Note: Descriptions are shown in the official language in which they were submitted.
CA 02758898 2011-11-17
SOFT START METHOD AND APPARATUS FOR A BIDIRECTIONAL DC TO DC
CONVERTER
BRIEF DESCRIPTION OF THE DRAWINGS
[0001] 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:
[0002] FIG. 1 illustrates an example system that provides an improved soft
start
technique for bidirectional converters;
[0003] FIG. 2 illustrates an example bidirectional direct current (DC)-DC step
down converter with improved soft start;
[0004] FIG. 3 illustrates an example system utilized for power generation in
hybrid electrical vehicle (HEV) and/or electrical vehicle (EV) systems;
[0005] FIG. 4 illustrates an example bidirectional DC-DC step up (boost)
converter with improved soft start;
[0006] FIG. 5 illustrates an example two-stage isolated bidirectional DC-DC
converter that reduces negative current flowing from the output to the input
of the
converter;
[0007] FIGs. 6A and 6B illustrate an example soft start circuit utilized to
control
the duty cycle of a passive switch, and signal waveforms at various nodes in
the soft start
circuit, respectively;
[0008] FIG. 7 illustrates an example system that soft starts a passive switch
in a
bidirectional DC-DC converter;
[0009] FIG. 8 illustrates an example methodology for reducing negative
transient
current in bidirectional DC-DC converters; and
[0010] FIG. 9 illustrates an example methodology for an improved soft start
mechanism in bidirectional DC-DC converters.
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CA 02758898 2011-11-17
DETAILED DESCRIPTION OF THE INVENTION
10011] 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.
100121 In one aspect, 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.
100131 Moreover, the word "exemplary" is used herein to mean serving as an
example, instance, or illustration. Any aspect or design described herein as
"exemplary"
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CA 02758898 2011-11-17
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.
[0014] Initially, referring to FIG. 1, there illustrated is an example
converter
control system 100 that provides an improved soft start technique for
bidirectional
converters, according to an aspect of the subject disclosure. In particular,
an embodiment
of system 100 can process large reverse transient inductor current during
start-up and
prevent the system from sustaining damage. The system 100 can be implemented
into
any bidirectional converter topology as well as any two stage synchronized
converter
topology, utilized in any applications, such as, but not limited to industrial
systems,
automotive systems, robotics, telecommunications, etc.
[0015] 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, uk, 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.
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[0016] 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 FIGs. 2 and 4) driven by a pulse
width modulated
(PWM) signal, generated by the PWM signal generator 106. Further, the input
stage can
be coupled to a output voltage at the output stage 112. In one example, the
switches 114
can be implemented by employing MOSFETs. Moreover, during a continuous
conduction mode (CCM) of operation, the active switch is "ON," when the
passive
switch is "OFF" and the active switch is "OFF," when the passive switch is
"ON." In
addition, there can be some "deadtime" in between, during which both the
active switch
and the passive switch are "OFF" to prevent current shoot-through through both
the
MOSFETs 114. To avoid negative current surges in the bidirectional DC-DC
converter
104, system 100 employs the soft start circuit 102. Moreover, the soft start
circuit 102
generates an output signal, based on a PWM signal provided by the PWM signal
generator 106, which controls the switching of the passive switch during
startup. For
example, at power up, the soft start circuit 102 adjusts the duty cycle of the
passive
switch and gradually increases the duty cycle of the passive switch from zero
to a steady
state. Since passive switch's duty cycle gradually increases, in the same way
as the
active switch's duty cycle, inductor current in the bidirectional DC-DC
converter 104,
changes smoothly and huge reverse or transient inductor current are prevented.
[0017] 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.
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[0018] Referring now to FIG. 2, there illustrated is an example bidirectional
DC-
DC step down converter 200 with improved soft start, according to an aspect of
the
specification. The step down converter typically includes an inductor (Lf) 214
and two
switches (e.g., comprising two transistors) that control the inductor. The
switches Qi 212
and Q2 208 can be MOSFET switches, illustrated in FIG. 2, with the body diodes
of the
MOSFETs shown. Specifically, the switches alternate between connecting the
inductor
to a source voltage to store energy in the inductor, and discharging the
inductor into the
load. In one example, switch Qi 212 is termed as an "active switch", since Qi
212 is a
switching element required for operation of the DC-to-DC converter
(unidirectional
and/or bidirectional). Additionally, Q2 208 is termed as a "passive switch",
since Q2 208
is an optional switching element required only during operation of a
bidirectional DC-to-
DC converter (e.g., a free wheeling diode can be utilized instead of a passive
switch for
unidirectional DC-to-DC converter operation).
[0019] 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 FIG. 2, the input and output of the converter are connected to
batteries,
wherein the input voltage is higher than the output voltage. Moreover, the
input stage is
termed as the high voltage side (VH) 202 and the output stage is termed as the
low voltage
side (VL) 204. As an example, the bidirectional converter 200, employing the
above
described configuration can be utilized in an electric automobile, in which
the battery at
VL (204) would be substituted for an electric motor, to propel the vehicle. In
addition,
the synchronous buck style bidirectional converter 200 includes a high side
capacitor
(CH) 206, a transistor (Q2) 208, and a low side capacitor (CL) 210 in parallel
to VH and
VL. A transistor (Q1) 212 and an inductor (Lf) 214 are in series between the
positive
terminal of CH 206 and node N, and node N and the positive terminal of CL 204.
[0020] In one embodiment, Qi (212) and Q2 (208) are complimentary switches,
wherein Qi (212) is defined as an active switch and Q2 (208) is defined as a
passive
switch. Moreover, when Qi (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 Qi (212) is gradually increased from
zero to
steady state, for example at start up, for example, by employing a PWM signal
generated
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CA 02758898 2011-11-17
by a PWM signal generator 106 to control the duty cycle of Qi (212). Because
the duty
cycle of Q2 (208) is complimentary (e.g., an inverted version) of the duty
cycle of Qi
(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.
[0021] 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 Qi (212), the inductor current ILF changes smoothly,
and a huge
reverse inductor current is avoided.
100221 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 Q (212) and Q2 (208) are depicted as MOSFETs, the subject
specification is not so limited and most any type of switch can be employed.
[0023] FIG. 3 illustrates an example system 300 utilized for power generation
in
hybrid electrical vehicle (HEV) and/or electrical vehicle (EV) systems. In one
aspect, a
200-400V high voltage battery stack 310 is used as energy storage at the input
stage in
the converter control system and a low voltage 12V battery 312 is connected to
the output
stage in the converter control system. The charging of the high battery pack
310 is done
through an isolated AC-DC converter 306, connected to the electric motor/
generator 304,
whereas the charging of the low battery pack 312 is done through an isolated
DC-DC
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CA 02758898 2011-11-17
converter within the converter control system 100. Given the large fluctuation
of the high
voltage battery pack 310, oftentimes a pre-regulator can be inserted between
the low
voltage battery 312 and the input of the isolated DC-DC converter within the
converter
control system 100, such that the transformer designs can be optimized.
[0024] 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.
[0025] FIG. 4 illustrates an example bidirectional DC-DC step up (boost)
converter 400 with improved soft start in accordance with an aspect of the
disclosure.
The step-up converter 400 can be a power converter with an output DC voltage
(VH) 404
greater than its input DC voltage (VI) 402. Typically, most any DC sources,
such as, but
not limited to, batteries, solar panels, rectifiers, DC generators, etc., can
be utilized at the
input and/or output side. Typically, system 400 can be utilized in various
applications,
such as, but not limited to, hybrid electric vehicles (HEV) and/or lighting
systems. In
addition, switches Qi (212) and Q2 (208) can be implemented by utilizing most
any
electrical circuit elements, such as, but not limited to transistors, (e.g.,
MOSFETs).
[0026] 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 Qi
(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
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CA 02758898 2011-11-17
=
inductor is discharged through the capacitor CH 410.
[0027] Typically, a soft start technique is utilized to gradually increase the
duty
cycle of the active switch Qi (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 Qi (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 Qi
(212) is switched "OFF".
[0028] Referring now to FIG. 5, there illustrated is an example two-stage
isolated
bidirectional DC-DC converter 500 that reduces negative current flowing from
the output
to the input of the converter in accordance with an aspect of the innovation.
In one
example, the first/input stage 502 can include, but is not limited to, a pre-
regulator,
bidirectional buck/boost converter, etc. and the second/output stage 504 can
include, but
is not limited to, a full bridge, half bridge, push pull circuits, etc.
Typically, isolation
between the two stages (502, 504) is achieved by employing a transformer 506.
As an
example, isolation can be provided to satisfy safety requirements, especially
for high
power levels. Further, the input stage 502 can be connected to a voltage
source, for
example, battery 508 with input voltage VI, and the output stage 504 can be
connected to
another voltage source, for example, battery 510 with output voltage Vo.
[0029] 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
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CA 02758898 2011-11-17
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.
[0030] 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
REV. 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.
[0031] Referring to FIGs. 6A and 6B, there illustrated is an example soft
start
circuit 102 utilized to control the duty cycle of passive switch Q2 208 (in
FIGs. 2, 4, and
5), and signal waveforms 690 at various nodes (650-658) in the soft start
circuit 102. In
one aspect the soft start circuit 102 reduces the large reverse transient
inductor current
during start-up and prevents damage to a bidirectional DC-DC converter system.
Soft
start circuit 102 is typically employed in most any bidirectional DC-DC
converters that
utilize switches (e.g., MOSFETs, bi-polar junction transistors (BJTs), etc.).
In general,
the soft start circuit 102 can ensure that the duty cycle of the passive
switch is
progressively increased from zero, during soft start of the active switch.
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[0032] 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.
[0033] 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 savvtooth
wave 618
and the signal 618 at node C 654 will be synced to the rising edge of the
inverted
PWM_In signal 602.
[0034] 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
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658, wherein the duty cycle is controlled to limit the time that the passive
switch is
initially turned "ON."
[0035] 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 FIGs. 1, 2, 4, and 5. Accordingly,
during soft start
of the active switch, the time for which the passive switch will remain turned
"ON" is
limited and gradually increased with each time period. Moreover, because the
duty cycle
of the passive switch increases gradually, in the same manner as the active
switch, the
inductor current changes smoothly and large reverse or transient inductor
currents are
avoided.
[0036] FIG. 7 illustrates an example system 700 that soft starts a passive
switch
208 in a bidirectional DC-DC converter 104. The bidirectional DC-DC converter
104
can include isolated and/or non-isolated topologies comprising an active
switch (as
shown in FIGs. 2 and 4) and passive switch 208 (e.g., implemented by MOSFETs,
BJTs,
etc.). During power up and/or reset, the active switch is soft started, for
example, the
duty cycle of the active switch is slowly increased from zero to a steady
state over several
time periods. During this time, the operation of the passive switch 208 is
controlled by
the soft start circuit 102. According to an aspect, the soft start circuit 102
can include a
digital signal processor (DSP) (e.g., a micro controller, micro processor,
etc.) 702.
Typically, DSP 702 can be utilized in lieu of the circuit 102 in FIG. 6A.
[0037] The DSP 702 can be programmed to generate a PWM _Out signal 628 (as
shown in FIG. 6B) that can be utilized to soft start the passive switch 208.
Initially, the
PWM _Out signal limits the time that the passive switch is turned "ON" when
the active
switch is "OFF" and thereafter gradually increases the time for which the
passive switch
is turned "ON" with every time period, until a steady state is reached. In
general, the
operation of the passive switch 208 is controlled by the PWM _Out signal 628
on power-
up/ reset, such that the passive switch 208 is soft started simultaneously or
concurrently
with the active switch. Moreover, since the passive switch's duty cycle
increases over
multiple time periods, in the same way as the active switch's duty cycle,
negative current
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CA 02758898 2011-11-17
issue in the bidirectional converter 104 is prevented.
[0038] FIGs. 8-9 illustrate methodologies and/or flow diagrams in accordance
with the disclosed subject matter. For simplicity of explanation, the
methodologies are
depicted and described as a series of acts. It is to be understood and
appreciated that the
subject innovation is not limited by the acts illustrated and/or by the order
of acts, for
example acts can occur in various orders and/or concurrently, and with other
acts not
presented and described herein. Furthermore, not all illustrated acts may be
required to
implement the methodologies in accordance with the disclosed subject matter.
In
addition, those skilled in the art will understand and appreciate that the
methodologies
could alternatively be represented as a series of interrelated states via a
state diagram or
events. Additionally, it should be further appreciated that the methodologies
disclosed
hereinafter and throughout this specification are capable of being stored on
an article of
manufacture to facilitate transporting and transferring such methodologies to
computers.
The term article of manufacture, as used herein, is intended to encompass a
computer
program accessible from any computer-readable device or computer-readable
storage/communications media.
[0039] FIG. 8 illustrates an example methodology 800 for reducing negative
transient current in bidirectional DC-DC converters in accordance with an
aspect of the
subject disclosure. Specifically, methodology 800 prevents generation of a
large negative
transient current that can damage the system and thus makes the system more
robust. At
802, the bidirectional DC-DC converter can be powered up (e.g., switched "ON",
reset,
re-started, etc.), for example, manually or automatically (e.g., in response
to an event).
Typically, the bidirectional DC-DC converter can include most any isolated or
non-
isolated topology, such as, but not limited to buck, boost, buck-boost, uk,
charge pump,
fly-back, fly-forward, half bridge, full bridge, dual full bridge, etc.
topologies, and can be
utilized in various applications, such as, but not limited to, industrial
automation systems,
automotive systems, robotics, etc.
[0040] 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
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CA 02758898 2011-11-17
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.
[0041] FIG. 9 illustrates an example methodology 900 for an improved soft
start
mechanism in bidirectional DC-DC converters, according to an aspect of the
subject
specification. Methodology 900 generates a soft start duty cycle to control a
passive
switch of the bidirectional DC-DC converter. Typically, the bidirectional DC-
DC
converter has an active switch and a passive switch, in which an active duty
cycle of the
active switch gradually increases from zero to a steady state value at start-
up. As noted
above, the operation of the active and passive switches is complimentary, such
that the
active switch is "OFF" when the passive switch is "ON" and the active switch
is "ON"
when the passive switch is "OFF."
[0042] 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
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CA 02758898 2011-11-17
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.
[0043] 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
FIG. 6B) is used as the duty cycle of the passive switch (instead of the
inverted active
duty cycle i.e. waveform 602 shown in FIG. 6B), such that the duty cycle of
the passive
switch increases gradually, in the same manner as the active switch, to avoid
large
reverse or transient inductor currents in the bidirectional converter.
[0044] 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.
[0045] 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.
[0046] 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,
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CA 02758898 2011-11-17
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.
[0047] 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.
[0048] 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.
15
