CA1276977C - Self-oscillating high frequency power converter - Google Patents

Abstract

SELF-OSCILLATING HIGH FREQUENCY POWER CONVERTER

Abstract A self-oscillating power converter utilizes a MOSFET power transistor switch with its output electrode coupled to a tuned network that operatively limits the voltage waveform across the power switch to periodic unipolar pulses. The transistor switch may be operated at a high radio frequency so that its drain to gate interelectrode capacitance is sufficient to comprise the sole oscillatory sustaining feedback path of the converter. A reactive network which is inductive at the operating frequency couples the gate to source electrodes of the transistor switch and includes a variable capacitance as a means of adjusting the overall reactance, and hence the converter's switching frequency in order to provide voltage regulation. A resonant rectifier includes a tuned circuit to shape the voltage waveform across the rectifying diodes as a time inverse of the power switch waveform. (FIG. 1).

Description

769~77 SELF-OSCILL~TING HIGH FREQUENCY POWER CONVE~TER

. Eield_of the_In~ention This inYention relates to self-oscillating DC-to-DC power converters operating in the high radio fre~uency range; and more particularly, to the oscillatins inverter, resonant rectifier circuitrY and associated regulatioD circuitrY.
B~round of the Invention __ __ ____ ______ __ A typical s~itching-type pover converter circuit operates by storing and releasing energy in various discre~e capacitive and inductive components during each cycle of operation, where the time interval for each cycle is determined by the switching frequency. An increase in switching freauency reduces the storage time interval and the level of energy stored in reactive components during any one particular cycle of operation. In principle this increase in frequency permits reduction of both the physical and electrical sizes of magnetic and capacitive storage elements for any particular power capacity.
Inasmuch as a significant increase in operatir.g fre~uency of a converter promi~es a significant size reduction in the circuit comPonents on the basis of energy storaqe per unit ~olume, the fact that the s~itching frequer.cy of power converters has not increased dramatically is indicative of other constraints on the increase of operating fre~uencies. For examPle, the svitching s~eed of bipolar semiconductor switching devices is limited by charge storage, thereby limiting the benefits to be achie~ed from high frequency operation.
nOSFET switching devices ~ay be u~ed in place of bipolar devices; hovever, their ..

7Çi977 switching speeds are limited by device capacitances and parasitic lead ~ire inductances.
Circuit comDonents generally include parasitic electrical parameters that produce undesirable effects at hi~h frequencies, and considerable design effort must be expended to cvmpensate for them. For example, at high frequencies, the parasitic inductance and resistance of a capacitor decrease its efficiencY. For inductors interwinding capacitance, ~inding resistance, and cor~
loss al~o limit the maximum practical s~itching frequency.
Circuit board laYouts also contribute numerous stray capacitances, inductances, and resistances which detract from power supply performance at hi~h frequency. Because of these complicating factors, it is extremely difficult to produce a traditional s~itching po~er supply circuit that operates at frequencies much above 500 Khz.
Despite the theoretical advantages of high frequency operation of po~er conversion circuits, these circuits have not been practical because of the manY
component and design proble~s related to operational difficulties at very high frequencies. One high frequencY
power supply ~hich surmounts these difficulties is disclosed iD U. S~ patent 4~449-174~ That patent discloses a High Freguency Resonant Po~er Converter that can operate at high radio frequencies.
That circuit ~as desi~ned to benefit from the advantages of high freguency operation by using the ~arasitic or adjunct reactive electrical characteristics of components as positi~e circuit elements. The term adjunct component is used herein to mean an electrical component characteristic inherent in a device, component, or length of conductor that is often considered a deleterious parasitic component but Yhich is fully and ~ositively utilized in the illustrative circuit herein 3~ embodying the princi~les of the invention. The switching device of the po~er train described in the Ziesse patent referenced above is driven by a separate or independent ,: .. . .. ,~ . ~,.

7~ 77 high frequency signal source. Voltage regulation is achieved by providing a range of frequency adjustment which is ad~usted either directly or by feedback means to attain a desired output voltage level. Hence, the signal source driving the power switching device must be capable of operating over a sufficiently wide band of frequencies to provide the converter with a regulated output voltage over a range of output current and input voltage that depends upon the converter's usage.
The added circuitry of a separate high frequency driver stage to drive the power switching device and provide frequency adjustment for regulation adds complexity to the converter in terms of the component count. If the drive circuit has a wide bandwidth to accommodate the frequency adjustment range, it cannot be precisely matched into the gate, and much of the drive energy is wasted. To achieve the desired high efficiency, a drive circuit must have a narrow instantaneous bandwidth and be tunable over the frequency adjustment range. A separate, tunable drive circuit, however, adds still further to circuit complexity or component count.
Summary_~f the Inventlon In accordance with an aspect of the invention there is provided a self-oscillating power converter comprising input means for accepting a DC voltage source, a semiconductor power swi~ch including first and second main conduction path electrodes and a control electrode and further including at least an interelectrode capacitance between the first main conduction path electrode and the control electrode and having its first main conduction path electrode coupled to the input means, a feedback network for sustaining oscillations in the power converter including: the interelectrode capacitance coupling the first main conduction path electrode and the control electrode, and a variable inductive circuit coupling the control electrode to the second main conduction path ' .... .

- 3a -electrode, the interelectrode capacitance and the variable inductive circuit having reactive values such that a sinusoidal voltage is generated at the control electrode causing oscillations to occur whereby the interelectrode capacitance is sufficient to be operative as an exclusive ~eedback path; a tuned network connected to the first main conduction path electrode and having an inductive reactive impedance and operative at a frequency of oscillation as established by the feedback network for controlling a voltage waveform at the first main conduction path electrode to be continuing for a portion of a cycle of operation while the semiconductor power switch is non-conducting and to be discontinued while the semiconductor power switch is conducting.
In accordance with another aspect of the invention there is provided a power converter comprising input means for accepting a ~C power source, output means, a MOSFET
power switch coupled for receiving power from the input means and including interelectrode capacitances, a frequency tuned network coupling an output electrode of the MOSFET power switch to the output means, and operative for maintaining current through the power switch exclusive of voltage across the power switch, an oscillatory feedback network including: an interelectrode capacitance coupling the output electrode to a control electrode of the MOSFET
power switch and a frequency control circuit including an inductor and variable capacitance coupling the control electrode to a third electrode of the MOSFET power switch, and operative for supplying a continuous sinusoidal drive signal at the con~rol electrode at a sufficiently high frequency so that the interelectrode capacitance coupling the output electrode to the control electrode is an exclusive feedback path.
The self-oscillating power train disclosed herein offers a solution to this problem in that the power switch ; is driven via a circuit having relatively few components 1~7~9'77 - 3b -and regeneratively deriving ~he drive power directly from the power train itself. Furthermore, its narrow tunable bandwidth permits operation at high overall efficiencies.
A self-oscillating power converter embodying the principles of the invention utilizes a MOSFET power switch ~insulated gate field effect transistor) with its output electrode coupled to a tuned network that operatively limits the voltage waveform across the power switch to a fraction of a cycle of operation. The MOSFET power switch may be operated at a high enough radio frequency so that its internal (i.e., parasitic or adjunct) drain to gate -``` 12769~7 capacitance is sufficient to comprise the sole oscillatorY
sustaining feedback path of the converter~ At loier frequencies, it may be necessary to add supplemen~ary external capacitance, ho~ever the principle of o~eration remains the same~
A reacti~e net~ork that is inductive at the operating freguency of the converter couples the gate to source electrodes of the ~OSFET sYitch and includes a ~ariable capacitance as a means of adjusting the oYerall inductive reactance. The variable capacitance maY
comprise a varactor diode arrangement. Another suitable arran~ement, or a directly variable inductance ~ay be used in ~lace of using variable capacitance control. The converter's s~itching frequency is su~stantially controlled at a value slightly less than the resonant freauency of the inductive network in shunt connection with the gate to source interelectrode caPacitance to obtain the correct phase of the feedback signal. The overall effect of the tuned circuitry connected to the ~OSF~T gate is to respond predominantly to the fundamental component of the drain to source Yoltage and produce a continuous and substantially sinusoidal ~aveform drive signal at the gate electrode having the proper amplitude and phase to sustain the self-oscillation.
The self-oscillating drive arrangement for the power s~itch has an inherent narrou bandwidth that is tuneable cver a vide frequency range. Rence, the frequency of operation of the po~er conYerter maY be varied for regulation purposes vithout losing the efficiency advantages of a narrow bandvidth drive. This self-oscillating arrangement is also simpler in construction and has fewer component parts than an ~gui~alent indePendent drive circuit for the po~er switch bandvidth.
A feature of the converter is the use of resonant rectifier that positively utilizes leakage inductance of the converter po~er transformer and its 5 ~ ~2769 77 parasitic lead inductances as well as the adjunct ca~acitances of the rectifier diodes as part of a tuned LC
circuit. This tuned circuit shapes the volta~e waveform across the diodes to appear substantially as a time reversal of the voltage vaveform across the inverter s~itching device.
It s readilY apparent that this converter circuit advantageously utilizes the adjunct reactances of the components in a positiYe manner as ~art of the operative converter and that by use of self-oscillation significantl~ im~roves the o~erall efficiency and reduces the oYerall parts count of the converter. This use of adjunct components permits a practical converter o~erating at high radio frequencies to be realized using fe~er discrete components.
BElef De CEl2t-Qn-of-th- Dra Y_n qs An understandiDg of the invention maY be obt2ined by reference to the following specification and the drawings in Yhich:
FIG. 1 is a functional block diagram of a high frequency DC to~DC po~er converter embodying the ~rinciples of the invention-FlGo 2 is a simPlified schematic of a self-oscillating power train of the high frequency DC-to-DC
po~er converter;
FIG. 3 is a circuit schematic of the power con~erte{ including a functional block diagram of a control circuit fcr voltage regulation and sho~ing the use of adjunct Parasitic elements to reduce the num~er of circuit components at high frequencies;
FIG. 4 discloses signal vaveforms to assist in describing the oPeration of the pover converter sh~vn in FIGS. 2 and 3;
FIGS. 5, 6 and 7 are schematics of al~ernate self-oscillating power inverter arrangements embodYing the principles of the invention; and , - 6 - ~769~7 FIGS. 8 and 9 are schematics of alternate resonant rectifying arrangements embodying the PrinciPles of the invention.
Detailed Descri~tion A high frequency DC-to-DC po~er converter embodying the principles of the invention is shown functior,ally in FIG. 1 and comPrises a power train circuit including a self-oscillatin~ resonant inverter 2, an irpedance transformer 10 and a resonant rectifier~filter net~ork 3. A contrvl circuit includin~ an error amplifier 6 is utilized to supply an error signal for controlling the fre~uencY of inYerter 2, and hence, achieve a regulated voltage at output 4. A DC voltage is a~plied to input terminal 1 and is coupled to a switching device in the inverter 2 and to a start-up circuit 8. The self-oscillating inverter 2 does not self-start, hence the start-up circuit 8 is included to respond to a ~C voltage ; at input 1 and applY a trigger signal to initiate oscillations in the self-oscillating inverter 2. The output of the inverter 2 is coupled to an i~pedance transformer 10 which, in turn, is connected to a rectifier filter circuit 3. The rectified output, a DC voltage, is COUD1ed, Via lead 4, to a load indicated herein for illustrative purposes as.resisti~e load ~.
The self-oscillating inverter circuit 2 and the impedance transformer 10 include a series ~-C circuit into which the transistor p~wer svitch opera~es. The oYerall power train netvork comprises a tuned network uhich controls the current and ~oltage vaveform across the pover 3~ s~itch of the inverter circuit so that there is minimal overlap during switching transition intervals and thereby ; reduced pover dissipation during these s~itching transitions. A co~plete discussion of a po~er converter having a si~ilar in~erter arrangement and ~hich is driven rather than self-oscillating is disclosed in the ; aforementioned U. S. patent 4,44~174. This patent discusses the details of the various pouer train _ 7 _ ~ ~ ~697~
components and their operation and it is not believed necessary to disclose ~hese matters in detail herein.
The power train of the po~er converter including the self-oscillating inverter is shown in more detail in FIG. 2. DC voltage is a~plied to input terminal 1 Yhich is cou~led to a filter circuit including an RF choke inductor 51 and capacitor 11. The RF choke 51 is coupled to a terminal 107 of a semiconductor Po~er s~i~ch 110 sho~n symbolically as a s~itch.
The output of ~he po~er s~itch 110 at electrode 107 is coupled into a series tuned LC circuit including caPacitOr 13 and inductor ~3 ~hich in conjunction ~ith the rest of the output net~ork and ca~acitor 15 constrains the current and voltage waveforms across the po~er s~itch 110 to assume certain desired characteristics. These ~aYeforms may be seen in FIG~ 4 ~here ~aveform 401 rePreseDtS the voltage ~aveform across the main po~er path of the power s~itch 110. The fundamental sinuscidal component of voltage ~aveform 401 is shown by uaveform 400. The voltage ~aveform 402 represents the drive signal applied betveen control termi~al 108 and terminal 109 of the po~er switch 110.
This ~olta~e ~aveform 402 apFroximates a sinusoidal vaveform that contains a DC component 4Q3 suPPlied as shoun bY the battery 50 or other DC voltage source. The s~itch 110 becomes conducting when the ~aveform 402 exceeds the threshold level 404 of s~itch 110~ It is apparent from these waveforms that current conduction through the po~er s~itch 110 (i.e., ~hile ~aveform 402 is above the threshold level 404) occurs onlY ~hen there is no volta~e drop across the suitch tlO (i.e., waveform 4Q1 is substantially zero). The si~ultaneous existence of substantial current through and ~oltage across the ssitch 110 is thus minimizedO qiving rise to little or no s~itching loss. The ~aveform of the current flo~ing through the series tuned circuit of capacitor 13 aDd inductor 53 has a quasi sinusoidal shape. The series - a ~ 76977 tuned network of capacitor 13 and inductor 53 is coupled t~ a shunt tuned network` includin~ capacitor 97 and inductor ~8 and in turn to the primary vinding 54 of an ideal isolating and impedance matching transformer 55.
The seccndary ~indin~ 56 is connected to a resonant fullwave rectifier including the rectifyinq diodes 131 and t32. This resonant rectifier builds upon the half~ave resonant rectifier disclosed in the aforementioned Zie-cse patent and operates on the sa~e principle. CaPacitOrs 133 and 134 are shovn shunting each diode. These maY be discrete or adjunct capacitances depending upon the diode de~ices used and the frequency of operation~ The inductors 57 and 58, together ~ith capacitors 133 and 134, shape the voltage across the diodes as shown in FIG. 40 The voltage ~avefor~ across rectifYing diode 131 is sho~n by ~a~eform 406 in FIG. 4 and the ~a~eform 407 rePresents the voltage vaveform across diode 1~2. These voltase ~avefor~s, as are apparent from FIG. 4, are substantiallY
a time reverse ~avefor~ of the Yoltage ~aveform 401 appearing across the MO~FET pover switch. The rectified output signal is applied to a filter comPrising inductor 5~ and capacitor 17 supplying a filtered DC
voltage to output terminals 4.
A practical major advantage of the resonant rectifier circuit for high radio frequencY operation is that it can utilize to advantage una70idable parasitic lead inductance a~d ~ransformer leakage inductance as part or all of the inductors 57 aDd 58. Furthermore, inductors 57 and 58 in conjunctioD vith the shunt tuned circuit of caPacitor 97 and inductor 98 act to make the ' input impedance of the rectifier as seen bet~een nodes 150 ; and 151 linear in nature thus ~aintaining a substantially sinusoidal voltage 405 and current at this point.
A Yariable i~ductor 75 is shoun,vith a battery 5~ as coupling the Po~er s~itch control electrode 108 to electrode 103 vhich is in Parallel ~ith capacitance 10. The feedback signal across capacitance 10 _ 9 _ ~76977 and the inductor 75 is a continuous quasi sinusoidal signal shown as ~aveform 402 in FIG. 4. This signal .s phzse dis~laced from the fundamental component 400 of the voltage ~aveform 401 appearing across power switch 110.
This feedback signal shown by ~aveform 402 is offset bY
bias voltage supplY 50 and applied to ~he po~er s~itch control electrode 108 to drive the power switch 110.
A diode 99 shunting the power sYitch 110 conducts reverse currents ~hich are present under some conditions of inPUt vol~age and output power when the po~er s~itch 110 is in its nonconducting or off state~
This allo~s the converter to operate over a ~ider range of input voltage and output po~er than if diode 99 ~ere not present.
The s~itching frequency of the Po~er switch 110 is controlled in`part by the value of inductance of the variable inductor 75. The phase displacement shown bY 0 in FIG. 4 of the driving Yaveform 402, normally leads ~aYeform 400 by 120 to nearlY 180. The resonant action of inductor 75 in parallel ~ith capacitance 10 responds through feedback capacitor 12 to the voltage Yaveform at the main po~er path electrode 107, ~hich is a periodic unipolar pulse-like vaveform 401, to produce the substantially sinusoidal drive signal at the control elactrode 108 of po~er svitch 110 as shoNn by ~aveform 402 in FIG. 4. The inductor 75 is controlled or Yaried as shown bY a signal aPplied to the control lead 106. This signal applied to lead 106 may be an error signal derived by voltage or current regulation circuitrY in response to a deviation of a Yoltage or current at output lead 4 from a regulated value.
The series I,-C circuit comprising capacitor 13 and inductor 53 acting in concert ~ith the shunt tuned circuit comprising ca~acitor S7 and inductance 98 converts the periodic unipolar signal at the drain electrode 107 into a substantially sinusoidal signal at node 150, ~hich is sho~n by ~aveform 405 in FIG. 4~ This voltage ~ave is ". .

~o _ ~ ~76977 transmitted through ideal transformer 55 and rectified bY
the action of the rectifying diode 131 and 132 each of ~hich produces a voltage signal having a waveform~ ~hich as sho~n by ~aveforms 406 and 407 ~ith a shape 5 characteristic similar to a time reverse of the voltage ~aYeform at electrode 107 of suitch 1100 These rectified signals are filtered by a filter circuit includiDg inductor 59 and capacitor 17 and a DC voltage appears at out~ut terminal 4 and across the capacitor 17.
The self-oscillating action of the inverter 100 does not begin automatically ~hen power is applied to the invention, and kence a start-up pulse must be supplied at terminal 105 to initiate the oscillating action of the power inverter 100.
An embodiment of a self-oscillating power train and associated control and signal processing circuitry to comprise a DC-to-DC con~erter is disclosed in FIG. 3.
This embodiment is suita~le for high radio frequencY
operation. It utilizes the adjunct caPaCitances of a MOSFET po~er switch 311 and Shottky rectifier diodes 331 and 332 as converter circuit elements. An inherent bodY
diode 399 of the power ~OSFET switch 311 operates to conduct reverse currents through the ~SFET s~itching device 311 ~hen it is in the off stats ~that is the channel is not conducting). Furthermore, the magnetizing inductance 354 (sho~n in dotted line) of the transformer 355 serves to replace shunt inductor 98 (sho~n in FIG~ 2), and the leakage and lead inductances 457 and 458 become dominant portions of the inductors 57 and 58, 30 shown in FIGo 2. Hence, inductors 357 and 358 may be smaller than inductors 57 and 58. This use of adjunct elements prGvides a unique advantage in that a practi~al realization of this converter reguires fe~ discrete components.
The inPut and output i~ coupled by a MOSFET
po~er s~itch 311 includins inherent or adjunct interelectrode capacitances 310, 312 and 315. The drain ~ 7~i,9 77 to gate inherent or adjunct capacitance, show~ as capacitor 312, supplies a feedback path fro~ drai~
electrode 307 to gate electrode 308 sufficie~t to sustain self-oscillation in the inverter circuit enclosed bY
dotted line 100 as described below, if the fre~uency of operation is sufficiently high. ~!hile a HOSFET po~er s~itch is shown herein, it is to be understood that other semiconductor po~er switches may be substituted for the HOSFET and the necessary adjunct elements supplied by discrete devices ~hen needed.
The Po~er converter of FIG. 3 is voltage regulated in response to an error signal supplied by a feedback network at control terminal 106. This error signal is supplied by ~he output of error signal am~lifier 6 vhich comPareS the converter's output voltage ~ith a reference voltage level supplied by reference voltage s~urce 5. This error voltage is coupled through amplifier 115 to a junction of two diodes 138 and 139 ~hich are connected in series ~ith the inductor 375 ~hich is, in turn, connected to the gate 30~ o~ power s~itch 31~. The diodes 138 and 139 each have significant nonlinear capacitance as shovn by capacitors 135 and 136.
The error ~oltage signal applied to the junction of the two diodes 138 aDd 139 varies their joint voltage ; 25 responsive caPacitance, and hence, alters ~he overall inductive reactance of the series connection of inductor 375 and ~he diodes 138 and 13~. The overall series circuit is designed to al~ays have an inducti~e reactance, and in conjunction ~ith the drain to gate capacitance 310 provides the desired phase shifted . feedback signal to the gate electrode 108 for oscillations. By altering the capaciti~e reactance of the ; tvo diodes 138 and 139 vith the error signal aPPlied to lead 106 the overall i~ductive reactance of the feedback netYork may be controlled to Permit Yariatisns in the fre~uency of oscillation of the inverter 100. The variations in frequency in combination ~ith the tuned ..~

-- 2 -- ~L276977 out~ut net~ork varies the DC output voltage level at lead 4, and he~ce, through the feedback control circuit achieves voltage regulation.
Another inverter circuit em~odiment suitable for application in the power train of the converter is sh~n in FIG~ 5. In this embodiment, the control is apPlied to a junction of diode 138 and a fixed caPacitance 19. The ~ariation of capacitance of diode 138 is sufficient in combination ~ith inductor 375 to achieve the desired reactance range.
In the inverter circuit embodiment shown in FIG. 6, the control signal is applied to a junction of the series connected diodes ~3B and 13~. The diodes are connected in shunt vith a series connection of ~5 inductor 375 and fixed DC blocking capacitor 77. As above, the control signal ~aries the diode capacitance to alter the overall inductive reactance of the net~ork as presented to ga~e ~erminal 30O- The inver~er embodiment of FIG. 7 applies the control signal to a junction of 20 series connected diode 138 and a fixed capacitor 18. Nany additional variations of both the inverter circuit and the power train will be readily apparent to those skilled in the art ~ithout deParting from the spirit and scoPe of the invention~
The converter in FIG. 3 includes a full~ave resonant rectifier comprising Schottky diodes 331 and 332.
Shown in dotted lines are t~o capacitors, 333 and 334, ~hich represent the inherent or adjunct c~pacitances of diodes 331 and 332, rsspectively. T~e secondary lead and transformer leakage inductances sho~n as inductors 457 and 458 of the secondary ~i~ding 356 combined vith discrete inductances 357 and 358 and caPacitances 333 and 334 form tuned circuits ~hich as described above ~ith reference to FIG. 2 shape the voltage ~aYeforms appearing across ghe diodes 331 and 332 to approximate a time reversed image of the vaveform across the ~OSFET po~er svitch 311.

., ~ '~7~;977 An alternative embodiment of a resonant rectifier is shown in FIG. 8 in ~hich the return lead 801 is connected to one terminal of the secondary ~inding 856 as opposed to being connected ~o a centertap of the secondary winding. The vaveform across diodes 831 and 832 is shaped in the same manner as for the rectifiers shown in FIGS~ 2 and 3.
A rectifier arrangement for multiple outPUts is sho~n in FIG. 9 in vhich a positive output voltage appears at lead 904 and a nega~ive output voltage appears at lead 905. The positive output section is identical in principle to the rectifier a~pearing in FIGo 3~ The negative output section utilizes added inductors 977 and 988, diodes S78 and 987 and output filter inductor 973.
This arrangement also utilizes both discrete and adiunct reactances to shape the Yoltage ~aveform across the diodes 978 and 987 in the same ~aY as described ~ith reference to FIG. 3~

.

Claims (23)

1. A self-oscillating power converter comprising:
input means for accepting a DC voltage source, a semiconductor power switch including first and second main conduction path electrodes and a control electrode and further including at least an interelectrode capacitance between the first main conduction Path electrode and the control electrode and having its first main conduction path electrode coupled to the input means, a feedback network for sustaining oscillations in the power converter including:
the interelectrode capacitance coupling the first main conduction path electrode and the control electrode, and a variable inductive circuit coupling the control electrode to the second main conduction path electrode, the interelectrode capacitance and the variable inductive circuit having reactive values such that a sinusoidal voltage is generated at the control electrode causing oscillations to occur whereby the interelectrode capacitance is sufficient to be operative as an exclusive feedback path;
a tuned network connected to the first main conduction path electrode and having an inductive reactive impedance and operative at a frequency of oscillation as established by the feedback network for controlling a voltage waveform at the first main conduction path electrode to be continuing for a portion of a cycle of operation while the semiconductor power switch is nonconducting and to be discontinued while the semiconductor power switch is conducting.

2. A self-oscillating power converter as defined in claim 1 and further including:
voltage regulation means comprising:

means for sensing an output voltage at the power converter, a reference voltage source, an error amplifier for comparing a voltage of the means for sensing and a voltage of the reference voltage source and generating an error voltage, and means for applying the error voltage to the variable inductive circuit in order to control a reactance thereof.

3. A self-oscillating power converter as defined in claim 1 or 2 wherein the variable inductance circuit comprises:
a fixed inductor, and first and second diodes having capacitive characteristics and being connected to each other at a common node oriented thereat with opposing polarity and further connected in series with the fixed inductor.

4. A self-oscillating power converter as defined in claim 1 or 2 wherein the variable inductance circuit comprises:
a series connection including a fixed inductor and a capacitor, first and second diodes having capacitive characteristics and being connected to each other at a common node oriented thereat with opposing polarity and further connected in parallel with the series connection of fixed inductor and the capacitor.

5. A self-oscillating power converter as defined in claim 1 or 2 wherein the variable inductance circuit comprises:
a fixed inductor, and a fixed capacitor and a diode having capacitive characteristics and the fixed capacitor and diode connected to each other at a common node and further connected in series with the fixed inductor.

6. A self-oscillating power converter as defined in claim 1 or 2 wherein the variable inductance circuit comprises:
a series connection of a fixed inductor and a capacitor, a fixed capacitor and a diode having capacitive characteristics and the fixed capacitor and diode being connected to each other at a common node and further connected in parallel with the series connection of fixed inductor and the capacitor.

7. A self-oscillating power converter as defined in claim 1 or 2 further including an output rectifier coupled to the tuned network and comprising:
a rectifying diode, and an LC network tuned so as to form part of the tuned impedance of the tuned network.

8. A power converter as defined in claim 1 or 2 wherein the semiconductor power switch includes second interelectrode capacitance between the second main conduction path electrode and the control electrode and the frequency of oscillation established by the feedback network is less than the resonant frequency of the network comprising the variable inductance and the second interelectrode capacitance.

9. A power converter as defined in claim 1 or 2 wherein the feedback network is operative for converting a periodic pulsed unipolar voltage waveform at the first main conduction path electrode to a continuous full cycle substantially sinewave voltage waveform at the control electrode.

10. A power converter comprising:
input means for accepting a DC power source, output means for accepting a load to be energized, a transistor switch coupled for receiving power from the input means and having a control electrode, first and second power carrying electrodes and having inter-electrode capacitance between the first and second power carrying electrodes and between each power carrying electrode and the control electrode, a tuned network coupling the first power carrying electrode to the output means and operative for controlling a voltage waveform at the first electrode lasting for portion of a half-cycle of operation of the transistor switch during its nonconducting state, a feedback network including an interelectrode capacitance coupling the first power carrying electrode and the control electrode and a frequency control network including an inductance and a variable capacitance coupling the control electrode to the second power carrying electrode, the interelectrode capacitance and the inductance and variable capacitance being tuned to generate a quasi-sinevave voltage at the gate electrode with a leading phase of 120° to 180° with respect to a fundamental component of the pulsed voltage waveform at the first power carrying electrode, and the feedback network being operative to generate the quasi sinewave voltage at the gate electrode at a frequency when the interelectrode capacitance is operative as an exclusive feedback path.

11. A power converter as defined in claim 10 wherein:
a frequency of operation of the converter established by the feedback network is less than the resonant frequency of the frequency control network comprising connected inductance and variable capacitance and an interelectrode capacitance between the second power carrying electrode and the control electrode.

12. A power converter as defined in claim 10 and further including:
voltage regulation means comprising:
means for sensing a voltage at the output means, means for comparing a voltage at the means for sensing with a reference voltage and generating an error voltage, and means for utilizing the error voltage for controlling the variable capacitance.

13. A power converter as defined in claims 10 or 11 or 12 wherein the variable capacitance comprises two diodes connected in series and having their cathode terminals joined to a common node.

14. A power converter as defined in claims 10 or 11 or 12 and further including:
an output rectifier coupled between the tuned network and the output means and comprising:
rectifying diodes, and an LC network tuned so as to form with the tuned network and the output means a composite tuned network operative for shaping a voltage waveform at the first power carrying electrode.

15. A power converter comprising:
input means for accepting a DC power source, output means, a MOSFET power switch coupled for receiving power from the input means and including interelectrode capacitances, a frequency tuned network coupling an output electrode of the MOSFET power switch to the output means, and operative for maintaining current through the power switch exclusive of voltage across the power switch, an oscillatory feedback network including:
an interelectrode capacitance coupling the output electrode to a control electrode of the MOSFET
power switch and a frequency control circuit including an inductor and variable capacitance coupling the control electrode to a third electrode of the MOSFET power switch, and operative for supplying a continuous sinusoidal drive signal at the control electrode at a sufficiently high frequency so that the interelectrode capacitance coupling the output electrode to the control electrode is an exclusive feedback Path.

16. A power converter as defined in claim 15 and further including:
voltage regulation means comprising:
means for sensing a voltage at the output means, means for comparing a reference voltage with a sensed voltage of the means for sensing and generate an error signal therefrom, and means for coupling the error signal to the variable capacitance for controlling a capacitance value thereof.

17. A power converter as defined in claims 15 and 16 wherein:
the variable capacitance comprises two diodes connected in series and having their main conduction paths oriented in opposite directions.

18. A power converter as defined in claims 15 and 16 wherein:
the frequency tuned network is inductively reactive at an operating frequency of the converter and includes components which limit a voltage waveform at the output electrode of the power switch to a part of a cycle of operation, and the interelectrode capacitance and the frequency control circuit of an inductor and variable capacitor being at values to cause a continuous quasi sinewave to occur at a control electrode of the power switch at a frequency below a resonant frequency of a circuit connection of the inductor and variable capacitor and an interelectrode capacitance between the control electrode and the third electrode of the power switch.

19. A self-oscillating power converter comprising:
input means for accepting a DC voltage source output means, a semiconductor power switch coupled for controlling energy flow from the input means to the output means and including first and second main conduction path electrodes and a control electrode for controlling current flow in a main conduction path of the power switch, a tuned network coupling the first main conduction path electrode to the output means and operative for controlling a voltage waveform at the first main conduction path electrode to be continuous for a portion of a half-cycle of operation of the semiconductor over switch during its nonconducting state, a tunable narrow bandwidth drive network for driving the power switch including;
capacitive means coupling the first main conduction path electrode to the control electrode, and inductive means having variable inductance in response to a control signal and coupling the second main conduction path electrode to the control electrode, control means for supplying a control signal varying the inductance of the inductive means, whereby the capacitive means and inductive means operate cooperatively to supply a quasi sinewave voltage at the control electrode and the control means is operative for varying a frequency of operation of the converter to control a signal amplitude at the output means.

20. A self-oscillating power converter as defined in claim 19 wherein the semiconductor power switch includes substantial interelectrode capacitance between the first main conduction path electrode and the control electrode and a capacitive value of the interelectrode capacitance being sufficient to comprise the capacitive means.

21. A self-oscillating power converter as defined in claim 20 wherein the control means includes:
voltage regulation means comprising:
means for sensing a voltage at the output means, means for comparing a voltage sensed by the means for sensing with a reference and generating an error voltage, and means for utilizing the error voltage as the control signal to vary inductance of the inductive means.

22. A self-oscillating power converter as defined in claim 21, wherein the inductive means includes a tunable network having an overall inductive reactance at a frequency of operation of the converter and including a fixed inductor and a variable capacitance connected together, whereby the variable capacitance responds to the control signal and the overall inductive reactance of the network is varied.

23. A self oscillating power converter as defined in claim 22 further including an output rectifier having diodes and having tuned circuit elements so that a voltage waveform across the diodes is substantially a time reversal of the voltage waveform across the semiconductor power switch.