LNK501
At very light or no-load, typically less than 2 mA of output
current, the output voltage rises due to leakage inductance peak
chargingofthesecondary.Thisvoltagerisecanbereducedwith
asmallpreloadwithlittlechangetono-loadpowerconsumption.
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).
Output Capacitor – C4
The output voltage load variation can be improved to be ±5%
across the whole load range by adding an optocoupler and
secondary reference (Figure 6). The secondary reference is
designed to only provide feedback above the normal peak
power point voltage to maintain the correct constant current
characteristic.
Capacitor C4 should be selected such that its voltage and ripple
current specifications are not exceeded.
LinkSwitch Layout considerations
Primary Side Connections
As the SOURCE pins in a LinkSwitch supply are switching
nodes, the copper area connected to SOURCE together with
C1, 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
transformer secondary output return pin(s) and the primary
bulk capacitor negative return. Such placement will maximize
the EMI benefit of the Y capacitor and avoid problems in
common-mode surge testing.
Control Pin Capacitor – C1
Quick Design Checklist
Capacitor C1 is used during start-up to power LinkSwitch and
setstheAuto-Restartfrequency. Fordesignsthathaveabattery
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
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.
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.
Note:InaLinkSwitchcircuit, theSOURCEisaswitchingnode.
This should be taken into consideration during testing.
Oscilloscope measurements should be made with probe
grounded to DC voltages such as primary return or DC rail but
not to SOURCE. Power supply input voltage should always be
suppliedusinganisolationtransformer. Thefollowingminimum
set of tests is strongly recommended:
Feedback Resistor – R1
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 the VOR
selected during design. Any 1%, 0.25 W resistor is suitable.
1. Maximum drain voltage – Verify that VDSS does not exceed
675 V at highest input voltage and peak output power.
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
2. Maximumdraincurrent–Atmaximumambienttemperature,
maximum input voltage and peak output power, verify drain
current waveforms at start-up for any signs of transformer
saturation and excessive leading edge current spikes.
F
9/02
9
POWERINT [ Power Integrations ]
