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A power supply system includes a transformer having a primary winding on its primary side for coupling to a power source and one or more secondary windings on its secondary side. A first control circuit is disposed on the primary side of the transformer for controlling a current flow in the primary

A power supply system includes a transformer having a primary winding on its primary side for coupling to a power source and one or more secondary windings on its secondary side. A first control circuit is disposed on the primary side of the transformer for controlling a current flow in the primary winding. A second control circuit disposed on the secondary side of the transformer, and the second control circuit is configured to provide a regulated output voltage. In the power supply system, the first control circuit is configured to generate a control signal for controlling the current flow in the primary winding without using a feedback control signal from the secondary side of the transformer.

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1. A power supply system, comprising: a transformer having a primary winding on its primary side for coupling to a power source and one or more secondary windings on its secondary side; a first control circuit disposed on the primary side of the transformer for controlling a current flow in the prim

1. A power supply system, comprising: a transformer having a primary winding on its primary side for coupling to a power source and one or more secondary windings on its secondary side; a first control circuit disposed on the primary side of the transformer for controlling a current flow in the primary winding, the first controller including a pulse-width modulation (PWM) control circuit configured to operate with 50% duty cycle at a frequency for maximum voltage gain for a given load condition; and a second control circuit disposed on the secondary side of the transformer, the second control circuit being configured to provide a regulated output voltage; wherein the first control circuit is configured to generate a control signal for controlling the current flow in the primary winding without using a feedback control signal from the secondary side of the transformer. 2. The power supply system of claim 1, wherein the first control circuit comprises an LLC resonant circuit. 3. The power supply system of claim 2, wherein the LLC resonant circuit comprises a leakage inductance provided by the transformer. 4. The power supply system of claim 1, wherein the first control circuit comprises an asymmetric half bridge circuit. 5. The power supply system of claim 1, wherein the first control circuit comprises a second transformer coupled between a pulse signal generator and two transistors. 6. The power supply system of claim 1, wherein the first control circuit comprises an integrated circuit that is configured to provide a pulsed control signal. 7. A backlight system for providing backlight to a display device, the backlight system comprising: a transformer having a primary winding on its primary side for coupling to a power source and one or more secondary windings on its secondary side;a first control circuit disposed on the primary side of the transformer for controlling a current flow in the primary winding; anda second control circuit disposed on the secondary side of the transformer, the second control circuit being configured to provide a regulated output voltage; anda backlight apparatus that includes one or more light devices;wherein the first control circuit is configured to generate a control signal for controlling the current flow in the primary winding without using a feedback control signal from the secondary side of the transformer;wherein the second control circuit comprises a pulse width modulated (PWM) control circuit. 8. The backlight system of claim 7, wherein the backlight apparatus comprises one or more light emitting diode (LED) devices. 9. The backlight system of claim 7, wherein the first control circuit comprises an LLC resonant circuit. 10. The backlight system of claim 9, wherein the LLC resonant circuit comprises a resonant inductor and a resonant capacitor. 11. The backlight system of claim 7, wherein the second control circuit is configured to receive a feedback signal from the backlight apparatus. 12. The backlight system of claim 7, wherein the second control circuit comprises a voltage boost circuit. 13. The backlight system of claim 7, wherein the first control circuit comprises a first power switch and a second power switch having substantially the same duty cycle. 14. The backlight system of claim 7, wherein the first control circuit comprises an asymmetric half bridge circuit. 15. The backlight system of claim 7, wherein the first control circuit comprises a first power switch and a second power switch having complementary duty cycles, D and 1-D, respectively. 16. A power supply system, comprising: a transformer having a primary winding on its primary side for coupling to a power source and one or more secondary windings on its secondary side; a first control circuit disposed on the primary side of the transformer for controlling a current flow in the primary winding, the first controller including an LLC resonant circuit, the first controller including a pulse width modulated (PWM) control circuit configured to operate at a frequency for a peak voltage gain for a target load condition; and a second control circuit disposed on the secondary side of the transformer, the second control circuit being configured to provide a regulated output voltage; wherein the first control circuit is configured to generate a control signal for controlling the current flow in the primary winding without using a feedback control signal from the secondary side of the transformer. 17. The power supply system of claim 16, wherein the first controller further comprises an LLC resonant circuit including a resonant inductor and a resonant capacitor. 18. The power supply system of claim 17, wherein the first controller includes a pulse width modulated (PWM) control circuit configured to operate at a frequency that is lower than a first frequency fr1 and higher than a second frequency fr2, wherein fr⁢⁢1=12⁢π·Lr·Cr⁢⁢andfr⁢⁢2=12⁢π·(Lr+Lm)·Cr wherein Cr is the capacitance of the resonant capacitor, Lr is the inductance of the resonant inductor, and Lm is a magnetizing inductance Lm of the transformer. 19. The power supply system of claim 16, wherein the pulse width modulated (PWM) control circuit is configured to operate at a substantially 50% duty cycle. 20. The power supply system of claim 16, wherein the second control circuit comprises a pulse width modulated (PWM) control circuit.