Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B41/00—Circuit arrangements or apparatus for igniting or operating discharge lamps
  • H05B41/14—Circuit arrangements
  • H05B41/26—Circuit arrangements in which the lamp is fed by power derived from DC by means of a converter, e.g. by high-voltage DC
  • H05B41/28—Circuit arrangements in which the lamp is fed by power derived from DC by means of a converter, e.g. by high-voltage DC using static converters

Definitions

• the present invention relates to a ballast system for operating a lamp, comprising a rectifier for rectifying AC input power to thereby produce a pulsating DC current; a DC-DC converter for converting said pulsating DC current into a substantially constant DC voltage; a DC-AC converter for converting said substantially constant DC voltage into an AC output current for driving the lamp.
• EBLs Electronic ballast lamps
• an EBL is a discharge lamp, e.g. , a fluores ⁇ cent lamp, which is coupled to an electronic ballast circuit (system) which converts an AC line voltage into a high frequency AC output voltage for operating the lamp, and which utilizes a lamp current feedback signal to regulate the sinusoidal lamp current.
• system electronic ballast circuit
• Fig. 1 there can be seen a block diagram of a conventional electronic ballast system 20 which receives its power from a utility AC line 22, e.g. , from a standard 60 Hz residential outlet.
• the ballast system 20 includes an EMI filter 24 which filters out high-frequency noise from the ballast circuit.
• the AC power from the utility line is rectified by a rectifier 26, which produces a pulsating DC output.
• the pulsating DC output from the rectifier 26 is smoothed out by a high-frequency power factor correction (PFC) boost converter 28, which produces a smooth DC output with highly attenuated (i.e., low percent) ripple.
• PFC power factor correction
• the PFC boost converter 28 functions to hold constant at near zero the phase angle between the current and voltage waveforms of the pulsating DC output from the rectifier 26, to thereby provide a near-unity power factor (pf).
• a gas discharge lamp ballast should draw power from the power line with a power factor of at least 90% and harmonic distortion of less than 20% .
• the smooth DC output from the PFC boost converter 28 is then converted by a high-frequency DC-AC inverter 30 into a high-frequency (e.g. , 25-50 kHz) AC voltage which is delivered to the lamp 32 for ignition thereof.
• a bulk capacitor C c is provided in the PFC boost converter 28 for energy storage, to thereby balance the input and output power. Isolation between the AC utility iine input and the lamp load is provided by the inverter 30.
• a control circuit A is utilized to control the switching frequency of the PFC converter, to thereby coarsely regulate the DC output from the PFC boost converter 28, and a control circuit B is utilized to control the switching frequency of the high-frequency DC-AC inverter 30, to thereby regulate the output power applied to the lamp 32. Since a fluorescent lamp acts as an antenna at high frequencies, the lamp current frequency is limited to about 100 kHz in order to prevent emission of excessive EMI radiation from the lamp. Typically, gas discharge lamps are operated at a frequency of 50 kHz.
• the conventional ballast system described above has at least one major shortcoming. Namely, the switching frequency of the DC-AC inverter are limited by the above-stated constraint on the lamp current frequency.
• This limitation on the switching frequency of the DC-AC inverter requires that magnetic components (e.g, inductors and isolation transformer), and other reactive elements (e.g. , capacitors) be designed for ⁇ 50- 100 kHz frequency, thereby imposing an unduly high lower limit on the size and weight of such components, thus unduly limiting the achievable miniaturization of the ballast system.
• the invention aims to provide an electronic ballast system for a lamp comp ⁇ sing a DC-AC converter which overcomes the above-described major shortcoming of conventional ballast systems.
• a ballast system as described in the opening paragraph is therefore according to the invention characterized in that the DC-AC converter comprises means I for generating a first high frequency voltage with frequency f out of said substantially constant DC voltage, comp ⁇ sing first switching means and control circuitry for generating a first control signal for switching said first switching means at frequency f; a high frequency transformer comp ⁇ sing a primary winding, coupled to the means I, and a secondary winding; means II for generating a second high frequency voltage comprising second switching means coupled to said secondary winding, control circuitry for generating a second control signal for switching said second switching means at frequency f and for controlling and modulating a phase shift between said first control signal and said second control signal at the frequency of said AC output current, and a junction node at which said second high frequency voltage is present du ⁇ ng operation, and demodulating means coupled to said junction node for generating said AC output current out of said second high frequency voltage.
• this frequency f can have a relatively high value. Because of this relatively high switching frequency f, a significant reduction in size and weight of reactive components comprised in the DC- AC converter can be realized.
• the frequency of said AC output current is preferably in the order of 10 kHz.
• said first switching means comprises a first pair of switches and said second switching means comprises a second pair of switches
• the DC- AC converter comprises first and second rails between which said substantially constant DC voltage is present during operation, a first switch and a second switch connected in series between said first and second rails forming the first switching means, a first capacitor and a second capacitor connected in series between said first and second rails, in parallel with said first pair of switches, and wherein said p ⁇ mary winding has first and second terminals, and said secondary winding has first and second terminals, and a center tap, a third switch connected between said first terminal of said secondary winding and a junction node, and a fourth switch connected between said second terminal of said secondary winding and said junction node, said third and fourth switches forming the second pair of switches, and wherein said demodulating means comprises an inductor connected between said junction node and a first output node, and a capacitor connected between said first output node and a second output no
• the demodulating means preferably comprise inductive means and capacitive means. Good results have been obtained in case the frequency f was chosen at least ten times the frequency of said AC output current and preferably higher than 1 MHz.
• the lamp current can be controlled during operation in case the ballast system comprises means for generating a lamp current feedback signal, means for adjusting the phase shift between the first and second control signal in dependency of the current feedback signal in a manner to regulate the lamp current
• control circuitry modulates the phase shift between the first control signal and the second control signal at a lamp ignition frequency for ignition of the lamp, and after ignition modulates the phase shift between the first control signal and the second control signal at the frequency of said AC output current
• the rectifier can be dispensed with Depending on the amplitude of the DC voltage supplied by the DC-supply, the DC-DC converter can also be dispensed with
• Fig 1 is a block diagram ot a conv entional electronic ballast system
• Fig 2 is a block diagram of an electronic ballast system which constitutes a preferred embodiment of the present invention
• Fig 3 is a schematic diagram of the high-frequency transformer linked DC-AC converter of the ballast sy stem of the preferred embodiment of the present invention depicted in Fig. 2;
• Fig. 4 is a diagram of the operating waveforms of the high-frequency transformer linked DC-AC converter depicted in Fig 3,
• Fig. 5 is a diagram depicting the relationship between the duty ratio and output voltage of the high-frequency transformer linked DC-AC converter depicted in Fig. 3;
• Fig. 6 is a diagram depicting the gain of the L-C filter of the ballast system of the present invention depicted in Fig 3 as a function of frequency.
• the ballast system 40 of the present invention includes the same elements as the conventional ballast system 20 depicted in Fig 1 , except that the standard DC-AC inverter 30 used in the conventional ballast is replaced with a high- frequency (HF) transformer-linked DC-AC converter 42
• the operation of the HF transformer-linked DC-AC converter 42 is controlled by the control circuit B to generate a well-regulated, high-frequency (e g , 25-50 kHz) sinusoidal (AC) current for driving the lamp 32.
• the HF transformer-linked DC-AC converter 42 is similar to the converter disclosed in U.S.
• Patent Number 3,517,300 issued to W McMurray, and the converter disclosed in an article by K Harada, H Sakamoto, and M Shoyama, entitled “Phase- Controlled DC-AC Converter with High-Frequency Switching” , IEEE Transacnons on Power Electronics, Vol. 3, No 4, October 1988
• the HF transformer-linked DC-AC converter 42 disclosed in both of these references is configured and controlled in such a manner as to generate low-frequency (50-60 Hz) output power, and to regulate the output voltage, as opposed to the output current
• the McMurray converter is just used as an inverter, and the Harada et al converter is used in an uninterrupted power supply (UPS) system.
• UPS uninterrupted power supply
• the HF transformer-linked DC-AC converter 42 of the preferred embodiment of the present invention is configured as follows A first pair of switches Ql and Q2 are connected in series between terminals 44 and 46. A pair of capacitors 48, 50 are connected in series between the terminals 44 and 46, in parallel with the first pair of switches Q l , Q2 A first terminal 52 of the primary winding 54 of a transformer T is connected to a node A intermediate the first and second switches Ql and Q2, and a second terminal 56 of the primary winding 54 of the transformer T is connected to a node B intermediate the capacitors 48 and 50.
• a switch Q3 is connected between a first terminal 58 of the secondary winding 62 of the transformer T and a node F, and a switch Q4 is connected between a second terminal 60 of the secondary winding 62 and the node F.
• the secondary winding 62 of the transformer T is center-tapped, with the center tap 64 being connected to a first output terminal 76.
• the switches Q3 and Q4 constitute a second pair of switches.
• the transformer T preferably has a turns ratio of N1 :N2:N3 — N: l : l . Of course, the particular value for N which is selected will depend upon the desired voltage transformation.
• An inductor L is connected between the node F and a second output terminal 74, and a capacitor C is connected between the first output terminal 74 and the second output terminal 76.
• the inductor L and capacitor C constitute a low-pass filter.
• the AC power (e.g. , 50 or 60 Hz) from the utility line is rectified by the rectifier 26, e.g. , a half-bridge or full- bridge rectifier, which produces a pulsating DC output.
• the pulsating DC output from the rectifier 26 is smoothed out by the PFC converter 28, which, under the control of the control circuit A, produces a smooth (constant) DC output with highly attenuated (i.e. , low percent) ripple.
• AC converter 42 under the control of the control circuit B, generates a well-regulated, high- frequency (e.g. , 25-50 kHz) sinusoidal (AC) current for driving the lamp 32.
• a well-regulated, high- frequency (e.g. , 25-50 kHz) sinusoidal (AC) current for driving the lamp 32.
• the first pair of switches Ql and Q2 are alternately turned on and off, in complementary fashion, i.e. , in opposite phase, and the second pair of switches Q3 and Q4 are also turned on and off in opposite phase.
• Each of the switches Q1-Q4 is operated at a duty ratio of 50% (0.5).
• the first pair of switches Ql and Q2 and the second pair of switches Q3 and Q4 are switched at a selected phase shift with respect to each other, i.e. , the switching phase between the two pairs of switches is shifted.
• the control circuit B functions in a manner explained hereinafter to sinusoidally modulate the phase-shift duty ratio D at a frequency which is lower than the cut ⁇ off frequency (fco) of the L-C filter.
• the value of the modulated phase-shift duty ratio D is determined by the following equation (2):
• D Dm sin (wt) + 0.5, where Dm is the maximum duty ratio.
• Fig. 5 illustrates the relationship between the phase-shift duty ratio D and the output voltage Vo.
• the control circuit B operates in the following manner. More particularly, the control circuit B produces control signals for switching the switches Q1-Q4 on and off in the above-described manner at a very high switching frequency, e.g. , > 1 MHz.
• a partial waveform of one of these control signals, labelled "B" is shown in Fig. 2.
• the phase-shift duty ratio D of these control signals is modulated by the control circuit B in accordance with a reference signal Fref generated by a reference signal generating circuit 68.
• the frequency of the reference signal Fref is the same as the desired lamp current frequency, e.g., 25 kHz.
• the control circuit B compares a lamp current feedback signal i with the reference signal Fref and, in response to any detected frequency/phase/amplitude error therebetween, adjusts the phase shift duty ratio D of the control signals in a manner to correct the error.
• a well-regulated output current is produced at the desired operating frequency for the lamp 32.
• the frequency of the reference signal Fref is preferably set at a frequency close to the cut-off frequency fco (e.g. , 250 kHz) of the L-C filter, and the phase-shift duty ratio D of the switches Q1-Q4 is preferably modulated at a frequency close to the cut-off frequency fco of the L-C filter.
• the control circuit B can detect when lamp ignition occurs.
• the frequency of the reference signal Fref is set to a lower frequency to modulate the phase-shift duty ratio D of the switches Q1-Q4, and thereby produce a sinusoidal lamp current having a lower frequency (e.g. , 25 kHz), for steady-state operation of the lamp 32.
• Fig. 6 illustrates the gain of the L-C circuit as a function of frequency, and particularly, at an exemplary lamp ignition modulation frequency (250 kHz), at an exemplary steady-state modulation frequency (25 kHz), and at an exemplary switching frequency (2.5 MHz) of the DC-AC converter 42.
• the switches Q1-Q4 can be switched on and off at a lamp ignition switching frequency (e.g. , 125 kHz) equal to one-half the cut-off frequency fco (e.g. , 250 kHz) of the L-C filter, while maintaining the phase-shift duty ratio D fixed at 50% .
• a lamp ignition switching frequency e.g. , 125 kHz
• fco cut-off frequency
• D phase-shift duty ratio
• the switching frequency of the DC-AC converter 42 is set to a higher frequency (e.g. , 2.5 MHz) well above the cut-off frequency fco of the L-C filter, and the phase-shift duty ratio D of the switches Q 1 -Q4 is modulated at a lower frequency (e.g. , 25 kHz), to thereby produce a sinusoidal lamp current having a lower frequency (e.g. , 25 kHz), for steady-state operation of the lamp 32.
• the switching frequency of the DC-AC converter 42 can be much higher than the lamp current frequency without increasing the EMI radiation from the lamp 32, the size of the transformer T and L-C low pass filter can be designed for very high frequency operation, thus greatly reducing the size and weight of these components.
• the switching frequency of the DC-AC converter 42 should be significantly higher than both the cut-off frequency fco of the L-C filter and the duty ratio modulation frequency, whereby the output voltage Vo can be derived like a buck-type converter.
• the switches Q3 and Q4 are preferably solid state switches, e.g. , metal-oxide-semiconductor field effect transistor (MOSFET) switches, and the control circuit B is preferably a solid state electronic control circuit.
• MOSFET metal-oxide-semiconductor field effect transistor
• the switches Q3 and Q4 should also be directional, e.g. , each of the switches Q3 and Q4 can be implemented with a pair of diode quads in combination with a MOSFET.
• the DC- AC converter 42 can be designed to be operated with zero-voltage- switching in order to reduce switching losses and switching noise.
• zero- voltage-switching can be realized by providing a short time interval ("dead time") during which both the switches Ql and Q2 are turned off.
• Other techniques, including soft ⁇ switching, can also be employed in order to improve the efficacy of the ballast system.

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• 1996
  • 1996-11-18 CN CN 96191841 patent/CN1173963A/zh active Pending
  • 1996-11-18 WO PCT/IB1996/001244 patent/WO1997022231A1/en not_active Application Discontinuation
  • 1996-11-18 JP JP9521872A patent/JPH11500860A/ja active Pending
  • 1996-11-18 EP EP96935277A patent/EP0808551A1/en not_active Withdrawn
  • 1996-12-23 TW TW85115896A patent/TW322677B/zh active

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