The numerous aspects, embodiments, objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:
The soft start techniques disclosed herein can be extensively employed in various industries, for example, industrial automation, automotive, etc., to reduce input inrushing current of direct current (DC) to DC (DC-DC) converters at startup. Typically, the systems and methods disclosed herein prevent large current surges, which can damage circuits, such as metal-oxide-semiconductor field-effect transistor (MOSFET) switches that depend on stable supply voltages. To avoid the damaging current surges, soft start circuits disclosed herein delay a complete startup of the converter by linearly increasing the duty cycle of a Pulse Width Modulator (PWM) until the output of the converter reaches a desired operational level (e.g., steady state value). Moreover, for a synchronous structure (e.g., bidirectional step-up converter, bidirectional step-down converter, two stage isolated bidirectional DC-DC converter, etc.) that employs a MOSFET instead of freewheeling diode, the systems and methods disclosed herein reduce/prevent a large negative current, which can damage the system because the energy can flow in both directions.
In one aspects, the systems and methods disclosed herein provide an improved soft start technique for a passive switch (e.g., MOSFET), utilized in any bidirectional DC-DC converter topology, that prevents a high negative transient inductor current during start-up/reset and thus avoids damaging system components. The subject matter is described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the subject innovation. It may be evident, however, that the subject matter may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the subject innovation.
Moreover, the word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the word “exemplary” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. In addition, the word “coupled” is used herein to mean direct or indirect electrical or mechanical coupling.
Initially, referring to
In bidirectional and/or synchronous structures with MOSFET switches 114 energy can flow in both directions causing huge negative current at power up and damage the system. To prevent these negative surges at power up, system 100 includes a soft start circuit 102 coupled to an input stage 110 of a bidirectional DC-DC converter 104. Typically, the bidirectional DC-DC converter 104 can include most any bidirectional topology, including, but not limited to, non-isolated and/or isolated topologies. In one example, the non-isolated topologies can comprise, but are not limited to, buck, boost, buck-boost, Ćuk, and/or charge pump converters, which are used for either step up or voltage inversion. In another example, the isolated topologies can comprise two-stage isolated bidirectional DC-DC converter, such as, but not limited to, fly-back, fly-forward, half bridge, full bridge and/or dual full bridge topologies.
Typically, the input stage 110 of the bidirectional DC-DC converter 104 can include two synchronous switches 114, namely, an active switch and a passive switch, (shown in detail with respect to
It can be appreciated that although the PWM signal generator 106 and the soft start circuit 102 are depicted to reside within a single integrated circuit (IC) chip, namely, controller IC 108, the PWM signal generator 106 and the soft start circuit 102 can reside on multiple ICs. Further, it can be appreciated that the mechanical design of system 100 can include different component selections, component placement, dimensions, topologies, etc., to achieve a control signal that gradually increases the duty cycle of the passive switch from zero to steady state. Furthermore, it can be appreciated that the soft start circuit 102, the bidirectional DC-DC converter 104 and the PWM signal generator 106 can include most any electrical circuit(s) that can include components and circuitry elements of any suitable value in order to implement the embodiments of the subject innovation. Furthermore, it can be appreciated that the system 100 can be implemented on one or more integrated circuit (IC) chips.
Referring now to
The exemplary converter 200 is employed in a variety of configurations in which a soft start method would be advantageous. In one exemplary configuration, depicted in
In one embodiment, Q1 (212) and Q2 (208) are complimentary switches, wherein Q1 (212) is defined as an active switch and Q2 (208) is defined as a passive switch. Moreover, when Q1 (212) is turned “ON,” Q2 (208) switches “OFF,” and when Q1 (212) is switched “OFF,” Q2 (208) is turned “ON.” Saturation and damage to the circuit can occur when the duty cycle of Q1 (212) is gradually increased from zero to steady state, for example at start up, for example, by employing a PWM signal generated by a PWM signal generator 106 to control the duty cycle of Q1 (212). Because the duty cycle of Q2 (208) is complimentary (e.g., an inverted version) of the duty cycle of Q1 (212), the duty cycle of Q2 (208) will be near 100% at the beginning of the soft start. As a result, voltage VH-VL is applied to the inductor Lf (214) with a low duty cycle (e.g., short time in an “ON state” per cycle) while voltage VL is applied to the inductor Lf (214) with a high duty cycle (e.g., long time in the “ON state” per cycle). Eventually, the negative inductor current ILF increases in a rapid manner, and the inductor Lf (214) becomes saturated, subjecting the converter to damage by a large uncontrolled reverse current.
In one aspect, the soft start circuit 102 employed by system 200, can prevent saturation of the inductor, by controlling the duty cycle of Q2 (208). As an example, the soft start circuit 102 drives the passive switch and gradually increases the duty cycle of the passive switch Q2 (208), from zero to a steady state value. Moreover, the soft start circuit 102 generates an output signal that initially switches Q2 (208) “ON” for only a fraction of time when Q1 (212) is “OFF”, and gradually increases the time for which Q2 (208) is kept “ON”, until Q2 (208) is kept “ON” for all or substantially all the time that Q1 is “OFF”. Since the duty cycle of Q2 (208) gradually increases in the same way as the duty cycle of Q1 (212), the inductor current ILF changes smoothly, and a huge reverse inductor current is avoided.
It can be appreciated that the capacitors CH 206 and CL 204 can have suitable capacitance values (or ratios) depending on the application. Further, inductor LF 214 can have most any inductance value depending on the application. In one example, although switches Q1 (212) and Q2 (208) are depicted as MOSFETs, the subject specification is not so limited and most any type of switch can be employed.
In one aspect, the converter control system 100 links the different DC voltage buses and transfers energy back and forth. For example, the converter control system 100 can facilitate conversion of the high voltage (e.g., 200-300V) in the main battery to low voltage (e.g., 12V) for use in electrical equipment in the HEV. In another example, the converter control system 100 can facilitate conversion of a battery voltage (e.g., 300V to 500V) and supply the converted voltage to a drive motor in the HEV. Specifically, the converter control system 100 ensures that large negative current surges at startup are avoided and/or substantially reduced by employing a soft start circuit, which controls the duty cycle of a passive switch of the converter control system 100, during a soft start of an active switch of the in the converter control system 100.
According to an aspect, the operation of the step-up converter 400 is based on the tendency of an inductor to resist changes in current. Moreover, when inductor LF 214 is charged, it stores energy, and when LF 214 is discharged, it acts as an energy source. The voltage generated by LF 214 during the discharge phase is a function of the rate of change of current, and not the original charging voltage, thus allowing different input and output voltages. Specifically, when Q2 (208) is “OFF” (e.g., open) and Q1 (212) is “ON” (e.g., closed), the inductor current increases. Alternatively, when Q2 (208) is “ON” (e.g., closed) and Q1 (212) is “OFF” (e.g., open), energy accumulated in the inductor is discharged through the capacitor CH 410.
Typically, a soft start technique is utilized to gradually increase the duty cycle of the active switch Q1 (212) from zero to steady state. At the beginning of the soft start, the duty cycle of the passive switch Q2 (208) (e.g., inverted active duty cycle) is too large (near 100%) and thus, the soft start circuit 102 is employed to prevent large negative inductor currents. The soft start circuit 102 gradually increases the duty cycle of Q2 (208) from zero to a steady state value. For example, the time for which Q2 (208) is closed (“ON”), is slowly increased over multiple switching cycles, until a steady state duty cycle is reached. In one aspect, the soft start circuit 102 modifies a PWM signal to control the duty cycle of Q2 (208), such that Q2 (208) is switched “ON” for only a portion of time when Q1 (212) is switched “OFF” and wherein the portion of time is gradually increased until Q2 (208) is switched “ON” for all or substantially all the time when Q1 (212) is switched “OFF”.
Referring now to
According to an aspect, the input stage 502 can include an active switch 212 and a passive switch 208 that are soft started during the same time, by employing soft start circuit 102. Moreover, during soft start of the active switch, soft start circuit 102 gradually increases the duty cycle of the passive switch from zero to steady state, by limiting the time for which the passive switch is turned “ON.” As an example, if switching frequency for the active and passive switches is 100 KHz, the switching period is 10 microseconds. Further, if the steady state duty cycle of the active switch is 20%, for example, the active switch is “ON” for approximately 2 microseconds and the passive switch is “ON” for approximately 8 microseconds (including about 100 nanoseconds-200 nanoseconds of “deadtime” for 100 kHz switching, when both switches are “OFF”). During power up, the duty cycle of the active switch is gradually increased from zero to 20%. Conventionally, at this stage, for example, for the first several cycles, the passive switch will remain “ON” for a large amount of time (with duty cycle 99% to 80%). This can cause a large negative current flow from the output stage 504 to the input stage 502 that can damage the battery 508 and/or other components of the system 500. However, soft start circuit 102 ensures that the time for which the passive switch is turned “ON” is limited during the first few cycles and provides a soft start for the passive switch simultaneously/concurrently during the soft start of the active switch.
In one example, system 500 can be utilized in a bidirectional DC-DC converter within HEVs for linking different DC voltage buses and transferring energy back and forth. For example, a DC-DC converter can convert the high voltage (e.g., 200-300V) in the main battery to low voltage (e.g., 12V) for use in electrical equipment in the HEV. In another example, a DC-DC converter can convert a battery voltage (e.g., 300V to 500V) and supply the converted voltage to a drive motor in the HEV. It can be appreciated that the input stage 502 and output stage 504 can include most any electrical circuits depending on the application. For example, system 500 can include fly-back and fly-forward converters that utilize energy stored in the magnetic field of an inductor and/or a transformer for low power applications. Further, system 500 can include a half bridge, full bridge and/or dual full bridge circuit for higher power applications.
Referring to
In one embodiment, the active and passive switches are driven by PWM signals that are inverted versions of each other. In other words, when the active switch is “ON” the passive switch is “OFF” and vice versa. During the soft start of the active switch, the duty cycle of the PWM signal driving the active switch gradually increases from zero to steady state over several cycles. Moreover, the soft start circuit 102 receives the inverted version 602 of this PWM signal at node A 650 and converts it to a PWM_Out signal 628 (at node E 658) that soft starts the passive switch.
According to an aspect, the inverted PWM In signal 602 is applied at the input of a positive triggered one-shot circuit 604. The output of the one-shot circuit 604 is used to set a latch 608 (e.g., by feeding the output into the set pin of the latch). In one example the latch 608 can comprise a Set-Reset (SR) latch implemented by a set of cross-coupled logic gates (e.g., NOR, NAND, etc.). In addition, the output of the one-shot circuit 604 can be provided to reset a saw-tooth signal generator 610. In one example, the saw-tooth signal generator 610 can be comprised of a constant current source 612 and a capacitor 614. Moreover, the constant current source 612 charges the capacitor 614 until the voltage across the capacitor is reset on the rising edge of the inverted PWM_In signal 602, by the utilizing the output of the one-shot circuit 604 to reset the switch 616. Accordingly, the voltage waveform at node C 654 will represent a sawtooth wave 618 and the signal 618 at node C 654 will be synced to the rising edge of the inverted PWM_In signal 602.
A soft start ramp 620, for example, utilized for soft starting the active switch, is received at node B 652 and is compared with the saw-tooth waveform 618 by employing comparator 622. Typically, the sawtooth signal 618 is provided to the non-inverting input terminal of the comparator 622, while the soft start ramp 620 is provided to the inverting input terminal of the comparator 622. Although comparator 622 is depicted as an operational amplifier (op-amp), it can be appreciated that most any electrical circuit for comparing/subtracting two or more input signals can be utilized. The output of the comparator 622 is employed to reset the latch 608. Further, the output 624 of the latch (at node D 656) is provided to an input of an AND gate 626. In addition, the inverted PWM_In signal 602 is provided to another input of the AND gate 626. Moreover, the output of the AND gate 626 provides a PWM_Out signal 628 at node E 658, wherein the duty cycle is controlled to limit the time that the passive switch is initially turned “ON.”
As seen from the waveforms 690, the PWM_Out signal 628 at node E 658 is synchronized to the original inverted PWM_In signal 602 at node A 650. However, the duty cycle of the PWM_Out signal 628 gradually increases from zero to a steady state value. The PWM_Out signal 628 is utilized to drive the passive switch in the bidirectional DC-DC converters of
The DSP 702 can be programmed to generate a PWM_Out signal 628 (as shown in
In one aspect, the bidirectional DC-DC converter can comprise an active switch and a passive switch, for example, implemented by MOSFETs, BJTs, etc. At 804, the active switch can be soft started on power up. For example, a PWM signal can be employed to control the duty cycle of the active switch, such that the duty cycle is gradually increased from zero to a steady state value. Typically, the signal driving the passive switch is an inverted version of the PWM signal driving the active switch. However, substantially simultaneously to 804, at 806, the passive switch is soft started, such that the duty cycle of the passive switch is also increased gradually from zero to steady state. In one aspect, the inverted version of the PWM signal driving the active switch is processed to generate an output signal that restricts the time for which the passive switch is kept “ON” and gradually increases the time for which the passive switch is kept “ON” over multiple time periods. The output signal, employed to drive the passive switch, is synchronized to the inverted version of the PWM signal and progressively increased from zero to a steady state value. Moreover, initially the passive switch is “ON” only for a portion of the time that the active switch is “OFF”, and over multiple time periods, the time that the passive switch is “ON” is gradually increased, until the passive switch is “ON” for the entire duration that the active switch is “OFF.” Accordingly, both the active switch and the passive switch are soft started concurrently/simultaneously and thus inductor current changes smoothly without generating a large reverse or transient inductor current.
At 902, a PWM signal is applied to a positive triggered one-shot circuit. Typically, the PWM signal has an inverted duty cycle of the active switch. At 904, a latch (e.g., SR latch) can be set based on the output of the positive triggered one-shot circuit. Accordingly, the latch is set on a leading/rising edge of an “ON” state of the inverted duty cycle. Further, at 906, a sawtooth signal can be generated based on the output of the positive triggered one-shot circuit. For example, the sawtooth signal resets on the leading/rising edge of the “ON” state of the inverted duty cycle. Furthermore, at 908, a soft start ramp signal that gradually increases from zero to a steady state value can be generated. At 910, the sawtooth signal and the soft start ramp signal can be compared. As an example, the soft start ramp signal can be subtracted from the sawtooth signal. Moreover, at 912, the latch can be reset based on the comparison. In one aspect, if the sawtooth signal equals or is greater than the soft start ramp signal, the latch can be reset.
At 914, the state of the output state of the latch and the PWM signal is input to an AND gate. The output from the AND gate provides a signal that is synchronized with the PWM signal and the duty cycle of the output signal progressively increases with each time period until a steady state duty cycle is reached. At 916, the signal output from the AND gate is utilized to control the duty cycle of the passive switch within the bidirectional DC-DC converter. For example, the waveform 628 (shown in
Accordingly, the embodiments of the soft start scheme are not complex, enabling comparatively less intensive implementation when compared to the implementing of more complicated circuits. The soft start scheme solves the issue of negative current in bi-direction converters and prevents damage to the converter system from excess current.
What has been described above includes examples of the subject disclosure. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the subject matter, but one of ordinary skill in the art may recognize that many further combinations and permutations of the subject disclosure are possible. Accordingly, the claimed subject matter is intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.
In particular and in regard to the various functions performed by the above described components, devices, circuits, systems and the like, the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (e.g., a functional equivalent), even though not structurally equivalent to the disclosed structure, which performs the function in the herein illustrated exemplary aspects of the claimed subject matter. Further, the components and circuitry elements described above can be of any suitable value in order to implement the embodiments of the present invention. For example, the capacitors can be of any suitable capacitance, inductors can be of any suitable inductance, amplifiers can provide any suitable gain, current sources can provide any suitable amperage, etc.
The aforementioned systems/circuits have been described with respect to interaction between several components. It can be appreciated that such systems/circuits and components can include those components or specified sub-components, some of the specified components or sub-components, and/or additional components, and according to various permutations and combinations of the foregoing. Sub-components can also be implemented as components communicatively coupled to other components rather than included within parent components (hierarchical). Additionally, it should be noted that one or more components may be combined into a single component providing aggregate functionality or divided into several separate sub-components, and any one or more middle layers, such as a management layer, may be provided to communicatively couple to such sub-components in order to provide integrated functionality. Any components described herein may also interact with one or more other components not specifically described herein but generally known by those of skill in the art.
In addition, while a particular feature of the subject innovation may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes,” “including,” “has,” “contains,” variants thereof, and other similar words are used in either the detailed description or the claims, these terms are intended to be inclusive in a manner similar to the term “comprising” as an open transition word without precluding any additional or other elements.
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/378,325, filed on Aug. 30, 2010, and entitled “SOFT START METHOD FOR A BI-DIRECTIONAL DC TO DC CONVERTER,” the entirety of which is incorporated by reference herein.
Number | Date | Country | |
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61378325 | Aug 2010 | US |