LinkSwitch-4
Switching Waveforms
Typical waveforms at the feedback and primary current sense inputs
are shown in Figure 8.
ꢅꢆꢇꢈꢀ
ꢋFꢊRꢌꢍ
Fꢊ
0 ꢋ
0 ꢋ
ꢅꢉꢆꢊ
ꢉꢆ
ꢋꢉꢆꢎꢏR
ꢅFꢔꢒ
ꢋꢋꢉꢉꢐRꢑꢒꢓ
0 ꢇ
ꢔꢒ
ꢊꢕ
ꢌꢕ
ꢔFF
ꢎꢖꢗꢘꢙꢚꢛꢖꢜeꢖ
Fꢝꢞꢟ
ꢀꢁꢂꢃꢄ58ꢂ010815
Figure 8. Typical Waveforms at the Feedback and Primary Current Sense Inputs.
Constant Voltage (CV) Regulation
approximate mains input voltage. If the input voltage is below
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):
V
MAINS(START) then the LinkSwitch-4 will not start. Instead it will pause
while VVCC discharges below VVCC(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:
NP
NF
-1
2
VMAINS
=
# IFBHT
# R
#
^
h
^
h
START
START
FB1
Input Undervoltage Protection
In Run mode, if the mains voltage falls to VMAINS(LO), the LinkSwitch-4
will stop issuing drive pulses, VVCC will reduce to VVCC(SLEEP) and the
LinkSwitch-4 will enter Sleep mode. VMAINS(LO) is set by RFB1 using this
equation:
NP
NF
-1
2
RFB1 NS
VMAINS
=
# IFBHT # R
#
^
h
^
h
LO
LO
FB1
VOUT CV = V
1 +
^
h
^
h
FB REG
RFB2 NF
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.
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:
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
^
h
VCS CC Typ
^
h
NP
NS
a
k
d
n
RCS
.
input voltage), the primary to feedback turns ratio, and resistor RFB1
.
IOUT CC Typ
^
h
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.
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).
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
5
Rev. F 05/18
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