LNK306DG Ver la hoja de datos (PDF) - Power Integrations, Inc

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LNK306DG

LinkSwitch-TN Family Lowest Component Count, Energy-Efficient Off-Line Switcher IC

Power Integrations, Inc 

LNK302/304-306

Pre-load Resistor R4

In high-side, direct feedback designs where the minimum load

is <3 mA, a pre-load resistor is required to maintain output

regulation. This ensures sufficient inductor energy to pull the

inductor side of the feedback capacitor C3 to input return via

D2. The value of R4 should be selected to give a minimum

output load of 3 mA.

In designs with an optocoupler the Zener or reference bias

current provides a 1 mA to 2 mA minimum load, preventing

“pulse bunching” and increased output ripple at zero load.

LinkSwitch-TN Layout Considerations

In the buck or buck-boost converter configuration, since the

SOURCE pins in LinkSwitch-TN are switching nodes, the copper

area connected to SOURCE should be minimized to minimize

EMI within the thermal constraints of the design.

In the boost configuration, since the SOURCE pins are tied

to DC return, the copper area connected to SOURCE can be

maximized to improve heatsinking.

The loop formed between the LinkSwitch-TN, inductor (L1),

freewheeling diode (D1), and output capacitor (C2) should

be kept as small as possible. The BYPASS pin capacitor

C1 (Figure 6) should be located physically close to the

SOURCE (S) and BYPASS (BP) pins. To minimize direct

coupling from switching nodes, the LinkSwitch-TN should be

placed away from AC input lines. It may be advantageous to

place capacitors C4 and C5 in-between LinkSwitch-TN and the

AC input. The second rectifier diode D4 is optional, but may

be included for better EMI performance and higher line surge

withstand capability.

Quick Design Checklist

As with any power supply design, all LinkSwitch-TN designs

should be verified for proper functionality on the bench. The

following minimum tests are recommended:

1) Adequate DC rail voltage – check that the minimum DC

input voltage does not fall below 70 VDC at maximum load,

minimum input voltage.

2) Correct Diode Selection – UF400x series diodes are

recommended only for designs that operate in MDCM at

an ambient of 70 °C or below. For designs operating in

continuous conduction mode (CCM) and/or higher ambients,

then a diode with a reverse recovery time of 35 ns or better,

such as the BYV26C, is recommended.

3) Maximum drain current – verify that the peak drain current

is below the data sheet peak drain specification under

worst-case conditions of highest line voltage, maximum

overload (just prior to auto-restart) and highest ambient

temperature.

4) Thermal check – at maximum output power, minimum

input voltage and maximum ambient temperature, verify

that the LinkSwitch-TN SOURCE pin temperature is

100 °C or below. This figure ensures adequate margin due

to variations in RDS(ON) from part to part. A battery powered

thermocouple meter is recommended to make measurements

when the SOURCE pins are a switching node. Alternatively,

the ambient temperature may be raised to indicate margin

to thermal shutdown.

In a LinkSwitch-TN design using a buck or buck boost converter

topology, the SOURCE pin is a switching node. Oscilloscope

measurements should therefore be made with probe grounded

to a DC voltage, such as primary return or DC input rail, and

not to the SOURCE pins. The power supply input must always

be supplied from an isolated source (e.g. via an isolation

transformer).

28-8

Rev. I 11/08

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