Method for controlling input voltage frequency of DC-DC converter

Abstract

The invention relates to a method (30) for controlling the input voltage frequency of a DC-DC converter (12), the method comprising a step (65) of calculating a control frequency value (F) of the DC-DC converter (12), wherein: DCM ) is greater than the upper limit voltage (V DCR +eps), the control frequency (F) corresponds to the minimum control frequency (FR MIN );‑If the measured voltage (V DCM ) is less than the lower limit voltage (V DCR ‑eps), the control frequency corresponds to the maximum control frequency (FR MAX ); and if the measured voltage (V DCM ) between the upper voltage limit (V DCR +eps) and the lower limit voltage (V DCR ‑eps), the control frequency (F) corresponds to the set point voltage value (V DCR ) and the measured voltage (V DCM ), the upper error value (eps) and the lower error value (‑eps), and the maximum control frequency value (FR MAX ) and the minimum control frequency value (FR MIN ) calculated the average frequency (F MOY ).

Description

Technical Field

[0001] The invention relates to the field of battery chargers, in particular for electric or hybrid vehicles.

[0002] More precisely, the invention relates to a method for controlling the frequency of an input voltage of a DC current to DC current converter for a battery charger. Background Art

[0003] Battery chargers (more commonly referred to as chargers) for electric vehicles require significant charging power, for example up to 22 kW in three-phase operation or up to 7 kW in single-phase operation.

[0004] These chargers typically include two power conversion stages: a first power factor correction stage (often abbreviated as PFC) that performs the AC to DC conversion of the grid voltage to the DC bus, and a second DC to DC conversion stage (referred to as DC to DC) that controls the output current required to charge the battery and electrically isolates the charger via a transformer.

[0005] Reference to prior art Figure 1 , the two output DC voltage buses at the terminals of the output capacitor are respectively coupled to the DC to DC converter.

[0006] like Figure 2 As shown, the DC to DC may in particular be an LLC comprising a transformer 22 which electrically isolates the charger.

[0007] Figure 3 Shows Figure 2 Simplified diagram of the components of a DC to DC converter, which includes a capacitor Cr and two inductors Lr and Lm. The input voltage corresponds to the DC bus, and the output voltage is the voltage of the battery. The gain then corresponds to the ratio of these two voltages.

[0008] The first MOSFET bridge 120 of LLC DC to DC operates at a 50% duty cycle and is frequency controlled. Specifically, the frequency control makes it possible to adjust the DC to DC gain and regulate the DC bus voltage at the charger input to a given set point value. Depending on the voltage of the battery and the required power, the frequency may fluctuate between, for example, 60kHz to 200kHz.

[0009] Solutions proposed in the prior art for controlling DC-to-DC converters of this type generally regulate the output voltage, e.g. Peter, Michal and The solution disclosed in Anna's publication A New Approach of Control System Design for LLC Resonant Converter. In: MATLAB for Engineers-Applications in Control, Electrical Engineering, IT and Robotics. InTech, 2011, in which the DC to DC output voltage is controlled using chopping frequency. The dynamic range of the output voltage response to the frequency increment is simulated using the PSPICE hardware module, and the transfer function between the duty cycle and the output voltage is derived according to the identification method. The controller is then designed based on the previously derived transfer function.

[0010] The transfer function can also be obtained using a so-called "small signal" method, which consists in deriving the transfer function from excitations around the operating point and measuring the DC to DC response, as described in YANG, Bo's 2003 doctoral dissertation Topology investigation of front end DC / DC converter for distributed power system. However, this transfer function is only valid for the considered operating point and becomes outdated every time the operating point changes. Therefore, the transfer function needs to be recalculated every time. Such a solution is also relatively complex to implement and expensive in terms of computational time.

[0011] Control operations are also known in terms of controlling the DC current when the output voltage varies within a low range.

[0012] Finally, the publication Control of an LLC Resonant Converter Using Load Feedback Linearization by FANG, Zhijian, WANG, Junhua, DUAN, Shanxu et al., IEEE Transactions on Power Electronics, 2018, Vol. 33, No. 1, pp. 887-898, is also known, in which a control is constructed by feedback linearization in order to control the output voltage of an LLC DC to DC. The publication describes a 7-state nonlinear model, which is subsequently simplified to 2 states, and proposes control by a PI loop. However, this solution involves complex and expensive hardware and software adaptations.

[0013] The fact is that the output voltage is applied by the battery. Furthermore, in particular in electric vehicle applications, the fact is that this output voltage varies within a large range of values, for example between 250V and 430V.

[0014] It is therefore desirable to control the DC voltage at the input, as this makes it possible to apply a DC voltage at the terminals of the capacitor at the output of the PFC.

[0015] However, controlling the DC voltage at the input of an LLC DC-to-DC converter is a subject for which the prior art does not provide any satisfactory solution.

[0016] Therefore, a solution is needed for quickly and reliably controlling the DC voltage at the LLC DC-to-DC input. Summary of the invention

[0017] A method for controlling the frequency of an input voltage of a DC current to DC current converter is proposed, the method comprising the following preliminary steps:

[0018] -Define the maximum control frequency value and the minimum control frequency value;

[0019] - define the set point voltage value;

[0020] - defining an upper error value and an associated upper voltage limit value; a lower error value and an associated lower voltage limit value, the upper voltage limit value and the lower voltage limit value defining an error margin around the set point voltage value;

[0021] The method further includes:

[0022] - a step of obtaining a measured value of an input voltage;

[0023] - A step of calculating a control frequency value of said DC current to DC current converter, wherein:

[0024] - if the measured voltage is greater than the upper limit voltage, the control frequency corresponds to the minimum control frequency;

[0025] - if the measured voltage is less than the lower limit voltage, the control frequency corresponds to the maximum control frequency; and

[0026] If the measured voltage is between the upper voltage limit and the lower voltage limit, the control frequency corresponds to an average frequency calculated based on the difference between the set point voltage value and the measured voltage, the upper error value and the lower error value, and the maximum control frequency value and the minimum control frequency value.

[0027] Hence, a fast and robust method for controlling the input voltage of a DC current to DC current converter may be achieved.

[0028] Advantageously but not limitingly, when the measured voltage is between the upper voltage limit and the lower voltage limit, the control frequency is calculated by applying the following equation:

[0029]

[0030] The error corresponds to the difference between the set point voltage and the measured voltage (V DCR -V DCM ). Therefore, it is ensured that the measured voltage converges accurately to the set point value and eliminates static errors when it approaches the set point voltage.

[0031] Advantageously but not limitingly, the control operation is at least partially controlled by an open loop controller. The calculation of the control frequency can thus be refined.

[0032] In particular, the control operation is controlled by the proportional-integral controller according to the frequency only when the measured voltage is between the upper limit voltage and the lower limit voltage. Therefore, when the measured voltage is close to the set point voltage, the control can be improved by selectively refining the control frequency calculation.

[0033] The invention also relates to a device for controlling the frequency of a DC current to DC current converter, the device comprising means for implementing the above method.

[0034] The present invention also relates to a charger for charging a storage battery, the charger comprising:

[0035] - Power factor correction stage;

[0036] - LLC resonant DC current to DC current converter; and

[0037] - A device for controlling the frequency of said DC current to DC current converter as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Other characteristics and advantages of the invention will become apparent on reading the following description of a particular embodiment of the invention given by way of indication and not limitation, with reference to the accompanying drawings, in which:

[0039] - Figure 1 is a schematic diagram of a battery charger known in the prior art;

[0040] - Figure 2 is used according to Figure 1 Detailed view of the charger's DC current to DC current converter;

[0041] - Figure 3 is based on Figure 2 A simplified diagram of the LLC circuit of a DC current to DC current converter;

[0042] - Figure 4a is a schematic depiction of a method according to one embodiment of the present invention;

[0043] - Figure 4b is based on Figure 4a A detailed view of the calculation steps of the method of an embodiment;

[0044] - Figure 5 is based on Figure 4a Schematic depiction of a method of applying frequency control as a function of a limit voltage, a set point voltage and a measured voltage, wherein the abscissa is time and the ordinate is volts; and

[0045] - Figure 6 is based on Figure 4a A flowchart of a method implemented by an embodiment of the present invention. DETAILED DESCRIPTION

[0046] Figures 1 to 6 The same embodiments are referred to and will be annotated simultaneously.

[0047] refer to Figure 1 , a charger 1 for a battery 13 connected to a three-phase grid 10 comprises a power factor correction stage 11 (also called PFC stage 11 ) and DC current to DC current converters DC to DC 12 a and 12 b , each having an inverter 212 .

[0048] The three-phase grid 10 is mounted on an input filter 14 , which transmits the filtered input current to the PFC stage 11 .

[0049] At the output of the PFC 11 , two DC voltage buses connected to the terminals of the output capacitor of the PFC stage 11 are coupled to DC to DC converters 12 a , 12 b , respectively, which are connected in parallel with a battery 13 at the output.

[0050] Each DC to DC 12a, 12b ( Figure 2 1 and 12 (only one example is shown in FIG. 1 ) include an input MOSFET bridge 120, an LLC circuit 121 (a simplified circuit diagram of which is shown in FIG. 1 ). Figure 3 As shown), transformer 22 and output diode bridge 122.

[0051] The charger 1 also comprises means 15 for controlling the DC current to DC current converter 12 , which means are able to implement a control method 60 according to the invention.

[0052] The control method 60 according to the invention aims at controlling the frequency of the input voltage of the DC current to DC current converter 12 .

[0053] Refer to Figure 4. Figure 5 and Figure 6 , a method for controlling a DC current to DC current converter comprises a plurality of preliminary steps 61, 62, 63. These preliminary steps 61 to 63 are independent of each other. The preliminary steps 61 to 63 are intended to define the operating parameters of the method; they can be performed before implementing the method (e.g. in a calibration phase) or dynamically at the beginning of the method.

[0054] These preliminary steps 61 to 63 can also be reproduced during the operation of the method 60 in order to dynamically modify the operating parameters of the method.

[0055] First, implement definition 61 of the maximum control frequency value FR MAX and minimum control frequency value FR MIN steps, for example, in this case, the maximum frequency FR MAX is 200kHz, and the minimum frequency FR MIN is 60kHz.

[0056] Then define the set point voltage value V DCR The input voltage should converge to the set point voltage value. Figure 5 In an exemplary embodiment, V DCR =450V.

[0057] Then an error zone 51 is defined 63 by two error values ​​(an upper error value eps and a lower error value −eps) which make it possible to define an upper voltage limit value V DRC +eps and lower voltage limit V DCR -eps.

[0058] In this exemplary embodiment, error voltages +eps=100 V and -eps=-100 V are defined.

[0059] Frame set point voltage V DCR These upper voltage values ​​V DRC +eps and lower voltage limit V DCR -eps thus defines the set point voltage V DCR The margin of error around that.

[0060] In this embodiment, the upper error value eps and the lower error value -eps have the same absolute value so that at the set point voltage value V DCR A symmetrical error zone is defined around . However, the invention is not limited to these same absolute values, and it may be provided that the upper error value eps and the lower error value -eps have different absolute values.

[0061] Then, the method is implemented to obtain 64 input voltage V DCM The measured value of the step.

[0062] A step of calculating 65 a control frequency value of the DC current to DC current converter is then implemented.

[0063] In the calculation step 65, the measured input voltage V DCM With the upper voltage value V DCR +eps and lower voltage limit V DCR -eps for comparison.

[0064] If the measured voltage V DCM Greater than or equal to the upper limit voltage V DCR +eps, then a control frequency equal to the minimum control frequency FR is applied MIN The control frequency F.

[0065] If the measured voltage V DCM Less than or equal to the lower limit voltage V DCR -eps, then apply a control frequency equal to the maximum control frequency FR MAX The control frequency F.

[0066] Finally, if the measured voltage V DCM Strictly between the upper voltage V DCR +eps and the lower limit voltage V DCR -eps, the control frequency F corresponds to the average frequency F calculated based on the difference between the measured voltage and the set point voltage, the values ​​of the upper error and the lower error, and the values ​​of the maximum control frequency and the minimum control frequency. MOY .

[0067] The average frequency F MOYUse the following equation to calculate:

[0068]

[0069] in:

[0070] - value error corresponds to the set point voltage value V DCR The measured voltage V DCM The difference, that is, error = V DCR -V DCM .

[0071] In other words, a pair of error parameters -eps, eps allows to set the point V DCR The error zone is defined around V, within which the control device calculates the value so that it can accurately converge to the set point value V DCR Frequency F MOY .

[0072] Specifically, when the error V DRC -V DRM When one of the thresholds -eps, eps is reached, the control frequency is calculated to accurately reach the set point and eliminate static errors.

[0073] As described in the following logic, the minimum frequency FR applies when above or below this error zone, respectively. MIN or maximum frequency FR MAX , to ensure effective convergence.

[0074] Reference Figure 4a , when the voltage V is measured DCM Strictly between the upper voltage V DCR +eps and the lower limit voltage V DCR -eps, the proportional-integral controller 42, usually called PI controller, is activated. This makes it possible to refine the calculation of the frequency F to be applied and improve the convergence of the measured voltage by a few volts.

[0075] The output of the first control stage 41 therefore arrives as a so-called feed-forward open-loop control command and is added 43 to the result obtained by the PI controller 42 .

Claims

1. A method for controlling (60) the frequency of an input voltage of a frequency controlled LLC DC current to DC current converter (12) operating at a 50% duty cycle, the method comprising the following preliminary steps: -Define (61) the maximum control frequency value (FR MAX ) and the minimum control frequency value (FR MIN ); -Define (62) the set point voltage value (V DCR ); - Define (63) the upper error value (eps) and the associated upper voltage limit value (V DCR +eps); lower error value (-eps) and the associated lower limit voltage value (V DCR -eps), the upper limit voltage value (V DCR +eps) and the lower voltage limit (V DCR -eps) defines the set point voltage value (V DCR ) around the margin of error (51); The method further includes: - Get (64) input voltage (V DCM ) of the measured value; - a step of calculating (65) a value of a control frequency (F) of said DC current to DC current converter (12), wherein: - If the measured voltage (V DCM ) is greater than the upper voltage limit (V DCR +eps), the control frequency (F) corresponds to the minimum control frequency (FR MIN ); - If the measured voltage (V DCM ) is less than the lower limit voltage (V DCR -eps), the control frequency corresponds to the maximum control frequency (FR MAX );and - If the measured voltage (V DCM ) between the upper voltage limit (V DCR +eps) and the lower limit voltage (V DCR -eps), the control frequency (F) corresponds to the set point voltage value (V DCR ) and the measured voltage (V DCM ), the upper error value (eps) and the lower error value (-eps), and the maximum control frequency value (FR MAX ) and the minimum control frequency value (FR MIN ) calculated the average frequency (F MOY ), When the measured voltage is between the upper limit voltage and the lower limit voltage, the control frequency is calculated by applying the following equation: The value error corresponds to the set point voltage value (V DCR ) and the measured voltage (V DCM ) difference (V DCR -V DCM ).

2. The method according to claim 1, characterized in that The control operation in the method is at least partially controlled by an open loop controller.

3. The method according to claim 2, characterized in that Only when the measured voltage is between the upper limit voltage and the lower limit voltage, the control operation is controlled according to the frequency by the proportional-integral controller.

4. An apparatus for controlling the frequency of a DC current to DC current converter, the apparatus comprising means for implementing the method as claimed in any one of claims 1 to 3.

5. A charger (1) for charging a storage battery (13), the charger comprising: - a power factor correction stage (11); - LLC resonant DC current to DC current converter (12); - Device for controlling the frequency of the DC current to DC current converter (12) according to claim 4.

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