PWM controller drives LEDs from high-voltage lines


Posted on Jun 26, 2012

The circuit forces a current to build up in the L1 coil and the LEDs until the voltage developed across R3 reaches VFB/3.3V. At this point, power switch Q1 turns off, and the magnetizing current keeps circulating in the coil and LEDs, thanks to freewheeling diode D1. To maintain a `clean` current in the LEDs, L1 must be large enough to keep the ripple to an acceptable value and to avoid pushing the controller to the minimum on-time (400 nsec) in high-line conditions. Because of the poor TRR (reverse-recovery time) of the LEDs, you must add an external filter, comprising R2 and C1 to the IC`s internal leading-edge-blanking circuitry. R1 sets the voltage-feedback level; keeping it lower than 3.3V prevents the NCP1200A`s internal short-circuit protection from tripping. In the example, the feedback voltage of 2.5V thus imposes a peak current of 2.5/3.3/4.7=161 mA.






Leave Comment

characters left:

New Circuits

.

 


Popular Circuits

Stablised Power Supply With Short Circuit Indication
25W HiFi Audio Amplifier with MOSFET
Logarithmic resistor ladder
pierce xtal oscillator circuit
white lights
3W FM Transmitter
Cheap And Cheerful Transistor Tester
Microcontroller In Circuit Serial Programming (ICSP) with Microchip PIC and Atmel AVR
High Voltage Electrolytic Capacitor Reformer



Top