Method for controlling a dc-dc converter of a bidirectional charger for charging a battery that is an electrical accumulator

Abstract

This invention relates to a method (4) for controlling the frequency of the input voltage of a DC-DC converter in a bidirectional charger, which operates in a discharge mode, the method comprising: - defining a setpoint voltage value (V DC req Preliminary steps; - Based on the output battery voltage (V) bat ), Input power setpoint (P) req ) and the setpoint input voltage (V DC req The steps include: calculating the control frequency value (fsw(ω)) of the DC-DC converter (12) (40), which is obtained by inverting the gain of the DC-DC converter (12); and fine-tuning the DC bus by increasing / decreasing the frequency. The step of applying the calculated control frequency to the converter.

Description

Technical Field

[0001] This invention relates to a DC-DC converter and its control method for a bidirectional charger for charging batteries of an energy storage device. Background Technology

[0002] Conventional chargers used to charge the batteries of storage devices are unidirectional because they only allow the batteries of storage devices to be recharged from an external power supply network; this is often referred to as the charging direction or forward direction.

[0003] Such a unidirectional charger used to charge the battery of an energy storage device typically includes a power factor correction stage (also known as its abbreviation PFC) and a DC-DC conversion stage (more generally referred to as the DC-DC stage).

[0004] However, advantageously, chargers used to charge storage devices can also act as current sources to supply stored electricity to the external power grid, or replace the grid and operate as a voltage source for connected loads; thus, these chargers are called bidirectional chargers. The flow of current from the storage device's battery to the external network is called the discharge direction or reverse direction.

[0005] Specifically, bidirectional chargers are known, such as the bidirectional charger from document FR 3014260 A1, which describes a charger with a resonant DC-DC converter of the series LC circuit type. However, such a circuit does not allow for changing the type of power conversion because its gain is always less than 1.

[0006] It is also known that, for example, Figure 1 The bidirectional (or reversible) charger shown is from the prior art for high power density applications, which implements a full-bridge LLC resonant type DC-DC converter.

[0007] according to Figure 1 The full-bridge LLC resonant converter 10 includes a fully switched bridge 11 that generates a square wave current or signal to excite an LLC circuit 12, which consists of a series capacitor Cr and two inductors: a series inductor Lr and an inductor Lm connected in parallel with the primary winding of a transformer 13. The LLC circuit 12 then generates a resonant sinusoidal current in the transformer 13, which is rectified by a bridge rectifier 14 and then transmitted to a battery 16, also shown as a voltage source 15 in the figure.

[0008] The component formed by LLC circuit 12 is referred to as the primary circuit or primary section of the converter, while the bridge rectifier 14 is referred to as the secondary circuit or secondary section of the converter. During forward or charging operation of the charger, current flows from the primary to the secondary.

[0009] Typically, in reversible chargers known from existing technology, the regulation frequency of the DC-DC converter in charging and discharging modes is limited to approximately 60kHz to 200kHz.

[0010] Now, in discharge mode, at battery voltage V bat In the case of high power and low efficiency, adjust to the desired gain V. dc / V bat (Typically less than 0.9) will cause the regulation frequency of the DC-DC converter to diverge towards a switching frequency higher than 200kHz. This will cause a significant decrease in the efficiency of the charger in discharge mode.

[0011] Therefore, a control suitable for better regulating the DC-DC converter is needed in discharge mode when the battery voltage is high and the power is low. Summary of the Invention

[0012] To this end, a method is proposed for controlling the frequency of the input voltage of a DC-DC converter in a bidirectional charger connected to a battery, the DC-DC converter operating in battery discharge mode with a 50% duty cycle, the method comprising:

[0013] - A preliminary step defining the setpoint input voltage value for the converter's input terminals, which are defined relative to the battery charging mode.

[0014] - The step of calculating the control frequency value of the DC-DC converter based on the output battery voltage, the input power setpoint, and the setpoint input voltage, wherein the value is obtained by inverting the gain of the DC-DC converter; and

[0015] - The step of applying the control frequency thus calculated to the converter.

[0016] Therefore, a DC-DC converter in a charger that operates in either discharge or reverse mode can be controlled, which is suitable for ensuring relatively efficient regulation when the battery voltage is high and the power is low. This control frequency is advantageously obtained by inverting the gain, i.e., by solving a third-order equation that expresses the gain as a function of this control frequency. This allows for optimization of the converter's efficiency.

[0017] Advantageously and non-limitingly, the DC-DC converter is of the series LLC resonant type, comprising a fully switched bridge at the input connected to an LLC resonant circuit, which itself is connected to a transformer connected to the battery via an H-bridge. The resonant circuit includes a series inductor, a switched inductor connected only to the output of the fully switched bridge in discharge mode, and a series capacitor. The control frequency value depends on the two inductors and the series capacitor. Therefore, the control frequency is calculated by approximating the operation of the DC-DC converter, thus allowing for simplified calculations and faster processing.

[0018] Advantageously and without limitation, the steps of applying the control frequency include:

[0019] - Define the frequency increment step size;

[0020] - The step of initializing the control frequency to an initial control value corresponding to the control frequency thus calculated;

[0021] - Define a first threshold and a second threshold, the opposite of the first threshold and the opposite of the second threshold, wherein the first threshold is strictly greater than the second threshold and the thresholds are strictly positive;

[0022] - The step of calculating the error between the measured input voltage value and the setpoint input voltage; and

[0023] - The step of comparing the error value with the threshold;

[0024] The method includes an adjustment step, in which:

[0025] - When the error value is between the first threshold and the opposite value of the first threshold, and when the error is greater than the second threshold or less than the opposite value of the second threshold, the control frequency is increased or the frequency increment step is decreased;

[0026] - When the error value is between the second threshold and the opposite value of the second threshold, the control frequency is maintained at its previous value;

[0027] - When the error value is greater than the first threshold or when the error value is less than the opposite value of the first threshold, the initial control value is applied as the control frequency.

[0028] Therefore, this method includes relatively simple, fast, and robust frequency control, which compensates for the inaccuracies associated with parameter dispersion.

[0029] Advantageously, and without limitation, the method further includes feedback regulation of the control frequency. This allows for more efficient and precise regulation.

[0030] Advantageously, once the control frequency reaches a value close to 200kHz and the measured input voltage deviates from the setpoint input voltage, a grouped adjustment mode is activated. This mode involves applying the maximum frequency while periodically stopping the chopping to allow the input voltage to return to its setpoint. This allows overcoming the operational limitations of LLC DC-DC converters in reversible mode.

[0031] The present invention also relates to a bidirectional charger for charging an energy storage device, the charger comprising a power factor correction stage, at least one DC-DC converter, and a device for implementing the methods described above.

[0032] The present invention also relates to a motor vehicle comprising a bidirectional charger for charging an energy storage device as described above. Attached Figure Description

[0033] Other features and advantages will become apparent from the description given with reference to the following figures and in an indicative but non-limiting manner:

[0034] [ Figure 1 [Illustration of a charger for charging an energy storage device, as known from the prior art;]

[0035] [ Figure 2 [Illustration] is a schematic diagram of a control method for alternating between nominal regulation mode and burst mode according to an embodiment of the present invention;

[0036] [ Figure 3a [Illustration of a bidirectional charger for charging an energy storage device according to an embodiment of the present invention;]

[0037] [ Figure 3b ]yes Figure 3a A simplified schematic representation of a charger;

[0038] [ Figure 4 [This is based on] Figure 2 A flowchart of the adjustment method in an embodiment. Detailed Implementation

[0039] refer to Figure 3a According to one embodiment of the invention, the charger (not shown in its entirety) includes a single-phase or multi-phase power factor correction rectifier input stage (referred to as PFC and not shown) and a DC-DC conversion device 1, which includes a bidirectional full-bridge LLC resonant converter 20. It should be noted that the components of the charger are described with reference to a charging mode. Therefore, the input terminal of the DC-DC converter corresponds to the connection of the converter opposite to that of the battery Batt. Similarly, the secondary winding of the converter's transformer belongs to the output stage containing the battery Batt.

[0040] The full-bridge LLC resonant converter 20 includes a fully switched bridge 21 that generates a square wave voltage or signal to excite an LLC circuit 22, which consists of a capacitor and two inductors. The LLC circuit 22 then generates a resonant sinusoidal current, which is transmitted by a transformer 23 and rectified by a bridge rectifier 24. The rectified and amplified current / signal is collected by a battery 26, and a battery 25 is also shown as a voltage source 25.

[0041] The component formed by the fully switched bridge 21 and LLC circuit 22 is referred to as the primary circuit or primary part of the converter, while the component formed by the rectifier 24 is referred to as the secondary circuit or secondary part of the converter.

[0042] In reverse mode, the impedances of the power factor correction stage, network, or load connected to the charger input are combined as a load resistor R. 负载 .

[0043] In bidirectional operation of the charger, when current flows from the primary to the secondary of converter 20, this is referred to as the forward operation direction of converter 20, which allows battery 26 to be recharged from an external power network connected to the primary. The charger is further configured for reverse operation, wherein the power stored by battery 26 is transferred from the secondary to the primary of converter 20 to supply power to an external power network by operating as a current source, or by operating as a voltage source to replace the network.

[0044] The switch bridge 21 includes four switch arms, each of which is formed by parallel structures 210, 210', 210”, 210'”, because the structures include electronic components connected in parallel to each other.

[0045] Each parallel structure 210, 210', 210”, 210'” includes a diode and a transistor.

[0046] The parallel structures 210, 210', 210”, and 210'” are connected as a full bridge in a configuration known to those skilled in the art.

[0047] LLC circuit 22 and transformer 23 are as previously referenced. Figure 1 The references are consistent with those in the existing technology.

[0048] The rectifier 24 of the secondary circuit includes a full bridge formed by four switching arms.

[0049] Each switch arm is formed by a parallel structure 240, 240', 240”, 240'”, because the structure includes electronic components connected in parallel to each other.

[0050] refer to Figure 3aIn the full-bridge rectifier circuit, each parallel structure 240, 240', 240”, 240'” includes a diode 302 and a transistor 301.

[0051] A parallel branch 28, connected in parallel with LLC circuit 22, is added between the full-switching bridge 21 and the LLC circuit 22. This branch 28 is connected to the two outputs of the switching bridge 21 upstream of LLC circuit 22 (the term "upstream" here refers to the forward charging direction).

[0052] In other words, the parallel branch 28 extends to the first junction between the output of the switch bridge 21 and the capacitor Cr of the LLC circuit, while the other junction is connected between the second output of the switch bridge 21 and the second inductor Lm of the two inductors Lr and Lm of the LLC circuit.

[0053] Parallel branch 28 includes an inductor L called a switching inductor. m_开关 The inductor is connected to LLC circuit 22 in discharge mode and disconnected in charging mode.

[0054] Therefore, a DC-DC converter in discharge operation mode is equivalent to an LLC circuit.

[0055] DC-DC converter 1 includes a control device (not shown), such as a microprocessor and / or FPGA, to control the opening and closing of switch k of parallel branch 28.

[0056] The control method 4 according to the present invention aims to control the frequency of the voltage Vdc of the input capacitor.

[0057] The purpose of this method is to implement a DC bus regulation mode, referred to as "burst" mode, which is envisioned through frequency hysteresis and the DC bus. This burst mode involves applying the maximum frequency to the packets and allowing the frequency to transition between 200kHz and 0Hz in order to enable regulation of the DC bus at its limits.

[0058] This means that the chopping will be stopped periodically to allow the DC bus (in...) to... Figure 3a The voltage (Vdc) returns to the set point.

[0059] Then resume chopping to maintain it as long as possible. Once the DC bus exceeds a certain maximum threshold defined by calibration, and once the frequency saturates, the control frequency is switched to zero and chopping stops. Repeat these steps.

[0060] Therefore, refer to Figure 2 The explanation addresses the burst mode during discharge, but the same principle applies to charging operations. An operating point is given as an example, where V... bat =400V or V bat=430V, the setpoint for voltage Vdc is equal to 450V. dc req .

[0061] exist Figure 2 In the middle, once the frequency F_ 标称 When a value of 50 greater than or equal to 190kHz is reached, or, according to the alternative, any value close to 200kHz is reached, and the DC bus deviates from the setpoint 51, burst mode is activated 52, thereby returning the DC bus to its setpoint, approximately 450V.

[0062] When the power rises again, causing the frequency F_ 标称 When the DC bus is no longer saturated (53) and converges toward the setpoint value again (54), the burst mode is stopped (55) and the nominal adjustment (56) continuously applies the control frequency.

[0063] Therefore, the method according to the invention includes calculating the chopping frequency of the DC-DC converter.

[0064] refer to Figure 3b The transfer function of an LLC DC-DC converter in the discharge mode according to the present invention is known to have the following form:

[0065]

[0066] Where G is the gain of the transfer function of the DC-DC converter (or at least the inverter section of the DC-DC converter up to the primary of the transformer);

[0067] η is the turns ratio of the transformer in the DC-DC converter;

[0068] V bat It is the voltage across the two ends of the battery, or the voltage at the output of the DC-DC converter;

[0069] V dc It is the DC voltage at the input terminal of the DC-DC converter;

[0070] Furthermore, according to general terminology, in discharge operation mode: V out It is the voltage at the output of the DC-DC converter in discharge mode, and V in It is the voltage at the input terminal of the DC-DC converter in discharge mode.

[0071] refer to Figure 3b (This diagram is a simplified view of a DC-DC converter), resistor R 负载 Corresponding to PFC and in reversible (release) Various loads or networks connected to the charger in (electric) mode The impedance. Therefore, R is calculated according to the following equation. 负载 :

[0072]

[0073] Where P is the power of the primary winding of the transformer.

[0074] Therefore, in order to calculate the gain of the transfer function of a DC-DC converter, the following calculations are performed:

[0075]

[0076] Rewrite equation (3) as angular frequency (ω=2πf) sw The function is set as s = jω.

[0077] Therefore, the gain equation can be written based on the following equation:

[0078]

[0079] or

[0080]

[0081] Calculate the transfer gain G so that the control frequency f can be obtained according to the equation. sw The expression:

[0082] f sw (ω)=fct(Vbat,Preq,Vdc(setpoint))(5)

[0083] Among them, V bat It is the battery voltage, V dc It is the voltage at the input terminal of the DC-DC converter, and P req It is the power setpoint at the input of the DC-DC converter.

[0084] Specifically, by using the setpoint V dc Value substitution of V in the G(s) expression dc It can calculate the frequency quantity, and the DC bus converges to the frequency of a given voltage (e.g., 450V).

[0085] The gain G is calculated as ηV bat The ratio of / Vdc, or in this embodiment, G = ηV bat / 450V.

[0086] It should be noted that the general expression for gain G is the same for discharge, but the gain value is different because the parameters themselves are different.

[0087] This leads to the conclusion that it depends on (ω=2πf) sw The third-order equation,

[0088] ω3 +Aω 2 +Bω+C=0(6)

[0089] Where parameters A, B, and C are V bat and P REQ L m_开关 and L r ( Figure 3b The inductance value of the inductor in the equivalent circuit of the DC-DC converter, and Cr ( Figure 3b The capacitance value of the equivalent circuit of the DC-DC converter in the circuit is a function of the capacitance value. In other words, Cr and Lr correspond to... Figure 3a LLC circuit 22 has a series capacitor and a series inductor.

[0090] Solving equation (6) for ω allows the calculation of the control frequency f of the DC-DC converter via feedforward control. sw (ω).

[0091] Due to parameter dispersion, computational accuracy, and the simplifying assumptions made when writing the transfer function of the DC-DC converter, applying this equation is insufficient to eliminate the steady-state error between the measured DC voltage and the setpoint. However, this error remains small, with a maximum value of 30V.

[0092] 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 is eliminated, thus making a slightly larger adjustment to the initial frequency generated in the previous calculations to obtain better accuracy.

[0093] The controller according to the first embodiment is a discrete controller, wherein:

[0094] eps1 is the threshold at which to start increasing / decreasing the frequency.

[0095] eps2 is the threshold for fixing the control frequency.

[0096] Therefore, according to the reference Figure 4 In an example embodiment, in the first step, the setpoint voltage V is determined as described above. DC req The control frequency f is calculated based on battery voltage and power. sw (ω), also known as the switching frequency f sw (ω).

[0097] Control frequency value f sw (k) Initialize 41 to the previously calculated initial frequency value f sw_前馈 .

[0098] Then, calculate the 44 setpoint voltage V. DC req The input voltage V measured at the input terminal of the DC-DC converter dc 测量 The error value ε between them.

[0099] The error value ε is compared with two error thresholds eps1 and eps2 (which are positive real numbers), such that eps1 > eps2.

[0100] If the error ε is between the adjustable limits eps1 and -eps1 (e.g., between 200V and -200V), and additionally if 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 adjusted according to the position of the DC bus relative to the setpoint. sw_前馈 Increase or decrease 43 by a frequency increment step ΔF, or:

[0101] f sw (k)=f sw_前馈 + / -ΔF(7)

[0102] k is a time integer.

[0103] After step 43, the method loops back to step 44.

[0104] If, after step 44, the error ε is between the limit eps2 and -eps2, then the frequency f will be... sw The value of (k) is fixed and maintained at 45 at the previous value (this ensures that the DC bus is within 5V close to the set point), or:

[0105] f sw (k)=f sw (k-1)

[0106] value f sw (k-1) equals f before condition 1 was previously verified. sw_前馈 Or, when step 45 is performed after k steps 43, it equals f. sw_前馈 +k*ΔF.

[0107] Finally, if error ε is greater than eps1 or less than -eps1, then the frequency value f calculated by feedforward in step 40 is used. sw (k). This value is updated periodically. As long as the condition regarding the error is not met, the control will continue to apply the frequency calculated via feedforward, and steps 43, 45, and 46 will loop back to step 44.

[0108] This invention is not limited to the given example error thresholds eps1 and eps2. In particular, depending on the feasibility of the operating point, eps2 can be set to 1V or 0V.

[0109] This method ensures that convergence occurs at a stable frequency—this is ensured by feedforward action and is effective with the help of the controller, thereby eliminating steady-state errors and enabling the DC bus to converge precisely to the setpoint value.

[0110] The present invention is not limited to the type of controller described in the first exemplary embodiment. Scale-integral or scale-integral-derivative type controllers, which are known to those skilled in the art to be implemented, may also be provided, although tuning these controllers is more complex than that of the controller in the first embodiment of the present invention.

Claims

1. A method (4) for controlling the frequency of the input voltage of a DC-DC converter (12) connected to a battery in a bidirectional charger, the DC-DC converter operating in a battery discharge mode with a duty cycle of 50%, the method comprising: - A preliminary step defining the setpoint input voltage value for the converter's input terminals, which are defined relative to the battery charging mode. - The step of calculating the initial control value of the DC-DC converter (12) based on the output battery voltage, the input power setpoint and the setpoint input voltage, wherein the initial control value is obtained by inverting the gain of the DC-DC converter (12); - The step of calculating the error between the measured input voltage value and the setpoint input voltage; - The step of adjusting the control frequency of the DC-DC converter (12) according to the error value; as well as - The step of applying the control frequency thus determined to the converter.

2. The method (4) as described in claim 1, characterized in that, The DC-DC converter is of the series LLC resonant type, which includes a fully switched bridge at the input connected to an LLC resonant circuit, which in turn is connected to a transformer connected to the battery via an H-bridge. The resonant circuit includes a series inductor, a switched inductor connected only to the output of the fully switched bridge in discharge mode, and a series capacitor. The initial control value depends on the series inductor, the switching inductor, and the series capacitor.

3. The method (4) as described in claim 1 or 2, characterized in that, The steps for applying this frequency control include: - Define the frequency increment step size; - The step of initializing the control frequency to an initial control value corresponding to the control frequency thus calculated; - Define a first threshold and a second threshold, the opposite of the first threshold and the opposite of the second threshold, wherein the first threshold is strictly greater than the second threshold, and the thresholds are strictly positive; and - The step of comparing the error value with the corresponding threshold; The method includes an adjustment step, in which: - When the error value is between the first threshold and the opposite value of the first threshold, and when the error is greater than the second threshold or less than the opposite value of the second threshold, the control frequency is increased or the frequency increment step is decreased; - When the error value is between the second threshold and the opposite value of the second threshold, the control frequency is maintained at its previous value; - When the error value is greater than the first threshold or when the error value is less than the opposite value of the first threshold, the initial control value is applied as the control frequency.

4. The method (4) as described in claim 1 or 2, characterized in that, The method further includes feedback adjustment of the control frequency.

5. The method (4) as described in claim 1 or 2, characterized in that, Once the control frequency reaches a value close to 200 kHz and the measured input voltage deviates from the setpoint input voltage, the group adjustment mode is activated. This mode involves applying the maximum frequency while periodically stopping chopping to allow the input voltage to return to its setpoint.

6. A bidirectional charger (1) for charging an energy storage device, the charger comprising a power factor correction stage, at least one DC-DC converter (12a, 12b), and a device for implementing the method (4) as claimed in any one of claims 1 to 5.

7. A motor vehicle comprising a bidirectional charger (1) for charging an energy storage device as described in claim 6.

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