U.S. patent number RE36,098 [Application Number 08/866,020] was granted by the patent office on 1999-02-16 for optimal resetting of the transformer's core in single-ended forward converters.
This patent grant is currently assigned to VLT Corporation. Invention is credited to Patrizio Vinciarelli.
United States Patent |
RE36,098 |
Vinciarelli |
February 16, 1999 |
Optimal resetting of the transformer's core in single-ended forward
converters
Abstract
The transformer's core in single ended forward converters is
reset by a "magnetizing current mirror" consisting of a capacitor
in series with an auxiliary switch which, during the OFF period of
the primary switch, couples the capacitor to one of the
transformer's windings to form a resonant circuit with the
transformer's magnetizing inductance. The resonant circuit recycles
the transformer's magnetizing energy by creating a mirror image of
the magnetic flux between ON periods. This maximizes the flux swing
available within a given core. The voltage stress on the primary
switch is minimized as the voltage across the switch during the OFF
period is approximately constant and automatically tailored to
avoid dead time for arbitrary values of the switch duty cycle.
Inventors: |
Vinciarelli; Patrizio (Boston,
MA) |
Assignee: |
VLT Corporation (San Antonio,
TX)
|
Family
ID: |
26994547 |
Appl.
No.: |
08/866,020 |
Filed: |
May 30, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
538351 |
Oct 3, 1995 |
|
|
|
Reissue of: |
345799 |
Feb 4, 1982 |
04441146 |
Apr 3, 1984 |
|
|
Current U.S.
Class: |
363/20;
363/56.11 |
Current CPC
Class: |
H02M
3/33576 (20130101); H02M 3/33569 (20130101); H02M
3/33538 (20130101); Y02B 70/10 (20130101) |
Current International
Class: |
H02M
3/335 (20060101); H02M 3/24 (20060101); H02M
003/335 () |
Field of
Search: |
;363/18-21,56,97
;323/312,313,318,324 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0350297 A2 |
|
Jan 1990 |
|
EP |
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56-88670 |
|
Mar 1981 |
|
JP |
|
56-141773 |
|
Nov 1981 |
|
JP |
|
55-6887 |
|
May 1989 |
|
JP |
|
892614 |
|
Dec 1981 |
|
SU |
|
959234 |
|
Sep 1982 |
|
SU |
|
989552 |
|
Jan 1983 |
|
SU |
|
1003067 |
|
Mar 1983 |
|
SU |
|
1107233 A |
|
Aug 1984 |
|
SU |
|
1136275 A |
|
Jan 1985 |
|
SU |
|
1224921 A |
|
Apr 1986 |
|
SU |
|
1367112 A1 |
|
Jan 1988 |
|
SU |
|
892614 |
|
Dec 1991 |
|
SU |
|
2 097 606 |
|
Nov 1982 |
|
GB |
|
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|
Primary Examiner: Berhane; Adolf
Attorney, Agent or Firm: Fish & Richardson P.C.
Parent Case Text
.Iadd.This application is a continuation of application Ser. No.
08/538,351, filed Oct. 3, 1995, now abandoned, which is a reissue
of Ser. No. 06/345,799 filed on Feb. 4, 1982, U.S. Pat. No.
4,441,146.
This application, Ser. No. 08/866,020, filed May 30, 1997, and
divisional application, Ser. No. 09/052,137, filed Mar. 31, 1998,
are both reissues of U.S. Pat. No. 4,441,146..Iaddend.
Claims
I claim:
1. In a single ended forward converter in which energy is
transferred from a primary winding to a secondary winding of a
transformer during the ON period of a primary switch, circuitry for
recycling the magnetizing energy stored in said transformer to
reset it during the OFF period of said primary switch,
comprising:
a storage capacitor;
an auxiliary switch connected in series with said storage
capacitor;
a switch control circuit operating said auxiliary switch in
accordance with a control logic such that (a) said auxiliary switch
is opened prior the ON period of said primary switch, (b) said
auxiliary switch remains open throughout the ON period of said
primary switch, (c) said auxiliary switch is closed after the ON
period of said primary switch.
2. The transformer resetting apparatus of claim 1 wherein said
circuitry is connected in parallel with said secondary winding.
3. The transformer resetting apparatus of claim 1 wherein said
circuitry is connected in parallel with said primary winding.
4. The transformer resetting apparatus of claim 1 wherein said
transformer further includes an auxiliary winding, wherein said
circuitry is connected in parallel with said auxiliary winding.
5. The transformer resetting apparatus of claim 1 wherein said
auxiliary switch is a MOSFET transistor with an integral reverse
diode.
6. In a single ended forward converter in which energy is
transferred across a transformer during the ON period of a primary
switch, an apparatus for recycling the magnetizing energy of said
transformer during the OFF period of said primary switch,
comprising:
a storage capacitor;
auxiliary switching means to selectively couple said storage
capacitor to said transformer, wherein said storage capacitor and
said transformer form a resonant circuit during the OFF period of
said primary switch. .Iadd.
7. The circuitry of claim 1 wherein said auxiliary switch is always
closed to currents in one direction and selectively closed to
currents in another direction..Iaddend..Iadd.8. The circuitry of
claim 1 wherein said auxiliary switch is adapted to selectively
couple said storage capacitor and said transformer to form a
resonant circuit during the OFF period of said primary
switch..Iaddend..Iadd.9. The circuitry of claim 1 wherein said
circuitry is connected to a winding in the
transformer..Iaddend..Iadd.10. The circuitry of claim 9 wherein
said winding is a primary winding..Iaddend..Iadd.11. The circuitry
of claim 9 wherein said winding is a secondary
winding..Iaddend..Iadd.12. The circuitry of claim 9 wherein said
winding is an auxiliary winding..Iaddend..Iadd.13. The circuitry of
claim 9, 10, 11 or 12 wherein said auxiliary switch is adapted to
selectively couple said storage capacitor and said transformer to
form a resonant circuit during the OFF period of said primary
switch..Iaddend..Iadd.14. The circuitry of claim 1 wherein an
equivalent circuit of said converter, during the OFF period of said
primary switch, comprises a resonant circuit formed by said
capacitor and a winding of said transformer..Iaddend..Iadd.15. The
circuitry of claim 14 wherein said winding comprises a primary
winding..Iaddend..Iadd.16. The circuitry of claim 14 wherein said
winding comprises a secondary winding..Iaddend..Iadd.17. The
circuitry of claim 14 wherein said winding comprises a auxiliary
winding..Iaddend..Iadd.18. The circuitry of claim 2 wherein said
auxiliary switch is adapted to selectively couple said storage
capacitor and said transformer to form a resonant circuit during
the OFF period of said primary
switch..Iaddend..Iadd.19. The circuitry of claim 3 wherein said
auxiliary switch is adapted to selectively couple said storage
capacitor and said transformer to form a resonant circuit during
the OFF period of said primary switch..Iaddend..Iadd.20. The
circuitry of claim 4 wherein said auxiliary switch is adapted to
selectively couple said storage capacitor and said transformer to
form a resonant circuit during the OFF period of said primary
switch..Iaddend..Iadd.21. The circuitry of claim 1, 7, 8, 9, 10,
11, 12, 18, 19 or 20 wherein said auxiliary switch comprises a
MOSFET transistor with an integral reverse diode..Iaddend..Iadd.22.
The circuitry of claim 1, 7, 8, 9, 10, 11, 12, 18, 19 or 20 wherein
the ON period of said primary switch begins a small delay period
after opening said auxiliary switch..Iaddend..Iadd.23. The
circuitry of claim 22 wherein said small delay period accommodates
charging and discharging of capacitances in said
converter..Iaddend..Iadd.24. The circuitry of claim 23 wherein said
capacitances comprise parasitic capacitances..Iaddend..Iadd.25. The
circuitry of claim 23 wherein said capacitances are associated with
said auxiliary switch..Iaddend..Iadd.26. The circuitry of claim 23
wherein said capacitances are associated with said primary
switch..Iaddend..Iadd.27. The circuitry of claim 23 wherein said
capacitances are associated with said primary
winding..Iaddend..Iadd.28. The circuitry of claim 23 wherein said
parasitic capacitances are associated with said secondary
winding..Iaddend..Iadd.29. In a single ended forward converter in
which energy is transferred across a transformer during the ON
period of a primary switch, an apparatus for recycling the
magnetizing energy of said transformer during the OFF period of
said primary switch, comprising:
a storage capacitor;
auxiliary switching means to selectively couple said storage
capacitor to said transformer, wherein said storage capacitor and
said transformer form a resonant circuit during the OFF period of
said primary switch; and
wherein the ON period of said primary switch begins a small delay
period after opening said auxiliary switching
means..Iaddend..Iadd.30. The apparatus of claim 29 wherein said
auxiliary switching means is always closed to currents in one
direction and selectively closed to currents in another
direction..Iaddend..Iadd.31. The apparatus of claim 29 wherein said
storage capacitor is connected to a winding of the
transformer..Iaddend..Iadd.32. The apparatus of claim 29 wherein
said auxiliary switching means is connected to a winding of the
transformer..Iaddend..Iadd.33. The apparatus of claim 29 wherein
said storage capacitor and said auxiliary switching means are
connected in parallel to a winding of the
transformer..Iaddend..Iadd.34. The apparatus of claim 31 wherein
said winding comprises a primary winding..Iaddend..Iadd.35. The
apparatus of claim 32 wherein said winding comprises a primary
winding..Iaddend..Iadd.36. The apparatus of claim 31 wherein said
winding comprises a secondary winding..Iaddend..Iadd.37. The
apparatus of claim 32 wherein said winding comprises a secondary
winding..Iaddend..Iadd.38. The apparatus of claim 31 wherein said
winding
comprises an auxiliary winding..Iaddend..Iadd.39. The apparatus of
claim 32 wherein said winding comprises an auxiliary
winding..Iaddend..Iadd.40. The apparatus of claim 29, 30, 31, 32,
33, 34, 35, 36, 37, 38, or 39 wherein said small delay period
accommodates charging and discharging of capacitances in said
converter..Iaddend..Iadd.41. The apparatus of claim 40 wherein said
capacitances comprise parasitic capacitances..Iaddend..Iadd.42. The
apparatus of claim 41 wherein said capacitances are associated with
said primary switch..Iaddend..Iadd.43. The apparatus of claim 41
wherein said capacitances are associated with said auxiliary
switching means..Iaddend..Iadd.44. The apparatus of claim 41
wherein said capacitances are associated with windings in the
transformer..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to DC-to-DC converters which process
electrical power from a source, at an input DC voltage, to deliver
it to a load, at an output DC voltage, by selectively connecting a
power transformer to the source and the load via solid state
switches. In particular, the invention relates to converters of the
forward type, in which the power transformer is simultaneously
connected to the source and the load. More particularly, the
invention relates to forward converters of the single ended type,
in which the power flow from source to load is controlled by a
single solid state switch.
2. Description of the Prior Art
This invention relates to the class of DC-to-DC converters which
incorporate the topology represented in FIG. 1. A converter in that
class is referred to as a "single ended forward" coverter because
power flow is gated by a single switch 10 and energy is transferred
forward, from the primary winding to the secondary winding of the
transformer 11, during the ON period of the switch 10.
Converters in this class present a unique problem, in that the
conversion topology does not inherently define the mechanism by
which the transformer's core is to be reset during the OFF period
of the switch. The solution to this problem is not unique, as
evidenced by the multiplicity of proposals found in the literature
which, in order to implement the reset function, complement the
topology represented in FIG. 1 by differing choices of auxiliary
circuitry. The differences are important since they affect the cost
of the converter, as well as its efficiency and power density.
The traditional approach, represented in FIG. 2a, has been to reset
the core via an auxiliary transformer winding connected with
inverted polarity in series with rectifier 13. The operation of
this reset mechanism is illustrated in FIG. 2b, where, in addition
to idealized component behavior, a one-to-one turn ratio between
auxiliary and primary windings has been assumed. This figure
exemplifies a sequence of two ON periods, separated by an OFF
period to enable the core to reset itself. The figure displays, as
functions of time, the state of the switch 10, the voltage V across
the switch, and the current I through the auxiliary winding.
The first ON period is given by the time interval between t.sub.1
and t.sub.2. During this interval, the voltage V across the switch
10 vanishes and the source voltage V.sub.o is impressed upon the
primary winding. The magnetizing inductance controls the slope of
the magnetizing current, flowing in the primary winding, and of the
magnetizing energy which accumulates in the transformer's core. The
current I vanishes, as the rectifier 13 is reverse biased, in a
blocking state, and thus keeps the auxiliary winding
inoperative.
At time t.sub.2, the opening of the switch 10 interrupts current
flow in the primary winding. Neglecting the effects of leakage
inductance between primary and auxiliary windings, the voltage V is
clamped to 2 V.sub.o as the rectifier 13 becomes forward biased and
begins to conduct the magnetizing current. The current I through
the auxiliary winding is then equal to the peak value I.sub.p of
the magnetizing current. Following time t.sub.2, I decays as
magnetizing energy is returned to the voltage source V.sub.o. At
time t.sub.3, the recycling of the magnetizing energy is completed,
the current I vanishes, and, neglecting hysteresis, the magnetic
flux through the transformer's core is reset to zero. The time
interval between t.sub.2 and t.sub.3 is the core reset period.
Having assumed a one-to-one primary to auxiliary turn ratio, this
period equals the ON period t.sub.2 -t.sub.1.
The remainder of the cycle, between times t.sub.3 and t.sub.4, is
the "dead" period. In this period, the switch 10 remains open, the
voltage V across it equals the source voltage V.sub.o, and the
current I vanishes. The circuit in FIG. 2a is not efficiently
functional during dead time.
The relative duration of the dead period depends upon the duty
cycle of the switch 10 (assumed to be 33% in FIG. 2b). At 50% duty
cycle, the dead period vanishes. Operation beyond 50% duty cycle
would lead to saturation of the transformer's core and
(catastrophic) converter failure.
Thus, the traditional reset mechanism, represented in FIG. 2a,
presents an inherent limitation in the available duty cycle range.
This is a significant drawback as it impairs the ability of the
converter to regulate against wide variations in the source voltage
or in the load. Another drawback of the traditional reset method is
that allowed values of the duty cycle are in general associated
with a non-vanishing dead time. The existence of a dead time causes
the switch 10 to experience a peak voltage greater than is in
principle necessary to reset the core in the time interval between
ON periods.
Similar limitations apply, to a varying degree, to any other reset
mechanism which involves a variable dead time to accomodate
variations in the switch duty cycle. Reset methods falling into
this category are found in S. Hayes, Proceedings of Powercon 8,
Power Concepts Inc. 1981 and in R. Severns, ibid..
To avoid these limitations, a different approach to the resetting
of the transformer's core in single ended forward converters was
proposed by S. Clemente, B. Pelly and R. Ruttonsha in "A Universal
100 KHz Power Supply Using a Single HEXFET", International
Rectifier Corporation Applications Note AN-939, December 1980.
These authors suggest a capacitor-resistor-diode network, as
represented in FIG. 3a. The network clamps the switch to the
minimal peak voltage consistent with a given source voltage and
switch duty cycle, eliminating the need for dead time while
allowing for a wide range of duty cycles. Attainment of these
design goals is actually dependent upon component characteristics
and values. In particular, the resistor 15 must be sized small
enough so that the transformer's magnetizing current does not ever
vanish.
With this assumption, the operation of this reset circuit is
illustrated in FIG. 3b. As in the example given to illustrate the
traditional reset mechanism, a sequence of two ON periods separated
by an OFF period, with a 33% duty cycle is considered. The figure
displays, as functions of time, idealized waveforms defining the
state of the switch 10, the voltage V across it, and the current I
through the rectifier 13. During the OFF period, the latter
coincides with the transformer's magnetizing current.
As exhibited in FIG. 3b, the voltage V across the switch 10 is now
a rectangular waveform with a peak value equal to 1.5 V.sub.o. The
current I through the rectifier 13 is a trapezoidal waveform which,
during the OFF period, decays from a peak value (I.sub.p +I.sub.o)
to a minimum value I.sub.o, a non-negative function of V.sub.o and
the duty cycle D.
A comparison of the voltage waveform of FIG. 3b with the
corresponding one in FIG. 2b emphasizes the main advantage of the
capacitor-resistor-diode clamp: a reduction in the voltage stress
applied to the switch 10. A related advantage is the elimination of
bounds resulting from core saturation on the duty cycle range,
enabling the converter to remain functional over wider ranges of
input voltage and output load. A further advantage is the avoidance
of auxiliary transformer windings which simplifies transformer
construction. Unfortunately, while attaining these benefits, the
reset mechanism of FIG. 3a compromises the converter's efficiency
and power density.
The reduction in the efficiency of the conversion process arises
principally from the dissipation of magnetizing energy accumulated
in the transformer during the ON period. Instead of being recycled,
this energy is converted into heat by the clamp circuit. This power
waste is significant, particularly in an otherwise efficiency
mindful conversion system.
The reduction in power density results mainly from an increase in
the size of the transformer which is rendered necessary by a
decrease in the available dynamic flux swing for the magnetic
material making up the transformer's core. This is evidenced in
FIG. 3b by the quantity referred to as I.sub.o, which represents a
non-negative, static component of the magnetization current. The
component shifts the peak value of the magnetizing current, leading
to an excitation of the transformer's core which brings the
magnetic material closer to saturation. The consequent decrease in
available flux swing reduces the power handling capability per unit
volume at a given frequency of a given core and, with it, the
converter's power density.
From these points of view, the traditional reset mechanism of FIG.
2a offers important, relative advantages. However, even from the
point of view of core utilization, it does not represent an optimal
reset mechanism, since the flux swing is still unipolar. This
unipolar character of the transformer's core excitation has often
been noted to be an inherent drawback of single ended forward
converters. In fact, it is not a general drawback of this class of
conversion topologies as it is only inherent to some reset
mechanisms which have been adopted to complement those
topologies.
These considerations suggest that the "optimal" reset mechanism for
single ended forward converters, yet to be invented, should
incorporate the following set of objectives:
it should be non-dissipative in nature, i.e. it should recycle the
core's magnetization energy;
it should maximize the available flux swing, i.e. it should lead to
a bipolar core excitation;
it should minimize the voltage stress on the switch, i.e. the
voltage waveform should be rectangular without involving a dead
period;
it should not introduce constraints on the switch duty cycle;
it should simplify transformer construction by eliminating the need
for auxiliary windings.
SUMMARY OF THE INVENTION
This invention provides new apparatus for resetting the
transformer's core in single ended forward converters. The
apparatus consists of a storage capacitor, an auxiliary solid state
switch (distinguished from the primary switch which controls the
converter's power flow), and of a switch control circuit. The
switch control circuit operates the auxiliary switch in its open
state during the converter's ON period, when the primary switch is
closed, and in its closed state during the converter's OFF period
when the primary switch is open. The auxiliary switch (operated by
such a control circuit) and storage capacitor are connected in
parallel with a transformer winding.
The apparatus defined above resets the transformer's core by
implementing the conceptual function of a "magnetizing current
mirror": it takes the magnetization at the end of the ON period and
creates a mirror image of it prior to the initiation of the
following conversion cycle. The image is created via the charging
and discharging of the storage capacitor which forms a resonant
circuit with the transformer's magnetizing inductance. The
capacitor is sized so that the period of this resonant circuit is
considerably greater than the conversion period. Consequently, the
capacitor's voltage and the voltage across the primary switch are
approximately constant during the OFF period.
The new apparatus provides optimal resetting of the transformer's
core in single ended forward converter topologies:
it is non-dissipative, as it recycles the core's magnetization
energy via intermediate storage in a resonant circuit;
it maximizes the available flux swing, as it creates a mirror image
of the magnetic flux between ON periods;
it minimizes the voltage stress on the (primary) switch, as it
applies to this switch during the OFF period an approximately
constant voltage which is automatically tailored to avoid dead
time;
it does not introduce constraints on the switch duty cycle due to
core saturation;
it simplifies transformer construction, as it can be implemented
without auxiliary windings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 defines the general class of single ended forward
converters.
FIG. 2a shows the auxiliary transformer winding which has been
traditionally employed to reset the core in single ended forward
converters.
FIG. 2b exemplifies the operation of the reset mechanism in FIG. 2a
by displaying a possible time sequence of states for the switch 10
and the corrresponding idealized waveforms for the voltage V,
across the switch 10, and the current I, in the auxiliary
transformer winding.
FIG. 3a shows a capacitor-resistor-diode network which has been
employed in the prior art as an alternative mechanism for resetting
the core in single ended forward converters.
FIG. 3b provides an example of the operation of the reset mechanism
in FIG. 3a analagous to that of FIG. 2b to allow for a direct
comparison of voltage and current waveforms.
FIG. 4a discloses a preferred embodiment of the new reset mechanism
for single ended forward converters, consisting of a "magnetizing
current mirror" connected across the transformer's secondary.
FIG. 4b discloses voltage and current waveforms useful in
describing the operation of the new reset mechanism.
FIG. 4c discloses a preferred implementation of the magnetizing
current mirror in which the auxiliary switch is realized in terms
of a MOSFET transistor and its integral reverse rectifier.
FIG. 4d discloses equivalent circuit diagrams characterizing the ON
and OFF periods of a single ended forward converter reset by a
magnetizing current mirror.
FIG. 4e discloses an embodiment of the new reset mechanism in which
the magnetizing current mirror is connected across the
transformer's primary.
FIG. 4f discloses yet another embodiment of the invention in which
the magnetizing current mirror is connected across an auxiliary
transformer winding.
FIG. 5 compares idealized waveforms exemplifying the time evolution
of the magnetic flux across the transformer's core in a single
ended forward converter reset by: (a) the traditional mechanism,
employing an auxiliary transformer winding; (b) the
capacitor-resistor-diode network; (c) the optimal reset mechanism,
utilizing a magnetizing current mirror.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 4a, the primary switch 10 selectively couples
the primary winding of transformer 11 to a source of voltage
V.sub.o. A rectifier 12 is connected in series with the secondary
winding of transformer 11 and is oriented to conduct a current
during the ON period of the primary switch 10. These are
conventional elements of a single ended forward converter. In order
to reset the transformer 11 during the OFF period of the primary
switch 10, these elements are complemented by a "magnetizing
current mirror."
The magnetizing current mirror comprises the storage capacitor 20,
the auxiliary switch 21 and the switch control circuit 22. The
capacitor 20 and the switch 21 are connected in series. The switch
21 is operated by the control circuit 22 in accordance with a
control logic requiring that the auxiliary switch 21 be opened
prior to the ON period of the primary switch 10, and closed after
this period. To accomplish this function, as suggested by the
arrows at the bottom of the box representing the switch control
circuit 22, this circuit is interfaced with primary switch control
circuitry, not represented in the figure. The implementation of the
control circuit and of its interface can be realized in a number of
ways which will become obvious to those skilled in the art.
In FIG. 4a, the magnetizing current mirror is connected in parallel
with the secondary winding of transformer 11. Assuming an equal
number of turns between this and the primary winding, and
neglecting the effects of leakage inductance between primary and
secondary windings, and parasitic effects including the ones
associated with winding capacitance or the capacitance of non-ideal
hardware realizations of the primary switch 10 and auxiliary switch
21, the operation of the magnetizing current mirror as a reset
mechanism is illustrated by an example in FIG. 4b.
As in the examples of FIG. 1b and 2b which were used to
characterize the operation of reset mechanisms found in the prior
art, FIG. 4b considers a sequence of two ON periods separated by an
OFF period, with a 33% duty cycle. The figure displays, as
functions of time, idealized waveforms defining the state of the
switch 10, the voltage V across it and the current I through the
auxiliary switch 21.
At time t.sub.1, the auxiliary switch 21 is opened and the primary
switch 10 is closed, initiating the first ON period. During this
period, the voltage V across the primary switch and the current I
through the auxiliary switch vanish. The source voltage V.sub.o is
impressed upon the primary winding of transformer 11, causing the
magnetic flux .phi. across the transformer's core to change with
time as dictated by Faraday's law. If N is the number of primary
(and secondary) turns, the total change in the flux .phi. is
V.sub.o (t.sub.2 -t.sub.1)/N. Concurrent with this is a change in
the magnetizing current given, in terms of the magnetizing
inductance L.sub.M, by V.sub.o (t.sub.2 -t.sub.1)/L.sub.M.
At time t.sub.2, the primary switch 10 is opened and the auxiliary
switch 21 is closed, initiating the OFF period. During this period,
the voltage across the auxiliary switch and the current through the
primary switch vanish. The voltage V across the primary switch is
clamped to a value V.sub.p =V.sub.o +V.sub.c, where V.sub.c is the
voltage across the storage capacitor 20. Conduction of the
magnetizing current is transferred to the secondary winding where
the current loop is closed by the storage capacitor 20 and the
auxiliary switch 21. Initially, this current, I, is negative in
sign and equal in magnitude to I.sub.p /2.
The evolution of the system during the OFF period, the time
interval between t.sub.2 and t.sub.4, depends upon the capacitance
value C chosen for the storage capacitor 20. C should be chosen to
be large enough so that the time dependence of the voltage,
V.sub.c, across the capacitor can be approximately neglected. This
accounts for the constancy of the voltage V, and the linear rise of
the current I, both displayed in FIG. 4b. Invoking once again
Faraday's law, the total change in the magnetic flux .phi. during
the OFF period is then approximately given by -V.sub.c (t.sub.4
-t.sub.2)/N. By equaling the magnitude of this change to the
corresponding flux change during the ON period, it follows that
where D=(t.sub.2 -t.sub.1)/(t.sub.4 -t.sub.2) is the primary switch
duty cycle. The total change in the magnetizing current I during
the OFF period is approximately given by -V.sub.c (t.sub.4
-t.sub.2)/L.sub.M. Since the integral of the magnetizing current I
during the OFF period must vanish (under steady state conditions),
it follows that
where t.sub.ON =(t.sub.2 -t.sub.1) is the ON time of the primary
switch.
The evolution of the system during the OFF period may be analyzed
further by dividing the period into two intervals, t.sub.2 -t.sub.3
and t.sub.3 -t.sub.4, of equal duration, characterized respectively
by negative and positive values of the magnetizing current I. In
the first interval, the magnetizing current charges the storage
capacitor 20, and storage of magnetizing energy is progressively
transferred from the transformer to the capacitor. This process is
completed at time t.sub.3 when the magnetizing current vanishes. In
the second interval, the magnetizing current discharges the storage
capacitor 20, and storage of magnetizing energy is progressively
transferred back from the capacitor to the transformer. This
process is completed at time t.sub.4 when a mirror image of the
magnetizing current has been formed and magnetizing energy has been
reflected into the transformer, resetting it for the next
cycle.
Because of the alternating character of the magnetizing current I,
the auxiliary switch 20 must be able to conduct negative as well as
positive currents, in addition to being able to block positive
voltages. This observation suggests MOSFET transistors as natural
candidates to implement the functions of the switch 21.
FIG. 4c shows a magnetizing current mirror in which the auxiliary
switch 21 is implemented with a MOSFET transistor. The reverse
rectifier inherent to the MOSFET is explicitly shown. Thus the
auxiliary switch 21 can be thought of as being always closed to
negative currents and selectively closed to positive currents. The
flow of positive currents is controlled by the switch control
circuit 22 which applies a suitable voltage to the gate of the
MOSFET.
Referring back to the example of FIG. 4b, it is apparent that the
MOSFET 21 can be turned on at any time between t.sub.2 and t.sub.3
without disrupting the operaion of the magnetizing current mirror.
Such a delay does not represent "dead" time since during this time
the reset mechanism is operational. On the other hand, a delay
between the opening of the auxiliary switch 21 and the closing of
the primary switch 10 represents dead time. For this reason it is
efficient to keep such a delay to a minimum, consistent with the
requirement to avoid an overlap between switches. However, a small
delay is useful to allow the magnetizing current to charge and
discharge parasitic capacitances associated with the switches and
windings.
A different perspective on the operation of the magnetizing current
mirror as a reset mechanism for single ended forward converters is
offered by FIG. 4d showing equivalent circuit diagrams
characterizing the converter's ON and OFF periods. The magnetizing
current mirror is operational only during the OFF period, when the
storage capacitor 20 is connected via the auxiliary switch 21, in
its closed position, across a winding of transformer 11. These
elements form a resonant circuit of period T.sub.res
=2.pi..multidot..sqroot.L.sub.M C, where L.sub.M is the
transformer's magnetizing inductance as seen from the mirror and C
is the mirror's capacitance. During the OFF period of the
converter's cycle, the resonant circuit undergoes a portion of its
natural cycle which, under steady state conditions, is centered
about a voltage maximum. Approximate constancy of the voltage seen
by the primary switch 10 during the OFF period translates into the
requirement that the OFF period t.sub.OFF be small relative to the
resonant period T.sub.res, t.sub.OFF <<T.sub.res.
The condition t.sub.OFF <<T.sub.res should, however, not be
interpreted to mean that the mirror's capacitance should be made
arbitrarily large, since the resonant period T.sub.res introduces a
time scale which limits the converter's transient response time. As
implied by Eq. (1), a change in the converter's duty cycle leads to
a change in the voltage across the storage capacitor 20. To effect
the latter, the integral of the magnetizing current during the OFF
period is non-vanishing. Consequently, under transient conditions
Eq.(2) ceases to be applicable. To avoid transformer saturation the
peak value of the magnetizing current must then be limited by
limiting the rate of change of the converter's duty cycle and,
therefore, the transient response time.
Aside from transient conditions, during which the voltage V.sub.c
across the storage capacitor 20 changes to adjust itself to a
varying duty cycle, the magnetizing current mirror and the
transformer's core define an essentially closed system: magnetizing
energy transferred from the transformer to the storage capacitor is
injected back into the transformer within the converter's OFF
period. This internal recycling is only incomplete to the extent
that non-ideal circuit elements give rise to energy losses. These
may be accounted for by modifying the resonant circuit of FIG. 4d
with the addition of resistive components representing the effects
of losses associated with the transformer's core, the winding
resistance and the equivalent series resistances of the storage
capacitor 20 and the auxiliary switch 21.
The equivalent circuits of FIG. 4d suggest that applications of the
magnetizing current mirror to the resetting of the transformer's
core in single ended forward converters need not be restricted to
the topology of FIG. 4a. Useful variations of the new reset
mechanism are indeed obtained by connecting the mirror in parallel
with different transformer windings.
In FIG. 4e, the magnetizing current mirror is connected in parallel
with the primary winding of transformer 11. The main advantage of
this topology, relative to that of FIG. 4a, originates from a
direct coupling of the mirror to the primary switch 10. This
eliminates a certain amount of ringing of the voltage V across the
primary switch, due to leakage inductance and parasitic
capacitances, which is present with the topology of FIG. 4a.
However, this is not a serious problem. On the other hand, a MOSFET
implementation of the switch 21 in FIG. 4e would require the use of
a p-channel device and/or a floating gate drive. This is a
relatively serious drawback for this topology.
In FIG. 4f, the magnetizing current mirror is connected in parallel
with an auxiliary transformer winding. Possible advantages to this
configuration originate from the flexibility provided by the choice
of turn ratio between auxiliary and primary windings, and the
possibility to magnetically couple these windings closely. The
trade off is added complexity in transformer construction.
These and other possible variations in the detailed implementation
of the new reset mechanism share the same equivalent circuits of
FIG. 4d and the same fundamental advantages when compared to reset
mechanisms known in the prior art. Some of these advantages are
made more evident by referring to FIG. 5, which compares the
idealized behavior of the magnetic flux as it would evolve in the
examples of FIGS. 2b, 3b and 4b, corresponding to: (a) the
traditional reset mechanism; (b) the capacitor-resistor-diode
network; (c) the new reset mechanism. The curves denoted
respectively by a, b and c define as a function of time the flux
across a core made of soft ferromagnetic material of negligible
hysteresis. The saturation flux is denoted by .phi..sub.sat.
Among the three reset mechanisms considered in FIG. 5, the
capacitor-resistor-diode network (b) is the one that brings the
core closest to saturation and does not recycle the core's
magnetization energy. This qualifies this reset mechanism as the
most inefficient in utilizing space (the volume of the core) and
energy. Its redeeming feature is the constancy in the slope of the
flux curve between t.sub.2 and t.sub.4 which, in view of Faraday's
law, implies minimal voltage stress on the converter's primary
switch.
Some of the drawbacks of the traditional reset mechanism (a) stem
from the greater slope of the flux curve between t.sub.2 and
t.sub.3 and vanishing slope between t.sub.3 and t.sub.4. These
observations imply greater voltage stress on the primary switch,
the presence of dead time, and a (50%) limitation on the duty
cycle. Other drawbacks stem from the asymmetry between positive and
negative flux, which signifies inefficient use of the core's volume
and increased core energy losses.
In light of these considerations, the nature of curve c,
characterizing the new reset mechanism, speaks for itself and
suggests that a magnetizing current mirror should provide optimal
resetting of the transformer's core in single ended forward
converters.
Other embodiments are within the following claims.
* * * * *