Design Idea DI-124
LinkSwitch-TN
Application
Metering / Industrial
Device
LNK304
®
Ultra-wide Input Range (57-580 VAC) Flyback Power Supply
Power Output
3W
Input Voltage
57-580 VAC
Output Voltage
12 V, 250 mA
Topology
StackFET Flyback
Design Highlights
•
•
•
•
•
•
StackFET
TM
flyback topology delivers full load over
extremely wide input voltage range
E-Shield
TM
transformer construction for reduced
common-mode EMI (>10 dBµV margin)
66 kHz switching frequency with jitter reduces
conducted EMI
Simple ON/OFF controller – no feedback compensation
required
Auto-restart function for automatic and self-resetting
open-loop, overload and short circuit protection
Built-in hysteretic thermal shutdown at 135 ºC
A 600 V MOSFET, Q1, and U1 are arranged in the
StackFET
configuration (cascode). The drain of U1 drives the source
of Q1 while the drain of Q1 drives the transformer primary.
The drain voltage of U1 is limited to 450 V by VR1-3. This
extends the maximum peak composite drain voltage of U1
and Q1 to 1050 V. The resistor chain R6-R8 provides startup
charge for the gate of Q1 and R9 dampens high-frequency
ringing. Once the converter is operating, the gate is largely
driven by the charge stored in the capacitance of VR1-3.
Zener VR4 limits the gate to source voltage of Q1. Leakage
inductance energy is clamped by VR5 and D9 with R10 added
to reduce ringing and thereby, EMI.
The operation of U1 is unaffected by the
StackFET
configuration. When the internal MOSFET turns on, Q1 is also
turned on, applying the input voltage across the transformer
primary. Once the primary current reaches the internal current
limit of U1, the MOSFET is turned off and the energy stored
is delivered to the output. Regulation is maintained using
ON/OFF control. Switching cycles are enabled/disabled based
on current into the FEEDBACK pin of U1. This is ideal as
it results in a lowering of the effective switching frequency
C1
2.2 nF
250 VAC
Operation
The AC input is rectified and filtered and the resultant DC
applied to one end of the transformer primary winding. The
450 V input capacitors are stacked with parallel balancing
resistors to meet the required voltage rating. Resistors R1 to
R4 provide fusing in case of a catastrophic failure. Inductor
L1, C1 and transformer
E-Shield
windings allow the design
to meet EN55022 B conducted limits with good margin.
D1
1N4007
D2
1N4007
D3
1N4007
D4
1N4007
C5
15
µF
450 V
J1
J2
J3
J4
R1
10
Ω
1 W
R2
10
Ω
1 W
R3
10
Ω
1 W
R4
10
Ω
1 W
D5
1N4007
D6
1N4007
D7
1N4007
D8
1N4007
R13
475 kΩ
1%
0.5 W
C6
15
µF
450 V
C9
5.6 nF
1 kV
C8
15
µF
450 V
R14
475 kΩ
1%
0.5 W
R6
680 kΩ
0.5 W
R7
680 kΩ
0.5 W
VR5
P6KE150A
D9
UF4007
EEL16
NC
4
5
7
9
10
T1
1
D10
UF4004
L2
Ferrite Bead
J5
C7
15
µF
450 V
R15
475 kΩ
1%
0.5 W
R16
475 kΩ
1%
0.5 W
R10
R8 200
Ω
680 kΩ 1%
0.5 W
R9
10
Ω
VR4
1N5245B
15 V
U1
LNK304P
VR2
P6KE150A
VR3
P6KE150A
S
C2
470
µF
16 V
C3
100
µF
16 V
J6
Q1
IRFBC20
R11
330
Ω
D
R5
1k
L1
1 mH
VR1
P6KE150A
FB
BP
U2B
PC817A
U2A
PC817A
R12
1 kΩ
C4
100 nF
50 V
VR6
BZX79-C11
11 V
PI-4487-081506
Figure 1. Schematic Diagram of 3 W Bias Supply using LinkSwitch-TN in StackFET Configuration.
DI-124
September 2006
POWERINT [ POWER INTEGRATIONS, INC. ]
