LNK500GNTL - Power Integrations, Inc.

Figure 1. Typical Application – Not a Simpliﬁed Circuit (a) and

Output Characteristic Tolerance Envelopes (b).

Product Highlights

Cost Effective Linear/RCC Replacement

• Lowest cost and component count, constant voltage (CV)

or constant voltage/constant current (CV/CC) solution

• Extremely simple circuit conﬁguration

• Up to 75% lighter power supply reduces shipping cost

• Primary based CV/CC solution eliminates 10 to 20 secondary

components for low system cost

• Combined primary clamp, feedback, IC supply, and loop

compensation functions–minimizes external components

• Fully integrated auto-restart for short circuit and open loop

fault protection–saves external component costs

• 42 kHz operation simpliﬁes EMI ﬁlter design

Much Higher Performance Over Linear/RCC

• Universal input range allows worldwide operation

• Up to 70% reduction in power dissipation–reduces enclosure

size signiﬁcantly

• CV/CC output characteristic without secondary feedback

• System level thermal and current limit protection

• Meets all single point failure requirements with only one

additional clamp capacitor

• Controlled current in CC region provides inherent soft-start

• Optional opto feedback improves output voltage accuracy

EcoSmart - Extremely Energy Efﬁcient

• Consumes <300 mW at 265 VAC input with no load

• Meets Blue Angel, Energy Star, and EC requirements

• No current sense resistors–maximizes efﬁciency

Applications

• Linear transformer replacement in all ≤3 W applications

• Chargers for cell phones, cordless phones, PDAs, digital

cameras, MP3/portable audio devices, shavers, etc.

• Home appliances, white goods and consumer electronics

• Constant output current LED lighting applications

• TV standby and other auxiliary supplies

Description

LinkSwitch is speciﬁcally designed to replace low power linear

transformer/RCC chargers and adapters at equal or lower system

cost with much higher performance and energy efﬁciency.

LNK500 is a lower cost version of the LNK501 with a wider

tolerance output CC characteristic. LinkSwitch introduces a

revolutionary patented topology for the design of low power

switching power supplies that rivals the simplicity and low

cost of linear adapters, and enables a much smaller, lighter, and

Table 1. Notes: 1. Output power for designs in an enclosed adapter

measured at 50 °C ambient. 2. See Figure 1 (b) for Min (CV only

designs) and Typ (CV/CC charger designs) power points identiﬁed

on output characteristic. 3. Uses higher reﬂected voltage transformer

designs for increased power capability – see Key Application

Considerations section. 4. For lead-free package options, see Part

Ordering Information.

attractive package when compared with the traditional “brick.”

With efﬁciency of up to 75% and <300 mW no-load consumption,

a LinkSwitch solution can save the end user enough energy

over a linear design to completely pay for the full power

supply cost in less than one year. LinkSwitch integrates a

700 V power MOSFET, PWM control, high voltage start-up, current

limit, and thermal shutdown circuitry, onto a monolithic IC.

LNK500

LinkSwitch® Family

Energy Efﬁcient, CV or CV/CC Switcher for

Very Low Cost Adapters and Chargers

December 2004

PI-3415-021103

Wide Range

HV DC Input

Output

(VO)

Min

(CV only)

Example Characteristic

Typ

(CV/CC)

(a)

(b)

For Circuit

Shown Above With Optional

Secondary Feedback**

*Estimated tolerance achievable in high volume production

including transformer and other component tolerances.

**See Optional Secondary Feedback section.

LinkSwitch

±10%

±25%*

±5%

±25%*

D S

IOIO

VOVO

OUTPUT POWER TABLE1

PRODUCT4230 VAC ±15% 85-265 VAC No-Load

Input

Power

Min2Typ2Min2Typ2

LNK500

P or G

3.2 W 4 W 2.4 W 3 W <300 mW

4.3 W 5.5 W 2.9 W 3.5 W <500 mW3

LNK500

12/04

Pin Functional Description

DRAIN (D) Pin:

Power MOSFET drain connection. Provides internal operating

current for start-up. Internal current limit sense point for drain

current.

CONTROL (C) Pin:

Error ampliﬁer and feedback current input pin for duty cycle

and current limit control. Internal shunt regulator connection

to provide internal bias current during normal operation. It is

also used as the connection point for the supply bypass and

auto-restart/compensation capacitor.

SOURCE (S) Pin:

Output MOSFET source connection for high voltage power

return. Primary side control circuit common and reference

point.

Figure 3. Pin Conﬁguration.

PI-3417-111802

P Package (DIP-8B)

G Package (SMD-8B)

LNK500

Figure 2. Block Diagram.

PI-3416-032603

SHUTDOWN/

AUTO-RESTART

PWM

COMPARATOR

CLOCK

SAW

OSCILLATOR

INTERNAL

SUPPLY

5.6 V

4.7

SOURCE

DMAX

CONTROL

5.6

IFB

EDGE

HYSTERETIC

THERMAL

SHUTDOWN

LEADING

EDGE

BLANKING

CURRENT

LIMIT

ADJUST

LOW

FREQUENCY

OPERATION

SHUNT REGULATOR/

ERROR AMPLIFIER

DRAIN

IDCS

CURRENT LIMIT

COMPARATOR

÷ 8

LNK500

12/04

LinkSwitch Functional Description

The duty cycle, current limit and operating frequency

relationships with CONTROL pin current are shown in

Figure 4. Figure 5 shows a typical power supply outline

schematic which is used below to describe the LinkSwitch

operation.

Power Up

During power up, as VIN is ﬁrst applied (Figure 5), the CONTROL

pin capacitor C1 is charged through a switched high voltage

current source connected internally between the DRAIN and

CONTROL pins (see Figure 2). When the CONTROL pin

voltage reaches approximately 5.6 V relative to the SOURCE

pin, the high voltage current source is turned off, the internal

control circuitry is activated and the high voltage internal

MOSFET starts to switch. At this point, the charge stored on

C1 is used to supply the internal consumption of the chip.

Constant Current (CC) Operation

As the output voltage, and therefore the reﬂected voltage

across the primary transformer winding ramp up, the feedback

CONTROL current IC increases. As shown in Figure 4, the

internal current limit increases with IC and reaches ILIM when IC

is equal to IDCT

. The internal current limit vs. IC characteristic

is designed to provide an approximately constant power supply

output current as the power supply output voltage rises.

Constant Voltage (CV) Operation

When IC exceeds IDCS, typically 2 mA (Figure 4), the maximum

duty cycle is reduced. At a value of IC that depends on power

supply input voltage, the duty cycle control limits LinkSwitch

peak current below the internal current limit value. At this point

the power supply transitions from CC to CV operation. With

minimum input voltage in a typical universal input design, this

transition occurs at approximately 30% duty cycle. Resistor R1

(Figure 5) is therefore initially selected to conduct a value of IC

approximately equal to IDCT when VOUT is at the desired value

at the minimum power supply input voltage. The ﬁnal choice

of R1 is made when the rest of the circuit design is complete.

When the duty cycle drops below approximately 4%, the

frequency is reduced, which reduces energy consumption under

light load conditions.

Auto-Restart Operation

When a fault condition, such as an output short circuit or open

loop, prevents ﬂow of an external current into the CONTROL

pin, the capacitor C1 discharges towards 4.7 V. At 4.7 V,

auto-restart is activated, which turns the MOSFET off and puts

the control circuitry in a low current fault protection mode. In

auto-restart, LinkSwitch periodically restarts the power supply

so that normal power supply operation can be restored when

the fault is removed.

Figure 4. CONTROL Characteristics.

Figure 5. Power Supply Outline Schematic.

VOUT

VIN

Link

witc

PI-2715-112102

PI-2799-112102

Internal Current Limit

CONTROL Current IC

IDCS

CONTROL Current IC

ICD1

CONTROL Current IC

Duty Cycle

Frequency

ILIM

IDCT

77%

30%

3.8%

fOSC

fOSC(low)

Auto-restart

LNK500

12/04

The characteristics described above provide an approximate

CV/CC power supply output without the need for secondary-

side voltage or current feedback. The output voltage regulation

is inﬂuenced by how well the voltage across C2 tracks the

reﬂected output voltage. This tracking is inﬂuenced by the

value of the transformer leakage inductance which introduces

an error. Resistor R2 and capacitor C2 partially ﬁlter the

leakage inductance voltage spike reducing this error. This

circuitry, used with standard transformer construction

techniques provides much better output load regulation than a

linear transformer, making this an ideal power supply solution

in many low power applications. If tighter load regulation is

required, an optocoupler conﬁguration can be used while still

employing the constant output current characteristics provided

by LinkSwitch.

Optional Secondary Feedback

Figure 6 shows a typical power supply outline schematic using

LinkSwitch with optocoupler feedback to improve output

voltage regulation. On the primary side, the schematic differs

from Figure 5 by the addition of R3, C3 and optocoupler U1.

R3 forms a potential divider with R1 to limit the U1 collector

emitter voltage.

On the secondary side, the addition of voltage sense circuit

components R4, VR1 and U1 LED provide the voltage feedback

signal. In the example shown, a simple Zener (VR1) reference

is used though a precision TL431 reference is typically needed

to provide ±5% output voltage tolerancing and cable drop

compensation, if required. Resistor R4 provides biasing for VR1.

The regulated output voltage is equal to the sum of the VR1

Zener voltage plus the forward voltage drop of the U1 LED.

Resistor R5 is an optional low value resistor to limit U1 LED

peak current due to output ripple. Manufacturerʼs speciﬁcations

for U1 current and VR1 slope resistance should be consulted

to determine whether R5 is required.

U1 is arranged with collector connected to primary ground and

emitter to the anode of D1. This connection keeps the opto in

an electrically “quiet” position in the circuit. If the opto was

Figure 6. Power Supply Outline Schematic with Optocoupler Feedback.

Figure 7. Inﬂuence of the Optocoupler on the Power Supply Output Characteristic.

LNK500

LinkSwitch

85-265

VAC

VOUT

RTN

CC1

VR1

PI-3418-071304

Output Voltage

Tolerance envelope

without optocoupler

Inherent

CC to CV

transition

point

Load variation

during battery

charging

Voltage

feedback

threshold

Characteristic with

optocoupler

Typical inherent

characteristic without

optocoupler

PI-2788-092101

Output Current

LNK500

12/04

Figure 8. Output Characteristic with Optocoupler Regulation (Reduced Voltage Feedback Threshold).

Output Voltage

Output Current

VO(MAX)

Tolerance envelope

without optocoupler

Characteristic with

optocoupler

Power supply peak

output power curve

Typical inherent

characteristic without

optocoupler

PI-2790-112102

Inherent

CC to CV

transition

point

Load variation

during battery

charging Characteristic observed with

load variation often applied during

laboratory bench testing

Voltage

feedback

threshold

instead placed on the cathode side of D1, it would become a

switching node, generating additional common mode EMI

currents through its internal parasitic capacitance.

The feedback conﬁguration in Figure 6 is simply a resistive

divider made up of R1 and R3 with D1, R2, C1 and C2 rectifying,

ﬁltering and smoothing the primary winding voltage signal. The

optocoupler therefore effectively adjusts the resistor divider ratio

to control the DC voltage across R1 and therefore, the feedback

current received by the LinkSwitch CONTROL pin.

When the power supply operates in the constant current (CC)

region, for example when charging a battery, the output voltage

is below the voltage feedback threshold deﬁned by U1 and

VR1 and the optocoupler is fully off. In this region, the circuit

behaves exactly as previously described with reference to

Figure 5 where the reﬂected voltage increases with increasing

output voltage and the LinkSwitch internal current limit is

adjusted to provide an approximate CC output characteristic.

Note that for similar output characteristics in the CC region,

the value of R1 in Figure 5 will be equal to the value of R1+R3

in Figure 6.

When the output reaches the voltage feedback threshold set by

U1 and VR1, the optocoupler turns on. Any further increase

in the power supply output voltage results in the U1 transistor

current increasing, which increases the percentage of the

reﬂected voltage appearing across R1. The resulting increase

in the LinkSwitch CONTROL current reduces the duty cycle

according to Figure 4 and therefore, maintains the output

voltage regulation.

Normally, R1 and R3 are chosen to be equal in value. However,

increasing R3 (while reducing R1 to keep R1 + R3 constant)

increases loop gain in the CV region, improving load regulation.

The extent to which R3 can be increased is limited by opto

transistor voltage and dissipation ratings and should be fully

tested before ﬁnalizing a design. The values of C2 and C3 are

less important other than to make sure they are large enough

to have very little inﬂuence on the impedance of the voltage

division circuit set up by R1, R3 and U1 at the switching

frequency. Normally, the values of C2 and C3 in Figure 6 are

chosen equal to the value of C2 in Figure 5, though the voltage

rating may be reduced depending on the relative values of R1

and R2 discussed above. See Applications section for typical

values of components.

Figure 7 shows the inﬂuence of optocoupler feedback on the

output characteristic. The envelope deﬁned by the dashed lines

represent the worst case power supply DC output voltage and

current tolerances (unit-to-unit and over the input voltage

range) if an optocoupler is not used. A typical example of an

inherent (without optocoupler) output characteristic is shown

dotted. This is the characteristic that would result if U1, R4 and

VR1 were removed. The optocoupler feedback results in the

characteristic shown by the solid line. The load variation arrow in

Figure 7 represents the locus of the output characteristic normally

seen during a battery charging cycle. The two characteristics

are identical as the output voltage rises but then separate as

shown when the voltage feedback threshold is reached. This

is the characteristic seen if the voltage feedback threshold is

above the output voltage at the inherent CC to CV transition

point also indicated in Figure 7.

Figure 8 shows a case where the voltage feedback threshold is

set below the voltage at the inherent CC to CV transition point.

In this case, as the output voltage rises, the secondary feedback

circuit takes control before the inherent CC to CV transition

occurs. In an actual battery charging application, this simply

limits the output voltage to a lower value.

LNK500

12/04

However, in laboratory bench tests, it is often more convenient

to test the power supply output characteristic starting from a

low output current and gradually increasing the load. In this

case, the optocoupler feedback regulates the output voltage until

the peak output power curve is reached as shown in Figure 8.

Under these conditions, the output current will continue to rise

until the peak power point is reached and the optocoupler turns

off. Once the optocoupler is off, the CONTROL pin feedback

current is determined only by R1 and R3 and the output current

therefore folds back to the inherent CC characteristic as shown.

Since this type of load transition does not normally occur in a

battery charger, the output current never overshoots the inherent

constant current value in the actual application.

In some applications it may be necessary to avoid any output

current overshoot, independent of the direction of load variation.

To achieve this goal, the minimum voltage feedback threshold

should be set at VO(MAX). This will ensure that the voltage at the

CC to CV transition point of the inherent characteristic will

always occur below the voltage feedback threshold. However, the

output voltage tolerance is then increased, since the inherent CV

characteristic tolerance below VO(MAX) is added to the tolerance

of the optocoupler feedback circuit.

Applications Example

The circuit shown in Figure 9 shows a typical implementation

of an approximate constant voltage / constant current (CV/CC)

charger using LinkSwitch. This design delivers 2.75 W with

a nominal peak power point voltage of 5.5 V and a current of

500 mA. Efﬁciency is greater than 70% over an input range

of 85 VAC to 265 VAC.

The bridge rectiﬁer, BR1, rectiﬁes the AC input. Resistor, RF1

is a fusible type providing protection from primary-side short

circuits. The rectiﬁed AC is smoothed by C1 and C2 with

inductor L1 forming a pi-ﬁlter in conjunction with C1 and C2

to ﬁlter conducted EMI. The switching frequency of 42 kHz

allows such a simple EMI ﬁlter to be used without the need for

a Y capacitor while still meeting international EMI standards.

When power is applied, high voltage DC appears at the DRAIN

pin of LinkSwitch (U1). The CONTROL pin capacitor C3 is then

charged through a switched high voltage current source connected

internally between the DRAIN and CONTROL pins. When

the CONTROL pin reaches approximately 5.6 V relative to the

SOURCE pin, the internal current source is turned off. The internal

control circuitry is activated and the high voltage MOSFET starts

to switch, using the energy in C3 to power the IC.

When the MOSFET is on, the high voltage DC bus is connected

to one end of the transformer primary, the other end being

connected to primary return. As the current ramps in the

primary of ﬂyback transformer T1, energy is stored. This

energy is delivered to the output when the MOSFET turns off

each switching cycle.

The secondary of the transformer is rectiﬁed and ﬁltered by D6

and C5 to provide the DC output to the load.

LinkSwitch dramatically simpliﬁes the secondary side by

controlling both the constant voltage and constant current regions

entirely from the primary side. This is achieved by monitoring

the primary-side VOR (voltage output reﬂected).

Diode D5 and capacitor C4 form the primary clamp network.

This both limits the peak drain voltage due to leakage inductance

and provides a voltage across C4, which is equal to the VOR plus

an error due to the parasitic leakage inductance. Resistor R2

ﬁlters the leakage inductance spike and reduces the error in the

value of the VOR. Resistor R1 converts this voltage into a current

that is fed into the CONTROL pin to regulate the output.

During CV operation the output is regulated through control of

the duty cycle. As the current into the CONTROL pin exceeds

approximately 2 mA, the duty cycle begins to reduce, reaching

30% at a CONTROL pin current of 2.3 mA.

Under light or no-load conditions, when the duty cycle reaches

approximately 4%, the switching frequency is reduced to lower

energy consumption.

If the output load is increased beyond the peak power point

(deﬁned by 0.5·LP·ILIM

2·f), the output voltage and VOR falls.

The reduced CONTROL pin current will lower the internal

LinkSwitch current limit (current limit control) providing an

approximately constant current output characteristic. If the

load is increased and the CONTROL pin current falls below

approximately 1 mA, the CONTROL pin capacitor C3 will

discharge and the supply enters auto-restart.

Current limit control removes the need for any secondary side

current sensing components (sense resistor, transistor, opto

coupler and associated components). Removing the secondary

sense circuit dramatically improves efﬁciency, giving the

associated beneﬁt of reduced enclosure size.

Key Application Considerations

Design Output Power

Table 1 (front page) provides guidance for the maximum

continuous output power from a given device under the

conditions speciﬁed.

The output of chargers (CV/CC) are normally speciﬁed at

the typical output peak power point. Conversely, non-charger

applications (CV only, which applies to many converters such

as adapters, standby/auxiliary supplies and other embedded

AC-DC converters) where CC operation is not required, are

normally speciﬁed at the minimum output power they will

supply under worst case conditions.

LNK500

12/04

Figure 9. 2.75 W Constant Voltage/Constant Current (CV/CC) Charger using LinkSwitch.

Figure 10. Measured Output Characteristic of the Circuit in Figure 9.

1000 200 300 400 500 600 700

Output Current (mA)

Output Voltage (V)

PI-3420-111802

VIN = 85 VAC

VIN = 115 VAC

VIN = 185 VAC

VIN = 265 VAC

4.7 µF

400 V

4.7 µF

400 V

RF1

10 Ω 1 W

Fusible

1 mH

20.5 kΩ

100 Ω

1N4937

0.1 µF

100 V

116 T

#34 AWG

EE13

LP = 2.55 mH

15 T

#30 AWG

TIW

11DQ06

470 µF

10 V

85-265

VAC

LNK500

LinkSwitch

5.5 V,

500 mA

RTN

BR1

1 A, 600 V

PI-3419-071304

PERFORMANCE SUMMARY

Output Power: 2.75 W

Efficiency: ≥72%

No Load

Consumption: 260 mW, 230 VAC

200 mW, 115 VAC

0.22 µF

50 V

LNK500

12/04

To aid the designer, the power table reﬂects these differences. For

CV/CC designs the typical power column and for CV designs

the minimum power column should be used, respectively.

Additionally, ﬁgures are based on the following conditions:

1. The minimum DC input bus voltage is 90 V or higher. This

corresponds to a ﬁlter capacitor of 3

µF/W for universal input

and 1 µF/W for 230 VAC or 115 VAC input with doubler

input stage.

2. Design is a discontinuous mode flyback converter.

Continuous mode designs can result in loop instability and

are therefore not recommended. For typical output power

ﬁgures, nominal values for primary inductance and I2f are

assumed. For minimum output power ﬁgures, primary

inductance minus 10% and the minimum I2f value are

assumed. For no-load consumption <300 mW, a VOR in the

range 40 V to 60 V is assumed. For no-load consumption

<500 mW and higher output power capability, a VOR in the

range 60 V to 100 V is assumed.

3. A secondary output of 5 V with a Schottky rectiﬁer diode.

4. Assumed efﬁciency of 70%.

5. The part is board mounted with SOURCE pins soldered to

sufﬁcient area of copper to keep the die temperature at or

below 100 °C.

6. An output cable with a total resistance of 0.2 Ω.

In addition to the thermal environment (sealed enclosure,

ventilated, open frame, etc), the maximum power capability

of LinkSwitch in a given application depends on transformer

core size, efﬁciency, primary inductance tolerance, minimum

speciﬁed input voltage, input storage capacitance, output voltage,

output diode forward drop, etc., and can be different from the

values shown in Table 1.

Transformer Design

To provide an approximately CV/CC output, the transformer

should be designed to be discontinuous; all the energy stored

in the transformer is transferred to the secondary during the

MOSFET off time. Energy transfer in discontinuous mode is

independent of line voltage.

The peak power point prior to entering constant current operation

is deﬁned by the maximum power transferred by the transformer.

The power transferred is given by the expression P = 0.5·LP·I2·f,

where LP is the primary inductance, I2 is the primary peak current

squared and f is the switching frequency.

To simplify analysis, the data sheet parameter table speciﬁes an

I2f coefﬁcient. This is the product of current limit squared and

switching frequency normalized to the feedback parameter IDCT

This provides a single term that speciﬁes the variation of the

peak power point in the power supply due to LinkSwitch.

As primary inductance tolerance is part of the expression

that determines the peak output power point (start of the CC

characteristic) this parameter should be well controlled. For

an estimated overall constant current tolerance of ±25% the

primary inductance tolerance should be ±10% or better. This

is achievable using standard low cost, center leg gapping

techniques where the gap size is typically 0.08 mm or larger.

Smaller gap sizes are possible but require non-standard, tighter

ferrite AL tolerances.

Other gapping techniques such as ﬁlm gapping allow tighter

tolerances (±7% or better) with associated improvements in

the tolerance of the peak power point. Please consult your

transformer vendor for guidance.

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 an LCR bridge should not be solely

relied upon; typically these instruments only measure at currents

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

ﬂux densities in the core to show uneven gapping.

For a typical EE13 core using center leg gapping, a 0.08 mm

gap (ALG of 190 nH/t2) 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. If ﬁlm gapping is used

then this increases to 3 W. Moving to a larger core, EE16 for

example, allows a 3 W output with center leg gapping.

The transformer turns ratio should be selected to give a VOR

(output voltage reﬂected through secondary to primary turns

ratio) of 40 V to 60 V. In designs not required to meet 300 mW

no-load consumption targets, the transformer can be designed

with higher VOR as long as discontinuous mode operation is

maintained. This increases the output power capability. For

example, a 230 VAC input design using an EE19 transformer

core with VOR >70 V, is capable of delivering up to 5.5 W typical

output power. Note: the linearity of the CC region of the power

supply output characteristic is inﬂuenced by VOR. If this is an

important aspect of the application, the output characteristic

should be checked before ﬁnalizing the design.

Output Characteristic Variation

Both the device tolerance and external circuit govern the overall

tolerance of the LinkSwitch output characteristic. Estimated

peak power point tolerances for a 3 W design are ±10% for

LNK500

12/04

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.

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.

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 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 (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.

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 VOR

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).

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

Since 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.

LNK500

12/04

Figure 11. Recommended Circuit Board Layout for LinkSwitch using P Package.

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 VDS 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.

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.

3. Thermal check – At peak output power, minimum input

voltage and maximum ambient temperature, verify that the

temperature speciﬁcations are not exceeded for LinkSwitch,

transformer, output diode and output capacitors. Enough

thermal margin should be allowed for part-to-part variation of

the RDS(ON) of LinkSwitch as speciﬁed in the data sheet. Under

low line, peak power, a maximum LinkSwitch SOURCE pin

temperature of 100 °C is recommended to allow for these

variations.

4. Centered output characteristic – Using a transformer with

nominal primary inductance and at an input voltage midway

between low and high line, verify that the peak power point

occurs at the desired nominal output current, with the correct

output voltage. If this does not occur then the design should

be reﬁned to ensure the overall tolerance limits are met.

Design Tools

Up to date information on design tools can be found at the

Power Integrations website: www.powerint.com.

HV DC

Input

PI-2900-070202

Transformer

DC Out

S S SS

LinkSwitch

Y1-

Capacitor

Input Filter

Capacitor

Output

Capacitor

LNK500

12/04

ABSOLUTE MAXIMUM RATINGS(1,4)

DRAIN Voltage .................................. ................ -0.3 V to 700 V

DRAIN Peak Current......................................400 mA

CONTROL Voltage ................................................ -0.3 V to 9 V

CONTROL Current (not to exceed 9 V)............100 mA

Storage Temperature .......................................... -65 °C to 150 °C

Operating Junction Temperature(3) ..................... -40 °C to 150 °C

Lead Temperature(4) ........................................................260 °C

Notes:

1. All voltages referenced to SOURCE, TA = 25 °C.

2. Normally limited by internal circuitry.

3. 1/16 in. from case for 5 seconds.

4. Maximum ratings speciﬁed may be applied, one at a time,

without causing permanent damage to the product.

Exposure to Absolute Maximum Rating conditions for

extended periods of time may affect product reliability.

THERMAL IMPEDANCE

Thermal Impedance: P or G Package:

(θJA) ........................... 70 °C/W(2); 55 °C/W(3)

(θJC)(1) ............................................... 11 °C/W

Notes:

1. Measured on pin 2 (SOURCE) close to plastic interface.

2. Soldered to 0.36 sq. in. (232 mm2), 2 oz. (610 g/m2) copper clad.

3. Soldered to 1 sq. in. (645 mm2), 2 oz. (610 g/m2) copper clad.

Parameter Symbol

Conditions

SOURCE = 0 V; TJ = -40 to 125 °C

See Figure 12

(Unless Otherwise Speciﬁed)

Min Typ Max Units

CONTROL FUNCTIONS

Switching

Frequency fOSC IC = IDCT

, TJ = 25 °C 34.5 42 49.5 kHz

Low Switching

Frequency fOSC(LOW)

Duty Cycle = DCLF

TJ = 25 °C 24 30 36 kHz

Duty Cycle at Low

Switching

Frequency

DCLF

Frequency Switching from fOSC to

fOSC(LOW), TJ = 25 °C 2.4 3.8 5.2 %

Low Frequency

Duty Cycle Range DC(RANGE) Frequency = fOSC(LOW), TJ = 25 °C 1.8 3.15 4.5 %

Maximum Duty

Cycle DCMAX IC = 1.5 mA 74 77 80 %

PWM Gain DCREG IC = IDCT, TJ = 25 °C -0.45 -0.35 -0.25 %/µA

CONTROL Pin

Current at 30%

Duty Cycle

IDCT

TJ = 25 °C

See Figure 4 2.21 2.30 2.39 mA

CONTROL Pin

Voltage VC(IDCT) IC = IDCT 5.5 5.75 6 V

Dynamic

Impedance ZCIC = IDCT, TJ = 25 °C 60 90 120 Ω

LNK500

12/04

Parameter Symbol

Conditions

SOURCE = 0 V; TJ = -40 to 125 °C

See Figure 12

(Unless Otherwise Speciﬁed)

Min Typ Max Units

SHUTDOWN/AUTO-RESTART

CONTROL Pin

Charging Current IC(CH) TJ = 25 °C VC = 0 V -4.5 -3.25 -2 mA

VC = 5.15 V -2.3 -1.3 -0.3

Control/Supply/

Discharge Current

ICD1 TJ = 25 °C Output MOSFET Enabled 0.95 1.06 1.14 mA

ICD2 TJ = 25 °C Output MOSFET Disabled 0.7 0.9 1.1

Auto-Restart

Threshold Voltage VC(AR) 5.6 V

Auto-Restart

Hysteresis Voltage VC(AR)hyst 0.9 V

Auto-Restart Duty

Cycle DC(AR)

Short Circuit Applied at

Power Supply Output 8 %

Auto-Restart

Frequency f(AR)

S2 Open

C1 = 0.22 µF (See Figure 12) 300 Hz

CIRCUIT PROTECTION

Self-Protection

Current Limit ILIM

TJ = 25 °C

di/dt = 90 mA/µs

See Note C

228 254 280 mA

I2f Coefﬁcient I2f

TJ = 25 °C

di/dt = 90 mA/µs

See Notes C, D

2412 2710 3008 A2Hz

Current Limit at

Auto-Restart ILIM(AR) IC = ICD1, TJ = 25 °C 158 mA

Power Up Reset

Threshold Voltage VC(RESET) 1.5 2.75 4.0 V

Leading Edge

Blanking Time tLEB IC = IDCT, TJ = 25 °C 200 300 ns

Current Limit Delay tIL(D) TJ = 25 °C 100 ns

Thermal Shutdown

Temperature IC = IDCT 125 135 °C

Thermal Shutdown

Hysteresis 70 °C

LNK500

12/04

NOTES:

A. For speciﬁcations with negative values, a negative temperature coefﬁcient corresponds to an increase in magnitude with

increasing temperature, and a positive temperature coefﬁcient corresponds to a decrease in magnitude with increasing

temperature.

B. Breakdown voltage may be checked against minimum BVDSS speciﬁcation by ramping the DRAIN pin voltage up to but not

exceeding minimum BVDSS.

C. IC is increased gradually to obtain maximum current limit at di/dt of 90 mA/µs. Increasing IC further would terminate the cycle

through duty cycle control.

D. This parameter is normalized to IDCT to correlate to power supply output current (it is multiplied by IDCT(nominal)/IDCT).

E. It is possible to start up and operate LinkSwitch at DRAIN voltages well below 36 V. However, the CONTROL pin charging

current is reduced, which affects start-up time, auto-restart frequency, and auto-restart duty cycle. Refer to the characteristic

graph on CONTROL pin charge current (IC) vs. DRAIN voltage (Figure 13) for low voltage operation characteristics.

Parameter Symbol

Conditions

SOURCE = 0 V; TJ = -40 to 125 °C

See Figure 12

(Unless Otherwise Speciﬁed)

Min Typ Max Units

OUTPUT

ON-State

Resistance RDS(ON) ID = 25 mA TJ = 25 °C 28 32 Ω

TJ = 25 °C 42 48

OFF-State Drain

Leakage Current IDSS

VC = 6.2 V

VD = 560 V, TA = 125 °C50 µA

Breakdown Voltage BVDSS

See Note B

VC = 6.2 V, TA = 25 °C700 V

DRAIN Supply

Voltage See Note E 36 50 V

LNK500

12/04

Figure 12. LinkSwitch General Test Circuit.

Figure 13. IC vs. DRAIN Voltage.

Figure 14. Duty Cycle Measurement.

120

100

0.0 2.0 4.0 8.06.0 10.0 12.0 14.0

CONTROL Pin Voltage (V)

PI-2895-102303

CONTROL Pin Current (mA)

Figure 15. CONTROL Pin I-V Characteristic.

2.15 2.25 2.35 2.45 2.55 2.65

CONTROL Pin Current (mA)

Duty Cycle (%)

PI-2902-051904

Figure 16. Duty Cycle vs. CONTROL Pin Current.

1.2

1.6

0 20 40 60 80 100

DRAIN Voltage (V)

CONTROL Pin

Charging Current (mA)

PI-2901-071602

0.4

0.8

VC = 5.15 V

LinkSwitch

PI-2894-031004

0.22 µF

40 V 40 V

750 Ω

10 kΩ

LNK500

12/04

Typical Performance Characteristics

Figure 17. Breakdown Voltage vs. Temperature.

1.200

1.000

0.800

0.400

0.600

0.200

0.000

-50 0 50 100 150

Junction Temperature (°C)

PI-2896-062802

Switching Frequency

(Normalized for 25 °C)

1.200

1.000

0.800

0.400

0.600

0.200

0.000

-50 -25 0 25 50 75 100 125 150

Junction Temperature (°C)

PI-2897-062802

Current Limit

(Normalized for 25 °C)

1.2

0.8

0.4

0.6

0.2

-50 0 50 100 150

Temperature (°C)

PI-2899-062802

PWM Gain (Normalized for 25 °C)

Figure 18. Switching Frequency vs. Temperature.

Figure 19. Current Limit vs. Temperature.

1.1

1.0

0.9

-50 -25 0 25 50 75 100 125 150

Junction Temperature (°C)

Breakdown Voltage

(Normalized to 25 °C)

PI-2213-012301

Figure 22. PWM Gain vs. Temperature.

1.200

1.000

0.800

0.400

0.600

0.200

0.000

-50 0 50 100 150

Junction Temperature (°C)

PI-2898-062802

IDCT (Normalized for 25 °C)

Figure 21. IDCT vs. Temperature.

Figure 20. I2f Coefﬁcient vs. Temperature.

1.2

0.8

1.0

0.0-50 -25 0 25 7550 100 150125

Junction Temperature (°C)

I2f Coefficient

(Normalized for 25 °C)

PI-2910-071602

0.2

0.6

0.4

LNK500

12/04

Typical Performance Characteristics (cont.)

Figure 23. Output Characteristics (DRAIN Current vs.

DRAIN Voltage).

Drain Voltage (V)

Drain Current (mA)

300

250

200

100

150

0 2 4 6 8 10

TCASE=25 °C

TCASE=100 °C

PI-2222-031401

LNK500

12/04

Notes:

1. Package dimensions conform to JEDEC specification

MS-001-AB (Issue B 7/85) for standard dual-in-line (DIP)

package with .300 inch row spacing.

2. Controlling dimensions are inches. Millimeter sizes are

shown in parentheses.

3. Dimensions shown do not include mold flash or other

protrusions. Mold flash or protrusions shall not exceed

.006 (.15) on any side.

4. Pin locations start with Pin 1, and continue counter-clock-

wise to Pin 8 when viewed from the top. The notch and/or

dimple are aids in locating Pin 1. Pin 6 is omitted.

5. Minimum metal to metal spacing at the package body for

the omitted lead location is .137 inch (3.48 mm).

6. Lead width measured at package body.

7. Lead spacing measured with the leads constrained to be

perpendicular to plane T.

.008 (.20)

.015 (.38)

.300 (7.62) BSC

(NOTE 7)

.300 (7.62)

.390 (9.91)

.367 (9.32)

.387 (9.83)

.240 (6.10)

.260 (6.60)

.125 (3.18)

.145 (3.68)

.057 (1.45)

.068 (1.73)

.120 (3.05)

.140 (3.56)

.015 (.38)

MINIMUM

.048 (1.22)

.053 (1.35)

.100 (2.54) BSC

.014 (.36)

.022 (.56)

-E-

Pin 1

SEATING

PLANE

-D-

-T-

P08B

DIP-8B

PI-2551-121504

D S .004 (.10)

⊕

T E D S .010 (.25) M

⊕

(NOTE 6)

.137 (3.48)

MINIMUM

PART ORDERING INFORMATION

LinkSwitch Product Family

Series Number

Package Identiﬁer

G Plastic Surface Mount DIP

P Plastic DIP

Lead Finish

Blank Standard (Sn Pb)

N Pure Matte Tin (Pb-Free)

Tape & Reel and Other Options

Blank Standard Conﬁgurations

TL Tape & Reel, 1 k pcs minimum, G package only

LNK 500 G N - TL

LNK500

12/04

SMD-8B

PI-2546-121504

.004 (.10)

.012 (.30)

.036 (0.91)

.044 (1.12)

.004 (.10)

0 -

° 8°

.367 (9.32)

.387 (9.83)

.048 (1.22) .009 (.23)

.053 (1.35)

.032 (.81)

.037 (.94)

.125 (3.18)

.145 (3.68)

-D-

Notes:

1. Controlling dimensions are

inches. Millimeter sizes are

shown in parentheses.

2. Dimensions shown do not

include mold flash or other

protrusions. Mold flash or

protrusions shall not exceed

.006 (.15) on any side.

3. Pin locations start with Pin 1,

and continue counter-clock-

wise to Pin 8 when viewed

from the top. Pin 6 is omitted.

4. Minimum metal to metal

spacing at the package body

for the omitted lead location

is .137 inch (3.48 mm).

5. Lead width measured at

package body.

6. D and E are referenced

datums on the package

body.

.057 (1.45)

.068 (1.73)

(NOTE 5)

E S

.100 (2.54) (BSC)

.372 (9.45)

.240 (6.10) .388 (9.86)

.137 (3.48)

MINIMUM

.260 (6.60) .010 (.25)

-E-

Pin 1

D S .004 (.10)

⊕

G08B

.420

.046 .060 .060 .046

.080

Pin 1

.086

.186

.286

Solder Pad Dimensions

LNK500

12/04

LNK500

12/04

Revision Notes Date

B 1) Released Final Data Sheet. 3/03

C 1) Added lead-free ordering information. 12/04

For the latest updates, visit our website: www.powerint.com

Power Integrations may make changes to its products at any time. Power Integrations has no liability arising from your use of any information, device or circuit

described herein nor does it convey any license under its patent rights or the rights of others. POWER INTEGRATIONS MAKES NO WARRANTIES HEREIN

AND SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY,

FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS.

PATENT INFORMATION

The products and applications illustrated herein (including circuits external to the products and transformer construction) may be covered by one or more U.S.

and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power Integrationsʼ patents

may be found at www.powerint.com.

LIFE SUPPORT POLICY

POWER INTEGRATIONSʼ PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS

WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF POWER INTEGRATIONS. As used herein:

1. Life support devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform, when

properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a signiﬁcant injury to the user.

2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life

support device or system, or to affect its safety or effectiveness.

The PI logo, TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch and EcoSmart are registered trademarks of

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