LNK520
Feedback Resistor – R4
Quick Design Checklist
The value of R4 is selected to give a feedback current into the
CONTROL pin of approximately 2.15 mA at the peak output
power point of the supply. The actual value depends on the bias
voltage, typically in the range 20 V to 35 V, selected during
design. Higher values for the bias voltage will increase no-load
power consumption. Any 1%, 0.25 W resistor is suitable.
As with any power supply design, all LinkSwitch designs
should be verified on the bench to make sure that component
specifications are not exceeded under worst-case conditions.
Performing the following minimum set of tests is strongly
recommended:
Output Diode – D7
1. Maximum drain voltage – Verify that VDS does not exceed
675 V at highest input voltage and peak output power.
PNfast, PNultra-fastorSchottkydiodescanbeuseddepending
on the efficiency target for the supply, Schottky diodes giving
higher efficiency than PN diodes. The diode voltage rating
should be sufficient to withstand the output voltage plus the
input voltage transformed through the turns ratio (a typical VOR
of 50 V requires a diode PIV of 50 V). Slow recovery diodes
are not recommended (1N400X types).
2. Maximumdraincurrent–Atmaximumambienttemperature,
maximuminputvoltageandpeakoutputpower, verifydrain
current waveforms at start-up for any signs of transformer
saturation and excessive leading edge current spikes.
LinkSwitch has a minimum leading edge blanking time of
200 ns to prevent premature termination of the on-cycle.
Verify that the leading edge current spike event is below
current limit at the end of the 200 ns blanking period.
Output Capacitor – C6
Capacitor C6 should be selected such that its voltage and
ripple current specifications are not exceeded. Selecting a
capacitor with low equivalent series resistance (ESR) will
reduce peak-peak output ripple and improve overall supply
operating efficiency.
3. Thermal check – At peak output power, minimum input
voltage and maximum ambient temperature, verify that the
temperaturespecifications are not exceededfor LinkSwitch,
transformer, output diode and output capacitors. Enough
thermalmarginshouldbeallowedforpart-to-partvariationof
theRDS(ON) ofLinkSwitchasspecifiedinthedatasheet.Under
lowline, peakpower, amaximumLinkSwitchSOURCEpin
temperature of 100 °C is recommended to allow for these
variations.
LinkSwitch Layout considerations
Primary Side Connections
The copper area connected to SOURCE should be maximized
to minimize temperature rise of the LinkSwitch device.
The CONTROL pin capacitor C5 should be located as close as
possible to the SOURCE and CONTROL pins.
4. Centered output characteristic – Using a transformer with
nominalprimaryinductanceandataninputvoltagemidway
between low and high line, verify that the peak power point
occurs at ~4% above the desired nominal output current,
with the correct output voltage. If this does not occur, then
the design should be refined (increase LP) to ensure the
overall tolerance limits are met.
TominimizeEMIcouplingfromtheswitchingDRAINnodeon
the primary to both the secondary andAC input, the LinkSwitch
shouldbepositionedawayfromthesecondaryofthetransformer
and AC input.
The length and copper area of all PCB traces connecting to the
switchingDRAINnodeshouldbekepttoanabsoluteminimum
to limit EMI radiation.
Selecting Between LNK500 and LNK520
The LNK500 and LNK520 differ in the circuit location of the
LinkSwitch device. The LNK500 is designed for high-side
operation and the LNK520 is designed for low-side operation.
TheLNK520can,however,beusedinthehigh-sideconfiguration
in certain applications. Refer to Figure 9 and supporting
description.Table2summarizestheconsiderationsforselecting
which device to use.
Y capacitor
If a Y capacitor is required, it should be connected close to the
transformersecondaryoutputreturnpin(s)andtheprimarybulk
capacitor positive terminal. Such placement will maximize the
EMI benefit of theYcapacitor and avoid problems in common-
mode surge testing.
Design Tools
Up to date information on design tools can be found at the
Power Integrations Web site: www.powerint.com.
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POWERINT [ Power Integrations ]
