LNK520P-TL datasheet(9/20 Pages) POWERINT

Part #	LNK520P-TL

Description	Energy Efficient, CV or CV/CC Switcher for Very Low Cost Adapters and Chargers

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Manufacturer	POWERINT [Power Integrations, Inc.]

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9

LNK520

E

2/05

Core gaps should be uniform. Uneven core gapping, especially

with small gap sizes, may cause variation in the primary

inductance with ﬂux density (partial saturation) and make the

constant current region non-linear. To verify uniform gapping,

it is recommended that the primary current wave-shape be

examined while feeding the supply from a DC source. The

gradient is deﬁned as di/dt = V/L and should remain constant

throughout the MOSFET on time. Any change in gradient of

the current ramp is an indication of uneven gapping.

Measurements made using a LCR bridge should not be solely

reliedupon;typicallytheseinstrumentsonlymeasureatcurrents

of a few milliamps. This is insufﬁcient to generate high enough

ﬂux densities in the core to show uneven gapping.

For a typical EE16 or EE13 core using center leg gapping, a

0.08 mm gap allows a primary inductance tolerance of ±10% to

be maintained in standard high volume production. This allows

the EE13 to be used in designs up to 2.75 W with less than

300 mW no-load consumption. Using outer leg ﬁlm gapping

reducesinductancetoleranceto±7%orbetter,allowingdesigns

up to 3 W. Using the larger EE16 allows for a 3 W output

with center leg gapping. The EE13 core size may be attractive

in designs were space is limited or if there is a cost advantage

over the EE16.

The transformer turns ratio should be selected to give a V

OR

(output voltage reﬂected through secondary to primary turns

ratio) of 40 V to 80 V. Higher V

OR increases the output power

capability of LinkSwitch but also increases no-load power

consumption. This allows even higher values to be used in

designs where no-load power is not a concern. However care

should be taken to ensure that the maximum temperature rise

of the device is acceptable at the upper limit of the output

characteristic when used in a charger application. In all cases,

discontinuous mode operation should be maintained and note

that the linearity of the CC region of the power supply output

characteristic is inﬂuenced by the bias voltage. If this is an

important aspect of the application, the output characteristic

should be checked before ﬁnalizing the design.

Output Characteristic Variation

Boththedevicetoleranceandexternalcircuitgoverntheoverall

tolerance of the LinkSwitch power supply output characteristic.

Estimated peak power point tolerances for a LNK520, 2.75 W

design are ±10% for voltage and ±24% for current limit for

overall variation in high volume manufacturing. This includes

device and transformer tolerances (±7.5% assumed) and line

variation. Lower power designs may have poorer constant

current linearity.

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.

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. The

output voltage load variation can be improved across the whole

load range by adding an optocoupler and secondary reference

(Figure 6). The secondary reference is designed to only provide

feedbackabovethenormalpeakpowerpointvoltagetomaintain

the correct constant current characteristic.

Component Selection

The schematic shown in Figure 10 outlines the key components

needed for a LinkSwitch supply.

Clamp diode – D5

Diode D5 can be an ultra-fast (t

rr < 50 ns), a fast (trr < 250 ns)

or standard recovery diode with a voltage rating of 600 V or

higher. Astandardrecoverydiodeisrecommendedasitimproves

the CV characteristic, but should be a glass-passivated type

(1N400xGP) to ensure a deﬁned reverse recovery time.

Clamp Capacitor – C4

Capacitor C4 should be in the range of 100 pF to 1000 pF,

500 V capacitor. A low cost ceramic disc is 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. 330 pF is a good initial value, iterated

with R1.

Clamp Resistor – R1

The value of R1 is selected to be the highest value that still

provides adequate margin to the DRAIN BV

DSS rating at high

line. As a general rule, the value of C4 should be minimized

and R1 maximized.

CONTROL Pin Capacitor – C5

Capacitor C5 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.

Bias Capacitor – C3

Capacitor C3 should be a 1 µF, 50 V electrolytic type. The

voltage rating is consistent with the 20 V to 30 V seen across

the bias winding. Lower values give poorer regulation.

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