DC-to-DC converter with two cascaded buck converter

Abstract

A method for operating a bi-directional DC-to-DC converter is provided. The bi-directional DC-to-DC converter can transfer energy from a high voltage network to a low voltage battery as well as from the low voltage battery to the high voltage network. The bi-directional DC-to-DC converter includes a first power stage including an LLC converter circuit, and a second power stage including two cascaded buck converters. The two cascaded buck converters are controlled by a single PWM in a same duty cycle. The method includes operating the DC-to-DC converter in a buck mode, switching from the buck mode to a boost mode, and operating the DC-to-DC converter in a boost mode. Operating the DC-to-DC converter in the buck mode and in the boost mode includes operating the LLC converter circuit in open loop at a resonant switching frequency and operating the two cascaded buck converters utilizing a same duty cycle.

Description

DC-TO-DC CONVERTER WITH TWO CASCADED BUCK CONVERTERCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of US Provisional Application No. 63 / 539,150 filed September 19, 2023.BACKGROUND

[0002] DC-to-DC converters are used in automotive applications to supply systems of different voltage levels throughout a vehicle. A common automotive application is to enable DC power from a high voltage battery to be used to supply lower DC voltages that power components such as headlights, interior lights, motorized windows, etc.

[0003] For example, a DC-to-DC converter is used in electric or hybrid vehicles, where a high voltage (HV) network having capacitors and a battery of several hundred volts (e.g., 400V or 800V) is used to provide energy to the electric motor, and a low voltage (LV) network having a battery (e.g., 14 V, 28 or 48V) is used to supply the control and comfort equipment of the vehicle. Such a DC-to-DC converter is typically inserted between the two HV and LV batteries with galvanic isolation for safety reasons and is used to transform and transfer the energy from the HV battery to the LV battery when the vehicle is running.

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

[0005] A bi-directional DC-to-DC converter and a method of operating a bi-directional DC- to-DC converter is presented. Through certain implementations of the described circuitry and methods, it is possible for the proposed circuit having a high input voltage of between 190V DC to 900V DC to deliver a high level of power to the low voltage battery with a high level of power density and efficiency.

[0006] A bi-directional DC-to-DC converter includes a first power stage including an LLC converter circuit and a second power stage including two cascaded buck converters. The two cascaded buck converters are controlled by a single PWM (pulse width modulation) in a same duty cycle.

[0007] A method for operating a bi-directional DC-to-DC converter is provided. The DC- to-DC converter includes an LLC converter circuit and two cascaded buck converters. The method includes the steps of operating the DC-to-DC converter in a buck mode and switchingfrom the buck mode to a boost mode. Operating the DC-to-DC converter in the buck mode and in the boost mode includes operating the LLC converter circuit in open loop at a resonant switching frequency and operating the two cascaded buck converters utilizing a same duty cycle.

[0008] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0009] To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the figure number in which that element is first introduced.

[0010] FIG. 1 illustrates a schematic diagram of an example implementation of a bidirectional DC-to-DC converter.

[0011] FIG. 2 illustrates a schematic diagram of an example implementation of a buck converter.

[0012] FIG. 3 illustrates a method operating a bi-directional DC-to-DC converter in accordance with one embodiment.

[0013] FIG. 4 illustrates a timing diagram for the two cascaded buck converter in accordance with one embodiment.DETAILED DESCRIPTION

[0014] A bi-directional DC-to-DC converter and a method of operating a bi-directional DC- to-DC converter is presented. Through certain implementations of the described circuitry and methods, it is possible for the proposed circuit having a high input voltage of between 190V DC to 900V DC to deliver a high level of power to the low voltage battery with a high level of power density and efficiency.

[0015] LLC converters are resonant converters that use a primary inductor and a capacitor coupled to a power transformer to make a resonant network for power conversion. An LLC converter is one of the most often used DC-to-DC converters in electric vehicle applications due to its higher efficiency and better EMI (electromagnetic interference) when they work at the resonant switching frequency in open loop. An LLC converter circuit is utilized thatoperates so that the output voltage delivered by the LLC converter circuit is proportional to the input high voltage with a ratio equal to the transformer ratio.

[0016] Buck converters are used to step down voltage from its input to achieve a required output. A single buck stage following an LLC converter circuit is not an efficient hardware architecture. Thus, a two stage (e.g., two buck stages cascaded) buck circuit that operates more efficiently is proposed. In this disclosure, a two cascaded buck power stage driven by a single PWM instead of having a separated PWM for both buck stages is described. A cascaded buck power stage driven by a single PWM offers better performance in terms of duty cycle variation and has lower voltage and current stress on the power switches and power inductor.

[0017] The presented DC-to-DC converter includes two power stages, a first power stage as an LLC converter circuit and a second power stage as a buck converter.

[0018] FIG. 1 illustrates a schematic diagram of an example implementation of a bidirectional DC-to-DC converter. The DC-to-DC converter 100 can be implemented with a first power stage and a second power stage. Referring to FIG. 1, DC-to-DC converter 100 includes an LLC converter circuit 104, e.g., the first power stage, a buck converter 106, e.g., the second power stage. A controller 102 can be integrated with the DC-to-DC converter 100 or be a separate component from the DC-to-DC converter 100. The controller 102 can be implemented using one or more processors (executing suitable software instructions), state machines, and / or logic circuits.

[0019] A high voltage battery 114 and a high voltage capacitor 116 are coupled to the LLC converter circuit 104. The high voltage battery 114 provides an input voltage to high voltage capacitor 116 and a first switching network 108 of the LLC converter circuit 104. The high voltage capacitor 116 can be charged under the control of controller 102 which operates switches in the first switching network 108. Through operations of the controller 102, the high voltage capacitor 116 can be charged according to the method 300.

[0020] The LLC converter circuit 104 includes the first switching network 108 comprising switches individually labeled QI, Q2, Q3, and Q4, a transformer, and a second switching network 118 comprising switches individually labeled 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 bi-directional full bridge LLC, the LLC components together form the resonant network 124. The first switching network 108 is connected to the resonant network 124 on its primary side. Resonant network 124 is connected on its secondary side to a second switching network 118. The second switching network 118can be considered a rectifier. The LLC converter circuit 104 provides electrical isolation and fixed voltage transfer ratio with high efficiency.

[0021] A buck converter 106 circuit is coupled to the LLC converter circuit 104. The buck converter 106 circuit regulates the voltage on the LV side, e.g., the voltage across a low voltage battery 122. Switches of the buck converter 106 can be operated using a gate signal of the controller 102. Output capacitor 126 is positioned in parallel to the low voltage battery 122. Buck converter 106 is shown in more detail in FIG. 2.

[0022] FIG. 2 illustrates a schematic diagram of an example implementation of a buck converter. Referring to FIG. 2, buck converter 106 includes two cascaded buck converter circuits. In the implementation shown in FIG. 2, buck converter 106 includes two cascaded buck converter circuits, a first buck circuit 202 and a second buck circuit 204. Buck converter 106 is controlled by a secondary control circuit 216 including logical operators as shown. The input ‘Gate’ signal is controlled by controller 102. In the shown embodiment, each of the first buck circuit 202 and second buck circuit 204 includes two switches, a first topside switch 206 and a first bottomside switch 208, and a second topside 210 switch and a second bottomside switch 212, respectively, and an inductor (having an associated DC resistance as shown).

[0023] The switches, first topside switch 206, first bottomside switch 208, second topside switch 210, and second bottomside switch 212, are controlled by secondary control circuit 216. Secondary control circuit 216 drives the switches by a single PWM (pulse width modulation) ‘Gate’ that drives the first topside switch 206 and the second topside switch 210. From this PWM ‘gate’ a complementary signal ‘Gate C’ is derived from ‘logical operator NOT and turn-on delay 3’ as shown in the logic of the secondary control circuit 216 in FIG. 2. The signal 'Gate C’ will drive the first bottomside switch 208 and the second bottomside switch 212. Thus, with a single PWM, the first topside switch 206, second topside switch 210, first bottomside switch 208 and second bottomside switch 212 can be driven with the same duty cycle. FIG. 4 illustrates a timing diagram for Gate and Gate C during the same duty cycle.

[0024] FIG. 3 illustrates a method for operating a bi-directional DC-to-DC converter in accordance with one embodiment. The method 300 can be performed when utilizing DC-to- DC converter 100 of FIG. 1. Method 300 includes operating (302) the DC-to-DC converter 100 in a buck mode. Method 300 further includes switching (304) from the buck mode to the boost mode. Method 300 further includes operating (306) the DC-to-DC converter 100 in the boost mode. The method 300 includes operating the LLC converter circuit in open loop at a resonant switching frequency in both the buck mode and in the boost mode. Method 300further includes operating the cascaded buck converters utilizing the same duty cycle in both the buck mode and in the boost mode. Controller 102 coupled to the DC-to-DC converter 100 can perform the switching and the operating steps of method 300.

[0025] DC-to-DC converter 100 as described herein is bidirectional and thus can operate in two modes: a buck, or forward, mode, and a boost, or reverse, mode. In buck mode, the energy is transferred from the high voltage side, e.g., high voltage battery, to the low voltage side, e.g., the low voltage battery. In boost mode, the low voltage battery 122 becomes a source to pre-charge the high voltage capacitor 116.

[0026] Operating the DC-to-DC converter 100 in buck mode, the DC-to-DC converter 100 can operate to deliver 14V, 28V, or 48V across the low voltage battery 122 from an input voltage range (e.g., from the high voltage battery 114) of 190V DC to 900V DC across the high voltage capacitor 116. The DC-to-DC converter 100 can also operate in boost mode, the boost mode describing a mode where the energy is transferred from the low voltage battery 122 to the high voltage side.

[0027] Controller 102 operates the LLC converter circuit 104 in open loop at the resonant switching frequency in both buck mode and boost mode for high efficiency. In order to operate the LLC converter circuit 104 at open loop at the resonant switching frequency, the controller 102 operates the switches, QI, Q2, Q3, Q4, Q5, Q6, Q7, and Q8 so that each one is in an open or closed position. In open loop, the output voltage of the LLC converter circuit 104, e.g., across high voltage capacitor 116, varies linearly with the input voltage from the high voltage battery 114. For example, when the input voltage varies from 190V to 900V, the output voltage of the LLC converter circuit, e.g., voltage across high voltage capacitor 116, varies from 17.3 V to 78 V with a constant gain equal to 0.0917.

[0028] Controller 102 operates the buck converter 106, e.g., cascaded first buck circuit 202 and second buck circuit 204 utilizing the same duty cycle in both the buck mode and the boost mode. A single PWM is generated by the controller 102 for the switches of the first buck circuit 202 and the second buck circuit 204, e.g., the first topside switch 206, the first bottomside switch 208, the second topside switch 210, and the second bottomside switch 212. The duty cycle can be defined as the amount of time in a given period that each switch is turned on. With the input voltage from high voltage battery 114 in the range of 190V DC to 900 V DC, the duty cycle will vary in a range of 0.4 to 0.9 which is an optimal duty cycle range for a buck converter.

[0029] In buck mode, the transfer function of the cascaded first buck circuit 202 and second buck circuit 204 is Vout / Vin = duty cycle * duty cycle, where Vout is the voltage of the lowvoltage battery 122 and Vin is the voltage at the input of the two cascaded buck converter 106, e g., across capacitor 214. The advantage of this regulation is that the variation of duty cycle is not linear but according to the square root of the ratio Vout / Vin. For this case, the minimum duty cycle is 0.4 and the maximum duty cycle is 0.9. An optimal performance for the buck converter 106 results when the duty cycle varies from 0.4 to 0.9. The two cascaded first buck circuit 202 and the second buck circuit 204 also gives an optimized voltage and current stress on the switches and the magnetic components in the DC-to-DC converter 100.

[0030] In precharge mode, the cascaded first buck circuit 202 and the second buck circuit 204 becomes two boost mode cascaded circuits controlled by the same duty cycle. In boost mode, the ratio Vout / Vin = 1 / (1-duty cycle)*(l-duty cycle), wherein Vout is the voltage across capacitor 214 and Vin is the voltage of the low voltage battery 122. The variation of the duty cycle is in a range of 0.1 to 0.6. The current and voltage stress across the switches and the power inductor are optimized in boost mode when the duty cycle is between 0 to 0.6.

[0031] FIG. 4 illustrates a timing diagram for the two cascaded buck converter in accordance with one embodiment. The timing diagram corresponds to the logic as shown in the secondary control circuit 216 of FIG. 2. Referring to FIG. 4, a timing diagram 400 for Gate and Gate C during the same duty cycle is illustrated. The duty cycle is shown in FIG. 4 by the period. Gate signal shows a pulse, pw, during the duty cycle. The Not operator is coupled to receive Gate and outputs an inverted Gate signal. Then, a turn-on delay operator is coupled to receive the inverted first control signal to adjust pulse width and output a delayed Delay 3 (PWM) signal.

[0032] Although the subject matter has been described in language specific to structural features and / or acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as examples of implementing the claims and other equivalent features and acts are intended to be within the scope of the claims.

Claims

CLAIMSWhat is claimed is:

1. A bidirectional DC-to-DC converter, comprising: a first power stage including an LLC converter circuit; and a second power stage including two cascaded buck converters, wherein the two cascaded buck converters are controlled by a single PWM in a same duty cycle.

2. The bidirectional DC-to-DC converter of claim 1, wherein the LLC circuit operates in open loop at a resonant switching frequency.

3. The bidirectional DC-to-DC converter of claim 1, wherein the first power stage is configured to receive an input voltage in a range of 190V DC to 900V DC.

4. The bidirectional DC-to-DC converter of claim 1, wherein each of the two cascaded buck converters, a first buck circuit and a second buck circuit, includes two switches, a first topside switch and a first bottomside switch and a second topside switch and a second bottomside switch, respectively, a shunt resistor and an inductor.

5. The bidirectional DC-to-DC converter of claim 4, further comprising: a not operator coupled to receive a first control signal and output an inverted first control signal; and a turn-on delay operator coupled to receive the inverted first control signal to adjust pulse width and output a second control signal, whereby the first control signal and the second control signal are active during the signal duty cycle, wherein the first control signal operates the first topside switch and the second topside switch and wherein the second control signal operates the first bottomside switch and the second bottomside switch.

6. A method of operating a bi-directional DC-to-DC converter, the DC-to-DC converter including an LLC converter circuit coupled to two cascaded buck converters, the method comprising:operating the DC-to-DC converter in a buck mode; switching from the buck mode to a boost mode; and operating the DC-to-DC converter in a boost mode, wherein operating the DC-to-DC converter in the buck mode and in the boost mode comprises: operating the LLC converter circuit in open loop at a resonant switching frequency, and operating the two cascaded buck converters utilizing a same duty cycle.

7. The method of claim 6, wherein the operating and switching is performed by a controller coupled to the DC-to-DC converter.

8. The method of claim 6, wherein each of the two cascaded buck converters, a first buck circuit and a second buck circuit, includes two switches, a first topside switch and a first bottomside switch, and a second topside switch and a second bottomside switch, respectively, a shunt resistor and an inductor.

9. The method of claim 8, wherein operating the two cascaded buck converters utilizing a same duty cycle includes operating the first topside switch and the second topside switch with a first gate signal and operating the first bottomside switch and the second bottomside switch with a second gate signal derived from the first gate signal using logical operators.

10. The method of claim 9, wherein the logical operators comprise a not operator followed by a turn-on delay operator.

11. The method of claim 6, wherein an input voltage to the LLC converter circuit in buck mode is in a range of 190 V DC to 900 V DC.

12. The method of claim 6, wherein the duty cycle is in a range of 0.4 to 0.9 in buck mode.

13. The method of claim 6, wherein the duty cycle is in a range of 0.1 to 0.6 in boost mode.

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