LinkSwitch-4 Family
Energy-Efficient, Accurate Primary-Side Regulated
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
Advanced Performance Features
• Dynamic base drive technology provides flexibility in choice of BJT
LinkSwitch-4
U1
LNK4xx2S
~
Compensates for input line voltage variations
Compensates for cable voltage drop
Compensates for external component temperature variations
Very accurate IC parameter tolerances using proprietary trimming
technology
• Frequency up to 65 kHz to reduce transformer size
• The minimum peak current is fixed to improve transient load response
Enhanced Performance Features
• Easy start for starting into capacitive loads (LNK4114D, LNK4115D)
• Constant power for high current start-up (LNK4214D, LNK4215D)
• 13003 drive improved efficiency with 13003 BJT’s (LNK4302S,
FB
GND
• Extends RBSOA of BJT
• Dramatically reduces sensitivity to BJT gain
•
•
•
•
VCC
CS
transistor by dynamically optimizing BJT switching characteristics
BD
ED
PI-7462-122315
Figure 1. Typical Application (SOT-23-6) (S).
+
BD
ED
~
VCC
LinkSwitch-4
U1
LNK40x3D
CS
LNK4322S)
SBD
FB
GND
Advanced Protection/Safety Features
• Single fault output overvoltage and short-circuit
• Over-temperature protection
• Active clamp
EcoSmart™– Energy Efficient
• Meets DoE 6 and CoC V5 2016 via an optimized quasi-resonant
switching PWM/PFM control
Output Power Table
85 - 265 VAC
Product3,4
• No-load consumption of
IFBHT(START)), the LinkSwitch-4 will enter Run mode and drive pulses will
be output on the BASE DRIVE pin. To achieve smooth power-up
(monotonic rise in VOUT), C VCC must be large enough to power the
control circuitry during Initialize mode and the first few cycles of Run
mode, until sufficient power is provided by the transformer voltage
supply winding.
If the input voltage falls below VMAINS(LO) (see Input Undervoltage
Protection), V VCC will fall below V VCC(SLEEP) and the LinkSwitch-4 will go
into Sleep mode, reducing its current consumption to IVCC(SLEEP). The
control circuitry will re-initialize if the input voltage is restored and
V VCC reaches V VCC(RUN).
VVCC(RUN)
VVCC
VVCC(SLEEP)
Off
Sleep
Initialize
Run
Sleep
Off
PI-7457-010815
Figure 7. VCC Waveforms.
Mode
Description
Sleep
From initial application of input power or from Run mode, if V VCC falls below V VCC(SLEEP), the LinkSwitch-4 goes to Sleep mode.
Non-essential circuits are turned off. Base and Emitter drives are turned off so BASE DRIVE and EMITTER DRIVE pins
become high impedance, allowing the bootstrap resistor (RHT) and BJT to start the circuit. Sleep mode is exited when V VCC
rises to V VCC(RUN) and the control circuitry goes to Initialize mode.
Initialize
Internal circuits are enabled and the LinkSwitch-4 issues one switching cycle to sample the input voltage via the FEEDBACK
pin. If VIN (hence VHT) is high enough, the LinkSwitch-4 changes to Run mode. If VIN is not high enough, no further base drive
pulses are issued and the LinkSwitch-4 returns to Sleep mode when V VCC falls below V VCC(SLEEP).
Run
Power conversion: The control circuitry is powered from the VCC rail and the internal VDD is regulated. If V VCC falls below
V VCC(SLEEP), the IC ceases power conversion and goes to Sleep mode.
Table 2.
Summary of LinkSwitch-4 Operating Modes.
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LinkSwitch-4
Switching Waveforms
Typical waveforms at the feedback and primary current sense inputs
are shown in Figure 8.
tSAMP
VFBREG
FB
0V
tCSB
0V
CS
VCSTHR
tFON
VVCC(RUN)
0A
BD
ON
OFF
ED
Transformer
Flux
PI-7458-010815
Figure 8. Typical Waveforms at the Feedback and Primary Current Sense Inputs.
Constant Voltage (CV) Regulation
Constant output voltage regulation is achieved by sensing the voltage
at the feedback input, which is connected to the voltage supply
winding as shown in Figure 10 or to a dedicated feedback winding.
An internal current source prevents the feedback voltage from going
negative. A typical feedback voltage waveform is shown in Figure 8.
The feedback waveform is continuously analyzed and sampled at time
tSAMP to measure the reflected output voltage. tSAMP is identified by the
slope of the feedback waveform and is coincident with zero flux in the
transformer. The sampled voltage is regulated at VFB(REG) by the
voltage control loop. The (typical) CV mode output voltage is set by
the ratio of resistors RFB1 and RFB2 (see Figure 10) and by the
transformer turns ratio, according to the following formula (where
output diode voltage is neglected):
N
R
VOUT^CV h = V FB^ REG h a 1 + R FB1 ka N S k
FB2
F
Where NF is the number of turns on the feedback (or voltage supply
if used for feedback) winding and NS is the number of turns on the
secondary winding. The tolerances of RFB1 and RFB2 affect output
voltage regulation and mains estimation so should typically be chosen
to be 1% or better.
The current required to clamp the feedback voltage to ground
potential during the on-time of the primary switch depends on the
primary winding voltage (approximately equal to the rectified mains
input voltage), the primary to feedback turns ratio, and resistor RFB1.
The controller measures feedback source current and so enables RFB1
to set the input voltage start threshold and the input undervoltage
protection threshold, as described below.
Input Voltage Start Threshold
In Initialise mode, the LinkSwitch-4 issues a single short-duration
drive pulse in order to measure the primary voltage and so the
approximate mains input voltage. If the input voltage is below
VMAINS(START) then the LinkSwitch-4 will not start. Instead it will pause
while V VCC discharges below V VCC(SLEEP) then it will begin a new power-up
cycle. If the input voltage exceeds VMAINS(START), the converter will
power-up. VMAINS(START) is set by RFB1 using this equation:
V MAINS^START h =
N
-1
# I FBHT^START h # R FB1 # N P
F
2
Input Undervoltage Protection
In Run mode, if the mains voltage falls to VMAINS(LO), the LinkSwitch-4
will stop issuing drive pulses, V VCC will reduce to V VCC(SLEEP) and the
LinkSwitch-4 will enter Sleep mode. VMAINS(LO) is set by RFB1 using this
equation:
V MAINS^LOh =
N
-1
# I FBHT^LOh # R FB1 # N P
F
2
Constant Current (CC Mode) Regulation
Constant current output (IOUT(CC)) is achieved by regulating the CS
input to the primary side estimate of the output current scaled by RCS,
VCS(CC). The regulated output current, IOUT(CC) is set by the value of the
current sense resistor, RCS, and the transformer primary to secondary
turns ratio (NP/NS). The value of RCS is determined using the formula:
VCS^CC h ^Typh
N
n
R CS . a N P kd
S
I OUT^CC h ^Typh
The tolerance of RCS affects the accuracy of output the current
regulation so is typically chosen to be 1%. The LinkSwitch-4 can
maintain CC regulation down to much lower levels of VSHUTDN(MAX)
normally specified for mobile phones chargers (see Figure 11).
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Rev. F 05/18
LinkSwitch-4
Cable Compensation
If required, LinkSwitch-4 adjusts the converter output voltage (VOUT)
to compensate for voltage drop across the output cable. The amount
of compensation applied (GCAB) is specified by using the formula below
to match cable compensation with output cable resistance (RCAB):
G CAB =
Or
G CAB =
I OUT^ CC h ^ Typ h # R CAB
# 100%
VOUT^ CV h ^ Typ h
I OUT^ CP h ^ Typ h # R CAB
# 100%
VOUT^ CV h ^ Typ h
Drive Pulse and Frequency Modulation
The LinkSwitch-4 control circuitry determines both the primary switch
peak current and the switching frequency to control output power,
ensuring discontinuous conduction mode operation at all times.
Primary current generates a voltage across the current sense resistor,
RCS, and is sensed by the primary current sense input. The voltage
on the primary CURRENT SENSE pin is negative-going, as shown in
Figure 8. When the voltage exceeds a (negative) threshold (VCSTHR)
set by the control circuitry, base drive is driven low to turn the
primary switch off. The primary current sense voltage threshold
(VCSTHR) varies from VCS(MIN) to VCS(MAX) during normal operation. The
switching frequency varies from fMIN at no-load, to the maximum
switching frequency, fMAX.
Minimum switching frequency occurs during no-load operation and
is typically in the range 1 to 3 kHz, depending on application design.
The periodic voltage waveform on the VCC input, which depends on
the current consumed by the control circuitry and the value of C VCC,
contributes to control of the switching frequency. In no-load
condition, C VCC must be large enough to ensure that ripple voltage on
VCC (∆V VCCPFM) is less than 1.6 V, and C VCC must be small enough to
ensure the ripple on VCC is greater than 50 mV:
C VCC =
I VCCNL
fMIN # DVVCCPFM
The switching frequency increases as the load increases, eventually
reaching fMAX at full load. For protection purposes in the event of
certain transitory conditions, the controller immediately issues a drive
pulse if VCC voltage falls to V VCC(LOW). This is not part of normal
operation or normal frequency control.
Base Drive Control
During the on-time of the BJT, the emitter is switched to GND via the
EMITTER DRIVE pin. Base current, IBD is controlled to achieve fast
turn-on, low on-voltage and fast turn-off to enable reduced power
dissipation and accurate timing of each part of the switching cycle.
As shown in Figure 9, the base drive current starts with a fixed pulse
of IF(ON)/tF(ON). Its amplitude and duration are then modulated to provide
sufficient charge for low BJT on-voltage, while allowing de-saturation
towards the end of on-time so as to enable fast turn-off. When VCSTHR
is detected on the primary CURRENT SENSE pin, the BASE DRIVE pin
is switched to GND and the emitter drive switch is opened.
LNK43x2S – drive optimized for high efficiency performance using
13003 transistors.
Duty Cycle Control
Maximum duty cycle is a function of the primary to secondary turns
ratio of the transformer (typically 16:1 for a 5 V output). For a
universal mains input power supply, maximum duty cycle is typically
chosen to be 50% at the minimum (including ripple) of the rectified
mains voltage (typically 80 V).
Quasi-Resonant Switching
The primary switch is turned on when the voltage across it rings
down to a minimum (voltage-valley, quasi-resonant switching). The
effect of this is to reduce losses in the switch at turn-on. It also
helps reduce EMI.
Primary Switch Over-Current Protection
The primary switch is turned off if the emitter current sensed by the
primary current sense input exceeds the effective threshold VCSOCP(EFF),
subject to the minimum on-time, TON(MIN). The effective threshold
VCSOCP(EFF) depends on a threshold VCS(OCP) predefined by the controller,
the primary current sense signal rate of rise (dVcs/dt), which is
dependent on the application design, and the primary CURRENT
SENSE pin turn-off response time, tCS(OFF). This gives pulse by pulse
over-current protection of the primary switch.
Output Overvoltage Protection
The on-time of the primary switch is reduced if the output voltage
tends to VOUT(OVP). The value depends on the set output voltage
(VOUT(CV)) and the feedback OVP ratio:
VOUT^OVPh = VOUT^CV h # G FB^OVPh
Supplementary Base Drive (LNK40x3D, LNK4114D, LNK4214D)
The resistor RSBD connects the SUPPLEMENTARY BASE DRIVE pin to
VOLTAGE SUPPLY pin. It supplements current to the base drive to
optimize the switching bipolar transistor turn-on and turn-off in high
power applications.
Suggested values for the supplementary base drive resistor RSBD are
between 220 Ω and 390 Ω.
Shunt Function (LNK40x3D, LNK40x4D, LNK4115D, LNK4215D)
The shunt function is intended to automatically limit the VCC voltage
and allow greater flexibility in transformer design. VOLTAGE SUPPLY
pin will be shunted via RSBD, the SUPPLEMENTARY BASE DRIVE pin
resistance RSBD(ON) and RBD(OFF) to the GROUND pin when the VCC
voltage is greater than V VCC(HI) and the transformer is discharging.
Output Undervoltage Protection (LNK40x3S/D, LNK43x3S/D)
The output undervoltage protection (UVP) function is used to
shutdown the converter when the output voltage is below VOUT(UVP).
At start-up this function is disabled during the first NSTARTUP switching
cycles and the output current is regulated allowing the output voltage
to rise from 0 V in a monotonic way.
Product
Output Undervoltage Protection Function
LNK40x2S
LNK43x2S
VOUT(UVP) Depends on V VCC(SLEEP)
LNK40x3S
LNK40x3D
LNK4323S
LNK4323D
VOUT(UVP) = 0.63 × VOUT(CV)
LNK40x4D
VOUT(UVP) Depends on V VCC(SLEEP)
LNK4114D
LNK4214D
LNK4115D
LNK4215D
VOUT(UVP) Depends on V VCC(SLEEP)
Table 3. Output Undervoltage Protection.
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LinkSwitch-4
If the output does not reach VOUT(UVP) during this time then the
controller will shutdown and restart.
VOUT(UVP) value depends on the set output voltage (VOUT(CV)) and the
feedback UVP ratio:
VOUT^UVPh = VOUT^CV h # G FB^UVPh
Easy Start (LNK4114D, LNK4214D, LNK4115D, LNK4215D)
The Easy Start feature guarantees start-up into large output
capacitances and allows the output voltage to work down the CC
chimney close to OV.
The Easy Start feature uses the BJT emitter current (equal to the
primary current) to charge the supply capacitor C VCC via an additional
Schottky diode.
This only occurs when the supply voltage has fallen below V VCCES and
is achieved by altering the sequencing of EMITTER DRIVE pin switching.
This allows the BJT emitter voltage to rise until the Schottky diode
conducts. Emitter current then charges the C VCC until the BJT is
turned off by the BASE DRIVE pin being pulled low.
If the supply voltage is above V VCC(ES), then Easy Start has no effect
on the operation of the controller.
Note V VCC(ES) = 6 V for LNK4114D and LNK4214D, V VCC(ES) = 10 V for
LNK43x3S, LNK43x3D, LNK4115D and LNK4215D during NSTARTUP cycles,
after which it reduces to 6 V.
Over-Temperature Protection
Temperature protection is internal to LinkSwitch-4. The sensor
measures the junction temperature TJ, which is the hottest part of
LinkSwitch-4.
At temperatures TJ ~ 140 °C, LinkSwitch-4 will shutdown and remain
in this state until a temperature of TJ ~ 70 °C is reached. Whereby
LinkSwitch-4 will power-up in the normal sequence.
If the supply voltage is below V VCC(ES), and when the base has received
enough charge, the EMITTER DRIVE pin is released at the same time
as the BASE DRIVE pin.
tFON
IFON
IBD
IBDSRC
Logic BD
Logic ED
BD Ground
PI-7459-090215
Figure 9. Base Drive Waveforms.
VCC
tFON
IBD
IFON
IBDSRC
Logic BD
Logic ED
BD Ground
CVCC Recharge Current
PI-7674-090215
Figure 9b. Base Drive Waveforms – Easy Start mode of Operation.
7
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Rev. F 05/18
LinkSwitch-4
Typical Application for LNK40x3D
Parameter
Symbol
Range or Value
Units
Supply Voltage
VIN
85 - 265
VAC
Output Voltage
VOUT(CV)
5.0 ± 5%
V
Constant voltage (CV) mode, at the load
Output Current
IOUT
2
A
Label rated output current
Switching Frequency at Full Load
fMAX
65
kHz
Cable Compensation
GCAB
6
%
No-load Power
PNL
75
%
Energy Star test method
TON
4
V
Average Efficiency
Turn-on Delay
Undershoot Voltage
Comment
Universal mains
Determined by the chosen variant
Load step from 0 A to 0.5 A
Table 4. 10 W Typical Application Results for Figure 10.
T1
DOUT
+
COUT
LFILT
ROUT
2 × RHT
DBRIDGE
Q1
BD
~
CIN1
RIN
RFB1
ED
CIN2
LNK40x3D
VCC
RSBD
CS
RCS2
RCS
GND
SBD
FB
C VCC
RFB2
PI-7679-071515
Figure 10. Typical Universal Input, 10 W Charger.
By sensing the primary-side waveforms of transformer voltage and
primary current, the LinkSwitch-4 achieves constant voltage and
constant current output within tight limits without the need for any
secondary-side sensing components. Figure 11 shows the output
characteristics of a typical charger implementation.
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Rev. F 05/18
www.power.com
LinkSwitch-4
PI-7471-121014
VOUT
100%
VOUTCV(TYP)
IOUTCC(TYP)
IOUTCC(MIN)
VSHUTDN(MAX)
IOUT
0
100%
Figure 11. Typical CV/CC Output Characteristic Achieved.
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Rev. F 05/18
LinkSwitch-4
Typical Networking Application for LNK4114D
Parameter
Symbol
Range or Value
Units
Supply Voltage
VIN
90 - 264
VAC
Output Voltage
VOUT(CV)
12.0 ± 5%
V
Constant voltage (CV) mode, at the load
Output Current
IOUT
1
A
Label rated output current
Load Capacitance
CLOAD
3000
mF
System capacitance
Switching Frequency at Full Load
fMAX
65
kHz
Cable Compensation
GCAB
3
%
No-load Power
PNL
83
%
Energy Star test method
TON