Rev. A 11/01/10

9

LNK454/456-458/460

www.powerint.com

Reduce the capacitance value to find the minimum acceptable

value. Reducing the capacitance value reduces power

dissipation and therefore increases efficiency.

If the bleeder circuit does not maintain conduction in the TRIAC,

then add a damper. The purpose of the damper is to limit the

inrush current (as the input capacitance charges) and

associated ringing that occurs when the TRIAC turns on.

Initially add a passive damper which is a simple resistor in series

with the AC input (R20 in Figure 7). Values in the range of 10

W

– 100

W are typical with the upper range being limited by the

allowed dissipation / temperature rise and reduction in

efficiency. Values below 10

W may also be used but are less

effective especially in high AC line input designs.

If a passive damper is insufficient to prevent incorrect TRIAC

operation then an active damper can be added. This is typical

in high line applications due to the much larger inrush current

that flows when the TRIAC turns on. A low cost active damper

circuit is formed by R3, R4, C3, Q3, R7 and R8 in Figure 7.

Resistor R7 and R8 limit the inrush current and can be a much

higher value than the passive case as they are in circuit for only

a fraction of the line cycle. Silicon controlled rectifier (SCR) Q3

shorts R7 and R8 after a delay defined by R3, R4 and C3. The

delay is adjusted to give the shortest time that provides

acceptable dimmer performance to minimize the dissipation in

the resistors. The SCR is a low current, low cost device

available in TO-92 packages with very low gate current

requirements. The gate drive requirement of the selected SCR

together with the minimum specified line voltage defines the

maximum value of R7 and R8.

It’s common for different dimmers to behave differently across

manufacturers and power ratings. For example a 300 W

dimmer requires less dampening and requires less power loss

in the bleeder than a 600 W or 1000 W dimmer due to the use

of a lower current rating TRIAC which typically have lower

holding currents. Line impedance differences can also cause

variation in behavior so during development the use of an AC

source is recommended for consistency however testing using

AC mains power should also be performed.

Electronic Trailing Edge Dimmers

Figure 11 shows the line voltage and current at the input of the

power supply with a trailing edge electronic dimmer. In this

example, the dimmer conducts at 90 degrees. This type of

dimmer typically uses a power MOSFET or IGBT to provide the

switching function and therefore no holding current is

necessary. Also since the conduction begins at the zero

crossing, high current surges and line ringing are not an issue.

Use of these types of dimmers typically does not require

damper and bleeder circuits.

Thermal Considerations

Lighting applications present unique thermal challenges for the

power supply designer. In many cases the LED load and

associated heatsink determine the power supply ambient

temperature. Therefore it is important to properly heatsink and

verify the operating temperatures of all devices. For the

LinkSwitch-PL device a SOURCE pin (D package) or exposed

pad (K or V package) temperature of <115

°C is recommended

to allow margin for unit to unit variation. Worst case conditions

are typically maximum output power, maximum external

ambient and either minimum or maximum input voltage.

Layout Considerations

Primary Side Connections

The BYPASS pin capacitor should be located as close to the

BYPASS pin and connected as close to the SOURCE pin as

possible. The SOURCE pin trace should not be shared with the

main power MOSFET switching currents. All FEEDBACK pin

components that connect to the SOURCE pin should follow the

same guideline as for the BYPASS pin capacitor.

It is critical that the main power MOSFET switching currents

return to the bulk capacitor with the shortest path possible. Long

high current paths create excessive conducted and radiated

noise.

50

100

150

200

250

300

350

Conduction Angle (°)

350

250

150

50

-50

-150

-250

-350

0.35

0.25

0.15

0.05

-0.05

-0.15

-0.25

-0.35

PI-5986-060810

Voltage

Current

0

Figure 11. Ideal Dimmer Output Voltage and Current Waveforms for a Trailing Edge

Dimmer at 90° Conduction Angle.