LNK562-564
L
J-1
D1
1N4937
L1
RF1*
8.2
Ω
3300
μH
2.5 W
C1
10
μF
400 V
2
T1
EE16 7
D4
UF4002
C5
220
μF
25 V
VR1*
R3 1N5240B
6 V,
2 kΩ
10 V
0.33 A
J3-2
90-265
VAC
J-2
N
D2
1N4005
1
6
4
5
D3
1N4005
R1
37.4 kΩ
C3
330 nF
50 V
R2
3 kΩ
PI-4106-010208
J3-1
RTN
D
LinkSwitch-LP
U1
LNK564PN
FB
BP
C4*
100 pF
250 VAC
S
C2
0.1
μF
50 V
*Optional components
Figure 5. 6 V, 330 mA CV/CC Linear Replacement Power Supply.
Applications Example
The circuit shown in Figure 5 is a typical implementation of
a 6 V, 330 mA, constant voltage, constant current (CV/CC)
output power supply.
AC input differential
fi
ltering is accomplished with the very
low cost input
fi
lter stage formed by C1 and L1. The proprietary
frequency jitter feature of the LNK564 eliminates the need for
an input pi
fi
lter, so only a single bulk capacitor is required.
Adding a sleeve may allow the input inductor L1 to be used as a
fuse as well as a
fi
lter component. This very simple
Filterfuse™
input stage further reduces system cost. Alternatively, a fusible
resistor RF1 may be used to provide the fusing function.
Input diode D2 may be removed from the neutral phase in
applications where decreased EMI margins and/or decreased
input surge withstand is allowed. In such applications, D1 will
need to be an 800 V diode.
The power supply utilizes simplified bias winding voltage
feedback, enabled by LNK564 ON/OFF control. The resistor
divider formed by R1 and R2 determine the output voltage across
the transformer bias winding during the switch OFF time. In the
V/I constant voltage region, the LNK564 device enables/disables
switching cycles to maintain 1.69 V on the FB pin. Diode D3 and
low cost ceramic capacitor C3 provide rectification and
fi
ltering
of the primary feedback winding waveform. At increased loads,
beyond the constant power threshold, the FB pin voltage begins
to reduce as the power supply output voltage falls. The internal
oscillator frequency is linearly reduced in this region until it
reaches typically 50% of the starting frequency. When the FB
pin voltage drops below the auto-restart threshold (typically
0.8 V on the FB pin, which is equivalent to 1 V to 1.5 V at the
output of the power supply), the power supply will turn OFF
for 800 ms and then turn back on for 100 ms. It will continue
in this mode until the auto-restart threshold is exceeded. This
function reduces the average output current during an output
short circuit condition.
No-load consumption can be further reduced by increasing C3
to 0.47
μF
or higher.
A
Clampless
primary circuit is achieved due to the very
tight tolerance current limit trimming techniques used in
manufacturing the LNK564, plus the transformer construction
techniques used. Peak drain voltage is therefore limited to
typically less than 550 V at 265 VAC, providing significant
margin to the 700 V minimum drain voltage specification
(BV
DSS
).
Output rectification and
fi
ltering is achieved with output rectifier
D4 and
fi
lter capacitor C5. Due to the auto-restart feature, the
average short circuit output current is significantly less than
1 A, allowing low cost rectifier D4 to be used. Output circuitry is
designed to handle a continuous short circuit on the power supply
output. Diode D4 is an ultra-fast type, selected for optimum
V/I output characteristics. Optional resistor R3 provides a
preload, limiting the output voltage level under no-load output
conditions. Despite this preload, no-load consumption is within
targets at approximately 140 mW at 265 VAC. The additional
margin of no-load consumption requirement can be achieved
by increasing the value of R4 to 2.2 kΩ or higher while still
maintaining output voltage well below the 9 V maximum
specification. Placement is left on the board for an optional
Zener clamp (VR1) to limit maximum output voltage under
open loop conditions, if required.
4
Rev. H 11/08
POWERINT [ POWER INTEGRATIONS, INC. ]
