LNK64x4-64x8
LinkSwitch-3
Family
Energy-Efficient, Accurate Primary-Side Regulation
CV/CC Switcher for Adapters and Chargers
Product Highlights
Dramatically Simplifies CV/CC Converters
•
Eliminates optocoupler and all secondary CV/CC control circuitry
•
Eliminates all control loop compensation circuitry
DC
Output
Advanced Performance Features
•
•
•
•
•
•
•
Compensates for transformer inductance tolerances
Compensates for input line voltage variations
Compensates for cable voltage drop
Compensates for external component temperature variations
Very accurate IC parameter tolerances using test trimming technology
Frequency jittering greatly reduces EMI filter cost
Programmable switching frequency up to 85 kHz to reduce trans-
former size
•
Minimum operation frequency fixed to improve transient load
response
Wide Range
High-Voltage
DC Input
LinkSwitch-3
D
FB
BP
S
PI-6907-020515
Figure 1.
Typical Application – Not a Simplified Circuit.
Advanced Protection/Safety Features
•
Auto-restart protection reduces power delivered by >90% for output
short-circuit and control loop faults (open and shorted components)
•
Hysteretic thermal shutdown – automatic recovery reduces power
Output Power Table
1,2,3,4
90-264 VAC
Product
5
LNK6404D / LNK6424D
LNK6405D / LNK6415D /
LNK6425D
LNK6406D / LNK6416D /
LNK6426D / LNK6436D /
LNK6446D
LNK6407D / LNK6417D /
LNK6427D
Product
5
supply returns from the field
•
Meets high-voltage creepage requirements between DRAIN and all
other pins both on the PCB and at the package
D (SO-8C) Package
Adapter
3.5 W
4.5 W
5.5 W
7.5 W
Open Frame
4.1 W
5.1 W
6.1 W
7.5 W
EcoSmart™– Energy Efficient
•
Easily meets all global energy efficiency regulations with no added
components
•
No-load consumption at 230 VAC input with bias winding <10 mW for
LNK64x4-LNK64x6 and <30 mW for LNK64x7-LNK64x8
•
ON/OFF control provides constant efficiency down to very light loads
– ideal for CEC regulations
•
No current sense resistors – maximizes efficiency
Green Package
Applications
•
Halogen free and RoHS compliant package
•
Chargers for cell/cordless phones, PDAs, MP3/portable audio
E (eSIP-7C) and
K (eSOP-12B) Packages
Adapter
Open Frame
9W
10 W
10 W
devices, adapters, etc.
Description
The LinkSwitch™-3 family of ICs dramatically simplifies low power CV/
CC charger designs by eliminating an optocoupler and secondary
control circuitry. The device introduces a revolutionary control
technique to provide very accurate output voltage and current
regulation, compen-sating for transformer and internal parameter
tolerances along with input voltage variations.
The device incorporates a 725 V power MOSFET, a novel ON/OFF control
state machine, a high-voltage switched current source for self biasing,
frequency jittering, cycle-by-cycle current limit and hysteretic thermal
shutdown circuitry onto a monolithic IC.
LNK6407K / LNK6417K /
LNK6427K
LNK6408K / LNK6418K /
LNK6428K / LNK6448K
LNK6408E / LNK6418E /
LNK6428E / LNK6448E
8.5 W
10 W
10 W
Table 1. Output Power Table.
Notes:
1. Assumes minimum input DC voltage >90 VDC, K
P
≥1 (Recommend K
P
≥1.15
for accurate CC regulation),
η
>78%, D
MAX
<55%.
2. Output power capability is reduced if a lower input voltage is used.
3. Minimum continuous power with adequate heat sink measured at 50
°C
ambient with device junction below 110
°C.
4. Assumes bias winding is used to supply BYPASS pin.
5. Package: D: SO-8C, E: eSIP-7C, K: eSOP-12B.
www.power.com
March 2016
This Product is Covered by Patents and/or Pending Patent Applications.
+
LNK64x4-64x8
BYPASS
(BP)
FB
OUT
6V
5V
REGULATOR
6V
+
+
-
DRAIN
(D)
FEEDBACK
(FB)
V
TH
-
D
Q
STATE
MACHINE
I
LIM
Reset
V
ILIMIT
Drive
t
SAMPLE-OUT
CABLE DROP
COMPENSATION
6.5 V
INDUCTANCE
CORRECTION
V
ILIMIT
DC
MAX
FAULT
AUTO-RESTART
OPEN-LOOP
THERMAL
SHUTDOWN
t
SAMPLE-OUT
t
SAMPLE-INPUT
OSCILLATOR
+
FB
t
SAMPLE-INPUT
DC
MAX
SAMPLE
DELAY
SOURCE
(S)
V
ILIMIT
LEADING
EDGE
BLANKING
SOURCE
(S)
CONSTANT
CURRENT
I
LIM
-
Current Limit
Comparator
PI-6660-020515
Figure 2
Functional Block Diagram.
Pin Functional Description
DRAIN (D) Pin:
This pin is the power MOSFET drain connection. It provides
internal operating current for both start-up and steady-state
operation.
BYPASS (BP) Pin:
This pin is the connection point for an external 1
mF
bypass capacitor
for the internally generated 6 V supply.
FEEDBACK (FB) Pin:
E Package
(eSIP-7C)
Exposed Pad
(On Back Side)
Internally
Connected to
SOURCE Pin
D Package (SO-8C)
8S
7S
6S
FB 1
12345 7
D
S
NC
NC
BP
FB
BP 2
During normal operation, switching of the power MOSFET is
controlled by this pin. This pin senses the AC voltage on the
bias winding. This control input regulates both the output
voltage in CV mode and output current in CC mode based on
the flyback voltage of the bias winding. The internal induc-
tance correction circuit uses the forward voltage on the bias
winding to sense the bulk capacitor voltage.
SOURCE (S) Pin:
D4
5S
Exposed Pad (On Bottom)
Internally Connected to
SOURCE Pin
K Package
(eSOP-12B)
12 S
11 S
10 S
9S
8S
7S
PI-6906-020515
FB 1
BP 2
NC 3
NC 4
D6
This pin is internally connected to the output MOSFET source
for high-voltage power and control circuit common returns.
Figure 3.
Pin Configuration.
2
Rev. C 03/16
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LNK64x4-64x8
LinkSwitch-3 Functional Description
The LinkSwitch-3 combines a high-voltage power MOSFET switch with
a power supply controller in one device. It uses an ON/OFF control to
regulate the output voltage. In addition, the switching frequency is
modulated to regulate the output current to provide a constant current
characteristic. The LinkSwitch-3 controller consists of an oscillator,
feedback (sense and logic) circuit, 6 V regulator, over-temperature
protection, frequency jittering, current limit circuit, leading-edge
blanking, inductance correction circuitry, frequency control for
constant current regulation and ON/OFF state machine for CV control.
Inductance Correction Circuitry
If the primary magnetizing inductance is either too high or low the
converter will automatically compensate for this by adjusting the
oscillator frequency. Since this controller is designed to operate in
discontinuous-conduction mode the output power is directly
proportional to the set primary inductance and its tolerance can be
completely compensated with adjustments to the switching
frequency.
Constant Current (CC) Operation
As the output voltage and therefore the flyback voltage across the
bias winding ramps up, the FEEDBACK pin voltage increases. The
switching frequency is adjusted as the FEEDBACK pin voltage increases
to provide a constant output current regulation. The constant current
circuit and the inductance correction circuit are designed to operate
concurrently in the CC region.
Constant Voltage (CV) Operation
As the FEEDBACK pin approaches 2 V from the constant current
regulation mode, the power supply transitions into CV operation.
The switching frequency at this point is at its maximum value,
corresponding to the peak power point of the CV/CC characteristic.
The controller regulates the FEEDBACK pin voltage to remain at
FEEDBACK pin threshold (V
FBTH
) using an ON/OFF state-machine.
The FEEDBACK pin voltage is sampled 2.5
ms
after the turn-off of the
high-voltage switch.
At light loads the current limit is also reduced to decrease the
transformer flux density and the FEEDBACK pin sampling is done
earlier.
Output Cable Compensation
This compensation provides a constant output voltage at the end of
the cable over the entire load range in CV mode. As the converter
load increases from no-load to the peak power point (transition point
between CV and CC) the voltage drop introduced across the output
cable is compensated by increasing the FEEDBACK pin reference
voltage. The controller determines the output load and therefore the
correct degree of compensation based on the output of the state
machine. The amount of cable drop compensation is determined by
the third digit in the device part number.
Auto-Restart and Open-Loop Protection
In the event of a fault condition such as an output short or an
open-loop condition the LinkSwitch-3 enters into an appropriate
protection mode as described below.
In the event the FEEDBACK pin voltage during the flyback period falls
below 0.7 V before the FEEDBACK pin sampling delay (~2.5
ms)
for a
duration in excess of ~300 ms (auto-restart on-time (t
AR-ON
) the
converter enters into auto-restart, wherein the power MOSFET is
disabled for 1500 ms. The auto-restart alternately enables and
disables the switching of the power MOSFET until the fault condition
is removed.
In addition to the conditions for auto-restart described above,
if the sensed FEEDBACK pin current during the forward period of the
conduction cycle (switch “on” time) falls below 120
mA,
the converter
annunciates this as an open-loop condition (top resistor in potential
divider is open or missing) and reduces the auto-restart time from
300 ms to approximately 6 clock cycles (90
ms),
whilst keeping the
disable period of 2 seconds.
Over-Temperature Protection
The thermal shutdown circuitry senses the die temperature. The
threshold is set at 142 °C typical with a 60 °C hysteresis. When the
die temperature rises above this threshold (142 °C) the power
MOSFET is disabled and remains disabled until the die temperature
falls by 60 °C, at which point the MOSFET is re-enabled.
Current Limit
The current limit circuit senses the current in the power MOSFET.
When this current exceeds the internal threshold (I
LIMIT
), the power
MOSFET is turned off for the remainder of that cycle. The leading
edge blanking circuit inhibits the current limit comparator for a short
time (t
LEB
) after the power MOSFET is turned on. This leading edge
blanking time has been set so that current spikes caused by
capacitance and rectifier reverse recovery time will not cause
premature termination of the MOSFET conduction. The LinkSwitch-3
also contains a “di/dt” correction feature to minimize CC variation across
the input line range.
6 V Regulator
The 6 V regulator charges the bypass capacitor connected to the
BYPASS pin to 6 V by drawing a current from the voltage on the
DRAIN, whenever the MOSFET is off. The BYPASS pin is the internal
supply voltage node. When the MOSFET is on, the device runs off of
the energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows the LinkSwitch-3 to
operate continuously from the current drawn from the DRAIN pin
however for the best no-load input power, the BYPASS pin should be
supplied current of I
S1
from the bias winding at no-load conditions.
A bypass capacitor value of 1
mF
is sufficient for both high frequency
decoupling and energy storage.
3
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Rev. C 03/16
LNK64x4-64x8
Applications Example
Circuit Description
This circuit shown in Figure 4 is configured as a primary-side
regulated flyback power supply utilizing the LNK6448K. With an
average efficiency of 78% and <30 mW no-load input power this
design easily exceeds the most stringent current energy efficiency
requirements.
Input Filter
AC input power is rectified by bridge BR1. The rectified DC is filtered
by the bulk storage capacitors C1 and C2. Inductors L2 and L3,
together with C1 and C2 form a pi (π) filter, which attenuates
conducted differential-mode EMI noise. This configuration along with
Power Integrations transformer E-Shield™ technology allows this
design to meet EMI standard EN55022 class B with good margin
without requiring a Y capacitor, even with the output connected to
safety earth ground. A ferrite bead for L3 is sufficient especially
when the output of the supply is floating. Fuse F1 provides protection
against catastrophic failure. NTC (Negative Thermal Coefficient)
thermistor RT1 is used to limit the rush current to below the peak
specification of BR1 during start-up especially at high-line input
voltage. High-line results in the highest current into C1 and C2. F1
and RT1 can be replaced by a single fusible resistor. If the reduction
in efficiency is acceptable, a bridge with a higher I
FSM
rating may also
allow removal of RT1. If a fusible resistor is selected, use a
flameproof type. It should be suitably rated (typically a wire wound
type) to withstand the instantaneous dissipation while the input
capacitors charge when first connected to the AC line.
LNK6448K Primary
The LNK6448K device (U1) incorporates the power switching device,
oscillator, CC/CV control engine, start-up, and protection functions.
The integrated 725 V MOSFET provides a large drain voltage margin
in universal input AC applications, increasing reliability and also
reducing the output diode voltage stress by allowing a greater
transformer turns ratio. The device is completely self-powered from
the BYPASS pin and decoupling capacitor C7. For the LNK64xx
devices, there are 4 options for different amount of cable drop
compensation determined by the third digit in the device part
number. Table 2 shows the amount of compensation for each device.
The LNK644x devices do not provide cable drop compensation.
The optional bias supply formed by D3 and C8 provides the operating
current for U1 via resistor R8. This reduces the no-load consumption
from ~200 mW to <30 mW and also increases light load efficiency.
The rectified and filtered input voltage is applied to one side of the
primary winding of T1. The other side of the transformer’s primary
winding is driven by the integrated MOSFET in U1. The leakage
inductance drain voltage spike is limited by an RCD-R clamp
consisting of D2, R3, R11, and C6.
Output Rectification
The secondary of the transformer is rectified by D1, a 10 A, 45 V
Schottky barrier type for higher efficiency, and filtered by C3, L1 and
C4. If lower efficiency is acceptable then this can be replaced with a
5 A PN junction diode for lower cost. In this application C3 and C4
are sized to meet the required output voltage ripple specification with
a ferrite bead L1, which eliminates the high switching noise on the
output. A pre-load resistor R2 is used to meet the regulation
specification. If the battery self-discharge is required, the pre-load
resistor can be replaced with a series resistor and Zener network.
Output Regulation
The LNK64xx family of devices regulates the output using ON/OFF
control in the constant voltage (CV) regulation region of the output
characteristic and frequency control for constant current (CC)
regulation. The feedback resistors (R6 and R7) were selected using
standard 1% resistor values to center both the nominal output
voltage and constant current regulation thresholds.
R10
4.7 kΩ
C5
R1 1 nF
10
Ω
50 V
R11
200 kΩ
C6
470 pF
250 V
6
FL2
4
T1
EPC17
L2
1 mH
BR1
B10S-G
1000 V
5
FL1
L1
Ferrite Bead
(3.5
×
7.6 mm)
D1
C3
SBR1045SP5-13 680
µF
10 V
C4
680
µF
10 V
R2
2.55 kΩ
1%
5 V, 2 A
J1-1
RTN
J1-4
PI-7309-020515
R3
150
Ω
D2
S1ML
F1
1A
RT1
10
Ω
C1
10
µF
400 V
C2
15
µF
400 V
LinkSwitch-3
U1
LNK6448K
FB
BP
6.0
3
5.0
115 VAC
230 VAC
Output Voltage (V)
C8
10
µF
25 V
PI-7209-020515
t
O
L
TP1
90 - 265
VAC
R6
44.2 kΩ
1%
R5
2.4
Ω
D3
RS1ML
4.0
3.0
2.0
1.0
N
TP2
D
L3
Ferrite Bead
(3.5
×
7.6 mm)
S
C7
1
µF
50 V
R7
10 kΩ
1%
R8
2.37 kΩ
1%
0.0
0
0.5
1
1.5
2
2.5
3
Output Current (A)
Figure 4.
Energy Efficient USB Charger Power Supply (78% Average Efficiency, <30 mW No-load Input Power).
4
Rev. C 03/16
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LNK64x4-64x8
Key Application Considerations
Output Power Table
The data sheet maximum output power table (Table 1) repre- sents
the maximum practical continuous output power level that can be
obtained under the following assumed conditions:
LinkSwitch-3 Layout Considerations
Circuit Board Layout
LinkSwitch-3 is a highly integrated power supply solution that
integrates on a single die, both, the controller and the high- voltage
MOSFET. The presence of high switching currents and voltages
together with analog signals makes it especially important to follow
good PCB design practice to ensure stable and trouble free operation
of the power supply. See Figure 5 for a recommended circuit board
layout for LinkSwitch-3.
When designing a printed circuit board for the LinkSwitch-3 based
power supply, it is important to follow the following guidelines:
Single Point Grounding
Use a single point (Kelvin) connection at the negative terminal of the
input filter capacitor for the LinkSwitch-3 SOURCE pin and bias
winding return. This improves surge capabilities by returning surge
currents from the bias winding directly to the input filter capacitor.
Bypass Capacitor
The BYPASS pin capacitor should be located as close as possible to
the SOURCE and BYPASS pins.
Feedback Resistors
Place the feedback resistors directly at the FEEDBACK pin of the
LinkSwitch-3 device. This minimizes noise coupling.
Thermal Considerations
The copper area connected to the SOURCE pins provides the
LinkSwitch-3 heat sink. A good estimate is that the LinkSwitch-3 will
dissipate 10% of the output power. Provide enough copper area to
keep the SOURCE pin temperature below 110 °C is recommended to
provide margin for part to part R
DS(ON)
variation.
Secondary Loop Area
To minimize leakage inductance and EMI the area of the loop connecting
the secondary winding, the output diode and the output filter capacitor
should be minimized. In addition, sufficient copper area should be
provided at the anode and cathode terminal of the diode for heat
sinking. A larger area is preferred at the quiet cathode terminal.
A large anode area can increase high frequency radiated EMI.
Electrostatic Discharge Spark Gap
A spark gap is created between the output and the AC input. The
spark gap directs ESD energy from the secondary back to the AC
input. The trace from the AC input to the spark gap electrode should
be spaced away from other traces to prevent unwanted arcing
occurring and possible circuit damage.
Drain Clamp Optimization
LinkSwitch-3 senses the feedback winding on the primary-side to
regulate the output. The voltage that appears on the feedback
winding is a reflection of the secondary winding voltage while the
internal MOSFET is off. Therefore any leakage inductance induced
ringing can affect output regulation. Optimizing the drain clamp to
1.
Assumes minimum input DC voltage >90 VDC, K
P
≥1 (Recom-
2.
Output power capability is reduced if a lower input voltage
is used.
50
°C
ambient with device junction below
110
°C.
4.
Assumes bias winding is used to supply BYPASS pin.
mend K
P
≥1.15 for accurate CC regulation),
η
>78%, D
MAX
<55%.
3.
Minimum continuous power with adequate heat sink measured at
Output Tolerance
LinkSwitch-3 provides an overall output tolerance (including
line, component variation and temperature) of ±5% for the output
voltage in CV operation and ±10% for the output current during CC
operation over a junction temperature range of 0 °C to 110 °C.
BYPASS Pin Capacitor Selection
A 1
mF
BYPASS pin capacitor is recommended. The capacitor
voltage rating should be greater than 7 V. The capacitor’s dielectric
material is not important but tolerance of capacitor should be ≤
±50%. The capacitor must be physically located adjacent to the
LinkSwitch-3 BYPASS pin.
Cable Drop Compensation
The amount of output cable compensation is determined by the third
digit in the device part number. Table 2 shows the amount of
compensation for each LinkSwitch-3 device.
The output voltage that is entered into PIXls design spreadsheet is
the voltage at the end of the output cable when the power supply is
delivering maximum power. The output voltage at the terminals of
the supply is the value measured at the end of the cable multiplied by
the output voltage change factor.
LinkSwitch-3 Output Cable Voltage
Drop Compensation
Device
LNK640x
LNK641x
LNK642x
LNK643x
LNK644x
Table 2.
Output Voltage Change Factor (±1%)
1.02
1.04
1.06
1.08
1.01
Cable Compensation Change Factor vs. Device.
5
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Rev. C 03/16
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