resonant tanks.

[0119] The controller 301 receives a measured current signal (Iprim) 306 representing the current flowing in the resonant tank. In this example, the current flowing through the primary transformer winding is sensed to provide the measured current signal (Iprim) 306. However, it should be appreciated that in other examples, the current in the resonant tank may be sensed at a different location. The controller 301 receives a measured voltage signal (Vcap) 307 representing the voltage at a predetermined point in the resonant tank. In this example, the voltage dropped across the resonant capacitor (Cr) is sensed to provide the measured voltage signal (Vcap) 307. However, it should be appreciated that in other examples, the voltage dropped across different components in the resonant tank may be sensed.

[0120] As will be discussed below, the controller 301 uses the resonant tank variables Vcap and Iprim to control the timing of the resonant tank.

[0121] Figure 3a Also shown is a parasitic capacitance (Cds) connected in parallel with the half-bridge node and the reference terminal 305. This parasitic capacitance (Cds) is essentially the MOSFET Cds capacitance (i.e., the capacitance between the drain and source of the second switch 303). The first switch 302 has a corresponding parasitic capacitance (Cds), which is also shown in FIG. Figure 3a . In some applications, additional capacitance is added in parallel with the parasitic capacitance (Cp) of the second switch 303, and thus the capacitance is not completely parasitic. The capacitance level of Cp is related to the energy required to charge the half-bridge node to the opposite power rail, and thus it is partially related to the threshold current level that can be compared with the resonant current to determine whether the half cycle should end.

[0122] like Figure 3b As shown in , the controller 301 also receives a power setting signal (Ps) 308. The power setting signal (Ps) 308 defines the requested power level of the load. In this example, the controller 301 receives the power setting signal (Ps) 308 from another component associated with the resonant converter 300. In another example, the power setting signal (Ps) 308 is generated by the controller 301 itself.

[0123] The controller 301 includes a first voltage comparator 309 that compares the measured voltage signal (Vcap) 307 with an upper voltage threshold (VCRH, which is Figure 3b The output signal of the first voltage comparator 309 indicates whether the measured voltage signal (Vcap) 307 is greater than the upper voltage threshold (VCRH). The controller 301 also includes a logic 313 (in Figure 3b309) is identified as output logic (OL), the logic 313 processes the output signal of the first voltage comparator 309 and provides: a first switch control signal (HS) 311 for setting the state of the first high-side switch 302; and a second switch control signal (LS) 312 for setting the state of the second low-side switch 303. In this way, in response to the measured voltage signal (Vcap) 307 exceeding the upper voltage threshold (VCRH), the logic 313 can disconnect the first switch 302 and close the second switch 303. As described above with reference to FIG. 2, this defines the end of the high-side switch half cycle and the beginning of the low-side half cycle. In some applications, the logic 313 may wait for a delay period between disconnecting the first switch 302 and closing the second switch 303, so as to implement a non-overlapping delay between the two switching operations, and thereby ensure that the two switches are not closed at the same time. If closed at the same time, the performance of the converter 300 will be reduced.

[0124] The controller 301 further includes a second voltage comparator 310, which compares the measured voltage signal (Vcap) 307 with a lower voltage threshold (VCRL, which is Figure 3b The output signal of the second voltage comparator 310 indicates whether the measured voltage signal (Vcap) 307 is less than the lower voltage threshold (VCRL). The logic 313 processes the output signal of the second voltage comparator 310 to set the value of the first switch control signal (HS) 311 and the second switch control signal (LS) 312. In this way, in response to the measured voltage signal (Vcap) 307 falling below the lower voltage threshold (VCRL), the logic 313 can open the second switch 303 and close the first switch 302. As described above with reference to FIG. 2, this defines the end of the low-side switch half cycle and the beginning of the high-side half cycle. In the same manner as described above, the logic 313 can wait for the delay period between opening the second switch 303 and closing the first switch 302 so as to implement a non-overlapping delay between the two switching operations.

[0125] The controller 301 also includes a first current comparator 314 and a second current comparator 315. The first current comparator 314 compares the measured current signal (Iprim) 306 with a positive current threshold (Iend_pos). The second current comparator 315 compares the measured current signal (Iprim) 306 with a negative current threshold (Iend_neg).

[0126] As described above, in some examples, the logic 313 can use the output signals of the first current comparator 314 and the second current comparator 315 to set the values ​​of the first switch control signal (HS) 311 and the second switch control signal (LS) 312. The positive current threshold (Iend_pos) is a positive value representing the minimum acceptable value of the resonant current at the end of the high-side switch half cycle. The negative current threshold (Iend_neg) is a negative value representing the minimum acceptable value of the resonant current at the end of the low-side switch half cycle. Therefore, if the measured current signal (Iprim) 306 drops below the positive current threshold (Iend_pos) at the end of the high-side switch half cycle, the logic 313 will disconnect the first switch 302 and close the second switch 303 to define the end of the high-side switch half cycle. Similarly, if the measured current signal (Iprim) 306 exceeds the negative current threshold (Iend_neg) at the end of the low-side switch half cycle, the logic 313 will disconnect the second switch 303 and close the first switch 302 to define the end of the low-side switch half cycle. In this way, the resonant converter 300 is controlled based on the level of the measured current signal (Iprim) 306 rather than the level of the measured voltage signal (Vcap) 307. That is, in response to the measured current signal (Iprim) 306 falling below the positive current threshold (Iend_pos) at the end of the high-side switching half cycle, the controller 301 opens the first switch 302 and closes the second switch 303; and in response to the measured current signal (Iprim) 306 exceeding the negative current threshold (Iend_neg) at the end of the low-side switching half cycle, the controller 301 opens the second switch 303 and closes the first switch 302.

[0127] However, as discussed above, in some applications, control of the resonant converter 300 based on the level of the measured current signal (Iprim) 306 may be disadvantageous. As we will now describe, Figure 3a and 3b The controller 301 may advantageously adjust the levels of the upper voltage threshold (VCRH) and the lower voltage threshold (VCRL) such that the controller 301 will cause the resonant converter 300 to transition from being controlled based on the level of the measured current signal (Iprim) 306 to being controlled based on the level of the measured voltage signal (Vcap) 307.

[0128] exist Figure 3bIn this case, logic 313 sets the power reduction mode to active or inactive and provides a mode signal 316 representing the power reduction mode. Logic 313 sets the power reduction mode to active if either or both of the following occur: i) at the end of the previous high-side switch half-cycle, the measured current signal (Iprim) 306 is less than the positive current threshold (Iend_pos); and ii) at the end of the previous low-side switch half-cycle, the measured current signal (Iprim) 306 is greater than the negative current threshold (Iend_neg). That is, when the resonant converter 300 is controlled based on the level of the measured current signal (Iprim) 306 of the previous switching cycle, logic 313 sets the mode signal 316 to represent "active". As will be discussed below, such a mode signal 316 is used to reduce the ΔV between the upper voltage threshold (VCRH) and the lower voltage threshold (VCRL), and thereby also reduce the power level at the output of the resonant converter 300.

[0129] If both of the following occur, logic 313 sets the power reduction mode to inactive: i) at the end of the previous high-side switch half-cycle, the measured current signal is not less than the positive current threshold; and ii) at the end of the previous low-side switch half-cycle, the measured current signal is not greater than the negative current threshold. That is, when the resonant converter 300 is controlled based on the level of the measured voltage signal (Vcap) 307 of the previous switching cycle, logic 313 sets the mode signal 316 to represent "inactive". As will be discussed below, such a mode signal 316 is used to potentially increase the ΔV between the upper voltage threshold (VCRH) and the lower voltage threshold (VCRL), and thereby also potentially increase the power level at the output of the resonant converter 300.

[0130] In this way, the power reduction mode can be defined as active as a result of at least one of the following events occurring within a full switching cycle:

[0131] ● Iprim < Iend_pos occurs during the high-side switch half-cycle; or

[0132] ● Iprim > Iend_neg occurs during the low-side switch half-cycle.

[0133] Alternatively, the power reduction mode can be defined as active as a result of at least one of the following events occurring within each half-cycle:

[0134] ● Iprim < Iend_pos occurs during the high-side switch half-cycle; or

[0135] ● Iprim > Iend_neg occurs during the low-side switch half-cycle.

[0136] These conditions may be satisfied at the beginning of each half cycle, and therefore the Iend control mechanism may include a feature to check the conditions only at the end of a half cycle. This may be implemented by first checking whether the condition disappears at the beginning of each half cycle, and then checking whether the condition is then satisfied at the end of the half cycle.

[0137] That is, the logic block 313 may generate the mode signal 316 to indicate whether one-sided or two-sided Iend control has occurred during the last switching cycle.

[0138] The mode signal 316 is received as an input signal by the regulator block (RL) 317. The regulator block (RL) 317 provides a reduced power signal (Pr) 318 to the power algorithm block (PA) 319. The power algorithm block (PA) 319 sets the deltaV (given by Figure 3b The control power device (CPD) 321 receives the Pv signal 320 from the power algorithm block (PA) 319 and sets the values ​​of the upper voltage threshold (Vcaph / VCRH) and the lower voltage threshold (Vcapl / VCRL) provided to the first voltage comparator 309 and the second voltage comparator 310.

[0139] In this way, when the requested power level of the load (represented by the power setting signal (Ps) 308) results in an upper voltage threshold (VCRH) and a lower voltage threshold (VCRL) that cannot be achieved by the measured voltage signal (Vcap) 307 (and therefore the outputs of the first current comparator 314 and the second current comparator 315 set the values ​​of the first switch control signal (HS) 311 and the second switch control signal (LS) 312), the upper voltage threshold (VCRH) and the lower voltage threshold (VCRL) can be defined based on the reduced power signal (Pr) 318 so that the resonant converter 300 will be controlled based on the level of the measured voltage signal (Vcap) 307. In this way, any control of the resonant converter 300 based on the level of the measured current signal (Iprim) 306 will only be temporary.

[0140] In other words, the power algorithm block (PA) 319 generates a signal Pv 320 as a function of the power feedback from the main regulation loop (power setting signal (Ps) 308) and the capacitive mode limit function (implemented by the regulator block (RL) 317 which provides the reduced power signal (Pr) 318 as an input to the power algorithm block (PA) 319). Subsequently, the power algorithm block (PA) 319 is configured to take the minimum of Ps 308 and Pr 319, so that if Ps 319 causes Iend control to occur within at least one of the half cycles, Pr 319 can take over.

[0141] This functionality is achieved by the regulator (RL) 317 appropriately setting the value of the reduced power signal (Pr) 318 as follows. When the mode signal 316 indicates that the power reduction mode is active, the regulator (RL) 317 gradually reduces the value of the reduced power signal (Pr) 318. In this way, when the resonant converter 300 is controlled based on the level of the measured current signal (Iprim) 306, the level of the reduced power signal (Pr) 318 will decrease. This will continue until the reduced power signal (Pr) 318 reaches a sufficiently low level so that the upper voltage threshold (VCRH) and the lower voltage threshold (VCRL) are close enough together (i.e., the value of ΔV decreases), so the resonant converter 300 can be controlled again based on the level of the measured voltage signal (Vcap) 307.

[0142] When the mode signal 316 indicates that the power reduction mode is inactive, the regulator (RL) 317 gradually increases the value of the reduced power signal (Pr) 318. This gradual increase continues at least until the reduced power signal (Pr) 318 is greater than the power setting signal (Ps) 308.

[0143] The gradual decrease and increase in the value of the reduced power signal (Pr) 318 may be expressed as a percentage of the nominal output power per unit time. In one example, it may be expressed as V / s (volts per second) because the signal unit may be volts. The rate at which the power is adjusted may be related to one or more of three requirements:

[0144] 1) The reduction should be less than the dynamics of the resonant tank itself. For example, the resonant trajectory may converge to the new operating point within 3 to 5 switching cycles. Furthermore, if the switching frequency is about 100khz-200khz (5us-10us), a good value for the power reduction may be 5% of the full load power reduction every 5 switching cycles. This is equal to roughly 5% / 50us=100% / ms.

[0145] 2) The system should be able to recover to normal operating conditions after the situation that caused the power reduction has disappeared. Such a situation could be a power dip, which may last approximately 20ms. Therefore, if we assume that the system recovers within 1ms, it should be able to increase power within this 1ms. Assuming 25% of power is lost, we need to increase 25% of full load within 1ms.

[0146] 3) The amount of irregular behavior can be limited to an acceptable value. The mechanism described herein can be considered a form of on / off control, which may cause some irregularities. By ensuring that the increase rate and the decrease rate are not too high, the possibility of irregular behavior can be limited to an acceptable value. The rates identified above can meet this requirement.

[0147] In one embodiment, when the power reduction mode is inactive, the regulator (RL) 317 gradually increases the value of the reduced power signal (Pr) 318 until the reduced power signal (Pr) is a predetermined amount greater than the power setting signal (Ps) 308. This predetermined amount may be a predetermined ratio of the power setting signal (Ps) 308, or it may be a predetermined absolute amount greater than the power setting signal (Ps) 308. Subsequently, the regulator (RL) 317 may maintain the value of the reduced power signal (Pr) 318 at a predetermined amount greater than the power setting signal (Ps) 308 until the power reduction mode becomes active.

[0148] In this way, the reduced power signal (Pr) 318 tracks the power setting signal (Ps) 308. This can be viewed as implementing a primary signal and replica function, which can limit the impact of drift of the uncontrolled variable (i.e., the reduced power signal (Pr) 318). That is, when the reduced power signal (Pr) 318 increases to a value greater than the power setting signal (Ps) 308, the action can be to set the reduced power signal (Pr) 318 to a function of the power setting signal (Ps) 308. For example, by controlling the reduced power signal (Pr) 318 so that it follows the power setting signal (Ps) 308 at a short distance. This can reduce the likelihood of unnecessary power overshoots during dynamic situations.

[0149] In this example, when the power reduction mode is active, the regulator (RL) 317 gradually decreases the value of the reduced power signal (Pr) 318 at a first rate of change. When the power reduction mode is inactive, the regulator (RL) 317 gradually increases the value of the reduced power signal (Pr) 318 at a second rate of change. The first rate of change is higher than the second rate of change, so that the value of the reduced power signal (Pr) 318 decreases relatively quickly, so the resonant converter 300 operates under Iend control only for a relatively short period of time, but the value of the reduced power signal (Pr) 318 increases more slowly, so the operation of the resonant converter 300 is gradually released back to Vcap control.

[0150] In this manner, the controller 300 may decrease the reduced power signal (Pr) 318 at a predefined rate if Mode=Active, and may increase the reduced power signal (Pr) 318 at a different predefined rate if Mode=Inactive.

[0151] exist Figure 3b In an example, the logic block 313 optionally generates a symmetry signal (sym) 322 related to the duty cycle of the switch of the high-side switching half cycle and the low-side switching half cycle. The control power device (CPD) block 321 can adjust the value of the upper voltage threshold (Vcaph / VCRH) and / or the lower voltage threshold (Vcapl / VCRL) based on the symmetry signal (sym) 322 to make the duty cycle closer to 50% when it is not 50%. In another example, the controller 301 can adapt the DC component of Vcap based on the symmetry signal (sym) 322 to make the duty cycle closer to 50%.

[0152] Figure 4 Another example embodiment of a controller 401 according to the present disclosure is shown. Figure 4 The controller 401 is also Figure 3b Features shown in will not necessarily be described again here.

[0153] Figure 4 The controller 401 also receives a protected power signal (Pr2) 423 defining a power level that is based (directly or indirectly) on a time delay between a change in state of one of the switches and a subsequent zero crossing of the measured current signal (Iprim) 406. More specifically, the power algorithm block (PA) 419 receives the protected power signal (Pr2) 423 along with the power level described above with reference to Figure 3bThe reduced power signal (Pr) 418 and the power setting signal (Ps) 408 are depicted. It should be appreciated that the system may be expanded with one or more additional protected power signals (e.g., Pr3, Pr4, etc.) such that the controller 401 may incorporate additional circumstances where the power to the resonant circuit should be reduced.

[0154] Subsequently, the power algorithm block (PA) 419 sets the value of ΔV (represented by the Pv signal 420) so that the control power device (CPD) block 421 sets the upper voltage threshold (VcapH) and the lower threshold (VcapL) based on the lower of the following: i) the reduced power signal (Pr) 418; ii) the power setting signal (Ps) 408; and iii) the protected power signal (Pr2) 423. In this way, as will be discussed below, another mechanism is introduced for overruling the power setting signal (Ps) 408 by reducing the difference between the upper voltage threshold (VcapH) and the lower threshold (VcapL) in order to improve the performance of the resonant converter.

[0155] In this example, the protected power signal (Pr2) 423 corresponds to the power level at which the measured voltage signal (Vcap) 407 will intersect: an upper voltage threshold (VcapH) during a high-side switching half cycle; and a lower voltage threshold (VcapL) during a low-side switching half cycle. In this way, the protected power signal (Pr2) 423 can control the associated resonant converter based on the detected distance from the capacitive mode boundary.

[0156] The protected power signal (Pr2) 423 may be set so that it corresponds to a detected distance from the capacitive mode boundary that is greater than the distance defined by the Iend_pos level and the Iend_neg level. Thus, in steady-state operation, when the protected power signal (Pr2) 423 limits the power of the resonant converter, the reduced power signal (Pr) 418 does not limit the power of the resonant converter. During certain transients, both the reduced power signal (Pr) 418 and the protected power signal (Pr2) 423 may be set to a value that will limit power (meaning that they are both lower than the power setting signal (Ps) 408).

[0157] This use of the protected power signal (Pr2) 423 can limit the energy converted by each switching cycle through a feedback loop that checks the distance from the capacitive mode limit based on the timing requirements or the area under the resonant current curve between the moment when the half cycle is completed and the moment when the resonant current crosses 0. When this distance becomes less than the predefined distance, the ΔV level is reduced, taking over the normal feedback loop. In this way, the power can be limited to a certain predefined distance from the capacitive mode point by maintaining the same mechanism of VcapH, VcapL that completes the half cycle. This can have the following advantages: there is no undesirable interaction between completing the half cycle by Iend_pos or Iend_neg during some half cycles and completing the half cycle by VcapH or VcapL at other half cycles. As indicated above, such switching may result in irregular switching behavior and may also cause complications for the symmetrical loop because the symmetrical loop relies on double-sided Vcap control (i.e., two half cycles completed by VcapH and VcapL).

[0158] In an embodiment, a timeout timer may be started when a capacitive mode reduction condition is detected. Such a timeout timer may be implemented as a counter that counts at a switching frequency (fsw). A capacitive mode reduction condition is a condition in which a normal regulation loop involving setting an upper voltage threshold (VcapH) and a lower voltage threshold (VcapL) based on a power setting signal (Ps) 408 is overruled by one or both of a reduced power signal (Pr) 418 and a protected power signal (Pr2) 423. If the timeout timer reaches an end value, the controller 401 may take corrective action. For example, it may decide to restart, latch, or keep running. The timeout timer may implement a protection time that may be configured by a user. Therefore, the end value may be considered a configurable delay. If the controller leaves the power reduction mode before the timeout timer reaches the end value, the timeout timer automatically resets.

[0159] One or more of the controllers described herein may also include a switch mode detector (not shown in the figures). Such a switch mode detector may detect whether the first switch and the second switch are being controlled based on the Vcap threshold / monitoring. That is, whether the output signals of the first and second voltage comparators 309, 310 cause the state change of the first switch control signal (HS) and the second switch control signal (LS). In some applications, this information may be obtained from a switch state machine that knows what control signals turn the gate driver on and off. In some examples, if the switch mode detector detects that the first switch and the second switch are not being controlled based on the Vcap threshold / monitoring, the controller may also set the power reduction mode to active. Other switch modes that may be used by the controllers described herein include: overcurrent conditions; and maximum on-time operation. The switch mode detector may also detect the reduced power signal (Pr) described herein.

[0160] Figure 5 An example embodiment of a method of operating a resonant converter according to the present disclosure is shown.

[0161] The resonant converter is used to supply electrical energy from a supply source to a load, and (as described in detail above) the resonant converter comprises: a first switch and a second switch, the first switch and the second switch are connected in series with each other between the supply source and a reference terminal, wherein when the first switch is closed and the second switch is open, the resonant converter has a high-side switching half-cycle, and wherein when the first switch is open and the second switch is closed, the resonant converter has a low-side switching half-cycle; and a resonant tank, the resonant tank being electrically connected to the first switch and the second switch, wherein the resonant tank comprises a resonant capacitor.

[0162] At step 530 , the method includes receiving a measured current signal (Iprim) representative of a current flowing in the resonant tank, and at step 531 , the method includes receiving a measured voltage signal (Vcap) representative of a voltage at a predetermined point in the resonant tank.

[0163] At step 532, the method includes receiving a power setting signal (Ps) defining a requested power level for the load.

[0164] At step 533 , the method compares the measured current signal (Iprim) to a positive current threshold and also compares the measured current signal (Iprim) to a negative current threshold.

[0165] At step 534, the method involves setting the power reduction mode to:

[0166] ●Activities whose constraints are the occurrence of either or both of the following:

[0167] At the end of the previous high-side switching half cycle, the measured current signal is less than a positive current threshold; and

[0168] At the end of the previous low-side switching half-cycle, the measured current signal is greater than the negative current threshold; or

[0169] ● Inactivity, where both of the following conditions occur:

[0170] At the end of the previous high-side switching half cycle, the measured current signal is not less than a positive current threshold; and

[0171] At the end of the previous low-side switching half cycle, the measured current signal is not greater than the negative current threshold;

[0172] When the power reduction mode is active: At step 535, the method involves gradually decreasing the value of the reduced power signal.

[0173] When the power reduction mode is inactive: At step 536, the method involves gradually increasing the value of the reduced power signal.

[0174] Subsequently, at step 537 , the method involves setting an upper voltage threshold and a lower voltage threshold based on the lower of: i) the reduced power signal; and ii) the power setting signal.

[0175] In response to the measured voltage signal exceeding the upper voltage threshold, at step 538 , the method involves opening the first switch and closing the second switch.

[0176] In response to the measured voltage signal falling below the lower voltage threshold, at step 539 , the method involves opening the second switch and closing the first switch.

[0177] Examples disclosed herein involve detecting a near-capacitive mode by a primary current crossing a threshold, and using the near-capacitive mode to override normal power regulation of a resonant converter controlled using a capacitor voltage control method by turning down power whenever the resonant converter is detected to be in near-capacitive mode.

[0178] Examples disclosed herein relate to capacitive mode limiting of a resonant converter for cycle-by-cycle control, where an actual switching half cycle is completed by an event where the resonant current becomes less than a predefined distance from zero, and where the instantaneous power of the converter is reduced based on the same event completing the half cycle.

[0179] One or more of the examples disclosed herein may be used in resonant converters for laptop adapters, personal computer (PC) power supplies, televisions, solar power inverters, on-board charger systems for automobiles, and the like.

[0180] Key features of examples of the present disclosure include one or more of the following:

[0181] 1. Resonant converter controller including capacitor mode protection based on Iend control mechanism

[0182] 2. A regulation loop that calls back the power level (via a ΔV reduction in the direction producing lower power) based on a detected single-sided or double-sided Iend crossing.

[0183] 3. Adjust power up based on no Iend crossing detected.

[0184] 4. An adjusted capacitive mode limit capable of maintaining a predefined distance from the Iend crossing.

[0185] Feature 4 ensures stable jitter-free operation, which may be desirable. As a result of feature 4, the switching frequency may be increased, and stable jitter-free operation may be achieved under double-sided Vcap control at a stable maximum possible power.

[0186] The examples disclosed herein can detect at least one side of Iend control and use the detection to call back ΔV. However, in some applications, if some half cycles are completed in response to Iprim crossing Iend_pos or Iend_neg, while other half cycles are completed in response to Vcap crossing Vcaph or Vcapl, this may cause irregularities. One or more of the examples described herein reduce such undesirable irregularities.

[0187] For implementations that do not require a symmetrical loop (eg, when sensing the resonant capacitor voltage when the resonant capacitor is placed between node HB and the transformer), the examples disclosed herein can control the resonant converter while reducing the risk of an asymmetrical shutdown condition.

[0188] Unless a particular order is explicitly stated, the instructions and / or flow chart steps in the above figures may be executed in any order. In addition, those skilled in the art will recognize that although an example instruction set / method has been discussed, the materials in this specification may be combined in a variety of ways to produce other examples, and should be understood within the context provided in this detailed description.

[0189] In some example embodiments, the instruction sets / method steps described above are implemented as functions and software instructions embodied as executable instruction sets, which are implemented on a computer or a machine programmed and controlled with the executable instructions. Such instructions are loaded for execution on a processor (e.g., one or more CPUs). The term processor includes a microprocessor, a microcontroller, a processor module or subsystem (including one or more microprocessors or microcontrollers), or other control or computing devices. A processor may refer to a single component or multiple components.

[0190] In other examples, the instruction sets / methods shown herein and the data and instructions associated therewith are stored in corresponding storage devices, which are implemented as one or more non-transitory machine or computer readable or computer usable storage media. This one or more computer readable or computer usable storage media is considered to be part of an article (or product). An article or product may refer to any manufactured single component or multiple components. A non-transitory machine or computer usable medium as defined herein does not include a signal, but this one or more media may be able to receive and process information from a signal and / or other transient media.

[0191] Example embodiments of the materials discussed in this specification may be implemented in whole or in part through network, computer, or data-based devices and / or services. These devices and / or services may include the cloud, the Internet, an intranet, a mobile device, a desktop computer, a processor, a lookup table, a microcontroller, a consumer device, an infrastructure, or other enabling devices and services. As may be used herein and in the claims, the following non-exclusive limitations are provided.

[0192] In one example, one or more instructions or steps discussed herein are automated. The term automation or automatic (and similar variations thereof) means the use of computers and / or mechanical / electrical devices to control the operation of a device, system, and / or process without human intervention, observation, effort, and / or decision making.

[0193] It should be understood that any components that are said to be coupled may be coupled or connected directly or indirectly. In the case of indirect coupling, additional components may be placed between the two components that are said to be coupled.

[0194] In this specification, example embodiments have been presented based on a selected set of details. However, one of ordinary skill in the art will appreciate that many other example embodiments may be practiced that include different selected sets of these details. It is intended that the appended claims cover all possible example embodiments.

Claims

1. A controller for a resonant converter, characterized in that: The resonant converter is used to supply electric energy from a supply source to a load, and the resonant converter comprises: a first switch and a second switch, the first switch and the second switch being connected in series with each other between the supply source and a reference terminal, wherein when the first switch is closed and the second switch is open, the resonant converter has a high-side switching half-cycle, and wherein when the first switch is open and the second switch is closed, the resonant converter has a low-side switching half-cycle; a resonant tank electrically connected to the first switch and the second switch, wherein the resonant tank includes a resonant capacitor; The controller is configured to: receiving a measured current signal representative of a current flowing in the resonant tank; receiving a measured voltage signal representative of a voltage at a predetermined point in the resonant tank; receiving a power setting signal defining a requested power level for the load; comparing the measured current signal to a positive current threshold; comparing the measured current signal to a negative current threshold; The power reduction mode is set to active if either or both of the following occur: At the end of a previous high-side switching half cycle, the measured current signal is less than the positive current threshold; and the measured current signal being greater than the negative current threshold at the end of a previous low-side switching half cycle; The power reduction mode is set to inactive if both of the following occur: At the end of the previous high-side switching half-cycle, the measured current signal is not less than the positive current threshold; and At the end of the previous low-side switching half cycle, the measured current signal is no greater than the negative current threshold; When the power reduction mode is active: gradually decreasing the value of the reduced power signal; When the power reduction mode is inactive: gradually increasing the value of the reduced power signal; setting an upper voltage threshold and a lower voltage threshold based on the lower of: i) the reduced power signal; and ii) the power setting signal; In response to the measured voltage signal exceeding the upper voltage threshold, opening the first switch and closing the second switch; In response to the measured voltage signal falling below the lower voltage threshold, the second switch is opened and the first switch is closed.

2. The controller according to claim 1, characterized in that: The controller is configured to: When the power reduction mode is inactive: gradually increasing the value of the reduced power signal until the reduced power signal is greater than the power setting signal.

3. The controller according to claim 1 or claim 2, characterized in that: The controller is configured to: When the power reduction mode is inactive: gradually increasing the value of the reduced power signal until the reduced power signal is a predetermined amount greater than the power setting signal.

4. The controller according to claim 1 or claim 2, characterized in that: The controller is configured to: When the power reduction mode is inactive: gradually increasing the value of the reduced power signal until the reduced power signal is a predetermined amount greater than the power setting signal; and The value of the reduced power signal is maintained at the predetermined amount greater than the power setting signal until the power reduction mode becomes active.

5. The controller according to claim 4, characterized in that: The predetermined amount is any one of: a predetermined proportion of the power setting signal; or a predetermined absolute amount greater than the power setting signal.

6. A controller according to any preceding claim, characterised in that The controller further comprises a timeout timer, wherein the controller is configured to: When the power reduction mode is active, starting the timeout timer; and If the timeout timer reaches an end value, corrective action is taken.

7. A controller according to any preceding claim, characterised in that The controller is configured to: When the power reduction mode is active: gradually decreasing the value of the reduced power signal at a first rate of change; When the power reduction mode is inactive: gradually increasing the value of the reduced power signal at a second rate of change; and The first change rate is higher than the second change rate.

8. A controller according to any preceding claim, characterised in that The controller is further configured to: in response to the measured current signal falling below the positive current threshold at the end of a high-side switching half-cycle, opening the first switch and closing the second switch; In response to the measured current signal exceeding the negative current threshold at the end of a low-side switching half-cycle, the second switch is opened and the first switch is closed.

9. A resonant converter, characterized in that: The resonant converter comprises a controller according to any preceding claim.

10. A method of operating a resonant converter, characterized in that The resonant converter is used to supply electric energy from a supply source to a load, and the resonant converter comprises: a first switch and a second switch, the first switch and the second switch being connected in series with each other between the supply source and a reference terminal, wherein when the first switch is closed and the second switch is open, the resonant converter has a high-side switching half-cycle, and wherein when the first switch is open and the second switch is closed, the resonant converter has a low-side switching half-cycle; and a resonant tank electrically connected to the first switch and the second switch, wherein the resonant tank includes a resonant capacitor; The method comprises: receiving a measured current signal representative of a current flowing in the resonant tank; receiving a measured voltage signal representative of a voltage at a predetermined point in the resonant tank; receiving a power setting signal defining a requested power level for the load; comparing the measured current signal to a positive current threshold; comparing the measured current signal to a negative current threshold; The power reduction mode is set to active if either or both of the following occur: At the end of a previous high-side switching half cycle, the measured current signal is less than the positive current threshold; and the measured current signal being greater than the negative current threshold at the end of a previous low-side switching half cycle; The power reduction mode is set to inactive if both of the following occur: At the end of the previous high-side switching half-cycle, the measured current signal is not less than the positive current threshold; and At the end of the previous low-side switching half cycle, the measured current signal is no greater than the negative current threshold; When the power reduction mode is active: gradually decreasing the value of the reduced power signal; When the power reduction mode is inactive: gradually increasing the value of the reduced power signal; setting an upper voltage threshold and a lower voltage threshold based on the lower of: i) the reduced power signal; and ii) the power setting signal; In response to the measured voltage signal exceeding the upper voltage threshold, opening the first switch and closing the second switch; In response to the measured voltage signal falling below the lower voltage threshold, the second switch is opened and the first switch is closed.

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