New-Tech Europe | March 2016 | Digital edition

slowly from initial startup. A similar soft-start feature is available on the SSL4101T Greenchip III+ from NXP. On this device, the soft-start period is determined through selection of an external resistor and capacitor. Able to work with conventional triac dimming techniques, and so support systems that need to handle a mixture of lighting sources, the SSL4101T was designed to provide high efficiency at all power levels; using a variety of PWM control techniques to suit the conversion to the desired output light level. The device moves from quasi- resonant operation at high power levels to quasi-resonant operation with valley skipping through to reduced-frequency operation at low power-output levels, reducing the current supplied to the LED to avoid generating audible noise from the ballast’s transformer. In quasi-resonant mode, the next converter stroke is started after demagnetization of the transformer that should be provided as part of the front-end circuitry to the SSL4101T. This helps to minimize losses, as the converter only switches on when the voltage across the external MOSFET is at its minimum. To reduce the frequency of operation at lower loads, the quasi-resonant operation changes to discontinuous mode operation with valley skipping in which the switching frequency is limited for EMI reasons, typically to around 125 kHz. Again, the external MOSFET is only switched on when the voltage across the MOSFET is at its minimum. At very low-power and standby levels, the frequency, which can be reduced

Figure 2: The FL7701 in a typical application

Figure 3: Block diagram of the SSL4101

to almost zero, is controlled down by a voltage-controlled oscillator (VCO). During frequency reduction mode, the primary peak current is kept at a minimal level of 25 percent of the maximum. In frequency reduction mode the PFC controller is switched off as the harmonics generated will be minimal. To prevent oscillation around the PFC-operation point, the device supports approximately 50 mV of hysteresis on the control pin.

The ability of the SSL4101T to support very-low input power in standby mode eliminates the need for an additional housekeeping power supply to power the lighting controllers or dimmers. Although design for LED smart lighting involves more complex power conversion than dimming control for incandescent lamps, integrated devices such as the FL7701 and SSL4101T can ease the design burden and achieve high efficiency.