EP1135849A1 - Isolating transformer - Google Patents

Isolating transformer

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Publication number
EP1135849A1
EP1135849A1 EP99973191A EP99973191A EP1135849A1 EP 1135849 A1 EP1135849 A1 EP 1135849A1 EP 99973191 A EP99973191 A EP 99973191A EP 99973191 A EP99973191 A EP 99973191A EP 1135849 A1 EP1135849 A1 EP 1135849A1
Authority
EP
European Patent Office
Prior art keywords
transistor
effect transistor
field effect
fet
flyback converter
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP99973191A
Other languages
German (de)
French (fr)
Inventor
Thomas Rothmayer
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Siemens AG Oesterreich
Original Assignee
Siemens AG Oesterreich
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Filing date
Publication date
Application filed by Siemens AG Oesterreich filed Critical Siemens AG Oesterreich
Publication of EP1135849A1 publication Critical patent/EP1135849A1/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33576—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer

Definitions

  • the invention relates to a flyback converter with a transformer, with a primary winding and with at least one secondary winding, the primary winding being connected in series with a controlled switch to an input voltage, and a rectifier element and a charging capacitor being connected downstream of the secondary winding.
  • Flyback converters as power supply devices have become known in a large number of designs, with a DC voltage, which may be obtained by rectification from an AC voltage network, being converted by means of the flyback converter into a generally electrically isolated DC output voltage.
  • the switching frequencies are generally above the hearing range.
  • Flyback converters are known, for example, from Hirschmann / Hauenstein, "Switching Power Supplies", Verlag Siemens 1990, Thiel, “Professional Switching Power Supply Applications”, Franzis Verlag 1996, Kilgenstein, “Switching Power Supplies in Practice", Vogel specialist book 1988, WO 94/22 207 and DE 196 13 136 AI.
  • flyback converter The function of a flyback converter is also assumed to be known, as are the control circuits which can be used to control the controlled switch and which are also described in more detail in the references mentioned above.
  • the rectifier element is designed as a field-effect transistor, which can be switched via a drive transistor, the gate being connected to a load resistor of the transistor, and the load resistor in series with its collector-emitter path on a transformer. Secondary voltage is.
  • the invention takes advantage of the peculiarity of a field effect transistor that it has an essentially constant and low resistance in the forward direction, whereas the diodes usually used for rectification have an essentially constant one Show forward voltage (lock voltage) on the order of half a volt. Because of the low gate-source resistance value, the losses in a field-effect transistor, for example at least in the case of not very high currents, remain a factor 5 below that of a rectifier diode.
  • the base current can be supplied to the base of the drive transistor via a parallel RC element. On the one hand, this results in the field-effect transistor being switched on very quickly, and on the other hand, the base current of the switching transistor is maintained for the required duration.
  • the breakdown voltage of the emitter-base section of the switching transistor is usually not very high, it is recommended in many cases if a protective diode is located in series with the emitter-collector section of the drive transistor, the forward direction of which corresponds to that of the emitter-base section.
  • a tried and tested embodiment of the invention is characterized in that the field effect transistor is an n-channel field effect transistor, which is connected between one end of a secondary winding and the negative terminal of the charging capacitor, whereas the other end of the secondary winding is connected to the positive terminal of the charging capacitor, the drive transistor is a pnp transistor, the collector of which is on the one hand connected to the negative terminal of the charging capacitor via the load resistor, and on the other hand is connected to the gate of the field effect transistor, and the parallel RC element between the drain of the field effect Transistor and base of the drive transistor is.
  • the protective diode can lie between the emitter of the drive transistor and the positive terminal of the charging capacitor.
  • FIG. 1 shows the basic circuit of a first embodiment of the invention
  • Fig. 2 shows a circuit of a second embodiment of the invention
  • Fig. 3 shows the course of the main currents and voltages of a flyback converter according to the invention in a diagram.
  • a flyback converter has a transformer Tr with a primary winding W p and a secondary winding W s .
  • a switch Sp controlled by a control circuit AST is connected to an input DC voltage Ue, which usually represents an intermediate circuit voltage, which is obtained by rectification and smoothing from an AC network.
  • Ue which usually represents an intermediate circuit voltage, which is obtained by rectification and smoothing from an AC network.
  • the corresponding rectifier and charging capacitor are not shown since they have nothing directly to do with the invention.
  • the field effect transistor FET with its source at the negative pole of the output voltage has an integrated diode D, and is to be controlled so that it switches in synchronized relationship with the controlled switch Sp and rectifies the secondary voltage across the winding W s .
  • a switching transistor T 8 is provided here, a pn-p transistor, the collector of which lies on the one hand at the gate of the field-effect transistor and on the other hand via a load resistor R GS at the negative pole of the output voltage.
  • the emitter is connected via a diode Ds connected in the forward direction to one end of the secondary winding W s or to the positive pole of the output voltage U A , whereas the other end of the secondary winding W "is connected to the drain of the field effect transistor.
  • the base of the switching transistor T s is connected via a protective resistor R, and a parallel RC element R, C to that end of the secondary winding at which the drain of the field effect transistor is also located.
  • the circuit also indicates that a control signal S R can be supplied to the control circuit AST.
  • the regulation of flyback converters is part of the state of the art and is well known to the person skilled in the art; it can be carried out, for example, by comparing the output voltage U A with a reference voltage and a control signal derived therefrom, an optocoupler often being used between the secondary and primary side, so that the control signal S R supplied to the control circuit AST is galvanically decoupled from the secondary side or the output voltage U A.
  • the control signal SR can also be derived from an additional winding of the transformer Tr, in particular if no high demands are made on the control accuracy.
  • a current control and a combined current / voltage control can also be carried out in the same way.
  • the primary current I P through the primary winding W P rises periodically, each time after switching on of the switch S p by the drive circuit AST substantially dVeieckfbrmig so long until the primary switch S p öflhet again. This results in a triangular current over a first time period Ti. During this time period the primary voltage across the winding W P> the voltage U P is constant. After opening the switch, the primary current Ip during a time period T 2 is equal to zero and the secondary current I s would drop in a triangular shape, complementary to the primary current I P, during this time period T 2 after the switch-off.
  • the secondary voltage Us is a rectangular voltage in phase opposition to the primary voltage U P.
  • the current at the beginning is high due to the capacitor C and causes the switching transistor T s to turn on, which subsequently also leads to the field-effect transistor FET turning on, since a correspondingly positive voltage across the load resistor R ⁇ of the switching transistor T s drops and lies at the gate of the field effect transistor FET.
  • T A which can only be in the nano-second range, if switching frequencies in the order of magnitude of, for example, 50 to 100 kHz are assumed.
  • a rectifier diode results in a power loss of around 0.5 W, in a field effect transistor with a path resistance of approx. 40 m ⁇ a power loss of only about 0.1 W.
  • a load resistor R L when there is a load on the secondary side, here a load resistor R L has been drawn in, the primary switch S p remains switched on longer, thus more energy is stored in the transformer and the secondary side is also demagnetized longer.
  • the field effect transistor FET remains switched on for a longer period, so that it is switched on for different lengths depending on the load.
  • the RC element R, C is of course dimensioned such that the field effect transistor is switched through as soon as possible (short time T A ) and that the field effect transistor is definitely switched off before the demagnetization is complete. Closing the field effect transistor too early, as well as opening it too late, would lead to a short-circuit-like state and thus to power losses. It should also be noted that even if the control of the field effect transistor FET fails, the circuit works in principle, even if with higher losses, since then the diode Di acts as a conventional rectifier diode.
  • the variant according to FIG. 2 is characterized in that a second secondary winding WS is provided in order to achieve a higher voltage for carrying out the switching operations if the output voltage U A is only low and is of the order of 3 V, for example.
  • the voltages of the diode D s and the transistor T s in the forward direction would lead to voltage drops, which may no longer guarantee a safe switching.
  • the protective diode Ds is intended to prevent a voltage breakdown of the base-emitter path of the switching transistor T s .
  • the breakdown voltage of this base-emitter path is only a few volts, so that this path would be endangered at higher voltages on the secondary side.
  • the protection diode Ds is therefore used, which can have a breakdown voltage of the order of 40 V, for example, in order to protect the base-emitter path of the transistor Ts.
  • the protective resistor Rs also protects the switching transistor T s , in this case from an excessively high base current.
  • time period T E is drawn longer than the time period T A at the beginning of the secondary current, which is to point out that time period T E is not particularly critical, since the current is already low here and any losses in the integrated diode Di no longer impact as much as at the beginning of the secondary current flow.

Landscapes

  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Dc-Dc Converters (AREA)
  • Rectifiers (AREA)
  • Power Conversion In General (AREA)

Abstract

The invention relates to an isolating transformer comprising a transformer (Tr), a primary winding (Wp) and at least one secondary winding (Ws), the primary winding being connected in series with a controlled switch (Sp) to an input voltage (Up) and a rectifier element (FET) and charge capacitor (Cs) being positioned downstream of the secondary winding. The rectifier element is configured as a field effect transistor (FET) which can be controlled via a control transistor (Ts). The gate is connected to a load resistance (Rgs) of the transistor and the load resistance is connected in series with the collector-emitter circuit of the transistor to a secondary voltage of the transformer.

Description

SPERRWANDLER LOCK CONVERTER
Die Erfindung bezieht sich auf einen Sperrwandler mit einem Übertrager, mit einer Primärwicklung sowie mit zumindest einer Sekundärwicklung, wobei die Primärwicklung in Serie mit einem gesteuerten Schalter an einer Eingangsspannung liegt, und der Sekundärwicklung ein Gleichrichterelement und ein Ladekondensator nachgeschaltet sind.The invention relates to a flyback converter with a transformer, with a primary winding and with at least one secondary winding, the primary winding being connected in series with a controlled switch to an input voltage, and a rectifier element and a charging capacitor being connected downstream of the secondary winding.
Sperrwandler als Stromversorgungsgeräte sind in einer großen Anzahl von Ausführungen bekannt geworden, wobei eine Gleichspannung, die gegebenenfalls durch Gleichrichtung aus einem Wechselspannungsnetz erhalten wird, mittels des Sperrwandlers in eine im allgemeinen galvanisch getrennte Ausgangsgleichspannung umgewandelt wird. Die Schaltfrequenzen liegen im allgemeinen oberhalb des Hörbereiches. Sperrwandler sind beispielsweise bekannt aus Hirschmann/Hauenstein, "Schaltnetzteile", Verlag Siemens 1990, Thiel, "Professionelle Schaltnetzteilapplikationen", Franzis Verlag 1996, Kilgenstein, "Schaltnetzteile in der Praxis", Vogel- Fachbuch 1988, WO 94/22 207 und DE 196 13 136 AI.Flyback converters as power supply devices have become known in a large number of designs, with a DC voltage, which may be obtained by rectification from an AC voltage network, being converted by means of the flyback converter into a generally electrically isolated DC output voltage. The switching frequencies are generally above the hearing range. Flyback converters are known, for example, from Hirschmann / Hauenstein, "Switching Power Supplies", Verlag Siemens 1990, Thiel, "Professional Switching Power Supply Applications", Franzis Verlag 1996, Kilgenstein, "Switching Power Supplies in Practice", Vogel specialist book 1988, WO 94/22 207 and DE 196 13 136 AI.
Die Funktion eines Sperrwandlers wird ebenso als bekannt vorausgesetzt, wie die für die Ansteuerung des gesteuerten Schalters verwendbaren Ansteuerschaltungen, die gleichfalls in den oben genannten Literaturstellen näher beschrieben sind.The function of a flyback converter is also assumed to be known, as are the control circuits which can be used to control the controlled switch and which are also described in more detail in the references mentioned above.
Wenngleich der Wirkungsgrad bekannter Sperrwandler als gut zu bezeichnen ist, versucht man ständig, die in dem Wandler auftretenden Verluste noch weiter zu verringern, einerseits um unnötigen Stromverbrauch zu verhindern und damit Energie zu sparen, andererseits um die Eigenerwärmung der entsprechenden Netzgeräte so gering wie möglich zu halten. Der Erfindung liegt demgemäß die Aufgabe zugrunde, einen Sperrwandler zu schaffen, der mit sehr geringen Verlusten arbeitet und dennoch ausgesprochen preiswert realisierbar ist.Although the efficiency of known flyback converters can be described as good, attempts are constantly being made to reduce the losses occurring in the converter even further, on the one hand to prevent unnecessary power consumption and thus save energy, and on the other hand to minimize the self-heating of the corresponding power supply units hold. The invention is therefore based on the object to provide a flyback converter which works with very low losses and is nevertheless extremely inexpensive to implement.
Diese Aufgabe wird erfindungsgemäß dadurch gelöst, daß das Gleichrichterelement als Feldeffekt-Transistor ausgebildet ist, der über einen Ansteuertransistor schaltbar ist, wobei das Gate an einem Arbeitswiderstand des Transistors liegt, und der Arbeitswiderstand in Serie mit dessen Kollektor-Emitter- Strecke an einer Übertrager-Sekundärspannung liegt.This object is achieved in that the rectifier element is designed as a field-effect transistor, which can be switched via a drive transistor, the gate being connected to a load resistor of the transistor, and the load resistor in series with its collector-emitter path on a transformer. Secondary voltage is.
Die Erfindung macht sich die Eigenheit eines Feldeffekttransistors zunutze, daß dieser in Durchlaßrichtung einen im wesentlichen konstanten und geringen Widerstand aufweist, wogegen die üblicherweise zur Gleichrichtung verwendeten Dioden eine im wesentlichen konstante Durchlaßspannung (Schleusenspannung) in der Größenordnung eines halben Volt zeigen. Wegen des geringen Gate-Source- Widerstandswertes bleiben - jedenfalls bei nicht sehr hohen Strömen - die Verluste bei einem Feldeffektransistor um beispielsweise einen Faktor 5 unter jenem einer Gleichrichterdiode.The invention takes advantage of the peculiarity of a field effect transistor that it has an essentially constant and low resistance in the forward direction, whereas the diodes usually used for rectification have an essentially constant one Show forward voltage (lock voltage) on the order of half a volt. Because of the low gate-source resistance value, the losses in a field-effect transistor, for example at least in the case of not very high currents, remain a factor 5 below that of a rectifier diode.
Zweckmäßig ist es, wenn der Basisstrom der Basis des Ansteuertransistors über ein Parallel- RC-Glied zuführbar ist. Dadurch erfolgt einerseits ein sehr rasches Einschalten des Feldeffekt- Transistors und andererseits wird dadurch für die benötigte Dauer der Basisstrom des Schalttransistors aufrecht erhalten.It is expedient if the base current can be supplied to the base of the drive transistor via a parallel RC element. On the one hand, this results in the field-effect transistor being switched on very quickly, and on the other hand, the base current of the switching transistor is maintained for the required duration.
Da die Durchbruchspannung der Emitter-Basis-Strecke des Schalttransistors meist nicht sehr hoch ist, empfiehlt es sich in vielen Fällen, wenn in Serie zur Emitter-Kollektor-Strecke des Ansteuertransistors eine Schutzdiode liegt, deren Durchlaßrichtung jener der Emitter-Basis- Strecke entspricht.Since the breakdown voltage of the emitter-base section of the switching transistor is usually not very high, it is recommended in many cases if a protective diode is located in series with the emitter-collector section of the drive transistor, the forward direction of which corresponds to that of the emitter-base section.
Gleichfalls zum Schutz des Schalttransistors ist es ratsam, wenn in der Basiszuleitung des Ansteuertransistors in Serie mit dem Parallel-RC-Glied ein Schutzwiderstand liegt.It is also advisable to protect the switching transistor if there is a protective resistor in the base lead of the drive transistor in series with the parallel RC element.
Eine praxisbewährte Ausfuhrungsvariante der Erfindung zeichnet sich dadurch aus, daß der Feldeffekt-Transistor ein n-Kanal-Feldeffekt-Transistor ist, welcher zwischen ein Ende einer Sekundärwicklung und den negativen Anschluß des Ladekondensators geschaltet ist, wogegen das andere Ende der Sekundärwicklung an dem positiven Anschluß des Ladekondensators liegt, der Ansteuertransistor ein p-n-p-Transistor ist, dessen Kollektor einerseits über den Arbeitswiderstand an dem negativen Anschluß des Ladekondensators liegt, und andererseits mit dem Gate des Feldeffekt-Transistors verbunden ist, und das Parallel-RC-Glied zwischen Drain des Feldeffekt-Transistors und Basis des Ansteuertransistors liegt.A tried and tested embodiment of the invention is characterized in that the field effect transistor is an n-channel field effect transistor, which is connected between one end of a secondary winding and the negative terminal of the charging capacitor, whereas the other end of the secondary winding is connected to the positive terminal of the charging capacitor, the drive transistor is a pnp transistor, the collector of which is on the one hand connected to the negative terminal of the charging capacitor via the load resistor, and on the other hand is connected to the gate of the field effect transistor, and the parallel RC element between the drain of the field effect Transistor and base of the drive transistor is.
Dabei kann die Schutzdiode zwischen dem Emitter des Ansteuertransistors und dem positiven Anschluß des Ladekondensators liegen.The protective diode can lie between the emitter of the drive transistor and the positive terminal of the charging capacitor.
Besonders bei Ausführungen mit niedriger Ausgangsspannung ist es von Vorteil, wenn zwei in Serie liegende Sekundärwicklungen vorgesehen sind, wobei Feldeffekt-Transistor und Ladekondensator einer Wicklung zugeordnet sind, die Ansteuerung der Gates über den Ansteuertransistor jedoch über die Summenspannung beider Sekundärwicklungen durchführbar ist.Particularly in the case of designs with a low output voltage, it is advantageous if two secondary windings are provided in series, field-effect transistor and charging capacitor being assigned to one winding, but the gates can be controlled via the drive transistor via the total voltage of both secondary windings.
Die Erfindung samt weiterer Vorteile ist im folgenden an Hand von Ausführungsbeispielen näher erläutert, die in der Zeichnung veranschaulicht sind. In dieser zeigenThe invention together with further advantages is explained in more detail below on the basis of exemplary embodiments which are illustrated in the drawing. In this show
Fig. 1 die prinzipielle Schaltung einer ersten Ausfuhrungsform der Erfindung, Fig. 2 eine Schaltung einer zweiten Ausführungsform der Erfindung und1 shows the basic circuit of a first embodiment of the invention, Fig. 2 shows a circuit of a second embodiment of the invention and
Fig. 3 den Verlauf der wichtigsten Ströme und Spannungen eines erfindungsgemäßen Sperrwandlers in einem Diagramm.Fig. 3 shows the course of the main currents and voltages of a flyback converter according to the invention in a diagram.
Gemäß Fig. 1 besitzt ein Sperrwandler nach der Erfindung einen Übertrager Tr mit einer Primärwicklung Wp und einer Sekundärwicklung Ws. In Serie mit der Primärwicklung liegt ein seitens einer Ansteuerschaltung AST gesteuerter Schalter Sp an einer Eingangsgleichspannung Ue, die meist eine Zwischenkreisspannung darstellt, welche durch Gleichrichtung und Glättung aus einem Wechselstromnetz gewonnen wird. Der entsprechende Gleichrichter und Ladekondensator sind nicht gezeigt, da sie mit der Erfindung nichts unmittelbar zu tun haben.1, a flyback converter according to the invention has a transformer Tr with a primary winding W p and a secondary winding W s . In series with the primary winding, a switch Sp controlled by a control circuit AST is connected to an input DC voltage Ue, which usually represents an intermediate circuit voltage, which is obtained by rectification and smoothing from an AC network. The corresponding rectifier and charging capacitor are not shown since they have nothing directly to do with the invention.
Sekundärseitig liegt in Serie mit der Sekundärwicklung Ws ein Feldeffekt-Transistor FET, im vorliegenden Fall ein N-Kanal FET an einem sekundären Ladekondensator Cs, an dem auch die Ausgangsgleichspannung UA abnehmbar ist.On the secondary side, in series with the secondary winding W s, there is a field effect transistor FET, in the present case an N-channel FET on a secondary charging capacitor C s , from which the DC output voltage U A can also be removed.
Der mit seiner Source an dem negativen Pol der Ausgangsspannung liegende Feldeffekt- Transistor FET besitzt eine integrierte Diode D, und soll so gesteuert werden, daß er in synchronisierter Beziehung mit dem gesteuerten Schalter Sp schaltet und die sekundäre Spannung an der Wicklung Ws gleichrichtet. Für diese Ansteuerung ist ein Schalttransistor T8 hier ein p-n- p— Transistor vorgesehen, der mit seinem Kollektor einerseits an dem Gate des Feldeffekt- Transistors und andererseits über einen Arbeitswiderstand RGS an dem negativen Pol der Ausgangsspannung liegt. Der Emitter ist über eine in Durchlaßrichtung geschaltete Diode Ds mit einem Ende der Sekundärwicklung Ws bzw. mit dem positiven Pol der Ausgangsspannung UA verbunden, wogegen das andere Ende der Sekundärwicklung W„ an dem Drain des Feldeffekt-Transistors liegt. Die Basis des Schalttransistors Ts ist über einen Schutzwiderstand R, und ein Parallel-RC-Glied R,C mit jenem Ende der Sekundärwicklung verbunden an welchem auch der Drain des Feldeffekt-Transistors liegt.The field effect transistor FET with its source at the negative pole of the output voltage has an integrated diode D, and is to be controlled so that it switches in synchronized relationship with the controlled switch Sp and rectifies the secondary voltage across the winding W s . For this actuation, a switching transistor T 8 is provided here, a pn-p transistor, the collector of which lies on the one hand at the gate of the field-effect transistor and on the other hand via a load resistor R GS at the negative pole of the output voltage. The emitter is connected via a diode Ds connected in the forward direction to one end of the secondary winding W s or to the positive pole of the output voltage U A , whereas the other end of the secondary winding W "is connected to the drain of the field effect transistor. The base of the switching transistor T s is connected via a protective resistor R, and a parallel RC element R, C to that end of the secondary winding at which the drain of the field effect transistor is also located.
In der Schaltung ist weiters angedeutet, daß der Ansteuerschaltung AST ein Regelsignal SR zugeführt werden kann. Die Regelung von Sperrwandlern ist dem Stand der Technik zuzurechnen und dem Fachmann wohl bekannt, sie kann beispielsweise durch den Vergleich der Ausgangsspannung UA mit einer Referenzspannung und einem daraus abgeleiteten Regelsignal durchgeführt werden, wobei oft zwischen Sekundär- und Primärseite ein Optokoppler verwendet wird, sodaß das der Ansteuerschaltung AST zugeführte Regelsignal SR galvanisch von der Sekundärseite bzw. der Ausgangsspannung UA entkoppelt ist. In ähnlicher Weise kann aber das Regelsignal SR auch von einer zusätzlichen Wicklung des Übertragers Tr abgeleitet werden, insbesondere wenn keine hohen Anforderungen an die Regelgenauigkeit zu stellen sind. In gleicher Weise kann auch eine Stromregelung und eine kombinierte Strom/Spannungsregelung durchgeführt werden.The circuit also indicates that a control signal S R can be supplied to the control circuit AST. The regulation of flyback converters is part of the state of the art and is well known to the person skilled in the art; it can be carried out, for example, by comparing the output voltage U A with a reference voltage and a control signal derived therefrom, an optocoupler often being used between the secondary and primary side, so that the control signal S R supplied to the control circuit AST is galvanically decoupled from the secondary side or the output voltage U A. In a similar way, however, the control signal SR can also be derived from an additional winding of the transformer Tr, in particular if no high demands are made on the control accuracy. In A current control and a combined current / voltage control can also be carried out in the same way.
Die Funktion der Schaltung, soweit sie für das Verständnis der Erfindung wesentlich ist, wird nachstehend unter Bezugnahme auch auf Fig. 3 erläutert.The function of the circuit, insofar as it is essential for an understanding of the invention, is also explained below with reference to FIG. 3.
Der Primärstrom IP durch die Primärwicklung WP steigt periodisch, jedesmal nach dem Einschalten des Schalters Sp durch die Ansteuerschaltung AST im wesentlichen dVeieckfbrmig so lange an, bis der Primärschalter Sp wieder öflhet. Es ergibt sich somit ein dreieckformiger Strom über eine erste Zeitdauer Ti. Während dieser Zeitdauer ist die Primärspannung an der Wicklung WP> die Spannung UP konstant. Nach dem Öffnen des Schalter beträgt der Primärstrom Ip während einer Zeitdauer T2 gleich Null und der Sekundärstrom Is würde nach dem Ausschalten dreieckförmig, komplementär zu dem Primärstrom IP während dieser Zeitdauer T2 abfallen. Die Sekundärspannung Us ist gegenphasig zu der Primärspannung UP eine rechteck- förmige Spannung.The primary current I P through the primary winding W P rises periodically, each time after switching on of the switch S p by the drive circuit AST substantially dVeieckfbrmig so long until the primary switch S p öflhet again. This results in a triangular current over a first time period Ti. During this time period the primary voltage across the winding W P> the voltage U P is constant. After opening the switch, the primary current Ip during a time period T 2 is equal to zero and the secondary current I s would drop in a triangular shape, complementary to the primary current I P, during this time period T 2 after the switch-off. The secondary voltage Us is a rectangular voltage in phase opposition to the primary voltage U P.
Gemäß der Erfindung wird nun nach dem Öffnen des Schalters Sp und entsprechend der Sekundärspannung Us ein Strom über die Sperr- bzw. Schutzdiode Ds und die Emitter-Basis- Strecke des Transistors Tr, über den Schutzwiderstand Rs und das RC-Glied R,C fließen, wobei der Strom zu Beginn wegen des Kondensators C hoch ist und ein Durchschalten des Schalttransistors Ts verursacht, was in der Folge auch zu einem Durchschalten des Feldeffekt- Transistors FET führt, da eine entsprechend positive Spannung an dem Arbeitswiderstand R^ des Schalttransistors Ts abfällt und an dem Gate des Feldeffekt-Transistors FET liegt. Das Einschalten erfolgt gemäß Fig. 3, beispielsweise nach einer Zeit TA, die lediglich im Nano- Sekunden-Bereich liegen kann, wenn man von Schaltfrequenzen in der Größenordnung von beispielsweise 50 bis 100 kHz ausgeht. Wenn nun durch den Transistor kein Strom mehr in das Parallel-RC-Glied fließt, schaltet der Schalttransistor Ts ab und der Feldeffekt-Transistor FET sperrt, doch kann wegen der technologisch bedingten, intregrierten Diode Di noch weiter ein Strom im Sekundärkreis fließen, bis der Stromfluß durch das Zurückgehen der Sekundärspannung beendet wird. In Fig. 3 ist gezeigt, daß während einer Zeit TE der Strom noch durch die Diode Dj des Feldeffekt-Transistors fließt, obwohl der Feldeffekt-Transistor selbst gesperrt ist. Die Zeitdauer während welcher Strom durch den Feldeffekt-Transistor fließt ist in Fig. 3 schraffiert eingezeichnet, und während dieser Zeit sind die Verluste aufgrund des geringen Bahnwiederstandes des Feldeffekt-Transistors FET entsprechend geringer, was bereits eingangs gezeigt wurde. Vergleicht man beispielsweise die Verluste einer Diode mit jenen eines Feldeffekt-Transistors bei einem Sperrwandler, dessen Ausgangsstrom 1 A beträgt, so ergibt sich bei einer Gleichrichterdiode eine Verlustleistung von etwa 0,5 W, bei einem Feldeffekt- Transistor mit einem Bahnwiderstand von ca. 40 mΩ eine Verlustleistung von lediglich von ca. 0,1 W. Es ist weiters zu beachten, daß bei einer Belastung an der Sekundärseite, hier wurde ein Lastwiderstand RL eingezeichnet der Primärschalter Sp länger eingeschaltet bleibt, somit mehr Energie im Übertrager gespeichert wird und auch sekundärseitig länger abmagnetisiert wird. Da aber auch in das RC-Glied R,C länger ein Strom fließt, bleibt der Feldeffekt-Transistor FET länger eingeschaltet, sodaß dieser lastabhängig unterschiedlich lang eingeschaltet wird.According to the invention, after opening the switch Sp and corresponding to the secondary voltage Us, a current through the blocking or protective diode D s and the emitter-base path of the transistor Tr, through the protective resistor Rs and the RC element R, C flow, the current at the beginning is high due to the capacitor C and causes the switching transistor T s to turn on, which subsequently also leads to the field-effect transistor FET turning on, since a correspondingly positive voltage across the load resistor R ^ of the switching transistor T s drops and lies at the gate of the field effect transistor FET. 3, for example after a time T A , which can only be in the nano-second range, if switching frequencies in the order of magnitude of, for example, 50 to 100 kHz are assumed. If current no longer flows through the transistor into the parallel RC element, the switching transistor T s switches off and the field effect transistor FET blocks, but because of the technologically-related, integrated diode Di, a current can still flow in the secondary circuit until the current flow is stopped by the decrease in the secondary voltage. In Fig. 3 it is shown that the current still flows through the diode Dj of the field effect transistor during a time T E , although the field effect transistor itself is blocked. The hatched period of time during which current flows through the field-effect transistor is shown in FIG. 3, and during this time the losses due to the low path resistance of the field-effect transistor FET are correspondingly less, which was already shown at the beginning. If, for example, you compare the losses of a diode with those of a field effect transistor in a flyback converter whose output current is 1 A, a rectifier diode results in a power loss of around 0.5 W, in a field effect transistor with a path resistance of approx. 40 mΩ a power loss of only about 0.1 W. It should also be noted that when there is a load on the secondary side, here a load resistor R L has been drawn in, the primary switch S p remains switched on longer, thus more energy is stored in the transformer and the secondary side is also demagnetized longer. However, since a current flows longer into the RC element R, C, the field effect transistor FET remains switched on for a longer period, so that it is switched on for different lengths depending on the load.
Das RC-Glied R,C wird natürlich so dimensioniert, daß der Feldeffekt-Transistor möglichst bald durchgeschaltet wird (kurze Zeit TA) und das auf jeden Fall der Feldeffekt-Transistor abgeschaltet wird, bevor die Abmagnetisierung abgeschlossen ist. Ein zu frühes Schließen des Feldeffekt-Transistors würde ebenso wie ein zu spätes Öffnen zu einem kurzschlußartigen Zustand und damit zu Leistungsverlusten führen. Zu beachten ist weiters, daß auch bei einem Ausfall der Ansteuerung des Feldeffekt-Transistors FET die Schaltung, wenngleich auch mit höheren Verlusten, prinzipiell funktioniert, da dann die Diode Di als übliche Gleichrichterdiode wirkt.The RC element R, C is of course dimensioned such that the field effect transistor is switched through as soon as possible (short time T A ) and that the field effect transistor is definitely switched off before the demagnetization is complete. Closing the field effect transistor too early, as well as opening it too late, would lead to a short-circuit-like state and thus to power losses. It should also be noted that even if the control of the field effect transistor FET fails, the circuit works in principle, even if with higher losses, since then the diode Di acts as a conventional rectifier diode.
Die Variante nach Fig. 2 zeichnet sich dadurch aus, daß eine zweite Sekundärwicklung Ws- vorgesehen ist, um eine höhere Spannung für die Durchführung der Schaltvorgänge zu erreichen, falls die Ausgangsspannung UA nur gering ist und beispielsweise in der Größenordnung von 3 V liegt. In diesem Fall würden nämlich die Spannungen der Diode Ds und des Transistors Ts in Durchlaßrichtung zu Spannungsabfallen führen, die ein sicheres Durchschalten möglicherweise nicht mehr gewährleisten.The variant according to FIG. 2 is characterized in that a second secondary winding WS is provided in order to achieve a higher voltage for carrying out the switching operations if the output voltage U A is only low and is of the order of 3 V, for example. In this case, the voltages of the diode D s and the transistor T s in the forward direction would lead to voltage drops, which may no longer guarantee a safe switching.
Die Schutzdiode Ds soll einen Spannungsdurchbruch der Basis-Emitter-Strecke des Schalttransistors Ts verhindern. Die Durchbruchspannung dieser Basis-Emitter-Strecke liegt nur bei wenigen Volt, sodaß bei höheren Spannungen an der Sekundärseite diese Strecke gefährdet wäre. Man verwendet daher die Schutzdiode Ds, die beispielsweise eine Durchbruchspannung in der Größenordnung von 40 V haben kann, um die Basis-Emitter-Strecke des Transistors Ts zu schützen. Auch der Schutzwiderstand Rs schützt den Schalttransistor Ts, in diesem Fall vor einem zu hohen Basisstrom.The protective diode Ds is intended to prevent a voltage breakdown of the base-emitter path of the switching transistor T s . The breakdown voltage of this base-emitter path is only a few volts, so that this path would be endangered at higher voltages on the secondary side. The protection diode Ds is therefore used, which can have a breakdown voltage of the order of 40 V, for example, in order to protect the base-emitter path of the transistor Ts. The protective resistor Rs also protects the switching transistor T s , in this case from an excessively high base current.
In der Zeichnung ist die Zeitdauer TE länger eingezeichnet als die Zeitdauer TA zu Beginn des Sekundärstromes, womit darauf hingewiesen werden soll, daß Zeitdauer TE nicht besonders kritisch ist, da hier der Strom schon gering ist und allfällige Verluste in der integrierten Diode Di nicht mehr so stark zu Buche schlagen wie zu Beginn des sekundären Stromflusses. In the drawing, the time period T E is drawn longer than the time period T A at the beginning of the secondary current, which is to point out that time period T E is not particularly critical, since the current is already low here and any losses in the integrated diode Di no longer impact as much as at the beginning of the secondary current flow.

Claims

PATENTANSPRÜCHE PATENT CLAIMS
1. Sperrwandler mit einem Übertrager (Tr), mit einer Primärwicklung (Wp) sowie mit zumindest einer Sekundärwicklung (Ws), wobei die Primärwicklung in Serie mit einem gesteuerten Schalter (Sp) an einer Eingangsspannung (Up) liegt, und der Sekundärwicklung ein Gleichrichterelement (FET) und ein Ladekondensator (C„) nachgeschaltet sind, dadurch gekennzeichnet, daß das Gleichrichterelement als Feldeffekt-Transistor (FET) ausgebildet ist, der über einen Ansteuertransistor (Ts) schaltbar ist, wobei das Gate an einem Arbeitswiderstand (R^) des Transistors liegt, und der Arbeitswiderstand in Serie mit dessen Kollektor-Emitter-Strecke an einer Übertrager-Sekundärspannung liegt.1. flyback converter with a transformer (Tr), with a primary winding (W p ) and with at least one secondary winding (W s ), the primary winding being connected in series with a controlled switch (S p ) to an input voltage (U p ), and the secondary winding is followed by a rectifier element (FET) and a charging capacitor (C "), characterized in that the rectifier element is designed as a field effect transistor (FET) which can be switched via a drive transistor (T s ), the gate being connected to a load resistor (R ^) of the transistor, and the load resistance in series with its collector-emitter path is at a transformer secondary voltage.
2. Sperrwandler nach Anspruch 1, dadurch gekennzeichnet, daß der Basisstrom der Basis des Ansteuertransistors (Ts) über ein Parallel-RC-Glied (RC) zuführbar ist.2. flyback converter according to claim 1, characterized in that the base current of the base of the drive transistor (T s ) via a parallel RC element (RC) can be supplied.
3. Sperrwandler nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß in Serie zur Emitter- Kollektor-Strecke des Ansteuertransistors (T2) eine Schutzdiode (Ds) liegt, deren Durchlaßrichtung jener der Emitter-Basis-Strecke entspricht.3. flyback converter according to claim 1 or 2, characterized in that in series with the emitter-collector path of the drive transistor (T2) is a protective diode (D s ), whose forward direction corresponds to that of the emitter-base path.
4. Sperrwandler nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, daß in der Basiszuleitung des Ansteuertransistors (Ts) in Serie mit dem Parallel-RC-Glied (RC) ein Schutzwiderstand (R,) liegt.4. flyback converter according to one of claims 1 to 3, characterized in that in the base lead of the drive transistor (T s ) in series with the parallel RC element (RC) is a protective resistor (R,).
5. Sperrwandler nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, daß der Feldeffekt-Transistor (FET) ein n-Kanal-Feldeffekt-Transistor ist, welcher zwischen ein Ende einer Sekundärwicklung (W8) und den negativen Anschluß des Ladekondensators (C) geschaltet ist, wogegen das andere Ende der Sekundärwicklung an dem positiven Anschluß des Ladekondensators liegt, der Ansteuertransistor (Ts) ein p-n-p-Transistor ist, dessen Kollektor einerseits über den Arbeitswiderstand (Rpj) an dem negativen Anschluß des Ladekondensators liegt, und andererseits mit dem Gate des Feldeffekt-Transistors verbunden ist, und das Parallel-RC-Glied (RC) zwischen Drain des Feldeffekt-Transistors (FET) und Basis des Ansteuertransistors (Ts) liegt. 5. flyback converter according to one of claims 1 to 4, characterized in that the field effect transistor (FET) is an n-channel field effect transistor, which between one end of a secondary winding (W 8 ) and the negative terminal of the charging capacitor (C ) is connected, whereas the other end of the secondary winding is connected to the positive terminal of the charging capacitor, the drive transistor (T s ) is a pnp transistor, the collector of which is connected to the negative terminal of the charging capacitor via the load resistor (Rpj), and the other is connected to the gate of the field effect transistor, and the parallel RC element (RC) lies between the drain of the field effect transistor (FET) and the base of the drive transistor (T s ).
6. Sperrwandler nach Anspruch 3 und Anspruch 5, dadurch gekennzeichnet, daß die Schaltdiode (D.) zwischen dem Emitter des Ansteuertransistors (T.) und dem positiven Anschluß des Ladekondensators (C„) liegt.6. flyback converter according to claim 3 and claim 5, characterized in that the switching diode (D.) between the emitter of the drive transistor (T.) and the positive terminal of the charging capacitor (C ").
7. Sperrwandler nach einem der Ansprüche 1 bis 6, dadurch gekennzeichnet, daß zwei in Serie liegende Sekundärwicklungen (W,_, Ws2) vorgesehen sind, wobei Feldeffekt- Transistor (FET) und Ladekondensator (C.) einer Wicklung (W,ι) zugeordnet sind, die Ansteuerung der Gates über den Ansteuertransistor (T.) jedoch über die Summenspannung beider Sekundärwicklungen (W,ι, W,2) durchführbar ist. 7. flyback converter according to one of claims 1 to 6, characterized in that two series-lying secondary windings (W, _, W s2 ) are provided, field-effect transistor (FET) and charging capacitor (C.) of a winding (W, ι ) are assigned, the control of the gates can be carried out via the control transistor (T.) but via the total voltage of both secondary windings (W, ι, W, 2 ).
EP99973191A 1998-12-02 1999-12-02 Isolating transformer Withdrawn EP1135849A1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
AT203798 1998-12-02
AT0203798A AT409689B (en) 1998-12-02 1998-12-02 FLYBACK CONVERTER
PCT/AT1999/000298 WO2000033450A1 (en) 1998-12-02 1999-12-02 Isolating transformer

Publications (1)

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EP1135849A1 true EP1135849A1 (en) 2001-09-26

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EP99973191A Withdrawn EP1135849A1 (en) 1998-12-02 1999-12-02 Isolating transformer

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EP (1) EP1135849A1 (en)
AT (1) AT409689B (en)
CZ (1) CZ20011923A3 (en)
HU (1) HUP0105222A3 (en)
NO (1) NO20012618L (en)
WO (1) WO2000033450A1 (en)

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3605417C1 (en) * 1986-02-20 1987-07-09 Ant Nachrichtentech Rectifier circuit
DE3727170A1 (en) * 1987-08-14 1989-02-23 Philips Patentverwaltung DC/DC voltage converter having a transformer
US4942510A (en) * 1989-12-04 1990-07-17 Motorola, Inc. Power and signal transfer interface circuit
DE4106915A1 (en) * 1991-03-05 1992-09-10 Ant Nachrichtentech Current supply circuit using switched regulator - has opto-coupler in pulse width modulation regulation stage and short-circuit current limitation
US5396412A (en) * 1992-08-27 1995-03-07 Alliedsignal Inc. Synchronous rectification and adjustment of regulator output voltage

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Title
See references of WO0033450A1 *

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NO20012618L (en) 2001-07-24
NO20012618D0 (en) 2001-05-29
ATA203798A (en) 2002-02-15
AT409689B (en) 2002-10-25
CZ20011923A3 (en) 2002-07-17
HUP0105222A2 (en) 2002-04-29
WO2000033450A1 (en) 2000-06-08
HUP0105222A3 (en) 2003-02-28

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