PWM controller drives LEDs from high-voltage lines

Powering LEDs from a wide dc range—say, 30 to 380V—without wasting a lot of power in the regulating block, is a difficult task when the LED current needs to be constant. Dedicated LED drivers are available, but they usually implement boost structures and are thus inadequate for high-voltage inputs. The NCP1200A, a high-voltage controller from On Semiconductor (www.onsemi.com), can serve as a constant-current generator if you add a simple coil in series with a power MOSFET. If you insert diodes between the coil and the MOSFET, the circuit becomes an economical light generator. Furthermore, there is no need for a transformer or any kind of external supply, because the controller directly connects to the rectified high voltage and thus supplies itself (Figure 1).

The circuit forces a current to build up in the L₁ coil and the LEDs until the voltage developed across R₃ reaches V_FB/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 T_RR (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.

In this application, the line goes as high as 380V dc. At steady state, L₁ and V_IN dictate the on-time, whereas the reset voltage applied to L₁ fixes the current decrease during off-time. This reset voltage, V_FTOTAL, equals the total LED forward voltage plus the forward drop of the freewheeling diode. The total reaches approximately 12V in this example. It can obviously vary, depending on the type of LED you want to drive, especially with white LEDs that incur significant forward drops of approximately 3V. To help derive the inductance value corresponding to your needs, a few lines of algebra suffice: t_OFF=L₁(ΔI/V_FTOTAL) and t_ON=L₁(ΔI/V_IN), where ΔI is the ripple current in L₁, V_FTOTAL is the previously described reset voltage, and V_IN is the dc input voltage. Because the circuit runs in continuous-current mode, the sum of on-time and off-time gives the switching period of the 1200AP60: L₁(ΔI/V_FTOTAL)+L₁(ΔI/V_IN)=1/f_S, where f_S is the switching frequency. Extracting L₁ yields L₁=(1/f_S)[(V_FTOTAL×V_IN)/(V_FTOTAL+V_IN)](1/ΔI).

If you select a ripple current of 20 mA peak-to-peak at 380V dc, then L₁=16.66×11.6×50=9.6 mH. From this value, you can check the minimum on-time using the equation: t_ON=9.6 mH×0.02/380=508 nsec, above the minimum limit. Figure 2 portrays typical signals captured on the prototype supplied with low line voltage.