BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high frequency DC to

AC switchmode power converters and specifically to high frequency ballasts for gas discharge devices. More specifically, the present invention relates to a high frequency ballast for high pressure sodium lamps.

2. Prior Art

Self-oscillating DC-to-AC converters have a significant position in the field of switchmode power converters, due to their simplicity and usefulness. Generally, DC-to-AC converters are configured as push-pull, half-bridge or full-bridge. One of the simplest, and oldest, DC-to-AC self-oscillating push-pull converters is the Royer circuit. Another topology similar to the Royer circuit, which removes the switch drive function from the main power transformer, is the self-oscillating voltage or current driven Jensen circuit. The common disadvantage of the push-pull configurations is the imbalance problem of the push- pull transformer, especially when applied to asymmetrical loads. An important application of the simple self-oscillating DC-to-AC switchmode power converters is supplying gas discharge devices, especially high pressure sodium (HPS) lamps in the range of 35 to 400 watts. In this case, the load impedance of the DC- to-AC converter is a HPS lamp connected in series with an inductor. In the case of a high frequency powering of the HPS lamp, the interaction between the high frequency ballast and the lamp is stronger than that of a conventional ballast. This high frequency ballast is significantly better than a conventional ballast due to its lessened weight and higher efficiency. Additionally, the high frequency ballast, utilized with an HPS lamp would have a longer life time, exhibit better light efficiency (lumen per watt) and display a better color tempera- ture.

Therefore, the critical design targets for high frequency ballasts supplying HPS lamps would be the following:

(a) very high efficiency (energy saving) ;

(b) ensuring that the lamp power is maintained between an allowed maximum and minimum power during the lifetime of the lamp at ±10% input voltage fluctuation;

(c) protection against the imbalance effect caused by the asymmetrical loading feature of the ignited HPS lamp;

(d) providing high voltage (3000V-4000V) ignition pulses;

(e) the relative simplicity of the ballast which would result in a lower cost; and

(f) reliability and longer life time.

The prior art is replete with many known push-pull configurations providing high frequency ballast for gas discharge lamps. A typical Jensen push-pull which can be used with HPS lamps is U.S. Patent No. 4,935,673 entitled "Variable impedance electronic ballast for gas discharge device", assigned to the assignee of the present invention, including an improved current driven Jensen push-pull converter.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a master-slave half-bridge DC-to-AC switchmode power converter which has a substantially improved efficiency and it is protected against the effect of an asymmetrical load.

A second object of the present invention is to provide a self-oscillating half-bridge switchmode converter which has an improved efficiency and in which the frequency depends linearly on the DC input voltage. A further object of the present invention is to provide a magnetically coupled MOSFET driver which has a substantially improved current sink capability, and therefore very short switching which is especially significant when the load is inductive.

A further object of the present invention is to provide a high frequency ballast for gas discharge devices having substantially improved efficiency, stability and reliability.

Another object of the present invention is to provide a high frequency ballast for HPS lamps which has a high voltage ignition circuit, providing imbalance protection against the effect of the asymmetrical feature of the ignited HPS lamp.

These and other objects, features and advantages of the present invention will be more readily apparent from the following detailed description, wherein reference is made to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A, IB, 1C and ID illustrate the evolution of the preferred master-slave half-bridge DC-to-AC switchmode power converter;

FIG. IE illustrates the two possible phase connections between the master and slave converters;

FIG. 2 shows a preferred embodiment of an improved self-oscillati:ιg half-bridge DC-to-AC switchmode converter as the master controller;

FIG. 3 shows a preferred embodiment of an improved magnetically coupled MOSFET-driver according to the present invention;

FIG. 4 shows a preferred embodiment of an improved half-bridge DC-to-AC switchmode power converter as a controlled slave;

FIG. 5 illustrates a schematic diagram of the preferred high frequency ballast gas discharge device; and

FIG. 6 shows a preferred embodiment of the high frequency ballast for HPS lamps combined with a high voltage ignition apparatus.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A shows a simplified diagram of a self-oscillat¬ ing half-bridge DC-to-AC switchmode converter used as a low power master controller connected to a DC power supply. The master

controller half-bridge configuration includes two electronicall controlled switches SI and S2 noted as master switches, a non saturated control transformer Tl provided with four secondar windings used as a master control transformer, and voltag divider capacitors Cl and C2. Two secondary feedback winding N _sl and N _s2 of the transformer Tl, provide control signals to two driver apparatuses Al and A2 controlling the master switches SI and S2, respectively. The remaining two secondary windings N _s3 and N _s4 of the transformer Tl, provide square wave AC signals for any other control purposes. The primary winding of the trans¬ former Tl is connected between the two switches SI, S2 and the two capacitors Cl, C2.

FIG. IB illustrates the half-bridge DC-to-AC switchmode converter as a controlled slave power converter connected to a DC power supply. The controlled slave power converter includes two electronically controlled switches S3 and S4 acting as slave switches, a non-saturated control transformer T2 having a primary winding and two secondary windings providing control signals to the driver apparatuses A3 and A4 of the slave switches S3 and S4 respectively. Furthermore, two voltage divider capacitors C3, C4 and a load impedance Z _L connected between the two capacitors C3, C4 and the slave switches S3, S4 is also included.

FIG. 1C shows a topological connection between the previously described master and slave half-bridge configurations in which a single DC power supply is shown. Furthermore, only a single set of voltage divider capacitors Cl and C2 are included.

FIG. ID illustrates the control connection between the topologically connected master and slave half-bridge configura- tions, in which a single control transformer Tl, having a single primary winding and four secondary windings, is included. Two of the secondary windings are connected to driver apparatuses Al and A2 and the remaining two secondary windings are connected to driver apparatuses A3 and A4. FIG. IE shows the two possible phase connections between the master and slave half-bridge configuration as a first phase connection (1) and a second phase connection (2) .

Utilizing the following equation:

U- * t _λ = U ₂ ^• t ₂ (1)

where t _λ and t ₂ are the ON times of the master switches SI and S2 respectively, U-,_ and U ₂ are the voltages of the identified voltage divider capacitors and u^ + U = input DC voltage. The phase connections in FIG. IE can be analyzed.

Assuming the first phase connection in which switches

SI and S4 are ON and switches S2 and S3 are OFF, the result is a negative feedback decreasing the effect of all asymmetry which can appear in the slave power converter, such as the effect of the polarity dependent load as in the case of an HPS lamp.

FIG. 2 shows the preferred embodiment of a self- oscillating half-bridge DC-to-AC switchmode converter including the voltage divider capacitors Cll and C21, a control transformer L31 provided with a main winding and four secondary windings Nil, N12, N21 and N22. Main switching transistors Til and T21 with two clamping rectifiers Dll and D21 respectively are also provided. We can assume that Til = T21, T12 = T22, Dll = D21, R12 = R22, R13 = R23, Nil = N21, N12 = N22, N13 = N23 and Cll = C21.

An important part of the circuit is a saturated transformer L32 having two parallel windings N13 and N23. The primary windings of transformer L32 is connected to the common point of transistors Tl and T21 and capacitors Cll and C21. Assuming that the voltage of the winding Nil, connected in series with resistor R12, is positive with respect to the point sign, transistor Til must be ON. Although the magnetizing current of transformer L32 flowing in the winding N13 and series resistor

Rll increases, if the voltage across the resistor Rll remains smaller than approximately 0.4V until the saturation of the transformer L32, the transistor T12 remains switched OFF. When the core of the transformer L32 is becoming saturated, the magnetizing current would quickly increase. Consequently, the voltage across the resistor Rll would also increase quickly to 0.7V, therefore opening the transistor T12 across resistor R13.

Additionally, the transistor Til would switch OFF, thereb reversing the voltage polarities in the windings of transformer 32. A similar process will be repeated in the upper part of the circuit. Based upon equation (1) , the on time t ₁ of transistor

Til depends on the voltage of capacitor Cll because U _C1 «U _N11 and ^U NII " -*i -- ^s constant. Similarly, the ON time t ₂ of transistor T22 depends on the voltage of capacitor C21 and since N13 = N23 we obtain

U _cl ^• t- _. = U _c2 ^• t ₂ (2)

The period time t = t _x + t ₂ and U _cl + U _c2 equals the input DC voltage.

If the voltages U _cl and U _c2 are not equal, for instance if U _cl > U _c2 , it follows that tl^ < tT ₂ . Conversely, if U _cl < U _c2 then t± > t ₂ . This voltage dependent ON time makes the previ¬ ously described self-oscillating half-bridge converter advanta¬ geous as the master controller in the master-slave half-bridge configuration.

FIG. 2 also shows a simple starter circuit including a resistor R32, a capacitor C31 and a DIAC S31. The windings N22 and N12 provide square wave AC signals if the circuit is designated as a master control half-bridge square wave oscilla¬ tor.

FIG. 3 shows a preferred embodiment of an improved MOSFET driver used with the present invention. The control transformer L31 provides a square wave AC control signal. During the positive half-period, with respect to the point sign of the secondary winding N12, a positive voltage is connected across the resistor R51 and rectifier D51 to the gate of an N-channel MOSFET T51 providing the ON state, while N-channel MOSFET T52 is in the OFF state. During the negative half-period, a positive voltage is connected across the resistor R52 and rectifier D52 to the gate of MOSFET T52 providing the ON state. Therefore, the gate of MOSFET T51 is short circuited to its source by MOSFET T52 ₇ providing an excellent current sink capability and a very short

switching time for MOSFET T51. The DC power loss of the described MOSFET driver is low because only a lower current I _R51 «U _D52 / R51 flows in the resistor R51 when the MOSFET T52 is ON. Comparing the described MOSFET driver to the conventional driver consisting of the control transformer L31, and a resistor R51 (D51 is short circuited) , a significant advantage is provided, particularly when the load current is inductive.

FIG. 4 shows a preferred embodiment of an improved half-bridge DC to AC switchmode power converter as the controlled slave using two equivalent MOSFET drivers as previously described as well as the electronically controlled MOSFET switches. Capacitors C51 and C61 are the voltage divider capacitors, Z _L is the load impedance and T51 = T61, T52 = T62, D51 = D61, D52 = D62, R51 = R61 R52 = R62, R53 = R63 and C51 = C61. FIG. 5 illustrates a schematic diagram of the preferred high frequency ballast for gas discharge devices. The high frequency ballast includes a previously described master-slave half-bridge configuration in which the load impedance is a gas discharge device G connected in series with an inductor L. It also includes a full-wave bridge rectifier D coupled to an AC source, shunted by a charge storage capacitor C and a filter apparatus F.

FIG. 6 shows a preferred embodiment of a high frequency ballast for an HPS lamp H. The high frequency ballast for the lamp H includes the previously described master-slave half-bridge DC to AC switchmode power converter in which the load impedance is the HPS lamp H connected in series with an inductor L71 including windings N71 and N72. The circuit is also provided with a high voltage ignition apparatus, in which winding N71 is connected in series with the HPS lamp H and the winding N72 is connected across a SIDAC S71 to a capacitor C71. The master control transformer L31 has a sixth winding N32 connected across a resistor R71 and a rectifier D71 to the capacitor C71, providing a charging current of capacitor C71. When the voltage of capacitor C71 reaches the switching voltage of SIDAC S71, the voltage of the capacitor C71 will reach the winding N72 and a high voltage impulse of between 3000V and 4000V will be induced

in the winding N71 which is required to initiate an arc. The capacitor C71 will be discharged very quickly and the SIDAC S71 will switch off providing a new charging period of the capacitor C7l. Thus, while preferred embodiments of the present invention have been shown and described in detail, it is to be understood that such adaptations and modifications as may occur to those skilled in the art may be employed without departing from the spirit and scope of the invention, as set forth in the claims.