LNK501P datasheet(9/20 Pages) POWERINT

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LNK501

At very light or no-load, typically less than 2 mA of output

current, the output voltage rises due to leakage inductance peak

charging of the secondary. This voltage rise can be reduced with

a small preload with little change to no-load power consumption.

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.

Component Selection

The schematic shown in Figure 5 outlines the key components

needed for a LinkSwitch Supply.

Clamp diode – D1

Diode D1 should be either a fast (t

rr <250 ns) or ultra-fast type

rr <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.

Clamp Capacitor – C2

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.

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

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

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 V

selected during design. Any 1%, 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 V

OR of 50 V requires a diode PIV of 50 V).

Slow recovery diodes are not recommended (1N400X types).

Output Capacitor – C4

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.

The CONTROL pin capacitor C1 should be located as close as

possible to the SOURCE and CONTROL pins.

To minimize EMI coupling from the switching nodes on the

primary to both the secondary and AC input, the LinkSwitch

should be positioned away from the secondary of the transformer

and AC input.

Routing the primary return trace from the transformer primary

around LinkSwitch and associated components further reduces

coupling.

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.

Quick Design Checklist

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: In a LinkSwitch circuit, the SOURCE is a switching node.

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

supplied using an isolation transformer. The following minimum

set of tests is strongly recommended:

1. Maximum drain voltage – Verify that V

DSS does not exceed

675 V at highest input voltage and peak output power.

2. Maximum drain current – At maximum ambient temperature,

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.

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