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Title:
SERIES-RESONANT CONVERTER
Document Type and Number:
WIPO Patent Application WO/1995/032544
Kind Code:
A1
Abstract:
The invention is based on a series-resonant converter, which consists of a self starting, free wheeling, internal non-dissipative passively damped series-resonant oscillator and guarantees a sinusoidal current, by using at least two electronic switches and a resonance network, comprising a series-resonant network, comprising an inductor and a capacitor, supplying energy of at least one source to a serial load, which can also consist of a source. Very strong energy fluctuations in the series-resonant network will be prevented by substitution of the resonant inductor by at least two transformers, the resonance transfomers and the driver transformer, each containing at least one primary and one secondary resonant coil, by which the primary coils are in series connected to at least one condenser and form the primary resonant network, of which the primary resonant coil induces a resonant current in the secondary coil, which together with at least one parallel connected condenser forms the secondary resonant circuit. If the induced current will be rectified, smoothed and stored in a buffer condenser, then it can be considered as the actual energy surplus. If the voltage across this buffer condenser is higher than the power source voltage, the buffer condenser itself will act as a source, by which the energy surplus will be passively transferred to the primary resonant network by means of a rectifier and the electronic switches. The energy surplus will be dissipated partially or entirely by the load depending on the amount of the energy surpplus and thus stabilisation of the energy balance will be achieved.

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Inventors:
LOUSBERG SEVRIEN HUBERT THOMAS (NL)
Application Number:
PCT/EP1995/001966
Publication Date:
November 30, 1995
Filing Date:
May 24, 1995
Export Citation:
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Assignee:
LOUSBERG SEVRIEN HUBERT THOMAS (NL)
International Classes:
H02M3/338; H02M7/5383; H05B41/282; (IPC1-7): H02M3/338; H02M3/28
Foreign References:
US5283727A1994-02-01
FR2696290A11994-04-01
EP0071284A11983-02-09
DE3917850A11989-12-07
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Claims:
CLAIMS
1. A seriesresonant converter supplies resonant electrical energy by using a seriesresonant network and at least two electronic switches, of a power source, which can consist of a direct power source or of one or more phasing alternating power sources, of which the alternating power sources are being rectified by at least one diode, to a load, which in itself can also consist of a source, with the characteristic that stabilization of the energy balance in the resonant network will be achieved by means of a buffer cir¬ cuit, whereby in the conventional situation of a rectified single phase alter¬ nated power source, the smoothing condenser can be omitted and is replaced by the buffer condenser in the buffer circuit to improve the power factor in comparison with the power factor in the conventional situation and by using an autonomous seriesresonant converter whether or not externally control¬ led, whereby the resonant network consisting of at least two transformers, the resonant transformer and at least one driver transformer, each contain¬ ing at least a primary and a secondary coil, of which the primary coils are connected in series to at least one condenser, thus forming the primary res onant circuit, of which the primary coil of the driver transformer induces resonant driver energy into the secondary coil, which forms with at least one in series connected condenser, the resonant driver circuit for the elec¬ tronic switches and whereby the resonant current in the primary coil of the resonant transformer induces resonant energy in the secondary coil, which forms with at least one parallel connected condenser the secondary resonant circuit and it is only when this induced energy will be rectified by at least one diode, smoothed by minimally one inductor, passively stored in at least one buffer condenser, which will be act as a source by itself if the voltage of this buffer condenser is higher than the voltage across the power source, it can be called the energy surplus, which is transferred the stored energy surplus by means of a rectifier and the electronic switches to the primary resonant circuit, in which this energy surplus will be fully or partly dissi¬ pated by the load depending on the magnitude of the load and the amount of the energy surplus, whereby the remnant amount of this surplus will be stored again in the buffer condenser and thus stabilization of the energy balance is achieved.
2. A series resonant converter according to claim 1 supplies resonant electrical energy to a load, with the characteristic that the two electronic switches are of a different type and without external control, therefore a resonant transformer and only one driver transformer with only one primary and one secondary coil will be used, of which the primary driver coil is con¬ nected in series to the resonant primary circuit and the secondary driver coil is connected in series to the driver condenser and forms the resonant driver circuit for both electronic switches.
3. A series resonant converter according to claim 2 supplies resonant electrical energy to a load, with the characteristic that the two electronic switches are of the same type and without external control, therefore, instead of using one driver transformer, two identical driver transformer each with only one primary and one secondary coil will be used, of which the primary driver coils are connected in series to the primary resonant cir¬ cuit and each secondary driver coil is in series connected to a driver con denser and thus form the resonant driver circuits for the electronic switches, to which one pair of in series connected diodes are coupled anti parallel to the driver terminals of each electronic switch to make reverse conduction of the resonant driver current possible and also for the possible protection of the driver electrodes of each electronic switch against reverse voltage polarisation.
4. A series resonant converter according to claim 3 supplies resonant electrical energy to a load, with the characteristic that the two electronic switches are of the same type and regulate the energy to the load by means of an external control circuit by blocking up one electronic switch, after it is put OFF by natural commutation and thus the conversion is stopped, therefore the driver circuit is extended with a blockup circuit, which must be conditionally controlled and blocks up the electronic switch, after it is put OFF by natural commutation, whereby the not regulated switch possibly be protected against reverse voltage polarisation by an antiparallel connect¬ ed diode.
5. A seriesresonant converter according to claim 4 supplies resonant electrical energy to a load, with the characteristic that the regulated current supplied to the load, which is ACcoupled to the primary resonant network, of which the regulated resonant current can consist of DC as well as AC energy, of which the frequency is lower then the resonant frequency, there¬ fore a full bridge circuit is used, of which each electronic switch will be regulated by a driver circuit, which whether or not by galvanic separation is regulated by an external control circuit, whereby only one electronic switch can be ON at the same time and whereby each electronic switch poss¬ ibly can be protected against reversed voltage polarisation by an antiparal¬ lel connected diode.
6. A seriesresonant converter according to claim 5 supplies resonant electrical energy to a load, with the characteristic that the regulated DC or relatively low frequency ACenergy current is bidirectional, uses a second bridge circuit with the quality that both bridge circuits have an ACcoupling to the primary series resonant circuit, of which the antiparallel diodes to the semiconductor switches can be used for rectifying the source as well as for protection of the electronic switches.
7. A seriesresonant converter according to claim 2 supplies resonant electrical energy to a resonant network, with the characteristic that the regulation will be achieved without blocking up the oscillation of the reson¬ ant network supplying energy, which can consist of DC as well as AC energy, of which the frequency is lower than the resonant frequency, to a load, which must act resistive and is separated together with the power source is galvanically separated from the regulation circuit, therefore a half bridge circuit and two identical resonant transformers are used, each con¬ taining at least three galvanic separated coils, of which the in antiseries connected control coils are coupled to a controllable direct current source or an alternating current source of relative low frequency compared to the res¬ onance frequency, whereby the induced difference in voltage between the two secondary in antiseries connected resonant coils depends on the magni¬ tude of the resonant current of the two in series connected primary reson¬ ant coils and on the magnitude of the premagnetization of the ferromagnetic cores, of which the premagnetization in its turn depends on magnitude of the control current, whereby the polarity of the induced voltage difference of the secondary resonant coils depends on the direction of the magnetic flux of the premagnetized ferromagnetic cores, of which the premagnetizat¬ ion in its turn depends on the direction of the control current and thus achieves controlling the energy supplied to the load. Rectifiers are being connected to the terminals of the secondary resonant coils and will conduct the energy surplus, so it is suitable for storing in the buffer capacitor and thus the balance of energy is realized.
8. A seriesresonant converter according to claim 7 supplies electrical energy to a load, with the characteristic that an optimized load voltage can be achieved by means of transformation and galvanic separation is accom¬ plished between the control circuit, the power source as well as the load cir¬ cuit, by using an additional in antiseries connected pair of identical coils, which wound around each ferromagnetic core of the two transformers and are called loadcoils, which are connected to the in series connected load circuit consisting of a smoothing coil and the dissipative load itself.
9. The prior described principles, techniques and designs, according to the above mentioned claims, like power converting and regulation or the integration of a buffer circuit in a resonance network for stabilisation of the energy balance, with the characteristic of not being used in connection with a series converter.
Description:
SERIES-RESONANT CONVERTER

The invention is based on a series-resonant converter, which uses at least two electronic switches and a series-resonance network, containing an inductor and a capacitor and supplying energy of at least one source to a serial load, which can also consist of a source.

BACKGROUND OF THE INVENTION

Such an energy converter is ideally suited for the purpose of converting electrical energy from a direct current source, called DC-source, or from a single phase or polyphase AC-sources, in the energy of different amplitudes and/or frequencies, with a pulsating or DC-voltage. The converter, which includes the electronic switches, called switches for short, in power elec¬ tronics well known as Transistors, Thyristors, GTOs, FETs, MOSFETs, IGBTs and the electronic switches which are being developed, such as the MCTs (Mos Controled Thyristors), supplies energy from at least one source to the prior mentioned serial load, through which the energy balance of the series- resonant network is achieved by active control of an external circuit and is known as a series-resonant inverter.

These series-resonant converters are known from articles published in IEEEE Transactions on Power Electronics. Vol. PE-1 No. 1, January 1986: "DC- to-AC Series-Resonant Converter System with High Internal Frequency Gener¬ ating Synthesized Waveforms for Multiwatt Power Levels" , IEEE Transactions on Power Electronics, Vol. 3, No. 2, April 1988: "Phase-Staggering Control of a Series-Resonant DC-DC Converter With Paralleled Power Modules" and in PolyTechnisch ti.idschrift - Energie. March 1992 "Frequentieomzetters vinden baat bij principe van soft-switching".

GENERAL ADVANTAGES OF THE SERIES-RESONANT CONVERTER

Because the switching takes place at the moment of a zero current cross- ing, also called natural commutation, without forcing the switches OFF and because of the limitation of the speed change of voltage ( du/dt) and current ( di/dt) as well as the product of current through and voltage across the switch during the switching time, so called soft switching, most of the here¬ after mentioned benefits are generated.

Restriction of (du/dt) and (di/dt) limits the dynamical losses in the electronic switches and together with the natural commutation, they are the cause of the relative better abilities in relation to "reverse recovery", therefore this converter is very suitable for heavy duty purposes.

The efficiency of the converter is high, due to both the absence of dissipative elements for limiting the output power and the limitation of the dynamical losses in the switches.

The resonant principle guarantees a sinusoidal current pulse, so high frequent electromagnetic interference remains relatively low.

- By synthesising voltages and currents to considerable higher fre- quencies than the synthesized frequency, the short-circuiting quality will be improved and can be limited to one current pulse, which con¬ tains only a small amount of energy.

- Because of the high internal switching frequency a very quick and accurate regulation of the converter can be achieved and this high switching frequency leads also to a reduction of mass and volume of the passive components.

STATE OF THE ART

In the prior mentioned articles different solutions are described for the inherent problem of the balance of energy in the series-resonant network, which implies that when an amount of energy, stored in the resonant net¬ work after each current pulse increases, this will result in an enormous cur- rent and voltage excitation, which will only be limited by the resistance of the converter.

- In the first solution a second power source voltage is used and the balance of energy in the series-resonant network will be achieved by means of controlling the switches by means of a microprocessor.

- A second solution uses a transformer comprising a primary coil, which is connected in series to the series-resonant network and a secondary coil, in which the induced current is rectified, stored and smoothed in

a condenser and subsequently transferred to a load.

In the first situation the microprocessor controlled system takes care of most of the elementary functions, such as obtaining resonant soft switching or protection against overloading and short-circuiting.

The advantages of the first solution are: controlling of the energy current only occurs by software

- the power factor can be controlled to the ideal value - by means of software controlling the possible external resonant net¬ works can be damped actively

- the prior mentioned advantages of the series-resonant principle

The disadvantages of the first solution are: - the necessary use of at least four electronic switches

- the necessary external control for protection and to accomplish reson¬ ant soft switching of the converter the control delay caused by applying the microprocessor system

- the necessary stand alone protection in case the microprocessor sys- tem fails the economical more expensive solution caused by having to use a microprocessor controlled system

In the second situation an additional transformer is used, of which the induced rectified current in the secondary coil will be stored in a con¬ denser and will be supplied to a load.

The advantages of the second solution are:

- the simple design of the converter - the mentioned advantages of the series-resonant principle

The disadvantages of the second solution are: the application is limited to DC-DC conversion the necessity to use an external control for functioning and protection

GENERAL DISADVANTAGES OF THE SERIES-RESONANT CONVERTER

Very strong energy fluctuations in the series-resonant network, which should stay controllable within certain but absolute limitations, are the cause

for most of the hereafter mentioned disadvantages.

- The necessity to u se an external mechanism to accomplis h soft switching. - The necessity to use an external circuit for stabilisation and control¬ ling of the energy in the resonant network.

- The necessity to use an external circuit to take care of protection if the converter is overloaded or short-circuited.

PURPOSE OF THE INVENTION

The invention achieves balance of energy in the series-resonant network by the integration of a buffer circuit comprising a capacitor, called buffer capacitor, in which the energy can be regarded as the energy surplus and will be passively supplied to the load by means of the electronic switches of the autonomous converter, if the voltage across this capacitor is higher than the source voltage.

In conventional designs, the rectified current of an alternating single phase voltage source will be smoothed by means of a smoothing condenser, which causes a bad power factor and a high switching ON current.

By integrating a buffer circuit in the series-resonant network, the conven¬ tional smoothing condenser can be omitted and this function will be over¬ taken by the buffer capacitor, which causes a less worse power factor and will also decrease the switching ON current, compared to the smoothing con- denser in the conventional situation.

PRINCIPLE OF THE INVENTION

The principle of the invention is a self starting, free wheeling, internal and non-dissipative passively damped series-resonant oscillator and guaran¬ tees a sinusoidal current pulse when the resistance is low compared to the impedance. Any external controlling circuit is restricted to only block up the electronic switch, after it is switching OFF by natural commutation and thus switching OFF the converter. Switching ON the converter is just restricted to unblock the electronic switch at any time. When, with the help of a special internal circuit is made sure that the electronic switch only will be blocked when it is OFF, then the external control circuit can switch OFF the con¬ verter at any time. The energy needed to open and close the switches will be derived from the resonant network itself.

DESCRIPTION OF THE INVENTION

The invention provides in the substitution of the inductor in the series- resonant network by at least two transformers, called resonance transformer and driver transformer, each containing a ferromagnetic core, which com¬ prises at least one primary and one secondary coil. The primary coils are connected in series to at least one condenser to form the primary resonant network.

The primary coil of The resonance transformer induces a resonant current in the secondary coil, which together to at least one parallel connected conden¬ ser, forms the secondary resonant circuit. When the induced current will be rectified by at least one diode, smoothed by at least one inductor and subsequently stored in a buffer condenser, then the energy in the buffer condenser can be considered as the actual energy surplus. When the voltage across this buffer condenser is higher then the power source voltage, the buffer condenser itself will act as a source. The energy surplus will be sup¬ plied to the primary resonant circuit by means of a rectifying diode and the electronic switches and is partially or entirely dissipated by the load depen- ding on the magnitude of the load and the amount of energy surplus, where¬ by the stabilisation of the energy balance is achieved.

The primary coil of the driver transformer, induces energy in the secondary coil, which is connected in series to at least one condenser and forms in principle the independent driver circuit for at least one switch. In the spe¬ cific situation, in which the switches are not controlled by an external cir¬ cuit, and the conventional anti-parallel connected free wheel diodes to the switches used in converters without natural commutation can be omitted. Thermal noise energy will start the oscillator, of which the impedance of the load, which is connected in series to the primary resonant circuit, can be resistive, inductive, capacitive as well as non-linear.

EXTERNAL REGULATION WITHOUT OSCILLATION BLOCK uτ>

Another possibility to control the energy supplied to a load by the series-resonant converter is based on pre-magnetization of two ferromagnetic cores and therefore the resonant transformer will be replaced by two iden¬ tical resonant transformers, each containing at least three galvanic separated coils. The coils of one resonant transformer are connected in series to the

corresponding coils of the other transformer.

The first pair of coils, called primary resonant coils are connected in series to each other and forms the inductive part of the prior discussed primary resonant network.

The second pair of coils, called secondary resonant coils are connected in anti-series to each other and forms together with to their parallel connected condensers, called secondary resonant condensers, the secondary resonant circuit, which is coupled to a series circuit containing a smoothing coil and the load, which must act resistive.

Rectifiers are connected to the terminals of the secondary resonant coils and will rectified the energy surplus, which makes it suitable for subsequently storing the energy surplus in the prior mentioned buffer capacitor.

The third pair of the corresponding coils are called control coils and are also connected in anti-series to each other and they are attached to a con¬ trollable direct or alternating current source of relative low frequency com¬ pared with the oscillation frequency, which results in pre-magnetization of the ferromagnetic cores possibly till saturation.

The combination of these two transformers showing strong resemblance with a transductor, because it also contains two pre-magnetized ferromagnetic cores of the same shape, including the use of the integrated EE or El cores. In such a transductor the pre-magnetization of the ferromagnetic cores will be used to vary the reactance and thus regulate the energy supply to the load. In the invention however, the energy supply to the load will be regu¬ lated by using the voltage difference of the in anti-series connected second¬ ary resonant coils. The magnitude of the voltage difference of the secondary resonant coils depends on the magnitude of the primary resonant current and the pre-magnetization of the ferromagnetic cores, which depends on the magnitude of the current in the control coils. The polarity of the induced voltage difference of the secondary resonant coils is defined by the direction of the current in the control coils and when the current is constant, then the load voltage will nearly be constant. Only a small variation in voltage will occur, depending on the inductance of the smoothing coil and the magnitude of the series connected load.

By using an additional in anti-series connected pair of identical coils, the so

called resonant load coils, each wound around the ferromagnetic core of the transformers, are connected to the prior mentioned serial circuit, containing the load and a smoothing inductor. This design will not only show some of the prior mentioned qualities like the regulation of the pulse form, magnitude and frequency, which is lower than the resonance frequency, but will add also the transformer qualities, such as transformation of voltage and galvanic separation between the control circuit, the load as well as the power source.

This design can be considered as a combined pre-magnetized transformer and magnetic amplifier, by which efficient heavy duty power converting can be combined with economical applications, like the use of audio amplifiers in the open air or in large rooms.

DESCRIPTION OF THE EMBODIMENTS OF THE SERIES-RESONANT CONVERTER

The purpose of these embodiments serves as examples for practical sol¬ utions and better understanding of the specific problems and extensive pos¬ sibilities for regulation of the series-resonant converter in the invention.

Description of Fig. 1.

Fig. 1. shows the embodiment of a basic circuit arrangement for the series resonant converter according to the present invention, in which an AC- power source (1) is rectified by the diode bridge (2) and supplies energy to a circuit, comprising a primary and a secondary resonant network containing a buffer and driver circuit. The primary resonant network con¬ sists of one resonant transformer and one driver transformer, containing one primary resonant coil (7) and one primary driver coil (17) and both primary coils are in series coupled to each other. The primary resonant coil (7) induces resonant energy in the secondary resonant coil (8), which forms with at least one parallel connected condenser (9) the secondary resonant circuit. This induced resonant energy can be considered as the energy surplus, when it is stored in capacitor (15), the buffer condenser, after being rec- tified by the diodes (10, 11, 12 and 13) and smoothed by inductor (14). If the voltage across buffer condenser (15) will be higher than the rectified voltage of the power source (1) and this buffer condenser will act by itself as a source. The stored energy surplus in the buffer condenser (15) will be transferred

by means of diode (16) and the electronic switches (3) and (4) to the pri¬ mary resonant circuit and depending on the amount of the energy surplus and the magnitude of load (20), this energy surplus will be dissipated com¬ pletely or partially by the load (20). The eventually remainder of the energy surplus will again be stored in the buffer condenser (15). This cycle will be repeated till the voltage across the buffer condenser is decreased to the level of the rectified voltage of the power source, by which the energy bal¬ ance in the series-resonant network is achieved and thus stabilisation of the energy supply to the load is realized. Driver energy will be obtained from the primary driver coil (17), by inducing resonant energy in the secondary driver coil (18) and forms together with in series connected driver condenser (19), the driver circuit for the electronic switches (3) and (4), through which the converter will function autonomously.

Description of Fig. 2.

Fig. 2 shows the embodiment of the series-resonant converter with ref¬ erence to Fig. 1 with the characteristic, that both switches are NPN transis- tors and supplies resonant energy to the load (20), in this example present¬ ing a gas discharge lamp with its parallel connected condenser (28) for igni¬ tion and lowering the electromagnetic interference of this type of lamp.

Therefore the converter in Fig. 1 will be expanded with a second driver transformer containing one primary driver coil (21), which is in series con- nected to the primary resonant circuit. The secondary driver coil (22) forms with the in series connected driver condenser (23) the second driver circuit for the NPN-transistor (4). Anti parallel to the base and emitter of the NPN- transistors (3) and (4) are attached two in series connected circuits contain¬ ing two pairs of diodes (24, 25) and (26, 27), in which the reverse resonant current can flow and will so protect the base emitter layer of the transistor (3) and (4) against reverse polarisation. By using of two pairs in series con¬ nected diodes, it will always be guaranteed that first the transistor switch will be OFF, before the other transistor switch can be ON and thus will pre¬ vent short-circuiting of the power source.

Description of Fig. 3.

Fig. 3 shows the embodiment of the series resonant converter with ref-

erence to Fig. 2 with the characteristic, that dimming the gas discharge lamp (20) is achieved by continuing to block the NPN transistor (4), after it is blocked by natural commutation and will damp the oscillation till it is blocked up and as such controlling the energy supply to the lamp. Therefore the driver circuit in Fig. 2 is extended with the protecting diode (34), which is anti-parallel connected to the collector and the emitter of the NPN transistor (3) and an internal blocking circuit This blocking cir¬ cuit consists of the shottky diode (31), the anti- parallel diode (29), the NPN signal transistor (30), of which the base of is connected together with resis- tor (32) to the control terminal (33). The other terminal of the resistor is connected together with shottky diode (31) and its anti-parallel diode to the resonant driver circuit containing the secondary driver coil (22) and the secondary driver condenser (23). Resistor (32) sets the signal transistor (30) in the conductional state, whereby the transistor switch will be blocked up and thus prevent operation convertion.

The signal transistor (30) will be blocked by means of short-circuiting the base and emitter of signal transistor (30) and as such starting or restarting of the converter. Unblocking signal transistor (30) must be done when the transistor (4) is switched OFF by means of natural commutation. Diode (29), as well as the in series connected diodes (26) and (27), assures the reverse polarisation of the resonance driver current.

Description of Fig. 4A and Fig. 4B.

Fig. 4A shows the embodiment of a series-resonant converter with refer¬ ence to Fig. 3, with the characteristic that the conditional regulated current supplied to the load (20), in this example representing a motor, can be a DC- current as well as an AC-current, of which the synthesized frequency is considerable lower than the resonant frequency, controls the speed and the rotation direction of the motor.

Therefore the design in Fig. 3 is expanded by two driver transformers, each containing a primary driver coil (17B) and (21B) and a secondary driver coil (18B) and (22B). These primary driver coils (17B) and (21B), which are connected in series to the primary resonant circuit, which induces resonant energy in the respective secondary driver coils (18B) and (22B), which in their turn are connected to the respective driver condensers (19B) and (23B) and thus form two driver circuits, which are supplying resonant energy to the respective NPN transistors switches (3B) and (4B). Anti-parallel to the

collector and the emitter terminals of the respective switches (3A) , (4A), (3B) and (4B), the respective diodes (27A) , (28A), (27B) and (28B) are connected to protect these transistor switches against reverse voltage polarisation if the source voltage is lower than the motor voltage. Two condensers (26A) and (26B) are connected in series and coupled in par¬ allel to the motor and thus possible voltage spikes will be smoothed. Because of the common coupling of these condensers to the primary series-resonant circuit an AC-link is achieved. By using conventional inverters two switches will always be OFF or ON at the same time, in contrast to the situation created by the invention in which only one transistor switch is ON at the same time.

The rotation of the motor can be reversed under the condition that first the blocked up transistor switch of the active transistor pair will remain OFF, by means of the regulation circuit, before the other pair of transistors will be switched ON and thus the converter can restart by excitation and the rota¬ tion will be reversed. The rotation speed can be adjusted by regulating the block-up time of the oscillator.

Fig. 4B shows the embodiment of the identical regulation circuits (24A), (24B), (25A) and (25B) in Fig. 4A, with reference to Fig. 3, with the charac¬ teristic that the controlling circuit is galvanically separated from the condi¬ tional regulated converter circuit.

Therefore the regulation circuit in Fig. 3. will be expanded with an optocoupler, which consists of two parts. The first part, called receiver con- taining the photo diode (33) also called detection diode and the fast detection transistor (35). The second part, called sender containing the fast diode (34), which in conduction will switch ON one of the power transistor switches, because detection transistor (35) acts as a current amplifier of the photo diode (33) and will block up the signal transistor (30), so one of the power transistor switches will go in the conduction state, whereby the converter will start or restart. To the base and emitter of the detection transistor (35) a resistor (36) is connected, which forms a compromise between the switching OFF speed of the detection transistor and the sensitivity of the detection current toward the sender current of the control circuit.

Description of Fig. 5.

Fig. 5 shows the embodiment of the resonant converter with reference to

Fig. 4A and Fig. 4B, with the characteristic that the energy current is bi¬ directional, because motor (20) can also act as a generator and therefore it will function as a source. In general one can speak about a situation of at least two sources, which can consist of AC-εourceε, DC-sources, or a co bi- nation of both.

Therefore Fig. 4A will be expand with extra switches, which contain the NPN-transistors (3C) , (3D), (4C) and (4D). Apart from being used as a recti¬ fier, the diode bridge (2) is also used to protect the switching transistors (3C), (3D), (4C) and (4D) against reverse voltage polarisation. The other way around it is clear that the diode bridge (19), used to protect the switching transistors (3A), (4A), (3B) and (4B) against reverse voltage polarisation, will also act as a rectifier for the motor source (20) acting as a generator. Analogous to the situation in Fig. 4A, in which the pair in series connected condensers ( 18A and 18B) are connected in parallel to the motor source (20) , another pair of in series connected condensers (18C and 18D) are connected to source (1), for a possible lowering of the impedance of the network, so that a quick start of the oscillator will be achieved and possible voltage spikes will be smoothed too. The common connection of both pairs of conden¬ sers is connected to the primary series resonant circuit and thus AC-coup- ling is being achieved. The in series connected primary driver coils (17) induce resonant energy in the secondary driver coils, of which neither the driver condensers nor the block-up circuit are displayed in Fig. 5.

Summarizing, the converter consists of two AC-sources (1) and (20) , which will be rectified and protected by means of the diode bridges (2) and (19). The electrical energy of source (1) will be rectified and transferred, with the help of the switches (3A) , (3B), (4A) and (4B), to motor (20). The other way around the energy of the motor source (20) will be rectified by diode bridge (19) and by means of the transistor switches (3C), (3D), (4C) and (4D) which are being protected by the diode bridge (2), is transferred to source (1). It may be evident that components like Asymmetric Thyristors and Mos-

Fets can be applied advantageously, because these components have built in anti-parallel diodes for protection, or because it is typical for the design. Furthermore these built in diodes can be used for rectifying alternated sources, which yields an additional economical profit.

Description of Fig. 6.

Fig. 6 shows the embodiment of the resonant converter according to the

invention with reference to Fig. 1 and Fig. 4A, with the characteristic that the control current, which is galvanically separated and proportional to the load current, which can be a direct or an alternating current of relative low frequency compared to the resonant frequency, will be used for pre-magnet- ization possibly till saturation of ferromagnetic cores.

Therefore the resonant transformer in Fig. 1 is being replaced by two identical ferromagnetic cores, of which each contains at least three coils, which are connected in series to the identical coils of the other ferromag¬ netic cores. The in anti-series connected control coils (21A and 21B) are con- nected to a controllable current source (25) to pre-magnetize the ferromag¬ netic cores. The in series connected primary resonant coils (7A and 7B) is connected to the autonomously functioning oscillator as discussed before. The in anti-series connected secondary resonant coils (8A and 8B) form, together with to them parallel connected secondary resonant condensers (9A and 9B), the secondary resonant circuit. This circuit is coupled to a in series con¬ nected circuit containing a smoothing coil (24) and load (20). Diodes (10, 11, 12, 13, 22 and 24) are connected to the terminals of the secondary resonant coils (8A and 8B) to rectify the energy surplus and thus it is suitable for storing in the buffer condenser.

Description of Fig. 7.

Fig. 7 shows the embodiment of the resonant converter with reference to Fig. 6 with the characteristic that a full galvanic separation between the control circuit, the power source as well as the load, of which the voltage can be as well higher as lower, because of transformation, then the voltage of the power source.

Therefore an extra pair of identical anti-series connected resonant load coils (26A and 26B) are used, which are connected in parallel to the resonant load condensers (27A and 27B) and thus supply energy, by means of the prior mentioned smoothing coil (24), to the series connected load (19).

ADVANTAGES OF THE INVENTION IN THE SERIES-RESONANT CONVERTER

- The general advantages of the series-resonant converter.

- The principle for achieving balance of energy in the series-resonant network is universal of purpose and application, because the power

source can consist of a direct current source as well as of one or more phase alternating current sources, of which energy will be sup¬ plied to a load, which also can consist of a source, by means of at least two switches, whether or not driven by an external regulation and by the integration of a passive buffer circuit in the resonant net¬ work, owning a buffer capacitor, which if charged higher then the voltage of power source, itself will function as a source.

Since the driver current is always in phase with the resonant current, both switches are protected against switching ON at the same times, so short-circuiting the power source will be prevented and because of the method in energy control in the series-resonant network, power limitation will always occur in case of overloading or short-circuiting the load, against which the converter is intrinsically protected.

- In spite of deterioration of the power factor by the integration of the buffer capacitor in the resonant network, it will be a considerable improvement in comparison to the conventional situation, in which a condenser is used for smoothing the voltage of a rectified single phase AC-source and an enormous reduction of the switch ON current will also be achieved.

- A well designed converter will start up because of the excitation of the thermal noise voltage and a conditional external control circuit will be restricted by blocking up and unblocking the oscillator, by which the converter is stopped and restarted.

Since the energy will be obtained directly from the resonant network, no special source is needed for the internal as well as the conditional external control circuit of the electronic switches.

- Because of the excellent stability and security of the resonant circuit, the load is allowed to be resistive, inductive, capacitive, as well as non-linear.

In the situation, which can be compared to at least two AC-sources, the diode bridges can be used as rectifiers as well as for protecting the semiconductor switches and components like Asymmetric Thyristors and MosFets can be applied advantageously, because these components

U have built in anti-parallel connected diodes for protection, or because it is typical for the design.

In case of natural commutation with or withtrut external regulation and if the electronic switches are suitable for reverse voltage polarisation, than the conventional free wheel diodes used in not resonant con¬ verters, also used for protection by inductive loads can be omitted. If natural commutation will be obtained without regulation of an exter¬ nal circuit, than the usual protecting diodes applied in combination with inductive loads can also be omitted.

The economical value will be obtained by the intrinsic protection, by which an external circuit will be saved and the simple design of the internal as well as the external driver circuits for both switches, in particular when an internal circuit will guarantee, that blocking up of the oscillator can only occur in case of absence of the resonant cur¬ rent in the switch.

Another economical advantage will be achieved by pre-magnetization of two ferromagnetic cores, which results in a very simple and uncompli¬ cated regulation circuit with galvanic separation between source as well as load and efficient heavy duty power conversion.