Frequency modulation for controlling a switched resonant converter

Abstract

An LED converter for supplying an LED load (18) is provided. The LED converter comprises a switched resonant converter (1) having at least one switch (11, 12) controlled by a control unit of the switched resonant converter (1). The control unit is configured to control a switching operation of the at least one switch (11, 12) such that the switched resonant converter (1) supplies an adjustable constant current (17) to an output terminal of the LED load (18). The control unit is further configured to generate the adjustable constant current (17) by alternatingly controlling the at least one switch (11, 12) at each one of at least two different and adjustable switching frequencies (44, 45) for a respective time period (54, 55) and by adjusting a relative duration of the time periods (54, 55). Thereby, a resolution of an output current of the LED converter for supplying the LED load (18) is improved, which is particularly relevant for dimming applications.

Description

Technical Field

[0001] This invention relates to LED converters for supplying LED loads, and more particularly to LED converters including switching resonant converters (such as, for example, LLC or LCC converters). Background Technology

[0002] Switching resonant converters, such as LLC or LCC converters, are well known in the art. In such LED converters with resonant topologies, the output current can be adjusted by changing the switching frequency. When low output current is controlled by feedback, particularly high switching frequencies above the peak resonant frequency of the LED converter occur. Therefore, when using digital control, dimming the LED load of such LED converters may suffer from differential output current. For example, even when considering switching frequencies as high as f = 2 MHz (1 / f = 500 ns), for a single dimming step, a controller clock frequency of f = 40 MHz (1 / f = 25 ns) may require changing the switching frequency by Δf = 1 / (500 ns) - 1 / (500 ns + 25 ns) ≈ 100 kHz. In other words, the output current can only be changed according to a low and limited number of steps.

[0003] EP 2484180B1 teaches a dimmable LED driver with a resonant DC / DC converter that can operate in PWM mode at two different and fixed frequencies. Summary of the Invention

[0004] Therefore, the object of the present invention is to improve the resolution of the output current of a switching resonant converter used to supply LED loads.

[0005] This invention is defined by the appended independent claims. Preferred embodiments are set forth in the dependent claims, as well as in the following description and drawings.

[0006] According to a first aspect, an LED converter for supplying an LED load is provided. The LED converter includes a switching resonant converter having at least one switch controlled by a control unit of the switching resonant converter. The control unit is configured to control the switching operation of the at least one switch such that the switching resonant converter supplies an adjustable constant current to the output terminal of the LED load. The control unit is further configured to generate the adjustable constant current by alternately controlling the at least one switch for a corresponding time period at one of at least two different switching frequencies and by adjusting the relative duration of the time periods. The at least two different switching frequencies are adjustable.

[0007] The LED converter may include an additional control unit, such as a microcontroller, which preferably communicates with the control unit via a bidirectional channel.

[0008] Another control unit can be connected to the LED converter's communication interface 101, such as, for example, a DALI interface, which communicates externally via a wireless or wired channel such as, for example, a DALI bus.

[0009] Another control unit can supply a signal indicating the output current or LED current.

[0010] Another control unit can send a signal to the control unit indicating the duty cycle of a discrete frequency used by the control unit to operate one or more switches.

[0011] The frequency to be used by the control unit can be indicated / modified by another control unit. Alternatively, in the absence of instructions from another control unit, the frequency to be used and / or its modification can be determined by the control unit.

[0012] At least two different switching frequencies can be constant.

[0013] At least two different switching frequencies can each exceed the peak resonant frequency of the converter.

[0014] At least two different switching frequencies can include two different switching frequencies.

[0015] The lower of the two different switching frequencies can be associated with the first load current.

[0016] The higher of two different switching frequencies can be associated with a second load current that is lower than the first load current.

[0017] The switching operation of at least one switch can be continuous.

[0018] According to a second aspect, a lighting system is provided. The lighting system includes an LED converter according to an embodiment of the first aspect and an LED load configured to be supplied with the output current of the LED converter.

[0019] According to a third aspect, a method for operating an LED converter for supplying an LED load is provided. The LED converter includes a switching resonant converter having at least one switch controlled by a control unit of the switching resonant converter. The method includes: controlling the switching operation of the at least one switch such that the switching resonant converter supplies an adjustable constant current to the output terminal of the LED load; and generating the adjustable constant current by alternately controlling the at least one switch for a corresponding time period at one of at least two different switching frequencies and by adjusting the relative duration of the time periods. When operating the LED converter, the at least two different switching frequencies are adjusted.

[0020] This method can be performed by an LED converter according to the implementation scheme of the first aspect.

[0021] According to a fourth aspect, a control unit for an LED converter is provided. This control unit is designed to implement the method according to the embodiment of the third aspect. Attached Figure Description

[0022] Other aspects, advantages, and objects of the invention will become apparent to a skilled reader when taken in conjunction with the accompanying drawings and the following detailed description of embodiments of the invention.

[0023] Figure 1 An LED converter for supplying an LED load according to an embodiment of the first aspect is shown, as well as a lighting system according to an embodiment of the second aspect;

[0024] Figure 2 and Figure 3 The following are examples of methods for using according to Figure 1 An exemplary resonant circuit circuit for an LED converter;

[0025] Figure 4 It shows the direction including according to Figure 3 The basis of the LLC resonant circuit Figure 1 The frequency dependence of the current supplied to the output terminals of the LED converter;

[0026] Figure 5 The diagram illustrates a switching operation that alternately controls at least one switch at one of at least two different switching frequencies for a given duration.

[0027] Figure 6 It shows that due to Figure 5 The alternating control of the switching frequency shown generates current supplied to the output terminals of the LED load;

[0028] Figure 7A method according to an embodiment of the third aspect is shown. This method is used to operate an LED converter for supplying an LED load. Detailed Implementation

[0029] The invention will now be described with respect to various embodiments. Unless otherwise stated, features of these embodiments may be combined with each other.

[0030] Figure 1 An LED converter 1 for supplying an LED load 18 according to an embodiment of the first aspect is shown, as well as a lighting system according to an embodiment of the second aspect.

[0031] As used herein, the term “LED converter” refers to a converter used to supply an LED load 18 as a power receiver.

[0032] As used herein, the term "switching converter" refers to an electronic power source that efficiently converts electrical power, particularly voltage and / or current characteristics, by periodically alternating between low-dissipation, fully-on, and fully-off states of the electronic switching elements of the switching converter to control the transfer of power from a power source to a power receiver via a reactive power storage circuit of the switching converter, including inductive and / or capacitive elements.

[0033] The LED converter includes a switching resonant converter 1 having at least one switch 11, 12, preferably two switches 11, 12, which are arranged in series as a half-bridge on the primary side of the switching resonant converter 1 (preferably isolated by a current isolation barrier) and supplied with DC voltage 10.

[0034] As used herein, the term "switching resonant converter" refers to a switching converter that includes a reactive power storage circuit, comprising inductive and capacitive elements, referred to as a "resonant loop" circuit. This resonant loop circuit can be designed to resonate at a given peak resonant frequency, i.e., to produce a peak response. The following section combines... Figure 2 and Figure 3 Exemplary resonant circuit circuits 13A and 13B are presented.

[0035] The switching resonant converter 1 includes a control unit 19, such as an ASIC that is provided with feedback signals and issues control signals for the primary-side switches 11, 12. Figure 1 In the example, the indication of the current 17 supplied to the output terminal 20 of the LED load 18 is shown as a feedback signal. It should be noted that the feedback signal 17, representing the current supplied to the LED, can be obtained from either the primary or secondary side of the isolation stage of the resonant converter.

[0036] The control unit 19 is configured to control the switching operation of at least one switch 11, 12, such that the switch resonant converter 1 supplies an adjustable and feedback-controlled constant current 17 to the output terminal 20 of the LED load 18.

[0037] Figure 1 The switching resonant converter 1 has a primary (source) side and a secondary (receiver) side connected via inductive / magnetic coupling, with a transformer 14 serving as a current isolation stage. On the primary side, the switching resonant converter 1 includes a DC voltage source 10 as a power supply. For example, the DC voltage can be 400V. The DC voltage can be generated, for example, by a PFC stage supplied with rectified AC mains voltage.

[0038] On the secondary side, the switching resonant converter 1 is configured to supply an adjustable (feedback-controlled) constant current 17 to the output terminal 20, where the LED load 18 can be connected as a power receiver.

[0039] For example, Figure 1 The switches 11 and 12 of the switch resonant converter 1 can be gallium nitride (GaN) or field-effect transistors.

[0040] Switches 11 and 12 control and regulate the power transmission from the primary side to the secondary side via resonant circuits 13, 13A, 13B and transformer 14.

[0041] Figure 1 The switching resonant converter 1 also includes four diodes and a capacitor 16. These diodes implement a full-bridge rectifier circuit 15 on the secondary side for rectifying the power transferred from the primary side to the secondary side. The capacitor smooths the output of the rectifier circuit 15 to supply an average current 17 at the output terminals of the switching resonant converter 1. Alternatively, the switching resonant converter 1 may implement a full-wave rectifier circuit 15 comprising only two diodes and a transformer 14 with a center-tapped secondary side.

[0042] Control unit 19 is configured to generate an adjustable constant current 17 by alternately controlling at least one switch 11, 12 for corresponding time periods 54, 55 at one of at least two different (spaced-out) switching frequencies 44, 45 each time, and by adjusting the relative duration of time periods 54, 55. Therefore, the duty cycle used for the different switching frequencies is adjusted to regulate the average frequency and thus the time average of the resulting secondary-side current.

[0043] Preferably, and therefore not necessarily, at least two frequencies are used at constant frequencies, and for the first frequency, only their duty cycles are adjusted from 0 to 100%, and for the second frequency, they are adjusted from 100% to 0% accordingly.

[0044] However, depending on the dimming level, one or more, or even all, of the discrete frequencies can be adjusted.

[0045] As used in this article, the term "adjustable constant current" refers to a current with an adjustable average value.

[0046] If available Figure 1 As shown, an additional control unit 100, such as a microcontroller 100, may be provided, which preferably communicates with the control unit 19 in bidirectional channels 103, 104, 17'.

[0047] The additional control unit 100 can be connected to a communication interface 101, such as a DALI interface, which communicates externally via a wireless or wired channel such as a DALI bus 102. Therefore, the additional control unit 100 can be supplied with external dimming control values.

[0048] Another control unit 100 supplies a signal indicating output current or LED current. Control unit 19 can send a signal 17' indicating output current or LED current to the other control unit 100. Alternatively, such signal 17' can also be supplied directly to the other control unit 100.

[0049] Another control unit can send a signal 103 to the control unit, which instructs the control unit 19 to operate the duty cycle of a discrete frequency used by one or more switches 11.

[0050] According to the invention, the values ​​of one or more discrete frequencies can be adjusted. For example, one or more of these values ​​can depend on the current dimming level. Furthermore, whenever the duty cycle of a frequency reaches a preset minimum, the resulting LED current will change in a way that at least one frequency (preferably in a frequency range above the resonant frequency) shifts. Subsequently, the PWM modulation of the discrete frequencies will be modified so that no discrete frequency is used with a duty cycle below the preset minimum.

[0051] The frequency to be used by control unit 19 can be indicated / modified by another control unit via signal 104. Alternatively, the frequency to be used and / or its modification can be determined by control unit 19 in the absence of instructions from another control unit 100.

[0052] Therefore, the value of at least one of the multiple discrete frequencies used can be modified during the ongoing operation of the converter, optionally in conjunction with the adjustment of the duty cycle. Thus, these two parameters are control values ​​used for feedback control of the converter's LED current or output current.

[0053] The following is combined Figures 4 to 6Further details of the operation of the switched resonant converter 1 are presented.

[0054] Figure 2 and Figure 3 The following are examples of methods for using according to Figure 1 Exemplary resonant circuit circuits 13A and 13B of LED converter 1.

[0055] Figure 2 The series inductor 21(L) shown RES ), series capacitor 22 (C RES,S ) and parallel capacitor 23 (C RES,P Example of implementing a resonant circuit 13A for a known LCC resonant topology.

[0056] Figure 3 The series capacitor 31(C) shown RES ), series inductor 32 (L RES,S ) and parallel inductor 33 (L RES,P Example of implementing a resonant circuit 13B for a known LLC resonant topology.

[0057] Those skilled in the art will know that similar converter structures can also be formed. For example, Figure 2 The parallel capacitor 23 of the LCC resonant circuit can optionally be arranged on the secondary side between the transformer 14 and the rectifier circuit 15. As another example, the series arrangement of the capacitor and inductor in the resonant circuit can also be reversed.

[0058] Figure 4 It shows the direction including according to Figure 3 The basis of the LLC resonant circuit Figure 1 The frequency dependence of the output current supplied to the output terminal of the LED converter is 17.

[0059] Figure 4 The curve shows the output / load current 17 varying with the switching frequency 43 of at least one switch 11, 12.

[0060] The output / load current 17 is determined by... Figure 3 The design of the resonant circuit 13B gives the peak value at the peak resonant frequency, and the frequency decreases from the peak resonant frequency as the frequency deviation increases.

[0061] If already combined Figure 1 As mentioned, at least one switch 11, 12 of the switched resonant converter 1 is alternately controlled at one of at least two different switching frequencies 44, 45 each time. The at least two different switching frequencies 44, 45 may each exceed the peak resonant frequency of the converter 1.

[0062] As an example, Figure 4 Two different switching frequencies, 44 and 45, above the peak resonant frequency are shown. In other words, Figure 4 It is shown that at least two different switching frequencies 44, 45 may include two different switching frequencies 44, 45.

[0063] Two different switching frequencies 44 and 45 are associated with corresponding output / load currents 17. More specifically, the lower of the two different switching frequencies 44 and 45, 44, can be associated with a first load current 42. Similarly, the higher of the two different switching frequencies 44 and 45, 45, can be associated with a second load current 41 that is lower than the first load current 42.

[0064] For example, the lower of the two different switching frequencies 44 and 45, 44, can be 0.5 MHz and associated with a first output / load current of 1 A, and the higher of the two different switching frequencies 44 and 45, 45, can be 2 MHz and associated with a second output / load current of 0.1 A. An average output / load current between 0.1 A and 1 A is generated alternately between the two different switching frequencies 44 and 45.

[0065] At least two different switching frequencies 44 and 45 can be constant or adjustable. Alternatively, at least two different switching frequencies 44 and 45 can be constant.

[0066] Figure 5 The diagram illustrates the switching operations of at least one switch 11, 12, which are alternately controlled at one of at least two different switching frequencies 44, 45, for corresponding time periods 54, 55.

[0067] Figure 5 The curve shows the switching frequency 43 and time 50 of at least one switch 11, 12.

[0068] If already combined Figure 1 As mentioned, the switching frequency 43 alternately controls the corresponding time periods 54 and 55 each time with one of two different switching frequencies 44 and 45.

[0069] exist Figure 5 In the example, the lower of two different switching frequencies 44 and 45 is applied during the time period 54 extending between times 51 and 52, and the higher of two different switching frequencies 44 and 45 is applied during the time period 55 extending between times 52 and 53. Then, the sequence of time periods 54 and 55 restarts according to the time period 56 that includes time periods 54 and 55.

[0070] In the case of a very short time period 54 or 55, the duration of time period 56, or its termed modulation frequency f, is... mod The reciprocal of the value can be fixed to a value of approximately 500 Hz to 100 kHz.

[0071] As an example, f mod It can be fixed at 20kHz, corresponding to a modulation time period 56 of 50μs. Considering the previously mentioned different switching frequencies 44 and 45 of f = 0.5 and 2MHz, the modulation time period 56 is equal to N = 25 and 100 switching cycles, respectively. In other words, with a fixed modulation frequency f... mod This ensures that the resolution of the switching frequency is higher than that of the modulation frequency f. mod The resolution is much higher. Furthermore, the maximum slope Δf / Δt between the subsequent time intervals 54 and 55 can be defined.

[0072] This avoids flickering issues in the case of very short time intervals of 54 and 55.

[0073] according to Figure 5 The switching operation of at least one switch 11, 12 can be continuous. This means that the switching frequency 43 controlling the switching operation of at least one switch 11, 12 is always greater than 0.

[0074] Figure 6 It shows that due to Figure 5 The alternating control of the switching frequency 43 shown generates a current 17 that is supplied to the output terminal of the LED load 18.

[0075] Figure 6 The curve shows the current 17a of the LED converter before it is flattened by capacitor 16 over time 50.

[0076] according to Figure 4 Each switching frequency 43 is associated with a corresponding output / load current 17. Therefore, Figure 6 The times 51, 52, and 53 correspond to Figure 5 The time shown, and in Figure 5 Applying two different switching frequencies during time periods 54 and 55 creates a situation where... Figure 6 The corresponding current 17a in the same time period 54 and 55.

[0077] If already combined Figure 1 As mentioned, the average value of the output / load current 17 can be adjusted by adjusting the relative duration of time periods 54 and 55.

[0078] Therefore, extending the duration of the higher first current 17a time period 54 to the duration of the second current 17a time period 55, which is lower than the first current 17a, increases the contribution of the higher first current 17a to the average output / load current 17 and reduces the contribution of the lower second current 17a to the average output / load current.

[0079] Conversely, extending the duration of the second current 17a, which is lower than the first current 17a, to the duration of the period 55 that impairs the first current 17a increases the contribution of the lower second current 17a to the average output / load current 17 and reduces the contribution of the higher first current 17a to the average output / load current 17.

[0080] In contrast to burst operation, the switching resonant converter 1 operates continuously at different frequencies. Low current is achieved by controlling the converter at a very high switching frequency instead of stopping its operation.

[0081] Different frequencies can be set using a feedforward method, rather than being generated by a feedback control algorithm.

[0082] Figure 7 A method 70 according to an embodiment of the third aspect is shown. Method 70 is used to operate an LED converter 1 for supplying an LED load 18.

[0083] The LED converter includes a switching resonant converter 1, which has at least one switch 11, 12 controlled by a control unit of the switching resonant converter 1.

[0084] Method 70 includes controlling the switching operation of at least one switch 11, 12, such that the switch resonant converter 1 supplies an adjustable constant current 17 to the output terminal of the LED load 18.

[0085] Method 70 further includes generating an adjustable constant current 17 by alternately controlling at least one switch 11, 12 for corresponding time periods 54, 55 at one of at least two different switching frequencies 44, 45 each time and by adjusting the relative duration of the time periods 54, 55.

[0086] The control unit is designed to implement method 70 according to the embodiment of the third aspect. Therefore, method 70 can be executed by the LED converter according to the embodiment of the first aspect.

Claims

1. An LED converter for supplying an LED load (18), the LED converter comprising: Switched resonant converter (1), the switched resonant converter having: At least one switch (11, 12), said at least one switch is made of The control unit of the switched resonant converter (1) controls, The control unit is configured to control the switching operation of the at least one switch (11, 12) such that the switch resonant converter (1) supplies an adjustable constant current (17) to the output terminal of the LED load (18). The control unit is further configured to generate the adjustable constant current (17) by alternately controlling the at least one switch (11, 12) for a corresponding time period (54, 55) at one of at least two different switching frequencies (44, 45) each time and by adjusting the relative duration of the time periods (54, 55). Wherein, the at least two different switching frequencies (44, 45) each exceed the peak resonant frequency of the converter (1), and the at least two different switching frequencies (44, 45) are adjustable, and The LED converter includes an additional control unit (100) that communicates with the control unit (19) via a bidirectional channel (103, 104, 17').

2. The LED converter according to claim 1, in, The additional control unit (100) is connected to the communication interface (101) of the LED converter, which communicates externally via a wireless or wired channel.

3. The LED converter according to claim 1 or 2, in, The additional control unit (100) provides a signal supply indicating the output current or LED current.

4. The LED converter according to claim 1, in, The additional control unit (100) sends a signal (103) to the control unit (19), the signal indicating the duty cycle of the discrete frequency used by the control unit (19) to operate the at least one switch (11).

5. The LED converter according to claim 1, in, The frequency to be used by the control unit (19) is indicated / modified by the other control unit (100), or In the absence of instructions from the other control unit (100), the frequency to be used and / or its modification can be determined by the control unit (19).

6. The LED converter according to claim 1, in, The at least two different switching frequencies (44, 45) include two different switching frequencies (44, 45).

7. A lighting system, the lighting system comprising: The LED converter according to any one of claims 1 to 6, and LED load (18), which is configured to be supplied with the output current of the LED converter.

8. A method (70) for operating an LED converter for supplying an LED load (18). The LED converter includes a switching resonant converter (1), which has: At least one switch (11, 12), said at least one switch is made of The control unit of the switched resonant converter (1) controls, The method (70) includes: According to the switching operation of at least one switch (11, 12) under duty cycle control (71), the switch resonant converter (1) supplies an adjustable constant current (17) to the output terminal of the LED load (18). The adjustable constant current (17) is generated (72) by alternately controlling the at least one switch (11, 12) to continue the duty cycle for a corresponding time period (54, 55) at one of at least two different switching frequencies (44, 45) each time and by adjusting the relative duration of the corresponding time period (54, 55) of the duty cycle. During the operation of the LED converter, the at least two different switching frequencies (44, 45) are adjusted, wherein the at least two different switching frequencies (44, 45) each exceed the peak resonant frequency of the converter (1). The adjustable constant current is measured and a feedback signal representing the measured adjustable constant current is generated. The duty cycle is adjusted using the feedback signal when controlling the switching resonant converter to provide the adjustable constant current to the output terminal for the LED load.

9. The method (70) according to claim 8. in, The method (70) is performed by an LED converter according to any one of claims 1 to 6.

10. A control unit for an LED converter, in, The control unit is designed to implement the method (70) according to claim 8 or claim 9.

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