LNK454/456-458/460
PI-5986-060810
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.
350
250
150
50
0.35
0.25
0.15
0.05
-0.05
-0.15
-0.25
Voltage
Current
0
50
100
150
200
250
300
350
-50
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.
-150
-250
-350
-0.35
Conduction Angle (°)
Figure 11. Ideal Dimmer Output Voltage and Current Waveforms for a Trailing Edge
Dimmer at 90° Conduction Angle.
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.
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.
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.
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.
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
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.
9
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POWERINT [ Power Integrations ]
