LNK3202/3204-7, LNK3294 & LNK3296
LinkSwitch-TN2
Family
Highly Energy Efficient Off-line Switcher IC with Integrated System Level
Protection for Low Component-Count Power Supplies
Product Highlights
Highest Performance and Design Flexibility
•
•
•
•
Supports buck, buck-boost and flyback topologies
Excellent load and line regulation
Selectable device current limit
66 kHz operation with accurate current limit
• Allows the use of low-cost off-the-shelf inductors
• Reduces size and cost of magnetics and output capacitor
•
Frequency jittering reduces EMI filter complexity
•
Pin-out simplifies PCB heat sinking
FB
BP/M
S
Wide Range
High-Voltage
DC Input
+
D
LinkSwitch-TN2
+
DC
Output
PI-7841-041816
Enhanced Safety and Reliability Features
Auto-restart for short-circuit and open loop faults
Output overvoltage protection (OVP)
Line input overvoltage protection (OVL)
Hysteretic over-temperature protection (OTP)
Extended creepage between DRAIN pin and all other pins improves
field reliability
• 725 V MOSFET rating for excellent surge withstand
• 900 V MOSFET rating series for industrial or extra safety margin
•
•
•
•
•
Figure 1.
Typical Buck Converter Application (See Application Examples
Section for Other Circuit Configurations).
EcoSmart
™
– Extremely Energy Efficient
•
•
•
•
•
•
•
•
Standby supply current <100
μA
On/Off control provides constant efficiency over a wide load range
Easily meets all global energy efficiency regulations
No-load consumption <30 mW with external bias
Appliances
Metering
Smart LED drivers and industrial controls
IOT, home and building automation
Figure 2.
Package Options. P: PDIP-8C, G: SMD-8C, D: SO-8C.
Output Current Table
1
725 V MOSFET
Product
4
LNK3202P/G/D
LNK3204P/G/D
LNK3205P/G/D
LNK3206P/G/D
LNK3207P/G/D
Product
4
LNK3294P/G
LNK3296P/G
230 VAC ±15%
MDCM
2
63 mA
120 mA
175 mA
225 mA
360 mA
CCM
3
80 mA
170 mA
270 mA
360 mA
575 mA
85-265 VAC
MDCM
2
63 mA
120 mA
175 mA
225 mA
360 mA
CCM
3
80 mA
170 mA
270 mA
360 mA
575 mA
Applications
Description
The LinkSwitch™-TN2 family of ICs for non-isolated off-line power
supplies provides dramatically improved performance compared to
traditional linear or cap-dropper solutions. Designs using the highly
integrated LinkSwitch-TN2 ICs are more flexible and feature increased
efficiency, comprehensive system level protection and higher reliability.
The device family supports buck, buck-boost and flyback converter
topologies. Each device incorporates a 725 V power MOSFET, oscillator,
On/Off control for highest efficiency at light load, a high-voltage
switched current source for self-biasing, frequency jittering, fast
(cycle-by-cycle) current limit, hysteretic thermal shutdown, and output
and input overvoltage protection circuitry onto a monolithic IC.
LinkSwitch-TN2 ICs consume very little current in standby resulting in
power supply designs that meet all no-load and standby specifications
worldwide. MOSFET current limit modes can be selected through the
BYPASS pin capacitor value. The high current limit level provides
maximum continuous output current while the low level permits using
very low cost and small surface mount inductors. A full suite of
protection features enables safe and reliable power supplies protecting
the device and the system against input and output overvoltage faults,
device over-temperature faults, lost regulation, and power supply output
overload or short-circuit faults.
900 V MOSFET
230 VAC ±15%
MDCM
2
120 mA
225 mA
CCM
3
170 mA
360 mA
85-265 VAC
MDCM
2
120 mA
225 mA
CCM
3
170 mA
360 mA
Table 1. Output Current Table.
Notes:
1. Typical output current in a non-isolated buck converter with devices operating
at default current limit and adequate heat sinking. Output power capability
depends on respective output voltage and thermal requirements. See Key
Applications Considerations Section for complete description of assumptions,
including fully discontinuous conduction mode (DCM) operation.
2. Mostly discontinuous conduction mode.
3. Continuous conduction mode.
4. Packages: P: PDIP-8C, G: SMD-8C, D: SO-8C.
The device family is available in three different packages: PDIP-8C,
SO-8C, and SMD-8C (725 V PNs only).
www.power.com
September 2020
This Product is Covered by Patents and/or Pending Patent Applications.
LinkSwitch-TN2
BYPASS
(BP/M)
REGULATOR
5.0 V
I
FBSD
I
FB
5.2 V
AUTO-RESTART
COUNTER
CLOCK
RESET
5.0 V
4.5 V
FAULT
PRESENT
BYPASS PIN
CAPACITOR
DETECT
+
-
DRAIN
(D)
OVL
BYPASS PIN
UNDERVOLTAGE
CURRENT LIMIT
COMPARATOR
+
-
VI
LIMIT
JITTER
CLOCK
DC
MAX
OSCILLATOR
THERMAL
SHUTDOWN
S
FEEDBACK
(FB)
2.0 V -V
T
R
Q
Q
LEADING
EDGE
BLANKING
OVP
DETECT
SOURCE
(S)
PI-7879-020819
Figure 3.
Functional Block Diagram.
Pin Functional Description
DRAIN (D) Pin:
Power MOSFET drain connection. Provides internal operating current
for both start-up and steady-state operation.
BYPASS (BP/M) Pin:
This pin has multiple functions:
• It is the connection point for an external bypass capacitor for the
internally generated 5.0 V supply.
• It is a mode selector for the current limit value, depending on the
value of the capacitance added. Use of a 0.1
μF
capacitor results
in the standard current limit value. Use of a 1
μF
capacitor results
in the current limit being reduced, allowing design with lowest cost
surface mount buck chokes.
•
It provides a shutdown function. When the current into the BYPASS
pin exceeds I
BPSD
for a time equal to 2 to 3 cycles of the internal
oscilator (f
OSC
), the device enters auto-restart. This can be used to
provide an output overvoltage protection function with external
circuitry.
FEEDBACK (FB) Pin:
During normal operation, switching of the power MOSFET is con-
trolled by the FEEDBACK pin. MOSFET switching is terminated when
a current greater than I
FB
(49
μA)
is delivered into this pin. Line
overvoltage protection is detected when a current greater than I
FBSD
(670
μA)
is delivered into this pin for 2 consecutive switching cycles.
SOURCE (S) Pin:
This pin is the power MOSFET source connection. It is also the
ground reference for the BYPASS and FEEDBACK pins.
P Package (DIP-8C)
G Package (SMD-8C)
BP/M
FB
1
2
8
7
6
D
4
5
S
S
S
S
D Package (SO-8C)
BP/M
FB
1
2
8
7
6
D
4
5
S
S
S
S
PI-7842-022416
Figure 4.
Pin Configuration.
2
Rev. M 10/20
www.power.com
LinkSwitch-TN2
PI-3660-081303
LinkSwitch-TN2 Functional Description
LinkSwitch-TN2 combines a high-voltage power MOSFET switch with
a power supply controller in one device. Unlike conventional PWM
(pulse width modulator) controllers, LinkSwitch-TN2 uses a simple
ON/OFF control to regulate the output voltage. The LinkSwitch-TN2
controller consists of an oscillator, feedback (sense and logic) circuit,
5.0 V regulator, BYPASS pin undervoltage circuit, over-temperature
protection, line and output overvoltage protection, frequency jittering,
current limit circuit, leading edge blanking and a 725 V or 900 V
power MOSFET. The LinkSwitch-TN2 incorporates additional circuitry
for auto-restart.
Oscillator
The typical oscillator frequency is internally set to an average of f
OSC
(66 kHz). Two signals are generated from the oscillator: the maximum
duty cycle signal (DC
(MAX)
) and the clock signal that indicates the
beginning of each cycle.
The LinkSwitch-TN2 oscillator incorporates circuitry that introduces a
small amount of frequency jitter, typically 4 kHz peak-to-peak, to
minimize EMI emission. The modulation rate of the frequency jitter is
set to 1 kHz to optimize EMI reduction for both average and quasi-
peak emissions. The frequency jitter should be measured with the
oscilloscope triggered at the falling edge of the DRAIN waveform.
The waveform in Figure 5 illustrates the frequency jitter of the
LinkSwitch-TN2.
Soft-Start
At power-up or during a restart attempt in auto-restart, the device
applies a soft-start by temporarily reducing the oscillator frequency
to fOSC(SS) (typically 33 kHz). Soft-start terminates either after 256
switching cycles or if the output voltage reaches regulation.
Feedback Input Circuit
The feedback input circuit at the FEEDBACK pin consists of a low
impedance source follower output set at V
FB
(2.0 V). When the
current delivered into this pin exceeds I
FB
(49
μA),
a low logic level
(disable) is generated at the output of the feedback circuit. This
output is sampled at the beginning of each cycle on the rising edge
of the clock signal. If high, the power MOSFET is turned on for that
cycle (enabled), otherwise the power MOSFET remains off (disabled).
The sampling is done only at the beginning of each cycle. Subse-
quent changes in the FEEDBACK pin voltage or current during the
remainder of the cycle do not impact the MOSFET enable/disable
status. If a current greater than I
FBSD
is injected into the feedback pin
while the MOSFET is enabled for at least two consecutive cycles the
part will stop switching and enter auto-restart off-time. Normal
switching resumes after the auto-restart off-time expires. This
shutdown function allows implementing line overvoltage protection in
flyback converters (see Figure 6). The current into the FEEDBACK pin
should be limited to less than 1.2 mA.
5.0 V Regulator and 5.2 V Shunt Voltage Clamp
The 5.0 V regulator charges the bypass capacitor connected to the
BYPASS pin to V
BP
by drawing a current from the voltage on the
DRAIN, whenever the MOSFET is off. The BYPASS pin is the internal
supply voltage node for the LinkSwitch-TN2. When the MOSFET is
on, the LinkSwitch-TN2 runs off of the energy stored in the bypass
capacitor. Extremely low power consumption of the internal circuitry
allows the LinkSwitch-TN2 to operate continuously from the current
drawn from the DRAIN pin. A bypass capacitor value of 0.1
μF
is
sufficient for both high frequency decoupling and energy storage.
In addition, there is a shunt regulator clamping the BYPASS pin
at
V
BP(SHUNT)
(5.2 V) when current is provided to the BYPASS pin through
an external resistor. This facilitates powering of LinkSwitch-TN2
externally through a bias winding to decrease the no-load consump-
tion to about 10 mW (flyback). The device stops switching instantly
600
500
V
DRAIN
400
300
200
100
0
68 kHz
64 kHz
0
Time (µs)
Figure 5.
Frequency Jitter.
20
and enters auto-restart when a current ≥I
BPSD
is delivered into the
BYPASS pin. Adding an external Zener diode from the output voltage
to the BYPASS pin allows implementing an hysteretic OVP function in
a flyback converter (see Figure 6). The current into the BYPASS pin
should be limited to less than 16 mA.
BYPASS Pin Undervoltage
The BYPASS pin undervoltage circuitry disables the power MOSFET
when the BYPASS pin voltage drops below V
BP
–V
BPH
(approximately 4.5 V).
Once the BYPASS pin voltage drops below this threshold, it must rise
back to V
BP
to enable (turn-on) the power MOSFET.
Over-Temperature Protection
The thermal shutdown circuitry senses the die temperature. The
threshold is set at T
SD
(142 °C typical) with a 75 °C (T
SDH
) hysteresis.
When the die temperature rises above T
SD
the power MOSFET is
disabled and remains disabled until the die temperature falls to
T
SD
–T
SDH
, at which point it 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 capaci-
tance and rectifier reverse recovery time will not cause premature
termination of the switching pulse. Current limit can be selected using
the BYPASS pin capacitor (0.1
μF
for normal current limit / 1
μF
for
reduced current limit). LinkSwitch-TN2 selects between normal and
reduced current limit at power-up prior to switching.
Auto-Restart
In the event of a fault condition such as output overload, output
short, or an open-loop condition, LinkSwitch-TN2 enters into
auto-restart operation. An internal counter clocked by the oscillator
gets reset every time the FEEDBACK pin is pulled high. If the
FEEDBACK pin is not pulled high for t
AR(ON)
(50 ms), the power
MOSFET switching is disabled for a time equal to the auto-restart
off-time. The first time a fault is asserted the off-time is 150 ms
(t
AR(OFF)
First Off Period). If the fault condition persists, subsequent
off-times are 1500 ms long (t
AR(OFF)
Subsequent Periods). The
auto-restart alternately enables and disables the switching of the
power MOSFET until the fault condition is removed. The auto-restart
counter is gated by the switch oscillator.
3
www.power.com
Rev. M 10/20
LinkSwitch-TN2
Hysteretic Output Overvoltage Protection
The output overvoltage protection provided by the LinkSwitch-TN2 IC
uses auto-restart that is triggered by a current >I
BPSD
into the BYPASS
pin. In addition to an internal filter, the BYPASS pin capacitor forms
an external filter providing noise immunity from inadvertent triggering.
For the bypass capacitor to be effective as a high frequency filter, the
capacitor should be located as close as possible to the SOURCE and
BYPASS pins of the device.
The OVP function can be realized in a flyback converter by connect-
ing a Zener diode from the output supply to the BYPASS pin. The
circuit example shown in Figure 6 describes a simple method for
implementing the output overvoltage protection. Adding additional
filtering can be achieved by inserting a low value (10
Ω
to 47
Ω)
resistor in series with the OVP Zener diode. The resistor in series
with the OVP Zener diode also limits the maximum current into the
BYPASS pin. The current should be limited to less than 16 mA.
During a fault condition resulting from loss of feedback, the output
voltage will rapidly rise above the nominal voltage. A voltage at the
output that exceeds the sum of the voltage rating of the Zener diode
connected from the output to the BYPASS pin and bypass voltage, will
cause a current in excess of I
BPSD
injected into the BYPASS pin, which
will trigger the auto-restart and protect the power supply from
overvoltage.
Line Overvoltage Protection
In a flyback converter LinkSwitch-TN2 can sense indirectly the DC bus
overvoltage condition during the power MOSFET on-time by monitor-
ing the current flowing into the FEEDBACK pin depending on circuit
configuration. Figure 7 shows one possible circuit implementation.
During the MOSFET on-time, the voltage across the secondary
winding is proportional to the voltage across the primary winding.
The current flowing through emitter and base of transistor Q3 is
therefore representing V
BUS
. Indirect line sensing minimizes power
dissipation and is used for line OV protection. The LinkSwitch-TN2
will go into auto- auto-restart mode if the FEEDBACK pin current
exceeds the line overvoltage threshold current I
FBSD
for at least 2
consecutive switching cycles.
In order to have accurate line OV threshold voltage and also for good
efficiency, regulation performance and stability, the transformer
leakage inductance should be minimized. Low leakage will minimize
ringing on the secondary winding and provide accurate line OVP
sampling. In some designs, a RC snubber across the rectifier diode
may be needed to damp the ringing at the secondary winding when
line voltage is sampled.
T1
V
O
+
V
BUS
D
FB
BP
S
+
V
OV
= V
BP
+ V
DOVP
R
BP
D
OVP
PI-8024-092916
LinkSwitch-TN2
C
BP
Figure 6. Non-Isolated Flyback Converter with Output Overvoltage Protection.
T1
n:1
R3*
n:1
V
BUS
Q3
FB
BP
S
V
O
+
VR3
D3
R4**
+
V
OV
= (V
VR3
+ V
D3
+ V
BE(Q3)
– V
BP
+ V
R3
)
×
n + V
DS(ON)
*R3 limits the current into the FEEDBACK pin.
A maximum current of 120% of I
FBSD
is
recommended.
**R4 is a pull-down resistor for Q3 to avoid
inadvertant triggering. 2 kΩ is a typical starting point
D
LinkSwitch-TN2
c
BP
PI-8025-092616
Figure 7. Line-Sensing for Overvoltage Protection by using FEEDBACK Pin.
4
Rev. M 10/20
www.power.com
LinkSwitch-TN2
Applications Example
R5
26.7 kΩ
R1
11.8 kΩ
1%
R3
2.49 kΩ
1%
C3
10
µF
25 V
L1
1 mH
280 mA
D2
1N4005GP
RF1
8.2
Ω
2W
D3
1N4007
D4
1N4007
L2
1 mH
FB
D
BP/M
S
C1
100 nF
12 V,
120 mA
85-265
VAC
C4
4.7
µF
400 V
C5
LinkSwitch-TN2
LNK3204
4.7
µF
400 V
D1
UF4005
C2
100
µF
16 V
R4
3.3 kΩ
1%
RTN
PI-7857-092616
Figure 8.
Universal Input, 12 V, 120 mA Constant Voltage Power Supply using LinkSwitch-TN2.
A 1.44 W Universal Input Buck Converter
The circuit shown in Figure 8 is a typical implementation of a 12 V,
120 mA non-isolated power supply used in appliance control such as
rice cookers, dishwashers or other white goods. This circuit may also
be applicable to other applications such as night-lights, LED drivers,
electricity meters, and residential heating controllers, where a
non-isolated supply is acceptable.
The input stage comprises fusible resistor RF1, diodes D3 and D4,
capacitors C4 and C5, and inductor L2. Resistor RF1 is a flame proof,
fusible, wire wound resistor. It accomplishes several functions:
A.
Inrush current limitation to safe levels for rectifiers D3 and D4;
B. Differential mode noise attenuation;
C. Acts as an input fuse in the event any other component fails
short-circuit (component fails safely open-circuit without emitting
smoke, fire or incandescent material).
The power processing stage is formed by the LinkSwitch-TN2,
freewheeling diode D1, output choke L1, and the output capacitor C2.
The LNK3204 was selected such that the power supply operates in
the mostly discontinuous-mode (MDCM). Diode D1 is an ultrafast
diode with a reverse recovery time (t
RR
) of approximately 75 ns,
acceptable for MDCM operation. For continuous conduction mode
(CCM) designs, a diode with a t
RR
of
≤35
ns is recommended.
Inductor L1 is a standard off-the-shelf inductor with appropriate RMS
current rating (and acceptable temperature rise). Capacitor C2 is the
output filter capacitor; its primary function is to limit the output
voltage ripple. The output voltage ripple is a stronger function of the
ESR of the output capacitor than the value of the capacitor itself.
Optional resistor R5 supplies the BYPASS pin externally for signifi-
cantly lower no-load input power and increased efficiency over all
load conditions.
To a first order, the forward voltage drops of D1 and D2 are identical.
Therefore, the voltage across C3 tracks the output voltage. The
voltage developed across C3 is sensed and regulated via the resistor
divider R1 and R3 connected to U1’s FEEDBACK pin. The values of R1
and R3 are selected such that, at the desired output voltage, the
voltage at the FEEDBACK pin is 2.00 V.
Regulation is maintained by skipping switching cycles. As the output
voltage rises, the current into the FEEDBACK pin will rise. If this
exceeds I
FB
then subsequent cycles will be skipped until the current
reduces below I
FB
. Thus, as the output load is reduced, more cycles
will be skipped and if the load increases, fewer cycles are skipped.
To provide overload protection if no cycles are skipped during a
50 ms period, LinkSwitch-TN2 will enter auto-restart, limiting the
average output power to approximately 3% of the maximum overload
power. Due to tracking errors between the output voltage and the
voltage across C3 at light load or no-load, a small pre-load may be
required (R4). For the design in Figure 8, if regulation to zero load is
required, then this value should be reduced to 2.4 kΩ.
Key Application Considerations
LinkSwitch-TN2 Design Considerations
Output Current Table
Data sheet maximum output current table (Table 1) represents the
typical practical continuous output current for both mostly discontinu-
ous conduction mode (MDCM) and continuous conduction mode (CCM)
of operation that can be delivered from a given LinkSwitch-TN2
device under the following assumed conditions:
1.
Buck converter topology.
2.
The minimum DC input voltage is ≥70 V. The value of input
capacitance should be large enough to meet this criterion.
3. For CCM operation a KRP* of 0.4.
4. Output voltage of 12 VDC.
5.
Efficiency of 75%.
6. A catch/freewheeling diode with t
RR
≤75
ns is used for MDCM
operation and for CCM operation, a diode with t
RR
≤35
ns is used.
7. The part is board mounted with SOURCE pins soldered to a
sufficient area of copper to keep the SOURCE pin temperature at
or below 100
°C.
*KRP is the ratio of ripple to peak inductor current.
5
www.power.com
Rev. M 10/20
京公网安备 11010802033920号
EEWORLD
