POWERINT DI-119

Design Idea DI-119
®
LinkSwitch-LP
Cordless Phone Linear Adapter Replacement
with 10 kV Surge Withstand
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
Cordless Phone/Adapter
LNK562P
1.6 W
85-265 VAC
7.7 V, 210 mA
Flyback
Design Highlights
• Simple, low-cost, low parts count CV/CC solution
• Low no-load input power: <180 mW at 265 VAC
• Dramatically improved regulation over line frequency
linear transformer
• Meets CEC / ENERGY STAR requirements for active
mode efficiency (63% vs. 53% requirement)
• Small, low-cost EE16 core size allows compact design
• >15 dBµV margin to EN55022B conducted EMI limits
• No Y-capacitor gives low (<10 µA) line frequency
leakage current
• Meets 10 kV common mode and 2 kV differential mode
surge (EN 1000-4-5 Class 4)
Operation
Supplies for cordless phones or answering machines often
require a 10 kV common mode surge withstand capability
to prevent damage to the telephone network during local
lightning strikes. The design shown in Figure 1 meets this
requirement while still being simple and low cost.
L
RF1
10 Ω
2.5 W
D1
1N4937
The LNK562 device (U1) provides primary side sensed output
voltage and current regulation, eliminating the need for an
optocoupler. Using the PI Transformer Designer software,
shield windings were included in the transformer design.
This allowed the circuit to meet EN55022 B conducted EMI
limits without the use of a Y-rated safety capacitor bridging
the primary to secondary isolation barrier.
The AC input is rectified and filtered by D1, D2 and C1,
C6. The input capacitance is split to form a π filter with
L1 and L3, with R5 damping the self resonance. Varistor
RV1 provides surge protection for differential surges while
RF1 provides filtering and fusing. The internal MOSFET of
U1 drives the transformer primary, but the normal primary
clamp network is not required due to the low current limit of
U1. Skipping switching cycles based on the voltage sensed
from the bias winding provides output regulation. Should
the output of the supply be overloaded, then U1 lowers the
switching frequency to limit the output current until ~2 V,
when the unit enters auto-restart.
L1
1 mH
1
T1
J1
C4
D4
100 µF
1N4933 25 V
J4
NC
R5
4.7 kΩ
J5
2
7.7 V, 210 mA
J3-1
R8
4.7 kΩ
J3-2
RTN
3
85-265
RV1
VAC
275 VAC
N
C6
3.3 µF
400 V
D2
1N4005
C1
3.3 µF
400 V
C3
10 µF
50 V
R7
4.7 kΩ
L3
1 mH
4
D3
1N4005
EE16
R6
100 kΩ
J2
LinkSwitch-LP
U1
LNK562
D
R1
22.1 kΩ
1%
FB
BP
S
C2
100 nF
50 V
R2
3.01 kΩ
1%
PI-4491-092806
Figure 1. LNK562 Linear Adapter Replacement Schematic.
DI-119
September 2006
DI-89 DI-119
Elimination of the optocoupler and Y1 capacitor allowed the
necessary printed circuit board (PCB) clearance and creepage
to be obtained to withstand a 10 kV surge. The use of triple
insulated wire for the secondary winding that terminates onto
the PCB as flying leads (J4 & J5) allows the necessary creepage
from primary to secondary. Figure 2 shows the PCB layout.
Key Design Points
•
•
TRANSFORMER PARAMETERS
Core Material
Bobbin
EE16, gap for ALG of 113 nH/T2
4+4 pin horizontal
Winding Details
Bias/Shield: 29T, 2 × 37 AWG
Primary: 176T, 37 AWG
Shield: 15T, 2 × 32 AWG
Secondary: 17T, 30 AWG TIW
Winding Order
(pin numbers)
Bias/Shield (3-4), tape, primary
(2-1), tape, shield (NC-1), tape,
7.7 V (FL-FL), tape
Inductance
Figure 2. PCB Layout – Extended Clearance and Creepage
Provided by Slot (A). Spark Gap Provided to Route High
Currents Back to Input and Around Electronics (B).
12
PI-4501-090606
•
•
Verify maximum drain voltage is <650 V at high
line, maximum overload. For EMI repeatability, the
transformer must be manufactured consistently. This is
especially important in designs with no Y-capacitor.
Using a fast diode for D1 improves EMI.
Make sure the PCB layout provides 10 mm clearance
and 15 mm creepage distance (use a slot in the PCB to
increase creepage distance).
Provide a path for surge discharge currents to go around
sensitive electronic components (see spark gap (B) in
Figure 2).
To prevent arcing, keep the PCB surfaces clean. Remove
flux and any other contaminants.
85 VAC
265 VAC
10
Output Voltage (V)
•
8
6
4
2
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Output Current (A)
Figure 3. Typical Output Characteristics.
Primary: 3.5 mH ±10%
Leakage: 105 µH (max)
Primary Resonant
250 kHz (min)
Frequency
Table 1. Transformer Design Parameters.
TIW = Triple Insulated Wire, NC = No Connect, FL = Flying Lead
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Rev. A 09/06