Note: Descriptions are shown in the official language in which they were submitted.
7~
CONVERTER CIRCUIT EMPLOYlNG. PULSE-WlDTH MODULATION
Background_of the Invention
The subject matter of the present invention
pertains to a circuit and method of same for convert-
ing and regulating a DC voltagé via the selective andperiodical energization of a tuned resonant circuit.
Pertinent examples of such circuits include those dis-
closed in Schwarz U.S. Patent No. 3,953,77~, an arti-
cle entitled "A 95 Percent Efficien. lkW DC Converter
with an Internal Frequen~y of 50kHz" by F. C. 5chwarz
and J. B. Klaassens of the Power Electronics Labora-
tory of the Department of Electrical Engineering,
State University of Technology, Delft, The Nether-
lands, and Andrews U. S. Patent No. 3,596, ~59 the
latter of which is assigned to the assignee of the
present invention. Such circuits operate, basically,
by periodically energizing a resonant circuit from a
source of direct current so as to produce in the tuned
circuit an alternating current, and then transforming
the alternating current to a level producing, upon
rectification, a desired ~C potential. Limited regula-
tion may also be obtained by altering the energization
rate in accordance with changes in the input supply or
output demand.
25In both the S~hwarz and Andrews circuits,
the tuned elements are energized at a rate offset a
variable distance from resonance to effect both conver-
` sion and regulation. A disadvantage of such circuits
is that their regulating capability is limited to a
load range of about 2 to 1; that is, from about
one-half load to about full load.
.
Summary of the lnvention
- The present invention is directed to a con-
~-~ verter circuit and method of same for converting and
regulating a DC voltage over a wide range of variation
in either the input supply or the output demand. More
particularly, the circuit of the present invention
comprises a converter circuit similar in many respects
.
-- 2 --
to that disclosed by the Andrews references cited in an
earlier part of this specification, except that the
operation of the circuit is controlled by pulse-width
modulation rather ~han frequency modulation.
In accordance with one aspect of the invention
there is provided an energy conv~rting and regulating
circuit comprising: (a) a tuned circuit having a capacitive
element connected in series with an inductive element; (b)
means for periodically energizing said tuned circuit from a
source of direct current so as to produce in said tuned
circuit an alternating current of constant frequency; ~c)
transformer means having a primary winding and a secondary
winding, said pLimary winding being connected in series with
said tuned circuit for providing a signal across said
secondary winding representative of said alternating
current; (d) output means coupled to said secondary winding
for providing an output signal representative of said
alternating current; and (e) means responsive to said output
signal for altering the duration of each periodic
energization of said tuned circuit depending on the level of
said output signalL
In accordance with another aspect of the invention
there is provided an energy converting and regulating method
comprising the steps of: ~a) providing a tuned circuit having
a capacitive element connected in series with an inductive
element; (b) periodically energizing said tuned circuit from
a source of direct current so as to produce in said circuit
an alternating current of constant frequency; (c) receiving,
via a transformer having its primary winding connected in
3Q series with said tuned circuit, an output signal represent-
ative of said alternating current produced in said tuned
circuit; and (d) responsive to said output signal, altering
the duration of each periodic energization of said tuned
circuit depending on the level of said output signal.
The circuit of the present invention includes a
tuned resonant circuit, switch means for periodically
energizing the circuit from a source of direct current so as
to produce in the circuit an alternating current having a
constant frequency substantially equal to the resonant
frequency of the circuit, output means for producing an
.
- 2a -
output in response to the alternating current, and a
pulse-width modulator for alterlng the duration of each
periodic energization depending on the level of the
output. The tuned circuit comprises an inductor and
capacitor connected in series with the primary winding of
an output transformer, the secondar~ winding of which is
connected in turn to a conventional diode-capacitor
rectifier circuit to produce an ultimate DC outputO
During operation, as the level of inpwt supply or output
demand changes so as to affect the level of the circuit
output, the pulse-width modulator operates to increase or
decrease the duration of periodic energization of the
tuned circuit in a manner tending to maintain the output
at a constant regulated level. As indicated earlier, with
the exception of the use of pulse-width modulation to
control the circuit energization, the circuit of the
present invention is similar in many respects to that
disclosed by the Andrews reference.
The principal advantages of the circuit and
method of the present invention are their simplicity o
understanding and maintenance and their ability to
maintain efficient voltage regulation over a relatively
wide range of variation in input and output levels; in
particular, over a range greater than that capable of
being handled by the ~ndrews circuit.
:
.
7~6
lt is, therefore, a principal ob]ective of
the present invention to provide a circuit and method
of same for converting and regulating a DC voltage
over a wide range of variation of input supply and
S output demand.
It is a feature of the present invention
that effective voltage conversion and regulation are
accomplished via a tuned resonant circuit, the reso-
nant elements of which are periodically energized from
a DC source aA, a predetermïned constant rate, but for
periods of varying duration depending on input supply
and output demand.
The foregoing objectives, features, and ad-
vantages of the present invention will be more readily
understood upon consideration of the following detail-
ed description of the invention taken in conjunction
with the accompanying drawings.
.
Brief Description of the Drawings
FIG. 1 is a simplified schematic representa-
tion of an exemplary embodiment of the converting and
regulating circuit of the present invention.
FIG5~ 2 and 3 are each a series of waveforms
illustrating the operation of the circuit of FIG. 1~
FlG. 4 is a simplified schematic representa-
tion of an alternate embodiment of the converting and
regulating circuit of the present invention.
Detailed Description of the Preferred Embodiment
Referring to FIG. 1, there is disclosed in
simplified schematic form an exemplary embodi-
ment of the converting and regulating circuit of the
present invention including: an inductor 22 and capaci-
tor 24, defining in combination a tuned resonant cir-
cuit 21, connected in series with the primary winding
25 of an output transformer 20, the secondary winding
41 of which is coupled to a pair of output terminals
44, 46 via a conventional diode-capacitor rectifier
arrangement; switch means 26 and 28 and a respective
40- pair of parallel-connected diodes 30 and 32 for selec-
tively energizing the tuned circuit 21 and transfQrmer
--4--
winding 25 from a source 34 of direct current; and a
pulse-width modulator 42 connected between the output
terminals 44, 46 and ~he switch means 26, 28 for
controlling the operation of the switch means in a
manner described more fully below. The two switch
means 26, 28 may be of any suitable design permitting
rapid switch rates in the area of 25 kHz, and electron-
ic control. An example of such means is the transistor
arrangement disclosed by, the Andrews reference ci$ed
in an earlier part of this specification. Similarly,
the pulsewidth modulator 42 may be any suitable unit
capable of controlling the on and off states of the
two switch means according to a voltage level present
at the two terminals 44, 46. An example of such a unit
is that marketed under the designation 3524 by such
manuf~cturers as Silicon General and Te~as Instruments.
ln operation, the pulse-width modulator 42 alter-
nately activates the two switch means 26, 28 in a
manner producing in the tuned circuit 21 and transform-
er winding 25 an alternating current of magnitude and
periodic duration sufficient to produce at the output
terminals 44, 46, after transformation by the trans-
former 20 and subsequent rectification, a DC potential
of predetermined, regulated magnitude. A more detailed
understanding of the circuit of FIG. 1 is obtained
upon consideration of the waveforms of FIGS. 2 and 3
wherein the curve labeled CLK defines a clock signal
(with each pair of adjacent pulses representing a
single cycle) for referencing the timed operation of
the pulse-width modulator 42, the curve labeled DRIVE
represents the ONioFF state of the two switch means
26, 23, the curve labeled el represents the voltage
- impressed across the circuit defined by the tuned
, .
circuit 21 and the transformer winding 25 during opera-
tion of the switch means, and the curves labeled epri
and ipri represent the voltage and current, respective-
ly, developed in the primary winding alone. The four
c FIG. 2 labeled ipri(1) through ipri(~) are
.,~ . : . ' .
3~6
component segments of the i ri curve that have been
separated for purposes of illustration. Each of the
voltage and current curves are related by a series of
dashed lines and timing marks tl through tll to indi-
cate the occurence of signiPicant events during theirgeneration. An arrow labeled T is included to indicate
~he relationship of a half cycle of the CLK curve to
the periodicity of the remaining curves in the fig-
ures. For simplicity, the inductor 22 and capacitor 24
forming the tuned circuit 21 are assumed to be purely
reactive and the primary winding 25 is considered to
be primarily a resistance in parallel wi~h a diode-
coupled capacitance. Note that, in the DRIVE curve of
FlGS. 2 and 3, the solid line labeled 526 represents
the state of switch means 26 and the t1ashed line
labeled 528 represents the state of switch means 28,
with a high level in each case indicating a closed or
ON state and a low level indicating an open or OFF
state. Note also that switch means 26 and 28, while
_ 20 potentially open at the same time, are never permitted
to be closed at the same time as such a condition
- would effectively short circuit the input source 34.
Assume as a starting point that, at a time just
before time tl, switch means 26 and 28 are open, as
indicated by the drive curve of FIG. 2, and current
ipr, is flowing in a positive direction, as indicated
by the arrow 50 in FIG. 1, through the circuit definld
by the inductor 22, the capacitor 24, the transformer
primary winding 25 and the forward-biased diode 32,
the~source of the current ipri being the release of a
charge stored in the capacitor 24 during a previous
operation of the circuit. At time tl, the pulse width
modulator 42 operates to close switch means 26, as
indicated by the DRIVE curve of FIG. 2, and cause the
voltage Ei of the VC source 34 to be impressed across
the tuned circuit 21 and primary winding 25, as indi-
cated by the el curve. This causes the current ipri to
increase sharply in the positive direction, as indicat-
ed by the Ipri(l) curve, until reaching a maximum at
.
~7~
--6--
time t2 and then decrease in accordance with the
typical transient response of a series LC circuit to a
step wave. (lt is assumed herein that the reader is
familiar with the typical transient response of such
an LC circuit to step functions of both ~he positive
and negative kind.) Note that the diode 32 is reverse-
biased by the application of the source voltage Ei and
the path for the current ipri is now through the
source 34. lf switch means 26 were left closed, the
current ipri would oscillate about zero at the reso-
nant frequency of the LC circuit while damping exponen-
tially as indicated by the dotted-line portion of the
pri(l) curve-
At time t3, though switch means 26 is opened and
the source voltage Ei removed from the circuit. Thecurrent ipri, however, rather than falling to zero, is
maintained for a short time by the collapsing field of
the inductor 22 and caused to continue flowing through
the circuit once again including the now forward-bias-
ed diode 32 until reaching zero at time ~4. At thattime, the field of the inductor 22 i5 dissipated and
~ the capacitor 24, charged to a maximum potential great-
er than that of the source 34 drives the current ipri
in a reverse direction, past the again reverse-biased
diode 32 and through the forward-biased diode 30, back
into the source 34 itself. This phenomenon is indicat-
ed by a comparison of the curves of FIG. 2 labeled e1
and ipri(2). (It will be recalled that high Q tuned LC
circuit driven at its resonant frequency will produce
a voltage across the capacitor with a peak value about
Q times the driving voltage; that is, for Ei = 300
volts and Q - lO, the voltage aross capacitor will be
about 3000 volts as a first approximation.)
As the charge on the capacitor 24 dissipates,
the current ipri increases in magnitude until reaching
a maximum at time t5 when it begins to decrease again
toward zero. If the circuit were left undisturbed, the
current ipri would decrease to zero and then resume
7~6
--7--
flowin~ in the positive direction through the then
forward-biased diode 32, as indicated by the dotted-
line portion of the curved ipri(2), until dissipating
to zero.
At time t6. switch means 28 is closed and the
current ipri flowing in the reverse direction through
the source 34 is suddenly shunted through the zero re-
sistance of the switch means. This causes the magni-
tude of the current i ~i~ as indicated in the curve
labeled ipri(3), to increase suddenly, still in the
reverse direction, until peakin~ at time t7 and then
decreasing toward zero. As before, if switch means 28
was left closed, the current ipri would oscillate
about zero in typical transient response to the remov-
al of a step wave from a series LC circuit. However,
at time t8, switch means 2~ is again opened and thecurrent i ri~ maintained for a short time again by the
collapsing field of the inductor 22, is forced a
second time through the forwara-biased diode 30 and
_ 20 into the source 34 until dropping to zero at time t9
when the capacitor 24, charged now to its maximum
-negative potential by the previously flowing reverse
current,begins to discharge and drive the current
again in the forward direction, as indicated by the
curve labeled ipri(4), through the diode 32. As the
charge on the capacitor 24 dissipates, the current
ipri reaches a maximum at time t1o and then begins to
decrease until time t11 when switch means 26 is again
closed and the process is repeated.
' 30 As mentioned earlier, the four current segments
pri(1) g ipri(4) form in combination the cur-
rent ipri actually flowing through the transformer
winding Z5 of the transformer 20. Although the current
i . is somewhat sinusoidal in nature, the voltage
prl
epri developed across the primary winding 25 follows
essentially the curve of a square wave primarily be-
cause of the load employed in the circuit wherein
diodes 36 and 38 operate to charge capacitor 40. For
an entirely resistive AC load, the voltage epri would
be less a square wave and more like the sinusoid of
the current ipri.
Control of the two switch means 26, 28 to a-
chieve the above-described operation is obtained via
the pulse-width modulator 42 connected between the
output t erminals 44, 46 of the circuit and the two
switch means. During normal quiescent opera~ion, the
pulse-width modulator 42 produces a series of pulses,
such as those depicted by the DRIVE curve of FIC. 2,
at a constant rate, preferably equal to the resonant
frequency of the tuned circuit 21, and with a width or
periodic duration sufficient to cause each switch
means 26, 28 to be closed for about 30% of each
operating cycle, defined as indicated earlier by two
cycles of the CLK curve, and open for the remainder of
the cycle. Each time the output level of the DC
voltage produced by the circuit is disturbed, such as
by a change in ei~her the output demand or the input
supply, the pulse-width modulator 42 automatically var-
~ies the width of the individual pulses fQrming the
pulse stream controlling the operation of the two
5Wi tch means 26, 2&.
For example, if the output voltage at the termi-
2S nals 44; 46 decreases, for example in response to anincrease in output demand, the pulse-width modulator
42 will operate to increase the width of the pulses in
the pulse stream. Such an occurrence is shown by the
curves of FlG. 3, individual ones-of which are labeled
,-- 30 to match corresponding curves of FlG. 2. As shown in
FIG 3, the frequency of the pulse stream is maintained
equal to that of FIG. 2, but the width or time
duration of each pulse is increased significantly. As
- a result, the current ipri, produced in the same
manner as discussed earlier, rises to an increased
magnitude and encloses a larger area under its curve.
Correspondingly, the output voltage across the two
terminals 44, 46 is also increased. The increase in
--9--
-
pulse-width is limited by feedback-loop operation of
the pulse-width modulator 42 to an amount just suffi-
cient to raise the output voltage of the circuit to
its previous undisturbed level.
If instead, the output voltage is increased,
such as by an increase in ~he input supply or a
decrease in the output demand, the width of the DRIVE
curve pulses will be narrowed below those of FIG. 2 to
produce a decreased current ipri in the ~ransformer
primary and a corresponding decrease in the voltage
produced at the output terminals. At very low load
demand, the operation of the circuit of FIG. 1 actu~l-
ly becomes discontinuous with the pulses of the drive
curve having a width just sufficient to maintain an
alternating charge on the capacitor 24.
In practice, very little change in pulse width
is required to compensate for changes in cutput volt-
age caused by a variation in output demand as most of
the change is due to a variation in input supply.
~ Typical values for the various components of the
circuit of FIG~ 1 include an inductor 22 of 0.,4 milli-
- henry, a capacitor 24 of 0.1 microfarad, a transforn~er
20 having a magnetizing inductance of around 400 milli--
henry, and a capacitor 40 of 5~000 microfarads. Typi-
cal operating voltages include an input voltage of
around 300 volts and an output voltage of around .100
volts, the difference being due primarily to the turns
ratio of the transformer 20. The preferred operating
frequency would be the approximately 25 kHz resonant
frequency of its tuned circuit 21. Such a circuit is
capable of providing a regulated output for input
voltages in the range from about 270 volts to about
` 500 volts and output demands in the range from about
zero to about full rated load.
Referring briefly to the circuit of FIG. ~,
there is disclosed a full-wave equivalent of the cir~
cuit of FIG. 1. For ease of correlation, the elements
of the circuit of FIG. 4 are labeled to match those of
. . .
-10-
the circuit of FlG. 1 except for the use of distin-
guishi~ng prime marks. The operation of the circuit of
-~ FlG. ~ is similar in most respects to that of FIG. 1
`` and will be readily understood by those persons famil-
iar with the art. The major difference in the voltage
and current curves produced by the operation of such
circuit is that the input voltage curve el alternates
between +Ei and -Ei, rather than ~Ei and zero as
before. Otherwise, the curves of FIGS. 2 and 3 apply
to the circuit of FIG. 4 except for changes in magni-
tude due to the fulI-wave versus half-wave character-
istics of the circuit.
Accordingly, there has been disclosed an energy
converting and regulating circuit operating by pulse-
width modulation to produce a DC voltage of predeter-
mined, regulated magnitude.
The terms and expressions which have been usedin the foregoing specification are used therein as
terms of description and not of limitation, and there
i 5 no intention, in the use of such expressions, of
excluding equivalents of the features shown and des-
cribed or portions thereof, it being recognized that
the scope ~f the invention is defined and limited only
by the claims which follow.
.
