Method for controlling the frequency of an input voltage of a DC to DC converter

Abstract

The invention relates to a method (4) for controlling the input voltage frequency of a DC-DC converter, the method comprising: defining a set point voltage value (V dc req ) of the previous step; and bat ), power set point (Preq) and the set point input voltage (V dc req ) to calculate the control frequency value (f sw (ω)) step (40); and the step of applying the control frequency to the converter.

Description

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

[0002] More particularly, the present invention relates to a method for controlling the frequency of an input voltage of a DC to DC converter for a battery charger.

[0003] Battery chargers (more commonly referred to as chargers) for electric vehicles require high levels of charging power, which may range, 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 (often abbreviated PFC) stage that performs AC to DC conversion of the grid voltage to a DC bus, and a second DC to DC conversion stage that controls the output current required to charge the battery and electrically isolates the charger with the aid of 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 converter may in particular be an LLC DC to DC converter comprising a transformer 22 which electrically isolates the charger.

[0007] Figure 3 Shows Figure 2 Simplified circuit diagram of a DC to DC converter comprising 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 the LLC DC to DC converter is operated at a duty cycle of 50% and is frequency controlled. In practice, the frequency control allows adjusting the gain of the DC to DC converter and setting the voltage of the DC bus at the input of the charger to a given set point. Depending on the voltage of the battery and the required power, the frequency may fluctuate between, for example, 60 kHz and 200 kHz.

[0009] Solutions proposed in the prior art for controlling DC-to-DC converters of this type generally involve the operation of regulating the output voltage, e.g. Peter, Michal, and The solution disclosed in the Anna publication A New Approach of Control System Design for LLC Resonant Converter. In MATLAB for Engineers-Applications in Control, Electrical Engineering, IT and Robotics. In Tech, 2011, in which the output voltage of a DC to DC converter is controlled by using a chopping frequency. The transfer function between duty cycle and output voltage is derived using an identification method by using a PSPICE hardware model that simulates the dynamic range of the output voltage response to a frequency step. The controller is then designed based on the previously derived transfer function.

[0010] The transfer function can also be obtained by a method called the "small signal" method, which consists in deriving the transfer function from excitations around the function point and measuring the response of the DC to DC converter, 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 at the operating point in question and becomes outdated every time the operating point changes. Therefore, the transfer function must be recalculated every time. Therefore, this solution is relatively complex to implement and expensive in terms of computational time.

[0011] Control operations involving regulating the DC current while the output voltage varies within a limited range are also known.

[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, also discloses feedback linearization control operation for controlling the output voltage of an LLC DC to DC converter. The publication describes a nonlinear model with 7 states, which is subsequently simplified to 2 states, and proposes PI loop control. However, this solution requires complex and expensive hardware and software adaptations.

[0013] In some cases, the output voltage is applied by a battery. In addition, in some cases, especially in electric vehicle applications, the output voltage varies within a large range of values, such as between 250V and 430V.

[0014] It is therefore desirable to regulate the DC input voltage as this allows a DC voltage to be applied across the terminals of the capacitor at the output of the PFC.

[0015] However, regulating the DC input voltage 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 for quickly and reliably controlling the DC voltage at the input of an LLC DC to DC converter is needed. A method for controlling the frequency of the input voltage of a frequency controlled LLC DC to DC converter operating at a 50% duty cycle is proposed.

[0017] The method includes:

[0018] - preliminary steps to define the set point voltage value,

[0019] - a step of calculating a control frequency value of said DC to DC converter based on the output battery voltage, the input power set point and said set point input voltage, the control frequency value being obtained by mathematically inverting the expression for the gain of said DC to DC converter; and

[0020] - A step of applying the control frequency calculated in this way to said converter.

[0021] Thus, input control of the DC to DC converter can be obtained relatively simply and quickly.

[0022] Advantageously and without limitation, the DC to DC converter is an LLC series resonant DC to DC converter defined by the parameters of an equivalent circuit comprising two inductors and one capacitor; the control frequency value is a function of the values ​​of the two inductors and the one capacitor. Thus, the calculation of the control frequency is obtained by approximating the operation of the DC to DC converter, allowing to simplify the calculation and speed up the method.

[0023] Advantageously and without limitation, said step of applying the control frequency calculated in this way comprises:

[0024] -Define the frequency increment step;

[0025] - a step of initializing the control frequency to an initial control value corresponding to the control frequency calculated in this way;

[0026] - defining a first threshold and a second threshold, and an additive inverse element of the first threshold and an additive inverse element of the second threshold;

[0027] - a step of calculating the error value between the measured input voltage value and the set point input voltage; and

[0028] - a step of comparing said error value with said threshold value;

[0029] - The method comprises a conditioning step during which:

[0030] - when the error value is between the first threshold and the additive inverse of the first threshold, and when the error is greater than the second threshold or less than the additive inverse of the second threshold, increasing the initial control frequency by the frequency increment step;

[0031] - when the error value is between the second threshold and the additive inverse of the second threshold, the control frequency is maintained at its previous value;

[0032] If none of these conditions are met, the initial control value is applied as the control frequency.

[0033] Thus, the method comprises a relatively simple, fast and robust frequency control.

[0034] According to a particular embodiment of the invention, the method comprises a feedback control of the control frequency.

[0035] The invention also relates to a device for implementing the method as described above.

[0036] The invention also relates to a charger for a battery, comprising a power factor correction stage, at least one DC to DC converter, and a device as described above.

[0037] The invention also relates to a motor vehicle comprising a charger for a battery as described above.

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

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

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

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

[0042] - Figure 4 is a flow chart of a control method according to an embodiment of the present invention.

[0043] because Figures 1 to 4 They relate to the same embodiment and therefore will be discussed simultaneously.

[0044] 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 referred to as a PFC stage 11 ) and DC to DC converters 12 a and 12 b , each of which comprises an inverter 212 .

[0045] The three-phase grid 10 is connected to an input filter 14 , which transmits a filtered input current to the PFC stage 11 .

[0046] 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, the outputs of which are connected in parallel with a series of batteries 13 .

[0047] Each DC to DC converter 12a, 12b ( Figure 2 Only one example is shown in FIG. 1 ) includes an input MOSFET bridge 120, an LLC circuit 121 ( Figure 3 ), transformer 22 and output diode bridge 122.

[0048] The charger 1 further comprises means 15 for controlling the DC-to-DC converter 12 , which means are able to implement the control method 4 according to the invention.

[0049] The control method 4 according to the invention aims at controlling the frequency of the input voltage of the DC-to-DC converter 12 .

[0050] To this end, the method according to the invention comprises calculating the chopping frequency of the DC-to-DC converter.

[0051] refer to Figure 3 It is known that the transfer function of the LLC DC to DC converter according to the present invention takes the following form:

[0052]

[0053] Where G is the gain of the transfer function of the DC to DC converter (or at least the inverter portion of the DC to DC converter up to the primary of the transformer);

[0054] η is the turns ratio of the transformer of the DC to DC converter;

[0055] V bat is the voltage across the battery terminals, i.e., the output voltage of the DC to DC converter;

[0056] V dc is the DC input voltage of the DC to DC converter;

[0057] And according to the general term: V 输出 is the output voltage of the DC to DC converter, and V 输入 is the input voltage to the DC to DC converter.

[0058] refer to Figure 3 , which is a simplified view of a DC to DC converter, the equivalent resistance R of the transformer of the DC to DC converter corresponds to the battery load known as the transformer primary. Therefore, R is calculated according to the following equation:

[0059]

[0060] Among them, N P and N S are the number of turns at the primary and secondary of the transformer, P is the power at the primary of the transformer, and V bat is the voltage at the secondary of the transformer.

[0061] Therefore, the transfer function of equation (1) can be written as follows:

[0062] The transfer function of this circuit is written as:

[0063]

[0064] Therefore, to calculate the gain of the transfer function of the DC to DC converter, the following calculation is performed:

[0065]

[0066] Rewrite equation (4) as a function of angular frequency ω (ω = 2πf sw ), where s = jω.

[0067] Therefore, the gain equation can be written as:

[0068]

[0069] or

[0070]

[0071] Calculate the transferred gain G so that the control frequency f is obtained according to the equation sw The expression is:

[0072] f sw (ω) = fct(V bat , Preq, Vdc (set point)) (5)

[0073] Among them, V bat is the battery voltage, V dc is the input voltage of the DC to DC converter, and Preq is the input power set point of the DC to DC converter.

[0074] In fact, using the set point V in the expression for G(s) dc Value replaces V dc This allows calculation of the frequency at which the DC bus converges at a given voltage (eg 450 V).

[0075] The gain G is calculated as ηV bat / Vdc ratio, that is, in this embodiment, G = ηV bat / 450V.

[0076] From this, it is deduced that it depends on (ω=2πf sω ) is the third-order equation:

[0077] ω 3 +Aω 2 +Bω+C=0 (6)

[0078] Among them, parameters A, B and C are V bat , Preq, Lm and Lr (inductance values ​​of the equivalent diagram of the DC to DC converter), and Cr (capacitance value of the equivalent diagram of the DC to DC converter).

[0079] Solving equation (6) for ω allows the calculation of the control frequency f of the DC to DC converter using feedforward control sw (ω).

[0080] Due to parameter dispersion and calculation accuracy and also due to simplifying assumptions made when writing the transfer function of the DC to DC converter, applying this direct calculation is not sufficient to eliminate the steady-state error between the measured DC voltage and the set point. However, the error is still small and is a maximum of 30V.

[0081] To overcome this problem, refer to Figure 4 , a controller has been added to the previous feedforward. This controller operates by increasing or decreasing the frequency until the steady-state error has been eliminated, and therefore adjusts the initial frequency generated by the previous calculation to a slightly greater extent to improve accuracy.

[0082] The controller according to a first embodiment is a discrete controller, wherein:

[0083] eps1 is the threshold from which the frequency increase / frequency decrease starts;

[0084] eps2 is the threshold that controls the frequency fixation.

[0085] According to one embodiment, also referring to Figure 4 In the first step, as described above, based on the set point voltage V DC req (e.g., 450V), the required power P req (For example, the range is 0<P req <11kW power) and based on the battery voltage (so that 250V<V bat <430V) to calculate 40 control frequency f sw (ω)(also called switching frequency f sw (ω)).

[0086] Control frequency value f sw (k) is initialized 41 to the previously calculated initial frequency value f sw_前馈 .

[0087] Then, calculate the set point voltage V of the 44DC to DC converter DC req The measured input voltage V dc 所测得 The error value ε between them.

[0088] This error value ε is compared with two error thresholds eps1 and eps2.

[0089] If (condition 1) the error ε is between the limit of eps1 and the limit of -eps1 (e.g., between 10V and -10V), and if in addition the error ε is greater than eps2 or less than -eps2, these thresholds are, for example, 5V and -5V, then the initial frequency value f is set to sw_前馈 Increase the frequency increment step size ΔF by 43, that is:

[0090] f sw (k) = f sw_前馈 +ΔF (7)

[0091] K is a time integer.

[0092] After this step 43 , the method loops back to step 44 .

[0093] If (condition 2) after step 44 the error ε is between the limit of eps2 and the limit of -eps2, then the frequency value f of the DC bus is ensured to be within 5V of the set value. sw (k) is fixed and is maintained at the previous value, namely:

[0094] f sw (k) = f sw (k-1)

[0095] Value f sw (k-1) is equal to f when condition 1 has not been met before sw_前馈 , or equal to f when step 45 is performed after k previous steps 43 sw_前馈 +k*ΔF.

[0096] If none of these conditions are met in step 44, the frequency value f calculated by feedforward in step 40 is used 46. sw (k). This value is updated periodically. As long as the condition on the error is not met, the control operation will continue to apply the frequency calculated by feedforward, and steps 43, 45 and 46 loop back to step 44.

[0097] The invention is not limited to the exemplary values ​​given for the error thresholds eps1 and eps2. In particular, eps2 may be set to 1 V or 0 V, depending on the feasibility of the operating point.

[0098] This approach ensures that convergence occurs at a stable frequency ensured by the feed-forward action and enabled by the action of the controller, thereby eliminating any residual steady-state error and ensuring that the DC bus converges accurately to the set point value.

[0099] The invention is not limited to the type of controller described in the first exemplary embodiment. A proportional-integral controller or a proportional-integral-derivative controller may also be provided, although the tuning of this controller is more complex than the tuning of the controller in the first embodiment of the invention, but its implementation is known to those skilled in the art.

Claims

1. A method (4) for controlling the frequency of an input voltage of a frequency controlled LLC DC to DC converter (12) operating at a 50% duty cycle, the method comprising: - Define the set point voltage value (V dc req ), -Based on the output battery voltage (V bat ), input power set point (Preq) and the set point input voltage (V dc req ) to calculate the control frequency value (f sw (ω)), the control frequency value is obtained by mathematically inverting the expression for the gain of the DC to DC converter (12); as well as - a step of applying the control frequency calculated in this way to said converter; The step of applying the control frequency calculated in this way comprises: - define the frequency increment step (ΔF); -The control frequency (f sw (k)) is initialized to the initial control value (f) corresponding to the control frequency calculated in this way. sw_前馈 ) step (41); - defining a first threshold (eps1) and a second threshold (eps2), and an additive inverse element of the first threshold (-eps1) and an additive inverse element of the second threshold (-eps2); -Calculate the measured input voltage value (V dc 所测得 ) and the set point input voltage (V dc req ) in step (44); and - a step (42) of comparing said error value with said threshold values ​​(eps1, -eps1, eps2, -eps2); - The method comprises a conditioning step during which: - When the error value is between the first threshold value (eps1) and the additive inverse element (-eps1) of the first threshold value, and when the error is greater than the second threshold value (eps2) or less than the additive inverse element (-eps2) of the second threshold value, the control frequency (f sw (k)) increasing (43) the frequency increment step (ΔF); - When the error value is between the second threshold value (eps2) and the additive inverse of the second threshold value (-eps2), the control frequency (f sw (k)) is maintained (45) at its previous value; If none of these conditions are met, the initial control value is applied as the control frequency.

2. The method (4) according to claim 1, characterized in that: The DC to DC converter is an LLC series resonant DC to DC converter defined by parameters of an equivalent circuit including two inductors (Lm, Lr) and one capacitor (Cr); The control frequency value (f sw (ω)) is a function of the values ​​of the two inductors (Lm, Lr) and the one capacitor (Cr).

3. The method (4) according to any one of claims 1 to 2, characterized in that The method further includes feedback control of the control frequency.

4. A device for carrying out the method (4) as claimed in any one of claims 1 to 3.

5. A charger (1) for a storage battery (13), comprising a power factor correction stage (11), at least one DC to DC converter (12a, 12b), and a device as claimed in claim 4.

6. A motor vehicle comprising a charger (1) for a battery (13) as claimed in claim 5.

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