CA1167916A

CA1167916A - Converter circuit employing pulse-width modulation

Info

Publication number: CA1167916A
Application number: CA000359037A
Authority: CA
Inventors: Robert E. White
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1979-10-10
Filing date: 1980-08-26
Publication date: 1984-05-22
Also published as: JPS5658777A; FR2467505A1; FR2467505B1; GB2060220A; GB2060220B; DE3038217A1

Abstract

Abstract:
A voltage conversion and regulation system and method of same utilizing pulse-width modulation to vary the periodic energization of a tuned circuit employed to drive a transformer and an associated rectifier. The tuned resonant circuit is periodically energized from a DC source at a fixed rate but for alterable periods of time during each cycle so as to produce in the tuned circuit an alternating current of fixed frequency for subsequent transformation and rectification into a DC
voltage of predetermined, regulated level. A pulse-width modulator is connected in feedback-loop fashion between the system output terminals and switch circuitry employed to effect the periodic energization. Varying the length of time during which the switch circuitry is activated responsive to changes in the level of output voltage produces the desired regulating control.

Description

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.

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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 .

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.

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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.
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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 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 .

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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 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 -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 . . .

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.

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Claims

Claims:

1. an energy converting 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 primary 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 signal.

2. The energy converting and regulating circuit of claim 1 wherein said means (b) includes switch means for selectively connecting said tuned circuit to said source of direct current, and wherein said means (e) includes means coupled to said switch means for maintaining each said connection for a period of time depending on the level of said output signal.

3. The energy converting and regulating circuit of claim 1 wherein said means (e) includes means for increasing the duration of each periodic energization of said tuned circuit in response to a decrease in the level of said output signal.

4. The energy converting and regulating circuit of claim 1 wherein said means (e) includes means for decreasing the duration of each periodic energization of said tuned circuit in response to an increase in the level of said output signal.

5. An improved energy converting and regulating circuit of the type including a tuned circuit having a capacitive element connected in series with an inductive element, means for periodically energizing said tuned circuit from a source of direct current so as to produce in said tuned circuit an alternating current, transformer means having a primary winding and a secondary winding, said primary winding being connected in series with said tuned circuit for providing a signal across said secondary winding representative of said alternating current, output means coupled to said secondary winding for providing an output signal representative of said alternating current, and means responsive to said output signal for altering the periodic energization of said tuned circuit depending on the level of said output signal, wherein the improvement comprises means for causing said energization of said tuned circuit to occur at a fixed rate so as to produce in said circuit an alternating current of constant frequency and means for causing the duration of each periodic energization to increase with a decrease in the level of said output signal and decrease with an increase in the level of said output signal.

6. 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 series with said tuned circuit, an output signal representative 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.

7. The energy converting and regulating method of claim 6 wherein said step (b) includes selectively connecting said tuned circuit to said source of direct current, and wherein said step (d) includes maintaining each said connection for a period of time depending on the level of said output signal.

8. The energy converting and regulating method of claim 6 wherein said step (d) includes increasing the duration of each periodic energization of said tuned circuit in response to a decrease in the level of said output signal.

9. The energy converting and regulating method of claim 6 wherein said step (d) includes decreasing the duration of each periodic energization of said tuned circuit in response to an increase in the level of said output signal.