Patent ID: 8461766

Claim:
A circuit for powering at least one light source, comprising: a transformer, comprising a primary winding, and a secondary winding coupled to provide electrical power to a light source; a sense resistor with a first terminal coupled with the primary winding and a second terminal coupled with a circuit ground; a first switching device comprising a first power terminal coupled with the primary winding, a second power terminal coupled with the sense resistor, and a first control input terminal, the first switching device being operative according to a control signal at the first control input terminal to selectively electrically couple the first and second power terminals to allow current to flow in the primary winding; a pulse width modulation circuit comprising: a pulse width modulation controller, including a PWM control input, a comparator input coupled with the first terminal of the sense resistor to receive a sense voltage, and a drive output providing a pulse width modulated control signal to the first control input terminal of the switching device at least partially according to the PWM control input, and an error amplifier, including an error input, and an output coupled to provide a signal to the PWM control input of the pulse width modulation controller; and an estimator circuit electrically isolated from the secondary winding of the transformer, the estimator circuit comprising: a capacitance with a first terminal coupled with the comparator input of the pulse width modulation controller, and a second terminal coupled with the error input of the error amplifier, and a second switching device, comprising a first power terminal coupled with the second terminal of the capacitance, a second power terminal coupled with the circuit ground, and a second control input terminal coupled with the drive output of the pulse width modulation controller, the second switching device operative according to the drive output of the pulse width modulation controller to selectively couple the second terminal of the capacitance with the circuit ground to allow the capacitance to charge based on the sense voltage when the first switching device allows current to flow in the primary winding, and to decouple the second terminal of the capacitance from the circuit ground to allow the capacitance to provide a signal to the error input of the error amplifier when the first switching device prevents current flow in the primary winding.