LinkSwitch-4
Cable Compensation
If required, LinkSwitch-4 adjusts the converter output voltage (VOUT
chosen to be 50% at the minimum (including ripple) of the rectified
mains voltage (typically 80 V).
)
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):
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.
^
h
IOUT
Typ # RCAB
^
h
CC
GCAB
=
=
# 100%
# 100%
^
h
VOUT
Typ
^
h
CV
Primary Switch Over-Current Protection
Or
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.
^
h
IOUT
Typ # RCAB
^
h
CP
,
GCAB
^
h
VOUT
Typ
^
h
CV
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.
Output Overvoltage Protection
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
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:
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
)
VOUT OVP = V
# G
^
h
^
h
^
h
OUT CV
FB OVP
(VCSTHR) varies from VCS(MIN) to VCS(MAX) during normal operation. The
switching frequency varies from fMIN at no-load, to the maximum
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.
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 CVCC
contributes to control of the switching frequency. In no-load
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 VVCC(HI) and the transformer is discharging.
condition, CVCC must be large enough to ensure that ripple voltage on
VCC (∆VVCCPFM) is less than 1.6 V, and CVCC must be small enough to
ensure the ripple on VCC is greater than 50 mV:
IVCCNL
CVCC
=
f
MIN # DVVCCPFM
Output Undervoltage Protection (LNK40x3S/D, LNK43x3S/D)
The output undervoltage protection (UVP) function is used to
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 VVCC(LOW). This is not part of normal
operation or normal frequency control.
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.
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.
Product
LNK40x2S
Output Undervoltage Protection Function
VOUT(UVP) Depends on VVCC(SLEEP)
LNK43x2S
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.
LNK40x3S
LNK40x3D
LNK4323S
LNK4323D
VOUT(UVP) = 0.63 × VOUT(CV)
VOUT(UVP) Depends on VVCC(SLEEP)
VOUT(UVP) Depends on VVCC(SLEEP)
LNK40x4D
LNK43x2S – drive optimized for high efficiency performance using
13003 transistors.
LNK4114D
LNK4214D
LNK4115D
LNK4215D
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
Table 3. Output Undervoltage Protection.
6
Rev. F 05/18
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