Half-bridge inverter

1. Field of the Invention

The present invention relates to a half-bridge inverter, and more particularly to a half-bridge inverter that accomplishes an inversion by switching an external-driven switch and a self-driven switch.

2. Description of Related Art

As electronic and computer devices are increasingly complicated, a power supply device becomes more important than ever. A power supply device can be divided into two main types: a linear type and a switching type. Since the linear type is less advantageous than the switching type, therefore, most power supply devices are the switching type.

The power supply for a backlight source of a thin film transistor (TFT) panel mainly uses a DC-AC inverter circuit to achieve the power conversion and drive a cold cathode fluorescent lamp (CCFL) to illuminate. With a different circuit topology, a conventional prior type of inverter circuit is generally divided into a half-bridge inverter circuit, a full-bridge inverter circuit, and a push-pull inverter circuit, which are inverter circuits for converting direct current (DC) into alternate circuit (AC).

U.S. Pat. No. 5,615,093 discloses a half-bridge inverter circuit. Referring to FIG. 1 , a schematic circuit diagram of a half-bridge inverter circuit driving a load is illustrated. In FIG. 1 , a transformer T 1 divides a circuit into a primary circuit and a secondary circuit, and a feedback network 30 is connected between these two circuits. The primary circuit comprises a direct current power supply VDD, two electronic switches Q 1 , Q 2 , a half-bridge control chip 10 and a LCC resonant network 40 , and the secondary circuit comprises a lamp load 20 . With reference to FIG. 2 together with FIG. 1 , schematic views of the output control signal of a half-bridge control chip and the voltage waveform of an alternate current (AC) according to a prior art are illustrated. The half-bridge control chip 10 uses two output ends LFET 1 , LFET 2 to output control signals D 1 , D 2 , and the control signals D 1 , D 2 respectively control the switching of two electronic switches Q 1 , Q 2 . By switching the two electronic switches Q 1 , Q 2 , the electric power of the direct current (DC) power supply is supplied to the primary winding of the transformer T 1 through the LCC resonant network 40 to define an AC power supply ac. The AC power supply ac is provided for supplying power to the transformer T 1 , and the transformer T 1 steps up and converts the AC power to the secondary winding for driving the lamp load 20 .

In the foregoing description, the half-bridge inverter circuit disclosed in the U.S. Pat. No. 5,61.5,093 requires a half-bridge control chip 10 having at least two outputs to drive directly the electronic switches Q 1 , Q 2 and control the alternating connection of the switches Q 1 , Q 2 . Further, the half-bridge control chip 10 also needs to control the first switch Q 1 and the second switch Q 2 to alternate for a short time by turning the switches on and off alternatingly. Therefore, it is necessary to have a little dead time for the control signals D 1 , D 2 as shown in FIG. 2 to avoid a concurrent electric connection of the first switch Q 1 and the second switch Q 2 , which will destroy both the first switch Q 1 and the second switch Q 2 .

Further, the control signals D 1 , D 2 need to change the signal cycle and control the switching of the first switch Q 1 and the second switch Q 2 to supply the electric power for the load according to the power requirement of the lamp load 20 . The half-bridge control chip 10 needs to protect the circuit according to the conditions fed back from the load end, such as the condition of a too-low voltage output, a too-high voltage output, an open circuit of the lamp or overheat etc. The output of the control signal D 1 , D 2 is terminated to switch off the first switch Q 1 and the second switch Q 2 to achieve a circuit protection function.

As described above, the conventional prior half-bridge inverter circuit comes with a half-bridge control chip 10 having two drive signals for driving the half-bridge circuit. Meanwhile, a protect circuit is needed to protect the secondary circuit of the transformer. Such requirement causes a high cost and an excessively large size of the product.

Therefore, it is a primary objective of the present invention to provide a half-bridge inverter that achieves an inversion by using any PWM integrated circuit and switching an external-driven switch and a self-driven switch. The external-driven switch is controlled by a square wave signal and works together with the self-driven switch for converting a DC power supply into an alternate current power and supplying the power for at least one lamp.

The present invention comprises a transformer having a primary winding, a second winding and an auxiliary winding. The transformer is coupled to the external-driven switch. The external-driven switch is coupled to the primary winding of the transformer and ground and controlled by the square wave signal. The self-driven switch is coupled to the primary winding, the auxiliary winding and the direct current power supply of the transformer and controlled by an output voltage of the auxiliary winding. A resonant capacitor is coupled to the secondary winding. Therefore, the present invention uses the square wave signal to control the switching of the external-driven switch and the output voltage of the auxiliary winding to control the switching of the self-driven switch. By alternately switching the switches, a direct current power supply is introduced to the primary winding of the transformer while sensing an alternating voltage at the second winding of the transformer. The alternating voltage is converted into alternating current power through the resonant capacitor. Further, the external-driven switch of the invention includes a first polar capacitor (or an additional capacitor) and a first interface diode (or an additional diode) between two output gates in the external-driven switch. The self-driven switch of the invention includes a second polar capacitor (or an additional capacitor) and a second interface diode (or an additional diode) between two output gates in the self-driven switch. Therefore, the square wave signal and the output voltage of the auxiliary winding respectively control the switching of the external-driven switch and the self-driven switch to charge or discharge the polar capacitor (or an additional capacitor) and the second polar capacitor (or an additional capacitor) by the DC power supply. The charging/discharging power is supplied to the primary winding of the transformer while sensing an alternating voltage at the secondary winding of the transformer and converted into the AC power by the resonant capacitor.

The foregoing aspects and many of the attendant advantages of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic circuit diagram of a half-bridge inverter circuit driving a load according to the prior art;

FIG. 2 is a schematic diagram of an output control signal of a half-bridge control chip and a voltage waveform of an alternate current power supply according to the prior art;

FIG. 3 is a schematic circuit diagram of a half-bridge inverter circuit driving a load according to the present invention;

FIGS. 4A to 4F are schematic diagrams illustrating the actions taken in a cycle of a circuit according to the present invention; and

FIG. 5 is a schematic diagram of a waveform in a cycle of a circuit according to the present invention.

Reference to FIG. 3 is made for a schematic circuit diagram of a half-bridge inverter in accordance with the present invention. A half-bridge inverter of the invention is controlled by a square signal S PWM for converting DC power supply VDC into AC power supplied for the use of at least one cold cathode fluorescent lamp (CCFL). The half-bridge inverter comprises a primary winding Np, a secondary winding Ns and an auxiliary winding Na of a transformer Tr. An external-driven switch Q 1 is coupled to the primary winding Np and ground G and controlled by the square wave signal S PWM . A self-driven switch Q 2 is coupled to the primary winding Np, the auxiliary winding Na and the direct current power supply VDC and controlled by an output voltage of the auxiliary winding Na. A resonant capacitor is coupled to the secondary winding Ns.

With the switching of the external-driven switch Q 1 controlled by the square wave signal S PWM and the switching of the self-driven switch Q 2 controlled by the output voltage of the auxiliary winding Na, the DC power supply VDC is introduced to the primary winding Np of the transformer Tr while an alternating voltage is sensed at the secondary winding Ns of the transformer Tr, and then converted into the AC power by the resonant capacitor C 3 .

Referring to FIG. 3 again, the present invention further comprises a fast diode D 1 coupled to a control end of the external-driven switch Q 1 for suppressing the current passing through the control end when the external-driven switch Q 1 is electrically connected and accelerating to turn off the external-driven switch Q 1 while the external-driven switch Q 1 is being turned off. A second accelerated diode D 2 is coupled to a control end of the self-driven switch Q 2 and the auxiliary winding Na for suppressing the current passing through the control end when the self-driven switch Q 2 is electrically connected and accelerating to turn off the self-driven switch Q 2 while the self-driven switch Q 2 is being turned off. In the foregoing description, the external-driven switch Q 1 is an N-channel field effect transistor (FET) having a first polar capacitor CQ 1 (or an additional capacitor) and a first interface diode (or an additional diode) disposed between two output gates of the external-driven switch. Further, the self-driven switch Q 2 is an N-channel field effect transistor (FET) having a second polar capacitor CQ 2 (or an additional capacitor) and a second interface diode (or an additional diode) disposed between two output gates of the self-driven switch.

Reference is made to FIGS. 4A to 4F for schematic views of the actions taken in a cycle of a circuit in accordance with the present invention, while referring to FIG. 5 simultaneously for a schematic diagram of the waveform in a cycle of a circuit of the invention. During period I, the square wave signal S PWM is high, and the external-driven switch Q 1 at that time is turned ON by the control of the square wave signal S PWM and the self-driven switch Q 2 is turned OFF. Therefore, the DC power supply VDC provides a path through the capacitor C 1 and transformer Tr to the external-driven switch Q 1 , the current is stored in the transformer Tr, and a positive alternating voltage Vo is sensed by the secondary winding Ns of the transformer Tr.

During period II, the square wave signal S PWM is turned OFF by the control of the external-driven switch Q 1 , and the self-driven switch Q 2 remains OFF. By that time, the electric power temporarily stored in the second polar capacitor CQ 2 or additional capacitor of the self-driven switch Q 2 will be discharged, and the current stored in the transformer Tr will charge the first polar capacitor CQ 1 or additional capacitor in the external-driven switch Q 1 . Thus, the alternating voltage Vo of the secondary winding Ns of the transformer Tr is at a low voltage.

During period III, both of the external-driven switch Q 1 and the self-driven switch still remain OFF. When the first polar capacitor CQ 1 or the additional capacitor is charged up to the voltage of the DC power supply VDC, and the second polar capacitor CQ 2 or additional capacitor is discharged to zero voltage, the second interface diode DQ 2 or additional diode is turned ON and the self-driven switch Q 2 is switched to zero voltage. Therefore, the alternating voltage sensed by the secondary winding Ns of the transformer Tr is at a negative alternating voltage Vo status.

During period IV, the self-driven switch Q 2 is turned ON by the control of the auxiliary winding Na of the transformer Tr. By that time, the direction of the current passing through the primary winding Np of the transformer Tr is reversed, and the energy will be accumulated in the transformer Tr. In this period, the alternating voltage sensed by the secondary winding Ns of the transformer Tr is at a negative alternating voltage status.

During period V, the self-driven Q 2 is switched OFF, and the external-driven switch Q 1 still remains OFF. By that time, the first polar capacitor CQ 1 or additional capacitor in the external-driven switch Q 1 discharges the temporarily stored electric power, and the current stored in the transformer will charge the second polar capacitor CQ 2 or additional capacitor in the self-driven switch Q 2 . Therefore, the alternating voltage Vo sensed by the secondary winding Ns of the transformer Tr is at a rising voltage status.

During period VI, if the second polar capacitor CQ 2 or additional capacitor is charged up to the voltage of the DC power supply VDC and the first polar capacitor CQ 1 or additional capacitor is discharged to zero voltage, then the first interface diode DQ 1 or additional diode will be turned ON. By that time, the external-driven switch Q 1 is switched to zero voltage. Therefore, the alternating voltage sensed by the secondary winding Ns of the transformer Tr is at a positive alternating voltage Vo status.

In the description above, the operation of the circuit returns to Period I after Period VI is finished to complete a cycle. The waveform of the alternating voltage Vo output in the cycle of the circuit is converted into the AC power Vac by the resonant capacitor C 3 . Therefore, the present invention can achieve an inversion according to the cycle of the foregoing circuit. Furthermore, the present invention only needs a square signal to drive the half-bridge circuit. If a short circuit or any other failure occurs at the secondary winding of the transformer, the self-driven switch Q 2 cannot continue its operation. The present invention does not require an additional peripheral circuit to achieve the circuit protection, and thus can reduce the manufacturing cost and the size of the product.

While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.