United States Patent [19]
`Weinmeier et al.
`
`[54] FIXED FREQUENCY FLYBACK
`CONVERTER
`
`[75] Inventors: Harald Weinmeier; Gunter Marka,
`both of Vienna, Austria
`
`[73] Assignee: Siemens AG Osterreich, Vienna,
`Austria
`
`[21] Appl. NO.Z 09/409,582
`[22]
`Filed,
`Sep_ 30 1999
`i
`’
`Foreign Application Priority Data
`
`[30]
`
`Austria .................................. .. 1631/98
`Sep. 30, 1998 [AT]
`[51]
`Int. Cl.7 ......................... .. H02H 7/125; H02M 7/44;
`HOZM 7/537
`[52] US. Cl. .............................. .. 363/97; 363/53; 363/131
`[58] Field Of Search .............................. .. 363/21, 97, 131,
`363/56
`
`[56]
`
`References Cited
`
`US- PATENT DOCUMENTS
`1/1997 Imamura et a1. ....................... .. 363/21
`2/1998 Sohner . . . . . . . . . . . . .
`. . . . .. 363/21
`
`5,598,324
`5,717,579
`
`5,729,443
`5,734,564
`5,768,118
`
`US006141232A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,141,232
`Oct. 31, 2000
`
`[57]
`
`ABSTRACT
`
`A ?yback converter including a transformer (Tr), having a
`primary Winding (WP) and having a secondary Winding
`(W5), Wherein the primary Winding is connected in series
`With a controlled sWitch (T) and an actuation circuit (A) to
`an input direct voltage (UP), and the secondary Winding is
`connected in series With a recti?er (D S) and, downstream of
`the secondary Winding, a charging capacitor (C5); the actua
`tion circuit (A) supplies start pulses of a ?xed frequency for
`the sWitch, has a current sensor (RP) for the current through
`the primary Winding. Coupled With the secondary Winding is
`a ?rst rectifying ampli?er (KU) for comparing the output
`voltage (Us) With an adjustable voltage reference value
`(URE ) and a Second rectifying ampli?er (KI) for Comparing
`the output current (IS) With a adjustable current reference
`value (URe?), Wherein as a control signal the combined and
`decoupled Output signals of the tWO rectifying ampli?ers and
`the Output signal of the Current sensor of the eetuetieh eireuit
`are supplied for the purpose of in?uencing the pulse duty
`radio, the maximum output of the converter-primary part is
`loWer than the product of the output voltage (UAmax) Which
`can be adjusted to a maximum degree and the output current
`(I Amwc) Which can be adjusted to a maximum degree, the
`reference Values (URe/w URe/I) which can be adjusted to a
`maXimllm degree to correspond to the Voltage and Current
`
`3/1998 Pavlin _ _ _ _ _ _
`3/1998 Brkovic
`6/1998 Faulk et a1
`
`_ _ _ _ __ 363/97
`.... .. 363/21
`363/72
`
`values With Which the secondary part can be loaded to a
`maximum, and the level of the control signal (Us) is limited
`by a limiter (DZ), Wherein the point (UZ) at Which the limiter
`
`5,903,452
`5/1999 Yang . . . . . . . . . . . . . . . . . . . .
`. . . . .. 363/97
`5,991,172 11/1999 Javanovic et al. ...................... .. 363/21
`
`Primary Examiner—Peter S. Wong
`Assistant Examiner—Bao Q. Vu
`Attorney, Agent, or Firm—Reed Smith ShaW & McClay
`LLP
`
`is used is Selected in Such a Way that a maximum breaking
`
`current in the primary circuit is established corresponding to
`the maximum permissible poWer loading of the primary
`circuit.
`
`17 Claims, 2 Drawing Sheets
`
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`
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`
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`
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`
`

`
`U.S. Patent
`
`Oct. 31, 2000
`
`Sheet 1 of2
`
`6,141,232
`
`

`
`U.S. Patent
`
`0a. 31, 2000
`
`Sheet 2 of2
`
`6,141,232
`
`35
`
`
`
`|[A] ——>
`
`25
`
`20
`
`10 15
`
`F|G.2
`
`

`
`6,141,232
`
`1
`FIXED FREQUENCY FLYBACK
`CONVERTER
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`This invention is concerned With a ?yback converter
`having a transformer, having a primary Winding and at least
`one secondary Winding, Wherein the primary Winding is
`connected, in series and at least one secondary Winding.
`More speci?cally, the invention relates to a ?yback con
`verter Wherein the primary Winding is connected, in series
`supplies start pulses of a ?xed frequency for the controlles
`sWitch, has a current sensor for the current through the
`primary Winding. Coupled With the secondary Winding is a
`?rst rectifying ampli?er for comparing the ampli?er for
`comparing the output current With an adjustable current
`reference value, Wherein as a control signal the combined
`and decoupled output signals of the tWo rectifying ampli?ers
`and the output signal of the current sensor of the actuation
`circuit are supplied for the purpose of in?uencing the pulse
`duty ratio, and the maximum output of the converter
`primary part is loWer than the product of the output voltage
`Which can be adjusted to a maximum degree and the output
`current Which can be adjusted to a maximum degree.
`2. Description of the Related Art
`Flyback converters are knoWn and are Widely used as
`mains poWer supply units for supplying current to electronic
`devices. For example, a knoWn ?yback converter is
`described as prior art in AT 403 535 B of the Applicant. A
`decisive cost factor for such mains poWer supply units is the
`poWer available to be draWn off since this, in particular,
`determines the costs for the transformer and smoothing
`means such as capacitors and throttles. A mains poWer
`supply unit of one construction type should, on the other
`hand, cover as many different applications as possible
`demanding different voltages and currents. For this reason
`mains poWer supply units are supplied With an adjustable
`voltage and adjustable current, for example a maximum of
`30 volts, 3 amps. Within these limits the purchaser can then
`adjust the current and voltage, Wherein as far as output is
`concerned the mains poWer supply unit, hoWever, is fully
`used only at the highest voltage, in this case 30 volts><3
`amps=90 Watts. If, on the other hand, the purchaser adjusts
`the device to a loW voltage, for example 3 volts, only a
`fraction of the poWer is available to him, namely in this
`example 3 volts><3 amps=9 Watts. The device is not
`exploited according to its capabilities, in this case only to a
`tenth of its capability, and is unnecessarily expensive for the
`application concerned.
`
`15
`
`25
`
`35
`
`45
`
`BRIEF SUMMARY OF THE INVENTION
`
`An object of the invention is to overcome the above-notes
`disadvantage and to create a mains poWer supply unit Which,
`even in the case of individual adjustment of current and
`voltage by the customer, fully exploits its capabilities.
`This object is achieved on the basis of a ?yback converter
`according to the invention and of the type mentioned in the
`introduction preamble before the description of the related
`art, so that in accordance With the invention the reference
`values Which can be adjusted to a maximum degree corre
`spond to the voltage and current values With Which the
`secondary part can be loaded to a maximum and the level of
`the control signal is limited by a limiter, Wherein the point
`at Which the limiter is used is selected in such a Way that a
`maximum breaking current in the primary circuit is estab
`
`55
`
`65
`
`2
`lished corresponding to the maximum permissible poWer
`loading of the primary circuit.
`The invention provides for a ?yback converter in Which
`the capacity is no longer limited by the maximum current in
`the case of the maximum voltage, Wherein the full capacity
`and therefore also a higher current is available even in the
`case of smaller voltages. The solution is also simple and
`inexpensive since With a ?xed-frequency ?yback converter
`the poWer to be transferred to the secondary side is depen
`dent only on the frequency and the primary current and in
`order to limit the capacity it is suf?cient to limit the
`sWitching current. It is suf?cient to dimension the primary
`part together With the transformer to suit the maximum
`capacity and only the less expensive secondary part must be
`dimensioned both With respect to the maximum current and
`to the maximum voltage.
`Aparticularly simple variation proposes that the limiter is
`a Zener diode.
`A variation Which is inexpensive to achieve and suitable
`for implementation makes provision for the outputs of the
`tWo rectifying ampli?ers to be decoupled by means of
`diodes and a pull-up resistor.
`It is also expedient that if the output signal of the current
`sensor is supplied to one input and the regulating signal is
`supplied to the other input of a third comparator, the output
`of Which is connected to the reset input of a ?ip-?op
`actuating the sWitch, then constant-frequency pulses are
`applied to the set input, since in this embodiment only a feW
`components are required.
`Finally, in terms of an inexpensive solution it is also
`advisable for the current sensor to be a resistor.
`The invention together With further advantages is
`explained in more detail hereinunder With the aid of an
`exempli?ed embodiment Which is illustrated in the draWing.
`The individual ?gure thereof shoWs a very schematic rep
`resentation of a circuit of a ?yback converter in accordance
`With the invention.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 as a circuit diagram of a ?yback converter in
`accordance With the invention; and
`FIG. 2 is a graphical representation of a diagram illus
`trating the maximum limits of adjustment of either the
`output voltage or output current.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`Referring noW more particularly to the draWings, the
`?yback converter is explained in detail in connection With
`FIG. 1.
`Speci?cally, the ?yback converter in accordance With the
`invention illustrated in the FIG. 1 has a transformer Tr, With
`a primary Winding WP and a secondary Winding W5,
`Wherein on the secondary side a recti?er D S, for example, an
`individual diode or a bridge recti?er, and a charging capaci
`tor C5 are provided. The output voltage US is connected to
`the charging capacitor and the output current IS determined
`by a load RL ?oWs via a sensor resistor R5.
`Returning to the primary side of the ?yback converter it
`Will be seen that a mains alternating voltage U” is recti?ed
`by means of a recti?er DP and is connected as a primary
`direct voltage UP to a primary capacitor C1,. A ?lter F, can
`also be provided in a knoWn manner to prevent or reduce the
`re?oW of interference into the mains.
`The primary or input direct voltage UP is connected via a
`controlled sWitch T, in this case a sWitching transistor, to the
`
`

`
`3
`primary Winding W P of the transformer Tr, wherein a current
`sensor R P is also disposed in this series circuit. An actuation
`circuit, designated as a Whole by A, actuates the sWitch T and
`in this exempli?ed embodiment contains a ?ip-?op FF, a
`frequency generator FG, a comparator K and a Zener diode
`DZ, With a Zener voltage UZ as a limiter.
`The pulses of the frequency generator FG are supplied to
`the set input of the ?ip-?op and initiate commencement of
`a sWitching pulse, Wherein each pulse is terminated by a
`signal at the reset input of the ?ip-?op FF. The pulse duty
`ratio of the pulse produced determines the current through
`the sWitch T, the primary Winding WP and the current sensor
`RP, Wherein in stationary operation the current Which is
`increasing in a substantially linear manner and is measured
`by the current sensor RP, from a certain level, namely the
`voltage applied to the non-inverting input of the comparator
`K, is supplied to reset the ?ip-?op and terminate the sWitch
`ing pulse.
`Returning noW to the secondary side it Will be seen that
`output voltage Us is compared in a ?rst rectifying ampli?er
`KU With an adjustable voltage reference value URefU. The
`same is carried out With the current Is measured at the
`resistor Rs and compared in a second rectifying ampli?er Ks
`With a reference value URe? Which can also be adjusted. The
`outputs of the tWo rectifying ampli?ers KU and Ks are
`supplied by decoupling diodes DU and D s to the non
`inverting input of the comparator K, Wherein the mutually
`connected anodes of the tWo diodes are connected by a
`pull-up resistor RH to an auxiliary direct voltage UH and the
`Zener diode DZ demands that a certain maximum voltage at
`this input cannot be exceeded. In this case the Zener voltage
`UZ=10 V and the auxiliary voltage UH=25 V.
`During operation the output voltage Us can be adjusted
`Within broad limits by the adjustable reference voltage
`source UREIU, and likeWise the current Is With the aid of the
`reference URe?, Wherein the adjustment of the current gen
`erally serves to limit the current and the operation is carried
`out With a constant voltage Us. In order not to exceed the
`maximum poWer—Which is set ?rst and foremost by the
`dimensioning of the transformer T, but also additionally by
`the dimensioning of very different components such as the
`sWitch T and the capacitor CP—during operation the Zener
`diode DZ is provided in accordance With the invention as a
`limiter. In normal operation in Which the maximum poWer
`value is not exceeded voltage regulation Will generally be
`used and a predetermined voltage value Will be connected to
`the output of the rectifying ampli?er KU but Will be loWer
`than the Zener voltage of the diode DZ. If then, With a
`predetermined voltage, the current is increased, for example,
`because the load resistance RL is loWered or because the
`reference voltage URe I is increased, the voltage regulation is
`re-regulated in order to compensate for the noW higher load,
`ie. the output voltage at the rectifying ampli?er KU Will
`increase. Above a certain current value this output voltage
`Will be so high that the Zener diode DZ becomes conductive
`Whereby the length of the sWitching pulse and the primary
`current, and therefore also the transferred poWer, is limited
`accordingly. The equivalent case arises if a constant current
`is draWn from the mains poWer supply unit and the voltage
`is increased.
`Reference is made hereinunder to FIG. 2 Which consti
`tutes a diagram of the maximum limits of adjustment of the
`output voltage or output current. With the aid of this diagram
`the invention Will be explained again using a speci?c
`example. The voltage is set to a maximum at one Working
`point at the top left in the diagram, Wherein the output of the
`rectifying ampli?er KU for the voltage is at a potential of 1
`
`15
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`65
`
`6,141,232
`
`4
`V, in contrast, the output of the rectifying ampli?er Ks for
`the current lies Within the positive overload point limit. The
`control signal Us amounts to about 1.7 V. At the Working
`point C the output of the rectifying ampli?er KU has a
`voltage of about 5 V, the output of the rectifying ampli?er Ks
`for the current is still Within the positive overload limit and
`the control signal amounts to 5.7 V. At the Working point C
`the output of the rectifying ampli?er KU for the voltage is at
`a potential of about 9.3 V, the output of the rectifying
`ampli?er Ks for the current is still Within the positive
`overload limit and the control signal is 10 V.
`At the Working point D the output of the rectifying
`ampli?er KU for the voltage has in the meantime reached its
`positive overload limit, the output of the rectifying ampli?er
`Ks for the current is also Within the positive overload limit
`and the control signal is limited by the Zener diode DZ to 10
`V. The Working point D is located on the poWer dissipation
`curve Which terminates toWards the right at the Working
`point E. The output of the rectifying ampli?er KU for the
`voltage is still Within the positive overload limit, the output
`of the rectifying ampli?er Ks for the current is, in contrast,
`at a potential of about 9.3 V and the control signal is still 10
`V. At the Working point F the output of the rectifying
`ampli?er KU for the voltage is still Within the positive
`overload limit, the output of the rectifying ampli?er Ks for
`the current is, in contrast, still only at a potential of about 2
`V and the control signal is 2.7 V. At the Working point G the
`output of the rectifying ampli?er KU for the voltage is Within
`the positive overload limit, the output of the rectifying
`ampli?er Ks for the current is at a potential of about 1 V and
`the control signal is 1.7 V.
`It Will be obvious to the person skilled in the art in the
`?eld of sWitched mode mains poWer supply units that the
`actuation circuit A can also be produced in a different
`manner provided that only the level of the control signal,
`Which is designated s2 and is derived from the decoupled
`outputs of the rectifying ampli?ers KU and Ks, is limited.
`The limiter can also be produced in a different Way than by
`using a Zener diode DZ. The rectifying ampli?ers KU and
`Ks, Which are designated herein as disconnected difference
`ampli?ers, are generally connected to additional circuit
`elements, in particular RC members for achieving the
`desired regulating characteristics, and likeWise in many
`cases the control signal Us is dc-isolated betWeen the
`primary and secondary side With the aid of an optical
`coupler.
`With the aid of the invention a ?yback converter in
`accordance With the invention can be used universally
`Within a certain poWer range since the user is free to choose
`the voltage and/or the current according to requirements,
`Wherein, hoWever, the maximum possible poWer is still
`available. The primary part of the ?yback converter must be
`dimensioned With respect to this poWer, but When dimen
`sioning the secondary part it is merely necessary to consider
`the maximum current Which may occur or the maximum
`voltage Which may occur.
`What is claimed is:
`1. A ?yback converter provided With a transformer, hav
`ing a primary Winding and having a secondary Winding, the
`primary Winding being connected in series With a controlled
`sWitch to an input direct voltage and the secondary Winding
`being connected in series to a recti?er and a charging
`capacitor connected doWnstream of the secondary Winding,
`comprising:
`an actuation circuit for supplying start pulses of a ?xed
`frequency for the controlled sWitch, said actuation
`circuit including a current sensor for current ?oWing
`
`

`
`6,141,232
`
`5
`through the primary Winding, a ?rst rectifying ampli?er
`for comparing the output voltage With an adjustable
`voltage reference value,
`a second rectifying ampli?er for comparing the output
`current With an adjustable current reference value as a
`control signal derived from the output signals of the
`tWo rectifying ampli?ers and the output signal of the
`current sensor of the actuation circuit is supplied for the
`purpose of in?uencing the pulse duty ratio, and the
`maximum output of the converter-primary part is loWer
`than the product of the output voltage Which can be
`adjusted to a maXimum degree and the output current
`Which can be adjusted to a maXimum degree; Wherein
`the reference values Which can be adjusted to a maXimum
`degree correspond to the voltage and current values
`With Which the secondary part can be loaded to a
`maximum, and
`the level of the control signal is limited by a limiter,
`Wherein the point at Which the limiter is used is selected
`in such a Way that a maXimum breaking current in the
`primary circuit is established corresponding to the
`maXimum permissible poWer loading of the primary
`circuit.
`2. The ?yback converter according to claim 1, Wherein the
`limiter is a Zener diode.
`3. The ?yback converter according to claim 1, Where the
`outputs of the tWo rectifying ampli?ers are decoupled by
`means of diodes and a pull-up resistor.
`4. The ?yback converter according to claim 2, Where the
`outputs of the tWo rectifying ampli?ers, are decoupled by
`means of diodes and a pull-up resistor.
`5. The ?yback converter according to claim 1 Wherein the
`output signal of the current sensor is supplied to one input
`and the regulating signal is supplied to another input of a
`third comparator, the output of Which is connected to the
`reset input of a ?ip-?op actuating the sWitch, Wherein the
`constant-frequency pulses are applied to the set input.
`6. The ?yback converter according to claim 2 Wherein the
`output signal of the current sensor is supplied to one input
`
`10
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`15
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`35
`
`6
`and the regulating signal is supplied to another input of a
`third comparator, the output of Which is connected to the
`reset input of a ?ip-?op actuating the sWitch, Wherein the
`constant-frequency pulses are applied to the set input.
`7. The ?yback converter according to claim 3 Wherein the
`output signal of the current sensor is supplied to one input
`and the regulating signal is supplied to another input of a
`third comparator, the output of Which is connected to the
`reset input of a ?ip-?op actuating the sWitch, Wherein the
`constant-frequency pulses are applied to the set input.
`8. The ?yback converter according to claim 4 Wherein the
`output signal of the current sensor is supplied to one input
`and the regulating signal is supplied to another input of a
`third comparator, the output of Which is connected to the
`reset input of a ?ip-?op actuating the sWitch, Wherein the
`constant-frequency pulses are applied to the set input.
`9. The ?yback converter according to claim 1, Wherein the
`current sensor is a resistor.
`10. The ?yback converter according to
`the current sensor is a resistor.
`11. The ?yback converter according to
`the current sensor is a resistor.
`12. The ?yback converter according to
`the current sensor is a resistor.
`13. The ?yback converter according to
`the current sensor is a resistor.
`14. The ?yback converter according to
`the current sensor is a resistor.
`15. The ?yback converter according to
`the current sensor is a resistor.
`16. The ?yback converter according to
`the current sensor is a resistor.
`17. The ?yback converter according to claim 1, Wherein
`in stationary operation the current through the controlled
`sWitch is measured by the current sensor and the voltage
`applied to the non-inverting input of the comparator is
`supplied to reset the ?ip-?op and terminate the sWitching
`pulse.
`
`claim 2, Wherein
`
`claim 9, Wherein
`
`claim 4, Wherein
`
`claim 5, Wherein
`
`claim 6, Wherein
`
`claim 7, Wherein
`
`claim 8, Wherein