RESONANT CONVERTER CIRCUIT

Abstract: A resonant converter which may be employed as an electronic ballast for a gas discharge lamp, the converter being in the form of a full or half bridge circuit which includes at least a pair of power switches such as MOSFETs. The gates of the switches are actuated by substantially square wave cyclic gating signals having complementary duty cycle ratios and a frequency somewhat above a resonant frequency of the converter circuit. The energy supply to the lamp is variable in accordance with the duty cycle ratios of the gating signals. In order to avoid generation of high voltage and current transients in the converter when a change in operating state requiring a substantial change in duty cycle ratios of the gating signals is to be performed, as when effecting turn-on or turn-off of the lamp, the changes in the duty cycle ratio are ramped to occur incrementally over several cycles of the gating signals rather than directly to required final values of the duty cycle ratios.

Description: Resonant converter circuit. This invention relates to resonant converters such as commonly used as an electronic ballast for fluorescent and other gas discharge lamps, and more particularly to a resonant converter which can provide a change in operating condition, such as lamp turn-on or turn-off, while suppressing generation of high voltage and current transients. A well-known half-bridge resonant converter circuit which is commonly used as an electronic ballast for fluorescent and other gas discharge lamps is shown in Fig. 1. Therein a power source 1 supplies a DC input voltage V m across a pair of series connected electronic switches Qi, Q 2 which may be power MOSFET's, the gates of which are actuated by substantially square wave gating signals V GSI and VQ S2 which are complementary ; i.e., V GS2 has a duty ratio D and V GSI has a duty ratio (1-D), where D is the ratio of ON time to period of the signal. The power source 1 may comprise a full wave rectifier followed by a preconditioner circuit, as shown in US Patent 5,742,134, issued April 21, 1998, assigned to Philips Electronics N. A. The frequency of the gating signals may be of the order of 45 kHz. Connected across switch Q 2 , in series, are a blocking capacitor C b , inductor r and the primary winding of a step-up isolation transformer T having a primary magnetizing inductance _ which is shunted by a capacitor C r . The inductor L r and capacitor C r have a resonant frequency which is somewhat below the gating signal frequency. An output voltage Vo is produced across the secondary winding of the transformer T, and is applied to a fluorescent lamp connected thereto represented by a resistance Ri in series with a current limiting capacitance Q. The circuit will be described starting from an initial state in which the lamp is off. It is assumed that the lamps is of the instant-start type which requires a starting voltage in the vicinity of 550 volts rms. When during a cycle of gating signal V GSI it is in the on or "1 " state it turns switch Qi ON. The input voltage Vj n , typically about 250 volts DC, then produces current in the path including C b , Lr and L π , and also charges capacitance C r . Capacitance C b is much larger than C r and serves only a blocking function to prevent DC from reaching transformer T. When gating signal V GSI returns to the "0" state it turns switch Qi OFF, and simultaneously gating signal V GS2 turns switch Q 2 ON. The energy stored in inductance Lr and capacitance C r then results in a very large reactive voltage across the transformer primary winding inductance L m , since the voltage gain in the vicinity of the resonant frequency of L r and C r can be 10 to 15 or even greater. That is still further amplified by the step-up turns ratio of transformer T, and so it is readily achieved that adequate starting voltage is applied to the lamp to cause it to start-up. After the lamp has ignited the lamp resistance Ri loads the resonant circuit, reducing the effective gain. Cyclic operation then continues at a frequency somewhat above the resonant frequency, producing a sufficiently high voltage to maintain the lamp in the ON condition. When it is desired to turn the lamp off the gating signal V GSI applied to switch Qi is turned off. It thereby switches to the "0" state (duty ratio = 0), remaining open, so that voltage is no longer supplied to the resonant circuit. At the same time, the common practice is to also turn gating signal V GS2 to the ON state (duty cycle = 1), so that switch Q 2 remains closed. The reactive energy existing in the resonant circuit at the instant turn-off is commenced will again result in a very large transient voltage across the transformer winding and also across inductance Lr, which will then decay over an interval determined by the time constant of the reactive circuit loop. The large reactive voltages and current produced in the converter circuit during lamp turn-on and turn-off requires the circuit designer to use circuit elements rated for voltages and currents many times the levels encountered during steady state circuit operation after the lamp has been turned on. That significantly increases the cost of the converter circuit. In addition, during turn-off the cyclic reactive voltage produced across the transformer winding may be sufficient to cause low frequency re-ignition of the lamp during several of the reactive cycles, causing repeated flickering which can be quite disturbing to an observer. Accordingly, an object of the present invention is to provide a resonant converter which, while still providing an adequate instant-start voltage for a fluorescent or other gas discharge lamp, suppresses generation of the excessive transient reactive voltages and currents conventionally produced during lamp turn-on and turn-off. A further object is to provide definite turn-off of the lamp, without cyclic production of reactive voltages sufficient to cause low frequency repetitive re-ignition of the lamp. Applicants have found that these objectives can be achieved by appropriate control of the duty ratios of the cyclic gating signals V GSI and V GS2 which are supplied to the switches in the converter circuit. In particular, instead of immediate transition of each gating signal to or from the completely on or completely off state, the duty ratios of the gating signals are swept so as to incrementally reach the required altered duty ratio over several cycles of the gating signals. For lamp turn-on, the duty cycle of gating signal V GSI is swept over several cycles from 0% (constant OFF) to 50% (ON time = OFF time), while simultaneously the duty ratio of gating signal V GS2 is swept over several cycles from 100% (constant ON) to 50% (ON time = OFF time). For lamp turn-off, the aforesaid sweeps of the duty cycles of the gating signals are reversed. Applicants have also found that control of the duty ratios of the gating signals may be employed to efficiently control the intensity of the light produced by the lamp, as contrasted with conventional control of light intensity by alteration of the cyclic frequency of the gating signals. Such gradual alteration of the duty ratios of the cyclic gating signals has been found to substantially suppress generation of high voltage and current transients during lamp turn-on or turn-off, as well as low frequency repeated re-ignition of the lamp during turn-off. The invention will now be described in more detail with reference to the accompanying drawings, wherein: Fig. 1 is a circuit drawing of a typical half-bridge resonant converter used as an electronic ballast for a gas discharge lamp; Figs. 2, 3 and 4 show, during start-up in accordance with the invention, the smooth transitions which are obtained in the output voltage Vo, the resonant current in inductor L r and the blocking capacitor voltage Vc b , respectively; Figs. 5 and 6 show the excessive low frequency oscillation which occurs in the inductor current and the blocking capacitor voltage during conventional start-up in which the switches Qi, Q 2 are directly driven from their initial states (0% and 100%) to a final state of D = 50%; and Figs. 7, 8 and 9 respectively show, during lamp turn-off in accordance with the invention, the smooth transitions which are obtained in the output voltage Vo, the resonant current in inductor L r and the blocking capacitor voltage Vc b - The converter circuit in Fig. 1 has a voltage gain A (• ) which can be readily evaluated by known approximation for the case in which there is symmetrical drive of switches Qi and Q 2 ; that is, each switch operating at a duty ratio of 50%. If asymmetrical but complementary gating signals are employed, e.g. V GS2 has a duty ratio D and V GSI has a duty ratio (1 - D), the converter voltage gain then becomes . . 1 - cos (2π ( 1 - D)) . . . A d (ω )= S— -ϊ ^ A(ω ). Correspondingly, the voltage across capacitor C b becomes V c = (l - D)V in during steady state operation of the converter. These equations are the foundation based on which control of the duty ratios can be used to achieve smooth start-up and shut-down of converter operation while suppressing generation of high voltage and current transients. From equation (1) it is seen that as D approaches either zero or 100% the gain approaches zero, whereas for D = 50% the gain is maximized. Start-up will be described from a starting condition in which all the energy storing components are fully discharged (except possibly an electrolytic capacitor included in the output of the power source 1). At time 0 the duty ratios of the power switches Qi, Q 2 begin to be swept asymmetrically from 0% to 50% for Qi and from 100% to 50% for Q 2 . The switching frequency • is constant at a value somewhat above the resonant frequency of the converter circuit, while the sweep frequency at which the duty ratio is changed is much lower and provides an approximately 5% step difference in D per cycle of the switching frequency. Figs. 2, 3 and 4 show the results thereby obtained, providing smooth start-up of the output voltage Vo, the resonant current in the inductor L r , and the blocking capacitor voltage V Cb , respectively. The mechanism of this improvement is based on the following. At time 0, the voltage across capacitor C b is 0. If the switches Qi and Q 2 are then at once driven by gating signals having a duty ratio D = 50%, a low frequency resonant mode of C b , L r and L m is excited by a step voltage input of magnitude 0.5 N u ,. This generates exaggerated low frequency oscillation of inductor current and capacitive voltage as shown in Figs. 5 and 6. In contrast thereto, in accordance with the invention , an equivalent ramp voltage is supplied to the oscillatory circuit to produce start-up. Appropriate choice of the slope of the ramp leads to greatly reduced low frequency oscillation. Consider now the operating conditions during turn-off of the lamp. The conventional procedure is to turn switch Qi off and turn switch Q 2 on at the same time. In that way, the energy stored in the high-frequency resonant circuit is released in part through the load and in part transferred to capacitor C b . It should be noted that the amount of energy stored in capacitor C b is much greater than that stored in the resonant circuit. Consequently, the charging and discharging of C b during start-up and shut-down are the main causes of the low frequency oscillation. This explains why ramping of the duty cycles of the switches Qi, achieves fast shut-down without low frequency oscillation. Assume that the converter is running in steady state with V Cb = 0.5 Vi n and D = 50%, and that at time 0 turn-off is commenced by starting to sweep the duty ratio D of switch Q 2 from 50% to 100% and of switch Qi from 50% to 0%. The sweep frequency is much below the switching frequency, with a 5% step difference in duty ratio per cycle. The results obtained are shown in Figs. 7, 8 and 9, providing smooth shut-down of the output voltage Vo, the current in inductor L r , and the voltage of blocking capacitor Cb, respectively. That is obtained because the power input to the oscillatory circuit becomes smaller as D increases, and so the output power is increasingly provided by capacitor C b . That gradually releases the energy stored therein. It should be noted that the described ramping of the duty cycle to provide start- up and shut-down applies equally to full-bridge as well as half-bridge converters. Also, the step difference of the duty ratio sweep per cycle can range anywhere between 1% and 10%. In the interest of clarity and completeness the invention has been described with reference to certain preferred features thereof. However, it will be apparent to those skilled in the art that various adaptations and modifications thereof may be made without departing from the essential teachings and scope of the invention as set forth in the ensuing claims.

Claims: CLAIMS: 1. A resonant converter which may be employed as a ballast circuit for controlling operation of a gas discharge lamp (RI) connected thereto, the converter being in a form of a full or half-bridge circuit having at least first and second power switches (Q1,Q2) which are respectively actuated by first and second cyclic gating signals (VGS1,VGS2) generated by a control circuit comprised in the resonant converter and of substantially the same frequency, the gating signal frequency being somewhat above a resonant frequency of said converter, the first and second gating signals having substantially complimentary duty ratios characterized in that the control circuit further comprises a circuit part for effecting a change in the operating state of said converter by increasing the duty ratio of the first gating signal by a selected incremental amount and decreasing the duty ratio of the second gating signal by said selected incremental amount; and the change in the duty ratio of each of said gating signals being ramped to occur incrementally over a plurality of cycles thereof, the incremental change per cycle being the same for both gating signals. 2. A resonant converter as claimed in claim 1, wherein the converter is employed as a ballast circuit for said lamp (RI) and the change in operating state of said converter corresponds to a change of said lamp to an ON state from an OFF state, in said OFF state the first gating signal (VGSI) having a duty ratio of zero % and the second gating signal (VGS2) having a duty ratio of 100%; lamps turn-on being effected by ramping the duty ratio of the first gating signal from 0% to 50% over a selected number of cycles thereof and concurrently ramping the duty ratio of the second gating signal from 100% to 50% over said selected number of cycles thereof. 3. A resonant converter as claimed in claim 1 , wherein the converter is employed as a ballast circuit for said lamp (RI) and the change in operating state of said converter corresponds to a change in the operating state of said lamp to the OFF state from an ON state, in said ON state the first and second gating signals (VGSI, VGS2) each having a duty cycle ratio of 50%; lamp turn-off being effected by ramping the duty cycle ratio of the first gating signal from 50% to 0% over a selected number of cycles thereof and concurrently ramping the duty cycle ratio of the second gating signal from 50% to 100% over said selected number of cycles thereof. 4. A resonant converter as claimed in claim 1, wherein the converter is employed as a ballast circuit for said lamp (RI) and the change in operating state of said converter corresponds to a change in the lighting intensity of said lamp, said change being produced from an initial condition in which the first gating signal (VGSI) has a duty ratio between 0% and 50% and the second gating signal (VGS2) has a duty ratio between 50% and 100%; said change in intensity being produced by increasing the duty ratio of one of the first and second gating signals by a selected incremental amount and concurrently reducing the duty cycle ratio of the other of the first and second gating signals by said selected incremental amount.

Inventors: BRUNING GERT, CHANG CHIN, CHANG JOSEPH E C, BRUNING, GERT, CHANG, CHIN, CHANG, JOSEPH, E., C.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: H05B37
IPC codes: H02M7, H05B41