This product is not recommended for new designs.
LNK520
LinkSwitch
Family
Energy Efficient, CV or CV/CC Switcher for
Very Low Cost Chargers and Adapters
Product Highlights
Cost Effective Linear/RCC Replacement
•
•
•
•
•
•
Lowest cost and component count, constant voltage (CV)
or constant voltage/constant current (CV/CC) solutions
Optimized for bias winding feedback
Up to 75% lighter power supply reduces shipping cost
Primary based CV/CC solution eliminates 10 to 20
secondary components for low system cost
Fully integrated auto-restart for short circuit and open loop
fault protection – saves external component costs
42 kHz operation with optimized switching characteristics
for significantly reduced EMI
Universal input range allows worldwide operation
Up to 70% reduction in power dissipation – reduces
enclosure size significantly
CV/CC output characteristic without secondary feedback
System level thermal and current limit protection
Meets all single point failure requirements with only one
additional bias capacitor
Controlled current in CC region provides inherent soft-start
Optional opto feedback improves output voltage accuracy
+
DC
Output
(V
O
)
Wide Range
HV DC Input
LinkSwitch
D
C
S
(a)
V
O
Example Characteristic
Min
Typ
(CV only) (CV/CC)
±10%
V
O
±5%
Much Higher Performance Over Linear/RCC
•
•
•
•
•
•
•
±24%*
For Circuit
Shown Above
I
O
(b)
±24%*
With Optional
Secondary Feedback**
I
O
EcoSmart ™ – Extremely Energy Efficient
•
•
•
*Estimated tolerance achievable in high volume production (external
components with
±7.5%
transformer inductance tolerance included).
**See Optional Secondary Feedback section.
PI-3853-030404
PI-3577-080603
Consumes <300 mW at 265 VAC input with no load
Meets California Energy Commission (CEC), Energy Star,
and EU requirements
No current sense resistors – maximizes efficiency
Figure 1. (a) Typical Application – not a Simplified Circuit and
(b) Output Characteristic Tolerance Envelopes.
Output Power Table
1
PRODUCT
LNK520
P or G
4
Applications
•
•
•
•
•
230 VAC ±15%
Min
2
3.3 W
4.2 W
Typ
2
4W
5.5 W
85-265 VAC
Min
2
2.4 W
Typ
2
3W
Linear transformer replacement in all ≤3 W applications
Chargers for cell phones, cordless phones, PDAs, digital
cameras, MP3/portable audio devices, shavers, etc.
Home appliances, white goods and consumer electronics
Constant output current LED lighting applications
TV standby and other auxiliary supplies
No-Load
Input
Power
<300 mW
2.9 W 3.5 W <500 mW
3
Description
LinkSwitch™ is specifically designed to replace low power
linear transformer/RCC chargers and adapters at equal or
lower system cost with much higher performance and energy
efficiency. LNK520 is equivalent to LNK500 but optimized for
use with bias winding feedback and has improved switching
characteristics for significantly reduced EMI. In addition, if bias
and output windings are magnetically well coupled, output
voltage load regulation can be improved. With efficiency of up
to 75% and <300 mW no-load consumption, a LinkSwitch
solution can save the end user enough energy over a linear
Table 1.
Notes: 1. Output power for designs in an enclosed
adapter measured at 50 °C ambient. 2. See Figure 1 (b) for
Min (CV only designs) and Typ (CV/CC charger designs) power
points identified on output characteristic. 3. Uses higher reflected
voltage transformer designs for increased power capability
–
see
Key Application Considerations section. 4. For lead-free package
options, see Part Ordering Information.
design to completely pay for the full power supply cost in less
than one year. LinkSwitch integrates a 700 V power MOSFET,
PWM control, high voltage start-up, current limit, and thermal
shutdown circuitry, onto a monolithic IC.
September 2016
LNK520
CONTROL
VC
ZC
SHUNT REGULATOR/
ERROR AMPLIFIER
-
+
0
DRAIN
INTERNAL
SUPPLY
1
SHUTDOWN/
AUTO-RESTART
+
5.6 V
4.7 V
-
÷
8
CURRENT
LIMIT
ADJUST
CURRENT LIMIT
COMPARATOR
-
+
5.6 V
I
FB
HYSTERETIC
THERMAL
SHUTDOWN
OSCILLATOR
D MAX
CLOCK
SAW
-
+
S
R
Q
PWM
COMPARATOR
IDCS
LOW
FREQUENCY
OPERATION
LEADING
EDGE
EDGE
BLANKING
RE
SOURCE
PI-2777-032503
Figure 2. Block Diagram.
Pin Functional Description
LNK520
DRAIN (D) Pin:
Power MOSFET drain connection. Provides internal operating
current for start-up. Internal current limit sense point for drain
current.
CONTROL (C) Pin:
Error amplifier and feedback current input pin for duty cycle
and current limit control. Internal shunt regulator connection to
provide internal bias current during normal operation. It is also
used as the connection point for the supply bypass and auto-
restart/compensation capacitor.
S
S
S
S
1
2
3
4
8
7
C
S
5
D
P Package (DIP-8B)
G Package (SMD-8B)
Figure 3. Pin Configuration.
PI-3790-121503
SOURCE (S) Pin:
Output MOSFET source connection for high voltage power
return. Primary side control circuit common and reference
point.
2
Rev. F 09/16
www.power.com
LNK520
LinkSwitch Functional Description
The duty cycle, current limit and operating frequency relation-
ships with CONTROL pin current are shown in Figure 4. Figure
5 shows a typical power supply schematic outline which is
used below to describe the LinkSwitch operation.
Power Up
During power up, as V
IN
is first applied (Figure 5), the CONTROL
pin capacitor C1 is charged through a switched high voltage
current source connected internally between the DRAIN and
CONTROL pins (see Figure 2). When the CONTROL pin voltage
reaches approximately 5.6 V relative to the SOURCE pin, the
high voltage current source is turned off, the internal control
circuitry is activated and the high voltage internal MOSFET
starts to switch. At this point, the charge stored on C1 is used
to supply the internal consumption of the chip.
Constant Current (CC) Operation
As the output voltage, and therefore the reflected voltage
across the transformer bias winding ramp up, the feedback
CONTROL current I
C
flowing through R1 increases. As shown
in Figure 4, the internal current limit increases with I
C
and
reaches I
LIM
when I
C
is equal to I
DCT
. The internal current limit vs.
I
C
characteristic is designed to provide an approximately con-
stant power supply output current as the power supply output
voltage rises.
Constant Voltage (CV) Operation
When I
C
exceeds I
DCS
, typically 2 mA (Figure 4), the maximum
duty cycle is reduced. At a value of I
C
that depends on power
supply input voltage, the duty cycle control limits LinkSwitch
peak current below the internal current limit value. At this point
the power supply transitions from CC to CV operation. With
minimum input voltage in a typical universal input design, this
transition occurs at approximately 30% duty cycle. Resistor R1
(Figure 5) is therefore initially selected to conduct a value of I
C
approximately equal to I
DCT
when V
OUT
is at the desired value at
the minimum power supply input voltage. The final choice of R1
is made when the rest of the circuit design is complete. When
the duty cycle drops below approximately 4%, the frequency is
reduced, which reduces energy consumption under light load
conditions.
Auto-Restart Operation
When a fault condition, such as an output short circuit or open
loop, prevents flow of an external current into the CONTROL
pin, the capacitor C1 discharges towards 4.7 V. At 4.7 V,
auto-restart is activated, which turns the MOSFET off and puts
the control circuitry in a low current fault protection mode. In
auto-restart, LinkSwitch periodically restarts the power supply
so that normal power supply operation can be restored when
the fault is removed.
Internal Current Limit
Auto-restart
I
LIM
CONTROL Current I
C
Duty Cycle
I
DCT
Auto-restart
77%
30%
3.8%
I
CD1
CONTROL Current I
C
Frequency
I
DCS
Auto-restart
f
OSC
f
OSC(low)
CONTROL Current I
C
PI-3579-031004
Figure 4. CONTROL Characteristics.
D1
R3
C3
C4
V
OUT
D3
V
IN
D2
LinkSwitch
D
I
C
C
S
R1
C1
R2
C2
PI-3578-021405
Figure 5. Power Supply Schematic outline.
3
www.power.com
Rev. F 09/16
LNK520
T1
D1
V
OUT
C4
VR1
RTN
R3
C3
85-265
VAC
D3
D2
R2
C2
R5
R4
LinkSwitch
LNK520
D
C
S
R1
U1
C1
PI-3703-030404
Figure 6. Power Supply Schematic Outline with Optocoupler Feedback, Providing Tight CV Regulation.
The characteristics described above provide an approximate
CV/CC power supply output without the need for secondary
side voltage or current feedback. The output voltage regulation
is influenced by how well the voltage across C2 tracks the
reflected output voltage. This tracking is influenced by the
coupling between transformer output and bias windings. Tight
coupling improves CV regulation and requires only a low value
for resistor R2. Poor coupling degrades CV regulation and
requires a higher value for R2 to filter leakage inductance spikes
on the bias winding voltage waveform. This circuitry, used
with standard transformer construction techniques, provides
much better output load regulation than a linear transformer,
making this an ideal power supply solution in many low power
applications. If even tighter load regulation is required, an
optocoupler configuration can be used while still employing the
constant output current characteristics provided by LinkSwitch.
When the power supply operates in the constant current
(CC) region, for example at start up and when charging a
battery, the output voltage is below the voltage feedback
threshold defined by U1 and VR1 and the optocoupler is fully
off. In this region, the circuit behaves exactly as previously
described with reference to Figure 5 where the voltage across
C2 and therefore the current flowing through R1 increases
with increasing output voltage and the LinkSwitch internal
current limit is adjusted to provide an approximate CC output
characteristic.
When the output reaches the voltage feedback threshold set
by U1 and VR1, the optocoupler turns on. Any further increase
in the power supply output voltage results in the U1 transistor
current increasing. The resulting increase in the LinkSwitch
CONTROL current reduces the duty cycle according to
Figure 4 and therefore, maintains the output voltage regulation.
Figure 7 shows the influence of optocoupler feedback on the
output characteristic. The envelope defined by the dashed lines
represent the worst-case power supply DC output voltage and
current tolerances (unit-to-unit and over the input voltage range)
if an optocoupler is not used. A typical example of an inherent
(without optocoupler) output characteristic is shown dotted.
This is the characteristic that would result if U1, R4, R5 and
VR1 were removed. The optocoupler feedback results in the
characteristic shown by the solid line. The load variation arrow
in Figure 7 represents the locus of the output characteristic
normally seen during a battery charging cycle. The two
characteristics are identical as the output voltage rises but then
separate as shown when the voltage feedback threshold is
reached. This is the characteristic seen if the voltage feedback
threshold is above the output voltage at the inherent CC to CV
transition point also indicated in Figure 7.
Optional Secondary Feedback
Figure 6 shows a typical power supply schematic outline
using LinkSwitch with optocoupler feedback to improve
output voltage regulation. On the primary side, the schematic
only differs from Figure 5 by the addition of optocoupler U1
transistor in parallel to R1.
On the secondary side, the addition of voltage sense circuit
components R4, VR1 and U1 LED provide the voltage
feedback signal. In the example shown, a simple Zener (VR1)
reference is used though more accurate references may be
employed for improved output voltage tolerancing and to
provide cable drop compensation, if required. Resistor R4
provides biasing for VR1. The regulated output voltage is
equal to the sum of the VR1 Zener voltage plus the forward
voltage drop of the U1 LED. Resistor R5 is an optional low
value resistor to limit U1 LED peak current due to output ripple.
Manufacturer’s specifications for U1 current and VR1 slope
resistance should be consulted to determine whether R5 is
required.
4
Rev. F 09/16
www.power.com
LNK520
Output Voltage
Voltage
feedback
threshold
Inherent
CC to CV
transition
point
Tolerance envelope
without optocoupler
Typical inherent
characteristic without
optocoupler
Characteristic with
optocoupler
Load variation
during battery
charging
Output Current
Figure 7. Influence of the Optocoupler on the Power Supply Output Characteristic.
PI-2788-092101
Output Voltage
Inherent
CC to CV
transition
point
V
O(MAX)
Tolerance envelope
without optocoupler
Typical inherent
characteristic without
optocoupler
Characteristic with
optocoupler
Power Supply peak
output power curve
Voltage
feedback
threshold
Load variation
during battery
charging
Characteristic observed with
load variation often applied during
laboratory bench testing
Output Current
Figure 8. Output Characteristic with Optocoupler Regulation (Reduced Voltage Feedback Threshold).
Figure 8 shows a case where the voltage feedback threshold
is set below the voltage at the inherent CC to CV transition
point. In this case, as the output voltage rises, the secondary
feedback circuit takes control before the inherent CC to CV
transition occurs. In an actual battery charging application,
this simply limits the output voltage to a lower value. However,
in laboratory bench tests, it is often more convenient to test
the power supply output characteristic starting from a low
output current and gradually increasing the load. In this case,
the optocoupler feedback regulates the output voltage until
the peak output power curve is reached as shown in Figure
8. Under these conditions, the output current will continue to
rise until the peak power point is reached and the optocoupler
turns off. Once the optocoupler is off, the CONTROL pin
feedback current is determined only by R1 and the output
PI-2790-092101
current therefore folds back to the inherent CC characteristic
as shown. Since this type of load transition does not normally
occur in a battery charger, the output current never overshoots
the inherent constant current value in the actual application.
In some applications it may be necessary to avoid any output
current overshoot, independent of the direction of load variation.
To achieve this goal, the minimum voltage feedback threshold
should be set at V
O(MAX)
. This will ensure that the voltage at
the CC to CV transition point of the inherent characteristic will
always occur below the voltage feedback threshold. However,
the output voltage tolerance is then increased, since the
inherent CV characteristic tolerance below V
O(MAX)
is added to
the tolerance of the optocoupler feedback circuit.
5
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Rev. F 09/16
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