Master-slave half-bridge DC-to-AC switchmode power converter

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- 5 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 10 Zl 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 ]5 shown. Furthermore, only a single set of voltage divider capacitors CI and C2 are included.

FIG. ID illustrates the control connection between the topologically connected master and slave halfbridge configurations, in which a single control trans- 2rj former 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. 25

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:

to 0.7V, therefore opening the transistor T12 across resistor R13. Additionally, the transistor Til would switch OFF, thereby reversing the voltage polarities in the windings of transformer L32. A similar process will be repeated in the upper part of the circuit.

Based upon equation (1), the on time ti of transistor Til depends on the voltage of capacitor Cll because UcisrU.vn and UMi-t] is constant. Similarly, the ON time t2 of transistor T22 depends on the voltage of capacitor C21 and since N13=N23 we obtain

30

(i)

where ti and t2are the ON times of the master switches SI and S2 respectively, Ui and U2 are the voltages of the identified voltage divider capacitors and U1 + U2= input DC voltage. The phase connections in FIG. IE 35 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 40 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 selfoscillating half-bridge DC-to-AC switchmode converter including the voltage divider capacitors Cll and 45 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 T11=T21, 50 T12=T22, D11=D21, R12 = R22, R13 = R23, N11 = N21, N12=N22, N13 = N23 and C11=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 55 to the common point of transistors Til 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 60 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 trans- 65 former L32 is becoming saturated, the magnetizing current would quickly increase. Consequently, the voltage across the resistor Rll would also increase quickly

The period time t=11 +12 and Uci+Uci equals the input DC voltage.

If the voltages Uci and Uc2 are not equal, for instance if Uci>Uc2, « follows that tTj<tT2. Conversely, if Uci <Uc2 then ti>t2- This voltage dependent ON time makes the previously described self-oscillating halfbridge converter advantageous 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 halfbridge square wave oscillator.

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 halfperiod, 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 Nchannel 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, 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 1*51 =U/)52/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, Z2. 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 5 H connected in series with an inductor L7 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 10 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 15 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 20 will switch off providing a new charging period of the capacitor C71.

Thus, while preferred embodiments of the present invention have been shown and described in detail, it is to be understood that such adaptations and modifica- 25 tions 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.

What is claimed is:

1. A master-slave half-bridge DC-to-AC switchmode 30 power converter comprising:

a DC power supply;

a self-oscillating half-bridge switchmode converter acting as a low power master converter connected to said DC power supply, said master converter 35 provided with a master control transformer having at least five windings, two controlled master switches, and first and second electronic control means for controlling said master switches, each of said electronic control means connected between 40 said master control transformer and each of said master switches; and

a half bridge switchmode converter acting as a controlled slave power converter connected to said DC power supply and said low power master con- 45 verter, said slave power converter provided with two slave switches, third and fourth electronic control means for controlling said slave switches, each of said electronic control means connected between said control transformer and each of said 50 slave switches, said slave power converter further including a load impedance; and

a pair of voltage divider capacitors common to said master converter and said slave converter;

wherein the ON and OFF states of each of said mas- 55 ter and slave switches are controlled by said selfoscillating half-bridge switchmode converter.

2. The master-slave half-bridge DC-to-AC switchmode power converter in accordance with claim 1, wherein said first winding of said master control trans- 60 former is connected between the common point of said master switches and said voltage divider capacitors, said second and third windings of said master control transformer, are respectively connected to said first and second electronic control means and said fourth and 65 fifth windings of said master control transformer are respectively connected to said third and fourth electronic control means.

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3. The master-slave half-bridge DC-to-AC switchmode power converter in accordance with claim 2 further including a self-saturated transformer provided with first, second and third windings, said first winding connected to the common point of said controlled master switches and said voltage divider capacitors, said controlled master switches provided with respective first and second transistors and said first and second electronic control means provided with first, second, third and fourth resistors and third and fourth transistors, wherein said second and third windings of said self-saturated transformer are respectively connected in series with said first and second resistors and respectively across said second and third windings of said master control transformer and further wherein said third and fourth transistors are connected respectively to said transistors of said controlled master switches.

4. The master-slave half-bridge DC-to-AC switchmode power converter in accordance with claim 2 wherein each of said slave switches is a MOSFET and each of said third and fourth electronic control means is provided with an additional MOSFET connected to said slave switch MOSFET, each of said third and fourth electronic control means provided with first and second rectifiers respectively connected to common sources of said slave switch MOSFET and said additional MOSFET, and each of said third and fourth electronic control means provided with first and second resistors connected between said.first and second rectifiers and said slave switch MOSFET and said additional MOSFET, and wherein said fourth and fifth windings are respectively connected to said third and fourth electronic control means provide a square wave AC control signal providing ON or OFF states to said slave switch MOSFET dependent upon the polarity of the square wave AC control signal.

5. The master-slave half-bridge DC-to-AC switchmode power converter in accordance with claim 3 wherein each of said slave switches is a MOSFET and each of said third and fourth electronic control means is provided with an additional MOSFET connected to said slave switch MOSFET, each of said third and fourth electronic control means provided with first and second rectifiers respectively connected to common sources of said slave switch MOSFET and said additional MOSFET and each of said third and fourth electronic control means provided with first and second resistors connected between said first and second rectifiers and said slave switch MOSFET and said additional MOSFET, and wherein said fourth and fifth windings are respectively connected to said third and fourth electronic control means provide a square wave AC control signal providing ON or OFF states to said slave switch MOSFET dependent upon the polarity of the square wave AC control signal.

6. The master-slave half-bridge DC-to-AC switchmode power converter in accordance with claim 1 further including a gas discharge device connected in series with an inductor acting as said impedance.

7. The master-slave half-bridge DC-to-AC switchmode power converter in accordance with claim 2 further including a gas discharge device connected in series with an inductor acting as said impedance.

8. The master-slave half-bridge DC-to-AC switchmode power converter in accordance with claim 1 wherein said load impedance is a high pressure sodium lamp and an inductor, and further including an ignition device connected to said high pressure sodium lamp,