(12) United States Patent
`Jacobs et al.
`
`USOO6304464B1
`US 6,304,464 B1
`Oct. 16, 2001
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`(54) FLYBACK AS LED DRIVER
`
`(56)
`
`References Cited
`
`(75) Inventors: Ronny A. A. M. Jacobs; Bertrand J.
`E. Hontele, both of Eindhoven (NL);
`John E. K. G. De Clercq, Oordegem
`(BE); Marcel J. M. Bucks, Weert;
`Engbert B. G. Nijhof, Eindhoven, both
`of (NL)
`
`(73) Assignee: U.S. Philips Corporation, New York,
`NY (US)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`USC 154(b) by 0 days.
`
`(21) Appl. No.: 09/610,717
`(22) Filed:
`Jul. 6, 2000
`(30)
`Foreign Application Priority Data
`
`Jul. 7, 1999
`
`(EP) ............................................... .. 99202228
`
`(51) Int. Cl.7 .................................................. .. H02M 3/335
`(52) U.S. Cl. ..................... .. 363/21.12; 363/23; 315/1693
`(58) Field of Search ................................ .. 363/21, 20, 56,
`363/97, 98, 131, 132; 315/205, 226, 169.3
`
`U.S. PATENT DOCUMENTS
`
`4,561,046 * 12/1985 Kuster .................................. .. 363/21
`
`* cited by examiner
`
`Primary Examiner—Rajnikant Patel
`(57)
`ABSTRACT
`
`The invention relates to a circuit arrangement for operating
`a semiconductor light source, comprising
`input terminals for connecting a supply voltage,
`input ?lter means,
`a converter provided with a switching element having a
`control circuit, and provided with inductive means, and
`output terminals for connecting the semiconductor light
`source.
`The switching element is periodically driven into conduction
`for a period to”. According to the invention, the converter is
`formed by a ?yback converter, and the inductive means are
`formed by a transformer, and the control circuit controls the
`I
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`7 Claims, 2 Drawing Sheets
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`WAC-1021
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`U.S. Patent
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`061. 16, 2001
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`Sheet 1 6f 2
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`US 6,304,464 B1
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`A
`Z
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`z
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`+
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`_
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`Q0
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`a
`(3
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`8452
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`E \ T?‘ E
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`M?
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`11
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`12
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`13
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`SC
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`14
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`0
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`R5
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`FIG. 2
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`

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`U.S. Patent
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`061. 16, 2001
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`Sheet 2 6f 2
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`US 6,304,464 B1
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`mlw
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`UT
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`IS 31
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`\
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`CS 33
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`FIG. 3
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`FIG. 4
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`C2
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`

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`US 6,304,464 B1
`
`1
`FLYBACK AS LED DRIVER
`
`The invention relates to a circuit arrangement for oper
`ating a semiconductor light source, comprising
`input terminals for connecting a supply voltage,
`input ?lter means,
`a converter provided With inductive means and With a
`sWitching element having a control circuit, Which
`sWitching element is periodically in a conducting state
`for a period to”, and
`output terminals for connecting the semiconductor light
`source.
`The invention also relates to a signal light provided With
`such a circuit arrangement.
`A circuit arrangement of the type mentioned in the
`opening paragraph is knoWn from WO 99/07188. The con
`verter serves as a sWitch-mode poWer supply of the semi
`conductor light source. In the case of the knoWn circuit
`arrangement, a multiresonant forWard/?yback converter is
`used as the converter. This enables a good, i.e. high, poWer
`factor to be achieved. Semiconductor light sources are
`increasingly used as signal lights. In such an application, the
`advantage of a semiconductor light source over a customary
`incandescent lamp is a substantial improvement in service
`life and a substantial reduction in energy consumption.
`Signal lights often form part of a complex signaling system,
`for example a traf?c control system With traf?c lights.
`Generally, the functioning of semiconductor light sources
`applied as a light source is determined by the value of the
`current supplied to the semiconductor. Consequently, the
`converter should serve as a current source (constant current
`generator). On the other hand, traffic light installations are
`often driven by a triac sWitching circuit. An important
`property of such a triac sWitching circuit is that a minimum
`current must ?oW through said triac sWitching circuit if the
`driver is to function properly in the conducting phase of the
`light source. This leads to a complex construction of both the
`converter and the control circuit of the knoWn circuit
`arrangement. This constitutes a draWback.
`It is an object of the invention to provide a circuit
`arrangement of the type described in the opening paragraph,
`Wherein the above-mentioned draWback is precluded. In
`accordance With the invention, this object is achieved in that
`the circuit arrangement of the type mentioned in the opening
`paragraph is characteriZed in accordance With the invention
`in that the converter is a ?yback converter and the inductive
`means are formed by a transformer, and in that the control
`circuit controls the period ton.
`It has surprisingly been found that the circuit arrangement
`in accordance With the invention can suitably be used to
`reliably operate a semiconductor light source as a replace
`ment for an incandescent lamp in a signal light, also in the
`case of loW temperatures, and said circuit arrangement is
`also characteriZed by great simplicity, Which is very advan
`tageous. Controlling the period ton in such a manner that it
`changes only little has the important advantage that the
`?yback converter is capable of achieving a very good poWer
`factor as Well as a very loW level of harmonic distortion
`(THD) of mains current extracted from the supply source.
`The poWer supply at the input terminals of the circuit
`arrangement Will generally take place from a mains voltage
`source and hence by means of an alternating voltage. If the
`alternating voltage has a period T, then, in a preferred
`embodiment of the circuit arrangement in accordance With
`the invention, the control circuit involves an integration over
`a period of at least 2.5 T. The realiZation of a very good
`poWer factor can thus be combined With a very fast feedback
`With respect to variations, such as temperature variations.
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`15
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`25
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`35
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`45
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`2
`In the case of alternating current poWer supply, preferably,
`recti?er means are present betWeen the input terminals and
`the converter. The loW level of THD can be further improved
`by incorporating only a high-frequency ?lter betWeen rec
`ti?er means and ?yback converter. This also precludes an
`adverse effect on the poWer factor, Which Would be caused
`by clipping of the recti?ed supply voltage.
`The converter can be driven into conduction at such a
`?xed frequency that current stops ?oWing simultaneously
`through the primary and secondary Windings for some time.
`Such a mode of operation is commonly referred to as
`discontinuous mode. Preferably, hoWever, the converter is
`operated in such a mode that a reduction to Zero of the
`current through the secondary transformer Winding causes
`the sWitching element to be driven into conduction. This
`mode of operation, Which is referred to as critical mode, can
`be realiZed by means of relatively simple means and addi
`tionally has the important advantage that peak currents are
`minimiZed and sWitching losses counteracted.
`These and other aspects of the invention Will be apparent
`from and elucidated With reference to the embodiments
`described hereinafter.
`In the draWings:
`FIG. 1 diagrammatically shoWs the circuit arrangement,
`FIG. 2 shoWs a diagram of a converter With a control
`circuit in greater detail,
`FIG. 3 shoWs the control circuit in greater detail, and
`FIG. 4 is a detailed vieW of input ?lter means.
`In FIG. 1, A, B are input terminals for connecting a supply
`source VB, for example an alternating voltage source having
`a period T, Which is provided, for example, With a solid-state
`relay. Input ?lter means are indicated by means of I, and a
`converter With a control circuit is indicated by means of III.
`C, D are output terminals for connecting the semiconductor
`light source LB. The input ?lter means I are provided With
`a positive pole + and a negative pole —, Which are connected
`to the output terminals C and D, respectively, via the
`converter III. The circuit arrangement also comprises a
`self-regulating current-limiting netWork, not shoWn, Which
`ensures, in a manner Which is knoWn per se, that in the
`off-state of the converter a leakage current generated in the
`supply source can ?oW aWay to a sufficient degree. By virtue
`thereof, an increase of the voltage at the input terminals and
`hence a Wrong status test are precluded. Such a netWork is
`disclosed in, for example, WO 99/07187.
`In FIG. 2, the converter and the associated control circuit
`are shoWn in greater detail. The converter is a ?yback
`converter With a sWitching element T1 connected in series
`With a transformer L2 provided With a primary Winding L21
`and a secondary Winding L22. The transformer forms the
`inductive means. The primary Winding L21 is shunted by a
`diode D1 Which is connected in series With a Zener diode Z1
`Which is oppositely poled. As a result, clamping takes place
`of any peak voltages as a result of a leakage self-inductance
`of the transformer L2 at a level beloW the breakdoWn voltage
`of the sWitching element T1. A current-measuring imped
`ance R3 Which is connected in series With the sWitching
`element T1 is also incorporated in the control circuit. The
`secondary Winding L22 is connected, via a diode D2, to an
`output terminal C and, via a current-measuring impedance
`R4, to the output terminal D. In addition, a buffer capacitor
`C4 is connected across the series-combination of diode D2
`and secondary Winding L22. The control circuit SC is
`connected With an output O to a control electrode g of the
`sWitching element T1. An input I1 of the control circuit SC
`is connected to the current measuring-impedance R3, an
`input I2 is connected to the secondary Winding L22, and an
`
`

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`US 6,304,464 B1
`
`3
`input I3 is connected to the current measuring-impedance
`R4. The control circuit is also provided With an input I4 to
`Which a temperature-sensitive element R5, such as a NTC
`impedance, is connected. This enables the current through
`the semiconductor light source to be compensated for tem
`perature changes of the light source and the direct surround
`ings thereof. For this purpose, a correction circuit 1532 in the
`control circuit is used to carry out a correction of the signal
`received at the input I3 by means of the signal received at the
`input I4, as is shoWn in FIG. 3. In this respect, a diode DS33
`in the path of the temperature-dependent signal ensures that
`the correction takes place only if the temperature-dependent
`signal has dropped beloW a threshold value. The corrected
`signal is supplied to connection pins I5a, 15b of a control-IC
`IS31, via a combination of a resistor R543 and a capacitor
`CS33. The resistor R543 and the capacitor CS33 in combi
`nation With an operational ampli?er present in the control IC
`ensure that the corrected signal is integrated. The siZe of the
`resistor R543 and the capacitor C533 is preferably chosen to
`be such that the integration time is at least 2.5 times the
`period T of the alternating voltage of the supply source. The
`control IC is connected through a connection pin I6 to the
`input I1, and through a connection pin I8 to the output O of
`the control circuit. The input I2 is connected to a connection
`pin I7 of the control IC 1531. The control IC is preferably a
`poWer factor correction IC.
`The ?lter means, shoWn in FIG. 4, comprise a fusistor R1,
`a varistor R2, a recti?er bridge DB1 and a at ?lter consisting
`of a self-inductance L1 and capacitors C1 and C2.
`In the eXample described hereinabove, the converter oper
`ates in the critical mode.
`In a practical embodiment of the circuit arrangement as
`described hereinabove, this circuit arrangement can suitably
`be connected to a driver having a voltage in the conducting
`state of at least 80 V, 60 HZ and at most 135 V, 60 HZ, and
`said circuit arrangement can also suitably be used to operate
`a semiconductor light source comprising a matriX of 6*3
`LEDs, made by HeWlett Packard, having a forWard voltage
`VF, de?ned at 250 mA and an ambient temperature of 25°
`C., betWeen 2 V and 3 V. If the converter is activated, the
`positive pole + of the input ?lter means carries a recti?ed
`voltage having an effective value of at least 80 V and at most
`135 V. In conditions Where the maXimum permissible tem
`perature is reached While the supply voltage is minimal, the
`circuit arrangement has a poWer ef?ciency of 75%. In
`comparable conditions, the poWer ef?ciency of the circuit
`arrangement in accordance With WO 99/07188 is only 65%.
`The ?yback converter has a 3NB60-type MOSFET, made
`by SGS-Thompson, as the sWitching element. The trans
`former consists of a ferrite core With a primary Winding of
`110 turns of 0.2 mm and a secondary Winding having 2
`sections of 15 turns of 0.355 mm each. The control circuit
`comprises a L6561-type poWer factor correction IC, made
`by SGS-Thompson, as the control IC, Wherein the connec
`tion pin I5a corresponds to the pin INV, I5b corresponds to
`COMP, I6 corresponds to CS, I7 corresponds to ZCD, and
`I8 corresponds to OUT. The correction circuit IS32 com
`prises the diode DS33 of the type LL4148. The comparator
`1533 is a dual comparator of the type LM293, Which is also
`made by SGS-Thompson. The current-measuring imped
`ances R3 and R4 are embodied so as to be a parallel
`arrangement of, respectively, 2 and 3 resistors of 1 Q each.
`The capacitor C4 is an electrolytic capacitor having a
`capacitance of 470 ME. The temperature-sensitive element
`R5 is a NTC-resistor of the type B57620C103, made by
`Siemens.
`The capacitors C1 and C2 are ?lter capacitors having a
`capacitance of 220 nF each. The self-inductance has a siZe
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`10
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`20
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`25
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`30
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`35
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`of 2.1 mH. The recti?er bridge DB1 is built up of 4 diodes,
`for example, of the type IN4007.
`The converter can suitably be used to reliably operate the
`LED array in an ambient temperature range from —40° C. to
`70° C. In the operating state of the ?yback converter, this
`converter supplies a minimum current of 540 mA to the
`connected LED array. The converter then WithdraWs a
`minimum poWer of 6 W, Which corresponds to a current
`consumption of at least 45 mA. This proves to be suf?cient
`as a holding current for triac sWitches forming part of the
`supply source. The correction of the instantaneous current
`through the LED array by means of the temperature signal
`originating from the NTC resistor is preferably such that it
`takes place at an ambient temperature from 25° C. upWards.
`In this manner, it is achieved in a very simple manner that,
`on the one hand, throughout the ambient temperature range
`from —40° C. to +70° C., the holding current required by the
`supply source is WithdraWn and, on the other hand, that the
`quantity of light generated by the LED array has a minimum
`siZe.
`The circuit arrangement described hereinabove can suit
`ably be used to operate a signal light comprising a light
`source consisting of a LED array With an installed poWer in
`the range from 6 W as a minimum to at least 15 W.
`What is claimed is:
`1. A circuit arrangement for operating a semiconductor
`light source, comprising:
`
`a. input terminals for electrically connecting a source of
`AC poWer having a period T;
`b. a ?lter electrically connected to the input terminals;
`c. a ?yback converter electrically connected to the ?lter
`and including a transformer electrically connected to a
`sWitching element;
`d. a control circuit for periodically effecting sWitching of
`the sWitching element into a conducting state having a
`period ton and for adjusting the duration of said period
`in response to variations in an ambient condition over
`an integration period Which is substantially longer than
`T; and
`e. output terminals for electrically connecting the circuit
`arrangement to the semiconductor light source.
`2. A circuit arrangement as claimed in claim 1 Where the
`ambient condition is ambient temperature.
`3. A circuit arrangement as claimed in claim 1 Where, in
`the operating state, the converter is operated in the critical
`mode.
`4. A circuit arrangement as claimed in claim Where the
`integration period is at least 2.5 T.
`5. A signal light arrangement as claimed in claim 1
`comprising an integral housing for the semiconductor light
`source and the circuit arrangement.
`6. Asignal light arrangement comprising a semiconductor
`light source and a circuit arrangement for operating said
`semiconductor light source, comprising:
`a. input terminals for electrically connecting a source of
`AC poWer having a period T;
`b. a ?lter electrically connected to the input terminals;
`c. a ?yback converter electrically connected to the ?lter
`and including a transformer electrically connected to a
`sWitching element;
`
`

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`US 6,304,464 B1
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`5
`d. a control circuit for periodically effecting switching of
`the switching element into a conducting state having a
`Period ton and for adjusting the duration of Said Period
`in response to variations in an ambient condition over
`an integration period Which is substantially longer than 5
`T; and
`
`6
`e. output terminals for electrically connecting the circuit
`arrangement to the semiconductor light source,
`7. A circuit arrangement as claimed in claim 6 Where the
`ambient Condition is ambient temperature
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`*
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