EMI reduction device and method for DC-DC converter

Abstract

The invention provides an EMI (Electro-Magnetic Interference) reducing device of a DC-DC converter. The DC-DC converter comprises: an input voltage terminal; an output voltage terminal; a plurality of switching devices modulating an output voltage of the DC-DC converter by adjusting an operation frequency of the switching devices; and a control terminal that changes an operation frequency of the switching device based on an output terminal voltage of the DC-DC converter. The EMI reduction device comprises a frequency adjustment input unit which is connected with the control end and injects a fluctuation modulation frequency to the operation frequency of the switching device, and the fluctuation modulation frequency is based on the output voltage of the output voltage end and is set according to expected EMI performance. According to the EMI reduction device, energy concentrated on a certain frequency point fc and high-frequency harmonic waves such as 2fc and 3fc can be dispersed to frequency bands around the frequency point fc and the high-frequency harmonic waves, so that the EMI amplitude on each frequency point is reduced, and the EMI performance of the DC-DC converter is improved.

Description

Technical Field

[0001] The invention relates to a switching power supply, and in particular to an EMI reduction device and method for a frequency modulated DC-DC converter. Background Art

[0002] A switching power supply is a high-frequency electric energy conversion device that mainly uses power semiconductor devices (such as bipolar transistors, MOS tubes, etc.) to periodically "turn on" and "turn off" electronic switching devices through control circuits, allowing power semiconductor devices to pulse-modulate the input voltage, thereby achieving voltage conversion and adjustable output voltage and automatic voltage regulation. For different switching power supply topologies, PWM and frequency modulation are two commonly used control methods for switching power supplies. For switching devices, high conversion efficiency and low electromagnetic interference (EMI) are desired.

[0003] For switching power supplies using PWM control, a known mechanism that can effectively reduce the average EMI noise level is direct switching frequency modulation. However, direct switching frequency modulation, which is a mechanism for reducing EMI noise for PWM-controlled switching power supplies, is not suitable for converters using switching frequency modulation. The reason is that direct switching frequency modulation is difficult to match the requirements of each load. For example, direct switching frequency modulation designed for light load conditions will cause large fluctuations in the output voltage under full load conditions.

[0004] Therefore, there is a need for an effective mechanism for frequency modulated switching power supplies that can reduce the average EMI noise under various load conditions and thereby reduce the size of the EMI filter. Summary of the invention

[0005] The present invention is completed in view of the above situation in the prior art, and its object is to provide an EMI reduction device and method for a DC-DC converter capable of improving EMI performance.

[0006] One aspect of the present invention provides an EMI reduction device for a DC-DC converter, the DC-DC converter comprising: an input voltage terminal; an output voltage terminal; a plurality of switching devices, the output voltage of the DC-DC converter is modulated by adjusting the operating frequency of the switching devices; and a control terminal, the operating frequency of the switching devices is changed based on the output terminal voltage of the DC-DC converter. The EMI reduction device comprises: a frequency adjustment input unit connected to the control terminal, injecting a fluctuating modulation frequency into the operating frequency, the fluctuating modulation frequency is based on the output voltage of the output voltage terminal, and is set according to the desired EMI performance.

[0007] According to an EMI reduction device of a DC-DC converter of an embodiment of the present invention, a fluctuating modulation frequency is injected through a control terminal through a frequency adjustment input unit. The fluctuating modulation frequency is based on the output voltage of the output voltage terminal and is set according to the desired EMI performance. As a result, the EMI reduction device of the present invention can disperse the energy concentrated on a certain frequency point fc and its high-frequency harmonics such as 2fc and 3fc to the frequency bands around them to reduce the EMI amplitude at each frequency point. Since the energy is dispersed to the baseband of the frequency point (i.e., frequency expansion), each frequency point does not exceed the limit value specified by EMI, thereby achieving the effect of improving EMI performance.

[0008] In addition, according to another embodiment of the present invention, the EMI reduction device of the DC-DC converter can also be that the frequency modulation input unit includes: a fluctuation output voltage detection unit, which detects the fluctuation output voltage of the output end of the DC-DC converter as the fluctuation feedback voltage; a fluctuation modulation voltage output unit, which outputs the fluctuation modulation voltage corresponding to the fluctuation modulation frequency; and a fluctuation error control unit, which inputs the fluctuation error signal of the fluctuation feedback voltage and the fluctuation modulation voltage, and outputs the fluctuation modulation frequency to the control end. In the EMI reduction device of the present invention, in order to make the actual fluctuation output voltage follow the preset fluctuation modulation voltage, the fluctuation error control unit adjusts the operating frequency of the switching device. Thereby, the desired frequency expansion is achieved.

[0009] In addition, according to another embodiment of the present invention, the EMI reduction device of the DC-DC converter can also be that the frequency modulation input unit has: a fluctuation modulation voltage output unit, which outputs a fluctuation modulation voltage corresponding to the fluctuation modulation frequency. The DC-DC converter has: an output voltage detection unit, which detects the output voltage of the output voltage terminal and outputs an output feedback voltage corresponding to the output voltage; and an error control unit, which inputs a superposition value of a constant reference voltage and the fluctuation modulation voltage, and an output feedback voltage, and outputs a frequency related to the fluctuation modulation frequency to the control terminal.

[0010] In addition, according to another embodiment of the present invention, the EMI reduction device of the DC-DC converter can also be that the corresponding relationship between the fluctuation modulation voltage and the fluctuation modulation frequency is: Vmod=Hout_fd*Fsw_d / Gout_f, wherein Vmod is the fluctuation modulation voltage, Fsw_d is the fluctuation modulation frequency, Gout_f is the transfer function of the DC-DC converter from the output voltage to the operating frequency, and Hout_fd is the transfer function of the output voltage detection unit.

[0011] In the DC-DC converter of this embodiment, since the DC-DC converter is a feedback-controlled DC-DC converter, that is, it has a feedback circuit that controls the switching device frequency according to the superposition value of the output voltage (including the DC output voltage and the fluctuating output voltage) of the DC-DC converter and the constant reference voltage. Therefore, there is no need to set up a feedback circuit for controlling the follow-up of the preset fluctuating modulation voltage, but the preset fluctuating modulation voltage is added to the original feedback control loop of the DC-DC converter. As a result, the sharing of devices is realized, and the effects of miniaturization and cost saving are achieved.

[0012] In addition, according to another embodiment of the present invention, the EMI reduction device of the DC-DC converter can also be that the fluctuation modulation voltage output unit includes: an operational amplifier, whose positive phase input terminal is connected to the output terminal via a first resistor, whose negative phase input terminal is connected to the ground terminal via a first capacitor, and connected to the output terminal via a second resistor; a constant voltage source, one end of which is grounded, and the other end is connected to the positive phase of the operational amplifier via a third resistor, and a series circuit of a fourth resistor and a second capacitor, one end of which is connected to the fluctuation output voltage detection unit or the output voltage detection unit, and the other end is connected to the connection point of the second resistor and the first capacitor. Through the fluctuation modulation voltage output unit, a preset fluctuation modulation voltage is injected into the output voltage terminal side of the DC-DC converter, so that the error control unit adjusts the operating frequency of the switching device. Thereby, the desired frequency expansion is achieved.

[0013] In addition, the EMI reduction device of the DC-DC converter according to another embodiment of the present invention may also be further provided with: a current detection unit for detecting the output current of the DC-DC converter. A plurality of current intervals are set according to the EMI performance of the DC-DC converter, and in at least one current interval, a fluctuating modulation frequency is injected into the switching device of the DC-DC converter, and in other current intervals other than the at least one current interval, a fluctuating modulation frequency is not injected into the switching device of the DC-DC converter. Thus, the activation of the EMI reduction device can be controlled according to the actual EMI performance of the DC-DC converter in different working intervals (here, the current interval). Thus, high EMI performance of the DC-DC converter in the entire working area is obtained.

[0014] In addition, according to another embodiment of the present invention, the EMI reduction device of the DC-DC converter may have the desired EMI performance being an average value of the EMI.

[0015] In addition, according to another embodiment of the present invention, the EMI reduction device of the DC-DC converter may be that the DC-DC converter includes an isolated DC-DC converter and a non-isolated DC-DC converter.

[0016] In addition, according to another embodiment of the present invention, the EMI reduction device of the DC-DC converter may be that the DC-DC converter includes: an LLC converter, a quasi-resonant boost converter, a quasi-resonant buck converter, and a quasi-resonant flyback converter.

[0017] One aspect of the present invention provides an EMI reduction method for a DC-DC converter, in which the operating frequency of a switching device of the DC-DC converter is adjusted via a control terminal based on the output voltage of the DC-DC converter. A fluctuating modulation frequency is injected into the operating frequency via the control terminal. The fluctuating modulation frequency is based on the output voltage and is set according to the desired EMI performance. Thus, the operating frequency of the switching device of the DC-DC converter can be expanded, thereby improving the EMI performance.

[0018] In addition, according to another embodiment of the present invention, the EMI reduction device of the DC-DC converter can also be, based on the EMI performance of the DC-DC converter, determine the working range where EMI needs to be improved, and inject the fluctuating modulation frequency into the switching device of the DC-DC converter in the working range to obtain the desired EMI performance, and do not inject the fluctuating modulation frequency into the switching device of the DC-DC converter outside the working range. Thus, the activation of the EMI reduction device can be controlled according to the actual EMI performance of the DC-DC converter in different working ranges. Thus, the high EMI performance of the DC-DC converter in the entire working area is obtained.

[0019] Effects of the Invention

[0020] The EMI reduction device and method of the DC-DC converter of the present invention expands the frequency of the switching device of the DC-DC converter by setting the fluctuation modulation frequency based on the output voltage of the output voltage terminal of the DC-DC converter and according to the desired EMI performance, thereby obtaining better EMI performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a functional block diagram schematically illustrating a frequency modulated DC-DC converter.

[0022] Figure 2 is a functional block diagram schematically showing a DC-DC converter having a fluctuation error control unit for controlling fluctuation of an output voltage according to an embodiment of the present invention.

[0023] Figure 3 is a functional block diagram schematically showing a DC-DC converter having an error control unit for controlling an output voltage according to an embodiment of the present invention.

[0024] Figure 4 It is schematically shown Figure 3The diagram is a diagram of an example of circuit implementation of a fluctuation modulation voltage output unit shown.

[0025] Figure 5 FIG. 1 is a diagram schematically showing an example of a general unit under test (UUT) of an LLC converter having output voltage feedback control.

[0026] Figure 6 It is shown Figure 5 Figure 1 shows the EMI test results of the structure under test.

[0027] Figure 7 is a diagram showing an example of a fluctuating modulation voltage.

[0028] Figure 8 It shows the injection Figure 7 A diagram showing the ripple voltage produced on the DC output voltage after the ripple modulation voltage is shown.

[0029] Fig. 9 It is shown Figure 4 Figure 1 shows the EMI test results of the structure under test.

[0030] Fig.10 It is shown Figure 5 The figure shows the EMI test results of the tested structure under output conditions of 48V / 1000W and 48V / 100W respectively.

[0031] Fig.11 is a flow chart showing whether to enable the EMI reduction device according to the load current.

[0032] Fig.12 It is shown in Figure 4 The structure of the DC-DC converter shown is further implemented Fig.11 A diagram of an example of the functionality shown. DETAILED DESCRIPTION

[0033] Hereinafter, an EMI reduction device and method for a DC-DC converter according to an embodiment will be described. In the description of each figure, the same or corresponding elements are denoted by the same reference numerals, and duplicate descriptions may be omitted.

[0034] Figure 1 FIG. 1 is a functional block diagram schematically showing a frequency modulated DC-DC converter. Figure 1As shown, in a DC-DC converter, the operating frequency of the switching device of the DC-DC converter is adjusted via the control terminal, thereby converting the DC input voltage at the input voltage terminal into a different DC output voltage at the output voltage terminal. As mentioned above, there are currently two control methods for the switching device: adjusting the duty cycle (PWM) and adjusting the frequency (i.e., width modulation and frequency modulation). In a frequency modulation DC-DC converter, the relationship between the DC output voltage and the DC input voltage is adjusted by the different frequencies of the pulses. Figure 1 The frequency modulated DC-DC converter shown may be any converter topology used in frequency modulated switching power supplies.

[0035] In a switching power supply, by operating the switching device at a high frequency, the power consumption of the switching device can be very small, thereby greatly improving the conversion efficiency of the power supply. On the other hand, the switching device will generate a large amount of electromagnetic interference. Therefore, an effective mechanism suitable for a frequency-modulated switching power supply that can reduce EMI noise and thus reduce the size of the EMI filter is needed.

[0036] A DC-DC converter according to an embodiment of the present invention includes: an input voltage terminal; an output voltage terminal; a plurality of switching devices, which modulate the output voltage of the DC-DC converter by adjusting the operating frequency of the switching devices; and a control terminal, which changes the operating frequency of the switching devices based on the output terminal voltage of the DC-DC converter. The EMI reduction device of the present invention includes: a frequency adjustment input unit, which is connected to the control terminal and injects a fluctuating modulation frequency into the operating frequency of the switching devices. The fluctuating modulation frequency is based on the output voltage of the output voltage terminal and is set according to the desired EMI performance.

[0037] According to an EMI reduction device of a DC-DC converter of an embodiment of the present invention, a fluctuating modulation frequency is injected through a control terminal through a frequency adjustment input unit. The fluctuating modulation frequency is based on the output voltage of the output voltage terminal and is set according to the desired EMI performance. As a result, the EMI reduction device of the present invention can disperse the energy concentrated on a certain frequency point fc and its high-frequency harmonics such as 2fc and 3fc to the frequency bands around them to reduce the EMI amplitude at each frequency point. Since the energy is dispersed to the baseband of the frequency point (i.e., frequency expansion), each frequency point does not exceed the limit value specified by EMI, thereby achieving the effect of improving EMI performance.

[0038] Figure 2 Schematically shows a functional block diagram of a DC-DC converter having a fluctuation error control unit for controlling fluctuation of an output voltage according to an embodiment of the present invention. Figure 2As shown, the frequency modulation input unit includes: a fluctuation output voltage detection unit 304, a fluctuation modulation voltage output unit, and a fluctuation error control unit 308. The fluctuation output voltage detection unit 304 detects the fluctuation output voltage at the output voltage end of the DC-DC converter 302 as a fluctuation feedback voltage 305. The fluctuation modulation voltage output unit outputs a fluctuation modulation voltage 306 corresponding to the fluctuation modulation frequency. In the fluctuation error control unit 308, the fluctuation feedback voltage 305 from the fluctuation output voltage detection unit 304 and the fluctuation modulation voltage 306 from the fluctuation modulation voltage output unit are differentiated to obtain a fluctuation error signal 307 as an input of the fluctuation error control unit 308. The error control unit 308 functions as a feedback control unit that makes the actual fluctuation feedback voltage 305 follow the preset fluctuation modulation voltage 306, and outputs a fluctuation modulation frequency 309. The fluctuation modulation frequency 309 is superimposed on other frequency setting signals 311 determined according to the working state of the DC-DC converter, and is input as a frequency adjustment input 310 to the control end of the DC-DC converter 302.

[0039] In a frequency-modulated DC-DC converter with a constant DC output voltage, since the operating frequency of the switching device is substantially constant, the EMI performance at the operating frequency and its high-frequency harmonics is poor. In the DC-DC converter 300 of the present invention, a fluctuating modulation voltage 306 is introduced at the DC output voltage terminal side. The fluctuation error control unit 308 adjusts the operating frequency of the switching device in order to make the actual fluctuating feedback voltage 305 follow the preset fluctuating modulation voltage 306. Thus, under various load conditions, the desired frequency expansion is achieved, and the average EMI noise level is reduced.

[0040] Figure 3 is a functional block diagram schematically showing a DC-DC converter having an error control unit for controlling an output voltage according to an embodiment of the present invention. Figure 3The DC-DC converter 500 shown is a feedback control type DC-DC converter, and has: an output voltage detection unit 404, and an error control unit 408. The output voltage detection unit 404 detects the output voltage 403 of the DC output voltage terminal, and outputs an output feedback voltage 405 corresponding to the output voltage 403. The fluctuation modulation voltage output unit outputs a fluctuation modulation voltage 501 corresponding to the desired fluctuation modulation frequency. In the error control unit 408, the superposition value of the constant reference voltage 406 and the fluctuation modulation voltage 501, and the output feedback voltage 405 are input. In order to make the output feedback voltage 405 follow the preset fluctuation modulation voltage 501, the error control unit 408 outputs a frequency 409 related to the fluctuation modulation frequency. The frequency 409 related to the fluctuation modulation frequency is superimposed on other frequency setting signals 411 determined according to the working state of the DC-DC converter, and is input as a frequency adjustment input 410 to the control end of the DC-DC converter 500.

[0041] Hereinafter, the correspondence relationship between Vmod, which is the ripple modulation voltage 501, and Fsw_d (desired frequency spread), which is the ripple modulation frequency, will be described.

[0042] In the power conversion circuit 402 of the frequency modulated DC-DC converter, there are:

[0043] Fsw_d = Vout_d*Gout_f (1)

[0044] Wherein, Vout_d is the fluctuation of the DC output voltage at the voltage output terminal of the DC-DC converter caused by Fsw_d, and Gout_f is the transfer function of the frequency modulated DC-DC converter 402 from the output voltage to the operating frequency.

[0045] In the output voltage detection unit 404, the relationship between Vout_fd, which is the feedback value of the output voltage fluctuation caused by Fsw_d, and Vout_d, which is the DC output voltage fluctuation value caused by Fsw_d, is:

[0046] Vout_d = Vout_fd / Hout_fd (2)

[0047] Wherein, Hout_fd is the transfer function of the output voltage detection unit 404 .

[0048] As described above, the error control unit 408 functions as a feedback control unit that causes the output feedback voltage 405 to follow the preset fluctuation modulation voltage 501, that is, there are:

[0049] Vmod=Vout_fd (3)

[0050] Combining the above formulas (1) to (3), we can obtain:

[0051] Vmod=Hout_fd*Fsw_d / Gout_f, wherein Vmod is the fluctuation modulation voltage 501 , Fsw_d is the fluctuation modulation frequency, Gout_f is the transfer function of the DC-DC converter 402 from the output voltage to the operating frequency, and Hout_f is the transfer function of the output voltage detection unit 404 .

[0052] In the DC-DC converter 500 with the EMI reduction device, since there is no need for Figure 2 As shown, the feedback circuit for controlling the follow-up of the preset fluctuating modulation voltage is additionally provided, but the preset fluctuating modulation voltage 501 is added to the original feedback control loop of the DC-DC converter (i.e., the error control unit 408). Thus, the sharing of components is realized, and the effects of miniaturization and cost saving are achieved.

[0053] Figure 4 It is schematically shown Figure 3 FIG. 1 is a diagram showing an example of a circuit implementation of a fluctuation modulation voltage output unit. Figure 4 In the DC-DC converter 700 with an EMI reduction device, as inputs of an error control unit 608, there are provided: a constant reference voltage 606, an output feedback voltage 605 from a fluctuating output voltage detection unit 604, and a fluctuating modulation voltage 701 from a fluctuating modulation voltage output unit 702.

[0054] like Figure 4 As shown, the fluctuation modulation voltage output unit 702 includes: an operational amplifier, whose non-phase input terminal is connected to the output terminal via the first resistor R6, whose inverting input terminal is connected to the ground terminal via the first capacitor C3, and connected to the output terminal via the second resistor R7; a constant voltage source U2, one end of which is grounded, and the other end is connected to the non-phase of the operational amplifier via the third resistor R5; and a series circuit of a fourth resistor R8 and a second capacitor C4, one end of which is connected to the fluctuation output voltage detection unit 604, and the other end is connected to the connection point of the second resistor R7 and the first capacitor C3. Through the fluctuation modulation voltage output unit 702, a preset fluctuation modulation voltage 701 is injected into the output terminal side of the DC-DC converter, so that the error control unit 608 adjusts the operating frequency of the switching device. Thus, under various load conditions, the desired frequency expansion is achieved.

[0055] It should be understood that Figure 4 This is a specific implementation of the fluctuation modulation voltage output unit according to the present invention. Other implementations such as a single chip microcomputer may also be used as long as the above-mentioned fluctuation modulation voltage function of the present invention (ie, outputting a fluctuation modulation voltage corresponding to the desired fluctuation modulation frequency) can be met.

[0056] Figure 5 FIG. 1 is a diagram schematically showing an example of a general unit under test (UUT) of an LLC converter having output voltage feedback control. Figure 5 The test structure shown is Figure 4 The difference is that in Figure 4 In the DC-DC converter with EMI reduction device of the present invention, a preset fluctuation modulation voltage 701 is injected through a fluctuation modulation voltage output unit 702 at the output voltage terminal side.

[0057] Will Figure 4 and Figure 5 The DC-DC converter shown in the figure is set to operate in the following modes: input is 390VDC, output is 48V / 100W, and EMI is tested according to EN55032 Class B requirements, and the results are shown in Figure 6 and Fig. 9 In the test, Figure 4 The DC-DC converter 700 shown in FIG. 1 is a pulsating modulated voltage 701, and the pulsating modulated voltage output unit 702 generates a pulsating modulated voltage. Figure 7 The peak-to-peak value shown is a 50mV fluctuation voltage. Figure 8 As shown, due to the superposition of Figure 7 The ripple modulation voltage shown in the figure generates a 400mV ripple voltage on the DC output voltage. When the ripple voltage increases, the DC-DC converter will adjust the operating frequency in order to stabilize the output voltage.

[0058] exist Figure 6 In Figure 1, the limits for the quasi-peak and mean values ​​are shown, as well as the quasi-peak trace and the measured mean trace of the DC-DC converter. Figure 6 As shown, Figure 5 Although the measurement results of the DC-DC converter 600 meet the EN55032 Class B limit, the average noise level around 182kHz and 455kHz does not meet the 3dB margin requirement according to the product specifications of this UUT.

[0059] On the other hand, for the DC-DC converter 700 to which the EMI reduction device of the present invention is applied, as shown in FIG. Fig. 9 As shown in the figure, the EMI noise spectrum is observed to be expanded and the average noise level is reduced by about 4dB, meeting the mean -3dB requirement. As a result, the EMI performance can be improved without changing the EMI filter. Figure 6 and Fig. 9 It can be seen from the comparison that the application of the EMI device of the present invention can effectively reduce the average EMI noise of the frequency modulated DC-DC converter.

[0060] It should be noted that the EMI reduction device of the present invention can be applied to a frequency modulated DC-DC converter of any topology. That is, the DC-DC converter can include an isolated DC-DC converter and a non-isolated DC-DC converter. In addition, the DC-DC converter can also be any of an LLC converter, a quasi-resonant boost converter, a quasi-resonant buck converter, and a quasi-resonant flyback converter.

[0061] In addition, the inventors have also found that the EMI performance of the DC-DC converter is different in different working ranges. Fig.10 It is shown Figure 5 The EMI test results of the structure under test under different working conditions are shown in the figure. Fig.10 (a) is the EMI performance at full load, i.e. output of 48V / 1000W. Fig.10 (b) shows the EMI performance under a smaller load, i.e., an output of 48V / 100W. Fig.10 (a) and Fig.10 As shown in (b), in particular, within the frequency range of 150k to 400k, the average EMI performance under a smaller load is close to the EN55032 Class B average value limit, which is significantly worse than the average EMI performance at a full load of 48V / 1000W. Therefore, it is considered not to enable the EMI reduction device in the entire operating range, but only in the medium and light load operating range where the EMI performance is poor.

[0062] Fig.11 FIG. 1 is a flow chart showing whether to enable the EMI reduction device according to the load current. Fig.11 As shown, in step S920, the output load current of the frequency modulated DC-DC converter is detected. In step S930, a judgment is made on the output load current detected in step S920, and step S940 is performed when the current meets the set conditions, and step S950 is performed when the current does not meet the set conditions. In step S940, the EMI reduction device of the present invention is enabled for the frequency modulated DC-DC converter, that is, the fluctuating modulation frequency is injected. In step S950, the EMI reduction device of the present invention is not enabled for the frequency modulated DC-DC converter, that is, the fluctuating modulation frequency is not injected. Thus, the activation of the EMI device can be controlled according to the actual EMI performance of the DC-DC converter in different working ranges (here, current). Thus, high EMI performance of the DC-DC converter in the entire working area is obtained.

[0063] Fig.12 It is shown in Figure 4 The structure of the DC-DC converter shown is further implemented Fig.11 An example of the functionality shown in FIG. Fig.12In the embodiment, an output current detection device 802 is provided for detecting the output current. 803 (U2) is used to set the current threshold value and segment the output current. When the detected load current is greater than the current value set by U2, Q1 is turned on, so that the fluctuation voltage generator 702 stops outputting the fluctuation modulation voltage 701, and does not inject the fluctuation modulation frequency into the frequency modulated LLC converter device 800. When the detected load current is less than the current value set by U2, Q1 is not turned on, the fluctuation voltage generator 702 outputs the fluctuation modulation voltage 701, and injects the fluctuation modulation frequency into the frequency modulated LLC converter device 800, that is, the frequency adjustment input 610 includes the fluctuation modulation frequency, so as to achieve the desired expansion of the operating frequency of the switching device, so that the average electromagnetic interference EMI is reduced.

[0064] It should be understood that the embodiments of the present disclosure do not limit the specific method of current detection, and resistance sampling or current transformer can be used. The embodiments of the present disclosure do not limit the preset current threshold and the number of load working intervals (here, 2 working intervals), which can be determined according to the noise reduction requirements of the average EMI.

[0065] The present disclosure also provides a method for reducing EMI of a DC-DC converter using the EMI reduction device as described in the above embodiment. It should be noted that the EMI reduction method of a DC-DC converter provided by the present invention is a method for applying the EMI reduction device of a DC-DC converter, which can achieve the same technical effect as the embodiment of the EMI reduction device. To avoid repetition, the above EMI reduction method embodiment will not be described in detail.

[0066] Although the present invention has been described in detail above in conjunction with the accompanying drawings and embodiments and variations, it is to be understood that the above description does not limit the present invention in any form. Those skilled in the art may deform and change the present invention as needed without departing from the essential spirit and scope of the present invention, and these deformations and changes all fall within the scope of the present invention.

Claims

1. An EMI reduction device for a DC-DC converter, characterized in that: The DC-DC converter comprises: an input voltage terminal; an output voltage terminal; a plurality of switching devices, the output voltage of the DC-DC converter is modulated by adjusting the operating frequency of the switching devices; and a control terminal, the operating frequency of the switching devices is changed based on the output terminal voltage of the DC-DC converter, The EMI reduction device includes: a frequency adjustment input unit connected to the control terminal, injecting a fluctuation modulation frequency into the operating frequency, wherein the fluctuation modulation frequency is based on the output voltage of the output voltage terminal and is set according to desired EMI performance.

2. The EMI reduction device of a DC-DC converter according to claim 1, characterized in that: The frequency modulation input unit comprises: a fluctuating output voltage detection unit, detecting a fluctuating output voltage at an output voltage terminal of the DC-DC converter as a fluctuating feedback voltage; a fluctuation modulation voltage output unit, outputting a fluctuation modulation voltage corresponding to the fluctuation modulation frequency; and The fluctuation error control unit inputs the fluctuation error signal of the fluctuation feedback voltage and the fluctuation modulation voltage, and outputs the fluctuation modulation frequency to the control end.

3. The EMI reduction device of a DC-DC converter according to claim 1, characterized in that: The frequency modulation input unit comprises: a fluctuation modulation voltage output unit, which outputs a fluctuation modulation voltage corresponding to the fluctuation modulation frequency, The DC-DC converter has: an output voltage detection unit, detecting an output voltage of the output voltage terminal and outputting an output feedback voltage corresponding to the output voltage; as well as The error control unit receives as input the superposition value of the constant reference voltage and the fluctuation modulation voltage, and the output feedback voltage, and outputs a frequency related to the fluctuation modulation frequency to the control terminal.

4. The EMI reduction device of a DC-DC converter according to claim 3, characterized in that: The corresponding relationship between the fluctuation modulation voltage and the fluctuation modulation frequency is: Vmod=Hout_fd*Fsw_d / Gout_f Among them, Vmod is the fluctuation modulation voltage, Fsw_d is the fluctuation modulation frequency, Gout_f is the transfer function of the DC-DC converter from the output voltage to the operating frequency, and Hout_fd is the transfer function of the output voltage detection unit.

5. The EMI reduction device of a DC-DC converter according to claim 2 or 3, characterized in that: The fluctuation modulation voltage output unit comprises: An operational amplifier, whose non-inverting input terminal is connected to the output terminal via a first resistor, whose inverting input terminal is connected to the ground terminal via a first capacitor, and connected to the output terminal via a second resistor; a constant voltage source, one end of which is grounded, and the other end of which is connected to the positive phase of the operational amplifier via a third resistor, and A series circuit of a fourth resistor and a second capacitor has one end connected to the fluctuating output voltage detection unit or the output voltage detection unit, and the other end connected to a connection point between the second resistor and the first capacitor.

6. The EMI reduction device for a DC-DC converter according to any one of claims 1 to 5, characterized in that: Further comprising: a current detection unit, detecting an output current of the DC-DC converter; A plurality of current intervals are set according to the EMI performance of the DC-DC converter, In at least one current interval, injecting the pulsating modulation frequency into the switching device of the DC-DC converter, In current intervals other than the at least one current interval, the ripple modulation frequency is not injected into the switching device of the DC-DC converter.

7. The EMI reduction device for a DC-DC converter according to any one of claims 1 to 6, characterized in that: The expected EMI performance is an average value of EMI.

8. The EMI reduction device for a DC-DC converter according to any one of claims 1 to 7, characterized in that: The DC-DC converter includes an isolated type and a non-isolated type DC-DC converter.

9. The EMI reduction device for a DC-DC converter according to any one of claims 1 to 8, characterized in that: The DC-DC converter includes: an LLC converter, a quasi-resonant boost converter, a quasi-resonant buck converter, and a quasi-resonant flyback converter.

10. A method for reducing EMI of a DC-DC converter, characterized in that: Based on the output voltage of the DC-DC converter, the operating frequency of the switching device of the DC-DC converter is adjusted via the control terminal. A ripple modulation frequency is injected into the operating frequency via the control terminal, the ripple modulation frequency being based on the output voltage and set according to desired EMI performance.

11. The EMI reduction method of a DC-DC converter according to claim 10, According to the EMI performance of the DC-DC converter, determine the working range where EMI needs to be improved. In the working range, the fluctuating modulation frequency is injected into the switching device of the DC-DC converter to obtain the desired EMI performance, Outside the working range, the ripple modulation frequency is not injected into the switching device of the DC-DC converter.

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