The present invention relates to the field of electric battery chargers, in particular for electric or hybrid motor vehicles.
The invention relates more precisely to a method for controlling the frequency of the input voltage of a DC current to DC current converter for an electric battery charger.
Electric battery chargers, more commonly called chargers, for electric motor vehicles require significant charging powers, possibly ranging for example up to 22 kW in three-phase operation or 7 kW in single-phase operation.
These chargers generally comprise two power conversion stages: a first power factor correction stage, generally abbreviated to PFC, performing the AC-to-DC conversion of the grid voltages to a DC bus, and a second DC-to-DC conversion stage, called DC-to-DC, controlling the output current required for charging the battery and galvanically isolating the charger through a transformer.
With reference to
The DC-to-DC may in particular be an LLC, as shown in
The first MOSFET bridge 120 of the LLC DC-to-DC operates with a duty cycle of 50% and is controlled in terms of frequency. Specifically, frequency control makes it possible to adjust the gain of the DC-to-DC and to regulate the voltage of the DC buses at the input of the charger to a given setpoint value. Depending on the voltage of the battery and the required power, the frequency may fluctuate for example between 60 kHz and 200 kHz.
The solutions proposed in the prior art for controlling this type of DC-to-DC converter generally regulate the output voltage, such as the one disclosed in the publication DRGOŇA, Peter, FRIVALDSKÝ, Michal and SIMONOVÁ, Anna. 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 output voltage of the DC-to-DC is controlled using the chopping frequency. A transfer function between duty cycle and output voltage is deduced from identification methods, using a PSPICE hardware module simulating the dynamic ranges of the output voltage responses to a frequency increment. A controller is then designed based on the previously deduced transfer function.
The transfer function may also be obtained using the method called “small signal”, which consists in deducing a transfer function from an excitation around an operating point, and measuring the response of the DC-to-DC, as described in the doctorate thesis by YANG, Bo. Topology investigation of front end DC/DC converter for distributed power system. 2003. However, this transfer function is applicable only to the operating point under consideration, and becomes obsolete upon each change of operating point. It is therefore necessary to recalculate it each time. Such a solution is also relative complex to implement and expensive in terms of computation time.
Control operations in terms of controlling DC current if the output voltage varies over a low range are also known.
Lastly, the publication FANG, Zhijian, WANG, Junhua, DUAN, Shanxu et. al. Control of an LLC Resonant Converter Using Load Feedback Linearization. IEEE Transactions on Power Electronics, 2018, vol. 33, no. 1, p. 887-898 is also known, in which control is constructed through feedback linearization so as to control the output voltage of an LLC DC-to-DC. This publication describes a 7-state nonlinear model, which is thereafter reduced to 2 states and proposes control through a PI loop. However, such a solution involves complex and expensive hardware and software adaptations.
It is the case that the output voltage is imposed by the battery. Furthermore, it is the case, in particular in electric motor vehicle applications, that this output voltage varies over a wide range of values, for example between 250 V and 430 V.
Therefore, it is desirable to control the DC voltage at input, since this makes it possible to impose a DC voltage at the terminals of the capacitors at the output of the PFC.
However, controlling the DC voltage at the input of the LLC DC-to-DC converter is a subject for which the prior art does not offer any satisfactory solution.
There is hence the need for a solution for controlling the DC voltage at the input of the LLC DC-to-DC quickly and reliably.
What is proposed is a method for controlling the frequency of the input voltage of a DC current to DC current converter, comprising preliminary steps of:
the method furthermore comprising:
It is thus possible to achieve a fast and robust method for controlling the input voltage of a DC current to DC current converter.
Advantageously and without limitation, when the measured voltage is between said upper limit voltage and said lower limit voltage, the control frequency is calculated by applying the following equation:
in which the value error corresponds to the difference VDCR−VDCM between the setpoint voltage value and the measured voltage. It is thus possible to ensure precise convergence toward the setpoint value when the measured voltage is close to the setpoint voltage, and cancel out the static error.
Advantageously and without limitation, the control operation is controlled at least in part by an open-loop controller. The control frequency calculation is thus able to be refined.
The control operation is in particular controlled in terms of frequency by a proportional-integral controller only when the measured voltage is between said upper limit voltage and said lower limit voltage. It is thus possible to improve the control by selectively refining the control frequency calculation when the measured voltage is close to the setpoint voltage.
The invention also relates to a device for controlling the frequency of a DC current to DC current converter comprising means for implementing the method as described above.
The invention also relates to a charger for charging an electric accumulator battery comprising:
Other features and advantages of the invention will become apparent on reading the description given below of one particular embodiment of the invention, given by way of indication but without limitation, with reference to the appended drawings, in which:
With reference to
The three-phase electricity grid 10 is fitted on an input filter 14 transmitting filtered input currents to the PFC stage 11.
At the output of the PFC 11, two DC voltage buses, connected to the terminals of the output capacitors of the PFC stage 11, are each coupled to a DC-to-DC converter 12a, 12b, connected at output in parallel with an accumulator battery 13.
Each DC-to-DC 12a, 12b, just one example of which is shown in
The charger 1 furthermore comprises means 15 for controlling the DC current to DC current converters 12, able to implement a control method 60 according to the invention.
The control method 60 according to the invention aims to control the frequency of the input voltages of the DC current to DC current converters 12.
With reference to
These preliminary steps 61-63 may furthermore be reproduced during the operation of the method 60 in order to dynamically modify the operating parameters of the method.
First of all, a step of defining 61 a maximum control frequency value FRMAX and a minimum control frequency value FRMIN is implemented, for example in this case a maximum frequency FRMAX of 200 kHz, and a minimum frequency FRMIN of 60 kHz.
A step of defining 62 a setpoint voltage value VDCM towards which the input voltage should converge is then implemented. In the exemplary embodiment of
An error zone 51, defined by two error values, an upper error value eps and a lower error value −eps, is then defined 63, these two error values making it possible to define an upper limit voltage value VDRC+eps and a lower limit voltage value VDCR−eps.
In this exemplary embodiment, an error voltage +eps=100 V and −eps=−100 V is defined.
These upper VDCR+eps and lower VDCR−eps limit voltage values framing the setpoint voltage VDCR thus define an error amplitude around the setpoint voltage VDCR.
In this embodiment, the upper error value eps and lower error value −eps have the same absolute value, so as to define a symmetrical error zone around the setpoint voltage value VDCR. However, the invention is not limited to these absolute values being the same, and there may be provision for an upper error value eps and a lower error value −eps having different absolute values.
The method then implements a step of obtaining 64 a measured value of the input voltage VDCM.
A step of calculating 65 a control frequency value of the DC current to DC current converter is then implemented.
In this calculation step 65, the measured input voltage VDCM is compared with the upper VDCR+eps and lower VDCR−eps limit voltage values.
If the measured voltage VDCM is greater than or equal to said upper limit voltage VDCR+eps, then a control frequency F equal to said minimum control frequency FRMIN is applied.
If the measured voltage VDCM is less than or equal to the lower limit voltage VDCR−eps, then a control frequency F equal to the maximum control frequency FRMAX is applied.
Lastly, if the measured voltage VDCM is strictly between said upper limit voltage VDCR+eps and said lower limit voltage VDCR−eps, the control frequency F corresponds to an average frequency FMOY that is calculated on the basis of the difference between the measured voltage and the setpoint voltage, the values of the upper and lower limit errors and the values of the maximum and minimum control frequency.
This average frequency FMOY is calculated using the following equation:
In other words, the pair of error parameters −eps, eps makes it possible to define an error zone close to the setpoint VDCR, in which the control means calculate a frequency FMOY that makes it possible to converge precisely on the setpoint value VDCR.
Specifically, when the error VDCR−VDRM reaches one of the thresholds −eps, eps, a control frequency is calculated so as to precisely reach the setpoint and cancel out the static error.
Above or below this error zone, respectively, the minimum frequency FRMIN or maximum frequency FRMAX is applied as described by the following logic in order to ensure effective convergence.
With reference to
The output of the first control stage 41 therefore arrives as what is called a feed-forward open-loop control command and is added 43 to the results obtained by the PI controller 42.
Number | Date | Country | Kind |
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18 55259 | Jun 2018 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2019/063690 | 5/27/2019 | WO |
Publishing Document | Publishing Date | Country | Kind |
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WO2019/238405 | 12/19/2019 | WO | A |
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