Rev. F 05/18

6

LinkSwitch-4

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Cable Compensation

If required, LinkSwitch-4 adjusts the converter output voltage (V

OUT)

to compensate for voltage drop across the output cable. The amount

of compensation applied (G

CAB) is specified by using the formula below

to match cable compensation with output cable resistance (R

CAB):

%

%

VTyp

ITyp

R

G

G

VTyp

ITyp

R

100

100

OUTCV

OUTC

CAB

CAB

P

CAB

OUTCV

OUTCCCAB

#

#

#

#

=

=

^

^

^

^

^

^

^

^

h

h

h

h

h

h

h

h

Or

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,

R

CS, 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 (V

CSTHR)

set by the control circuitry, base drive is driven low to turn the

primary switch off. The primary current sense voltage threshold

(V

CSTHR) varies from VCS(MIN) to VCS(MAX) during normal operation. The

switching frequency varies from f

MIN at no-load, to the maximum

switching frequency, f

MAX.

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 CVCC must be small enough to

ensure the ripple on VCC is greater than 50 mV:

VCCPFM

C

fV

I

VCC

MIN

VCCNL

# D

=

The switching frequency increases as the load increases, eventually

reaching f

MAX 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, I

BD 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 I

F(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 V

CSTHR

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 V

CSOCP(EFF),

subject to the minimum on-time, T

ON(MIN). The effective threshold

V

CSOCP(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, t

CS(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 V

OUT(OVP). The value depends on the set output voltage

(V

OUT(CV)) and the feedback OVP ratio:

VV

G

OUTOVP

OUTCVFBOVP

#

=

^^

^

hh

h

Supplementary Base Drive (LNK40x3D, LNK4114D, LNK4214D)

The resistor R

SBD 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 R

SBD 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 R

SBD, the SUPPLEMENTARY BASE DRIVE pin

resistance R

SBD(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 V

OUT(UVP).

At start-up this function is disabled during the first N

STARTUP switching

cycles and the output current is regulated allowing the output voltage

to rise from 0 V in a monotonic way.

Table 3. Output Undervoltage Protection.

Product

Output Undervoltage Protection Function

LNK40x2S

LNK43x2S

V

OUT(UVP) Depends on VVCC(SLEEP)

LNK40x3S

LNK40x3D

LNK4323S

LNK4323D

V

OUT(UVP) = 0.63 × VOUT(CV)

LNK40x4D

V

OUT(UVP) Depends on VVCC(SLEEP)

LNK4114D

LNK4214D

LNK4115D

LNK4215D

V

OUT(UVP) Depends on VVCC(SLEEP)