Dimmable 660mA Boost/Buck for three LEDs in series (AL8811EV2)

AL8811EV2 User Guide
General Description
The MR16 LED Driver Module Evaluation
board shows how to use the new AL8811 as
a Boost LED driver for an inexpensive PFC
front end and the new AL8807A as a Buck
LED driver for a cost effective MR16 LED
Driver circuit from which high PFC (~0.9)
can be achieved.
Key Features





Parameter
Value
Input Voltage
LED Current
Number of LEDs
12VAC
660mA (Adjustable)
3 LEDs in series
(Under Tested)
0.8 ” x 0.88”
XY Dimension
EVB (Rev 1.2)
Non-Dimmable or Triac Dimmable
Front end Constant On time PFC circuit
using the AL8811 Boost LED Driver
New AL8807A Buck LED Driver
PFC for the 12VAC input allowing
multiple MR16 units on one
transformer
Compatible with Electronic
Transformers
Applications



AL8811EV2 Specifications
Figure 1: Top View
MR16 LED Bulb
Desktop lamps
Under the counter lamps
Figure 2: Bottom View
Connection Instructions
Input Voltage: 12VAC (AC+, AC-)
LED Outputs: LED+ (Red), LED- (Black)
AL8811EV2 Rev3
March 2013
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Page 1 of 17
AL8811EV2 User Guide
Block Diagram:
12VAC
Input
AL8811EV2
PFC Boost
LED Driver
AL8811
Bridge
rectifier
Buck LED Driver
AL8807A
LEDs
Phase Detect
Triac Dimming
Figure 3: Block Diagram
Evaluation Board Schematic
Figure 4: Evaluation Board Schematic
AL8811EV2 Rev3
March 2013
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Page 2 of 17
AL8811EV2 User Guide
Evaluation Board Layout
Figure 5: PCB Board Layout Top View
Figure 6: PCB Board Layout Bottom View
AL8811EV2 Rev3
March 2013
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Page 3 of 17
AL8811EV2 User Guide
Quick Start Guide
1. By default, the evaluation board is preset at 660mA LED Current by R11.
2. Ensure that the AC source is switched OFF or disconnected.
3. Connect the 12VAC AC line wires of power supply to two test points of “12VAC” on the left
side of the board.
4. Connect the anode wire of external LED string to LED+ output test point.
5. Connect the cathode wire of external LED string to LED- output test point.
6. Turn on the main switch. LED string should light up.
Bill of Material
#
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Name
IC1
IC2
Q1
Q2
D1-D4,
D6,D8,D9
D5,D7
L1
L2
R1
R2
R3
R4
R5, R10
R6
R7, R8,
R12
15
16 R9
17 R11
18 C1, C3
19
C2, C5,
C6, C10,
C11
20 C4, C7
21 C8, C9
AL8811EV2 Rev3
March 2013
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Quantity
1
1
1
1
Part number
7
2
1
1
1
1
SBR3U30P1
1
1
2
1
MCR25JZHFLR330
3
1
1
2
ERJ-2RKF4701X
5
2
2
AL8811
AL8807A
MMBT3904-7-F
ZXMN6A11Z
BAV70-7-F
SRR1280-221K
ASPI-104S-101M-T
ERJ-2RKF10R0X
ERJ—2RKF4700X
ERJ-2RKF2201X
ERJ-2RKF1001X
ERJ-2RKF5902X
ERJ-3GEY0R00V
RL1220S-R15-F
C0402C102J5GACTU
C2012X5R1E106K125
AB
C1005X7R1H104K050
BB
ELXZ500ELL680MF15
Manufacturer
Diodes Inc
Diodes Inc
Diodes Inc
Diodes Inc
Description
Boost IC
Buck LED Driver
Diodes Inc
Diodes Inc
Bourns
Abracon
Panasonic
Panasonic
Rohm
Semiconductor
Panasonic
Panasonic
Panasonic
Super Barrier Rectifiers
Panasonic
Panasonic
Susumu
Kemet
4.7kΩ Resistor 1/10W 1% 0402 SMD
TDK
10µF Cer Cap 25V 10% X5R 0805
TDK
United Chemi
0.1µF Cer Cap 50V 10% X7R 0402
NPN transistor
N-MOSFET
BAV70 Dual diodes
220µH, 1.6A Inductor for PFC stage
100µH, 1.35A Inductor for Buck Stage
10Ω Resistor 1/10W 1% 0402 SMD
470Ω Resistor 1/10W 1% 0402 SMD
0.33Ω Resistor 1/2W 1% 1210 SMD
2.2kΩ Resistor 1/10W 1% 0402 SMD
1kΩ Resistor 1/10W 1% 0402 SMD
59kΩ Resistor 1/10W 1% 0402 SMD
0Ω Resistor 1/10W 1% 0603 SMD
0.15Ω Resistor 1/3W 1% 0805 SMD
1000pF Cer Cap 50V 5% 0402 SMD
68µF Aluminum Cap 50V 20% Radial
Page 4 of 17
AL8811EV2 User Guide
Functional Performance (Three series LEDs @660mA)
AL8811EV2 Module Board Performance (without dimmer and electronic transformer)
Manuf
Board Type
VIN
(VRMS)
IIN
(IRMS)
PIN
(W)
PF
VLED
(V)
ILED
(mA)
PLED
(W)
Diodes
Inc
AL8811EV2
Module
Board
12
694.5
7.77
0.95
8.65
659.1
5.7
ILED
Ripple
(%)
8
Efficiency
(%)
73.4
Functional Waveforms
For 120VAC dimmable MR16 design bench testing:
The electronic transformer type is Hatch RS12-150 / 150W.
The dimmer type is Lutron SELV-300P.
Following is a block diagram of the bench circuit that indicates voltage and current designations
where the scope plots are functionally captured on the bench set-up. The bench set-up is used
in the evaluation of the AL8811EV2 module dimmable MR16 design.
Figure 7: Bench Set-up Circuit
AL8811EV2 Rev3
March 2013
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Page 5 of 17
AL8811EV2 User Guide
Waveform #1 (Maximum Dimming) => Channel 1: V1; Channel 2: I1
Waveform #2 (Dimming Control from full to lowest brightness) => Channel 1: V1
AL8811EV2 Rev3
March 2013
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Page 6 of 17
AL8811EV2 User Guide
Waveform #3 (Maximum Dimming) => Channel 1: V2; Channel 2: I2
Waveform #4 (Zoom-in for Maximum Dimming) => Channel 1: V2; Channel 2: I2
AL8811EV2 Rev3
March 2013
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Page 7 of 17
AL8811EV2 User Guide
Waveform #5 (Maximum Dimming) => Channel 1: V2; Channel 2: I2
Waveform #6 (Dimming Control from full to lowest brightness) => Channel 1: V2
AL8811EV2 Rev3
March 2013
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Page 8 of 17
AL8811EV2 User Guide
Waveform #7 (Dimming Control from full to lowest brightness) => Channel 2: I2
Waveform #8 (Maximum Dimming) => Channel 1: V2; Channel 2: I2; Channel 3: I3
AL8811EV2 Rev3
March 2013
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Page 9 of 17
AL8811EV2 User Guide
Waveform #9 (Maximum Dimming) => Channel 3: I3; Channel 4: V3
Waveform #10 (Dimming Control from full to lowest brightness) => Channel 3: I3
AL8811EV2 Rev3
March 2013
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Page 10 of 17
AL8811EV2 User Guide
Functional Data Curves
AL8811EV2 Rev3
March 2013
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Page 11 of 17
AL8811EV2 User Guide
Transformer Compatibility List
1) 120VAC to 12VAC Electronic Transformers without dimmers in 3 LEDs in series:
Electronic Transformers
(120VAC to 12VAC)
Index
Brand
Model
1
HATCH
RS12-30M-LED (30W)
2
RSA
RT60A (60W)
3
HATCH
RS12-150 (150W)
4
HATCH
RS12-60M-LED (60W)
5
HATCH
VS12-75W (75W)
6
LIGHTECH
LET303 (300W)
7
HATCH
RS12-60M (60W)
8
HATCH
RS12-80M (80W)
9
HATCH
VS12-60M (60W)
10
HATCH
RS12-105 (105W)
11
LIGHTECH
LET75 (75W)
12
HATCH
VS12-60W (60W)
13
HATCH
RS12-15M-LED (15W)
AL8811EV2 Rev3
March 2013
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Performance Result
(No flicker)
√
√
√
√
√
√
√
√
√
√
√
√
√
Page 12 of 17
AL8811EV2 User Guide
2) 120VAC to 12VAC Electronic Transformers with dimmers in 3 LEDs in series
Electronic Transformers
(120VAC to 12VAC)
Index
Brand
Model
1
HATCH
RS12-30M-LED (30W)
2
HATCH
RS12-150 (150W)
3
HATCH
RS12-60M-LED (60W)
4
HATCH
RS12-150/150W
5
LIGHTECH
VS12-75W (75W)
6
HATCH
RS12-60M (60W)
7
HATCH
LET75 (75W)
8
HATCH
VS12-60W (60W)
9
HATCH
RS12-15M-LED (15W)
Note:
√ = No Flicker
AL8811EV2 Rev3
March 2013
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120VAC Dimmer Type
LUTRON
SELV-300P / 300W
LUTRON
MAELV-600 / 600W
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
Page 13 of 17
AL8811EV2 User Guide
Application Information
Circuit Description
This design consists of three sections:
1) The input PFC circuit converts the 12VAC 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.
Buck LED Driver
The AL8807A is a step-down DC-DC converter designed to drive LEDs with a constant current.
The current through the LED is controlled by R11. In the present Evaluation board, the current
is set to around 660mA based on a resistor value of 0.15Ω. The current is set using the “CTRL”
input pin which in this new version of the IC can vary from 0 to 2.5V, controlling the current
from 0mA to the maximum current at 2.5V. This control input pin is used to lower the LED
current as the TRIAC dims the LED. In this way, the energy stored in the two output capacitors
AL8811EV2 Rev3
March 2013
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Page 14 of 17
AL8811EV2 User Guide
of the PFC circuit will be able to provide current throughout the AC cycle.
TRIAC Phase Detection Circuit
The phase of the TRIAC is detected by using an additional rectifier circuit that generates a
voltage in proportional to the phase of the TRIAC Driver. This is done by rectifying the input AC
voltage and averaging the energy using a resistor to charge a capacitor. Two additional
resistors in series, R12 and R8, slowly discharge this circuit so it will follow the input phase
change. The two resistors, R12 and R8, are used to scale the voltage so the range is from 0 to
2.6V to the Buck LED driver control pin.
A simple transistor emitter follower circuit is used to drive a 1K resistor in the emitter circuit.
This low resistance is needed to drive the input control pin of the AL8807A LED driver because
the pin outputs a small current of 50uA, which limits the lowest control voltage to around 50
mV.
Setting the LED output current (AL8807A):
The LED output current is set using resister R11 and the formula:
ILED = VTH / R11 where VTH is equal to 0.1V
For a current of 660mA, R11 is about 0.15Ω.
Setting the PFC Variables (AL8811)
The choice for the size of the boost converter inductor selected in this design is based on a
compromise which it is able to support a peak current to around 1.5A since the average input
voltage will be around 12-14V.
The boost converter (AL8811) includes a current limit resistor R3 which will limit the current
through the inductor and thus the power delivered to the output load. The formula for the
resistor is:
IPK(switch) = 0.33V / R3
For a current limit of 1A, R3 is 0.33Ω.
In this evaluation design, this value was selected based on having three LEDs in series drawing
about 660mA. It was found that two 68µF capacitors mounted in parallel would just fit into the
cavity of the MR16 bulb. The important design goal is to have the PFC circuit, which is used to
always draw current from the Electronic Transformer.
AL8811EV2 Rev3
March 2013
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Page 15 of 17
AL8811EV2 User Guide
Performance Description
The evaluation board allows the testing of different combinations of circuit component values
to match the final design specifications. The main design goal is to have a constant load on the
Electronic Transformer so that it will be operating throughout the AC cycle. This is accomplished
when the input power is about the same as the output power.
Overall, there are three major components that are essential to the operation of the circuit.
The first component to select is the resistor (R11) in the Buck LED driver (AL8807A) that sets the
final current through the LED string. This will set the amount of power the system needs.
The second component is the size of the R3 which limits the current provided to the output
filter capacitors. This should be adjusted so that the boost input circuit by AL8811 LED driver is
always running and thus providing a load to the Electronic Transformer. This usually means
that the output voltage of the PFC circuit will have a large ripple. This will be okay as long as
the lowest voltage is higher than the maximum final LED string voltage.
The third component is the output capacitors (C8 and C9) of the PFC circuit. These should not
be too large that the PFC circuit stops working. If it happens, the resonant circuit of the
Electronic Transformer will become erratic and cause the LEDs to flicker.
AL8811EV2 Rev3
March 2013
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Page 16 of 17
AL8811EV2 User Guide
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