POWERINT DI-131

Design Idea DI-131
®
LinkSwitch-TN
LED Driver for Halogen Light Bulb Retrofit
Application
Device
Power Output
Input Voltage
LED Lighting
LNK306DN
3W
85-265 VAC
Design Highlights
• Fits inside standard GU10 lamp base
• Constant current (CC) output
• Drive current independent of LED VF variations
• Tight line regulation (±5%) for constant light output
• No annoying line frequency flicker
• Universal input voltage range
• A single design can be sold worldwide
• Consistently high efficiency across entire range
• Meets EN55022 B conducted EMI limits
• Complete overvoltage and over temperature protection
• Operates unloaded: can be tested without the LEDs
• Auto-restart: withstands shorted output indefinitely
• Thermal shutdown: protects entire lamp assembly
Operation
The non-isolated buck converter shown in Figure 1 was
designed around a member of the LinkSwitch-TN IC family,
the LNK306DN (U1). A LNK306 was selected since its
minimum current limit (450 mA) ensures that it will be able
to deliver the required 330 mA of output current.
Output Voltage
10 VTYP (9-15 V), 300 mA Non-isolated Buck
Diode bridge BR1 rectifies the AC input while capacitors C1
and C2 filter it. Inductor L1 and RF1 attenuate conducted EMI. In case of a component failure, RF1 will act as a fuse.
The MOSFET within U1, diode D3, inductor L2 and capacitor
C3 form the buck converter stage. The controller within
U1 regulates the output current by enabling and disabling
MOSFET switching cycles.
During normal operation, the output current develops a
voltage drop across R8 and R10, which is impressed across
C6. When the voltage across C6 exceeds the VBE of Q1,
Q1 and Q2 both turn on. When Q2 turns on, current is
injected into the FEEDBACK (FB) pin of U1 from C5, via
R3. Whenever the current into the FB pin exceeds 49 mA,
MOSFET switching is disabled. The controller adjusts the
ratio of enabled and disabled cycles to regulate the output
current at 330 mA.
During no-load operation, the output voltage is sensed by
D1, D2 and C2. The voltage across C5 feeds current into
the FB pin of U1 through divider R7 and R1. The FB pin
D1
BAV21WS-7-F
R7
30 kΩ
Q2
MMST3906
R3
10 kΩ
R1
4.7 kΩ
C4
100 nF
25 V
FB
L
85-265
VAC
N
RF1
10 Ω
2.5 W
D
BR1
MB6S
600 V
C1
4.7 µF
380 V
C2
4.7 µF
380 V
Topology
R9
4.7 kΩ
D2
BAV21WS-7-F
C5
100 nF
25 V
R6
10 kΩ
R4
10 kΩ
Q1
MMST3904
C6
100 nF
25 V
BP
S
R8
2Ω
LinkSwitch-TN
U1
LNK306DN
R10
12 Ω
D3
MURS160T3
9-15 V,
300 mA
J3
L2
1 mH
C3
2.2 µF
25 V
R11
1 kΩ
L1
1 mH
J4
PI-4575-030707
Figure 1. 3 W LED, Halogen Bulb Replacement using LinkSwitch-TN in Non-Isolated Buck Converter Technology.
DI-131
March 2007
voltage is specified as 1.63 V at 49 mA, which allows it to be
used as a reference. The output voltage is regulated to <18 V
by disabling MOSFET switching cycles as the current into the
FB pin exceeds 49 mA.
80
Key Design Points
40
QP
60
AV
50
dBµV
• To prevent EMI coupling between the switching nodes on
the main board and the input filter board, a shield board
(connected to C2 negative) was placed between the two
PC boards (see Figure 3). This greatly improved the
margin to the conducted EMI limits.
• Diodes D1 and D2 can be replaced by a single 600 V
diode. Two 250 V diodes were used because they took up
less space than a single 600 V diode.
• Increasing the value of C3 will reduce the ripple current
through the LED load at the expense of increased
component size and cost.
• Pre-load resistor R11 ensures correct no-load operation,
which enables the power supply to be tested without the
load connected to it.
• Verify that L2 does not saturate heavily at the maximum
current limit of U1 (647 mA). Refer to the Inductor
Manufacturer’s L – 20% current rating.
• In less thermally challenged enclosures, the FB pin can
be used to directly sense the voltage across resistors
R8 and R10. This can eliminate Q1, Q2 and associated
components, but requires the current limit sense voltage
to increase from 0.65 V to 1.65 V. This will increase the
sense resistor dissipation by about 300 mW and cause the
internal ambient temperature of the supply to increase.
• Maximize the PCB area that the U1 SOURCE pins
connect to, for optimal heatsinking.
• Since this design does not provide input to output
isolation, the load LEDs must be isolated from the user via
the light bulb enclosure.
70
PI-4579-021207
DI-89 DI-131
QP
30
AV
20
10
0
-10
-20
0.15
1.0
10.0
100.0
MHz
Figure 2. Worst Case Conducted EMI (230 VAC).
0.780”
TOL ± 0.050”
1.080”
1.024”
0.742”
0.614”
PI-4577-030707
Figure 3. Assembled Unit, Dimensions: 0.78 × 1 inch
(19.8 × 25.4 mm).
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Rev. B 03/07