DC-DC converter with two cascaded downconverters

Abstract

A method for operating a bidirectional DC-DC converter is provided. The bidirectional DC-DC converter can transfer energy both from a high-voltage network to a low-voltage battery and from the low-voltage battery to the high-voltage network. The bidirectional DC-DC converter comprises a first power stage with an LLC converter circuit and a second power stage with two cascaded buck converters. The two cascaded buck converters are controlled by a single PWM with the same duty cycle. The method includes operating the DC-DC converter in a buck mode, switching from buck mode to boost mode, and operating the DC-DC converter in a boost mode.The operation of the DC-DC converter in step-down mode and step-up mode includes the operation of the LLC converter circuit in open-loop operation at a resonant switching frequency and the operation of the two cascaded step-down converters using the same duty cycle.

Description

REFERENCE TO RELATED REGISTRATIONS

[0001] This application claims priority over preliminary US application No. 63 / 539,150, filed on September 19, 2023. BACKGROUND

[0002] DC-DC converters (direct current-to-direct current converters) are used in automotive applications to power systems with varying voltage levels throughout the vehicle. A common automotive application is to use direct current from a high-voltage battery to provide lower DC voltages that power components such as headlights, interior lights, power windows, and so on.

[0003] For example, a DC-DC converter is used in electric or hybrid vehicles, where a high-voltage (HV) network with capacitors and a battery of several hundred volts (e.g., 400 V or 800 V) supplies power to the electric motor, and a low-voltage (LV) network with a battery (e.g., 14 V, 28 V, or 48 V) supplies power to the vehicle's control and comfort equipment. For safety reasons, such a DC-DC converter is typically connected with galvanic isolation between the two HV and LV batteries and serves to convert and transfer energy from the HV battery to the LV battery while the vehicle is running.

[0004] There is a need for a DC-DC converter circuit that can deliver a high power level to the LV network and operate bidirectionally as efficiently as possible. BRIEF SUMMARY

[0005] A bidirectional DC-DC converter and a method for operating a bidirectional DC-DC converter are presented. Through certain embodiments of the described circuits and methods, the proposed circuit can operate with a high input voltage between 190 V DC and 900 V DC and deliver a high power level to the low-voltage battery with high power density and efficiency.

[0006] A bidirectional DC-DC converter comprises a first power stage with an LLC converter circuit and a second power stage with two cascaded buck converters. The two cascaded buck converters are controlled by a single PWM (pulse width modulation) with the same duty cycle.

[0007] A method for operating a bidirectional DC-DC converter is provided. The DC-DC converter comprises an LLC converter circuit and two cascaded buck converters. The method includes the steps of operating the DC-DC converter in a buck mode and switching from buck mode to boost mode. Operation of the DC-DC converter in buck and boost modes includes operating the LLC converter circuit in open-loop mode at a resonant switching frequency and operating the two cascaded buck converters using the same duty cycle.

[0008] This summary serves to present a selection of concepts in simplified form, which are explained in more detail below. This summary is not intended to identify main features or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter. BRIEF DESCRIPTION OF THE DIFFERENT VIEWS OF THE DRAWINGS

[0009] To easily identify the discussion of a particular element or process, the highest-value digit(s) of a reference number refer to the figure number in which that element is first introduced. Fig. Figure 1 shows a schematic diagram of an exemplary implementation of a bidirectional DC-DC converter. Fig. Figure 2 shows a schematic diagram of an exemplary implementation of a downward converter. Fig. Figure 3 shows a method for operating a bidirectional DC-DC converter according to one embodiment. Fig. Figure 4 shows a timing diagram for the two cascaded step-down converters according to one embodiment. DETAILED DESCRIPTION

[0010] A bidirectional DC-DC converter and a method for operating a bidirectional DC-DC converter are presented. Through certain embodiments of the described circuit and method, it is possible for the proposed circuit, which has a high input voltage between 190 V DC and 900 V DC, to deliver a high power level to the low-voltage battery with high power density and efficiency.

[0011] LLC converters are resonant converters that use a primary inductor and a capacitor coupled to a power transformer to form a resonant network for power conversion. Due to its higher efficiency and improved EMI (electromagnetic interference) performance when operating at the resonant switching frequency in open-loop operation, an LLC converter is one of the most widely used DC-DC converters in electric vehicle applications. The LLC converter circuit operates such that the output voltage supplied by the LLC converter circuit is proportional to the input high voltage, with the ratio corresponding to the transformer ratio.

[0012] Buck converters are used to step down an input voltage to achieve a required output voltage. A single buck stage following an LLC converter circuit is not an efficient hardware architecture. Therefore, a two-stage buck circuit (e.g., two cascaded buck stages) is proposed, which operates more efficiently. This disclosure describes a cascaded buck power stage driven by a single PWM, instead of using separate PWMs for each buck stage. A cascaded buck power stage driven by a single PWM offers better duty cycle variation performance and results in lower voltage and current loads on the power switches and power inductor.

[0013] The presented DC-DC converter comprises two power stages, a first power stage as an LLC converter circuit and a second power stage as a step-down converter.

[0014] Fig. Figure 1 shows a schematic diagram of an exemplary implementation of a bidirectional DC-DC converter. The DC-DC converter 100 can be implemented with a first power stage and a second power stage. Referring to Fig. The DC-DC converter 100 comprises an LLC converter circuit 104, e.g., the first power stage, and a buck converter 106, e.g., the second power stage. A controller 102 can be integrated into the DC-DC converter 100 or be a separate component. The controller 102 can be implemented using one or more processors (executing appropriate software instructions), state machines, and / or logic circuits.

[0015] A high-voltage battery 114 and a high-voltage capacitor 116 are connected to the LLC converter circuit 104. The high-voltage battery 114 supplies an input voltage to the high-voltage capacitor 116 and to a first switching network 108 of the LLC converter circuit 104. The high-voltage capacitor 116 can be charged by control of the controller 102, which actuates the switches in the first switching network 108. By operating the controller 102, the high-voltage capacitor 116 can be charged according to method 300.

[0016] The LLC converter circuit 104 comprises a first switching network 108, consisting of switches individually designated Q1, Q2, Q3, and Q4, a transformer, and a second switching network 118, consisting of switches individually designated Q5, Q6, Q7, and Q8. The switches of the first switching network 108 and the second switching network 118 are semiconductor switches. The LLC converter circuit 104 is a bidirectional full-bridge LLC, with the LLC components together forming the resonant network 124. The first switching network 108 is connected to the resonant network 124 on its primary side. The resonant network 124 is connected to the second switching network 118 on its secondary side. The second switching network 118 can be considered a rectifier. The LLC converter circuit 104 provides electrical isolation and a fixed voltage transfer ratio with high efficiency.

[0017] A buck converter circuit 106 is coupled to the LLC converter circuit 104. The buck converter circuit 106 regulates the voltage on the low-voltage side, e.g., the voltage at a low-voltage battery 122. The switches of the buck converter 106 can be actuated by means of a gate signal from the controller 102. The output capacitor 126 is connected in parallel to the low-voltage battery 122. The buck converter 106 is in Fig. 2 shown in more detail.

[0018] Fig. Figure 2 shows a schematic diagram of an exemplary implementation of a step-down converter. Referring to Fig. The step-down converter 106 comprises two cascaded step-down converter circuits. In the Fig. In the embodiment shown, the buck converter 106 comprises two cascaded buck converter circuits, a first buck converter 202 and a second buck converter 204. The buck converter 106 is controlled by a secondary control circuit 216, which, as shown, contains logic operators. The input "gate" signal is controlled by the control circuit 102. In the embodiment shown, both the first buck converter 202 and the second buck converter 204 each comprise two switches, namely a first upper switch 206 and a first lower switch 208, and a second upper switch 210 and a second lower switch 212, as well as an inductor (with an associated DC resistance, as shown).

[0019] The switches, the first upper switch 206, the first lower switch 208, the second upper switch 210, and the second lower switch 212, are controlled by the secondary control circuit 216. The secondary control circuit 216 controls the switches via a single PWM (pulse-width modulation) gate that drives the first upper switch 206 and the second upper switch 210. From this PWM gate, a complementary signal, "Gate C," is derived from the "logical operator NOT and the turn-on delay 3," as described in the logic of the secondary control circuit 216. Fig. Figure 2 shows that the signal “Gate C” controls the first lower switch 208 and the second lower switch 212. Thus, with a single PWM signal, the first upper switch 206, the second upper switch 210, the first lower switch 208, and the second lower switch 212 can be controlled with the same duty cycle. Fig. Figure 4 shows a timing diagram for Gate and Gate C during the same duty cycle.

[0020] Fig. Figure 3 illustrates a method for operating a bidirectional DC-DC converter according to one embodiment. Method 300 can be used with the DC-DC converter 100. Fig. Method 300 comprises operating (302) the DC-DC converter 100 in a step-down mode. Method 300 further comprises switching (304) from step-down mode to step-up mode. Method 300 further comprises operating (306) the DC-DC converter 100 in step-up mode. Method 300 comprises operating the LLC converter circuit in open-loop mode at a resonant switching frequency in both step-down and step-up modes. Method 300 further comprises operating the cascaded step-down converters using the same duty cycle in both step-down and step-up modes. The controller 102 coupled to the DC-DC converter 100 can perform the switching and operating steps of Method 300.

[0021] The DC-DC converter 100 described here is bidirectional and can therefore be operated in two modes: a step-down or forward mode and a step-up or reverse mode. In step-down mode, energy is transferred from the high-voltage side, e.g., the high-voltage battery, to the low-voltage side, e.g., the low-voltage battery. In step-up mode, the low-voltage battery 122 serves as the source for pre-charging the high-voltage capacitor 116.

[0022] In step-down mode, the DC-DC converter 100 can be operated to supply 14 V, 28 V, or 48 V to the low-voltage battery 122 from an input voltage range (e.g., from the high-voltage battery 114) of 190 V DC to 900 V DC via the high-voltage capacitor 116. The DC-DC converter 100 can also be operated in step-up mode, where step-up mode describes a mode in which energy is transferred from the low-voltage battery 122 to the high-voltage side.

[0023] The controller 102 operates the LLC converter circuit 104 in both step-down and step-up modes in open-loop operation at the resonant switching frequency to achieve high efficiency. To operate the LLC converter circuit 104 in open-loop operation at the resonant switching frequency, the controller 102 controls switches Q1, Q2, Q3, Q4, Q5, Q6, Q7, and Q8 so that each one is in an open or closed position. In open-loop operation, the output voltage of the LLC converter circuit 104, e.g., across the high-voltage capacitor 116, varies linearly with the input voltage from the high-voltage battery 114. For example, if the input voltage changes from 190 V to 900 V, the output voltage of the LLC converter circuit, e.g., the voltage across the high-voltage capacitor 116, varies from 17.3 V to 78 V at a constant gain of 0.0917.

[0024] The controller 102 controls the buck converter 106, for example, the cascaded first buck converter 202 and second buck converter 204, using the same duty cycle in both buck and boost modes. The controller 102 generates a single PWM signal for the switches of the first buck converter 202 and the second buck converter 204, for example, for the first upper switch 206, the first lower switch 208, the second upper switch 210, and the second lower switch 212. The duty cycle can be defined as the time within a given period that each switch is on. With an input voltage of the high-voltage battery 114 in the range of 190 V DC to 900 V DC, the duty cycle varies in a range of 0.4 to 0.9, which represents an optimal duty cycle range for a buck converter.

[0025] In step-down mode, the transfer function of the cascaded first step-down converter 202 and second step-down converter 204 is Vout / Vin = duty cycle * duty cycle, where Vout is the voltage of the low-voltage battery 122 and Vin is the voltage at the input of the two cascaded step-down converters 106, e.g., via the capacitor 214. The advantage of this control is that the change in the duty cycle is not linear, but corresponds to the square root of the ratio Vout / Vin. In this case, the minimum duty cycle is 0.4 and the maximum duty cycle is 0.9. Optimal performance of the step-down converter 106 is achieved when the duty cycle varies between 0.4 and 0.9. The two cascaded circuits, the first step-down circuit 202 and the second step-down circuit 204, also ensure an optimized voltage and current load on the switches and the magnetic components in the DC-DC converter 100.

[0026] In pre-charge mode, the cascaded first step-down circuit 202 and second step-down circuit 204 become two step-up circuits cascaded, controlled by the same duty cycle. In step-up mode, the ratio Vout / Vin = 1 / (1-duty cycle)*(1-duty cycle), where Vout is the voltage across capacitor 214 and Vin is the voltage of the low-voltage battery 122. The duty cycle varies between 0.1 and 0.6. The current and voltage load on the switches and the power inductor are optimized in step-up mode when the duty cycle is between 0 and 0.6.

[0027] Fig. Figure 4 shows a diagram for the two cascaded step-down converters according to one embodiment. The timing diagram corresponds to that in the secondary control circuit 216 of Fig. 2. Logic shown. With reference to Fig. Figure 4 shows a 400-second timing diagram for Gate and Gate C during the same duty cycle. The duty cycle is shown in Fig. The period is represented by 4. The gate signal shows a pulse pw during the duty cycle. The NOT operator is coupled to receive the gate signal and output an inverted gate signal. Subsequently, a turn-on delay operator is coupled to receive the inverted first control signal to adjust the pulse width and output a delayed Delay-3 (PWM) signal.

[0028] Although the subject matter has been described in language relating to structural features and / or actions, it is understood that the subject matter defined in the attached claims is not necessarily limited to the specific features or actions described above. Rather, the specific features and actions described above are disclosed as examples of the implementation of the claims, and other equivalent features and actions are intended to fall within the scope of the claims. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] US 63 / 539,150

[0001]

Claims

[1] Bidirectional DC-DC converter, comprising: a first power stage that includes an LLC converter circuit; and a second power stage comprising two cascaded step-down converters, with the two cascaded step-down converters being controlled by a single PWM with the same duty cycle. [2] Bidirectional DC-DC converters according to claim 1, wherein the LLC circuit operates in open-loop control at a resonant switching frequency. [3] Bidirectional DC-DC converters according to claim 1, wherein the first power stage is configured to receive an input voltage in a range of 190 V DC to 900 V DC. [4] Bidirectional DC-DC converter according to claim 1, wherein each of the two cascaded step-down converters comprises a first step-down circuit and a second step-down circuit, two switches, a first upper switch and a first lower switch or a second upper switch and a second lower switch, a shunt resistor and an inductor. [5] Bidirectional DC-DC converter according to claim 4, further comprising: a NOT operator coupled in such a way that it receives a first control signal and outputs an inverted first control signal; and a turn-on delay operator coupled such that it receives the inverted first control signal to adjust the pulse width and outputs a second control signal, with the first control signal and the second control signal being active during the signal duty cycle, wherein the first control signal actuates the first upper switch and the second upper switch and the second control signal actuates the first lower switch and the second lower switch. [6] Method for operating a bidirectional DC-DC converter, wherein the DC-DC converter comprises an LLC converter circuit coupled to two cascaded step-down converters, the method comprising: Operating the DC-DC converter in a step-down mode; Switching from downward mode to upward mode; and Operating the DC-DC converter in a boost mode, the operation of the DC-DC converter includes both step-down and step-up modes: Operating the LLC converter circuit in an open-loop system at a resonant switching frequency and Operating the two cascaded step-down converters using the same duty cycle. [7] Method according to claim 6, wherein the operation and switching are carried out by a control coupled to the DC-DC converter. [8] Method according to claim 6, wherein each of the two cascaded step-down converters comprises a first step-down circuit and a second step-down circuit, two switches, a first upper switch and a first lower switch, and a second upper switch and a second lower switch, a shunt resistor and an inductor. [9] Method according to claim 8, wherein the operation of the two cascaded step-down converters using the same duty cycle comprises operating the first upper switch and the second upper switch with a first gate signal and operating the first lower switch and the second lower switch with a second gate signal derived from the first gate signal using logical operators. [10] Method according to claim 9, wherein the logical operators comprise a NOT operator followed by a turn-on delay operator. [11] Method according to claim 6, wherein an input voltage to the LLC converter circuit in step-down mode is in a range of 190 V DC to 900 V DC. [12] Method according to claim 6, wherein the duty cycle in down mode is in a range of 0.4 to 0.

9. [13] Method according to claim 6, wherein the duty cycle in up mode is in a range of 0.1 to 0.6.

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