[1]United States Patent Office 31413,539 3,413,539 DIRECT,CURRENT-ALTERNATING CURRENT INVERTERS HAVING A PAIR OF CONTROLLE,,D RECT IFIERS 5 Karl Heinz Gustav Lopitzsch, Emmasin,,ael, Eindhoven, Nethe rlands, assignor to North America@ Philips Com. pany, Inc., New York N.Y., a corporation of Delaware Fiied Nov. 23, 1966, Ser. No. 596,619 Claims priority, application Netherlands, Nov. 24, 196'51 651520 5 10 4 Claims. (Cl. 321-45) The inveition relates to a parallel-tuned direct ci-irreiitalterna ting current inverter. This type of inverter generally includ es a pair of controlled rectifiers, the main current 15 paths of which are connected in the forward direction between a first terminal of a direct volta.-e supply source and the respective ends of a windin.- wbich is incorporated, at least partly, in a symmetric parallel resonant circuit that is colipled to a load. A centre tap of said wind- 20 ing is connected, through a c@,oke coil, to the otlier terminal of the direct voltage supply source. The inverter also includes a source of pulses for controlling the rectifiers. Such inverters are known -Linder t-he name of "porallel ,5 if,vert ers," for example, from the book "Wechselric@later ' and Umrichter" by W. Schilling (1940) ar@d from Conimunic ation No. 963 of the "Proceedings of the International Conference on Semic(>nductor Devices" held in Paris in 1961. As is cltarly explained in this article by A. 10 E. Jackets and K. Wiison they may be constructed both a as "parallel-tuned" inverters and as "sei-ies-tuned" inverter s. In addition, there exisis a mixed form havin- both series and parallel resonance, as described in a paper by Ir. J. J. Wiltin.- in '@Philips Technisch Tijdschrift" No. 35 7/8 of 1961, pp. 255-261. In the series-tuned in@verters and in the mixed form, the choike coil is connected to a centre tap of the windin.- of the output transformer and its inductance is small relative to that of each half of said winding. Eacli ctirrent 40 pulse through one of the controlled rectifiers terniinates wit@ a corresponding half cycle of the current throu,-"i a seriesresonant circuit which is imptilsed by the current pulses a.,id consists of the choke coil and the effective capacit ance of the parallel resonatit circiiit relative to the 45 corres pondin@g half of the -winding. The parallel resodant circuit inter alia ful@fils the functioii of a commutation capacit or. The current through the clioke coil is a direct curren t which is strongly modulated with the resona-,it freque ncy of the tuned series circuit, or even a pulslitory 50 direct current in the mixed form, and the olitput altemating voltage strongly diepends -upon the load as a result of the fact that it is produced by oscillations of a series resonant circuit. In the parallel-tlined inverters, the inductance of t,@e 55 choke coil is large relative to that of each half of the windin g so that in certain circumstances, the choke coil can also be connected to a point other than a cer@tre tap of the wi-nding of said circiiit. Every current pulse through or@e of the controlled rectifiers ends sometime before a 60 corres pondin,- half cycle of an oscillating voltage produced thereby across- the parallel resonant cirortit. The capacit ance of said circuit serves in particular as a tuning capacitor and, in addition, also as a commutation capacit or. The current througli the choke coil is a direct ' 65 curren t having a ripple compoiient with a peak-to-peak amplit ude, for exa@mple, smaller thaii one fourth of the avera ge direct current, inversely proportional to the ir@ipedan ce of the choke coil and to ti-.e frequency of the alterna ting voltage produced, and the output alt,,rnat:ng 70 voltag e depends comparatively little upon the load. Owin- to its more favourable load characteristic, the Patented Nov. 26, 1968 2 so-called parallel-tuned inverter is preferred for niany applications, for example, in the case of induction heating by means of intermediate freqtiency currents. However, this inverter usually presents startin@,- difficulties because the capacitor of the parallel resonant circuit can charge only slowly during the first i.@nition of one of the controlled rectifiers, since the increase of its ehar@in,.@ current is limited by the lar.-e iiductance of the choke coil. As a result, at the end of the first half cycle of the oscillations produced across the parallel resonant circuit, and damped principally by the load, coiumutation of the reciifier which was first rendered conduc,ive does not take place because the ener.-y accuriiulated in the capacitor of the paraltel resonant circuit is too small to extinguish said rectifier. In the paper by Jackets and Wilson already cited, it is proposed to prevent said difficulty, by conpectin- a thermistor in series with the load when starting under a heavy load. In this case, the inverter is started in a substantially unloaded condition. Alternatively, it is possible lo cause the a-tnplitude of the cortrol pulses to increase slowly by means of an RC-,network so that the more sensitive of the controlled rectifiers is first rendered conductive and the other oi-ily after several further time intervals betweeii successive co@itrol pulses, so that sufficient eneray ;s then present i@i the capacitor of th@- parallel resonant circliit. For this p,,irpose oiie of the controlled rectifiers is usually additionally rendered artificially less conductive ti-,an the other. In a paper by W. Berens and H. Glimski in "Fachberichte der Haupt@versamniltiiig des VDE," pp. 258-264, 1964, further methods of starting series-ti-ined in,verters are described: (a) A coritrol-logic prodl,.ces the conductivity of one of the rectifiers and ensures that the oth@-r ca-.i only be rendered conductive after reversal of the direction of the load current; (b) Energizatioii of th,- load by an auxiliary source of alteriiatin.- voltage before startin.,@ the inverter (this is possible only if an auxiliary source of sufficient po,wer and of the required frequency is available); (c) @Interruption of the load circuit and cbar,@in.- the capacitor of the resonant circuit by n-ieaiis of an auxbiary direct voltage source before startin.- the inverter; and (d) Dividing the capacitor of the resonant circuit into two symnietrical parts and loading the capacitors of said parts by means of a direct volta.ac source and then shortcircuiting the capacitor of one of the parts tliroligh a switch and an inductance through iwhich said capacitor is oscillatorily reversely charged. As a result, a voltage equal to twice that of the auxiliary direct voltage source suddenly becomes operative across the whole resonant circuit so that this is powerflilly inipulsed and the inverter is started. However, startin.- ol' these series-tuned inverters or of the said mixed form cannot be compared directly with the startiig of a parallel-tuned inverter, because tl-,ey do not operate,with compulsory commutation and the choke coil may even be lacking entirely. In French patent specification 1,325,224 parallel-tuned inverter circuits are described in which the startin- measures described above in para-raph c and/or d are more or less employed. Two of these circuits are ccmparatively complicated a-id compi-ise a large number of elei-i-lents wbich axe included in the parallel resonapt circiiit or are ' conne--ted thereto, while the other can only be tised for inductive loads having a phase angle smaller than 45' and, as a result of the measures used for faci!itating startin,-, Do longer constitute real parallel-tuned inverters, so that their output alternating voltage is presumabiy coinparative'@y stror,.gly depe,.ident upon the load. lt is an object of the invention to provide a compara@
[2]3)413,539 3 tively simple, entirely electronic and easily starting direct current-alternating current inverter of the parallel-tuned type already defined. The inverter according to the invention is characterized in that a third controlled rectifier is connected in the for- 5 ward direction between the end of the choke coil remote from the winding and one end of the winding and is controlled so that, when starting the inverter, it becomes conductive together with the controlled rectifier of the pair of rectifiers the main current path of which is connected to 10 the other end of the winding and thus strongly accelerates the first charging of the capacitor of the parallel resonant circuit by the direct voltage supply source. The third control-led rectifier is preferably controlled through a gate circuit which is cut off by the alternating ]5 voltage produced across the parallel resonant circuit. In order that the invention may readily be carried into effect, one embodiment thereof will now be described in greater detail, by way of example, with reference to the accompanying drawing, in which- 20 FIGURE I is the circuit diagram of an embodiment of the inverter according to the invention, and FIGURE 2 shows current and voltage time diagrams to explain the operation of this embodiment. The embodiment shown diagrammatically and partly in 25 block-schematic form in FIGURE 1 may be used, for example, for the induction beating of goods to be treated by means of intermediate frequency etirrents, for example, steel components to be hardened. It comprises a pair of controlled rectifiers I and 2 shown @is controlled senii- 10 conductor rectifiers. The main current cii-cuits of rectifiers 1 and 2 are connected in the forward direction between a first (negative) terminal of a direct voltage supply source 4, and the respective ends of a winding 5. Together with a capacitor 6, the winding 5 constitutes a sym- 3;@-) metric parallel resonant circuit. The winding is coupled to a load 7 and an auxiliary winding 8 includes a centre tap wbich is connected to the other terminal of the supply source 4 through a choke coil 9. The embodiment shown furtber comprises a source of pulses for controlling the 40 rectifiers I and 2, which will be described hereinafter. Small induction coils 11 and 12 are connected between the cathodes of each of the rectifiers I and 2 and the negative terminal of the supply source 4. The object of these coils is to limit the steepness of the increase of the cur- 45 rent when the respective controlled rectifier becomes conductive. The inductance of the choke coil 9 is large with respect to that of each half of the winding 5 of the parallel resonant circuit 5-6, which half is closely coupled to the 50 other half. In other words, the inverter is of the paralleltuned type in which the frequency of the alternating voltage produced across the parallel resonant circuit is somewhat higher than and substantially equal to the natural frequency of said circuit. Inverters of this type have a 55 very good load characteristic and are consequently highly suitable for induction heating in which strong and sudden load variations may occur. The parallel resonant circuit determines the operating frequency since the inverter is self-controlled and its ffy-wheel effect, as well as the fact 60 that the direct current flowing through the choke coil 9 is uninterrupted and shows only a comparatively weak rippie component, benefit the loading properties. However, these inverters operate with compulsory commuta. tion, that is to say, that each controlled rectifier is ren- 65 dered non-conductive as a result of the other - controlled rectifier becoming conductive. The capacitor 6 of the parallel resonant circuit also plays the role of a commutation capacitor. At the instant that one of the - controlled 70 rectifiers becomes conductive it must be sufficiently charged to render the other controlled rectifier nonconductive by bringing its anode to a potential which is negative relative to that of its cathode, to absorb charge carriers which emerge from the said electrode, and never75 4 theless to maintain said electrode at a reverse potential during at least the whole recovery time of the controlled rectifier. This recovery time is the time which elapses between the instart at whic@h the current through a controlled rectifier has become zero and the instant at which it has regained its cut-off properties relative to a voltage directed in the forward direction. In operation, the capacitor 6 is repeatedly charged in the other direction through the small induction coil 11 or 12, the controlled rectifier 1 or 2 and through the lower or upper half of the winding 5 and the choke coil 9. The value of the direct current flowing through the choke coil varies very little so that, practically speaking, the impedance of the choke coil 9 does not hamper the discharging and charging of the capacitor 6. The situation on starting the inverter is quite different. When connecting the supply source 4, nothing happens for the time being. A switching-on pulse, if any, is suppressed by the high impedance of the choke coil 9 to such an extent that, even in the case of a direct and/or strong feedback coupling, it cannot supply the energy to impulse the resonant circuit 5, 6 sufficiently strongly to produce a pulse for making one of the controlled rectifiers 1 and 2 conductive. A first starting pulse has thus to be supplied between the control electrode and the cathode of one of the control4ed rectifiers 1 and 2, for example, of rectifier 1. This starting pulse renders the controlled rectifler I conductive and the capacitor begins to char.-e through the coil 11, the rectifier 1, the lower half of the winding 5 and the choke coil 9. Its lower electrode becomes positive relative to its upper electrode. However, the charging current is strongly inhibited by the comparatively high impedance of the choke coil 9, so that it can increase only very slowly. This slowly increasing current corresponds to a weak initial current pulse which may be insufficient to impulse the parallel resonant circuit 5, 6 sufficiently strongly, particularly if starting has to take place in the loaded condition. Even if the initial current pulse is sufficient to impulse strong oscillations in the parallel resonant circuit 5, 6 and to produce a pulse for rendering the second rectifier 2 conductive, the charge of the capacitor 6, at the instant the second rectifier 2 becomes conductive, is usually insufficient to render the first rectifier I non-conductive, to absorb the charge of said rectifier and to hold the voltage between cathode and anode of said rectifier in the reverse direction during the required recovery time. The supply source 4 is therefore substantially short-circuited through the choke coil 9 and the controlled rectifiers I and 2, which may thereby be damaged. In order to overcome said starting difficuity, according to the invention, a third controlled rectifier 3 is connected in the forward direction between the end of the choke coil 9 remote from the winding 5 and the lower end of the winding 5. This third controlled rectifler is controlled by the pulse source to be described, in such manner that, when starting the inverter, it becomes conductive together with the controlled rectifier I of the pair of rectifiers, the main current path of which is connected to the upper end of the winding 5. Thus, the first charging operation of the capacitor 6 of the parallel resonant circuit 5, 6 by the direct voltage supply source 4, is strongly accelerated by short-circuiting the circuit consisting of the choke coil 9 and the lower half of the winding 5 by means of the third controlled rectifier 3. The pulse source for controlling the controlled rectifiers 1, 2, and 3, substantially comprises a pulse shaper 20, for example, a monostable trigger circuit or a controlled blocking oscillator having two inputs. One input 21 receives a starting pulse, for example, tery and a push-button switch. One feedback coupling voltage from the through a double rectifier composed 14, and a phase control network 24 the phase of the pulses produced by 20 can be varied relative to the half by means of a batinput 22 receives a auxiliary winding 8 of rectifiers 13 and by means of which the pulse generator sinusoidal pulses of
[3]5 the direct voltage produced between the centre tap of the auxiliary winding 8 and the common point of the rectifiers 13 and 14. By this phase adjustment, the time between commutation and passage through zero of the voltage across the capacitor 6 may be adjusted to the value required for the recovery of the controlled rectifiers. The output 23 of the pulse shaper 20 is connected to the primary windings of three control transformers 15, 16 and 17 throu,ah gates 31, 32 and 33, respectively. The secondary windin.-s of said control transformers are shunted by diodes 25, 26 and 27 respectively, which suppress the negative excursions of the control pulses and are connected between the cathodes and the control electrodee of the corresponding controlled rectifiers 1, 2 and 3, respectively. The gates 31, 32 and 33 are controlled by a logical device 28 having an input 29, 30 connected to one half of the auxiliary winding 8, and outputs connected to said gates. This logical device is constructed so that the gates 31 and 33 are open as long as no voltage appears across the winding 8, so that they pass the positive Pulse produced by the pulse shaper 20, as a result of a startiiig pulse appl;ed at its input 21, to the control electrodes of the controlled rectifiers I and 3. These rectifiers are thus rendered conductive by said control pulse and the capacitor 6 is rapidly oscillatorily cbarged to nearly twice the voltage of the supply source 4. The capacitor char,-e path includes the induction coil 11, the controlled rectifier 1 and the controlled rectifier 3. This is shown in FIGURE 2 by the variatioii of the voltage V, across the capacitor 6 and the first peak of the currents I, and 13 through the controlled rectifers I and 3, respectively, during the beginning of the first half cycle of the voltage V, (until the time tl). During the charging of the capacitor 6, a comparatively small increasing curre-@it 10 also begins to flow throu.-h the choke coil 9, via the controlled rectifier I and the upp,-r half of the windin, 5. At the instaiit t, at which the charging current I3 again passes the value zero, the controlled rectiler 3 becomes non-cond,ictive again ind is kept in the non-conductive condition by the fact that the voltage across the capacitor 6 at the instant t, and up to a later instant t,, (first line of FIGURE 2) remains lar.-er than the voltage V4 of the supply source 4, so that a negative voltage is set up at the anode of the controlled rectifier 3 between the instants t, and t-,. The controlled rectifier I remains conductive after t, because, at this instant, the ctirrent 1( though the choke coil 9 has already becor-@ie lar,@er than its holding current. From the instant t, the inverter begins to operate as s@,ich and the source 4 further stipplies ener,@y to the parallel resonant circuit 5, 6 through the controlled rectifier I and the choke coil 9. The voltage across the auxiliary winding 8 varies of course like the voltage Vc so that, for example, the upper input conductor 29 of the logical device 28 has become positive relative to the other input conductor 30 during the charging of the capacitor 6. As a result, the gates 31 and 33 are blocked, while the gate 32 is opened. The same first voltage peak across the auxiliary winding 8 is applied to the input 22 of the pulse shaper 20 through the rectifier 14 and after adjiistment of its phase by the network 24. At an in-tant t2 at which the voltage across the capacitor 6 has not yet changed sign, a second control pulse is consequently prodticed by the pulse shaper 20. This second pulse is applied through the gate 32 between the control electrode and the cathode of the controlled rectifier 2 and renders the said rectifier conductive. The voltage still present across the capacitor 6 is thereby applied in the reverse direction across the controlled rectifler 1, so that the said rectifier stops conducting. The charge carriers accumulated therein produce a small peak,of ctirrent ffowing in the reverse direction and which ceases at an instant t3, still before the voltage across the capacitor 6 changes sign at the iistant t4. If the first commutation jtist described has succeeded, which is always the case provided that the controlled rectifier 3 can recover d,,iring the time tl-t., and the controlled 3)413,539 6 rectifier I can recover during the time t2-t4, the actual starting is completed and the inverter further operates in the normal manner. However, a build-up phenomenon still follows beca-use, up to the instant tl, the capacitor 6 has not yet been char.-ed to the ifull voltage to which it should be charged for the given direct supply voltage. This buildilig up occurs at a relatively low frequency determined by a series resonant circuit consistiiig of the parallel resonant circuit with the winding 5 and the capac10 itor:6, and of the choke coil 9, and has a strongly attenulated character, even when the inverter is not loaded. However, it is associated with a temporary excessive voltage across the capacitor ;6. It is therefore obvidus not to start witli the full direct supply voltage but with a supply volt15 age reduced to, for example, one half or even one third, so that the temporary excessive voltages occurring durin.the build-up do not exceed the volta.-cs occurring duriiig normal operation. A much smaller and cheaper type may be used for the controlled rectifier 3 than for the con20 trolled rectifiers I and 2. During the second half cycle, after a comparatively weak initial peak, the current 12 through the controlled rectifier 2 increases slowly up to an instant t2'. The volta.-e across the auxiliary winding 8 changes sign after the in25 stait t4 and the logical device a.-ain blocks the gate 32 and opens the gate 31, but r@ot the gate 33. At the instant t2' a third pulse is produced by the pulse shaper under the control of the voltage across the auxiliary winding 8. This pulse again renders the cortrolled rectif@er I con30 ductive through the gate 31. The controlled rectifier 2 is rendered non-conductive by this conduction and regains its cut-off property in the forward direction before the instant t4' when the voltage across the capacitor 6 again changes sign and after the end, at the instant t3', of the 35 small reverse cdrrent i)eak produced by the charge carriers accumulated in t.,L- said rectifier 2. The current 10 throigh the choke coil 9 decreases before the instant t2, but is not anulled, and increases more strongly after the instant t2 and again after the instant t2' 40 and so on. Therefore, after the first half cycle of the oscillatioii of the volta,-e across the capacitor 6, the impedance of the choke coil 9 no loiger hampers the alternate charging of the capacitor 6. In fact, the current 10 through said choke coil remains nearly constant and only 45 periodically chan.-es its path. Once it flows through the controlled rectifier I and the upper half of the wirding 5, then it is transferred to the controlled rectifier 2 and the lower half of the winding 5, and so on. It will be clear that the control device described can be 50 considerably simplified. In cases where control of the power supplied to the load 7 by phase shifting of the control pulses is not recessary or desired, a very simple phase control network 24 may be used. In addition, the logical device 28 together with the gates 31, 32, 33 may 55 have a very simple construction and may consist, for example, of only two switcliing transistors with time constant networks controlled by the rectified voltage of the auxiliary winding 8. The first of said transistors prevents the transfer of the -first pulse pr<)duced by the pulse 60 shaper 20 to the control transformer 16. The other transistor passes said first pulse but does not pass all the subsequent pulses to the control tranformer 17. If the frequency is not too high, every short control pulse may be applied to the respective control electrodes of the two 65 controlled rectifiers I and 2 after startin,-, that is to say, from the second half wave orf the voltage across th-capacitor 6 onward. In case of higher operating frequencies, said met-hod might cause reignitions. Therefore, the output 23 of the pulse shaper 20 may then be connected 70 to the iiiput of a bistable multivibrator operating as a -frequency divider, and having two outputs. One of said outputs is connected through the first switchin- transistor of the simplified logical and gate device to the control transformer 16. The other is connected directly to the 7,:,) control transformer 15 and, through the other switching
[4]7 transistor of the logical and gate circuit, to the control transformer 17. Said bistable multivibrator also forms part of the logical and gate device. An inverter of the type diagrammatically shown in FIGURE I was constructed with the following components: Controlled rectifiers I and 2: Controlled semiconductorrectifiers Philips BTX 12-400R. Controlled rectifier 3: Controlled semiconductor rectifiers Philips BTX 34-600R. Capacitor 6: 4pf.; inductance of the winding 5: 106,,uh. Inductance of the choke coil 9: 500,uh. -Inductance of the coils 11 and 12: 4/.th. each. This inverter presented no starting difficulties even with full load and operated very satisfactorily at a frequency of 7.4 kc./s. with a direct supply voltage of nominally 50 volts. The power supplied to the load 7 could be raised up to 1.25 kva. at an efficiency of 90%, the operating frequency decreasing to 7.1 kc./s. The controlled rectifier 3 may be ichosen of a type having a considerably smaller maximumdissipation and peak current t-han the controlled rectifiers I and 2, since this rectifier need pass only one strong current peak at every starting operation. Therefore, the use of a third controlled rectifier is practically not disadvantageous with respect to other inverters of the parallel-tuned type having only two controlled rectifiers, but which require comparatively complicated starting devices' What is
