Patent ID: 8970122
Filing Date: 2015-03-03
Classification: H05B

Abstract:
1. A LED driver for driving LED strings, comprising: a power switch having a first terminal, a second terminal and a control terminal; a transformer comprising a primary winding, a secondary winding and a third winding, wherein each winding has a first terminal and a second terminal, and the first terminal of the primary winding is configured to receive an input signal and the second terminal of the primary winding is coupled to the first terminal of the power switch, the secondary winding is configured to supply power to a load; a current sense circuit configured to sense a current flowing through the power switch and generates a current sense signal based thereupon; a zero cross detecting circuit configured to detect a current flowing through the secondary winding, and generates a zero cross detecting signal based thereupon; a compensation circuit coupled to the third winding to receive a current flowing through the third winding, wherein based on the current flowing through the third winding, the compensation circuit generates a compensation signal to compensate the current sense signal by feeding forward a voltage across the secondary winding to eliminate a variation of a current flowing through the LED strings when a number of LEDs in the LED strings varies; and a control circuit configured to receive the compensated current sense signal and the zero cross detecting signal, wherein based on the compensated current sense signal and the zero cross detecting signal, the control circuit provides a control signal to the control terminal of the power switch to control the power switch.