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 Vin across a pair of series connected electronic switches Ql, Q2 which may be power MOSFET's, the gates of which are actuated by substantially square wave gating signals VGS1 and VGS2 which are complementary; i. e., VGS2 has a duty ratio D and VGS1 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 Q2, in series, are a blocking capacitor Cb, inductor Lr and the primary winding of a step-up isolation transformer T having a primary magnetizing inductance Lg, which is shunted by a capacitor Cr. The inductor Lr and capacitor Cr 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 Rl in series with a current limiting capacitance Cl.

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 VGSi it is in the on or"1" state it turns switch Q1 ON. The input voltage Vin, typically about 250 volts DC, then produces current in the path including Cb, Lr and Lm and also charges capacitance Cr.

Capacitance Cb is much larger than Cr and serves only a blocking function to prevent DC from reaching transformer T. When gating signal VGS1 returns to the"0"state it turns switch Q1 OFF, and simultaneously gating signal VGS2 turns switch Q2 ON. The energy stored in inductance Lr and capacitance Cr then results in a very large reactive voltage across the

transformer primary winding inductance Lm, since the voltage gain in the vicinity of the resonant frequency of Lr and Cr 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 Rl 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 VGS1 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 VGS2 to the ON state (duty cycle = 1), so that switch Q2 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 VGS1 and VGS2 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 VGS1 is swept over several cycles from 0% (constant OFF) to 50% (ON time = OFF time), while simultaneously the duty ratio of gating signal VGS2 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 Lr and the blocking capacitor voltage Vcb, 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 Ql, Q2 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 Lr and the blocking capacitor voltage Vcb.

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 Q1 and Q2; that is, each switch operating at a duty ratio of 50%. If asymmetrical but complementary gating signals are employed, e. g. VGS2 has a duty ratio D and VGS1 has a duty ratio (1-D), the converter voltage gain then becomes ,, 7-cos (27r (7-D,,, 2 Correspondingly, the voltage across capacitor Cb becomes

Vcb = (l-D) Vin 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 Ql, Q2 begin to be swept asymmetrically from 0% to 50% for Q1 and from 100% to 50% for Q2. 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 Lr, and the blocking capacitor voltage Vcb, respectively.

The mechanism of this improvement is based on the following. At time 0, the voltage across capacitor Cb is 0. If the switches Q1 and Q2 are then at once driven by gating signals having a duty ratio D = 50%, a low frequency resonant mode of Cb, Lr and Lm is excited by a step voltage input of magnitude 0.5 Vjn. 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 Q1 off and turn switch Q2 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 Cb. It should be noted that the amount of energy stored in capacitor Cb is much greater than that stored in the resonant circuit. Consequently, the charging and discharging of Cb 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 Ql, achieves fast shut-down without low frequency oscillation.

Assume that the converter is running in steady state with

Veb = 0.5 Vin and D = 50%, and that at time 0 turn-off is commenced by starting to sweep the duty ratio D of switch Q2 from 50% to 100% and of switch Q1 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,, 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 Cb. 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.