U.S. patent application number 12/907188 was filed with the patent office on 2011-07-21 for power converter with isolated and regulation stages.
This patent application is currently assigned to SynQor, Inc.. Invention is credited to Richard W. Farrington, Martin F. Schlecht.
Application Number | 20110176333 12/907188 |
Document ID | / |
Family ID | 46324325 |
Filed Date | 2011-07-21 |
United States Patent
Application |
20110176333 |
Kind Code |
A1 |
Schlecht; Martin F. ; et
al. |
July 21, 2011 |
Power Converter with Isolated and Regulation Stages
Abstract
In a power converter, the duty cycle of a primary winding
circuit causes near continuous flow of power through the primary
and secondary winding circuits during normal operation. By
providing no regulation during normal operation, a very efficient
circuit is obtained with a synchronous rectifier in the secondary
operating at all times. However, during certain conditions such as
start up or a short-circuit, the duty cycle of the primary may be
reduced to cause freewheeling periods. A normally non-regulating
isolation stage may be followed by plural non-isolating regulation
stages. To simplify the gate drive, the synchronous rectifiers may
be allowed to turn off for a portion of the cycle when the duty
cycle is reduced. A filter inductance of the secondary winding
circuit is sufficient to minimize ripple during normal operation,
but allows large ripple when the duty cycle is reduced. By
accepting large ripple during other than normal operation, a
smaller filter inductance can be used.
Inventors: |
Schlecht; Martin F.;
(Lexington, MA) ; Farrington; Richard W.; (Heath,
TX) |
Assignee: |
SynQor, Inc.
Boxborough
MA
|
Family ID: |
46324325 |
Appl. No.: |
12/907188 |
Filed: |
October 19, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11901241 |
Sep 14, 2007 |
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12907188 |
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11407699 |
Apr 20, 2006 |
7272021 |
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11901241 |
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10729430 |
Dec 5, 2003 |
7050309 |
|
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11407699 |
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10812314 |
Mar 29, 2004 |
7072190 |
|
|
11901241 |
|
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10359457 |
Feb 5, 2003 |
6731520 |
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10812314 |
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09821655 |
Mar 29, 2001 |
6594159 |
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10359457 |
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09417867 |
Oct 13, 1999 |
6222742 |
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09821655 |
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09012475 |
Jan 23, 1998 |
5999417 |
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09417867 |
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60431673 |
Dec 6, 2002 |
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60036245 |
Jan 24, 1997 |
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Current U.S.
Class: |
363/17 |
Current CPC
Class: |
Y02B 70/1475 20130101;
Y02B 70/10 20130101; H02M 3/33592 20130101; H02M 3/33561
20130101 |
Class at
Publication: |
363/17 |
International
Class: |
H02M 3/335 20060101
H02M003/335 |
Claims
1. A power converter system comprising: a normally non-regulating
isolation stage comprising: a primary winding circuit; a secondary
winding circuit coupled to the primary winding circuit, the
secondary winding circuit comprising a secondary transformer
winding in series with a controlled rectifier having a parallel
uncontrolled rectifier; and a control circuit which controls duty
cycle of the primary winding circuit, the duty cycle causing
substantially uninterrupted flow of power through the primary and
secondary winding circuits during normal operation; and a plurality
of non-isolating regulation stages, each receiving the output of
the isolation stage and regulating a regulation stage output.
2-48. (canceled)
Description
RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. application Ser.
No. 11/901,241, filed Sep. 14, 2007, which is a Continuation of
U.S. application Ser. No. 11/407,699, filed Apr. 20, 2006, now U.S.
Pat. No. 7,272,021, which is a Continuation-in-Part of U.S.
application Ser. No. 10/729,430, filed on Dec. 5, 2003, now U.S.
Pat. No. 7,050,309, which claims the benefit of U.S. Provisional
Application No. 60/431,673, filed Dec. 6, 2002 and a
Continuation-in-Part to U.S. application Ser. No. 10/812,314, filed
Mar. 29, 2004, now U.S. Pat. No. 7,072,190, which is a continuation
of application Ser. No. 10/359,457, filed Feb. 5, 2003, now U.S.
Pat. No. 6,731,520, which is a continuation of application Ser. No.
09/821,655, filed Mar. 29, 2001, now U.S. Pat. No. 6,594,159, which
is a divisional of application Ser. No. 09/417,867, filed Oct. 13,
1999, now U.S. Pat. No. 6,222,742, which is a divisional of
09/012,475, filed Jan. 23, 1998, now U.S. Pat. No. 5,999,417, which
claims the benefit of U.S. Provisional Application 60/036,245 filed
Jan. 24, 1997.
[0002] The entire teachings of the above applications are
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0003] This invention pertains to switching power converters.
A specific example of a power converter is a DC-DC power supply
that draws 100 watts of power from a 48 volt DC source and converts
it to a 5 volt DC output to drive logic circuitry. The nominal
values and ranges of the input and output voltages, as well as the
maximum power handling capability of the converter, depend on the
application.
[0004] It is common today for switching power supplies to have a
switching frequency of 100 kHz or higher. Such a high switching
frequency permits the capacitors, inductors, and transformers in
the converter to be physically small. The reduction in the overall
volume of the converter that results is desirable to the users of
such supplies.
[0005] Another important attribute of a power supply is its
efficiency. The higher the efficiency, the less heat that is
dissipated within the supply, and the less design effort, volume,
weight, and cost that must be devoted to remove this heat. A higher
efficiency is therefore also desirable to the users of these
supplies.
[0006] A significant fraction of the energy dissipated in a power
supply is due to the on-state (or conduction) loss of the diodes
used, particularly if the load and/or source voltages are low (e.g.
3.3, 5, or 12 volts). In order to reduce this conduction loss, the
diodes are sometimes replaced with transistors whose on-state
voltages are much smaller. These transistors, called synchronous
rectifiers, are typically power MOSFETs for converters switching in
the 100 kHz and higher range.
[0007] The use of transistors as synchronous rectifiers in high
switching frequency converters presents several technical
challenges. One is the need to provide properly timed drives to the
control terminals of these transistors. This task is made more
complicated when the converter provides electrical isolation
between its input and output because the synchronous rectifier
drives are then isolated from the drives of the main, primary side
transistors. Another challenge is the need to minimize losses
during the switch transitions of the synchronous rectifiers. An
important portion of these switching losses is due to the need to
charge and discharge the parasitic capacitances of the transistors,
the parasitic inductances of interconnections, and the leakage
inductance of transformer windings.
SUMMARY OF THE INVENTION
[0008] In certain embodiments of the invention, a power converter
system comprises a normally non-regulating isolation stage and a
plurality of non-isolating regulation stages, each receiving the
output of the isolation stage and regulating a regulation stage
output. The non-regulating isolation stage may comprise a primary
winding circuit and a secondary winding circuit coupled to the
primary winding circuit. The secondary winding circuit comprises a
secondary transformer winding in series with a controlled rectifier
having a parallel uncontrolled rectifier. A control circuit
controls duty cycle of the primary winding circuit, the duty cycle
causing substantially uninterrupted control of power through the
primary and secondary winding circuits during normal operation.
[0009] The duty cycle of the primary winding circuit may be reduced
to cause freewheeling periods in other than normal operation. Duty
cycle might be reduced during the startup or to limit current and
may be a function of sensed current.
[0010] The primary winding circuit may include a single primary
winding, and the secondary winding circuit may include plural
secondary windings coupled to the single primary winding. The
primary winding may be in a full bridge circuit having a capacitor
in series with the primary winding. In one implementation of the
full bridge circuit, during freewheeling, only two top FETs or two
bottom FETs are turned off.
[0011] A control signal of the controlled rectifier may be derived
from a waveform of the secondary winding circuit. The secondary
winding circuit may include a filter inductor and have a capacitor
coupled across its output.
[0012] The isolation stage may be a step down stage. For example,
it may provide an output of about 12 volts from a DC power source
that provides a voltage varying over the range of 36-75 volts. The
regulation stages may be down converters to provide outputs of
voltage levels to drive logic circuitry. A regulation stage output
may, for example, be 5 volts or less, such as 3.3 volts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0014] FIG. 1 shows a full-bridge, single-transformer, voltage-fed
isolation stage that incorporates concepts of the '417 patent.
[0015] FIG. 2 illustrates the addition of a capacitor to the
primary winding of FIG. 1.
[0016] FIG. 3 illustrates the addition of an output filter inductor
to the circuit of FIG. 2.
[0017] FIGS. 4A-4C show a control circuit for the circuits of FIGS.
1-3 and embodying the present invention, and FIG. 4D shows an
alternative to the circuit of FIG. 4B.
[0018] FIG. 5 shows an Intermediate Bus Architecture (IBA)
implementation of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] A description of preferred embodiments of the invention
follows.
[0020] FIG. 1 shows a full-bridge, single-transformer, voltage-fed
isolation stage that incorporates synchronous rectification and the
concepts of the '417 patent. The operation of this isolation stage
is as follows. For the first half of the cycle, MOSFETs 101 and 103
are turned on while MOSFETs 102 and 104 are left off, and the
voltage V.sub.B is applied positively (according to the "dot"
convention) across the transformer's primary winding 107. This
voltage, modified by the transformer's turns-ratio, appears across
the secondary windings with the appropriate polarity. Power flows
into the transformer's primary winding, and out of the first
secondary winding 108 to the output. The voltage at Node B is
approximately twice the output voltage, and it causes the MOSFET
synchronous rectifier 105 to be turned on. The voltage at Node A is
therefore slightly below ground, which causes the MOSFET
synchronous rectifier 106 to be turned off. These states of the
rectifier switches are consistent with the power flowing out of the
first secondary winding.
[0021] During the second half of the cycle, MOSFETs 102 and 104 are
turned on while MOSFETs 101 and 103 are left off, and the voltage
V.sub.B is applied negatively across the transformer's primary
winding. This negative polarity causes MOSFET 106 to be turned on,
MOSFET 105 to be turned off, and power to flow into the primary
winding and out of the second secondary winding 109 to the output
across capacitor 110.
[0022] The secondary windings are not tightly coupled to each
other, as indicated with the parasitic inductances 113 and 114, to
achieve the advantages discussed in the '417 patent. A similar
setup was shown in the topology of FIG. 9 of the '417 patent since
it also used a single transformer.
[0023] Care must be taken in this isolation stage topology to
insure that the magnetizing inductance of the transformer does not
saturate. One way to do this is to place a large capacitor 215 in
series with the primary winding, as shown in FIG. 2. This capacitor
will assume a dc voltage across it that counters any imbalance
there may be in the positive and negative volt-seconds of the
waveforms created by MOSFETs 101-104. Alternatively, several
well-known techniques to sense the magnetizing inductor's current
could be used to modify the durations of the first and second
halves of the cycle.
[0024] The filters at the output of the isolation stages in the
'417 patent are composed of one or more capacitive and inductive
elements. When the isolation stage is voltage-fed, it may be
desirable to have the output filter begin with an inductor 316, as
shown in FIG. 3. One benefit this approach provides is that the
voltage-fed isolation stages can now be operated with a variable
duty cycle control strategy to provide a soft-start capability or
to limit current flow in a short-circuit condition. These functions
could be provided by the regulation stages in the topologies
depicted in the '417 patent, but if the isolation stage is not
combined directly with a regulation stage in a single product, then
it may be desirable to include these functional capabilities in the
isolation stage, as well.
[0025] Under variable duty cycle control, the percentage of the
overall cycle (the duty cycle) that MOSFETs 101 and 103 (or MOSFETs
102 and 104) conduct is reduced from the 50% value described above.
For the remaining, freewheeling fraction of the half-cycle, either
all of the primary-side MOSFETs are turned off, or at least the two
top MOSFETs 101 and 104 or the two bottom MOSFETs 102 and 103 are
turned off. During the freewheeling part of the cycle, both diodes
111 and 112 conduct the current flowing through inductor 316, and
the voltage across the transformer windings is approximately zero.
As is well know, this additional portion of the cycle permits the
output voltage to be less than V.sub.B divided by the transformer's
turns-ratio. How much less depends on the duty cycle. Since during
normal operation the isolation stage is operated at a fixed duty
cycle in which power is always flowing from input to output (except
during the brief switch transitions), the value of inductor 316 can
be relatively small to achieve an acceptable output ripple. This
reduces the size, cost, and power dissipation of this inductor
compared to what it might have been. During those times when the
isolation stage is operated under a variable duty cycle, the ripple
in the inductor current may then become large, but the larger
output voltage ripple that results can usually be tolerated for
start-up and short-circuit conditions.
[0026] As mentioned above, during the freewheeling part of the
cycle the diodes are carrying the inductor current. This is because
the gate drive scheme shown in FIG. 3 would cause the MOSFET
synchronous rectifiers to be off during this part of the cycle. The
additional power dissipation that occurs due to the higher on-state
voltage of the diodes compared to that of the MOSFETs can usually
be tolerated for the start-up and short-circuit conditions because
they are normally short in duration.
[0027] If the output voltage is high, then it may be desirable to
use a capacitive divider technique described in the '417 patent to
reduce the voltages applied to the gates of the MOSFET synchronous
rectifiers below that of the voltages appearing at Nodes A and B.
FIGS. 4A-4C show a circuit schematic of a product based, in part,
on the ideas presented here and in the '417 patent. The function of
the product is to provide isolation and a transformation of the
input voltage to the output voltage according to the turns-ratio of
the transformer. It does not, in its normal state of operation,
provide regulation. As such it is a very efficient product. One
example of its use is to convert a 48V input to a 12V output by
using a turns-ratio of 4:1. Since there is no regulation, if the
input voltage varies +/-10%, so too will the output voltage vary
+/-10%. In certain applications, this variation in the output is
acceptable, and well worth the very high efficiency of the
converter, which is 96% in this example.
[0028] In addition, since the converter of FIG. 4 does not provide
regulation, its output voltage demonstrates a droop characteristic.
By this it is meant that for any given input voltage, the output
voltage drops slightly as the output current increases. For
instance, the output voltage may drop 5% as the output current
varies from 0% to 100% of the rated maximum value. This droop
characteristic provides automatic current sharing between two or
more such converters that might be place in parallel.
[0029] Note in this schematic that the IC labeled U100 is a pulse
width modulator (PWM) control chip that is normally operated such
that the gate drive signals that pass through gate drivers U101 and
U105 give the fixed duty cycle operation of the full-bridge
described above. If the current sensing amplifier U104-A senses
that the current flowing on the primary side of the circuit exceeds
a threshold value, it commands the PWM control chip to reduce its
duty cycle by an amount determined by how large the current gets
above the threshold value. This provides a current limiting scheme
for the product that protects against a short-circuit
condition.
[0030] Note also that comparator U106-A senses the duty cycle
output of the PWM control chip, and compares it to a threshold. If
the duty cycle falls below this threshold value, the output of the
comparator causes the PWM control IC to shut down. The circuitry
around this comparator, including transistors Q111 and Q114,
provides a latching mechanism such that the PWM control IC remains
off once this condition is observed.
[0031] As described in the '417 patent and illustrated in FIG. 5,
in some situations, it may be desirable to place the isolation
stage first in the power flow, and to have the regulation stage
follow. For example, when there are many outputs sharing the total
power, the circuit might be configured as one isolation/step-down
(or step-up) stage 501 followed by several DC-DC switching or
linear regulators 503.
[0032] The DC power source to the full bridge primary circuit may
provide a voltage that varies over the range of 36-75 volts. The
output of the isolation stage may be 12 volts, and the regulation
stage output may be 5 volts or less. In particular, the regulation
stage output may be 3.3 volts. Typically, the regulation stage
output is of a voltage level to drive logic circuitry.
[0033] Because the isolation stage uses synchronous rectifiers, it
is possible for the current to flow from the output back to the
input if, for a given input voltage and duty cycle, the output
voltage is too high. This condition might, for example, occur
during start-up where the duty cycle is slowly raised from its
minimum value to its maximum value, but the output capacitor is
already pre-charged to a high voltage, perhaps because it had not
fully discharged from a previous on-state condition. It might also
occur when the input voltage suddenly decreases while the output
voltage remains high due to the capacitors connected to this
node.
[0034] The negative current that results could cause destructive
behavior in the converter or in the system if it is not kept small
enough.
[0035] One way to avoid this condition is to turn off either just
the top two primary-side MOSFETs 101 and 104, or just the bottom
two primary-side MOSFETs 102 and 103, during the freewheeling
period, as described above. By leaving the other two primary-side
MOSFETs on, the voltage across the primary and secondary windings
of the transformer is guaranteed to be essentially zero during the
freewheeling period. Given the gate drive scheme shown in FIG. 3,
this, in turn, ensures the controlled rectifiers will be off during
this part of the cycle.
[0036] With the controlled rectifiers off, negative current cannot
flow during the freewheeling period. Negative current can flow
during the non-freewheeling part of the cycle, but since it must
always start at zero, its value is limited to the ripple that the
inductor permits, which is typically small enough to not cause a
problem. This negative current will be reset to zero at the start
of each freewheeling period, either by providing a clamp circuit,
as shown in FIG. 4D, or by allowing the controlled rectifiers to
avalanche and act as their own clamp. Since the clamp circuit must
only work for a short duration, it need not recover its absorbed
energy and so can be simple, such as the one shown in FIG. 4D.
[0037] To limit the negative current, the isolation stage could
operate in a reduced duty-cycle mode. While the control circuit is
typically designed to achieve this mode during start-up and
shutdown of the isolation stage, it is not the normal mode of
operation. If, during normal operation, the input voltage drops
suddenly, a large negative current can flow because there are no
freewheeling periods.
[0038] To avoid this condition, the current flowing through the
converter can be sensed, either by sensing the load current
directly, or by sensing a signal indicative of the load current.
When the load current falls below some threshold, the duty cycle of
the isolation stage can be reduced from its maximum value to
provide freewheeling periods. Given the drive scheme for the
primary-side MOSFETs outlined above, the negative current will then
be kept small since the controlled rectifiers will be turned off
for a portion of the cycle.
[0039] 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
scope of the invention encompassed by the appended claims. For
example, whereas the Figures show the secondary side rectification
circuit arranged in a center tapped configuration with two
secondary windings and two synchronous rectifiers, as is well known
it could be a full wave rectification configuration. One could use
a full-bridge rectification circuit in which there is only one
secondary winding and four synchronous rectifiers. Such a circuit
reduces voltage stress on the synchronous rectifiers when they are
off by a factor of two during normal operation of the
converter.
* * * * *