CA1118837A

CA1118837A - Converter including oscillatory circuit

Info

Publication number: CA1118837A
Application number: CA000306116A
Authority: CA
Inventors: David Gurwicz; Keith Pacey
Original assignee: Individual
Current assignee: Individual
Priority date: 1977-06-24
Filing date: 1978-06-23
Publication date: 1982-02-23
Also published as: NL7806798A; AU3741778A; GB1602613A; US4227243A; FR2395634B1; DE2827693A1; AU527438B2; SE430453B; US4196469A; JPS5418030A; SE7807118L; FR2395634A1

Abstract

ABSTRACT
A converter including a resonant oscillatory tank circuit comprising a capacitor and inductor, at least one switching circuit, including a solid state switching device in series or in parallel with a diode, connected to apply a predetermined potential difference from a DC supply to the inductor, and synchronizing means responsive to the oscillation of the oscillatory circuit for switching on the switching means for a fraction of a cycle only when the instantaneous value of the oscillatory potential difference across the inductor is already not less than a predetermined value, to delay the fall of the said oscillating potential difference and inject energy from the supply into the oscillatory circuit, and a load circuit coupled or connected to the tank circuit either in parallel or in series with the inductor.

Description

~L~11!315 37 ~ his inv~ntion relates to converters. The invention is particularly applicable, although not limited, to high-frequency converters incorporated in electric battery charging apparatu~s supplied from a mains supply. In the usual form of charger if a step down or step up of voltage is required a -transformer is necessary and even where there is no change of v~ltage a trans-former will generally be required to ensure isol~tion of the battery from direct connection to the mains supply. In the case of a high-power charger the weight and buLk oE a transformer :~or ~ains frequency will be very substantial, but if the frequ~ncy involved is increased to some kiloherz, for example 25 KHz, the weight and bulk are greatly reduced. In the case of a battery-driven vehicle this may make it practicable to carry the charger on the vehicle.
An object of the pr2sent invention is to provide a con-verter having a more nearly sinusoidal output than has been hitherto available.
According to the present invention a converter includes a resonant tank oscillatory circwit ccmprising a capacitor and an inductor, switching means including a solid state switching dev:ice connected to apply a predetermined potential difference from a DC
supply to the inductor~ and synchronising means responsive to the ~;r oscillation of the oscilla-tory circ~it for switching on the switch-ing means for a fraction of a cycle only when the instantaneous value of the oscillatory potential difference across the inductor is already not less than a predetennined valuel to delay the fall of the said oscillatory potential difference and inject energy frc~
the supply into the oscillatory circuit, and a load circuit coupled or c~ mected to the tank circuit.
me switching circuit may include a solid state switching device in series with a diode. Alternatively or in addition it may include a solid state switching device in parallel with a diode.
In one fonn of the invention, the tank circuit comprises an inductor connected in series with a capacitor across the DC
supply and the switching circuit is connected in p æ allel with the capacitor.
In another form of the invention, of push pull fonn, the converter includes a pair of supply capacitors connected in series with each other across the DC supplyr and a pair of solid state switching devices eac~l in series or in parallel with a diode, also connected in series with each other across the supply, and the inductor of the tank circuit is connected between the junction of the supply capacitors and the junction of the switching circuits.
me capacitor of the tank circuit may be ccnnected be-tween the junction of the switching circuits and c~e or each sun~ly terll:ir~al, or it may ~e connected in paralleJ
with the :;nductor.
'l'he~ capacitor of the tank circui-t may be shunted by a ~esistor, and m~ e co~lec-ted :in series wi-th a 5. solid state swi-tching device ~or s-tarting oscillation.
In one ~orm o~ the lnvention the load circult is connected or coupled to the tank clrcult in parallel with the inductor.
Thus the load circui-t may be connected to a 10. secondary winding of a trans~ormer having a primary winding in parallel with th2 inductor o~ the tar~ circuit.
Al-ternalively~ the load circuit may be con~ected to the secondary ~inding of a trans~ormer of which the lnductor o~ the tank circuit forms a primary winding. Again 15. -the load circuit may be coupled or connected to the tank . -circuit in serles with the inductor, for e}ample i-t may be connected to a secondary winding o~ a trans~ormer having primary winding in series with the inductor of the ~ ~ank circuit.
20. The invention also embraces a battery charger incorporatin~ the con~erter in combination with a recti~ier to supply it ~rom AC mains, a high-~requency tra~s~ol~lner, and a recti~ier ~or supplying uni.directional charglnp, current.
25. ~urther ~eatures and details o~ ~hç invention will be , ~M/AS 4. `~-.

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arJparent Ircm the fo~lo~ring cL~scription o~ ccrtain spe~cific e~bod:iments that wiJl be g:iven by way oI' exaMp.~e with reference to the accornpallying dr.awi.ngs in ~1hich:
Figure 1 is a diagram of a kn~ arrangement;
5. Figure 2 îs a wave ~orm diagram o:~ the kno~n arrangeMent of Figure 1;
Figure 3 is a schematic clrcuit diagram of one embodimen-t of the invention showing a single-ended parallel-load arrangement;
10. Figure ~ i5 a wave form diagram of the arrangement of Figure ~;
~ igure 5 is a s~hematic circuit diagram of a push-pull parallel-load arrangement;
~ igure 6 is a wave ~orm diagram o~ the arrangement 15. of Figure 5;
Figure 7 is a more detailed circuit dia8ram o~ an arrangement similar to that of ~igure 5;
~ Figure 8 is a clrcuit dlagram of a push-pull series-- load arrangement;
20. Figures 9 and 10 are equi.~alent circuit diagrams illus-trating the operation of the arrangement of Figure 8 in different portions of a cycle and;
Figllres 11 and 12 are wave form dia~rams illustrating the operation of the arrange1nent o~ F:lgure~ 8.,~
25. Fi~ure 1 illustrates~a typlcal known hig~ requency , . . ............. .

EW~M/AS 5.

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~ 37 COn'~el~'ter o])cr.ltin~ f`rom rl ~V source, sho~m as being deriv~cl from ~C rnains 'l5 -through a recti~ier 16 the ; ' purpose be:ing;~ to produce an i.so.Lated 10W vol-tage source o:~ small clim~nsio~s. rL'hi.'~ technique is kno~ generally , 5. as a chopped mocle power supply. As shol~n in Figure 1, a pair o~ capacitors C1 and C2 are connected in series across the positive and ne~ga-tive~3~ supply terminals 10 and 11, the reactance o~ these capacitors being . negligible at the conver-ting frequency (which might 10. t~pically be 30 KHz). A pair o:~ tran.slstors T1 and T2 are also connected in series across the terminals of D~' thel~ supply, while the primary winding o~ a transformer T~ connected between -the junction of the capacitors and the Junction A o~ -the -transis-to,rs. The transistors 15. T1 and T2 are controlled by their base drives to switch on alternately. The outpu-t voltage of the transformer l'R1 will be controlled by the mark space ,ratio of the conduction time o~ each of the two transistors.
~igure 2: illustrates the voltage wave ~orm appearlng 20. at the point A for t~o di~ferent output voltage conditions.
Due to the sharply ohangin~ voltage waveforms across the transistors the operating frequency,and power output o.~ such converters is .severely limited by the - perm:lssible dissipati.on appear:lng across the swi-tching 25. device,s on s~:ltch ~on~l ancl switch ~o~ . Specl~ica~ly .

EWI'~.T~/A,S , 6.

~ 3 7 on switch "o:~f" the -voltage appears across the de~iee when -th~ curren-t .Clow -through it is only sligh-tly reducedJ resulting in the high po~er condition of coinciclen-t high currcn-t and high voltage. ~ ~urther undesirable feature o~ these converters,again due to the ve~ hi~h rates of change of ~oltage, is the radiated R.F. e~ergy associated wi-th the harmonics unavoidably pres~n-t.
i It .is an objec-t o~ this .inven-tioll to produce a converter operat:Lng in a more nearly sinusoidal form, thus avoiding the drawbacks described above, ancl st}ll achieve the necessary control oP the outpu-t voltage.
~ igure 3 shows an embodiment o~ -the invell-tion employing a single-ended for~n wi.th parallel-co~mected load. The arrangement comprises a resonant oscillatory tank circuit consis-ting o~ an induc-tor L1 connected in series with a capaci-tor C3 across -the supply terminals.
~s shown the supp~.y terminals may be the terminals of - ~ a smootILing capacitor C8 connected -to the output o~ a 20. bridge rectifier 16 whereof the inpu-t is connected to A.C. suppl~ terminals 150 At the flequency of the tank elrcuit, (con~en~entl~ 25 KHz) the reactanee o~ -the smoothin~ capaeitor is negl.igib.le so that although the capacitor C3 and :lnductor L1 are connected in series 25. across the supply, they are e~ee~ivel~ a parallel tuned EW~M/AS 7 --~ 83 ~

c:ircu.it at t;he.i.:r resonan-t frequency.
A load clrcuit RL is connected in parallel with the :inductor whilst a -transistor T2 in series with a diode D2 are conrlected in paral~ol with the capaci-tor 5. C3. Th~ dio(le ensure.s tha-t the transistor canno-t be reverse biasecl even i~ the point A, nam~ly -the junc-tion between the capaci-tor and inductor7 goes nega-tive.
The load R~ mayin fac-t be represented by the primary winding of a transformer connected in parallel with the ~o. induc-tor L1 or it may be the re:~lected lmpedence across L1 due to a loaded secondary w.inding magne-tically coupled to L1, in other words L1 may be the primary winding of a trans~ormer whose secondary wind.ing is connec-ted to a load, for example a rectifier connected to charge a 15. secondar~ bat-tery. The e~fect of this load is to damp the natural oscillation of the t,ank circuit by extracting energy ~rom it.
The operation is as follows9 consi~ering the capacItor C~. discharged on initiating the supply. The transistor ~20. T2 is switched "on" transferring energy ~ro~ the supply to ~he inductor. Cur~ent rises in the inductor at a rate deter~ ed b~ the va.lue o:~ lnductance and the supply td~e- Aîter a preset interval the transistor ~ is tux~ed ~OIII'. The current flowi.ng in th~ i~ductor ls 25. now l;ran.s:~erred to the capaci-tor C3 and' the potentiQï at .

E~M/AS 8.

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the ~)Gi~ r:lses sinusoidally. I~igure ~ ilIustra-tes the ~1avefolln. Provid~ad ~hat RL does no-t load the tuned circui t heavil~ the point A clescribes the oscilla-tory pa th as shown . The e.~cursions at~ove~ and helow zero are ; 5. dependent on two ~actors, the darnping e~`~ect o.f the load and -the en~rgy fed intv the system .in the peri~od during which -the transistor T2 conduc ts. Thi~; period is 1 variable and under the control o~ a feed back system, thus I the amplitude o:~ oscil.Lation can be concrolled to -take 1 10~ account of load and supply variations. It should be noted that conduction of the transistor is o~ly i~itiated when the voltage across it is zero or nearly so (this is deter~ined by a sensin~ circuit which may take any of a number o:E well known forms for exarnple as describe~ below 15. wi-th reference to Figure 7). E~urthermore at termination of conduction the rate o~ rise of voltage .is determined ~by the charging time of the capacitor C3 associated with ~he current flowlng in both the incluctor L1 and the load resistor RL. The high dissipation conditior mentioned 20. above is avoided.
If ~UL were to be short circuited -the oscillation would ce~a~e and due to the action o~ ~he sensing clrcuit ment:lo~led abo~e the trallsi.stor T2 would no-t be turned , on. On removal o~ the short circui.t, oscillations may 251 be r~~initiated by a nwnber of alternative methods.

i3WEM/AS 9. - -,, }l`.i~lr~l.re 5 sho~s .l l~USIl~pUl l v~rs:ion of the above, rc 6 l~ .in~, the ~l~v~ n~
'~he c:ircuit colllprises a pair ~.E supply capaci-tors Cl and C2 oonnected in series aoross -the supply termi~a:ls 11 5. and 10, a pair o:f trans:is-tors Tl and T2 each i~ series with a d.iodc Dl or D2, a:l.so connected in series across the supply -te~rminals. An inductor L]. in par~ Lel (or e:E~ectively in parallel) wi.kh a load resistor RL ls connected bet~lee~ tlle junctiorl o~ the capacitors Cl and C2, and -the jU~C'tiOll 10. between the transis;tor Tl and diode D2. To complete the tanl~
' circuit a capacitor C3 shunted by a resistor Rl, is co~nected ;
through a -triac Xl across the transistor T2 and diode D2. The pusll-pull clrcuit operates in a si~ilar manner to tha~ o~
. Figures 3 and 1-~ but with o~ course ~o conduc-ting periods 15. per cycle, instead o~ one, as i.ndica-ted in Figure 60 The triac X1 i5 provided -to i~itiate oscillations~ The capacitor C3 discharged by v.irtuc oE -the resistor Rl, is charged when the triac i5 fired into conduction and oscilla~
. tions commence. I~le to the high :Erequencies involved9 it 20. would be unnecessary to re:~ire the triac clurin~ normaI
... . .
. operation.
.
Clamping diodes D3 and D4, sllown on Figure 5 are connecl;ed betwen each o~:' the suppl~y ra:ils ~nd a suil;able tappin~; poin~ OIl the irlductor L1. These diode~ li.mi-t the ~5. voltage appQarin~ across the inductor Ll, and thereforet s ~ , .

~ 10 ' : .

the tl~ar)~;:i.stons T:L and T2, -to a pre~(letermined sa~e value in the e~ent o~ f~n ope~-c,ircui t appear.ing on the out,put 0 ~he converl~r or, during the initiati.on o~ circui.t oscilla-. t:ions.
5. The .switchin~ devices re~err~d to above have been ~l].ustra-ted as transistQrs~ but may be a.ny suitable electronic swi-tching devices such as thyristors of various types etc.
A starting circuit compr:ising a thyristor T~ connscted in parallel to a reverse~conncctcd diode D3, as shown in 10. Figure 7~ cc~n be used to replace the tri.ac ~1 o~ Figure 5.
The operation of thi.s start.in~ circuit is descrlbed :I.ater~

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~ lOh ~ ure 7 sllo~Js al~ arxarlgement similar to that of Fi~ure 5 in gr~a-ter de~ il. Thus the capacitors C1 and C2 ~ilL be seen on ~e rigilt o~ tlle ~igure co~ected in scr.;es across t~le po~er supply terminals 11 and 10~
5. Similarly, l;he -transi.stors T1 and T~ each :i.n serles with a diocle ~1 or D2 will ~e seen con~ected in series wi th each other across the same pair o:~ terminals, whilst the junction bet~ieen the transistors and -the junction between the cal~ici-tors are connected through the inductor 10. L1 ~hich is shunted by a resistor RI" which, QS illdiCated above, may be a primary ~rinding o.~ a trans.~Qrmer or a windirlg inductively coupled to ~lle inductor L1.
The capacitors C1 and C2 share the supply voltage resistors R3 and R4 servi~g to compensate for unequal 15. leakage curren-ts ,.so that the potential at the juncti.on of C1 and C2 wlth respect to the earthed terminal 10 is hal~ the potent:i.al at the terminal 11. The ., ~nductor L1 and a capacitor C~ ~orm a tuned tank circuit.
Power is ~ed into the "~ank" circuit by the alternate 20, conduction of' the transistors T1 and T2, the diode,s D1 and D2 preventing reversc transistor current, It ls believed ~hat the remainder o~ th~ c.i.rcuit ; ~ill beGlear ~rom the di.agra~, ~alcen with the ~ollow.ing descr:lption of .its operation.
25. Starting .is achieved by :~irill~ a Thyristor Th1, As , ..
~W~M/AS 11, ' ' . ' 8;~
descr~ cl belo~ a logic s~stcm on thc left o:~ the d:i.agram senses ~he po~;e:~!tl~l at ~the point A; i:f this is ntgative ~r:l1;h respect to -the li.ne 10 the drive cirsu:it to the tran.sistor T2 is initia~ed, whercas i:f tlle point ~ is positive with respect 5. to the ~ e 11 the drivc circui.t to -the -transistor Tl is ini tiated. Ini-ti.ally a s~Jitch Sl :is open, the ~hyristor Thl non-conduc-t.ing, ~nd the capacitor C3 charges via a resistor nl and -the inductor Ll 50 that its hottom plate is positive with respect to -the point ~. A capaci-tor Cl~ also charges via 10. resi.stors Rl and n2. Closin~r the swi-tch Sl (whlch ma~ be effected manv.all~ or electronically) fires the thyristor Thl ~hich "gro~ds" the bot-tom pla-te o~ the capac:i-kor C~; the ~ - .
capaci.-tor C~ provides la tching current ~or the ~h~rrlstor Thl.
The poin-t A is thus driven negative with xespect to ground~
15. ini-tiating the drive to transistor T2. Thereaf-ter, the thyris-tor rhl rem~1s in conduction, reverse currents being by-passed by a diode D3.
.
Clamping diodes D8 and D9 are included as in ~igure 5 -to limit ~he vol-tage appearing across the tral~sistors Tl and 20. T2 durin~ initiatioll of circuit oscillations and in the event o~ an open-circuit appearing on the output o~ the converter.
Considering now the logic system, two identical circui ts are employed providing the drive to each of the translstors Tl and 1'2 :Ln I;urn. ~en ~he point ~ :~all,s 25. below the potent;ial of the negat:Lve :I.lne 10, current ~low,s vla a resistor R5 a diode D5 and a light emitting ~liode D7. ~nlen th~ po~n-t A rises above the poten~ial .

3L~Li~3133~

- of the posit:ive line 11 current flows via the resi.stor R~ a diode D~ and a li~ht emitting diode D6. The L.E.~.'.s for.m part o~ opto-isolators, the d~ode D7 being i assoo:ia-~ed ~rith a transis-tor T101 and the diode D6 5. with a tran~istor T201~
Conduction of the -transistor T101.-turns on a transis-tor '1'102 the output of whicl1 is di~erentia-teQ
by a capacitor C102 into the base o~ a transistor T103.
The resultanl: negative going pulse at -the collector 10, passes through a diode D102 to ;trigger an integrated circuit IC1 CoQperating with a resistor VR, and a capacitor C6 to ~orm Q timer of whioh the timing period now coNmences. Simultaneously with -the trig~ering o~
. the timer, a fu.rtherintegra-ted circuit IC101 here 15. conn~cted as a ~lip-~lop is triggered by the pulse frorn the collector o~ the translstor T103. The output of ~thei.ntegratecl circuit IC1"~oing hlg~'removes the reset signal, triggering being therefore effected directly . .
by the transistor T103. The output o~ the integrated 20. circuit XC101 "going high" turns on a transistor T104 ~rlerg:isillg aprlmarywin(ling of a trans~or.mer TR1 which pro~ides base drive to the transistor T~. With the circult IC101 high,,a transistor T107 is held on, ' remo~rin~ the base drive o~ a transistor T106.`
25. At the end o~ the tlming per.iod o~ the ~lmer IC19 .its .. ' E~ S '~ 3 .

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ou-tput "goe.~ low" resett:in~ ~he in-te~rrated ci.rcui~ IC101 removi.n~r the base ~ ive to the ~ransis-tors T104 and '1'107. Thc transls-tor T'106 -thus "turns on", energising a second prirnary wi.ncl:i.ng oI' the -transformer T~1 and 5. applylng reverse base ~r.i.ve -to r~.'2 turning lt off.
~ he circuit associated wlth the drive to the transistor T1 per~orms .in an identical manner be.ing initiated by the opto-isolator consisting o~ the dlode DG and tran.s.istor T201, (o-ther corresponding components 10. bearing numbers 200 odd instead o~ 100 odd)~ I-t should be noted that common timing components i.e. YR1sR75 C6 and IC1 are used,for both circui-ts.
It should further be noted tha-t the timing periods co~nence when -the potential o~ -the point A becomes more 15- positive -than the positive line 11 or more negati.ve than the ~egatLve line 10, but due to the diodes D5 and D6 power can only be tra~sferred from the supply to the tank circuit when the said potential a~ain has passed through that of the sa.id line and the respective transistor 20. and diode become forward biased. Thus the circuit is t`o'a large degree self regulating under conditions of CQ~stant load and supply voltage. Any increase in the amplitude of osclllation will reduce the ~oncluction peri.od and hence energy trans~er and vice versa.
25. ' The power output ca~ be varied by adjustment o~ the EWEM/~S 14. -~ ' : . .

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variable resls-tor VR1 to var~ the resis-tarlce of the resistox c~paci-tor t.imer :~G1.
Xn the arrang~ments o~ F:igures 3 to 7 witl1 -the load in paral:lel with the .inductor of the tank circu:it, 5. control i.s ~ffected by increasing or decreas:ing the amplitude of the tank oscil:Latlo~s. In the Gase wherc the load i 5 a ba-t-tery the amplitu~e must vary in response to the state o~ charge of the battery and thus its terminal voltage. Perrni-tting the volta~e to rise 10. above -the positlve rail or fall below the neKative achic~ves this objective. This implies the use o~ swl-tching devices ~ated to wi-thstand voltages considerably ir~ exce~s o.~ the supply.
In the circuit shown in Figure ~, totall~ di.~feren-t 15. considerations apply. The voltage excursioIl of the point A is co-nstrained to the supply rails by diodes D9 and D10, with the load illusirated by a. transformer, connected in .series with the inductor of the tank circuit.
The tank capacitor C3 is connected in parallel with the ~o. combination.
~ igure 9 shows the e~uivalent c:lrcuit durin~ the conduction o~ one or other o~ the swltching transistors t.
~;~scd ~or ch~r~3~n~ a ba~ry ~he .ioa~l voltage sho~n as VL is o~
coursc the re.~lec-ced voltage appearin~r across the trans~ormer prllnary o~ ~he .. ..
25. actua:L battery connect~d across the recti.~icd .secondar~

. EWEM/AS 15.

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~ 3i7 OlltpUt. R cleno-tes tlle~ system resistance ~Jhich can be i~,J,noretl . ' Fi~ure 10 5ho~/S ~he equ.ivalent clrcuit during the non conducti.on periods of the~ .sw.i.tcl1ing transistors.
5. Thus with the -trans:is~or T2 conducting current IL
will rise in -tlle induc1;or El,linearly a~ a ra~e determirled by -the ~iff'ere~lc,e between V/2 and the reflected load voltage ancl ~he circuit constan-ts. A~ter a given -time ~rans:istor cond~lc-tion is termi,na-ted ancl the potential 1~. of the poin-t A rises sinusoiclally towards the positive :: :
supply line. It will be assumed that sufficient energy ; ' has been supplied during -the conduc-tion of onea device to ensure thac the point ~ attains -the potential of the opposite rail ~hen or before -the current in the tank - 15~ circuit ~alls to zero. If zero, -the ~second dev.ice conducts im~ledlal,ely the point A attains the ra:il ; po-ten~ial, if not, current is initially passed back to he supply via -the parallel diode, the trans:istor~then - turns on. Load current, i.e. ta~ c,urrent, can be 20. controlled by varying the conduction time of the swi-tch:i.ng devices. Figure 11 illusl;ra-tes the voltage and c,urrent wave~orms ~or z~ero tanlc currents at reversa~ ure 12 shows ~he same wav~forms ~or non~^~ero condition.

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~WEM/AS 16, : ' :

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A converter including a resonant tank oscillatory circuit comprising a capacitor and an inductor, switching means including a solid state switching device connected to apply a predetermined potential difference from a DC supply to the inductor, and synchronising means responsive to the oscillation of the oscillatory circuit for switching on the switching means for a fraction of a cycle only when the instantaneous value of the oscillatory potential difference across the inductor is already not less than a predeter-mined value, to delay the fall of the said oscillatory potential difference and inject energy from the supply into the oscillatory circuit, and a load circuit coupled or connected to the tank circuit.

2. A converter as claimed in claim 1 in which the switching means includes a solid state switching device in series with a diode.

3. A converter as claimed in claim 1 in which the switching means include a solid state switching device in parallel with a diode.

4. A converter as claimed in any one of claims 1, 2 or 3 in which the tank oscillatory circuit comprises an inductor connected in series with a capacitor across the DC supply and the switching means is connected in parallel with the capacitor.

5. A converter as claimed in claim 1 of push pull form which includes a pair of supply capacitors connected in series with each other across the DC
supply, and a pair of solid state switching devices each in series with a diode also connected in series with each other across the supply, and in which the inductor of the tank oscillatory circuit is connected between the junction of the supply capacitors and the junction of the switching devices.

6. A converter as claimed in claim 1 of push pull form which includes a pair of supply capacitors connected in series with each other across the DC
supply, and a pair of solid state switching devices each in parallel with a diode also connected in series with each other across the supply, and in which the inductor of the tank oscillatory circuit is connected between the junction of the supply capacitors and the junction of the switching devices.

7. A converter as claimed in claim 5 or claim 6 in which the capacitor of the tank oscillatory circuit is connected between the junction of the switch-ing devices and one or each supply terminal.

8. A converter as claimed in claim 5 or claim 6 in which the capacitor of the tank oscillatory circuit is connected in parallel with the inductor.

9. A converter as claimed in claim 5 or claim 6 in which the capacitor of the tank oscillatory circuit is shunted by a resistor.

10. A converter as claimed in claim 5 or claim 6 in which the capacitor of the tank oscillatory circuit is connected in series with a solid state switching device for starting oscillation.

11. A converter as claimed in Claim 1 in which the load circuit is connected or coupled to the tank oscillatory circuit in parallel with the inductor.

12. A converter as claimed in Claim 11 in which the load cir-cuit is connected to a secondary winding of a transformer having a primary winding in parallel with the inductor of the tank oscillatory circuit.

13, A converter as claimed in Claim 11 in which the load cir-cuit is connected to the secondary winding of a transformer of which the inductor of the tank oscillatory circuit forms a prim-ary winding.

14. A converter as claimed in Claim 1 in which the load circuit is coupled or connected to the tank oscillatory circuit in series with the inductor.

15. A converter as claimed in Claim 14 in which the load circuit is connected to the secondary winding of a transformer having a primary winding in series with the inductor of the tank oscill-atory circuit.

16. A converter as claimed in Claim 1 in which the frequency of the tank oscillatory circuit is at least 1 KHz.

17. A converter as claimed in Claim 16 in which the frequency of the tank oscillatory circuit is of the order of 25 KHz.

18. A battery charger incorporating a converter as claimed in Claim 1 in combination with a rectifier to supply it from A.C.
mains, a high-frequency transformer and a rectifier for supply-ing unidirectional charging current.