Diodes AL8811 - Manual (Page 6)

AL8811
Document number: DS36090 Rev. 2 - 2
6 of 11
www.diodes.com
June 2013
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AL8811
Application Circuit
(cont.)
(2)
Electronic Transformer compatible triac dimmable MR16/GU5.3 lamp for 120Vac
Circuit Description
This design consists of three sections:
1) The input PFC circuit converts the 12V
AC
input voltage to a DC voltage around 30V (AL8811).
2) The output Buck LED Driver drives the three LEDs in series at a fixed current (AL8807A).
3) Finally, the phase-detect circuit generates a voltage proportional to the phase of the incoming AC voltage (when triac dimming is used).
PFC Circuit
The AL8811 Boost converter is a simple “Constant ON time controller”. By keeping the same ON time throughout the AC cycle, the circuit will
draw a current that will closely match the voltage and result in a constant input current. This eliminates the classic peak current problem with a
bridge rectifier and a large input filter capacitor.
The PFC circuit includes the input bridge rectifier, EMI filter (if needed) and the AL8811 Boost converter. The AC input voltage is rectified by the
bridge circuit and filtered by C1, R1, C4, and C5. This first filter removes the high frequency that is generated by the Electronic Transformer in the
range of 20-30 KHz. An additional diode rectifier circuit (D5, C2) is used to generate a voltage that is used to power the circuit that will turn on/off
the external MOSFET of the Boost converter. This circuit is very important as the gate drive of the MOSFET has to be greater than 3-4 volts
throughout the AC cycle. The external MOSFET is used to reduce the heat dissipation in the AL8811.
The AL8811 has a current limit resistor R3 which sets the maximum current allowed through the inductor L1. The output voltage is set by the
divider R6, R5 to an output of around 35 volts. The output voltage is filtered by the two capacitors C8 and C9. These two capacitors store energy
that will be used when the input voltage is low during the AC cycle.