HIGH-BRIGHTNESS LEDS ARE AN inexpensive, robust, and green replacement for halogen light bulbs. LEDs offer a much longer lifetime and eliminate the safety hazards of the inert gas, the expense of the UV filter encasement, and the handling sensitivity of halogens.
Since halogen bulbs typically are driven with 12 or 24 V due to their excellent efficacy at those voltages, buildings have been wired with 12- and 24-V ac transformers for halogen lighting. Therefore, replacing existing halogen lighting with LEDs requires only an LED driver to convert the 12- or 24-V ac to an appropriate dc voltage.
A suitable circuit is a switchmode- regulator LED driver designed for ac lighting that requires a high power factor (PF). The ideal load, with 100% power factor, is a pure resistor. In contrast to the resistive load, an LED driver with constant (dc) LED current, as is often used to power LEDs from a dc source, has a very poor power factor when used with an ac source.
A constant dc load requires dc power from the input. With an ac source, dc power creates the highest input current when the input voltage is lowest and vice versa. This creates voltage and current input waveforms with a large phase shift and a very low power factor.
The goal is to develop an LED driver with input current in phase with the input voltage. Thus, a 120- Hz LED current waveform is created to demand the highest input current when the input voltage is the highest and vice versa. The 120-Hz frequency is high enough to not be perceived by the human eye.
The circuit in Figure 1 uses an LT3755 ac LED driver with 98.1% power factor. The topology is buckboost mode since the wide-ranging input of 0 to 18 V (12 V rms) and the wide-ranging four-LED string voltage of 9 to 14 V cross over each other.
The 12-V ac rms input is rectified by D3-D6 to create a 120-Hz, 0- to 18-V PVIN supply. The PVIN and VIIN nodes are separated by diode D2 since the LT3755 works best when the VIN pin is held above 7 V, maintaining 7 V on the INTVCC pin and driving power MOSFET M1 with proper gate voltage. The diode makes it possible to peak charge the VIN capacitor, CVIN, which is large enough to maintain its charge when PVIN drops down to 0 V.
LED current foldback via the CTRL pin voltage creates a high PF in the LED driver and ensures a soft startup during which inrush currents don’t reduce the PF. RS2 sets the maximum LED current at 680 mA, but the CTRL pin monitors the PVIN voltage and shapes the LED current waveform to match PVIN.
When PVIN drops below the shutdown-pin threshold, the IC goes into shutdown, switching stops, and the soft-start feature is reset. The LED current trails off as the output capacitors are discharged and soon PVIN rises above the shutdown-pin threshold and the LT3755 starts back up. A light soft-start capacitor allows the LT3755 to start up quickly and keep the PF high.
Figure 2 shows the LED string current and voltage waveforms at 120 Hz. Multiplying the average LED current and the average LED voltage measured by the oscilloscope will provide the approximate LED power:
356 mA × 11.175 V ≈ 4 W
which is enough for an efficient light-bulb replacement.
The circuit’s power factor of 98.1% was measured using an Agilent 6811B ac power source/ analyzer. Figure 3 shows the 60-Hz ac line voltage and current measurements.
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