Design Idea DI-124
LinkSwitch®-TN
Ultra-wide Input Range (57-580 VAC) Flyback Power Supply
Application
Device
Power Output Input Voltage
3 W 57-580 VAC
Output Voltage
Topology
Metering / Industrial
LNK304
12 V, 250 mA
StackFET Flyback
Design Highlights
A600 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
inductanceenergyisclampedbyVR5andD9withR10added
to reduce ringing and thereby, EMI.
•
•
•
•
•
•
StackFETTM flyback topology delivers full load over
extremely wide input voltage range
E-ShieldTM 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
The operation of U1 is unaffected by the StackFET
configuration. WhentheinternalMOSFETturnson,Q1isalso
turned on, applying the input voltage across the transformer
primary. Oncetheprimarycurrentreachestheinternalcurrent
limit of U1, the MOSFET is turned off and the energy stored
is delivered to the output. Regulation is maintained using
ON/OFFcontrol. Switchingcyclesareenabled/disabledbased
on current into the FEEDBACK pin of U1. This is ideal as
it results in a lowering of the effective switching frequency
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.
C1
2.2 nF
250 VAC
D1
D2
D3
D4
D10
VR5
1N4007 1N4007 1N4007 1N4007
R6
EEL16
NC
UF4004
R13
475 kΩ
1%
R14
475 kΩ
1%
P6KE150A
L2
J5
J6
680 kΩ
0.5 W
4
1
C5
C6
Ferrite Bead
15 µF
450 V
15 µF
450 V
D9
UF4007
0.5 W
0.5 W
J1
J2
J3
R1
5
R7
680 kΩ
0.5 W
C2
470 µF
16 V
C3
100 µF
16 V
10 Ω 1 W
C9
5.6 nF
1 kV
7
9
R10
200 Ω
1%
R2
R8
680 kΩ
0.5 W
R15
475 kΩ
1%
R16
475 kΩ
1%
10 Ω 1 W
C8
15 µF
450 V
C7
15 µF
450 V
R3
10
0.5 W
0.5 W
T1
10 Ω 1 W
R9
10 Ω
Q1
IRFBC20
R4
J4
10 Ω 1 W
R5
1 k
R11
VR4
330 Ω
1N5245B
15 V
U2B
PC817A
U2A
PC817A
VR1
P6KE150A
D5
D6
D7
D8
D
L1
1 mH
1N4007 1N4007 1N4007 1N4007
FB
U1
LNK304P
BP
R12
1 kΩ
VR2
P6KE150A
S
VR6
BZX79-C11
11 V
C4
100 nF
50 V
VR3
P6KE150A
PI-4487-081506
Figure 1. Schematic Diagram of 3 W Bias Supply using LinkSwitch-TN in StackFET Configuration.
September 2006
DI-124
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