LNK500
voltage and ±25% for current limit for overall variation in high
volume manufacturing. This includes device and transformer
tolerances and line variation. Lower power designs may have
poorer constant current linearity.
time during start-up for the output voltage to reach regulation.
Any capacitor type is acceptable with a voltage rating of
10 V or above.
Feedback Resistor – R1
As the output load reduces from the peak power point, the
output voltage will tend to rise due to tracking errors compared
to the load terminals. Sources of these errors include the
output cable drop, output diode forward voltage and leakage
inductance, which is the dominant cause. As the load reduces,
the primary operating peak current reduces, together with the
leakage inductance energy, which reduces the peak charging
of the clamp capacitor. With a primary leakage inductance of
50 µH, the output voltage typically rises 30% over a 100% to
5% load change.
The value of R1 is selected to give a feedback current into the
CONTROL pin of approximately 2.3 mA at the peak output
power point of the supply. The actual value depends on theVOR
selected during design. Any 0.25 W resistor is suitable.
Output Diode – D2
Either PN fast, PN ultra-fast or Schottky diodes can be used,
depending on the efficiency target for the supply, Schottky
diodes giving higher efficiency then 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).
Atverylightorno-load,typicallylessthan2mAofoutputcurrent,
theoutputvoltagerisesduetoleakageinductancepeakcharging
of the secondary. This voltage rise can be reduced with a small
preload with little change to no-load power consumption.
Output Capacitor – C4
Capacitor C4 should be selected such that its voltage and ripple
current specifications are not exceeded.
The output voltage load variation can be improved across the
whole load range by adding an optocoupler and secondary
reference(Figure6). Thesecondaryreferenceisdesignedtoonly
provide feedback above the normal peak power point voltage
to maintain the correct constant current characteristic.
LinkSwitch Layout considerations
Primary Side Connections
Since the SOURCE pins in a LinkSwitch supply are switching
nodes,thecopperareaconnectedtoSOURCEtogetherwithC1,
C2 and R1 (Figure 5) should be minimized, within the thermal
contraints of the design, to reduce EMI coupling.
Component Selection
The schematic shown in Figure 5 outlines the key components
needed for a LinkSwitch supply.
The CONTROL pin capacitor C1 should be located as close as
possible to the SOURCE and CONTROL pins.
Clamp diode – D1
Diode D1 should be either a fast (trr <250 ns) or ultra-fast
type (trr <50 ns), with a voltage rating of 600 V or higher. Fast
recovery types are preferred, being typically lower cost. Slow
diodes are not recommended; they can allow excessive DRAIN
ringing and the LinkSwitch to be reverse biased.
To minimize EMI coupling from the switching nodes on the
primary to both the secondary and AC input, the LinkSwitch
shouldbepositionedawayfromthesecondaryofthetransformer
and AC input.
Clamp Capacitor – C2
Routing the primary return trace from the transformer primary
around LinkSwitch and associated components further reduces
coupling.
Capacitor C2 should be a 0.1 µF, 100 V capacitor. Low cost
metallized plastic film types are recommended. The tolerance
of this part has a very minor effect on the output characteristic
so any of the standard ±5%, ±10% or ±20% tolerances are
acceptable. Ceramic capacitors are not recommended. The
common dielectrics used such as Y5U or Z5U are not stable
with voltage or temperature and may cause output instability.
Ceramic capacitors with high stability dielectrics may be used
but are expensive compared to metallized film types.
Y capacitor
If a Y capacitor is required, it should be connected close to the
transformersecondaryoutputreturnpin(s)andtheprimarybulk
capacitor negative return. Such placement will maximize the
EMI benefit of theYcapacitor and avoid problems in common-
mode surge testing.
CONTROL Pin Capacitor – C1
Capacitor C1 is used during start-up to power LinkSwitch and
sets the auto-restart frequency. For designs that have a battery
load, this component should have a value of 0.22 µF and for
resistive loads, a value of 1 µF. This ensures there is sufficient
D
2/05
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POWERINT [ Power Integrations ]
