U.S. patent application number 13/435679 was filed with the patent office on 2013-03-07 for intermediate bus architecture with a quasi-regulated bus converter.
This patent application is currently assigned to SynQor, Inc.. The applicant listed for this patent is Richard W. Farrington, Martin F. Schlecht. Invention is credited to Richard W. Farrington, Martin F. Schlecht.
Application Number | 20130058133 13/435679 |
Document ID | / |
Family ID | 39732573 |
Filed Date | 2013-03-07 |
United States Patent
Application |
20130058133 |
Kind Code |
A1 |
Farrington; Richard W. ; et
al. |
March 7, 2013 |
Intermediate Bus Architecture with a Quasi-Regulated Bus
Converter
Abstract
A dc-dc converter system comprises a quasi-regulated bus
converter and plural regulation stages that regulate the output of
the bus converter. The bus converter has at least one controlled
rectifier with a parallel uncontrolled rectifier. A control circuit
controls the controlled rectifier to cause a normally non-regulated
mode of operation through a portion of an operating range of source
voltage and a regulated output during another portion. The bus
converter may be an isolation stage having primary and secondary
transformer winding circuits. For the non-regulated output, each
primary winding has a voltage waveform with a fixed duty cycle. The
fixed duty cycle causes substantially uninterrupted flow of power
during non-regulated operation. Inductors at the bus converter
input and in a filter at the output of the bus converter may
saturate during non-regulated operation.
Inventors: |
Farrington; Richard W.;
(Heath, TX) ; Schlecht; Martin F.; (Lexington,
MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Farrington; Richard W.
Schlecht; Martin F. |
Heath
Lexington |
TX
MA |
US
US |
|
|
Assignee: |
SynQor, Inc.
Boxborough
MA
|
Family ID: |
39732573 |
Appl. No.: |
13/435679 |
Filed: |
March 30, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12870326 |
Aug 27, 2010 |
8149597 |
|
|
13435679 |
|
|
|
|
11982327 |
Nov 1, 2007 |
7787261 |
|
|
12870326 |
|
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|
60855971 |
Nov 1, 2006 |
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Current U.S.
Class: |
363/17 |
Current CPC
Class: |
H02M 3/33592 20130101;
H02M 2001/008 20130101; Y02B 70/10 20130101 |
Class at
Publication: |
363/17 |
International
Class: |
H02M 3/335 20060101
H02M003/335 |
Claims
1. (canceled)
2. A dc-dc converter system comprising: an isolated dc-dc bus
converter that receives a dc source voltage and converts the dc
source voltage to a dc intermediate voltage output, the dc-dc bus
converter: in normal operation, having a non-regulated mode of
operation, over a low voltage portion of an operating range of the
dc source voltage, where the dc intermediate voltage output is not
regulated by the bus converter and the dc intermediate voltage
output displays a droop characteristic; and in normal operation,
having a fully regulated mode of operation, over a high voltage
portion of the operating range of the dc source voltage, where the
dc intermediate voltage output is fully regulated by the bus
converter; and a plurality of non-isolated switching regulators,
each receiving the dc intermediate voltage output of the dc-dc bus
converter and regulating a switching regulator output.
3. A dc-dc converter system as claimed in claim 2 wherein the fully
regulated mode of operation of the dc-dc bus converter creates a
tightly regulated dc intermediate voltage output by feeding back a
signal from the dc intermediate voltage output.
4. A dc-dc converter system as claimed in claim 3 wherein the dc-dc
bus converter comprises: at least one transformer with at least one
primary winding and at least one secondary winding; a primary
winding circuit that receives the dc source voltage; and a
secondary winding circuit having plural controlled rectifiers with
parallel uncontrolled rectifiers and an output filter having an
output inductor.
5. A dc-dc converter system as claimed in claim 4 wherein the
controlled rectifiers are driven from signals derived from the at
least one secondary winding.
6. A dc-dc converter system as claimed in claim 4 wherein the
non-regulated mode of operation has a duty cycle of the controlled
rectifiers to cause substantially uninterrupted flow of power
through the dc-dc bus converter.
7. A dc-dc converter system as claimed in claim 4 wherein each
controlled rectifier is turned on and off in synchronization with a
voltage waveform across a primary transformer winding to provide
the dc intermediate voltage output, each primary winding having a
voltage waveform for the non-regulated mode of operation with a
fixed duty cycle and transition times which are short relative to
the on state and off state times of the controlled rectifiers.
8. A dc-dc converter system as claimed in claim 7 wherein the dc-dc
bus converter further comprises a filter having an inductor that is
allowed to saturate during the non-regulated mode of operation.
9. A dc-dc converter system as claimed in claim 7 wherein the dc-dc
bus converter further comprises an output filter having an inductor
that is allowed to saturate during the non-regulated mode of
operation.
10. A dc-dc converter system as claimed in claim 4 wherein the
transformer has a turns ratio of 4:1.
11. A dc-dc converter system as claimed in claim 3 wherein the
non-regulated mode of operation has a fixed duty cycle.
12. A dc-dc converter system as claimed in claim 2 wherein the
dc-dc bus converter comprises: at least one transformer with at
least one primary winding and at least one secondary winding; a
primary winding circuit that receives the dc source voltage; and a
secondary winding circuit having plural controlled rectifiers with
parallel uncontrolled rectifiers and an output filter having an
output inductor.
13. A dc-dc converter system as claimed in claim 12 wherein the
controlled rectifiers are driven from signals derived from the at
least one secondary winding.
14. A dc-dc converter system as claimed in claim 12 wherein the
non-regulated mode of operation has a duty cycle of the controlled
rectifiers to cause substantially uninterrupted flow of power
through the dc-dc bus converter.
15. A dc-dc converter system as claimed in claim 12 wherein each
controlled rectifier is turned on and off in synchronization with a
voltage waveform across a primary transformer winding to provide
the dc intermediate voltage output, each primary winding having a
voltage waveform for the non-regulated mode of operation with a
fixed duty cycle and transition times which are short relative to
the on state and off state times of the controlled rectifiers.
16. A dc-dc converter system as claimed in claim 15 wherein the
dc-dc bus converter further comprises a filter having an inductor
that is allowed to saturate during the non-regulated mode of
operation.
17. A dc-dc converter system as claimed in claim 15 wherein the
dc-dc bus converter further comprises an output filter having an
inductor that is allowed to saturate during the non-regulated mode
of operation.
18. A dc-dc converter system as claimed in claim 12 wherein the
transformer has a turns ratio of 4:1.
19. A dc-dc converter system as claimed in claim 2 wherein the
dc-dc bus converter further comprises a filter having an inductor
that is allowed to saturate during the non-regulated mode of
operation.
20. A dc-dc converter system as claimed in claim 2 wherein the
dc-dc bus converter further comprises an output filter having an
inductor that is allowed to saturate during the non-regulated mode
of operation.
21. A dc-dc converter system as claimed in claim 2 wherein the
non-regulated mode of operation has a fixed duty cycle.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 12/870,326, filed Aug. 27, 2010, now U.S. Pat. No. 8,149,597,
which is a continuation of U.S. application Ser. No. 11/982,327,
filed Nov. 1, 2007, now U.S. Pat. No. 7,787,261, which claims the
benefit of U.S. Provisional Application No. 60/855,971, filed on
Nov. 1, 2006.
[0002] The entire teachings of the above applications are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] U.S. Pat. No. 5,999,417 (the '417 patent) and U.S. Pat. No.
7,050,309 (the '309 patent) describe what is here referred to as
the "Intermediate Bus Architecture" and "bus converters." The
entire teachings of these patents are incorporated herein by
reference.
[0004] The Intermediate Bus Architecture (IBA) has become a popular
approach for providing multiple output voltages (for loads such as
digital circuits) from a single input voltage source. A first dc-dc
converter (sometimes called a "bus converter"), usually providing
isolation through a transformer, is used to change the source
voltage, say 48V, to an intermediate voltage, say 12V. This
intermediate voltage is then used as the input to several
non-isolated dc-dc converters (sometimes called "P.O.L.
converters," for "point-of-load") or linear regulators, each of
which create a regulated output voltage appropriate for their
respective loads.
[0005] When the range of the source voltage is narrow enough, the
bus converter can be a device that normally does not regulate. It
simply isolates and converts the source voltage to the intermediate
voltage by virtue of the turns-ratio of its transformer. For
instance, it may have a turns-ratio of 4:1, so that a 48V source
becomes a 12V intermediate voltage. As the source voltage ranges
from 38V to 56V, the intermediate voltage correspondingly ranges
from 9V to 14V. This type of bus converter will be referred to
herein as a "non-regulated bus converter."
[0006] Since the non-regulated bus converter does not normally
regulate (it regulates only during a turn-on or turn-off transient,
or during a current-limit condition, and the like), the
intermediate voltage displays a "droop" characteristic. By this it
is meant that the value of the intermediate voltage decreases as
the current flowing out of the bus converter increases. For the
example given above, this might make the intermediate voltage range
from 8.5V to 14V over the full range of source voltage and bus
converter output current.
[0007] This variation of the intermediate voltage is acceptable
since the P.O.L.'s can typically operate over such a range of their
input voltage.
[0008] When the range of the source voltage is wider, such as 36V
to 75V, or even 36V to 100V, then a different type of bus converter
is often used because the non-regulated bus converter would give
too much variation in the intermediate voltage for the P.O.L.'s to
handle. This second type of bus converter, referred to herein as a
"semi-regulated bus converter," provides regulation (as well as
isolation) so that the intermediate voltage does not vary
proportionally to the source voltage. To first order it holds the
intermediate voltage approximately constant, although it does
permit this voltage to droop as the bus converter's output current
increases so that some costs might be saved. For this reason, this
type of bus converter is referred to as "semi-regulated." The droop
in the intermediate voltage, which might be around 5% to 10% of the
nominal voltage as the bus converter's output current ranges from
zero to full rated current, is well within the range of what a
typical P.O.L. can handle for its input voltage.
[0009] A semi-regulated bus converter has a lower level of
performance, in terms of efficiency and power handling capability,
than the non-regulated bus converter as a result of its design to
provide regulation over the full range of the source voltage.
SUMMARY OF THE INVENTION
[0010] To address the problem of reduced performance of the bus
converter for an application where the source voltage range is
relatively wide, a new type of bus converter is presented here.
This bus converter, herein called a "quasi-regulated bus
converter," is normally non-regulating over some portion of the
source voltage operating range, and regulating (either fully
regulating or semi-regulating) over another portion of the
operating range. The operating range is the intended source voltage
range where the system is expected to operate and meet its
specifications and is generally specified for each converter and/or
for the application in which it is applied. Typically, the
converter only receives a source voltage outside its operating
range during a transient such as start up or shut down.
[0011] For instance, in a system where the input voltage ranges
from 36V to 100V, the quasi-regulated bus converter might be
designed so that it does not normally regulate when the source
voltage is between 36V and 56V. If the transformer turns ratio is
4:1, the intermediate voltage would then vary from 9V to 14V if we
do not account for the droop characteristic, and from perhaps 8.5V
to 14V if we do account for the droop.
[0012] When the source voltage is between 56V and 100V, the
quasi-regulated bus converter would then regulate its output,
perhaps with a droop characteristic. In one example, the
intermediate voltage would remain constant at 14V (perhaps minus a
droop) over this 56V-100V range of source voltage.
[0013] With such an approach, the quasi-regulated bus converter
keeps the intermediate voltage within a range that is acceptable
for typical P.O.L.'s, but it does not try to regulate or
semi-regulate the bus voltage to a range as tight as 10% (or so),
as the semi-regulated bus converter would. As such, the
quasi-regulated converter is capable of achieving an efficiency and
power handling capability that is higher than the semi-regulated
bus converter.
[0014] The exact details of how the source voltage range should be
divided between these two modes of operation in the quasi-regulated
converter are flexible, and they depend on the design of the
converter and on the details of the application.
[0015] For instance, the non-regulated mode of operation might be
at the high end of the source voltage range instead of the low end,
as mentioned in the example above. It might even be in a middle
section of the source voltage range, with regulation occurring at
either end of the range. The acceptable range of the intermediate
voltage might be different than the example given above based on
the needs of the P.O.L.'s or on a desire to optimize the
performance of the total system. For instance, the onset of the
regulation range might occur in the 50V-52V level, instead of the
56V level mentioned previously.
[0016] Provisions for handling start-up and shut-down, and
protection features such as over-voltage, over-current,
over-temperature, and back-drive current limiting would be added to
the quasi-regulated bus converter as required.
[0017] A dc-dc converter system may comprise a bus converter that
receives a source voltage and converts the source voltage to an
output. The bus converter may include a control circuit that
normally controls the bus converter to cause a non-regulated mode
of operation, over a portion of an operating range of a source
voltage, where the output is non-regulated. The control circuit
normally causes a regulated mode of operation, over another portion
of the operation range of the source voltage, where the output is
regulated. A plurality of regulation stages each receive the output
of the bus converter and regulate a regulation stage output.
[0018] The bus converter may be an isolation stage while the
regulation stages are non-isolating. The regulated mode of
operation of the bus converter may be a semi-regulated mode. The
regulation stages may be switching regulators.
[0019] The bus converter may comprise at least one transformer with
at least one primary winding and at least one secondary winding.
The primary winding circuit receives the source voltage and has an
input filter with an input inductor. The secondary winding circuit
has plural controlled rectifiers with parallel uncontrolled
rectifiers, and output filter having an output inductor and the bus
converter output.
[0020] For the non-regulated mode of operation, the control circuit
may control the duty cycle of the controlled rectifiers to cause
substantially uninterrupted flow of power through the dc-dc
converter. Each controlled rectifier of the secondary winding
circuit may be turned on and off in synchronization with a voltage
waveform across a primary transformer winding to provide the output
. Each primary winding may have a voltage waveform for the
non-regulated mode of operation with a fixed duty cycle and
transition times which are short relative to the on state and off
state times of the controlled rectifiers.
[0021] The bus converter may comprise a filter having an inductor
that saturates during a non-regulating mode of operation. Such
inductors may be included in an input filter and/or in an output
filter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The foregoing will be apparent from the following more
particular description of example embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different
views.
[0023] The drawings are not necessarily to scale, emphasis instead
being placed upon illustrating embodiments of the present
invention.
[0024] FIG. 1: Block diagram of an Intermediate Bus Architecture
(IBA).
[0025] FIG. 2: One example of a Bus Converter power circuit
topology which, based on how it is controlled and on the component
selection, could be used for all three types of Bus Converters.
[0026] FIGS. 3A-C: Three possible ways to divide the total range of
V.sub.S into non-regulating and (semi-)regulating regions.
DETAILED DESCRIPTION OF THE INVENTION
[0027] FIG. 1 shows a block diagram of an Intermediate Bus
Architecture. The source of power 101 provides a dc voltage
V.sub.S, which may nominally be, for example, 48 volts, but which
may vary over a range above and below this nominal, the width of
the range being dependent on the application. The bus converter 102
then converts this voltage to a different nominal voltage; for
example 12V. It also provides the electrical isolation needed for
safety regulations and to avoid ground loop noise problems.
[0028] The output of the bus converter provides an intermediate
voltage V.sub.I 103. Capacitors 104 are typically connected between
this intermediate node and ground to provide additional filtering.
Also connected to the intermediate node are the inputs of several
non-isolated switching regulators 105, which are called P.O.L.'s.
These P.O.L.'s are usually "buck-converters," and in today's
technology they often incorporate synchronous rectifiers to improve
efficiency. They each create an output voltage 106 that is held
constant (i.e. regulated) even as the intermediate voltage or their
output current varies. These output voltages are typically supplied
to digital and analog load circuitry.
[0029] It is also possible for one or more of the P.O.L.'s to be of
a design that gives an output voltage higher than the intermediate
voltage, or to give an output voltage that is negative with respect
to the ground potential. Various topologies such as a
"boost-converter," a "buck-boost converter," and a "Sepic
converter" could be used for these purposes, and they (and others)
are well known in the art.
[0030] The bus converter of FIG. 1 could be a non-regulated bus
converter, a semi-regulated bus converter, or, as described herein,
a "quasi-regulated bus converter." The choice depends, in part, on
how much the source voltage varies and on how wide a range of
intermediate voltage the P.O.L.s can tolerate. Some sources stay
within a relatively tight range; for example 48V .+-.10%. Others
vary over a moderate range; for example 36V to 56V. Yet others have
a very wide range, for example 36V to 100V. Some P.O.L.'s are
designed to handle an input voltage that is relatively tight; for
example 12V .+-.20%. Others are designed to handle a wider range;
for example 7V to 15V.
[0031] One example of a non-regulated bus converter is described in
the '309 patent. As shown in FIG. 2 here, it uses a full-bridge
topology for the switches 201-204 connected to the primary winding
209 of the isolation transformer, and a center-tapped topology for
the secondary side. Synchronous rectifiers, composed of controlled
rectifiers 205 and 206 and their corresponding uncontrolled
rectifiers 207 and 208 are connected to the center-tapped secondary
windings 210 and 211, respectively. The synchronous rectifiers are
typically MOSFETs, where the channel is the controlled rectifier
and the parasitic body diode is the uncontrolled rectifier. An
external diode could also be used for the uncontrolled
rectifier.
[0032] A capacitor 214 may be placed in series with the primary
winding 209 to ensure flux balance. Other means of achieving flux
balance are well known in the art.
[0033] Inductors 212 and 213 represent the leakage inductance of
the transformer.
[0034] Inductor 216 and capacitor 217 form a low-pass output
filter. The voltage across capacitor 217 is the intermediate
voltage 103 shown in FIG. 1. Capacitor 218 and inductor 219 provide
a low-pass filter for the bus converter's input. It may be
connected directly to the source voltage 101, or there may be
additional filter elements between the two.
[0035] A control circuit 220 provides the gate drive signals for
the various power switches. It typically senses voltages and
currents within the power circuit, and it provides the desired duty
cycle/switch timing for the switches.
[0036] During normal operation the circuit is operated at a fixed
duty cycle where a positive voltage (of value V.sub.S) is applied
across the transformer's primary winding for the first half of the
cycle, and a negative voltage (-V.sub.S) is applied across the
winding for the second half of the cycle. Except for the relatively
short switch transition times between these two half cycles, power
is always flowing from the source, through the transformer, and
then to the output of the bus converter. Except for the short
switch transitions, there is no explicit "freewheeling" portion of
the cycle where the power flow through the transformer is
interrupted and the flow of power to the output is maintained by
the output inductor 216 and capacitor 217 alone.
[0037] The synchronous rectifiers in the secondary circuit rectify
the output of the transformer and create a dc voltage. This is than
passed through the low-pass filter 216,217. The components in this
filter, particularly the inductor, are relatively small due to the
fact that during normal operation they only need to filter the
interruption of power during the short switch transition times at
the end of each half-cycle. These interruptions might last only
about 50 ns, which is very short compared to the half-cycle, which
may be 2 us long, depending on the details of the design.
[0038] Consider the case where the source voltage varies over a
relatively narrow range from 36V to 56V. Assume the turns-ratio of
the transformer is 4:1 (primary to secondary). If there were no
load current flowing, the output voltage of the bus converter,
V.sub.I, would be approximately 1/4 that of the source voltage,
V.sub.S. In other words, V.sub.I would range from 9V to 14V.
However, as the load current builds, the V.sub.I falls, or
"droops," due to the resistances of the power path and the voltage
drops required across the leakage inductances to commutate the
current from one secondary winding to the other each half cycle.
This droop might typically be about 0.5V at full load current, so
the output voltage would range from approximately 8.5V (at full
load and V.sub.S=36V) to 14V (at zero load and V.sub.S=56V).
[0039] This non-regulated bus converter can be very efficient
(typically 97%) for several reasons, all associated with the fact
that it does not normally make use of a freewheeling portion of the
cycle to regulate. First, power is transferred from source to
output for nearly 100% of the cycle and as a result, the rms value
of the current flowing through all the components of the converter
is minimized. Second, the turns-ratio of the transformer can be
higher than it otherwise would be, so that the currents flowing on
the primary side are smaller for a given output current and the
off-state voltage ratings of the synchronous rectifiers are smaller
for a given maximum source voltage. Third, the switch transitions
between one half-cycle and the next are nearly lossless because
there is not a need to recover from a freewheeling period. Fourth,
the output inductor 216 is relatively small in value, and as a
result can be relatively low in losses for a given volume of the
device.
[0040] In addition, the non-regulated bus converter does not need
to have control signals that bridge the isolation barrier between
the primary and secondary sides of the circuit. As mentioned in the
'417 patent, there is no need to feed back a signal representing
the output voltage since there is no attempt to regulate it. There
is also no need to send control signals to the synchronous
rectifiers. Their control signals can be easily derived from the
voltages on the secondary windings, as described in the '309 and
the '417 patents. Besides saving cost, this lack of circuitry that
bridges the isolation barrier permits the transformer to occupy the
entire physical width of the converter, and therefore it can have a
lower winding resistance, higher efficiency, and better thermal
performance.
[0041] During transients such as startup and shutdown, or in
situations where the output current must be limited from getting
either too high or too negative, the non-regulated bus converter
may be operated with a duty cycle less than 100%, and therefore
with an associated freewheeling period. For instance, at startup
the duty cycle may be slowly ramped from 0% to 100% to cause the
output voltage V.sub.I to slowly rise to its final value. As
explained in the '309 patent, there are additional losses during
these abnormal conditions due to dissipative switch transitions and
the fact that the controlled rectifiers may not be conducting
during the freewheeling period, depending on the control strategy
used. In addition, the output ripple voltage may be larger than
normal. But these conditions can be tolerated since they are
transient in nature and do not represent the normal use of the
non-regulated bus converter.
[0042] When the input voltage varies over a range that is too wide
to use a non-regulated bus converter with a given P.O.L.
technology, a semi-regulated bus converter is often used. Such a
bus converter could have the same full-bridge topology shown in
FIG. 2. The difference is that now the converter is designed to
have its duty cycle vary as the source voltage varies so that the
intermediate voltage V.sub.I stays near a nominal value. For
instance, if the source voltage V.sub.S varies from 36V to 100V,
the semi-regulated bus converter might be designed to have a duty
cycle near 100% when V.sub.S=36V, and a duty cycle of about 36%
when V.sub.S=100V.
[0043] Since it is not necessary, in an IBA application, to have
the intermediate bus voltage be fully regulated, a semi-regulated
converter is usually designed such that the feedback signal that
determines the duty cycle is derived from one or more signals
available on the primary side of the converter that are indicative
of the output voltage. One possible signal is the source voltage,
V.sub.S. Another possible signal is the voltage across the primary
transformer winding.
[0044] These signals do not account for voltage drops across
resistances in the power path, nor do they account for the voltage
drops across leakage inductances that are required to commutate the
load current from one secondary winding to the other. As such, a
semi-regulated bus converter will have an output voltage that
falls, or droops, as the load current is increased. A droop of
5%-10% is typical as the load current varies from zero to full
rated current.
[0045] If the droop is larger than desired, then it is possible to
reduce it by measuring a signal on the primary side that is
indicative of the current flowing through the power stage. This
signal, multiplied by an appropriate gain, can be used to modify
the duty cycle to give a higher output voltage to compensate for
the droop, as described in the '417 patent. Since the resistances
of the power path are temperature dependent, it might also be
desirable to adjust the effect of this compensation circuitry as a
function of the converter's temperature.
[0046] A semi-regulated bus converter cannot be as efficient as a
non-regulated bus converter, all other things held constant. By
definition, it has a freewheeling period for all values of the
source voltage except, perhaps, the lowest. It therefore isn't able
to fully utilize the transformer and the power switching devices.
There is also a significant switching loss at the end of each
freewheeling period due to the leakage inductance of the
transformer and the parasitic capacitances of the power switches.
Third, the transformer's turns ratio would be lowered, which means
that the currents on the primary side are higher for a given load
current and the off-state ratings of the synchronous rectifiers are
higher for a given maximum source voltage. Finally, the output
inductor would be made relatively large in value to minimize the
ripple in its current and the intermediate voltage. This large
value of inductance translates into a physically larger and more
dissipative device.
[0047] For instance, if the source voltage ranges from 36V to 100V,
and the output voltage is to be nominally 12V (not counting the
droop), the turns ratio for a semi-regulated bus converter must be
3:1. If the source voltage had been 48V .+-.10% and a
non-regulating bus converter were used, it could have a turns ratio
of 4:1 to create a nominal 12V output. This difference is
significant with regard to the bus converter's efficiency.
[0048] The output filter must do much more filtering on the
semi-regulated bus converter than it has to on the non-regulated
bus converter. Instead of just brief interruptions of power flow
during the switch transitions each half cycle, now the output
filter must deal with interruptions that last for the duration of
the freewheeling period. In general, this means that the output
inductor must be much larger in value, peak energy storage, and
physical size. As a result, more power is dissipated in this
inductor, and there is less room available on the converter for
other components.
[0049] For example, consider the size of the non-regulated bus
converter's filter inductor. During the switch transitions it will
see the output voltage across it, but these transitions will last
only about 50 ns. In comparison, for the semi-regulated bus
converter, the filter inductor will see the output voltage across
it for the length of the freewheeling period. In the example given
above, the longest freewheeling period is 64% of the cycle when
V.sub.S is 100V. If we assume a switching frequency of 250 kHz, or
2 .mu.s for each half cycle, then this maximum freewheeling period
would be 1.28 .mu.s long. To maintain the same current ripple in
the inductor, the value of inductance would therefore need to be
increased by a factor of 25 for the semi-regulated bus converter as
compared to the non-regulated bus converter. This increase is so
large that a more reasonable design might call for more inductor
ripple current (to limit the size of the inductor) and more output
capacitor to achieve the required ripple level in V.sub.I.
[0050] Overall, holding all other things constant, a semi-regulated
bus converter that must handle a wide range of source voltage might
be 94% efficient, while a non-regulated bus converter that is
designed to handle a narrower range of source voltage might be 97%
efficient. This difference translates into about twice the
dissipated power for the former compared to the latter, and
therefore results in a lower power handling capability for the
semi-regulated bus converter.
[0051] An alternate approach, newly described here, is the
quasi-regulated bus converter. As mentioned earlier, this bus
converter operates as a non-regulated bus converter over a portion
of the range of the source voltage, but then regulates its output
over the rest of the range. In general, when the quasi-regulated
converter is regulating its output, it may do so with or without a
droop characteristic.
[0052] The portion of the source range over which the bus converter
is non-regulating may be the low end of the range, the high end of
the range, or some other portion, as depicted in FIG. 3, depending
on the design of the converter. FIG. 3a depicts the case where the
non-regulating range is at the lower end of the total range of
V.sub.S. FIG. 3b depicts the case where the non-regulating range is
at the higher end. And FIG. 3c depicts the case where the
non-regulating range is in the middle of the total range of
V.sub.S.
[0053] As one example, a quasi-regulated bus converter could have
the same full-bridge topology shown in FIG. 2. Since this topology
is inherently a down-converter when it is regulating, the converter
would be designed to be non-regulating over the lower end of the
source range, and then regulating, or semi-regulating, over the
higher end of the range.
[0054] Assume the source voltage ranges from 36V to 100V and that
the P.O.L.'s can tolerate an intermediate voltage that varies
between 7V and 15V. If the transformer is given a turns-ratio of
4:1, then when the quasi-regulated bus converter is operated in a
non-regulating manner where its duty cycle is nearly 100% (except
for the short switch transitions), the intermediate voltage V.sub.I
will be approximately 1/4 that of the source voltage V.sub.S, minus
the droop. Assume that this mode of operation exists whenever the
source voltage is in the 36V to 56V range. This would result in an
output voltage that varies from 9V to 14V at zero bus converter
output current. If we consider the droop, the output voltage might
vary from 8.5V to 14V over the full range of source voltage and bus
converter output current. This range can be handled by the P.O.L.'s
mentioned above with appropriate margins at both ends of the
range.
[0055] Once the source voltage gets above 56V, the control circuit
220 of the quasi-regulated bus converter will reduce the duty cycle
appropriately to keep the output voltage from rising any higher
than, say, 14V. The control strategy during this range of source
voltage could be the same as is used in the semi-regulating bus
converter, in which case the output voltage will display a droop
characteristic in this mode of operation, as well. Just as for the
semi-regulated bus converter, this droop could be reduced by making
use of a signal indicative of current (and of temperature, for
further accuracy). Or the control strategy could, if desired,
create a tightly regulated output by feeding back a signal from the
output. The former approach is simpler, cheaper, and all that is
needed for many IBA applications.
[0056] The advantages gained by using the quasi-regulated bus
converter approach instead of the semi-regulated bus converter are
several. First, the turns-ratio of the transformer is 4:1 instead
of 3:1. This reduces both the current levels in the primary side of
the circuit and the voltage stresses on the secondary side by
25%.
[0057] Second, the maximum freewheeling period will be only 44% of
the half-cycle period for this example of the quasi-regulated bus
converter, as compared to a 64% value for the corresponding
semi-regulated bus converter. This permits the output inductor to
be reduced by approximately 33% in value.
[0058] Because V.sub.I can go as low as 9V for the example given
above, the output inductor of the quasi-regulated bus converter
must carry a higher maximum dc current than does the inductor in
the semi-regulated bus converter whose output voltage remains at
12V (ignoring droop in both cases). For instance, if we assume the
output power of the bus converter is 240W, the quasi-regulated bus
converter's inductor would have to carry a dc current of 27A;
whereas, the semi-regulated bus converter's inductor would have to
carry only 20A. This would appear to require that the inductor of
the quasi-regulated bus converter be designed to store much more
peak energy, even though its inductance value is smaller.
[0059] However, it is possible to let the quasi-regulated bus
converter's inductor saturate at the higher current levels
associated with V.sub.S below 56V and V.sub.I therefore falling
below 14V (again, ignoring droop) since for that condition, the
converter is being operated at nearly 100% duty cycle and very
little output inductance is therefore required. The residual
inductance that remains after the core saturates will typically be
sufficient. Doing this makes the quasi-regulated bus converter's
output inductor much smaller than can be achieved with the
semi-regulated bus converter.
[0060] Similarly, the input filter inductor 219 could be made
physically smaller by allowing it to saturate when V.sub.S is low
enough such that that quasi-regulated bus converter is operating at
nearly 100% duty cycle. In this condition there is much less ripple
caused at the input compared to when there is a freewheeling
period, and so less input filter inductance is needed.
[0061] An additional advantage of the quasi-regulated bus converter
is the fact that it avoids the switching losses of recovering from
a freewheeling period over a significant portion of the range of
the source voltage. For instance, in the example given above, when
V.sub.S is low and the currents are therefore relatively high, the
quasi-regulated bus converter is operating with nearly 100% duty
cycle, which keeps the switching losses small just when the
conduction losses are at their highest. At higher values of
V.sub.S, when the duty cycle is reduced below 100% and the
switching losses increase, the currents are then lower so that the
conduction losses are smaller. Overall, the total dissipation in
the semiconductor devices is reduced compared to the semi-regulated
bus converter for any given operating point.
[0062] Overall, holding all other things constant, a
quasi-regulated bus converter able to handle a source voltage range
of 36V to 100V will be about 96% efficient, as compared to a
semi-regulated bus converter which would be only 94% efficient.
This results in only two-thirds the power dissipation for the same
output power, and will permit a higher power density
capability.
[0063] The description of the quasi-regulated bus converter
presented above has been based on a power circuit topology that is
an isolated down-converter. It is also possible to base a
quasi-regulated bus converter on a topology that is an isolated
up-converter, such as a center-tapped, push-pull topology. For this
topology, the control strategy might be to maintain 100% duty cycle
for the high end of the source voltage, and then reduce the duty
cycle (and therefore increase the output voltage) when the source
voltage is at the lower end of its range.
[0064] Similarly, one skilled in the art could configure a
quasi-regulated bus converter based on an isolated up-down
converter. For this topology the control strategy might be to
maintain 100% duty cycle for the middle portion of the range of the
source voltage, and then regulate at both the low and the high end
of the range.
[0065] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
spirit and scope of the invention as defined by the appended
claims. Those skilled in the art will recognize or be able to
ascertain using no more than routine experimentation many
equivalents to the specific embodiments of the invention described
specifically herein. Such equivalents are intended to be
encompassed in the scope of the claims. For instance, other
topologies besides the full-bridge topology, such as the
half-bridge topology and others known in the art could be used. In
addition, the ideas presented above for isolated bus converters
could also be used for bus converters based on non-isolated power
circuit topologies.
* * * * *