LNK3205PD datasheet(4/28 Pages) POWERINT

4 / 28 page

Rev. P 06/22

LinkSwitch-TN2

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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.

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.

VBUS

LinkSwitch-TN2

PI-8024-092916

DOVP

RBP

CBP

VOV = VBP + VDOVP

LinkSwitch-TN2

PI-8025-092616

VOV = (VVR3 + VD3 + VBE(Q3) – VBP + VR3) × n + VDS(ON)

*R3 limits the current into the FEEDBACK pin.

A maximum current of 120% of IFBSD is

recommended.

**R4 is a pull-down resistor for Q3 to avoid

inadvertant triggering. 2 kΩ is a typical starting point

R3*

VR3

n:1

R4**

cBP

VBUS

n:1

Figure 6. Non-Isolated Flyback Converter with Output Overvoltage Protection.

Figure 7. Line-Sensing for Overvoltage Protection by using FEEDBACK Pin.

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